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A Methodology for Tribological Examination of Thin Film in the Nanodisplacement Regime


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A METHODOLOGY FOR TRIBOL OGICAL EXAMINATION OF THIN FILMS IN THE N ANODISPLACEMENT REGIME By GERALD R. BOURNE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2006

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Copyright 2006 by Gerald R. Bourne

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To Wanda and Gabrielle

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iv ACKNOWLEDGMENTS First and foremost, I would like to tha nk my wife, Wanda, for seeing this long journey to the end with unwavering support. Without her encouragement, I would not have embarked on this path, let alone seen it to completion. I would like to thank Greg Sawyer for his advice and support. His dedication to his students and research is insp iring and unparalleled. Thanks to Carol Sawyer for putting up with Greg. Thanks to Amelia Dempere; I could not ask for a more understanding and flexible boss. Thanks to Mike Kaufma n for showing me what fun hovering over a phosphorous screen for several hours could be I appreciate Kevin Jones accepting a committee position at this late stage in the project. Thanks to Nam-Ho Kim for his contributions in finite element analysis. I would like to thank Jeff Bardt for his contribution on many levels, from keeping me young with our juvenile banter and back and forth hijinks, to the extensive assistance with my computer incompetence. Thanks to Ben Boesl, Matt Hamilton, Pam Dickrell, Nicole McCook, Kerry Siebein, and Junghun Ja ng for contributions to and useful discussions about this project. Thanks to the students of the Tribology Lab for support and comic relief. Thanks to the MAIC facility and its sta ff for use of the instruments, financial support, and discussions about te chniques and analysis. Finall y, I would like to thank the faculty at the Materials Science and Engineering Department of the University of Florida. As an undergraduate, I was inspired to pursue research in this field from the knowledge,

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v enthusiasm, and support from Richard Conne ll, Robert DeHoff, Mike Kaufman, and Fereshteh Ebrahimi.

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES.............................................................................................................ix LIST OF FIGURES.............................................................................................................x ABSTRACT.......................................................................................................................xv CHAPTER 1 INTRODUCTION AND BACKGROUND.................................................................1 Introduction...................................................................................................................1 Background...................................................................................................................3 Early Friction Studies...................................................................................................4 Mechanism for Metallic Friction..................................................................................5 Sliding Wear.................................................................................................................7 Prow Formation.....................................................................................................7 Delamination Theory.............................................................................................8 Subsurface Wear....................................................................................................9 Film Thickness Wear and Friction Dependence...................................................9 Modeling.....................................................................................................................11 2 EXPERIMENTAL MATRIX.....................................................................................13 Sample Preparation.....................................................................................................14 Bulk Gold Foil............................................................................................................15 Ion Beam Sputtered Gold on Silicon..........................................................................16 Electron Beam Evaporated Gold on Silicon...............................................................19 Carbon Arc Evaporation.............................................................................................20 Ion Beam Etched Area of Interest..............................................................................21 3 TEST EQUIPMENT...................................................................................................23 Hysitron Triboindenter...............................................................................................25 CSM Microtribometer................................................................................................28 4 EXPERIMENTAL PROCEDURES...........................................................................30

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vii Hysitron Triboindenter Procedures.............................................................................30 Tip Preparation....................................................................................................30 Sample Mounting................................................................................................32 Electrostatic Force Constant and Zero Volt Gap Calibration..............................33 Tip Optics Offset Calibration..............................................................................33 Standard tip optics offset procedure.............................................................33 Modified tip optics offset procedure............................................................35 Quick Approach...................................................................................................43 Tip Area Function Calibration.............................................................................43 Machine Compliance Calibration........................................................................44 Testing Modes.....................................................................................................45 Depth sensing indentation............................................................................46 Rastering wear test.......................................................................................47 Reciprocating wear test................................................................................48 CSM Microtribometer................................................................................................48 Tip Preparation....................................................................................................48 Sample Mounting................................................................................................48 Reciprocating Wear Test.....................................................................................49 5 DATA ANALYSIS....................................................................................................50 Indentation Testing.....................................................................................................50 Oliver-Pharr Method...........................................................................................50 Compliance Method (Stones Method)...............................................................54 Modified Winkler Method...................................................................................57 Rastering Wear Test...................................................................................................63 Triboindenter Reciprocating Wear Test.....................................................................65 CSM Microtribometer................................................................................................66 6 CHARACTERIZATION............................................................................................67 Tip Characterization...................................................................................................67 Indentation Method.............................................................................................67 Scanning Method.................................................................................................68 Transmission Electron Microscopy Method.......................................................71 X-ray Diffraction........................................................................................................74 Crystallographic Texture.....................................................................................76 Grain Size............................................................................................................78 Scanning Electron Microscopy...................................................................................79 Focused Ion Beam Milling.........................................................................................80 Transmission Electron Micr oscopy Sample Thinning........................................80 Slice and View Serial Sectioning........................................................................83 Fixed sample method...................................................................................84 Rotated sample method................................................................................85 Transmission Electron Microscopy Analysis.............................................................87 7 RESULTS AND DISCUSSION.................................................................................89

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viii Static Indentation Testing...........................................................................................89 Oliver and Pharr Analysis Results.......................................................................89 Stone Analysis Results........................................................................................92 Modified Winkler Approach...............................................................................95 Rastering Wear Tests..................................................................................................96 Reciprocating Wear Tests...........................................................................................99 Film Failure.........................................................................................................99 Gold on silicon without carbon..................................................................101 Carbon on gold on silicon..........................................................................103 Electron Microscopy Cross S ections of Wear Scars.........................................105 Friction Measurements......................................................................................110 Hysitron Triboindenter friction measurements..........................................110 CSM microtribometer friction coefficient measurements..........................114 8 CONCLUSIONS......................................................................................................117 APPENDIX A SCANNING ELECTRON MICROSCOPY IMAGES............................................119 B SCANNING ELECTRON MICROSCOPY CROSS SECTIONS...........................123 LIST OF REFERENCES.................................................................................................127 BIOGRAPHICAL SKETCH...........................................................................................131

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ix LIST OF TABLES Table page 1-1 Summary of studies of thin metallic films on substrates.........................................10 2-1 Sample naming designations....................................................................................14 7-1 Hardness values calculated for sa mples from the Hall-Petch equation...................92 7-2 Hardness values calculated for samples from the Stone method.............................95 7-3 Results of SEM micrographs from gold on silicon samples with 100 N normal load.........................................................................................................................101 7-4 Results of SEM micrographs from gold on silicon samples with 500 N normal load.........................................................................................................................102 7-5 Results of SEM micrographs from carbon on gold on silicon samples with 100 N normal load.......................................................................................................104 7-6 Results of SEM micrographs from carbon on gold on silicon samples with 500 N normal load.......................................................................................................104

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x LIST OF FIGURES Figure page 1-1 Slider and areas from Equation 1-3............................................................................6 1-2 Three slider geometries from Bowden and Tabors experiment................................6 1-3 Effect of film thickness on friction coefficient of metallic film solid lubricants on harder substrates. ...............................................................................................10 2-1 South Bay Technology 8 polishing wheel..............................................................16 2-2 Ion beam sputtering process.....................................................................................17 2-3 Gatan Model 681 High Reso lution Ion Beam Coater..............................................17 2-4 Electron beam evaporation process..........................................................................19 2-5 Carbon arc evaporation............................................................................................20 2-6 Rastering wear test 4 m x 4 m with low contact pressure....................................21 2-7 Etched grid lines for wear test location....................................................................22 3-1 A typical pin-on-di sc type tribometer......................................................................23 3-2 Linear reciprocating tribometer................................................................................24 3-3 Hysitron Triboindenter with upper ther mal-acoustic isolation cover removed to show interior details.................................................................................................25 3-4 Capacitive transducer assembly schematic..............................................................26 3-5 Enlarged view of Tr iboindenter components...........................................................27 3-6 CSM Microtribometer............................................................................................28 4-1 Diamond indenter tip and assembly of a large radius sapphire tip..........................32 4-2 Optical CCD microscope im age of H-pattern indents.............................................34

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xi 4-3 Spherical indenter on flat surface and cross sectional geometry to solve for depth of indentation............................................................................................................35 4-4 Patterned modified tip optics offset sample.............................................................37 4-5 Modified tip optics offset samp le centered in the video window............................39 4-6 Single indent selected...............................................................................................39 4-7 Manual height adjustment window..........................................................................40 4-8 Search option window..............................................................................................40 4-9 Imaging window and control panel with image of modified tip optics offset sample.......................................................................................................................41 4-10 Actual tip optics offset seen in the video window...................................................42 4-11 Triboindenter testing modes.....................................................................................46 4-12 Rastering wear test showing fiduc ial indentation marks in circles..........................47 4-13 CSM Microtribometer............................................................................................48 5-1 Load versus displacement curve along with a cross secti on of an indented surface. The range of is indicated for various indenter geometries......................51 5-2 Measured compliance versus square root of maximum load for a load range of 5mN to 10mN on quartz, Al (001), and Si (001).....................................................55 5-3 Indent cross sections from Oliver-Pha rr and Modified Winkler compared to an actual indent.............................................................................................................58 5-3 Differential element described with cy lindrical coordinates and tip displacement based on a spherical tip geometry............................................................................60 5-4 Differential element illustration prior to (l eft) and after load (right) with variable notation indicated.....................................................................................................61 5-5 Loading code flow chart...........................................................................................62 5-6 Unloading code flow chart.......................................................................................63 5-7 Rastering wear test data plot showing a trenched area on the right of the test area and a pile-up area on the left....................................................................................64 5-8 Reciprocating wear test showing the transient areas near the ends of the reciprocation and the area over which th e friction force data is analyzed...............66

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xii 6-1 Large radius tip scanning a small radius asperity....................................................68 6-2 Sharp asperity milled on silicon to scan indenter tip...............................................69 6-3 Scanning image produced with a sharp asperity by rastering first in the x direction, and then in the y direction........................................................................70 6-4 Transmission Electron Micr oscope 200 CX sample holde r with tip holder fixture and indenter tip mounted..........................................................................................71 6-5 The upper images are taken at 40 CC W, 0, and 40 CW respectively. The lower image shows data point s plotted along the profile.........................................72 6-6 Kikuchi map observed in diffraction mode..............................................................73 6-7 Top down view of the si ngle crystal diamond indenter tip with crystallographic orientations and slidi ng direction indicated..............................................................73 6-8 X-ray diffractometer with a representation of Braggs law.....................................75 6-9 X-ray scans of the six different gold films shown with indexed peaks....................77 6-10 Grain size and strain versus gold film thickness......................................................79 6-11 Wear track in center of image to be pr epared with cross sectional TEM parallel to the wear direction.................................................................................................81 6-12 Focused ion beam cross s ection sample preparation................................................82 6-13 Cross section removed from the sa mple trench with a glass rod.............................83 6-14 Trenched area in preparation of fixe d sample method slice and view serial sectioning procedure................................................................................................84 6-15 Slice and view rotating sample method....................................................................86 6-16 Gold films with wear track s run for half cycles at 500 N normal load. Film thicknesses are indicated on each image..................................................................88 7.1 Hardness determined by the Oliver and Pharr method versus maximum tip displacement for five samples of varying thickness.................................................89 7-2 Stone plot including lin ear curve fits for sample EBE 20 with a gold film thickness of 27 nm....................................................................................................92 7-3 Stone plot including lin ear curve fits for sample IBS 100 with a gold film thickness of 105 nm..................................................................................................93

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xiii 7-4 Stone plots including linear curve fi ts for samples EBE 200, IBS 300, and EBE 500 with a gold film thickness of 180 nm, 315 nm, and 537 nm, repectively.........94 7-5 Hardness determined by four methods pl otted versus sample thickness for five samples.....................................................................................................................96 7-6 Scanning electron micrographs from a ra stering wear test matrix with a 40 N normal load varying tip geometry and number of cycles as indicated.....................97 7-7 Scanning electron micrograph showing a ra stering wear anomaly. The wear scar extends outside of the prescr ibed area especially in the horizontal direction..........98 7-8 Scanning electron micrograph showing a rastering wear anomaly. A trench develops on the right and lowe r edge of the wear scar............................................98 7-9 Scanning electron micrographs of sa mples IBS 300 (left) and EBE 200 (right) run for 30 cycles at 500 N normal loads................................................................99 7-10 Scanning electron micrographs of sample EBE 200 C run for 3 cycles (left) and 30 cycles (right) at 500 N normal load.................................................................100 7-11 Scanning electron micrograph of sample IBS 100 after 1 cycl e of sliding with 500 N normal load................................................................................................102 7-12. Nominal film thickness versus number of cycles prior to film failure..................103 7-13 Vertical tip displacement versus lateral displacement for a 30-cycle reciprocating test....................................................................................................105 7-14 Transverse cross section of a wear scar run on sample EBE 500 for 10 cycles with a 500 N normal load.....................................................................................106 7-15 Depth of wear scar versus numbe r of cycles for samples run at 500 N normal load.........................................................................................................................108 7-16 Fraction of coating depth penetrated versus number of cycles for samples run at 500 N normal load................................................................................................109 7-17 Friction coefficient versus number of cycles in log scale for gold on silicon samples without carbon run at 500 N normal load..............................................111 7-18 Friction coefficient versus number of cycles on a log scale for gold on silicon samples without carbon coating run at 100 uN normal load..................................112 7-19 Friction coefficient versus number of cycles for gold on silicon samples with carbon coating run at 500 uN normal load.............................................................113 7-20 Friction coefficient versus number of cycles for gold on silicon samples with carbon coating run at 100 uN normal load.............................................................114

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xiv 7-21 Friction coefficient versus number of cycles for a contact pressure of 92 MPa run on sample EBE 200..........................................................................................115 A-1 Wear scar of gold on silicon at 100 N load normal load.....................................119 A-2 Wear scar of gold on silicon at 500 N load normal load.....................................120 A-3 Wear scar of carbon on gold on silicon at 100 N load normal load....................121 A-4 Wear scar of carbon on gold on silicon at 500 N load normal load....................122 B-1 Sample IBS 100 at 500 N normal loads. A)1 cycle. B) 3 cycles.......................123 B-2 Sample EBE 200 at 500 N normal loads. A)1 cycle. B) 3 cycles......................123 B-3 Sample EBE 200 at 500 N normal loads and 10 cycles.......................................124 B-4 Sample IBS 300 at 500 N normal loads. A)1 cycle. B) 3 cycles.......................124 B-5 Sample IBS 300 at 500 N normal loads. A) 10 cycle. B) 30 cycles..................124 B-6 Sample IBS 300 at 500 N normal loads. A) 100 cycle. B) 300 cycle................125 B-7 Sample EBE 500 at 500 N normal loads. A) 1 cycle. B) 3 cycle.......................125 B-8 Sample EBE 500 at 500 N normal loads. A) 10 cycle. B) 30 cycle...................125 B-9 Sample EBE 500 at 500 N normal loads. A) 100 cycle. B) 300 cycle...............126

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xv Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy A METHODOLOGY FOR TRIBOL OGICAL EXAMINATION OF THIN FILMS IN THE N ANODISPLACEMENT REGIME By Gerald R. Bourne August 2006 Chair: Luisa A. Dempere Cochair: W. Gregory Sawyer Major Department: Materials Science and Engineering This research aims to develop a met hodology for studying tribology of thin films on the nanodisplacement level for both fundame ntal and practical w ear applications. A Hysitron Triboindenter was chosen to perform the majority of the wear testing of this study. Thin film gold on silicon is chosen fo r its applications in micro-electrical mechanical systems (MEMS) devices, el ectrical switching c ontacts, low surface roughness, and its thermodynamic stability at ambient temperature and pressure. Samples are prepared with a variety of depos ition techniques to vary film thickness. Various techniques are developed to overcome challenges with testing setup. An optics offset procedure solves resolution problems w ith light optical micr oscopy. Tip holders are designed to alter contact pressures and w ear conditions. Load functions are designed to operate the equipment in a linear reciprocating motion.

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xvi Numerous characterization techniques including scanning electron microscopy (SEM), focused ion beam (FIB) transmi ssion electron microscopy (TEM), and x-ray diffraction (XRD) are employed to characte rize testing parameters and results. Geometrical and mechanical properties of the tip are characterized using TEM. The sample thickness and microstructure are char acterized using XRD, SEM, FIB, and TEM. From the characterization, elastic and plasti c properties are approximated using elastic theory and the Hall-Petch rela tionship. For quantifying wear, SEM and FIB are used for depth measurements. An indentation model is propos ed to include material pi le-up exhibited by plastic films on elastic substrates which has been i gnored in other models Results show good agreement with the Hall-Petch relationship. Displacement and friction results show good reproducibility with repe at testing. Friction coefficient proves to be a goo d indication of film failure.

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1 CHAPTER 1 INTRODUCTION AND BACKGROUND Introduction The term tribology, coined circa 1965 and defined as, The branch of science and technology concerned with interacting surfaces in relative motion and with associated matters (as friction, wear, l ubrication, and the design of bearings), by the Oxford English Dictionary, is centuries newer than the study of friction and wear. For much of history up to and including th e present day, tribological problems are often handled with an alchemistic approach by choosing a variety of materials and empirically testing. One might wonder if some studies were designed wi th a periodic table of the elements and a handful of darts. Much of the problem has been the lack of experimental, computational and characterization equipment with the capabilities required to study fundamental tribological interactions. The past decade ha s seen advances in electron microscopy to sub-nanometer resolutions, computers a nd software simulation to represent 106 atoms, and mechanical testing inst rumentation to resolve 10-7 N loads and nanometer displacement. Tribological testing on th e nanometer displacement level, supported by high resolution characterization is necessary to validate finite element modeling and molecular dynamic (MD) simulations of surface interactions. In addition to fundamental studies of tr ibology, wear on the nanodisplacement level has applications in microelectromechanic al system (MEMS) power transmission and ohmic contact switching. With steady trend towards mi niaturization, MEMS devices have received much attention and intere st. Due to the extensive research and

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2 development of fabrication technology for in tegrated circuit (IC) chips, silicon was a convenient material for pioneering work in MEMS devices. It was quickly discovered that for mechanical sliding as in power tr ansmission devices, silicon components will fail with limited cycles and could not be used outside of laborator y conditions. Wear resistant thin film coatings may provide solutions for silicon MEMS and will require tribology testing equipment and methodol ogies to support research [1]. Interest in tribological study of thin f ilms extends beyond MEMS devices. Often in successful wear resistant materials combinations a transfer film from one of the materials develops on the other. The result is a self -mated contact which after an initial run-in period exhibits good wear resistance and low friction. Typically when these transfer films exhibit desirable wear properties, they are thin, on the micron to sub-micron scale, and very adherent. A methodology to study a nd probe properties of transfer films is useful in the design of contacts that will exhibit them. A set of standards for the mechanical testing of thin film coatings is necessary for future developments in the field [2]. This research aims to develop a methodology for studying tribology on the nanodisplacement level for both fundamental a nd practical wear applications. A Hysitron Triboindenter was chosen to perform the major ity of the wear testi ng of this study for the following reasons: The range of load and displacement. The Triboindenters practical load range is from 1 N to 10 mN and displacement is from 1 nm to 5 m. These ranges cover loads and displacements in MEMS devices thin film applications, and modeling simulations. The flexibility of tip geometry and mate rials. The tip holder can accept a wide range of materials and geometries. Ther e is no need for elaborate manufacturing techniques to produce tips.

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3 The accurate stage positioning. Accurate pos itioning of test areas is possible to sub-micron resolution. This allows for precise location of test ar ea prior to testing, and post-test characteriza tion of the test area. The scratch mode. The scratch mode of the Triboindenter can be tailored to reproduce motions expected in MEMS devi ces and other practi cal applications. Reciprocating and unidirec tional sliding is a fundamental geometry used in tribological applications a nd computational simulations. The experimental reproducibility. Tests on the Triboindenter show good reproducibility. This study will present a new approach for tribological testing w ith a characterized standard which will be available for furt her studies. It will provide a foundation on which to expand research on various mate rials, geometries, modes of wear, and applications. Background Friction and wear are by no means new phe nomena. From the earliest times, man has experienced friction and wear. To early man, friction was benefici al in creating fire by frictional heating of wood, and wear was used to sharpen tools for hunting and working. From the early 1700s to the early 1900 s, friction and wear are studied mostly as a scientific curiosity. As the industrial revolution progresses friction and wear are seen as engineering problems which lead to energy loss and machine failure. Therefore the interest in studying the phenomena greatly incr eases. As the resolution of measuring and characterization devices improve and computer simulation power incr eases, the study of friction and wear progresses from the macroto micro-, to nano-, and approaching atomic displacement levels. The driving forces for tribological research involve device longevity, energy conservation, economy, a nd fundamental understanding [3].

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4 Early Friction Studies The study of friction and wear dates back to the beginnings of th e scientific era. Amontons (1699) is credited with the first obs ervation that friction force is proportional to the normal load and independent of the contact area, hence Amontons Law: fnFF (1-1) where Ff is the frictional force, tangential to the surface, Fn is the force normal to the surface, and is the friction coefficient. Actual ly Amontons concluded that the friction force was always equal to one third of the normal load [4]. He explained this as interlocked asperities on th e surfaces which caused a lifting motion to move relative to each other which caused an energy loss and th erefore a friction force. This theory became known as the roughness hypothesis. Subsequent researches found that the friction coefficient did not equal one third for all materials, but agreed that the normal load and friction force were proportional. The roughness theory remained accepted for hundreds of years. Alternate theories invol ving adhesion were propos ed, but could not be validated without relating the friction force dependence on cont act area [3]. A significant contribution to the adhesion theory came from Holm (1938) when he showed that the real area of contact is related to the normal load divided by the yield pressure (or hardness) of the material [5]. It was therefore realized th at the real area of cont act, rather than the apparent area of contact, was responsible for the friction force and si nce the real area of contact was proportional to the normal lo ad, the friction force is subsequently proportional to the normal load.

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5 Mechanism for Metallic Friction The discovery by Holm paved the way for Bowden and Tabors development of the mechanism for metallic friction. Bowden and Tabor showed that the frictional for force to translate a slider along a surface is co mprised of two factors: the force required to shear the contact between the two intimate surfaces and the force to plough, or plastically deform the material in front of the slider. Their equation for friction force takes the form: fdsFFF (1-2) where Ff is the frictional force, Fs is the interfacial shearing force, and Fd is the force of plastic deformation (or ploughing). They expand this equation by breaking down the interfacial shearing force to an interfacial shear stress times the area it is acting upon, and breaking down the force of plastic deforma tion to the pressure required to yield the material times the projected area of the slider in contact with the surface in the direction of sliding and Equation 1-2 takes the form: fpsrealFHAA (1-3) where H is the hardness of the material which is taken as the pressure required to deform the material, Ap is the projected area of the slider in the direction of sliding and in contact with the surface, s is the interfacial shear stress, and Areal is the area of the slider in contact with the surface (Figure 1-1).

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6 Figure 1-1. A schematic diagram representing the slider and areas from Equation 1-3. Bowden and Tabor tested this equation with a set of three sliders, one with the geometry of a circular spade, one sphe rical, and one cylindrical, all three with the same radius (Figure 1-2). The tests were made with st eel sliders on an indium surface with a slow sliding speed of 0.1 mm/s to avoid strain ra te sensitivity. From the geometry of the sliders, it can be seen that the Fs term from Equation 1-2 or sAreal term from Equation 1-3 will be negligible for the spade slider and will be a maximum for the cylindrical slider. Figure 1-2. Schematic drawing of the three sl ider geometries from Bowden and Tabors experiment.

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7 Results from the experiments showed a defo rmation pressure equal to 1.5 times the material hardness measured by indentation. Bowden and Tabor suggest this result may be due to the fast sliding speed relative to the slow indentation sp eed, further suggesting some strain hardening effect. The experi ment was repeated on f our different surfaces, indium, lead, steel, and copper, and the re sults of the interfaci al shear stress were compared with the shear strength of the ma terial. The interfacial shear strength was on the same order of magnitude as the shear st rength of the material, but 1.5 to 2 times higher than the bulk shear strength. Sliding Wear Prow Formation A mechanism for wear of a metallic surface by a linear slider is described by Antler as a severe adhesive process called prow forma tion [6]. A net metal transfer occurs from the surface to the slider. This transferred material is described as a lump of severely work hardened metal (the prow) transferred to the slider which in turn continues to wear the surface by plastic shearing or cutting while the slider remains unworn. Prows are removed from the slider by transferring back to the surface or becoming loose debris. If the slider continues to ride over virgin material, prow fo rmation continues throughout the wear test. With dissimilar materials, fricti on and contact resistance characteristics of the system are determined by the prow metal, therefore a diamond slider on a gold surface should behave as a self-mated gold contact [7 ]. Antler found that the size of prows is inversely related to the hardness, and metals th at do not work harden, fail to form prows. He attributes this to the prow becoming harder than the surface from the severe deformation and then being able to cut the surface [8].

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8 Cocks proposed the following mechan ism [9] for prow formation: adhesion occurs between the slider and the surface plastic deformation occurs in a volume of metal from the surface adhesive forces are sufficient to remove the metal from the surface the metal transfers to the slider as a chip the process repeats on the chip the chip becomes too large and detaches from the slider Antler found that with a linear reciprocati ng slider, prows would be deposited at the ends of the tracks and prow formation woul d eventually stop when the surface material work hardened to a hardness higher than that of the slider in whic h case would cause the slider to begin wearing. From empirical observations, the following equation was developed: prow nx(cm) N1.7 F(N) (1-4) where Nprow is the number of cycles for which prow formation will continue, x is the track length in centimeters, Fn is the normal load in Newtons [5]. Rabinowicz and co-workers have suggested that materials that show little to no solid solubility and form immiscible li quids would make good candidates for mated materials in adhesive wear slid ing applications [3]. Antler challenges this theory based on experiments with gold mated with plati num and gold mated with rhodium [5] that show similar performance to gold on gold. Antl er suggestions for adhesive wear contacts are high hardness to limit initial contact ar ea, and low ductility to limit junction growth. Delamination Theory Delamination wear occurs in reciprocati ng sliding when cracks nucleate below the surface and result in liberating sheets of material. Crack nuc leation can occur in areas of high plastic deformation at disloc ation pile-ups particularly at hard precipitates in a softer

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9 matrix or between material in terfaces like a substrat e/film interface [10]. Suh discovered delamination wear typically occurs with harder sliders on soft surfac es. Prow formations and delamination wear are different mechan isms and are mutually exclusive [5]. Delamination wear has been reported in re ciprocating sliding on gold plated contacts [11]. Subsurface Wear Gold plated systems have shown subsur face plastic deformation leading to the formation of buckles on the surface layer. With continuous sliding exposure of the sublayers can occur with or without thinning of the surface layer. Increasing normal load results in less number of cycles to produce buckling and exposure of the sub-layer. It was found that increasing layer thickness can re duce or eliminate the mechanism [12]. Film Thickness Wear a nd Friction Dependence Several research groups have investigated th in metallic films as a solid lubricant in sliding contact applications [ 4, 13-17]. All found that fric tion and wear were initially high for ultra thin coatings but as coating thickness was increased, both friction and wear went through minima values at some critical coating thickness (Figure 1-3). Table 1-1 summarizes the results from the various resear ch groups. In some studies, tribometer geometries were not well defined and experime ntal details were not mentioned. Two of the research groups studied sputtered coatings versus evaporated coatings and both found the sputtered coatings exhibited lower fric tion coefficients and longer life than the evaporated coatings [14, 17]. Both groups attrib uted this behavior to better film adhesion and finer grain structure, re sulting in higher coating har dness, exhibited by sputtered coatings. Antler improved his coating by alloyi ng gold with cobalt [13] The cobalt acts as a solid solution strengthener to harden the gold film.

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10 Figure 1-3. Schematic representation of the effect of film thickness on friction coefficient of metallic film solid lubri cants on harder substrates (reproduced from [17]). The critical thickness where (the friction coefficient) goes through a minimum is denoted as hc. Table 1-1. Summary of studies of thin metallic films on substrates. film/substrate hc ( m) tip radius ( m) load (N) reference In/steel 0.1-1 0.5 3000 39.2 [4] Pb/steel 0.5 0.05 3175 29 [15] In/steel 0.5 0.05 3175 29 [15] Sn/iron/glass 0.1 0.15 3175 2.4 [16] Au/steel 2.0-3.5 0.1 2375 2.45 [17] Pb/steel 1.8-2.5 0.1 2375 2.45 [17] Co-Au/Pd/Ni 0.05 0.4 N/A 1.96 [13] Au/Si (111) 0.01 0.2 1600 0.001 [14] Ag/Si (111) 0.007 0.2 1600 0.001 [14]

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11 Modeling Several attempts have been made to deve lop models to describe friction and wear phenomena in thin films on substrates. Bowers and Zisman modeled the friction coefficient of a thin gold film on a steel s ubstrate based on a pressure dependent shear strength which showed good agreement with ex perimental data [18] They calculated a friction coefficient of 0.1 assuming no plastic deformation of the steel substrate. ElShafei et. al. investigated thin films of lead on steel with a contact radius larger than the film thickness [19]. They found good agreemen t with a Hertzian elastic contact model once their contact radius exceeded five times the film thickness. Finkin sought to explain the ultra-thin to thin film tr ansition phenomenon seen in Fi gure 1-3 [20]. He developed two theories to explain the two regimes. The ultra-thin regime is explained as stiffening due to coupled stresses with a quantitative expression verified by data from literature. The coefficient of friction in thin film regime is shown to be proportional to the square root of the film thickness divided by the nor mal load based on a thin film with modulus much less than the substrate. Jang and Ki m, and Schiffmann and Hieke incorporate a ploughing term in their wear models which ta kes plastic deformation into account [14, 21]. For these models, tip ge ometry, contact geometry, and plastic properties of the film and substrate are necessary parameters. Difficulty in measuring these parameters is attributed to discrepancies with th e models and experimental results. The combinations of substrate properties, film properties and thicknesses, sliders, and geometry lead to different modes of wear Due to the complexity of the problem, no single model or solution is expected to e xplain all possible wear combinations and geometries. A model that describes the fric tion coefficient of a thin soft film on an

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12 elastic substrate of significantl y higher modulus is not expected to describe a hard elastic film on a soft compliant substrate. A variety of models need to be developed to explain specific cases.

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13 CHAPTER 2 EXPERIMENTAL MATRIX An experimental matrix was designed to test various microstructures, film thicknesses, number of reciprocations, and nor mal load effects on wear resistance of gold films on silicon substrates. Gold was chosen for its applications in MEMS devices, electrical switching contacts, and its therm odynamic stability at ambient temperature and pressure. Standard metallographic polishing techniques were initia lly used to prepare sample surfaces. Wear testing and profilome try revealed surface roughness effects which excluded the use of bulk gold. Polishing media particles were embedde d into the surface changing the properties of the material and the roughness produced was on the scale of the measurements taken. Therefore commerc ially available polished silicon wafer was chosen as a substrate material for its lo w surface roughness. In addition, silicon has direct applications in MEMS research. A variety of microstructures and sample thicknesses were produced using deposition t echniques including ion beam sputtering (IBS) and electron beam evaporation (EBE). An amorphous carbon layer of approximately 60 nm in thickness was deposited on half of the samples to explore an electrically conductive friction modifying substance. The samples with and without carbon will be referred to as co ated and uncoated respectively. Samples were named based on the deposition technique and the targeted approximate film thickness in nanometers (Tab le 2-1). Normal loads were chosen of 100 N and 500 N because the instrument shows good re peatability and low signal to noise ratio at these levels. With the 100 N normal load setting, number of reciprocations was

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14 varied from one forward pass, referred to as a half cycle, to 300 cycles in the following increments: half cycle, one cycle, three cycles, 10 cycles, 30 cycles, 100 cycles, and 300 cycles. Although the reciprocating tip is diam ond and is therefore mu ch harder and more wear resistant than the gold foils, some precautions were taken to avoid possible tip damage. For the 500 N normal load setting, the 100-cy cle and 300-cycle tests were omitted if film failure was detected on samples at 30 cycles to avoid excessive number of reciprocations on the harder si licon substrate. The combin ation of loads and number of cycles results in 12 different te st parameters. Each set of tests was repeated four times on all 12 samples for a total of 576 wear tests. Table 2-1. Sample naming designations. Approximate sample thickness Without amorphous carbon With amorphous carbon 20 nm EBE 20 EBE 20C 100 nm IBS 100 IBS 100C 200 nm EBE 200 EBE 200C 300 nm IBS 300 IBS 300C 400 nm EBE 400 EBE 400C 500 nm EBE 500 EBE 500C Sample Preparation Gold was chosen as the test material for its applications in th e electronics industry, and its mechanical and chemical properties de scribed previously. A variety of samples were produced and can be categorized into th ree basic types includi ng bulk gold foils, ion beam sputtered gold on silicon wafer, and electron beam evaporat ed gold on silicon wafer. These various techniques were c hosen to introduce microstructure, surface roughness, and film thickness vari ables into the test matrix.

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15 Bulk Gold Foil Initial experimentation was perfor med on bulk gold samples. A 100 m thick 99.9975% gold foil was cut into 10 mm x 10 mm sections and mounted onto steel gauge blocks using a cyanoacrylate based adhesive Samples were polished using a South Bay Technologies manual polishing wheel (Figure 21) beginning with grinding discs of 240 grit silicon carbide rotating at a speed of 200 rpm. After a short time of material removal, the polishing wheel was cleaned and the gri nding media was changed to 320 grit and the sample was rotated 90 to th e previous polishing directi on. The sample was polished until no scratches from the previous step were detectable using an optical stereoscope. This procedure was repeated using 400 a nd 600 grit respectively. Upon completion of the grinding steps, the media was removed and the wheel was cleaned. Next a billiard cloth was mounted on the polishing wh eel and water-based slurry of 15 m alumina (Al2O3) was used as a polishing media. The slurry was used to wet the billiard cloth and samples were polished by rotating the samp le manually in the opposite direction of rotation of the polishing wheel. Samples were polished until scratches from the previous steps were not detectable. The wheel was cleaned and the process was repeated with fresh cloth and 5 m slurry. The process was then continued using 1 m, 0.3 m, and 0.05 m slurries respectively on velvet clot h. Upon completion of the final polishing step, the samples were examined in a Wyko optical profilometer to verify consistent surface roughness.

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16 Figure 2-1. South Bay Tec hnology 8 polishing wheel Ion Beam Sputtered Gold on Silicon Ion beam sputter coating is often used to produce thin films for conductivity to aid in imaging in electron microscopy. The advant age to ion beam sputtering versus other techniques is that the samples are not e xposed to extreme temperatures. The low temperature deposition is beneficial in producing and retaining a fine grained or amorphous structure. Producing coatings on po lished silicon wafers will yield surface roughness values that would be exceptionally challenging to obtain by metallographic polishing techniques. The ion beam sputtering process is illustr ated in the schematic in Figure 2-2. An argon ion beam is produced by dual Penning guns The guns are aimed to strike the target of desired deposition ma terial. Material from the ta rget is sputtered off, and deposited on the sample substrate. The s ubstrate can be rotated to provide uniform deposition. This system is contained in a ch amber that can be evacuated to levels of 10-6 Torr.

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17 Figure 2-2. Ion beam sputtering process Commercially available 0.6 mm thick single crystal silicon wafer with (001) plan view orientation was scored and cleaved along <110> directions to produce substrates of approximately 25 mm2. The substrates were cleaned by sonicating in acetone. This was repeated three times using new acetone each time. The substrates were then sonicated in methanol three times using new methanol each time. Finally the substrates were blown dry using laboratory grade Freon spray. Severa l substrates were mounted for coating in a Gatan Model 681 High Resolution Ion Beam Coater seen in Figure 2-3. Figure 2-3. Gatan Model 681 High Resolution Ion Beam Coater

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18 It is well known that gold has poor adhesi on when deposited on silicon and a thin titanium bond coat will greatly enhance gold film adhesion. A target of 99.99% gold was mounted on one side of the dual target holder using conductive silver epoxy and a target of 99.99% titanium was mounted on the opposite side. After sufficient curing time, the ion beam coater chamber was pumped down to a level of 2 x 10-4 Torr and the ion guns were purged with argon following the instruct ion manual. After purging, the ion beam was set to 9 keV at 600 A for 60 minutes with a sample rotation speed of 30 rpm. Based on a deposition rate of 3.5 /s the f ilm thickness should be on the order of 1 m. The first batch of samples was run w ithout a titanium bonding layer to examine the effect of film/substrate adhesion. A second se t of samples were coated by first sputtering titanium with a 9keV ion beam at 300 A for 15 minutes followed by gold with a 9 keV ion beam at 300 A for 60 minutes. The lower current was used due to beam stability issues which could not be resolved over se veral repeated attemp ts. Once the coating process was complete, the silicon substrates were glued to 15 mm di ameter atomic force microscopy (AFM) sample disks using a cya noacrylate based adhesive. The AFM disks are ferritic steel which causes them to be attracted to magnets embedded in the testing equipment stage. It was discovered early in th is project that the magnetic attraction of the stage was not sufficient to rigidly affix the samples to the stage, and an adhesive was needed when mounting samples on the stage of the test equipment. Late in this study it was determined that the AFM disks signifi cantly hindered scanning electron microscope (SEM) examination due to the effect of magnetic fields on the electron beam. In hindsight, the AFM disks should have been eliminated.

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19 Electron Beam Evaporated Gold on Silicon Electron beam evaporation is another coat ing technique for producing high quality thin films on substrates. In contrast to ion beam coating, electron beam evaporation exposes the sample high temperatures due to the energy required to ev aporate the metal. This leads to grain growth in the film which results in a coarser grained structure. The elevated temperature can also result in diffu sion between the film and substrate. Again a bonding layer is used to enhance film adhesion. Titanium and chromium layers are often used for gold deposition on silicon. Since tita nium has a lower diffusivity in gold than chromium, titanium was selected. The electron beam evaporation process is illustrated schematically in Figure 2-4. An electron beam is generated by producing a large electrical bias between the electrode and the crucible. The crucible contained th e desired film material. The electron beam heats the material in the crucible to a liquid and then to a gas which fills the chamber. The gas condenses on the substrate thereby producing a thin film on the substrate. Figure 2-4. Electron beam evaporation process

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20 Attempts were made to use an on-site electron beam evaporation system, but coatings were of a poor quality with many spheres of gold on the sample surface. Fortunately, high quality electron beam evaporated coatings on silicon (001) substrate are available commercially at economical prices Wafers of 100 mm diameter with gold films of 20 nm and 200 nm were purchased and sectioned into samples as described previously. The sectioned sa mples were then glued to AFM disks using a cyanoacrylate based adhesive. Carbon Arc Evaporation To investigate an electrically conductiv e friction modifying coating, all samples were duplicated with a thin amorphous car bon layer using a carbon arc evaporation system. This procedure is similar to the el ectron beam evaporation except that a bias is applied across two carbon rods to evaporate carbon. The resu ltant condensation yields a thin amorphous carbon film. This process is illustrated schematically in Figure 2-5. Figure 2-5. Carbon arc evaporation

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21 Ion Beam Etched Area of Interest Many of the tests conducted in this research leave residu al damage smaller than the resolution limits of optical microscopy. Therefore, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) ar e used to characterize the damage from testing. Even with the resolution obtainab le with electron microscopy, locating micron and submicron surface features with displacements on the orde r of tens of nanometers or less is extremely challenging due to the low am ount of contrast they produce, see Figure 2-6. Scanning at a low magnification to locate such features is not possible because the feature will not be visible. Scanning th e sample surface at a high magnification without an indication of the approximate area is not practical because the field of view is much smaller than the entire area of the sample. A method to accu rately position wear tests and to subsequently locate the te sts in an SEM is necessary. Figure 2-6. SEM image of 4 m x 4 m rastering wear test with low contact pressure.

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22 A dual beam focused ion beam SEM was used to layout and etch grids on the samples surface. Each grid square is 100 m x 100 m and the grid lines are approximately 1 m wide and 1 m deep, see Figure 2-7. Figure 2-7. SEM image of etched gr id lines for wear test location The grid lines were aligned with <110> directions on the silicon substrate by setting the intersection of the cleavage planes {111} and the plan view plane (001) to be parallel with the x and y axis in the micros cope. These markings are easily located in both the SEM and optical microscope and can be used situate the wear test and to identify the area for characterization. The markings allow wear tests to be run approximately parallel to a <110> direction on the substrat e which will aid in locating a major zone for high resolution transmission electron micros copy (HRTEM) analysis in the JEOL 2010F which is limited to ~20 tilt.

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23 CHAPTER 3 TEST EQUIPMENT A wide variety of tribological equipment exists to test various wear conditions and contact geometries. By and large this equipm ent aims to prescribe specific sliding paths while controlling an applied normal load and me asuring the frictional forces generated at the sliding interface. Most of this equipm ent does not make in situ measurements of wear; rather, wear is measured ex situ us ing gravimetric, metrol ogical, or microscopy techniques. The most common sliding motions are unidir ectional, with the standard pin-on-disk tribometer being a device that prescribes a continuous rotary mo tion to a disk while holding a loaded pin stationary on the surface (Figure 3-1). Figure 3-1. A typical pin-ondisc type tribometer

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24 The reciprocating pin-on-flat tr ibometer is another common tr ibological testing appartus. In this configuration, the sample reciprocates in a linear direction against a stationary pin through which a normal load is transmitted (Figure 3-2). Figure 3-2. Linear reciprocating tribometer Several researchers have begun to use at omic force microscopes (AFM) to study tribological surface interactions. Normal a nd lateral forces acting upon an AFM tip are calculated from deflections of the tip cantileve r. From these forces, sliding friction is calculated. Accurate measurements of sliding friction from AFMs are exceptionally challenging, requiring well charac terized cantilevers with well know material properties. There exists a gap in tribological test equipment from the AFM scale with nanonewton normal loads and nanometer to sub-nanom eter normal displacements, to the

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25 microtribometer scale with millinewton norma l loads and micrometer displacements. Tribological conditions in MEMS devices and thin films fit in this regime and remain largely unstudied. In this study, the Hysitr on Triboindenter is used to mimic a linear reciprocating tribometer and bridge the gap in scale. Hysitron Triboindenter The majority of wear testing done in this study was performed on a Hysitron Triboindenter. The Triboi ndenter is a commercially available nanodisplacement mechanical testing system whose primary function is nanoindentation (Figure 3-3). Figure 3-3. Hysitron Triboinde nter with upper thermal-acous tic isolation cover removed to show interior details.

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26 Loads, displacements, and measurements are performed by a capacitive transducer (Figure 3-4). The capacitive transducer assembly contains two parallel fixed plates with a parallel center plate supported by springs. An indenter tip is mounted to the center plate. To apply a load a DC potential is applied between the lower plate and the center plate. An electrostatic attraction displaces the center plate towards the bottom plate. Based on the spring constant of the support sp rings, the voltage can be calibrated to a force. For displacement measurements, an AC signal is applied between the center plate and top plate, and an AC signal of equal ma gnitude 180 out of phase is applied to the lower plate and the center plate. When the center plate is equidistant from the top and bottom plates, the net signal is zero. When the center plate is di splaced a voltage is recorded and calibrated to a distance. A similar transduc er is mounted to provide lateral displacements. Figure 3-4. Capacitive tran sducer assembly schematic The capacitive transducer assembly is mount ed to a three axis piezo tube scanner similar to those found in AFMs. The piezo tu be scanner allows for precise tip placement and can be used in conjunction with the tr ansducer as a scanning probe microscope.

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27 Using the tube scanner to raster the indenter tip in contact with a su rface, the transducer can collect z displacements and produce an AFM-like image. The tube scanner is mounted to a z-axis st epper motor which provides coarse translations for tip approach. An optical CCD microscope is also attached to the z-axis stepper motor. The optical microscope allows for accurate sample locati on and test placement. The sample stage is attached to x-y stepper motors for translat ion (Figure 3-5). All systems are computer controlled. Figure 3-5. Enlarged view of Triboindenter components The components are mounted on granite to da mpen vibrational effects. The equipment frame also contains an active vibration isolation device. The working components of the Triboindenter are enclosed in a thermal-acous tical isolation chamber with provisions for environmental control.

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28 CSM Microtribometer The CSM Microtribometer is a commercia lly available tribological testing device that operates on scales orders of magnitude larger than the Triboindenter, but well below those of conventional test equipment (Table 31). This tribometer is available with a linear reciprocating stage or an interc hangeable rotating stage to mimic both reciprocating and pin-on-disc tribometers. Rough stage translations are handled by stepper motors and fine transl ations and reciprocations are performed by piezos. A dual flexure both applies normal load and reacts to frictional forces between the pin and the counterface. Mirrors are mounted on this flexure in the horizontal and vertical directions. Optical light intensity sensors are positioned at a distance from the mirrors to read the magnitude of deflection of the flexure in the frictional and normal lo ad directions (Figure 3-6). Figure 3-6. Schematic of CSM Microtribometer [22].

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29 Table 3-1. Comparison of equi pment loads and displacements Conventional Reciprocating Tribometer Hysitron Triboindenter CSM Microtribometer maximum normal load 1.6 kN 10 mN 1 N maximum lateral load 1.6 kN 2 mN 1 N maximum normal displacement 50 mm 5 m 0.1 mm maximum lateral displacement 0.2 m 15 m 0.6 mm

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30 CHAPTER 4 EXPERIMENTAL PROCEDURES Hysitron Triboindenter Procedures Tip Preparation For the most part, tips in indentation tes ting are not considered to be a parameter involved in a test. Typically tip s are chosen so that they ar e significantly harder than the material being tested so that it can be assumed that they experience no plastic deformation. Diamond is the material of choi ce for indentation testing because it is the hardest material and its elastic properties ar e well characterized so that any elastic deformation experienced by the tip can be back calculated from the test data. Diamond tips are expensive costing approximately $2000 eac h. For indentation testing, this cost is not a huge factor because the tips will last for an exceptionally long time. In hardness testing, a sharp tip is often desirable to re ach high contact pressu res and induce plastic deformation. In wear testing, the two materials in intim ate contact are both considered. Often wear tests consist of self mated materials, gold on gold for example, or materials from a specific application, e.g., a bearing on a race. It is often found that both materials experience some change. In the case of a ha rd pin (or tip) moving across a softer flat, transfer of the softer material to the hard pi n is of interest. Observing this material transfer in cross section is de sirable, but would require destru ction of the pin. Due to the high cost of diamond tips, it is not practical to section them. It is not uncommon for the pin to experience wear along with the surface. For diamond tips, wear of the tip should

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31 be negligible and no transfer film of gold on to the diamond tip has been observed in the scanning electron microscope (SEM). Self mated materials could be tested by using coating techniques on the tip, bu t again, the high tip cost makes this prohibitive. An alternative to high cost diamond tips is needed. A crucial parameter in wear testing is th e contact pressure. Contact pressure is used to target desired wear modes, and to re plicate applications. The factors involved in contact pressure are normal lo ad and contact area. The nor mal load can be controlled by the Triboindenter, and the contact area varies with the tip geometry. In contrast to hardness testing, wear tests are often performed with mu ch lower contact pressures requiring a blunt rather than sharp tip. By sel ecting tips of various ra dii, a wide range of contact pressure can be achieved. For the reasons mentioned above, tips of relatively large radii are produced in addition to the sharp tips available from the manufacturer. Sapphire spheres of 100 m to 1 mm are commercially available for a nomin al cost of approximately $10.00 each. The original equipment manufacturers (OEM) tip holder is machined on a lathe with a conical cut such that when a sapphire sphere is place in the tip hol der, it will self-center (Figure 4-1). A small amount of cyanoacrylat e based adhesive is used to attach the sapphire sphere to the OEM tip holder. The ad hesive is allowed to cure overnight. The transducer piezo assembly is then removed from the Triboindenter for easier access. Once the tip has sufficiently cured, it is thread ed on to the capacitive transducer with a special torque limiting wrench.

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32 Figure 4-1. Illustration of an OEM diamond indenter tip and assembly of a large radius sapphire tip Sample Mounting The Triboindenter has a stage that contains six positions which are magnetized by rare earth magnets. Samples mounted on atom ic force microscope (AFM) disks can be placed on the stage and will be sufficiently he ld in place for indentation testing. In preliminary tests, it was determined that th is mounting system may not be effective for tests involving lateral translational forces. Therefore, samples are held in position with an alcohol soluble adhesive in addition to the magnetic forces. Once the samples are mounted and the stage replaced on the stage motor, the machine is powered on. The samples are imaged and focused on using th e Triboindenters optical charge coupled device (CCD) camera, and a safety area is generated by plotting the outline of each sample and recording the points. This will defi ne an area of safe travel for the indenter tip.

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33 Electrostatic Force Constant and Zero Volt Gap Calibration The electrostatic force consta nt is a conversion used by th e Triboindenter to convert an applied voltage to a force. This constant is determined by the ar ea of the plates and the distance between them squared. Because the center plate is suspended by springs, and different mass tips can be used, the pl ate gap spacing can cha nge. The calibration procedure is as follows: A load of 600 uN is set in the load functi on editor. This load will displace the tip through a large range of motion. Perform advanced z-axis calibration is selected from the drop down menu. The plate is moved through its range of motion and a pl ot is generated of the electrostatic force calibration based on the spring constant and applied voltage versus displacement. The curve is fit to a quadratic and the e quilibrium position of the plate is recorded and tared to zero and the calibration is complete. Tip Optics Offset Calibration All tests on the Triboindenter are set up on the sample using the optical CCD microscope camera. The computer then translat es that area to the indenter tip to perform the test. Therefore a calibrati on procedure is needed to reco rd the differences in the x, y, and z positions of the center of the optical focus and the indenter tip. The standard procedure developed by the OEM relies on residual indents large enough to image optically and works well in indentation applic ations with sharp, small radius tips. For tribological applications a blunt radius tip is often desired, so an alternate method had to be developed. Standard tip optics offset procedure As mentioned above, this optics offset procedure involves optically imaging residual indents. A relatively soft, polished sample is required for this procedure. A single crystal aluminum sample with [001] out-of-plane orie ntation is included with the

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34 Triboindenter. This sample is mounted on th e stage. The open loop load function editor is set with a maximum load of 10 mN. Th is load with a sharp tip of less than 5 m radius will leave a residual indent that can be easil y imaged with the optical CCD microscope. A safety zone is defined around the aluminum sample, and an area free of indents is chosen to perform the calibration. The optic s offset procedure is chosen from the software menu, and the machine automatically translates the tip to the approximate position above the center of the optical focus. The user is prompted to use the z height control to lower the indenter tip to within 1 mm of the sample surface. Once the user does so, an automated pattern of seven indent s in the shape of an H is performed on the sample at the prescribed load (Figure 4-2). The machine then translates the sample back to the optical microscope, and the user is prom pted to set the microsc ope reticule over the center indent. The computer then records this difference in the x, y, and z positions of the center of optical focus and the indenter tip. Once this procedure is completed, an eighth indent is added to the pattern so not to conf use it with another pattern in a later tip optics offset calibration. Figure 4-2. Optical CCD microscope image of residual H-pattern indents in Al (001) sample

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35 Modified tip optics offset procedure This procedure was developed for large ra dius tips where the residual indentation left by the maximum load would not be reso lvable in the optical microscope. By calculating the depth of a residual impression left by a s pherical indenter of radius R, one can see how indenters of sharp radii less than 5 m leaves an impression that is resolvable by optical microscopy, but impressi ons from indenters of blunt radii greater than 100 m cannot be resolved optically. Hard ness was defined earlier as the normal load divided by the projected area of th e residual impression. Figure 4-3 shows a schematic of a spherical inde nter on a flat surface and the cross sectional geometry to solve for the depth of impression, where Fn is the normal load, 2a is the diameter of the residual impression, a is the radius of re sidual impression, and R is the radius of the indenter tip. Figure 4-3. Representation of spherical inde nter on flat surface and cross sectional geometry to solve for depth of indentation The hardness equation can be rearranged as follows: 2 nnn F FF Ha A aH (4-1)

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36 For the relatively soft aluminum sample H = 500 MPa. With the machine set at the maximum load of 10 mN, solving for a yields a = 2.5 m. Using geometry from Figure 4-3, the following can be written: 2RRa (4-2) For tips of radius R = 5 m, the depth of residual impression is 670 nm, which is within the limits of resolution in the optical microscope. C onsidering a tip of radius R = 100 m, the depth of the residual impression is 30 nm which cannot be resolved in an optical microscope. Fortunately, the Triboindenter has a sca nning probe imaging feature which works much like an AFM in that the tip can be held in contact with a surf ace at a low load and the piezo tube scanner can raster the tip ove r the surface. At each point, or pixel, a zheight is recorded along with th e x and y position of the tip. Th is data is used to generate an image by applying contrast values to the zheight values. This scanning feature has a resolution limit below 1 nm in the z direction be cause the capacitive transducer is used to record the z-height. The following method was developed to work around the resolution limits of the optical microscope. This method uses a low load which can be beneficial in fields outside of tribology. If one were to design a delicate functional tip th at could not survive the high load used in the standard optic offs et procedure, yet stil l needed accurate tip positioning, the modified tip optics offset method could be employed. Since a scanning probe technique would be used to calibrate the tip optics offset, a sample was designed and produced to contain fe atures that would be easily imaged and located in both the optical CCD microscope and the scanning probe microscope feature

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37 of the Triboindenter. A silicon wafer was c hosen because of its hi ghly polished surface, and its compatibility with the focused ion beam milling technique. The wafer was cleaved into a 5 mm x 5 mm section a nd mounted on a 12 mm AFM disk with a cyanoacrylate based adhesive. This specimen will now be referred to as the modified tip optics offset sample and is shown in Figure 4-4. The following features were carefully chosen and patterned on the wafer using an FEI DB-235 focused ion beam (FIB) SEM. series of concentric circles 200 m outer diameter 20 m radial spacing 40 m inner diameter two sets of orthogonal crosshairs rotated 45 from each other 1 m patterned line width Figure 4-4. SEM image of the patterned modified tip optics offset sample The series of concentric circles were chos en because it is easy to determine, from an arc, the direction of travel required to h ead towards the center. A pattern size of 200 m outer diameter was chosen because it has been observed that from a tip change the tip optics offset rarely varied by more than 100 m and a 200 m feature is easily located in an optical microscope. When the scanning imaging function of the Triboindenter is

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38 started, a rastering pattern of 40 m x 40 m is a typical scan size, therefore a radial spacing of 20 m will ensure that at least one line is caught in the initial scan. The inner diameter of 40 m was chosen because it could be imag ed in the initial scan size once located. The two sets of orthogonal crosshairs rotated 45 from each other were chosen because if only one set was present, and one of the lines were aligned closely with the rastering direction, that line would not be imaged. The two sets ensure at least 3 of the 4 lines will be imaged. The reason for the 1 m line width is two-fold. First, a line of 1 m in width can be resolved in an optical microscope. For the second reason, we return to Figure 4-3 and Equation 4-2. Equation 4-2 can be rearranged and solved for a to give: 22() aRR (4-3) From the geometry of Figure 4-3, 2a will be the minimum width of the line spacing required for a given R and The largest radius tip of in terest in this study is R = 250 m. Hysitron publishes the z axis resolution lim it to be 0.04 nm with a noise load floor of 0.2nm. A value of = 0.5 nm should be within the reso lution of the equipment. Using these values in Equation 4-4 gives a value for a = 0.5 m, making the line width 2a = 1 m. If a FIB is not available, a similar tip op tics offset sample could be produced using a soft polished sample such as the Al (001) and a sharp indenter ti p. Using the automated methods feature of the Triboindenter, one coul d set up a similar patter n to that described above and run the pattern with a sharp tip, hi gh load, and a soft sample. Once the sample is produced, the sharp tip can be exchange for a blunt tip and the modified tip optics offset procedure could be followed.

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39 Once the modified tip optics offset sample is produced and mounted on the stage with a safety limits area defined, the blunt tip installed on the capacitive transducer, and the electrostatic force calibra tion and zero volt gap calibrati on completed, the modified tip optics offset procedure is initiated. Th e samples concentric rings are located and centered to the video window reticule usi ng stage translations (Figure 4-5). Figure 4-5. Modified tip optics offset sample centered in the video window From the main positioning window, tip optics offs et is selected. Then new single indent is selected from the window seen in Figure 4-6. Figure 4-6. At this point, new single indent is selected In the load function editor, a low load of 2 N or less is programmed. By setting a low load, the machine will go through the standard procedure of performing an indent, but no

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40 damage to the sample or the tip will occur. The machine will translate the sample so that the tip is over the sample and the user wi ll be prompted by the window in Figure 4-7. Figure 4-7. Manual height adjustment window From this window the indenter tip is lo wered to approximately 1 mm from the sample surface. The machine then slowly a pproaches the sample surface until contact is made, and the low load indent is executed. The sample is then translated back to the optical CCD microscope and th e window in Figure 4-8 pops up. Figure 4-8. Search option window At this point, no translation is made even though the machine prompts the user to locate the pattern. This set is used only to se t the z-axis offset. The x and y offset is set

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41 in following steps. The ok button is pre ssed without any x or y translation. Now from the Triboscan toolbar, the im aging mode is selected. Th e windows in Figure 4-9 will open. Initial scan parameters of 50-80 m scan size, 2 N set point, and 1 Hz scan rate are entered. Topography and gradie nt imaging windows are selected. Figure 4-9. Imaging window and control panel with image of modifi ed tip optics offset sample Tip approach is selected and once the Triboi ndenter has indicated that the tip is in contact with the surface, the scanning is star ted. Once one complete raster is finished, some of the tip optics offset sample should be visible. Now the stage controls are used to translate the sample towards the center of the ci rcles. It is critical to use only the stage controls and not the x-y offset to translate the sample. The x-y offset is a bias applied to the piezo tube scanner which will give an inacc urate tip optics offset calibration if used

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42 here. Determining the direction for transla tion is easily identified by moving radially inward from the patterned arcs. For translati on it is easiest to think of the sample being stationary and the reticule move s in the direction indicated on the x-y translate arrows. Relative direction lengths can be entered and are conveni ent for homing-in on the center. As the center is approached, the raster size is reduced which effectively zooms in on the image. Once the center of the sample is located on the imaging reticule, the tip is withdrawn from the sample and the sample is translated automatically back to the optics window. Now the amount of tip optics offset can be visualized in the video window (Figure 4-10). Figure 4-10. Actual tip optics o ffset seen in the video window No stage translations are made at this point. Again calibra te optics is selected from the drop down menu, and the window in Figur e 4-6 appears. Ne w single indent is selected again and a low load is entered in the load function editor window. The sample

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43 is automatically translated beneath the inde nter tip, and the window in Figure 4-7 appears again. The tip is manually lowered to w ithin 1 mm of the surface and the machine performs the low load indent. Upon completi on, the sample is translated back to the optical CCD microscope and the auto search window seen in Figure 4-8 reappears. Now stage translations are used to center the tip optic s offset sample pattern to the center of the video window reticule, and the ok button is pressed in the auto search window. The calibration is now complete and can be ch ecked by returning to the scanning imaging mode to verify the pattern is at the center of the imaging reticule. This entire procedure was completed with no load greater than 2 N. Quick Approach Now that all calibrations are complete a nd the samples are mounted with safety zones defined, an area of interest is located us ing grid lines etched by the FIB. A quick approach is performed which involves set ting a load threshold and approaching the sample surface. Once the load threshold, called the set point is reached, the Triboindenter records the z height as the samp le height. This procedure is performed for each sample safety area. Tip Area Function Calibration With depth sensing indentation machines load and displacement are measured, however, an area of residual impression is necessary for analysis of modulus and hardness as can be seen in Equation 4-1. From the analys is above, it is shown that a residual impression cannot be imaged or measur ed optically. It would be convenient to be able to extract area calc ulations based on displacement data. Therefore a tip area function relating the residual ar ea to the depth of indentati on is needed. For micron and larger scale indentation depths ideal tip geometry can be assumed and an area function

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44 can be generated from simple geometric rela tionships. The ideal area function for an indenter with pyramidal Berkovich geometry is A=24.5*hc 2 where hc is the contact depth and A is the residual area (see Data Analysis section for in-depth explanation). At the nanometer level, the tip of a pyramidal inde nter is rounded, therefor e the assumption that the three faces of a pyramid intersect at a point is invalid, and the ideal area function breaks down and greatly underestimates the resi dual area based on the depth. A method has been devised to generate a tip area f unction by creating a seri es of indents in a standard of known elastic modulus. In this section, the mechanics of generating a tip area function are explained. For a de tailed mathematical description of the procedure, refer to the Data Analysis section. A fused quartz standard is loaded on the st age of the Triboindenter. A series of approximately 100 indents are made targeti ng a depth range to cover the series of experiments to be run. The series is run and the data is saved. A power law curve fit is performed on the unloading portion of the curv es. A tangent line to the power law curve is used to find the stiffness from which an area is calculated. The calculated area versus displacement for all the indents is plotted, and a curve fit is performed to generate an area versus displacement function. This function is saved for this tip and displacement range. A new function must be generated for each different tip and the functions must be verified periodically on a st andard to ensure the tip geometry has not changed. Machine Compliance Calibration In a depth sensing indentation device, th e desired measurement is the position of the tip relative to the sample surface. This is determined by displacement, however measured displacements include compliance of the system along with the tip and the sample.

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45 There are several factors that can contri bute to the system compliance such as: Indenter tipthe indenter tips are handmad e from various materials and adhesives. Sample mountingif the sample mounting procedure varies in materials, the compliance will vary. Transducer mountingthe transducer must be mounted the same way each time and should be level and properly seated. Therefore it is necessary to determine th e system compliance and remove it from the measurements. This section only explains th e procedure for the compliance calibration. The mathematics are explained in de tail in the Data Analysis section. Once the other procedures have been comple ted, the transducer calibration constant is set so that the machine compliance is 0.00. A set of 15 indents are made with loads varying from 5 mN to 10 mN. Curves from the data are ploted and the multiple file analysis is executed. A plot is generated of the inverse of the stiffness versus the inverse of the square of the load. A linear fit is generated and the machine compliance in nm/mN is equal to the y-intercept times 1000. This number is entered for machine compliance. A new machine compliance is generated for each tip and any time the conditions mentioned above might change. Testing Modes The Triboindenter has three basic modes of operation which are illustrated in Figure 4-11. The static indentation mode br ings the tip into contact with the surface, applies a normal load, and records load and di splacement. The scratch mode brings the tip into contact, applies a normal load, tran slates the tip latera lly, and records normal load, normal displacement, lateral load, and late ral displacement. The rastering wear test mode applies a normal load, and uses the piezo t ube scanner to raster the tip over an area. No quantitative data except the prescribed normal load is collected in this test.

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46 Figure 4-11. Schematic illustration of Triboindenter testing modes. Depth sensing indentation Depth sensing indentation tests yield data in the form of loads and displacements which can be analyzed to obtain material pr operties such as elastic modulus and hardness as described in the data analysis section. Once the calibrations have been completed, load versus displacement versus time functions are generated in the lo ad function editor. The area of interest is located, and the test is initiated. Indentati on tests can be operated in load control, displacement control, or ope n loop where a DC bias voltage is set based on the electrostatic force calibration to appr oximate a maximum load. Once the test is started, the tip approaches the surface and make s contact at the set point. The load is increased and load, displacement, and time are recorded. A general rule of thumb has been established for testing thin films on s ubstrates which suggests maximum indentation

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47 depth should not exceed 10% of the film thickness to avoid a contribution from the substrate properties. Rastering wear test For data analysis, fiducial marks are placed using the indentation mode prior to running the test. An automated pattern of eight indents surrounds the test area (Figure 412). An atomic force microscope AFM scan of this area with the fiducial marks is necessary prior to testing. In this test, a normal load is prescribed in the wear function editor along with an area to rast er over and a scan rate. The number of raster lines is set at 256. The area of interest is selected in th e optical CCD microscope. Once the test is started, the tip approaches a nd contacts the surface, appl ies the desired load, and the piezo rasters the tip over the prescribed area for a given number of cycles. Figure 4-12. SEM image of a rastering wear test showing fiducial indentation marks in circles.

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48 Reciprocating Wear Test The scratch test mode is modified to re plicate a reciprocating linear tribometer. This mode was originally designed to place a normal load on the tip, and using the 2D transducer, translate the tip to produce a scratch. The test mode is often employed for testing relative coating adhesion. By usi ng this method with a low normal load, and programming multiple passes in the f unction editor, a nanodisplacement scale reciprocating tribometer is emulated. CSM Microtribometer Tip Preparation Tips on the CSM Microtribometer were m ounted on the cantilever flexure using a cyanoacrylate based adhesive [22] (Figure 4-13). The tips are commercially available BK7 optical glass lenses with a semi-spherical geometry. Figure 4-13. Schematic of the CSM Microtribometer. Sample Mounting The sample is mounted on a SEM micros cope stub using a cyanoacrylate based adhesive. The SEM stub is fixed to the lin ear reciprocating stage. Stepper motors are used for coarse positioning of test area with the aid of a portable microscope.

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49 Reciprocating Wear Test The Microtribometer has a normal load ra nge of 0.5 mN 1 N. This range of applied normal loads is achieved by having th ree different types of dual flexures, with varying stiffness values. Low range norma l loading (0.5-100 mN) is achieved with a glass flexure, middle range loading (50 mN300 mN) is achieved with a stainless steel flexure, and high range loadi ng (200 mN-1N) is achieved with a thicker stainless steel flexure. The appropriate flexure for the desi red load range is selected, and the testing parameters of normal load, track length, slid ing speed, and sampling rate are entered in the computer. The machine records lateral lo ad and lateral position. From this data, friction coefficient and slidi ng speed can be calculated.

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50 CHAPTER 5 DATA ANALYSIS Indentation Testing Indentation testing is a conve nient, relatively non-dest ructive method to determine mechanical properties such as hardness and elastic modulus. Hardness is defined as: / HPA (5-1) where P is the normal load and A is the proj ected residual area in the direction of the load. Microhardness tests are typically performed by applying a known load and calculating the area of the residual indent from measurements taken in an optical microscope. Nanoindentation has been define d as tests involving normal displacements on the order of nanometers [23]. Residual indents from tests at this scale have dimensions smaller than the resolution limits of optical microscopy and would require electron microscopy for measurements. Sin ce electron microscopy is costly and time consuming, requiring a subsequent characte rization of the sample, post-indentation, methods have been developed to calculate the residual area based on a measured depth of indentation. The Oliver-Pharr Method The most widely accepted method for de pth sensing nanoindentation analysis involves a model first proposed by Doerner and Nix and later refined by Oliver and Pharr [24, 25]. This analysis is ofte n referred to as the Oliver and Pharr Method. Once load and displacement data has been collected a nd plotted as shown in Figure 5-1, a power law curve fit of the form:

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51 ()m f P Ahh (5-2) Figure 5-1. Schematic of load versus displacem ent curve along with a cross section of an indented surface. The range of is indicated for various indenter geometries. where P is the load, h is the depth and hf is the depth of the resi dual impression, is fit to the unloading portion of the data. Sneddon de rived the following equation to describe the

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52 load as a function of displacement for an infi nitely rigid cylindrical flat punch into an elastic half-space: 4 1 Ga Ph (5-3) where P is the load, G is the shear modulus, a is the radius of the cylinder, h is the depth of penetration, and is the Poisson ratio [26]. By substituting: 2(1) E G (5-4) for G, Equation 5-3 becomes: 22 (1) Ea P h (5-5) Rearranging the equation for the area of contact of the indenter to: A a (5-6) and substituting, Equation 5-5 becomes: 22 (1)E PAh (5-7) and differentiating with respect to h yields: 22 (1)dPE A dh (5-8) where dP/dh is, by definition, the stiffness. One must now consider the elastic propertie s of the indenter. Defining the modulus of the system as the reduced modulus Er yields: 22(1)(1) 1 s i rsiEEE (5-9)

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53 where Es and s are the Youngs modulus and Poisson ratio for the indented surface and Ei and i are the Youngs modulus and Poisson ratio for the indenter. Substituting into Equation 5-8 yields: 2rdP A E dh (5-10) Solving for Er gives: 2rES A (5-11) Oliver, Pharr, and Brotzen have shown that this equation holds true for any axisymmetric indenter, and will extend to pyramidal indenters with a correction factor, for different geometries [27]. A function to describe contact area from displacement along with Equation 5-11 gives a convenient method to measure the reduced modulus from data shown in Figure 5-1. The Poisson ra tio of the material of interest is often unknown, therefore reduced modulus rather than Youngs modulus is often reported. For a Berkovich indenter of ideal geomet ry, the relationship between contact area and contact depth can be calculated from simple geometry: 224.5c A h (5-12) However, due to the inability to produce a perfectly sharp indenter, some radius is expected at the tip apex. For large indent s, this radius is negligible, but for nanodisplacements, it cannot be neglected. An area function to relate contact area as a function of contact depth can be generate d by indenting on a fused quartz sample of know elastic modulus, E = 72 GPa. A seri es of indentation tests of increasing displacement over the desired range are run on the standard sample. Load versus

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54 displacement plots are generated to find the s tiffness as described above. By solving for hc from the relationship in Figure 5-1: max max cP hh S (5-13) and Equation 5-10 solved for area and plotting Ac versus hc, a curve fit is generated in the form: 11 11 816 242 012345()cccccccc A hChChChChChCh (5-14) Indentation testing data can now be analyzed from eqn. 5-1 and 5-11 to obtain hardness and reduced modulus. The Oliver and Pha rr method includes the following assumptions: Material does not pile-u p outside the indent. The unloading behaves as an isotropic elastic half-space. The tip area function is well approximat ed over the range of displacements. Compliance Method (Stones Method) When designing mechanical testing equipmen t, engineers choose stiff materials to reduce the effect of machine complia nce upon sample data, however, machine compliance cannot be ignored. Compliance is defined as the inverse of stiffness. Compliance of the entire system, CT, can be written as: TmsCCC (5-15) where Cm is the compliance of the load frame (machine) and all other compliances except the sample, and Cs is the compliance of the sample. The load frame compliance includes sample mounting, tip, frame, and stage. Th e following method is suggested by Hysitron to determine the compliance of the machine [28]. The inverse of Equation 5-10 can be substituted in Equation 5-15 for Cs to give:

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55 2Tm rCC EA (5-16) solving Equation 5-1 for area and substituting yields: max2Tm rH CC E P (5-17) Assuming hardness is constant at large indentations, a series of large indents can be run and plotting the inverse of the tota l measured compliance versus 1/Pmax will give yintercept equal to Cm. This method can be run on various mounting techniques and different tips to determine the compliance fo r any given testing conditions. Figure 5-2 shows a plot of the total complia nce measured versus the invers e of the square root of the maximum load for various large loads on a qua rtz standard, an aluminum standard, and a (001) silicon wafer. Figure 5-2. Measured complian ce versus square root of maximum load for a load range of 5mN to 10mN on quartz, Al (001), and Si (001). A least squares fit line is generated to determine Cm. Both the quartz standard and the silicon sample show good agreement in the machine compliance of 2.6 nm/mN. The

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56 aluminum sample shows a slightly highe r compliance of 3.0 nm/mN which may be attributed to differences in sample mounting. A similar yet alternative method to an alyze machine compliance and extract hardness and modulus information has been deve loped by Stone et al. [29]. Stone et al. begin by defining a material compliance parameter: 2 0max 2 s rH JPC E (5-18) where Pmax is the maximum load, Cs is the compliance of the sa mple, H is the hardness of the sample, and Er is the reduced modulus. Note that the parameter J0 is independent of the tip geometry. This parameter was original ly developed by Joslin and Oliver [30] who noted that by substituting for the definiti on of hardness from Equation 5-1 and stiffness from Equation 5-11, one could obtain: max 224r P H E S (5-19) Stones parameter is missing the /4 term from Joslin and Olivers analysis. This /4 term arises from the geometry in Sneddons [ 26] analysis of the cylindrical punch, which Pharr et al. have shown to apply to any axis symmetric indenter [27]. Stone incorporates a geometrical factor in his Er term which is adjusted de pending on tip geometry to account for the /4 term. The indenter used in this study is modeled as an axisymmetric indenter and no geometrical term is incor porated in the reduced modulus; therefore a parameter will be substituted in Stones analysis to better fit this study. This parameter, K0, is defined as: 2 0max 24s r H KPC E (5-20)

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57 By taking Equation 5-15 and multiplying by the square root of Pmax yields: maxmaxmax TmsCPCPCP (5-21) and substituting for the material parameter: maxmax0 TmCPCPK (5-22) Now plotting the CTPmax versus Pmax yields a line with a slope of the machine compliance and an intercept of K0 provided the material sh ows no indentation size effect. This analysis is useful in that the material parameter can be verified for the calibration standard and materi al data can be extracted wi thout knowing the indenter tip geometry. Just as in the Oliver and Pharr an alysis, the unloading is assumed to behave as an isotropic elastic half-space. Modified Winkler Method The Oliver and Pharr model works well on materials that do not exhibit significant pile-up. However, their model greatly overe stimates hardness and modulus in materials that exhibit pile-up due to the underestim ation of the contact area [31-33]. This overestimation of properties is amplified in soft thin films on hard substrates due to excessive pile-up. The compliance method of Stone et. al. extracts a material parameter that requires knowledge of eith er the hardness or the modulus to determine the other. The Stone model also assumes no indentation size effect. An alternate model is developed base d on the Winkler model of a bed of independent springs to account for materi al pile-up. Figure 5-3 shows schematic representations of indent cr oss sections from the Olive r-Pharr model, the modified Winkler model, and the output fr om an actual indent in a mate rial exhibiting pile-up.

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58 Figure 5-3. Schematic illust ration of indent cross sect ions from Oliver-Pharr and Modified Winkler compared to an actual indent. It is assumed that each differential elem ent is constrained or thogonal to the loading direction due to surrounding material. The justifications for this assumption are the contact area radius is much larger than the film thickness and the Poisson ratio of gold is 0.42 which is nearly incompressible. The inde nter tip is assumed to have axisymmetric geometry and is modeled based on a curve fit to a 4th order polynomial from micrographs (see Chap. 6). To account for pile-up and plas tic deformation neglected in the Oliver and Pharr model, each discretized element is ex amined to determine the onset of plastic deformation based on the hardness. Upon plas tic deformation, the volume deformed is transferred to the next differential element. Once the maximum displacement is reached, elastic unloading is assumed. This model is developed to attempt curve fits with load versus displacement data collected from the nanoindenter. The first assumption of constraint due to the surrounding material and the indenter tip/displacement scale results in no elastic strain in the directions orthogonal to the loading direction such that xx= yy=0. From this assumption, and effective modulus can be calculated from: {} [] {} S (5-23) where is the strain, S is the compliance matrix, and is the stress. Expanding yields:

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59 1 000 1 000 1 000 1 00000 1 00000 1 00000xx y y zz y zyz zxzx xyxyEEE EEE EEE G G G (5-24) where E is the Youngs modulus. Solving for the stiffness and assuming no shear coupling yields: 1(1) 1 (1) (1)(12) (1) EEE CSEEE EEE (5-25) The assumption of no strain in the x and y directions: 0 {}0 z (5-26) can be substituted into: {}[]{} C (5-27) which yields: (1) (1)(12)zz E (5-28) The coefficient:

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60 (1) (1)(12) E (5-29) is Lames constant and can be thoug ht of as an effective modulus. Each element of the model film is described in cylindrical coordinates as illustrated in Figure 5-3. For the illustration, the tip geometry is shown as spherical; however, for calculations, the 4th order polynomial fit from micrographs of the tip characterization in Chapter 6 is used. Figure 5-3. Schematic illust ration of a differential elemen t described with cylindrical coordinates and tip displacement ba sed on a spherical tip geometry. A MATLAB code is developed to incr ement tip displacement and calculate differential element elas tic-plastic response se quentially. Figure 5-4 shows the variable designations for the loading and unloading response.

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61 Figure 5-4. Differential element illustration pr ior to (left) and after load (right) with variable notation indicated. At each displacement increment, the stress on each element is calculated by: 0L (5-30) where is the stress, is the effective modulus, is the incremented displacement, and L0 is the film thickness. The incremental load is calculated by: dFdA (5-31) If =H, the hardness, the amount of volume from that incremental displacement over the differential element is transferred to the next element in the outward radial direction. The force is summed and the total displacement is compared to the maximum displacement. The displacement is incremented until the maximum displacement is reached, upon which, the unloading cycle begins. The loadi ng code is represented in the flow chart in Figure 5.5.

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62 Figure 5-5. Flow chart diagram of loading code. The unloading is assumed to be elastic. Each element begins compressed to the point Lmin in Figure 5-4. The material will only recover to the point Lf. Figure 5-6 shows a flow chart of the elastic unloading code.

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63 Figure 5-6. Flow ch art of unloading code. Rastering Wear Test In the rastering wear test mode, no da ta is collected during the test by the Triboindenter. The input normal load is th e only quantitative information available without subsequent analysis. Often in wear tes ting, a wear rate is re ported to quantify the material performance. The wear rate, K, is the volume of material removed per normal load per distance of sliding. The normal load is the input parameter of the test, and the distance of sliding can be calculated by the s quare root of the area rastered over times

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64 256, the number of raster lines, times the number of passes over the raster area. The volume can be calculated in the following manner: Fiducial marks are placed around the area to be worn prior to testing. An area larger than the test area is sca nned in the AFM to include the fiducial marks. The desired rastering test area is run for a given lo ad and number of passes. The larger area including the fiducial ma rks is again scanned with the AFM. The digitized data from the final scan is aligned to the data from the initial scan using the fiducial marks. The initial surface is then subt racted from the final surface. The data below the zero surface plane is integrated to calculate a volume of material removed. This volume can now be used in the wear rate calculation. The subtracted data can be plotted to reveal wear mor phology as shown in Figure 5-7. Figure 5-7. Rastering wear test data plot showing a trenched area on the right of the test area and a pile-up area on the left.

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65 Triboindenter Reciprocating Wear Test The reciprocating wear test mode, or sc ratch mode, is a normal load control and lateral displacement control test. The Tri boindenter collects normal displacement, and lateral force data during the experiment. Th e normal displacement data is subject to errors from sample tilt and thermal drift. To compensate for sample tilt, a low load prescan of the wear track is colle cted and this displacement data is subtracted from the data collected during the test. In the indentation mode, there is a provision to monitor thermal drift for a selected amount of time, and then subtract the recorded drift from the displacement data. A typical indentation test is run over a short period of time. The drift monitor can be set to record drift over a si milar amount of time a nd wait until the drift reaches some minimum threshold, or until th e drift is sufficiently linear. In the reciprocating wear mode, this is not practical due to the length of time that the test is run. Therefore, the displacement of the test will be verified by cross sect ions taken from wear tracks and measured in SEM or TEM and adjusted accordingly. Lateral force data is also subject to errors due to sample tilt. To remove tilt error from the lateral force data, the initial low load pre-scan friction data is subtracted from the data. During the reciprocating tests, mate rial piles up at the e nds of the tracks, and near the endpoints of reciprocation, displacement is increased due to smaller initial contact areas. The frictional data is filtere d to examine sliding conditions excluding these transient areas (Figure 5-8). The friction fo rce is the average per sliding pass and can then be examined versus sliding cycle.

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66 Figure 5-8. Data from a 10-cy cle reciprocating wear test sh owing the transient areas near the ends of the reciprocation and the ar ea over which the friction force data is analyzed. CSM Microtribometer Frictional force data is collected and analyzed in a similar manner to the reciprocating test in the Hy sitron Triboindenter. An exte nsive development of data collection and analysis was presented by Dickrell [22].

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67 CHAPTER 6 CHARACTERIZATION Tip Characterization The indenter tip used for the majority of testing in this study is a single crystal diamond. Diamond is a convenient material due to its high hardness and resistance to wear. In addition, it is well known that gold does not wet carbon which will be beneficial to avoid material transfer from the film to the indenter tip. Due to high costs and limited available geometries, indenter tips of various materials may be sought out. The following methods can be applied to most tip materials. Tip geometry is critical in analysis of indentation and wear testing. Properties such as elastic modulus, hardness, shear stress, and friction coefficient are al l calculated from contact ar eas based on tip geometry. Inaccuracies in tip geometry will cause signi ficant errors in cont act area calculations which will propagate to the above mentioned properties. Methods for determining tip geometry include: the indentation me thod, the scanning method, and electron microscopy imaging. Indentation Method This method, developed by Oliver and Ph arr [25], involves cr eating a series of indents of various depths on a standard samp le of known elastic modulus, typically fused quartz. By generating a plot of area versus contact depth as described in Chapter 5, a function in the form of Equation 5-14 can be generated to describe the contact area as a function of contact depth. This method has proven reliable for samp les that exhibit high hardness with little plastic deformation; how ever, it greatly undere stimates the contact

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68 area in samples that exhibit pi le-up which leads to errors in hardness and modulus [32]. This method is also lacking in three-dime nsional information which is necessary in calculating properties derived from lateral translation of the tip. Scanning Method The scanning method is an adaptation of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) techniqu es that involve s canning the tip over a sample with well characterized f eatures or features of much smaller radii than the tip [3436]. By scanning a feature with a radius much smaller than the tip, the image produced is representative of th e tip (Figure 6-1). Figure 6-1. A schematic representation of a large radius tip scanning a small radius asperity. An FEI DB235 dual beam focused ion beam (FIB ) microscope (described later in greater detail) was used to produce a sharp pyramidal as perity with a radius of less than 100 nm. A silicon wafer was mounted in the FIB and th e normal to the sample was tilted to 20 with respect to the ion beam. A trench was milled, and the sample was rotated 120 about the ion beam. Another trench was milled and the sample was again rotated 120. This

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69 procedure was repeated at progressively lo wer beam currents to sharpen the asperity radius (Figure 6-2). Figure 6-2. Sharp asperity milled on silicon to scan indenter tip. The sharp asperity sample is mounted on the Triboindenter and the desired tip is installed on the transducer. Th e Triboindenter is set to imag ing control which places the transducer into load feedback mode An imaging set point load of 1 N is selected. Using the piezo tube scanner, the tip raster s over the sharp asperity feature maintaining the 1 N load and recording x, y, and z positional da ta. The x, y, and z data is entered in scanning imaging software to produce the image seen in Figure 6-3. This procedure is attractive because it can be run just prior to and upon completion of testing to verify tip geometry. Because it is an in-situ method, removal of the tip for characterization is not required. This method suffers from lo ngevity of the sharp asperity

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70 feature. The feature should be made from the hardest, toughest material available. A diamond sample of the sharp asperity would be optimum. Figure 6-3. Scanning image produced with a sh arp asperity by rastering first in the x direction, and then in the y direction.

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71 Transmission Electron Microscopy Method A direct measure of the tip geometry is the best method for characterization. Imaging the tip in a transmission electron mi croscope provides a hi gh resolution profile of the geometry. A fixture to mount the indenter tip in a TEM sample holder was fabricated and is seen in Fi gure 6-4. The indenter tip is fixed in a polymeric mounting device with a blind threaded hole as receive d from the equipment manufacturer. The polymeric fixture is glued into the TEM moun ting fixture, using a conductive adhesive. The mounting fixture is attached to the TEM sample holder with countersunk set screws. Figure 6-4. Schematic drawi ng of TEM 200 CX sample holde r with tip holder fixture and indenter tip mounted. The normal loading axis of the indenter tip is oriented such that it is parallel with the sample holder tilt axis. The sample holder is inserted into a JEOL 200CX TEM and the tip is imaged at 50 kX and 200 kV. The samp le is adjusted so the normal loading axis and sample tilt axis are at the eucentric height. The indenter tip can be rotated from 60 counter-clockwise (CCW) to 60 clockwise (C W) to produce profile images through 120 of rotation. A series of images are collected at 40 CCW, 0, and 40 CW, the images are digitized and points are pl otted along the profile. The data is fit to a curve to approximate an axisymmetric geometry to describe the tip shape, see Figure 6-5.

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72 Figure 6-5. The upper images are taken at 40 CCW, 0, and 40 CW respectively. The lower image shows data points plotte d along the profile and subsequently curve fit to a 4th order polynomial. This method can also provide crystallographic orientation information in addition to tip geometry. By entering diffraction mode with the beam focused on the indenter tip, a Kikuchi map image is produced. The crysta llographic planes that are orthogonal to the tilt axis remain parallel to the electron beam and therefore diffract through the range of tilt. Since the normal loading axis of the inde nter tip is coincident with the tilt axis the planes that remain diffracting through the tilt, or the planes indexed to correspond to the Kikuchi band upon which the tilt occurs, correspond to the planes normal to the loading axis. One degree of freedom remains to desc ribe the crystallographi c orientation of the indenter tip. By noting the tilt angle at zone axes along the Kikuchi band, the

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73 crystallographic orientation of the tip is characterized, see Figure 6-6. Figure 6-7 shows a schematic representation of the crystallographi c orientations of the indenter tip and the direction of reciprocation in the wear tests. Once the crystallographic orientation of th e indenter tip is characterized, elastic properties of the tip can be calculated from th e stiffness matrix. Single crystals are often elastically anisotropic a nd therefore the Youngs modulus and Poisson ratio are orientation dependent [37]. Figure 6-6. Schematic representation of th e Kikuchi map observed in diffraction mode indexed such that the (100) plane is orthogonal to the tip loading axis. The major zone axes are labeled with the sample holder tilt angle readings. Figure 6-7. Schematic representation of a top down view of the single crystal diamond indenter tip with crystallogr aphic orientations and slid ing direction indicated. Note the loading axis of the indenter corresponds to the [100] direction as indexed in the image.

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74 The orientation dependent Y oungs modulus of a cubic si ngle crystal is given by: 222222 1111124411 2[()]() 2uvwSSSSlmmnln E (6-1) where Euvw is the Youngs modulus for the direction of interest; S11, S12, and S44 are the coefficients of the compliance matrix; and l, m, and n, are the direction cosines of the [uvw] with respect to the unit cell axes [38] Equation 6-1 contains the stiffness matrix for diamond [39] 1079124124000 1241079124000 1241241079000 00057800 00005780 00000578GPaijC which can be converted to the compliance matix by: 0.9490.09790.0979000 0.09790.9490.0979000 0.09790.09790.949000 0001.7300 00001.730 000001.7311 TPa ijijSC From figure 6-7, the loading or ientation of the indenter tip is characterized and therefore the Youngs modulus in the loading dire ction is calculate d at 1053 GPa. X-ray Diffraction X-ray diffraction is used to characterize the grain size and the texture of the gold films. A Philips APD 3720 diffractometer with a Cu K ( =1.54178 ) x-ray source is used to scan the samples. Figure 6-8 shows a schematic drawing of the diffractometer.

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75 Figure 6-8. Schematic drawing of an x-ra y diffractometer with a representation of Braggs law. The x-ray source is in a fixed positio n, and the sample is rotated through The x-ray detector is rotated 2 fo r every 1 of the sample and is therefore rotated 2 with respect to the x-ray source. A plot is generated c ontaining the number of counts recorded by the detector versus 2 From Braggs law: 2sin nd (6-2) where n is an integer (1, 2, 3.), is the wavelength of the radiation, d is the spacing of the diffracting planes, and is the Bragg angle, the spacing of the diffracting planes can be determined. Samples are scanned from 30 to 130 2 and data is collected to determine crystallographic texture and to choos e a family of planes to analyze for grain size approximation.

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76 Crystallographic Texture Often in processing a certain crystall ographic orientation develops due to conditions. These conditions may be due to mechanical processing, thermal gradients upon solidification, or preferred growth orientati on. For thin films, the latter is often the case. Since most single crystals exhibit an isotropic mechanical properties, it tends to reason that polycrystalline samples with a preferred orientation will also exhibit similar anisotropy. Analysis of crystallographic textur e by x-ray diffraction can range from exceptionally simplistic when only one out-of-pl ane orientation exists, to rather complex as the orientation becomes more random. For the simple case when only one out-ofplane orientation exists, Braggs law will onl y be satisfied for planes parallel to the surface and they will be the only in tensities that show up in the 2 scan. To analyze the texture of a more random sample, one must c onsider the integrated intensities of the various peaks that show up in the scan. This is not a simple direct comparison because the intensity is a function of the diffrac ting planes and the Bragg angle through the structure factor, multiplicity factor, and the Lorentz-polarization factor [40]. A simple method to approximate the sample texture is obtained in the JCPDS files. Each file contains space group, lattice parameter, planar spacings, and relative peak intensities for perfect powder samples. A perfect powder sample is equiaxed, or randomly oriented, therefore by comparing relative peak intensitie s collected from a sample of interest with those of the JCPDS file, one can draw conclusions about the texture. Samples were scanned from 30 to 135 2 with a step size of 0.02. Figure 6-9 shows 2 scans for the six different gold coatings us ed in the test matrix. All six show a strong (111) texture with no othe r significant orientations.

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77 Figure 6-9. Scans of the six different gold films shown with indexed peaks. Equiaxed polycrystalline gold has a Y oungs modulus of 78.5 GPa [41], whereas single crystal gold is el astically anisotropic with the <111> orientation being the stiffest. It is reasonable to assume that the textured film will exhibit the elastic properties of the single crystal rather than t hose of an equiaxed polycryst alline sample, therefore the Youngs modulus is calculated to be EAu<111> = 117 GPa using Equation 6-1 and the stiffness matrix for gold:

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78 19042.342.3000 42.319042.3000 42.342.3190000 00016100 00001610 00000161GPaijC Grain Size The grain size of a metal affects the yiel d strength by the Hall-Petch relationship [37]: 1 2 0 ykD (6-3) where y is the yield strength, D is the grain size, and 0 and k are material parameters. To elucidate the plastic response of a material, the grain size should be well characterized. Scherrer first showed that d ecreasing particle size resulted in slight deviations from Braggs law in diffraction due to incomplete destructive interference, which results in peak broadening [42]. Peak broadening can be due to particle size, or grain size, and residual strain, and is usually a combination of the two. Warren and Averbach devised a method to represent diffraction peaks as Fourie r series. They showed that by comparing families of peaks, (111) and (222) for example, coefficients due to strain and grain size could be separated because the strain coe fficient is dependent upon the index of the planes, whereas the grain size coefficient is not. These calculations have been incorporated into software packages for stain and grain size analysis. To collect data for the grain size and strain measurements, the diffractometer was set to scan 2.5 at 38.2 2 and 2.5 at 81.7 2 which correspond to the (111) and

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79 (222) respectively. To increas e resolution, the step size was set at 0.05. The data was entered in the Philips Line Profile software along with scans from standard samples with similar d-spacing, and the Warren-Averbach grain size and strain analysis was performed. Figure 6-10 shows results of the f ilm grain size and strain with respect to the film thickness. The grain size is increasing with film thickness up to 45 nm grains in films over 300 nm thick. All films show 0. 1% compressive strain except the sample designated EBE20 with a film thickness of 27 nm and 0.22% compressive strain. Figure 6-10. Plots of grain size a nd strain versus gold film thickness. Scanning Electron Microscopy Samples were imaged to examine wear tracks and determine film failure in an FEI DB235 field emission gun (FEG) microscope by fi rst locating the test area from the grid lines that were milled prior to testing. Once the area was located, magnification was increased and the sample was oriented so that the direction of wear was along the horizontal, or x-axis. The sample was then ti lted about that axis 52 to provide better perspective. Test areas were imaged at 20-25 kX magnification using a secondary electron detector, 30 m aperture, 5 keV accelerating voltage and number 2 spot size. Some samples displayed beam sensitivity, so im age quality was sacrificed to protect the

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80 sample from repeated rastering with the b eam. Each wear test from the matrix was imaged and examined for coating damage and breakdown. Focused Ion Beam Milling A Dual Beam FEI focused ion beam (FIB) DB235 microscope was used for grid line etching as mentioned earlier. This micr oscope is equipped with a field emission gun (FEG) high resolution SEM column situated ve rtical. Mounted 52 with respect to the electron column is a liquid metal ion source (LMIS) FIB column using gallium for the ion source. The microscope has a gas inje ction system (GIS) which allows platinum deposition by heating the organic-metallic compound methylcyclopentadienyl (trimethyl) platinum to 313 K and then opening the inject ion system in the microscope chamber. The low pressure of the microscope chamber (~ 10-5 Torr) causes the compound to out gas and adsorb on the sample surface. Scanning with the ion beam reduces the compound leaving a platinum layer on the surf ace. The area of platinum deposition can be controlled by the rastering area. The micr oscope is also equipped with an Omniprobe Autoprobe 200 which provides in-situ mechanical micro-manipulation. Transmission Electron Microscopy Sample Thinning Samples for TEM examination must be at least less than 200 nm thick for most materials. For HRTEM, samples must be ty pically less than 100 nm thick. Producing cross sectional TEM samples of a uniform th in film using conventional techniques is challenging. Attempting to produce site specific cross sectional TEM samples with conventional techniques of micron sized featur es would require a degree of luck that would be better suited for playing the Florida Lottery. Fortunately, the FIB microscope has the capability of locating micron sized sites and milling thin cross sections from them.

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81 Once the wear tests were finished, sample s were selected for TEM examination. On those samples, the wear tests were locate d using the grids lines. The wear test was oriented such that the cross section would be orthogonal to the sample surface, and either orthogonal or parallel to the wear direction. For cro ss sections that were parallel to the wear direction, indicator mark s were etched for final thi nning process (Figure 6-11). Fiducial x marks were milled for automated milling. Figure 6-11. SEM micrograph of wear track in center of image to be prepared with cross sectional TEM parallel to the wear di rection. Both indicator and fiducial marks milled by ion beam are indicated with arrows. Platinum was deposited 2 um x 15 um x 1 um high to protect the area of interest while milling. Trenches approximately 15 m x 5 m were milled at 5 nA beam current to 6 m deep at the base a nd sloped out to about 1 m deep on both sides of the desired cross section area. Progressively lower beam currents were then used to remove more material and reduce ion damage. Once the samp le is thinned to approximately 1 m thick, the stage is tilted and the sample is undercut wi th only small tabs of material keeping the

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82 cross section connected to the bulk. The sample is then tilted back and the final automated polishing steps are performed at 300 pA current to thin the sample to approximately 300 nm. Once the automated script is complete, the thinning is controlled manually at 30 pA to thin the sample to less th an 100 nm thick. The cross section is then cut free with the 30 pA beam current, and the sample is removed from the FIB. Figure 612 illustrates the steps. Figure 6-12. FIB cross section sample preparat ion. A) area of interest is located and fiducial marks are milled. B) platinum is deposited. C) front trench is milled. D) back trench is milled. E) cross section is undercut. F) cross section is cut free. The sample is then transferred to an op tical microscope equipped with hydraulic micromanipulators. Glass rods are heated and pulled in tension to form sharp radius tips of approximately 1 m radius. The rods are attached to the hydraulic micromanipulators for precise movement. The cross section area is imaged with the optical microscope and the glass rod is brought into contact with the cr oss section. The cro ss section is attached to the rod by static forces and it is lifted out of the sample trench. The sample is removed

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83 and replaced with a carbon film coated coppe r TEM grid. The static attraction between the carbon film and the cross section is gr eater than the attraction betw een the cross section and the glass rod so the cross secti on can be deposited on the carbon film. The process is illustrated in Figure 6-13. The cross section is now ready for TEM examination. Figure 6-13. Cross section is removed from th e sample trench with a glass rod attached to a hydraulic micromanipulator and th en deposited on a carbon film coated copper TEM grid. Slice and View Serial Sectioning The FEI DB235 FIB microscope is capable of milling with the ion beam, followed by imaging with the electron beam. Using thes e techniques, a series of images can be produced similar to tomography. From these images, movies can be assembled to produce a view of moving through the bulk of a sample and with the aid of computer software, three dimensional reconstructions of the sectioned sample can be generated. There are alternate methods for producing a seri es of slice and view images. The first

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84 described will be referred to as the fixed sample method. The second technique will be referred to as the rotated sample method. Fixed sample method In this method, the sample is not rota ted once the sectioning process has begun. The sample is oriented orthogonal to the i on beam or 52 from the electron beam. A sloped trench is milled to clear material so the sectioning area can be imaged with the electron beam (Figure 6-14). Some bulk mate rial from around the area of interest can be removed prior to sectioning to aid in imaging. Figure 6-14. SEM image of trenched area in preparation of fixed sample method slice and view serial sectioning procedure. A series of ion beam slices, each followed by an electron beam image, is recorded. The fixed sample method can be automated using sc ripting software which requires initial set up but then runs to completion without user interaction.

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85 The ion beam is mounted 52 with respect to the electron beam so the image height will need to be corrected if the sample is not rotated when imaging. The correction is as follows: y y1.27y sin52image actualimage (6-4) where yactual is the actual dimension in the y-ax is or height of the image area, yimage is the dimension of the projected hei ght in the image. The imag e can be corrected in image processing software by changi ng the height on the image by 1.27 times while keeping the width the original value. In addition to pr oducing a series of images, this technique can be used to produce a single cross section image. This is much less time consuming, and greater emphasis can be place on image quality. Rotated sample method An alternative to correcting the image is ti lting the sample so that the electron beam is orthogonal to the imaging surface. This requires a free, unobstructed view of the surface. The following method, illustrated in Figure 6-15, was devised to section and image wear tracks. The wear test is located with the electron beam and tilted to 52 with respect to the e-beam. A platinum layer is deposited approximately 1 m thick and twice as long as the wear scar. Trenches are milled on each side and in front of the wear scar to about 4 m deep. The trench on one side is milled with the sloping cut, so an undercut can be performed. The sample is tilted to 0 with respect to the electron column, and an undercut is made the entire length of the wear scar up to the platinum protection layer.

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86 The Omniprobe in-situ micromanipulator is inserted in the front trench and the material containing the wear scar is lifted up to approximately 45 using the platinum as a plastic hinge. Now the sample is tilted 7 and the firs t slice is taken off with the ion beam. The sample is rotated 38 and an image is taken with the electron beam. Step 6 and 7 are repeated until the entire wear track has been sliced and imaged. Figure 6-15. Schematic illustration of slice and view rotating sample method.

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87 This process has advantages over the fixed sample process in that no correction is needed and the image retains the same resoluti on in the height and width. Since there is no bulk surrounding the area, imaging is less challenging because ther e is no signal loss due to imaging down into a trench. The expos ed surface can also f acilitate electron back scattered diffraction (EBSD) if the FIB is e quipped with such analys is. The process has disadvantages due to complications with so me geometries and the time required is considerably longer than the fixed sample method. Once the images are collected, they can be loaded into Macromedia Flash or similar processing software. The images can be registered to each other and generate a movie of slices through the material. In addition to movie making software, three dimensional reconstructure software, such as TGSs Amir a, can be used to produce three dimensional images and slices in different orientations from the slicing orientation. These techniques provide information about surface and subsur face damage, such as material volume exuded from the contact, plastic deformation zone characteristics, and sub-surface crack formation and delamination, that is unobtai nable by top-down microscopy techniques. Transmission Elelctron Microscopy Analysis After cross sections are produce on the FIB, they are analyzed in a TEM. Film thickness is determined in a JEOL 200CX microscope at 200kV. Figure 6-16 shows cross sectional images of the six different film thicknesses with half cycle wear scars run at 500 N normal load. Samples are named base d on the deposition techniques, electron beam evaporation (EBE) and ion beam s puttered (IBS), and the targeted coating thickness. Acutal coating thickne ss is indicated on the images.

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88 Figure 6-16. Cross sectional TEM images of gold films with wear tracks run for half cycles at 500 N normal load. Film thicknesse s are indicated on each image.

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89 CHAPTER 7 RESULTS AND DISCUSSION Static Indentation Testing A series of fifty displacement control indents were performed from 10 nm to 100 nm on each sample varying the displacement incrementally from test to test. The tests were analyzed using all three methods menti oned in Chapter 5 namely: Oliver and Pharr (O-P), Stone, and Modified Winkler (MW). For each sample, hardness is plotted versus maximum tip displacement. Oliver and Pharr Analysis Results A plot of the hardness results based on the Oliver and Pharr model is shown in Figure 1. Figure 7.1. Plot of hardness determined by the Oliver and Pharr method versus maximum tip displacement for five samples of varying thickness.

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90 As mentioned earlier, the Oliver and Ph arr model overestimates hardness of materials that exhibit significant plastic flow. The Oliver and Pharr model assumes no pile-up. For indentations on materials that e xhibit large plastic flow resulting in pile-up, the load is supported by a portion of the mate rial that has been exuded. Oliver and Pharrs assumption of no pile-up results in an underestimated contact area. Since hardness is calculated from H=Pmax/A, where H is the hardness, Pmax is the maximum normal load, and A is the area, underestimati ng the area will result in overestimating the hardness. Indentation tests analyzed with the O-P model results show an increase in hardness for each sample below 15 nm in maximum displacement. This result could arise from an inaccurate tip area function for these low displacements or from excessive scatter observed at indentation tests perf ormed below 15 nm maximum displacement. The exceptionally high hardness shown by the EBE 20 sample (27 nm thick) is more representative of hardness va lues for silicon (100) [43], which is due to the maximum displacement for the tests being well beyond 10% of the film thickness. Indentation well below 15 nm maximum displacement is required to examine the hardne ss of the gold film in sample EBE 20 without a substrate cont ribution. Although the manufacturer claims resolution of 0.2 nm displacement on the Hysitr on Triboindenter, special tips are required to achieve repeatable results below approximately 10-15 nm. Figure 7-1 shows the hardness for the remainder of the samples ranging from over 2 GPa for sample EBE 500 up to 5 GPa for sample IBS 100. Although the general rule of thumb for indentation suggests indenti ng only 10% of the film thickness to avoid substrate contribution, the ha rdness values are not greatly affected by indentation depth for samples other than EBE 20 even on the IBS 100 sample with a film thickness of 105

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91 nm. At a maximum displacement of 85% of the film thickness, the hardness has only increased by 25%. For materials that exhibit significant plastic deformation, hardness is frequently approximated by the yield strength with the equation: 3y H (7-1) where H is the hardness and y is the yield strength [44]. Yield strength values for bulk gold range from 100 MPa to 200 MPa [41] gi ving a hardness range of 300 MPa to 600 MPa, well below the values calculated from Oliver and Pharrs model shown in Figure 71. However, the published yield strength va lues from handbooks ar e for coarse grained microstructures. It is well documented that yield strength increases with decreasing grain size by the Hall-Petc h equation [37]: 1 2 0 ykD (7-2) where y is the yield strength, 0 and k are material dependent parameters, and D is the grain size. Combining E quation 7-1 and 7-2 gives: 1 2 03() H kD (7-3) Conrad and Jung found 0=216 MPa and k=0.06 MPa m for gold [45]. Table 7-1 shows Hall-Petch calculated hardness using Conrad and Jungs constants and the grain size of the sample measured by XRD from Figure 6-10. The values for hardness range from 1.48 to 1.70 GPa. The reduction in grain si ze does not justify the high hardness values obtained from the Oliver and Pharr analysis method. Thus, inaccurate approximations of contact area due to material pile-up is the likely cause of over estimation of hardness.

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92 Table 7-1. Hardness values calculated for samples from the Hall-Petch equation. Sample Hardness (GPa) EBE 20 2.26 IBS 100 1.70 EBE 200 1.55 IBS 300 1.48 EBE 500 1.49 Stone Analysis Results The same load displacement data that was used in the Oliver and Pharr method above is also analyzed using Stones appro ach. If the assumptions of no indentation size effect and unloading as an isotropic elastic half-space are valid, th e plots of compliance times the square root of load versus the s quare root of load sh ould fall on one line as described in Chapter 5. Figure 7-2 and 73 show Stone plots for samples EBE 20 and IBS 100 respectively. Figure 7-2. Stone plot incl uding linear curve fits for samp le EBE 20 with a gold film thickness of 27 nm.

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93 Figure 7-3. Stone plot incl uding linear curve fits for samp le IBS 100 with a gold film thickness of 105 nm. Both plots do not fit well to a single line indicating a problem w ith the model. The assumption of an isotropic elastic half-space is questionable for multiple reasons. It is reasonable to expect some contribution from the substrate on both of these samples considering the ratio of inde ntation depth to film thickne ss is 74-330% on sample EBE 20 and 19-84% on sample IBS 100. The assumption of isotropy is problematic in that single crystals and textured surfaces are often elastic ally anisotropic. A measure of the degree of anisotropy is given by [46]: 1112 442 CC C (7-4) The compliance matrix for gold is given in Chapter 6, and the value for C11, C12, and C44 for silicon are 165 GPa, 64 GPa, and 79.2 GPa, respectively [41]. Calculating the degree of anisotropy for gold and silicon gives 0.46 and 0.64 where 1.0 is the result for an

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94 isotropic material. In addition to these fact ors, there exists a pressure induced phase transformation in silicon which may o ccur in the upper bounds of the maximum displacement [47]. Figure 7.4 shows Stone pl ots for the remaining three samples, EBE 200, IBS 300, and EBE 500. Figure 7-4. Stone plots in cluding linear curve fits for samples EBE 200, IBS 300, and EBE 500 with a gold film thickness of 180 nm, 315 nm, and 537 nm, repectively. The data from three samples from Figure 7.4 fits reasonably well to linear curves suggesting the assumption of no indentation size effect may be valid for these samples. The hardness for samples EBE 20 and IBS 100 will be calculated from the curve fits for the lower values of the square root of Pmax because these values correspond to smaller indentation depths. Table 72 shows the sample versus th e hardness calculated by the Stone method.

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95 Table 7-2. Hardness values calculated for samples from the Stone method. Sample Hardness (GPa) EBE 20 2.57 IBS 100 3.19 EBE 200 1.96 IBS 300 2.83 EBE 500 2.21 The values in Table 7-2 from the Stone analysis are lower than the values determined by Oliver and Pharr, but they are still higher than those predicted by the HallPetch equation. Modified Winkler Approach The Modified Winkler method was employed as described in Chapter 5 to account for the material pile-up exhibited in this system. This model assumes axisymmetric tip geometry based on the TEM characterization. Input parameters ar e film modulus and film thickness. The same load versus displacement data is used from the two previous analyses. Figure 7-5 shows a plot of the hardness versus the sample thickness for the Oliver and Pharr method, the Stone method, the Modified Winkler model, and the predicted values based on the Hall-Petch e quation and sample grain size. The Modified Winkler predictions show excellent agreemen t with Hall-Petch for sample IBS 100, EBE 200, and IBS 300. Stones method shows good agreement with Hall-Petch for sample EBE 20, however this is probably a coinci dence. As mentioned previously, the assumptions for Stones model are questiona ble for large displacement depth to film thickness ratios. The more likely agr eement shows up on sample EBE 500 where Modified Winkler determines a value some what lower than Hall-Petch, and Stone calculates a higher value. The larger thickness of EBE 500 at 537 nm gives an

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96 indentation depth to film ratio from 3. 7% to 16.6% making the elastic half-space assumption much more reasonable. Figure 7-5. Hardness determined by four me thods plotted versus sample thickness for five samples. Excluding the low film thickness sample due to previously mentioned concerns, the Modified Winkler method shows the best ag reement with the Ha ll-Petch equation for analyzing highly plastic thin film hardness on elastic substrates. Rastering Wear Tests Several sets of test matrices varying tip geometry, normal load, and number of cycles, were run using the rast ering wear mode as described in Chapter 4. This mode collects no quantitative data during the test. A method for quantitative wear measurements was developed in Chapter 5. This testing mode suffers from difficulties due to machine control and wear debris. Figu re 7-6 shows SEM micrographs from a test matrix run at a 40 N normal load while varying tip geom etry and number of cycles. The

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97 images in Figure 7-7 and Figure 7-8 show pr oblems that could not be resolved after numerous attempts, discussions, and an exte nded visit to the equi pment manufacturer. The image in Figure 7-7 shows a proble m with wear scar extending beyond the prescribed area. This is most likely due to a drift or control problem with the piezo scanner. The image in Figure 7-8 shows a trench that sometimes develops during test runs. No correlation was made due to sample variation, and the phenomenon would occur intermittently on the same sample. In addition to the control problems, Figure 7-8 shows wear debris that was transferred into the wear s car which could confound a wear volume measurement. Figure 7-6. SEM micrographs from a ra stering wear test matrix with a 40 N normal load varying tip geometry and number of cycles as indicated.

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98 Figure 7-7. SEM micrograph showing a rasterin g wear anomaly. The wear scar extends outside of the prescribed area especi ally in the horizontal direction. Figure 7-8. SEM micrograph showing a raster ing wear anomaly. A trench develops on the right and lower edge of the wear scar Debris is also transferred to the wear area. The problems with the rastering wear mode make quantification of tests virtually impossible. At best this method can be us ed as a qualitative examination method for a relative study. Even in this application, the researcher must be cautious about true results

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99 versus artifacts created by th e problems mentioned above. If the rastering problems can be solved, this method along with the wear volume measurement described in Chapter 5 could be useful testing tools. Reciprocating Wear Tests Film Failure Film failure will be defined as the point where the gold film has been removed and the silicon substrate is exposed. The wear track will still contain gold in the contact, and gold will be supporting a portion of the load, but the indenter tip will be in intimate contact with the silicon substr ate at the point of failure. Each wear area was examined upon completion of testing in a FEG SEM to determine if the coating survived the test. When the film has failed, atomic number contrast between the silicon substrate (dar k) and the gold film (bright) shows in the micrographs of the wear scar, see Figure 7-9. Figure 7-9. SEM micrographs of samples IBS 300 (left) and EBE 200 (right) run for 30 cycles at 500 N normal loads. Note the dark contrast from the silicon substrate in the center of the wear tr ack on sample EBE 200 due to complete removal of the gold film.

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100 The results in Figure 7-9 will be denoted with an S for coatings that survive the number of cycles of the give n test, and denoted with an F for coatings that show removal of the gold to expose the silicon substr ate. One half of the test matrix was run with an evaporated carbon coating on top of the gold coating. Some of these samples exhibited a delamination of th e carbon coating which could o ccur with or without failure of the gold film. Figure 710 illustrates each case. Figure 7-10. SEM micrographs of sample EBE 200 C run for 3 cycles (left) and 30 cycles (right) at 500 N normal load. On the left, the carbon film has delaminated and the gold is worn but not compromised. On the right, the gold film has been worn through, and the silicon substrate is visible. Results from Figure 7-10 will be denoted with a D for samples that show delamination of the carbon wit hout exposure of silicon below the gold or denoted with an F if the silicon is exposed. Each test condi tion was repeated four times to produce a test matrix with twelve different load and cycle combinations on five different samples both with and without carbon coating on top of the gold coating yiel ding a total of 480 tests. Every wear scar was imaged to determine wh ether the coating survived, delaminated, or failed. Micrographs from the repeat tests were very consistent with similar results in at

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101 least three of every set of four. Table 73 through Table 7-6 shows results of the micrographs from the entire test matrix. Se lected micrographs show ing transitions from survived coatings, to delamination, and/or to failure are included in Appendix A. Gold on silicon without carbon Five different target thicknesses of gold on silicon were produced by either electron beam evaporation or ion beam sputtering as described in Chapter 2. Table 7-3 summarizes the micrograph re sults for tests with a 100 N normal load using the designations described above. Table 7-3. Results of SEM micrographs from gold on silicon samples with 100 N normal load. Designations are S for coatings that survived to the number of cycles shown, or F for coatings that have exposed silicon. As seen in Table 7-3, all coatings surviv ed up to 300 cycles with the exception of the EBE 20 coating with a target thickness of 20 nm which had failed by the first cycle. Table 7-4 summarizes the microscopy results fo r tests with a higher normal load of 500 N. Here again the sample EBE 20 did not surv ive any number of reciprocations. This is an expected result because as in most appl ications, higher normal loads result in more material removal. Sample IBS 100 shows that the coating survived the first cycle but the table entry for the one cycle test is S/F.

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102 Table 7-4. Results of SEM micrographs from gold on silicon samples with 500 N normal load. Designations are S for coatings that survived to the number of cycles shown, or F for coatings that have exposed silicon. The designation S/F is chosen because th e sample shows small exposed areas of silicon sporadically in the track and is the ons et of film failure. The gold film is still intact through most of the cont act and is expected to exhib it the tribological properties of the diamond indenter tip sliding on a gold film, see Figure 7-11. Figure 7-11. SEM micrograph of sample IBS 100 after 1 cycle of sliding with 500 N normal load. Note the small dark spots in the center at each end of the wear track indicating exposed silicon substrate. A note of interest from Table 7-4 is that the survival of the gold coating based on number of cycles is not linear ly dependent on film thickness. Figure 7-12 is a plot of the targeted film thickness versus the maximum number of cycles obs erved without coating failure. Since 300 was the maximum number of cycles in the tests, samples IBS 300 and

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103 EBE 500 data points are marked with arrows to show the samples may have survived beyond the indicated data point. Figure 7-12. Nominal film thickness versus num ber of cycles prior to film failure. Note that the 300 nm and 500 nm thick films did not show failure at the maximum 300 test cycles. Number of cy cles are plotted on log scale. Carbon on gold on silicon A layer of amorphous carbon, approximately 60 nm in thickness, was deposited on a set of samples as described in Chapter 2 to explore the effects of an electrically conductive friction modifying layer. The resu lts from the SEM examination of the wear tests with 100 N normal load are presented in Table 75. In this tabl e the designation of D for coatings that show delamina tion as described above is included.

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104 Table 7-5. Results of SEM micrographs from carbon on gold on silicon samples with 100 N normal load. Designations are S fo r coatings that survived to the number of cycles shown, D for coati ngs that exhibited delamination of the amorphous carbon, or F for coatings that have exposed silicon. The effect of amorphous carbon is signifi cant when comparing Table 7-5 with Table 7-3. The sample EBE 20C appears to have survived for at least 30 cycles with the amorphous carbon where without it failed before one complete cycle. With the addition of amorphous carbon, the carbon film has de laminated by 30 cycles on the EBE 200C sample, and gold film failure is seen at 300 cycles. Table 7-6 shows the results from microgra phs taken from a sample matrix run on carbon on gold on silicon. The results are sim ilar to those seen in Table 7-4 with the exception of the IBS 100C sample. With the car bon coating, the life of this sample has been extended beyond 3 cycles. Table 7-6. Results of SEM micrographs from carbon on gold on silicon samples with 500 N normal load.

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105 Electron Microscopy Cross Sections of Wear Scars A dual beam FIB SEM was used to make cross sections of the wear tracks as described in Chapter 6. While the Hysitr on Triboindenter is capable of measuring displacement, the instrument is sensitive to thermal drift during the extended amount of time required to run a reciprocating wear test Figure 7-13 shows a pl ot of vertical tip displacement versus lateral tip disp lacement for a 30-cycle test. Figure 7-13. Plot of vertical tip displacem ent versus lateral displacement for a 30-cycle reciprocating test. Points are plotted in red at time=0 and progress in color through the spectrum (ROYGB) as time increases. The data points are shown in color beginning in red at the start of the test and as time goes on, the color of the points transition through the spectrum to end in blue at the end of the test. The initial vertical displacement of the tip is in the 100 nm range and

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106 increases in displacement initially. Therma l drift eventually overtakes the vertical displacement and the tip appears to be at a nega tive displacement at the end of the test. This contradicts force measurements and elec tron microscopy of cross sections. For these reasons, displacement is measured using electron microscopy by taking cross sections from interrupted tests. Figure 7-14 shows a transverse cross section from a wear scar. The image was produced and tilt co rrected as described in Chapter 6. Figure 7-14. FIB/SEM transverse cross sect ion of a wear scar run on sample EBE 500 for 10 cycles with a 500 N normal load. Measurements of the film thickness and the depth of the wear scar from the original surface are indicated. In addition to film thickness and wear scar measurements, the cross sectional image shows the morphology of the film damage. Note the curling of the material exuded from the contact area on the right side Features such as these and sub-surface damage including fracture and delamination coul d not be investigated with a top-down microscopy technique. The microstructure of the gold film is revealed in this image through channeling contrast. Se veral researchers have demo nstrated and explained FIB secondary electron channeling co ntrast using the ion beam as the incident source and a secondary electron detector [4850]. Samples from this study could not be characterized using channeling contrast produced with th e ion beam due to extreme damage caused

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107 even at low beam currents. The image in Figure 7-14 is produced by polishing the surface with a low current (30 pA) ion beam and switching the incident beam to the electron beam with a 5 kV accelerating voltage, 30 m aperture and #2 spot size. This type of image is challenging to produce in conventional SEMs because the technique is sensitive to surface contamination and topograp hy. In the FIB, a nascent polished surface can be easily produced. The microstructure revealed in Figure 714 shows columnar grains typical of a deposition process. Presumably, grains nucle ate on the surface with random orientations, but will be overtaken by grains oriented in th e fast growth direction. The results from XRD in Chapter 6 show a strong (111) orientatio n, suggesting this is the preferred growth orientation for gold which is substantiated in the literature [51] A highly deformed plastic zone is visible below the contact area. A detailed study of plastic zone evolution could be conducted with this imaging technique. Uncoated samples from the wear matrix with a 500 N normal load were chosen for measurements using the cross sectiona l FIB/SEM technique. Figure 7-15 shows a plot of displacement below the original surface versus number of cycles as measured from cross sections. Wear tests were run with incremental number of cycles so data could be collected for various cycles. The numbe rs of cycles were as follows: 0.5, 1, 3, 10, 30, 100, and 300. If penetration of th e film was observed in a cross section, displacement was not plotted because the actual number of cycles at penetration can not be determined. Sample EBE 20 is not plotted because film penetration was apparent after the first half cycle. The figures used to measure the displacement data have been included in Appendix B.

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108 Figure 7-15. Depth of wear scar versus number of cy cles for samples run at 500 N normal load. It is interesting to note th at both electron beam evaporated (EBE) samples behave nearly identical up to the failure of EBE 200. The ion beam sputtered (IBS) samples also behaved similarly with IBS 100 showing slig htly more displacement than IBS 300. Considering displacement into the film below the original surface as a measure of wear, the EBE samples showed more wear than bo th IBS samples and IBS 100 showed slightly more wear than IBS 300. A plot of fraction of the coa ting penetrated versus number of cycles is constructed in Figure 7-16. In this plot the displacement is normalized by the coating thickness to give the fraction of penetration. Again, if the coating had failed, the number of cycles at the observed failure was not plot ted, and since sample EBE 20 had failed at the first half pass it does not show up in the plot.

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109 Figure 7-16. Plot showing fraction of coating depth penetrated versus number of cycles for samples run at 500 N normal load. Now it appears the two thinner film samples, IBS 100 and EBE 200, behave similarly, as do the two thicker film sa mples IBS 300 and EBE 500. Figure 7-16 shows good agreement with Table 7-4 between the two characterization methods to determine the point of film failure with the exception of Sample IBS 100, appearing to have failed before 3 cycles in Table 7-4, and after 3 cycl es in Figure 7-16. Th is may be explained by the coating first breaking down sporadically al ong the wear track and slight variation in cycle number at failure from tests run with re peat conditions. The fr action of penetration appears to be linear when plotted versus the log of the number of cycles to failure which could lead to a coating lifetime prediction ba sed on an extrapolation. The EBE 200 data appears to fall on a line nearly all of the way to failure, however, the last point plotted for IBS 100 and IBS 300 deviated from a line fit through the rest of the points for these

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110 samples. The deviation occurs somewhere be tween 50% and 80% of the film thickness. More data is needed to determine if this could be a viable method for life prediction. Friction Measurements Metallic friction, as described in Chapter 1, is comprised of two components, the force of deformation, and the force to slide one interfacial surface [4]. Reducing the contact pressure below the yield point of the material will result in an elastic contact, and the force of plastic deformation should be eliminated. The contact pressure can be reduced by two methods, increasing the contac t area or reducing the normal load. The tests were run using the well characterized diamond indenter previously described and the load resolution is such that a pure elastic cont act with this tip geometry is not possible on the Hysitron. To examine the friction from an elastic contact, tests were run on the CSM Microtribometer in addition to the te sts run on the Hysitron Triboindenter. Hysitron Triboindenter friction measurements Lateral force is collected during the wear tests on the Hysitron Triboindenter, and is normalized with the normal load to determin e a friction coefficient. Figure 7-17 shows friction coefficient versus the number of cycles for gold on silicon samples without carbon coating run at 500 N. Testing on samples EBE 20, IBS 100, and EBE 200 was terminated at 30 cycles because the film had been compromised by this point on all three. Table 7-4 shows that sample EBE 20 had failed by the first half cycle. The friction data from this sample is much lower than for the others. The friction starts out at 1.6 and decreases to 1.0. This low friction coeffi cient is probably more representative of diamond sliding on silicon than diamond sliding on gold. Sample IBS 100 shows friction above 0.25 for less than 3 cycles of sliding. Both cross sectional microscopy and top down microscopy show the film failure betw een 1 and 3 cycles for IBS 100. Sample

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111 EBE 200 shows failure between 10 and 30 cycles and its friction coefficient drops below 0.25 after 10 cycles. Sample IBS 300 has not failed at 300 cycles but the cross sectional image shows the coating is close to penetra tion, see Figure 7-16. The dashed line shown on the plot represents friction values where the gold film has been penetrated and the sliding contact is comprised of a combination of gold and silicon. Below the grey region, diamond sliding on silicon is a major component of the friction coeffiecent. For gold film samples with a 500 N normal load, a friction coeffi cient below 0.25 indicates film failure. Figure 7-17. Friction coefficient versus numbe r of cycles in log scale for gold on silicon samples without carbon run at 500 N normal load. Friction results from the gold on silicon sa mples without carbon coating run at 100 N normal load are plotted in Figure 7-18. Ag ain the friction coefficient is much lower for the sample EBE 20 that failed as early as th e first half cycle, see Table 7-3. All other samples survived the entire test at 100 N normal load and their friction coefficient remained above 0.25.

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112 Figure 7-18. Plot of friction coefficient vers us number of cycles on a log scale for gold on silicon samples without carbon co ating run at 100 uN normal load. Figure 7-19 shows friction coefficient re sults for gold on silicon samples with carbon coating run at 500 N normal load. The dashed line indicates a friction coefficient of 0.25. Again samples exhibit fri ction coefficients below 0.25 at or near the number of cycles when the gold film is pene trated. Comparing Tabl e 7-6 to Figure 7-19, EBE 20C shows failure immediately and the friction coefficient begins below 0.25. Sample IBS 100C fails between 3 and 10 cycles from the table, and the friction coefficient drops below 0.25 at 5 cycles. Table 7-6 shows sample EBE 200C failed between 10 and 30 cycles, and the friction coe fficient drops below 0.25 at 14 cycles. The remaining two samples survived beyond 30 cy cles and their friction coefficients remained above 0.25. Friction data was lost for these samples beyond 30 cycles due to an error in the wear cycle load f unction code. Friction data does not seem to be sensitive to the carbon coating at the 500 N normal load, and the data does not show a correlation to the carbon delamination event.

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113 Figure 7-19. Plot of friction coefficient ve rsus number of cycles for gold on silicon samples with carbon coating run at 500 uN normal load. Dashed line indicates a friction coefficient of 0.25. The friction data for gold on silicon sa mples coated with carbon run at 100 N load shows significant differences from the pr evious results. Table 7-5 shows the carbon coating survived on all samples up to 30 cycl es with the exception of a delamination of EBE 200C at the 30-cycle micrograph. The fr iction data for these tests is plotted in Figure 7-20. The friction coefficient for a ll samples ranged from 0.12 to 0.17 and the point of delamination on sample EBE 200 was not detected in the friction data. Friction data beyond 30 cycles was lost for these sample s due to the same error in the wear cycle load function mentioned previously The lower normal load of 100 N allows friction measurements of the diamond indenter tip sliding against the amorphous carbon coating to be measured whereas the 500 N normal load friction is dominated by properties of the gold.

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114 Figure 7-20. Plot of friction coefficient ve rsus number of cycles for gold on silicon samples with carbon coating r un at 100 uN normal load. CSM microtribometer friction coefficient measurements As mentioned previously, an elastic contact cannot be accomplished with the current tip geometry on the Hysitron Triboi ndenter. Although a technique has been developed in this study to target low c ontact pressures in the Hysitron, a CSM Microtribometer was employed as a matter of convenience. To target a completely elastic contact, the maximum pressure should be below the hardness. Sample EBE 200 was chosen for te sting. From the hardness analysis, the lowest calculated hardness for this sample was determined to be 1.4 GPa. If the relationship H=3 y holds true for this material, the yield strength should be 470 MPa. Coarse grained annealed gold is reported to have a yield strength of 100 MPa 120 MPa [41, 52]. The onset of plasticity can occur at pressures as low as 1.1 y [37]. With all these considerations, a maximum pressure below 100 MPa was targeted to ensure an elastic contact. Based on a 2 mm diameter pin of Al2O3, with a normal load of 1.5 mN determined by a cantilever flexure, in cont act with gold, the maximum pressure was

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115 calculated to be 92 MPa using Herztian elastic contact mechanics. Reciprocating tests were run for 1, 3, 7, 10, 30, 70, and 100 cycles and are plo tted in Figure 7-21. Figure 7-21. Friction coefficient versus num ber of cycles plotted on log scale for a contact pressure of 92 MP a run on sample EBE 200. The friction coefficient from Figure 721 should contain no pl astic deformation component and should be representative of the interfacial sliding term from Bowden and Tabors model [4]. Virtually no deform ation was detected in SEM examination of the wear area on this test sample. This result suggests for the samples that failed on the Hysitron, the contact remained plastic, rem oving material until failure. A combination of work hardening and increased contact area might eventually develop an elastic contact on

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116 thicker samples resulting in a more wear resistant contact, but testing for higher numbers of cycles is required to investigate this possibility.

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117 CHAPTER 8 CONCLUSIONS Considerations for testing thin films on the nanodisplacement level were examined and addressed. A test matrix was designed to investigate thin film wear testing over a range of film thickness, normal load, and number of repeat cycles. Gold was chosen as a test film for its well characterized propertie s and applications in MEMS. A variety of sample preparation techniques were empl oyed with best results obtained from commercially processed electron beam evapor ated and in-house ion beam sputtered coatings. Various testing modes and parameters were explored. Reciprocating linear testing was demonstrated in the Hysitron Triboindent er and proved to be the best method for data collection and reproducibility. A method to mount a wide range of tip materials and geometries was developed to test various contacts and pressures. A systematic approach was designed to lo cate wear regions throughout testing and characterization. An optics offset calib ration method was developed to overcome the limitations of light optical microscopy. Wear test placement was repeatable within 0.5 m. A variety of characterization techni ques including nanoindentation, x-ray diffraction, ion beam milling, and electron microscopy were employed to analyze sample properties and quantify results. Established indentation mo dels overestimate hardness in materials that exhibit pile-up. An alternative model was formulated and applied to load displacement data. Results of this altern ative model show good ag reement with the Hall-

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118 Petch relationship for the grain size of the films tested. Wear test lateral load was measured with the Hysitron Triboindenter transducer and showed good repeatability. Displacement data was characterized by dire ct measurement from SEM micrographs due to drift issues during extende d testing times. Friction coe fficient showed good agreement with an alternate testing method and prove d to be a good indication of film failure. The methodology developed in this study gives well characterized, reproducible, quantitative data from reciproc ating wear tests on the nanodisplacement level. The data set from this study will be extended to predic t wear rates and fricti on coefficients using finite element analysis.

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119 APPENDIX A SEM IMAGES OF EXPERIMENTAL WEAR MATRIX A) B) C) D) E) Figure A-1. Images showing the wear scar of gold on silicon at 100 N load normal load. Conditions showing transitions fr om Table 7-3 are included. A) EBE 20, half cycle. B) IBS 100, 300 cycle. C) EBE 200, 300 cycle. D) IBS 300, 300 cycle. E) EBE 500, 300 cycle.

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120 A) B) C) D) E) F) G) Figure A-2. Images showing the wear scar of gold on silicon at 500 N load normal load. Conditions showing transitions from Table 7-4 are included. A) EBE 20, half cycle. B) IBS 100, half cycle. C) IBS 100, 1 cycle. D) EBE 200, 10 cycle. E) EBE 200, 30 cycle. F) IBS 300, 300 cycle. G) EBE 500, 300 cycle.

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121 A) B) C) D) E) F) G) H) I) J) Figure A-3. Images showing the wear sc ar of carbon on gold on silicon at 100 N load normal load. Conditions showing transitions from Table 7-5 are included. A) EBE 20, 30 cycle. B) EBE 20, 100 cycle. C) IBS 100, 300 cycle. D) EBE 200, 10 cycle. E) EBE 200, 30 cycle. F) EBE 200, 100 cycle. G) EBE 200, 300 cycle. H) IBS 300, 300 cycle. I) EBE 500, 100 cycle. J) EBE 500, 300 cycle.

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122 A) B) C) D) E) F) G) H) I) J) K) Figure A-4. Images showing the wear scar of carbonon gold on silicon at 500 N load normal load. Conditions showing transitions from Table 7-6 are included. A) EBE 20, half cycle. B) IBS 100, 1 cycle. C) IBS 100, 3 cycle. D) IBS 100, 10 cycle. E) EBE 200, 10 cycle. F) EBE 200, 30 cycle. G) IBS 300, 1 cycle. H) IBS 300, 3 cycle. I) IBS 300, 30 cycle. J) EBE 500, half cycle. K) EBE 500, 30 cycle.

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123 APPENDIX B SEM CROSS SECTIONS USED FOR DISPLACEMENT MEASUREMENTS A) B) Figure B-1. SEM/FIB cross s ection of sample IBS 100 at 500 N normal loads. A)1 cycle. B) 3 cycles. A) B) Figure B-2. SEM/FIB cross sect ion of sample EBE 200 at 500 N normal loads. A)1 cycle. B) 3 cycles.

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124 Figure B-3. SEM/FIB cross sect ion of sample EBE 200 at 500 N normal loads and 10 cycles. A) B) Figure B-4. SEM/FIB cross s ection of sample IBS 300 at 500 N normal loads. A)1 cycle. B) 3 cycles. A) B) Figure B-5. SEM/FIB cross s ection of sample IBS 300 at 500 N normal loads. A) 10 cycle. B) 30 cycles.

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125 A) B) Figure B-6. SEM/FIB cross s ection of sample IBS 300 at 500 N normal loads. A) 100 cycle. B) 300 cycle A) B) Figure B-7. SEM/FIB cross sect ion of sample EBE 500 at 500 N normal loads. A) 1 cycle. B) 3 cycle A) B) Figure B-8. SEM/FIB cross sect ion of sample EBE 500 at 500 N normal loads. A) 10 cycle. B) 30 cycle

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126 A) B) Figure B-9. SEM/FIB cross sect ion of sample EBE 500 at 500 N normal loads. A) 100 cycle. B) 300 cycle

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127 LIST OF REFERENCES 1. Walraven, J. Failure mechanisms in mems. in International Test Conference (ITC 2003). 2003. Charlotte, NC: IEEE Computer Society. 2. Saunders, S. R. J. and H. R. Vetters, Standardisation of test methods for the mechanical properties of thin coatings. Thin Solid Films, 1997. 299(1-2): p. 82. 3. Rabinowicz, E., Friction and Wear of Materials. 2nd ed. 1995, New York: John Wiley & Sons, Inc. 315. 4. Bowden, F. P. and D. Tabor, The Friction and Lubrication of Solids. Vol. 1. 1950. 391. 5. Slade, P. G., Electrical Contacts. 1999, New York: Marcel Dekker. 1073. 6. Antler, M., Processes of metal transfer and wear. Wear, 1964. 7: p. 181. 7. Antler, M., Wear, friction, and electrical nois e phenomenon in severe sliding systems. ASLE Transactions, 1962. 5: p. 297. 8. Antler, M., Metal transfer and wedge forming mechanism. Journal of Applied Physics, 1963. 34(2): p. 438. 9. Cocks, M., Interaction of sliding metal surface. Journal of Applied Physics, 1962. 33: p. 2152. 10. Suh, N. P., The delamination theory of wear. Wear, 1973. 25: p. 111. 11. Tian, H., N. Saka, and E. Rabinowicz, Friction and failure of electroplated sliding contacts. Wear, 1991. 142(1): p. 57. 12. Bayer, R. G., A general model for sliding we ar in electrical contacts. Wear, 1993. 162(Part 2): p. 913. 13. Antler, M. and M. Feder, Friction and wear of electrodeposited palladium contacts: Thin film lubricat ions with fluids and with gold. IEEE Trans., 1986. CHMT-9(4): p. 485. 14. Jang, D.S. and D. E. Kim, Tribological behavior of ultra-thin soft metallic deposits on hard substrates. Wear, 1996. 196(1-2): p. 171.

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128 15. Sherbiney, M. A. and J. Halling, Friction and wear of ion-plated soft metallic films. Wear, 1977. 45(2): p. 211. 16. Shimura, Y., T. Ito, Y. Taga, and K. Nakajima, Frictional properties of sputtered tin film. Wear, 1978. 49(1): p. 179. 17. Spalvins, T. and B. Buzek, Frictional and morphological c haracteristics of ionplated soft metallic films. Thin Solid Films, 1981. 84(3): p. 267. 18. Bowers, R. C. and W. A. Zisman, Pressure effects on the friction coefficient of thin-film solid lubricants. Journal of Applied Physics, 1968. 39(12): p. 5385 5395. 19. El-Shafei, T. E. S., R. D. Arnell, and J. Halling, An experimental study of the hertzian contact of surfaces covered by soft metallic films. ASLE Transactions, 1983. 26(4): p. 481. 20. Finkin, E. F., A theory for the effects of film thickness and normal load in the friction of thin films. Transactions of the ASME Journal of Lubrication Technology, 1969(July): p. 551. 21. Schiffmann, K. I. and A. Hieke, Analysis of microwear experiments on thin dlc coatings: Friction, wear and plastic deformation. Wear, 2003. 254(5-6): p. 565. 22. Dickrell, P. L., Tribological behavior and gas -surface interactions of hydrogenated carbon films. 2005: Gainesville, FL. 23. Fischer-Cripps, A. C., Nanoindentation, ed. F.F. Ling. 2002, New York: SpringerVerlag. 197. 24. Doerner, M. and W. D. Nix, A method for interpreting the data from depth sensing indentation instruments. Journal Of Materials Research, 1986. 1(4): p. 601. 25. Oliver, W. C. and G. M. Pharr, An improved technique for determining hardness and elastic-modulus using load and di splacement sensing indentation experiments. Journal Of Materials Research, 1992. 7(6): p. 1564. 26. Sneddon, I. N., The relation between load and pene tration in the axisymmetric boussinesq problem for a punch of arbitrary profile. International Journal of Engineering Science, 1965. 3(1): p. 47. 27. Pharr, G. M., W. C. Oliver, and F. R. Brotzen, On the generality of the relationship among contact stiffness, contact area, and elastic-modulus during indentation. Journal Of Materials Research, 1992. 7(3): p. 613.

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129 28. Wyrobek, T. J., Triboindenter User's Manual, Minneapolis, MN: Hysitron Inc. 29. Stone, D. S., K. B. Yoder, and W. D. Sproul, Hardness and elastic modulus of tin based on continuous indentati on technique and new correlation. Journal of Vacuum Science and Technology A, 1991. 9(4): p. 2543. 30. Joslin, D. L. and W. C. Oliver, A new method for analyzing data from continuous depth-sensing microindentation tests. Journal Of Materials Research, 1990. 5(1): p. 123. 31. Saha, R. and W. Nix, Effects of the substrate on the determination of thin film mechanical properties by nanoindentation. Acta Materialia, 2002. 50(1): p. 23 38. 32. Saha, R. and W. D. Nix, Soft films on hard substrat es -nanoindentation of tungsten films on sapphire substrates. Materials Science and Engineering A, 2001. 319: p. 898. 33. Yoder, K., D. Stone, R. Hoffman, and J. Lin, Elastic rebound between an indenter and a layered specimen: Part ii. Using contac t stiffness to help ensure reliability of nanoindentation measurements. Journal Of Materials Research, 1998. 13(11): p. 3214. 34. Reiss, G., J. Vancea, H. Wittmann, J. Zweck, and H. Hoffmann, Scanning tunneling microscopy on rough surfa ces: Tip-shape-limited resolution. Journal of Applied Physics, 1990. 67(3): p. 1156. 35. Villarrubia, J. S., Algorithms for scanned probe microscope image simulation, surface reconstructi on, and tip estimation. Journal of Research of the National Insititute of Standard s and Technology, 1997. 102(4): p. 425. 36. Xu, S. and M. F. Arnsdorf, Calibration of the scanning (atomic) force microscope with gold particles. Journal of Microscopy, 1994. 173: p. 199. 37. Dieter, G. E., Mechanical Metallurgy. 1986, New York: McGraw-Hill. 38. Hertzberg, R. W., Deformation and Fracture Mechanics of Engineering Materials. 1996, New York: J. Wiley & Sons. 39. McSkimin, H. J. and P. Andreatch, Jr., Elastic moduli of diamond as a function of pressure and temperature. Journal of Applied Physics, 1972. 43(7): p. 2944. 40. Cullity, B. D., Elements of X-ray Diffraction. 2nd. ed. 1959, Reading, MA: Addison-Wesley Publishing Company, Inc.

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130 41. Brandes, E. A. and G. B. Brook, eds. Smithell's Metals Reference Book. 7th ed. 1998, Butterworth-Heinemann: Oxford. 42. Patterson, A. L., The scherrer formula for x-ray particle size determination. Physical Review, 1939. 56: p. 978. 43. Williams, J. S., Y. Chen, J. Wong-Leung, A. Kerr, and M. V. Swain, Ultra-microindentation of silicon and compound se miconductors with sp herical indenters. Journal Of Materials Research, 1999. 14(6): p. 2338. 44. Kelly, A., Strong Solids. 1966, Oxford: Oxford University Press. 45. Conrad, H. and K. Jung, Effect of grain size from mm to nm on the flow stress and plastic deformation kinetics of au at low homologous temperatures. Materials Science and Engineering: A, 2005. 406(1-2): p. 78. 46. Nye, J. F., Physical Properties of Crystals. 1998, Oxford: Claredon Press. 47. Hebbache, M., Nanoindentation: Depth dependen ce of silicon hardness studied within contact theory. Physical Review B, 2003. 68(12). 48. Phaneuf, M. W., Applications of focused ion be am microscopy to materials science specimens. 1999. 30(3): p. 277. 49. Reyntjens, S. and C. Kubel, Scanning/transmission electron microscopy and dualbeam sample preparation for the an alysis of crystalline materials. Proceedings of the 14th International Conf erence on Crystal Growth a nd the 12th International Conference on Vapor Growth and Epitaxy, 2005. 275(1-2): p. e1849. 50. Uchic, M. D., M. A. Groeber, D. M. Dimiduk, and J. P. Simmons, 3d microstructural characteriza tion of nickel superalloys vi a serial-sectioning using a dual beam fib-sem. In Press, Corrected Proof. 51. Barrett, C. S. and T. B. Massalski, Structure of Metals. 3rd ed. 1966, New York: McGraw-Hill. 52. Davis, J. R., P. Allen, S. R. Lampman, and T. B. Zorc, eds. Asm Handbook Volume 2, Properties and Selection: N onferrous Alloys and Special-Purpose Materials. 1998, ASM-International: Materials Park, OH.

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131 BIOGRAPHICAL SKETCH Gerald Bourne graduated high school in 1982 and worked in the automotive parts industry until 1986. He then began an en try level position in a serigraphic textile company. He learned the trade and ope ned his own company in 1988. By 1990, competition with foreign manufacturers lead to declining profits and caused him to seek other avenues. He joined a small service company and helped them to expand to a nationwide parts and service supp lier. In 1992, he met Wanda Hix and fell in love. They married in the spring of 1993, and their da ughter, Gabrielle was born in November. Wanda had the opportunity for a new position in 1995, so they moved to Gainesville in the Fall. Gerald went to Santa Fe Comm unity College and comp leted his associates degree in 1996. In 1997, he transferred to th e University of Florid a. He completed his bachelors degree in 1999. He accepted the Un iversity of Florida Alumni F ellowship in 1999 and completed his masters degree in 2001. He worked at the Major Analytica Instrumentation Center throughout his graduate career as a te aching assistant, microscope technician and class lecturer.


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Permanent Link: http://ufdc.ufl.edu/UFE0015180/00001

Material Information

Title: A Methodology for Tribological Examination of Thin Film in the Nanodisplacement Regime
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0015180:00001

Permanent Link: http://ufdc.ufl.edu/UFE0015180/00001

Material Information

Title: A Methodology for Tribological Examination of Thin Film in the Nanodisplacement Regime
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0015180:00001


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Full Text












A METHODOLOGY FOR TRIBOLOGICAL EXAMINATION OF
THIN FILMS IN THE NANODISPLACEMENT REGIME
















By

GERALD R. BOURNE


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2006

































Copyright 2006

by

Gerald R. Bourne

































To Wanda and Gabrielle















ACKNOWLEDGMENTS

First and foremost, I would like to thank my wife, Wanda, for seeing this long

journey to the end with unwavering support. Without her encouragement, I would not

have embarked on this path, let alone seen it to completion.

I would like to thank Greg Sawyer for his advice and support. His dedication to his

students and research is inspiring and unparalleled. Thanks to Carol Sawyer for putting

up with Greg. Thanks to Amelia Dempere; I could not ask for a more understanding and

flexible boss. Thanks to Mike Kaufman for showing me what fun hovering over a

phosphorous screen for several hours could be. I appreciate Kevin Jones accepting a

committee position at this late stage in the project. Thanks to Nam-Ho Kim for his

contributions in finite element analysis.

I would like to thank Jeff Bardt for his contribution on many levels, from keeping

me young with our juvenile banter and back and forth hijinks, to the extensive assistance

with my computer incompetence. Thanks to Ben Boesl, Matt Hamilton, Pam Dickrell,

Nicole McCook, Kerry Siebein, and Junghun Jang for contributions to and useful

discussions about this project. Thanks to the students of the Tribology Lab for support

and comic relief.

Thanks to the MAIC facility and its staff for use of the instruments, financial

support, and discussions about techniques and analysis. Finally, I would like to thank the

faculty at the Materials Science and Engineering Department of the University of Florida.

As an undergraduate, I was inspired to pursue research in this field from the knowledge,









enthusiasm, and support from Richard Connell, Robert DeHoff, Mike Kaufman, and

Fereshteh Ebrahimi.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ............. ......... .. ......................... ......... ........ ix

LIST OF FIGURES ................................. ...... ... ................. .x

A B S T R A C T .......................................... ..................................................x v

CHAPTER

1 INTRODUCTION AND BACKGROUND ..................................... ...............

In tro d u ctio n .................................................................................. 1
B ack g rou n d .................................................................................................. .3
Early Friction Studies ................... .. ........... ... ............ ... .............
M echanism for M etallic Friction............................................................. ... ...... 5
S lid in g W ear ................................................................... ............................... . 7
Prow Form action .................................................................. .......... 7
D elam in ation T h eory ................................................................... ........... .. .. 8
Subsurface Wear ...............................................................
Film Thickness Wear and Friction Dependence .................................................9
M modeling ..................................... .................. ................ ......... 11

2 EXPERIM ENTAL M ATRIX ......................................................... ............... 13

Sam ple Preparation ............................................................. .............. 14
Bulk Gold Foil ................ ................. .................................... .......... 15
Ion B eam Sputtered G old on Silicon .................................... .................................... 16
Electron Beam Evaporated Gold on Silicon.......................................................19
Carbon A rc Evaporation ............................................................. ......20
Ion B eam Etched A rea of Interest ...... ... ....................................... .....2.......... .21

3 T E ST E Q U IPM E N T ............................................................................ .............. 23

H ysitron Triboindenter ...................... .................. ................... .. ...... 25
C SM M icrotrib om eter ........................................................................ .................. 2 8

4 EXPERIMENTAL PROCEDURES....................................... ........................ 30









Hysitron Triboindenter Procedures........................... ....................... ............... 30
Tip Preparation ....................... .................................... ........... .. 30
Sam ple M counting .......................... .............. ..................... .... .. ....... .....32
Electrostatic Force Constant and Zero Volt Gap Calibration............................33
Tip O ptics O ffset C alibration ........................................ ......................... 33
Standard tip optics offset procedure.......................... ..... .......... ........33
M odified tip optics offset procedure ........................................ .................35
Q uick A approach ............. ............................................................ .... .... ..... .. 43
Tip A rea Function Calibration....................................... .......................... 43
M machine Com pliance Calibration.................................... ........................ 44
T estin g M o d es ................................................... ................ 4 5
D epth sensing indentation ........................................ ........................ 46
Rastering wear test ....................................... ............................ 47
Reciprocating wear test ............................... .......... ........ .......... 48
C SM M icrotrib om eter ........................................................................ .................. 4 8
T ip P rep aration ..............................................................4 8
Sam ple M counting ........................................ ................... ..... .... 48
Reciprocating Wear Test ................... ......... .................... 49

5 D A T A A N A L Y SIS .......................................................................... .................... 50

Indentation Testing ........... .. ......................... .. ................ .. ...... .... 50
Oliver-Pharr Method ............................ ........ .................... 50
Compliance Method (Stone's Method) .................................... ............... 54
Modified Winkler Method............. .............. ...... ............57
R astering W ear Test ................ ....................................... ....... ....63
Triboindenter Reciprocating W ear Test .......................................... ............... 65
C SM M icrotrib om eter ........................................................................ .................. 66

6 CHARACTERIZATION ......... .................. ................. .................... ............... 67

T ip C h aracterization ......................................................................... ....................67
Indentation M ethod ....................... .. ......................... .. ..... ........... 67
Scanning M ethod ............... .. .... ........ .. .................... .. ................ .. 68
Transmission Electron Microscopy Method ................................................. 71
X -ray D iffractio n .............................................................................. ................ .. 7 4
Crystallographic Texture ............................................................................... 76
G rain S ize .........................................................................7 8
Scanning Electron M icroscopy ........................................................ ............... 79
Focused Ion Beam M killing ................................................................................. 80
Transmission Electron Microscopy Sample Thinning .......................................80
Slice and V iew Serial Sectioning ............................................. ............... 83
Fixed sam ple m ethod ............................................................................84
R otated sam ple m ethod ........................................ .......................... 85
Transmission Electron M icroscopy Analysis .................................. .................87

7 RESULTS AND DISCU SSION .......................................... ........................... 89









Static Indentation Testing .................................... ............................ ............... 89
Oliver and Pharr Analysis Results...... ................. ...............89
Stone A analysis R results ............................................... ............................. 92
M odified W inkler A approach ........................................ .......... ............... 95
R astering W ear Tests ....................................................... ................. 96
R eciprocating W ear Tests........................................................................... 99
Film Failure ........................................99
Gold on silicon without carbon ............ ............................................. 101
Carbon on gold on silicon ............................................... ............... 103
Electron Microscopy Cross Sections of Wear Scars .............. ................105
Friction M easurements ...... ........ ..................... ..... ............... 110
Hysitron Triboindenter friction measurements ............... ..................110
CSM microtribometer friction coefficient measurements..........................114

8 CONCLU SION S ............................................. ....................... ........ 117

APPENDIX

A SCANNING ELECTRON MICROSCOPY IMAGES ...........................................119

B SCANNING ELECTRON MICROSCOPY CROSS SECTIONS ...........................123

L IST O F R E F E R E N C E S ...................................................................... ..................... 127

BIOGRAPHICAL SKETCH ......................................................................... 131
















LIST OF TABLES


Table p

1-1 Summary of studies of thin metallic films on substrates. .......................................10

2-1 Sam ple nam ing designations .................................................................................. 14

7-1 Hardness values calculated for samples from the Hall-Petch equation. .................92

7-2 Hardness values calculated for samples from the Stone method ...........................95

7-3 Results of SEM micrographs from gold on silicon samples with 100 pN normal
lo ad ................................................................................ 10 1

7-4 Results of SEM micrographs from gold on silicon samples with 500 pN normal
lo a d .. ............................................................................. 1 0 2

7-5 Results of SEM micrographs from carbon on gold on silicon samples with 100
pN norm al load.................................................................... .......... ...... 104

7-6 Results of SEM micrographs from carbon on gold on silicon samples with 500
pN norm al load.................................................................... .......... ...... 104
















LIST OF FIGURES

Figure page

1-1 Slider and areas from Equation 1-3..................................... .......................... 6

1-2 Three slider geometries from Bowden and Tabor's experiment.............................6

1-3 Effect of film thickness on friction coefficient of metallic film solid lubricants
on harder substrates. ....................................... ........ ............... 10

2-1 South Bay Technology 8" polishing wheel..........................................................16

2-2 Ion beam sputtering process......................................................................... ... ... 17

2-3 Gatan Model 681 High Resolution Ion Beam Coater ............................................17

2-4 Electron beam evaporation process.................................... .......................... 19

2-5 C arbon arc evaporation ................................................. ............................... 20

2-6 Rastering wear test 4 [m x 4 [tm with low contact pressure............................... 21

2-7 Etched grid lines for wear test location ...................................... ...............22

3-1 A typical pin-on-disc type tribom eter ............................................. ............... 23

3-2 Linear reciprocating tribom eter........................................ ............................ 24

3-3 Hysitron Triboindenter with upper thermal-acoustic isolation cover removed to
show interior details. ............................ ........... ...... ...... ...... ...... 25

3-4 Capacitive transducer assembly schematic ................................... .................26

3-5 Enlarged view of Triboindenter components................................... ... ................ ...27

3-6 C S M icrotribom eter. ............................................. ......................................... 28

4-1 Diamond indenter tip and assembly of a large radius sapphire tip ........................32

4-2 Optical CCD microscope image of H-pattern indents ......................... ..........34









4-3 Spherical indenter on flat surface and cross sectional geometry to solve for depth
of indentation.................................... ................................ .........35

4-4 Patterned modified tip optics offset sample .................................. .................37

4-5 Modified tip optics offset sample centered in the video window ..........................39

4-6 Single indent selected ............................................. .... .... ................. 39

4-7 M annual height adjustm ent window ........................................ ....... ............... 40

4-8 Search option w window ..........................................................................40

4-9 Imaging window and control panel with image of modified tip optics offset
sam p le ......................................................... .................. 4 1

4-10 Actual tip optics offset seen in the video window ............................................. 42

4-11 Triboindenter testing m odes .................................. ............... ............... 46

4-12 Rastering wear test showing fiducial indentation marks in circles ........................47

4-13 C S M icrotribom eter. ............................................. ........................................ 48

5-1 Load versus displacement curve along with a cross section of an indented
surface. The range of ; is indicated for various indenter geometries....................51

5-2 Measured compliance versus square root of maximum load for a load range of
5mN to 10mN on quartz, Al (001), and Si (001). ............................................. 55

5-3 Indent cross sections from Oliver-Pharr and Modified Winkler compared to an
actual indent. .........................................................................58

5-3 Differential element described with cylindrical coordinates and tip displacement
based on a spherical tip geom etry. ........................................ ....................... 60

5-4 Differential element illustration prior to (left) and after load (right) with variable
notation indicated. .....................................................................61

5-5 L loading code flow chart ............................................................................ ..... ... 62

5-6 U nloading code flow chart ....................................................................... 63

5-7 Rastering wear test data plot showing a trenched area on the right of the test area
and a pile-up area on the left. ............................................................................. 64

5-8 Reciprocating wear test showing the transient areas near the ends of the
reciprocation and the area over which the friction force data is analyzed ..............66









6-1 Large radius tip scanning a small radius asperity. ................................................68

6-2 Sharp asperity milled on silicon to scan indenter tip. ............................................ 69

6-3 Scanning image produced with a sharp asperity by rastering first in the x
direction, and then in the y direction ..................................................................... 70

6-4 Transmission Electron Microscope 200 CX sample holder with tip holder fixture
and indenter tip m mounted ............................................................................ ... .... 7 1

6-5 The upper images are taken at 400 CCW, 0, and 400 CW respectively. The
lower image shows data points plotted along the profile......................................72

6-6 Kikuchi map observed in diffraction mode................................... .................73

6-7 Top down view of the single crystal diamond indenter tip with crystallographic
orientations and sliding direction indicated................................... ............... 73

6-8 X-ray diffractometer with a representation of Bragg's law. ................................75

6-9 X-ray scans of the six different gold films shown with indexed peaks ..................77

6-10 Grain size and strain versus gold film thickness ............................................... 79

6-11 Wear track in center of image to be prepared with cross sectional TEM parallel
to the w ear direction. ...................... .... .......... ................. ........... 1

6-12 Focused ion beam cross section sample preparation ............................................82

6-13 Cross section removed from the sample trench with a glass rod ...........................83

6-14 Trenched area in preparation of fixed sample method slice and view serial
sectioning procedure. ....................................... ............... .... ....... 84

6-15 Slice and view rotating sample method......................................... ............... 86

6-16 Gold films with wear tracks run for half cycles at 500 atN normal load. Film
thicknesses are indicated on each image. ...................................... ............... 88

7.1 Hardness determined by the Oliver and Pharr method versus maximum tip
displacement for five samples of varying thickness .............................................89

7-2 Stone plot including linear curve fits for sample EBE 20 with a gold film
thickness of 27 nm .............. .......... ......... ...... .... ................92

7-3 Stone plot including linear curve fits for sample IBS 100 with a gold film
thickness of 105 nm.......................................... ............ 93









7-4 Stone plots including linear curve fits for samples EBE 200, IBS 300, and EBE
500 with a gold film thickness of 180 nm, 315 nm, and 537 nm, repectively.........94

7-5 Hardness determined by four methods plotted versus sample thickness for five
sam ples. ..............................................................................96

7-6 Scanning electron micrographs from a rastering wear test matrix with a 40 taN
normal load varying tip geometry and number of cycles as indicated...................97

7-7 Scanning electron micrograph showing a rastering wear anomaly. The wear scar
extends outside of the prescribed area especially in the horizontal direction. .........98

7-8 Scanning electron micrograph showing a rastering wear anomaly. A trench
develops on the right and lower edge of the wear scar.......................................98

7-9 Scanning electron micrographs of samples IBS 300 (left) and EBE 200 (right)
run for 30 cycles at 500 atN normal loads. .................................... .................99

7-10 Scanning electron micrographs of sample EBE 200 C run for 3 cycles (left) and
30 cycles (right) at 500 atN norm al load.................................... ............... 100

7-11 Scanning electron micrograph of sample IBS 100 after 1 cycle of sliding with
500 naN norm al load................... ..................... .............. ........ .. .. .....102

7-12. Nominal film thickness versus number of cycles prior to film failure................03

7-13 Vertical tip displacement versus lateral displacement for a 30-cycle
reciprocating test. ....................................................................105

7-14 Transverse cross section of a wear scar run on sample EBE 500 for 10 cycles
w ith a 500 atN norm al load .......................................................................... ..... 106

7-15 Depth of wear scar versus number of cycles for samples run at 500 atN normal
lo ad ................................................................................ 10 8

7-16 Fraction of coating depth penetrated versus number of cycles for samples run at
500 naN norm al load................... ..................... .............. ........ .. .. .....109

7-17 Friction coefficient versus number of cycles in log scale for gold on silicon
samples without carbon run at 500 atN normal load. ............................................111

7-18 Friction coefficient versus number of cycles on a log scale for gold on silicon
samples without carbon coating run at 100 uN normal load ..............................112

7-19 Friction coefficient versus number of cycles for gold on silicon samples with
carbon coating run at 500 uN norm al load ...........................................................113

7-20 Friction coefficient versus number of cycles for gold on silicon samples with
carbon coating run at 100 uN normal load .........................................................114









7-21 Friction coefficient versus number of cycles for a contact pressure of 92 MPa
run on sam ple EB E 200 ......... ................. ................... ................. ............... 115

A-1 W ear scar of gold on silicon at 100 [[N load normal load...................................119

A-2 Wear scar of gold on silicon at 500 [[N load normal load..................................120

A-3 Wear scar of carbon on gold on silicon at 100 [[N load normal load..................121

A-4 Wear scar of carbon on gold on silicon at 500 [[N load normal load..................122

B-l Sample IBS 100 at 500 aN normal loads. A)1 cycle. B) 3 cycles. ....................123

B-2 Sample EBE 200 at 500 aN normal loads. A)1 cycle. B) 3 cycles....................123

B-3 Sample EBE 200 at 500 aN normal loads and 10 cycles..................................... 124

B-4 Sample IBS 300 at 500 aN normal loads. A)1 cycle. B) 3 cycles. ....................124

B-5 Sample IBS 300 at 500 aN normal loads. A) 10 cycle. B) 30 cycles ................124

B-6 Sample IBS 300 at 500 aN normal loads. A) 100 cycle. B) 300 cycle..............125

B-7 Sample EBE 500 at 500 aN normal loads. A) 1 cycle. B) 3 cycle.....................125

B-8 Sample EBE 500 at 500 aN normal loads. A) 10 cycle. B) 30 cycle.................125

B-9 Sample EBE 500 at 500 aN normal loads. A) 100 cycle. B) 300 cycle.............126















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

A METHODOLOGY FOR TRIBOLOGICAL EXAMINATION OF
THIN FILMS IN THE NANODISPLACEMENT REGIME

By

Gerald R. Bourne

August 2006

Chair: Luisa A. Dempere
Cochair: W. Gregory Sawyer
Major Department: Materials Science and Engineering

This research aims to develop a methodology for studying tribology of thin films

on the nanodisplacement level for both fundamental and practical wear applications. A

Hysitron Triboindenter was chosen to perform the majority of the wear testing of this

study.

Thin film gold on silicon is chosen for its applications in micro-electrical

mechanical systems (MEMS) devices, electrical switching contacts, low surface

roughness, and its thermodynamic stability at ambient temperature and pressure.

Samples are prepared with a variety of deposition techniques to vary film thickness.

Various techniques are developed to overcome challenges with testing setup. An optics

offset procedure solves resolution problems with light optical microscopy. Tip holders

are designed to alter contact pressures and wear conditions. Load functions are designed

to operate the equipment in a linear reciprocating motion.









Numerous characterization techniques including scanning electron microscopy

(SEM), focused ion beam (FIB) transmission electron microscopy (TEM), and x-ray

diffraction (XRD) are employed to characterize testing parameters and results.

Geometrical and mechanical properties of the tip are characterized using TEM. The

sample thickness and microstructure are characterized using XRD, SEM, FIB, and TEM.

From the characterization, elastic and plastic properties are approximated using elastic

theory and the Hall-Petch relationship. For quantifying wear, SEM and FIB are used for

depth measurements.

An indentation model is proposed to include material pile-up exhibited by plastic

films on elastic substrates which has been ignored in other models. Results show good

agreement with the Hall-Petch relationship. Displacement and friction results show good

reproducibility with repeat testing. Friction coefficient proves to be a good indication of

film failure.














CHAPTER 1
INTRODUCTION AND BACKGROUND

Introduction

The term 'tribology', coined circa 1965 and defined as, "The branch of science and

technology concerned with interacting surfaces in relative motion and with associated

matters (as friction, wear, lubrication, and the design of bearings)," by the Oxford

English Dictionary, is centuries newer than the study of friction and wear. For much of

history up to and including the present day, tribological problems are often handled with

an alchemistic approach by choosing a variety of materials and empirically testing. One

might wonder if some studies were designed with a periodic table of the elements and a

handful of darts. Much of the problem has been the lack of experimental, computational

and characterization equipment with the capabilities required to study fundamental

tribological interactions. The past decade has seen advances in electron microscopy to

sub-nanometer resolutions, computers and software simulation to represent 106 atoms,

and mechanical testing instrumentation to resolve 10-7 N loads and nanometer

displacement. Tribological testing on the nanometer displacement level, supported by

high resolution characterization is necessary to validate finite element modeling and

molecular dynamic (MD) simulations of surface interactions.

In addition to fundamental studies of tribology, wear on the nanodisplacement level

has applications in microelectromechanical system (MEMS) power transmission and

ohmic contact switching. With steady trend towards miniaturization, MEMS devices

have received much attention and interest. Due to the extensive research and









development of fabrication technology for integrated circuit (IC) chips, silicon was a

convenient material for pioneering work in MEMS devices. It was quickly discovered

that for mechanical sliding as in power transmission devices, silicon components will fail

with limited cycles and could not be used outside of laboratory conditions. Wear

resistant thin film coatings may provide solutions for silicon MEMS and will require

tribology testing equipment and methodologies to support research [1].

Interest in tribological study of thin films extends beyond MEMS devices. Often in

successful wear resistant materials combinations, a transfer film from one of the materials

develops on the other. The result is a self-mated contact which after an initial run-in

period exhibits good wear resistance and low friction. Typically when these transfer

films exhibit desirable wear properties, they are thin, on the micron to sub-micron scale,

and very adherent. A methodology to study and probe properties of transfer films is

useful in the design of contacts that will exhibit them. A set of standards for the

mechanical testing of thin film coatings is necessary for future developments in the field

[2].

This research aims to develop a methodology for studying tribology on the

nanodisplacement level for both fundamental and practical wear applications. A Hysitron

Triboindenter was chosen to perform the majority of the wear testing of this study for the

following reasons:

* The range of load and displacement. The Triboindenter's practical load range is
from 1 aN to 10 mN and displacement is from 1 nm to 5 tm. These ranges cover
loads and displacements in MEMS devices, thin film applications, and modeling
simulations.

* The flexibility of tip geometry and materials. The tip holder can accept a wide
range of materials and geometries. There is no need for elaborate manufacturing
techniques to produce tips.









* The accurate stage positioning. Accurate positioning of test areas is possible to
sub-micron resolution. This allows for precise location of test area prior to testing,
and post-test characterization of the test area.

* The scratch mode. The scratch mode of the Triboindenter can be tailored to
reproduce motions expected in MEMS devices and other practical applications.
Reciprocating and unidirectional sliding is a fundamental geometry used in
tribological applications and computational simulations.

* The experimental reproducibility. Tests on the Triboindenter show good
reproducibility.

This study will present a new approach for tribological testing with a characterized

standard which will be available for further studies. It will provide a foundation on

which to expand research on various materials, geometries, modes of wear, and

applications.

Background

Friction and wear are by no means new phenomena. From the earliest times, man

has experienced friction and wear. To early man, friction was beneficial in creating fire

by frictional heating of wood, and wear was used to sharpen tools for hunting and

working. From the early 1700s to the early 1900s, friction and wear are studied mostly as

a scientific curiosity. As the industrial revolution progresses friction and wear are seen as

engineering problems which lead to energy loss and machine failure. Therefore the

interest in studying the phenomena greatly increases. As the resolution of measuring and

characterization devices improve and computer simulation power increases, the study of

friction and wear progresses from the macro- to micro-, to nano-, and approaching atomic

displacement levels. The driving forces for tribological research involve device

longevity, energy conservation, economy, and fundamental understanding [3].









Early Friction Studies

The study of friction and wear dates back to the beginnings of the scientific era.

Amontons (1699) is credited with the first observation that friction force is proportional

to the normal load and independent of the contact area, hence Amontons' Law:

Ff = F (1-1)

where Ff is the frictional force, tangential to the surface, Fn is the force normal to the

surface, and [t is the friction coefficient. Actually Amontons concluded that the friction

force was always equal to one third of the normal load [4]. He explained this as

interlocked asperities on the surfaces which caused a lifting motion to move relative to

each other which caused an energy loss and therefore a friction force. This theory

became known as the "roughness hypothesis". Subsequent researches found that the

friction coefficient did not equal one third for all materials, but agreed that the normal

load and friction force were proportional. The roughness theory remained accepted for

hundreds of years. Alternate theories involving adhesion were proposed, but could not be

validated without relating the friction force dependence on contact area [3]. A significant

contribution to the adhesion theory came from Holm (1938) when he showed that the real

area of contact is related to the normal load divided by the yield pressure (or hardness) of

the material [5]. It was therefore realized that the real area of contact, rather than the

apparent area of contact, was responsible for the friction force and since the real area of

contact was proportional to the normal load, the friction force is subsequently

proportional to the normal load.









Mechanism for Metallic Friction

The discovery by Holm paved the way for Bowden and Tabor's development of

the mechanism for metallic friction. Bowden and Tabor showed that the frictional for

force to translate a slider along a surface is comprised of two factors: the force required

to shear the contact between the two intimate surfaces and the force to plough, or

plastically deform the material in front of the slider. Their equation for friction force

takes the form:

F, = F,+ F (1-2)

where Ff is the frictional force, Fs is the interfacial shearing force, and Fd is the force of

plastic deformation (or ploughing). They expand this equation by breaking down the

interfacial shearing force to an interfacial shear stress times the area it is acting upon, and

breaking down the force of plastic deformation to the pressure required to yield the

material times the projected area of the slider in contact with the surface in the direction

of sliding and Equation 1-2 takes the form:

F = HA + rAreal (1-3)

where H is the hardness of the material which is taken as the pressure required to deform

the material, Ap is the projected area of the slider in the direction of sliding and in contact

with the surface, Ts is the interfacial shear stress, and Areal is the area of the slider in

contact with the surface (Figure 1-1).














lateral (frontal) area, Areai


Figure 1-1. A schematic diagram representing the slider and areas from Equation 1-3.

Bowden and Tabor tested this equation with a set of three sliders, one with the geometry

of a circular spade, one spherical, and one cylindrical, all three with the same radius

(Figure 1-2). The tests were made with steel sliders on an indium surface with a slow

sliding speed of 0.1 mm/s to avoid strain rate sensitivity. From the geometry of the

sliders, it can be seen that the Fs term from Equation 1-2 or TsAreal term from Equation 1-3

will be negligible for the spade slider and will be a maximum for the cylindrical slider.









circular spade sphere cylinder

Figure 1-2. Schematic drawing of the three slider geometries from Bowden and Tabor's
experiment.









Results from the experiments showed a deformation pressure equal to 1.5 times the

material hardness measured by indentation. Bowden and Tabor suggest this result may

be due to the fast sliding speed relative to the slow indentation speed, further suggesting

some strain hardening effect. The experiment was repeated on four different surfaces,

indium, lead, steel, and copper, and the results of the interfacial shear stress were

compared with the shear strength of the material. The interfacial shear strength was on

the same order of magnitude as the shear strength of the material, but 1.5 to 2 times

higher than the bulk shear strength.

Sliding Wear

Prow Formation

A mechanism for wear of a metallic surface by a linear slider is described by Antler

as a severe adhesive process called prow formation [6]. A net metal transfer occurs from

the surface to the slider. This transferred material is described as a lump of severely

work hardened metal (the prow) transferred to the slider which in turn continues to wear

the surface by plastic shearing or cutting while the slider remains unworn. Prows are

removed from the slider by transferring back to the surface or becoming loose debris. If

the slider continues to ride over virgin material, prow formation continues throughout the

wear test. With dissimilar materials, friction and contact resistance characteristics of the

system are determined by the prow metal, therefore a diamond slider on a gold surface

should behave as a self-mated gold contact [7]. Antler found that the size of prows is

inversely related to the hardness, and metals that do not work harden, fail to form prows.

He attributes this to the prow becoming harder than the surface from the severe

deformation and then being able to cut the surface [8].









Cocks proposed the following mechanism [9] for prow formation:

* adhesion occurs between the slider and the surface
* plastic deformation occurs in a volume of metal from the surface
* adhesive forces are sufficient to remove the metal from the surface
* the metal transfers to the slider as a chip
* the process repeats on the chip
* the chip becomes too large and detaches from the slider

Antler found that with a linear reciprocating slider, prows would be deposited at the

ends of the tracks and prow formation would eventually stop when the surface material

work hardened to a hardness higher than that of the slider in which case would cause the

slider to begin wearing. From empirical observations, the following equation was

developed:

17x(cm) (1-4)
Nprow = 1.7
po" F (N)

where Nprow is the number of cycles for which prow formation will continue, x is the

track length in centimeters, Fn is the normal load in Newtons [5].

Rabinowicz and co-workers have suggested that materials that show little to no

solid solubility and form immiscible liquids would make good candidates for mated

materials in adhesive wear sliding applications [3]. Antler challenges this theory based

on experiments with gold mated with platinum and gold mated with rhodium [5] that

show similar performance to gold on gold. Antler suggestions for adhesive wear contacts

are high hardness to limit initial contact area, and low ductility to limit junction growth.

Delamination Theory

Delamination wear occurs in reciprocating sliding when cracks nucleate below the

surface and result in liberating sheets of material. Crack nucleation can occur in areas of

high plastic deformation at dislocation pile-ups particularly at hard precipitates in a softer









matrix or between material interfaces like a substrate/film interface [10]. Suh discovered

delamination wear typically occurs with harder sliders on soft surfaces. Prow formations

and delamination wear are different mechanisms and are mutually exclusive [5].

Delamination wear has been reported in reciprocating sliding on gold plated contacts

[11].

Subsurface Wear

Gold plated systems have shown subsurface plastic deformation leading to the

formation of buckles on the surface layer. With continuous sliding exposure of the sub-

layers can occur with or without thinning of the surface layer. Increasing normal load

results in less number of cycles to produce buckling and exposure of the sub-layer. It

was found that increasing layer thickness can reduce or eliminate the mechanism [12].

Film Thickness Wear and Friction Dependence

Several research groups have investigated thin metallic films as a solid lubricant in

sliding contact applications [4, 13-17]. All found that friction and wear were initially

high for ultra thin coatings but as coating thickness was increased, both friction and wear

went through minima values at some critical coating thickness (Figure 1-3). Table 1-1

summarizes the results from the various research groups. In some studies, tribometer

geometries were not well defined and experimental details were not mentioned. Two of

the research groups studied sputtered coatings versus evaporated coatings and both found

the sputtered coatings exhibited lower friction coefficients and longer life than the

evaporated coatings [14, 17]. Both groups attributed this behavior to better film adhesion

and finer grain structure, resulting in higher coating hardness, exhibited by sputtered

coatings. Antler improved his coating by alloying gold with cobalt [13] The cobalt acts

as a solid solution strengthener to harden the gold film.












g for bare metals


critical or
minimum [t


thick coating


increasing film thickness (h)


Figure 1-3. Schematic representation of the effect of film thickness on friction
coefficient of metallic film solid lubricants on harder substrates (reproduced
from [17]). The critical thickness where t (the friction coefficient) goes
through a minimum is denoted as he.


Table 1-1. Summary of studies of thin metallic films on


substrates.


film/substrate he ([m) [ tip radius ([m) load (N) reference
In/steel 0.1-1 0.5 3000 39.2 [4]
Pb/steel 0.5 0.05 3175 29 [15]
In/steel 0.5 0.05 3175 29 [15]
Sn/iron/glass 0.1 0.15 3175 2.4 [16]
Au/steel 2.0-3.5 0.1 2375 2.45 [17]
Pb/steel 1.8-2.5 0.1 2375 2.45 [17]
Co-Au/Pd/Ni 0.05 0.4 N/A 1.96 [13]
Au/Si (111) 0.01 0.2 1600 0.001 [14]
Ag/Si (111) 0.007 0.2 1600 0.001 [14]


*OOOOrMOOOOOTTTTTTTTTTT











Modeling

Several attempts have been made to develop models to describe friction and wear

phenomena in thin films on substrates. Bowers and Zisman modeled the friction

coefficient of a thin gold film on a steel substrate based on a pressure dependent shear

strength which showed good agreement with experimental data [18]. They calculated a

friction coefficient of 0.1 assuming no plastic deformation of the steel substrate. El-

Shafei et. al. investigated thin films of lead on steel with a contact radius larger than the

film thickness [19]. They found good agreement with a Hertzian elastic contact model

once their contact radius exceeded five times the film thickness. Finkin sought to explain

the ultra-thin to thin film transition phenomenon seen in Figure 1-3 [20]. He developed

two theories to explain the two regimes. The ultra-thin regime is explained as stiffening

due to coupled stresses with a quantitative expression verified by data from literature.

The coefficient of friction in thin film regime is shown to be proportional to the square

root of the film thickness divided by the normal load based on a thin film with modulus

much less than the substrate. Jang and Kim, and Schiffmann and Hieke incorporate a

ploughing term in their wear models which takes plastic deformation into account [14,

21]. For these models, tip geometry, contact geometry, and plastic properties of the film

and substrate are necessary parameters. Difficulty in measuring these parameters is

attributed to discrepancies with the models and experimental results.

The combinations of substrate properties, film properties and thicknesses, sliders,

and geometry lead to different modes of wear. Due to the complexity of the problem, no

single model or solution is expected to explain all possible wear combinations and

geometries. A model that describes the friction coefficient of a thin soft film on an






12


elastic substrate of significantly higher modulus is not expected to describe a hard elastic

film on a soft compliant substrate. A variety of models need to be developed to explain

specific cases.














CHAPTER 2
EXPERIMENTAL MATRIX

An experimental matrix was designed to test various microstructures, film

thicknesses, number of reciprocations, and normal load effects on wear resistance of gold

films on silicon substrates. Gold was chosen for its applications in MEMS devices,

electrical switching contacts, and its thermodynamic stability at ambient temperature and

pressure. Standard metallographic polishing techniques were initially used to prepare

sample surfaces. Wear testing and profilometry revealed surface roughness effects which

excluded the use of bulk gold. Polishing media particles were embedded into the surface

changing the properties of the material and the roughness produced was on the scale of

the measurements taken. Therefore commercially available polished silicon wafer was

chosen as a substrate material for its low surface roughness. In addition, silicon has

direct applications in MEMS research. A variety of microstructures and sample

thicknesses were produced using deposition techniques including ion beam sputtering

(IBS) and electron beam evaporation (EBE). An amorphous carbon layer of

approximately 60 nm in thickness was deposited on half of the samples to explore an

electrically conductive friction modifying substance. The samples with and without

carbon will be referred to as coated and uncoated respectively.

Samples were named based on the deposition technique and the targeted

approximate film thickness in nanometers (Table 2-1). Normal loads were chosen of 100

kN and 500 kN because the instrument shows good repeatability and low signal to noise

ratio at these levels. With the 100 kN normal load setting, number of reciprocations was









varied from one forward pass, referred to as a half cycle, to 300 cycles in the following

increments: half cycle, one cycle, three cycles, 10 cycles, 30 cycles, 100 cycles, and 300

cycles. Although the reciprocating tip is diamond and is therefore much harder and more

wear resistant than the gold foils, some precautions were taken to avoid possible tip

damage. For the 500 [aN normal load setting, the 100-cycle and 300-cycle tests were

omitted if film failure was detected on samples at 30 cycles to avoid excessive number of

reciprocations on the harder silicon substrate. The combination of loads and number of

cycles results in 12 different test parameters. Each set of tests was repeated four times on

all 12 samples for a total of 576 wear tests.

Table 2-1. Sample naming designations.
Approximate sample Without amorphous With amorphous
thickness carbon carbon
20 nm EBE 20 EBE 20C
100 nm IBS 100 IBS 100C
200 nm EBE 200 EBE 200C
300 nm IBS 300 IBS 300C
400 nm EBE 400 EBE 400C
500 nm EBE 500 EBE 500C



Sample Preparation

Gold was chosen as the test material for its applications in the electronics industry,

and its mechanical and chemical properties described previously. A variety of samples

were produced and can be categorized into three basic types including bulk gold foils, ion

beam sputtered gold on silicon wafer, and electron beam evaporated gold on silicon

wafer. These various techniques were chosen to introduce microstructure, surface

roughness, and film thickness variables into the test matrix.









Bulk Gold Foil

Initial experimentation was performed on bulk gold samples. A 100 atm thick

99.9975% gold foil was cut into 10 mm x 10 mm sections and mounted onto steel gauge

blocks using a cyanoacrylate based adhesive. Samples were polished using a South Bay

Technologies manual polishing wheel (Figure 2-1) beginning with grinding discs of 240

grit silicon carbide rotating at a speed of 200 rpm. After a short time of material removal,

the polishing wheel was cleaned and the grinding media was changed to 320 grit and the

sample was rotated 900 to the previous polishing direction. The sample was polished

until no scratches from the previous step were detectable using an optical stereoscope.

This procedure was repeated using 400 and 600 grit respectively. Upon completion of

the grinding steps, the media was removed and the wheel was cleaned. Next a billiard

cloth was mounted on the polishing wheel and water-based slurry of 15 atm alumina

(A1203) was used as a polishing media. The slurry was used to wet the billiard cloth and

samples were polished by rotating the sample manually in the opposite direction of

rotation of the polishing wheel. Samples were polished until scratches from the previous

steps were not detectable. The wheel was cleaned and the process was repeated with

fresh cloth and 5 atm slurry. The process was then continued using 1 [am, 0.3 [am, and

0.05 [am slurries respectively on velvet cloth. Upon completion of the final polishing

step, the samples were examined in a Wyko optical profilometer to verify consistent

surface roughness.

























Figure 2-1. South Bay Technology 8" polishing wheel

Ion Beam Sputtered Gold on Silicon

Ion beam sputter coating is often used to produce thin films for conductivity to aid

in imaging in electron microscopy. The advantage to ion beam sputtering versus other

techniques is that the samples are not exposed to extreme temperatures. The low

temperature deposition is beneficial in producing and retaining a fine grained or

amorphous structure. Producing coatings on polished silicon wafers will yield surface

roughness values that would be exceptionally challenging to obtain by metallographic

polishing techniques.

The ion beam sputtering process is illustrated in the schematic in Figure 2-2. An

argon ion beam is produced by dual Penning guns. The guns are aimed to strike the

target of desired deposition material. Material from the target is sputtered off, and

deposited on the sample substrate. The substrate can be rotated to provide uniform

deposition. This system is contained in a chamber that can be evacuated to levels of 10-6

Torr.











f Penning Gun
Target I-beam






Substrates

L'


Vacuum Chamber


Figure 2-2. Ion beam sputtering process

Commercially available 0.6 mm thick single crystal silicon wafer with (001) plan

view orientation was scored and cleaved along <110> directions to produce substrates of

approximately 25 mm2. The substrates were cleaned by sonicating in acetone. This was

repeated three times using new acetone each time. The substrates were then sonicated in

methanol three times using new methanol each time. Finally the substrates were blown

dry using laboratory grade Freon spray. Several substrates were mounted for coating in a

Gatan Model 681 High Resolution Ion Beam Coater seen in Figure 2-3.


Figure 2-3. Gatan Model 681 High Resolution Ion Beam Coater









It is well known that gold has poor adhesion when deposited on silicon and a thin

titanium bond coat will greatly enhance gold film adhesion. A target of 99.99% gold was

mounted on one side of the dual target holder using conductive silver epoxy and a target

of 99.99% titanium was mounted on the opposite side. After sufficient curing time, the

ion beam coater chamber was pumped down to a level of 2 x 10-4 Torr and the ion guns

were purged with argon following the instruction manual. After purging, the ion beam

was set to 9 keV at 600 atA for 60 minutes with a sample rotation speed of 30 rpm.

Based on a deposition rate of 3.5 A/s the film thickness should be on the order of 1

am. The first batch of samples was run without a titanium bonding layer to examine the

effect of film/substrate adhesion. A second set of samples were coated by first sputtering

titanium with a 9keV ion beam at 300 [aA for 15 minutes followed by gold with a 9 keV

ion beam at 300 uaA for 60 minutes. The lower current was used due to beam stability

issues which could not be resolved over several repeated attempts. Once the coating

process was complete, the silicon substrates were glued to 15 mm diameter atomic force

microscopy (AFM) sample disks using a cyanoacrylate based adhesive. The AFM disks

are ferritic steel which causes them to be attracted to magnets embedded in the testing

equipment stage. It was discovered early in this project that the magnetic attraction of the

stage was not sufficient to rigidly affix the samples to the stage, and an adhesive was

needed when mounting samples on the stage of the test equipment. Late in this study it

was determined that the AFM disks significantly hindered scanning electron microscope

(SEM) examination due to the effect of magnetic fields on the electron beam. In

hindsight, the AFM disks should have been eliminated.









Electron Beam Evaporated Gold on Silicon

Electron beam evaporation is another coating technique for producing high quality

thin films on substrates. In contrast to ion beam coating, electron beam evaporation

exposes the sample high temperatures due to the energy required to evaporate the metal.

This leads to grain growth in the film which results in a coarser grained structure. The

elevated temperature can also result in diffusion between the film and substrate. Again a

bonding layer is used to enhance film adhesion. Titanium and chromium layers are often

used for gold deposition on silicon. Since titanium has a lower diffusivity in gold than

chromium, titanium was selected.

The electron beam evaporation process is illustrated schematically in Figure 2-4.

An electron beam is generated by producing a large electrical bias between the electrode

and the crucible. The crucible contained the desired film material. The electron beam

heats the material in the crucible to a liquid and then to a gas which fills the chamber.

The gas condenses on the substrate thereby producing a thin film on the substrate.


Figure 2-4. Electron beam evaporation process









Attempts were made to use an on-site electron beam evaporation system, but

coatings were of a poor quality with many spheres of gold on the sample surface.

Fortunately, high quality electron beam evaporated coatings on silicon (001) substrate are

available commercially at economical prices. Wafers of 100 mm diameter with gold

films of 20 nm and 200 nm were purchased and sectioned into samples as described

previously. The sectioned samples were then glued to AFM disks using a cyanoacrylate

based adhesive.

Carbon Arc Evaporation

To investigate an electrically conductive friction modifying coating, all samples

were duplicated with a thin amorphous carbon layer using a carbon arc evaporation

system. This procedure is similar to the electron beam evaporation except that a bias is

applied across two carbon rods to evaporate carbon. The resultant condensation yields a

thin amorphous carbon film. This process is illustrated schematically in Figure 2-5.


Bell Jar







Graphite Rod




Substrates,

NN


Figure 2-5. Carbon arc evaporation









Ion Beam Etched Area of Interest

Many of the tests conducted in this research leave residual damage smaller than the

resolution limits of optical microscopy. Therefore, scanning electron microscopy (SEM)

and transmission electron microscopy (TEM) are used to characterize the damage from

testing. Even with the resolution obtainable with electron microscopy, locating micron

and submicron surface features with displacements on the order of tens of nanometers or

less is extremely challenging due to the low amount of contrast they produce, see Figure

2-6. Scanning at a low magnification to locate such features is not possible because the

feature will not be visible. Scanning the sample surface at a high magnification without

an indication of the approximate area is not practical because the field of view is much

smaller than the entire area of the sample. A method to accurately position wear tests and

to subsequently locate the tests in an SEM is necessary.


Figure 2-6. SEM image of 4 pm x 4 pm rastering wear test with low contact pressure.








A dual beam focused ion beam SEM was used to layout and etch grids on the

samples' surface. Each grid square is 100 [tm x 100 [m and the grid lines are

approximately 1 [m wide and 1 am deep, see Figure 2-7.


Figure 2-7. SEM image of etched grid lines for wear test location

The grid lines were aligned with <110> directions on the silicon substrate by

setting the intersection of the cleavage planes { 111 and the plan view plane (001) to be

parallel with the x and y axis in the microscope. These markings are easily located in

both the SEM and optical microscope and can be used situate the wear test and to identify

the area for characterization. The markings allow wear tests to be run approximately

parallel to a <110> direction on the substrate which will aid in locating a major zone for

high resolution transmission electron microscopy (HRTEM) analysis in the JEOL 2010F

which is limited to -200 tilt.


20m















CHAPTER 3
TEST EQUIPMENT

A wide variety of tribological equipment exists to test various wear conditions and

contact geometries. By and large this equipment aims to prescribe specific sliding paths

while controlling an applied normal load and measuring the frictional forces generated at

the sliding interface. Most of this equipment does not make in situ measurements of

wear; rather, wear is measured ex situ using gravimetric, metrological, or microscopy

techniques.

The most common sliding motions are unidirectional, with the standard pin-on-disk

tribometer being a device that prescribes a continuous rotary motion to a disk while

holding a loaded pin stationary on the surface (Figure 3-1).

pin holder

o compression
0-35 mm diameter load cell
sample .- I
30,000~~~.1c R M o ---- ---- --= --- -, -=--- = ------------ -'- II .
30,000 RPM
spindle --j- -_. i__
-I-

i ... --_ :--- o -o o .




dead weight loads (1-50 N)

Figure 3-1. A typical pin-on-disc type tribometer










The reciprocating pin-on-flat tribometer is another common tribological testing appartus.

In this configuration, the sample reciprocates in a linear direction against a stationary pin

through which a normal load is transmitted (Figure 3-2).

pneumatic cylinder











6 channel
load cell
(Fx, Fy, Fz,
Mx, My, M z)


a pin
counter- g sample

sample





stepper motor
and ball-screw linear table

Figure 3-2. Linear reciprocating tribometer

Several researchers have begun to use atomic force microscopes (AFM) to study

tribological surface interactions. Normal and lateral forces acting upon an AFM tip are

calculated from deflections of the tip cantilever. From these forces, sliding friction is

calculated. Accurate measurements of sliding friction from AFMs are exceptionally

challenging, requiring well characterized cantilevers with well know material properties.

There exists a gap in tribological test equipment from the AFM scale with nanonewton

normal loads and nanometer to sub-nanometer normal displacements, to the









microtribometer scale with millinewton normal loads and micrometer displacements.

Tribological conditions in MEMS devices and thin films fit in this regime and remain

largely unstudied. In this study, the Hysitron Triboindenter is used to mimic a linear

reciprocating tribometer and bridge the gap in scale.

Hysitron Triboindenter

The majority of wear testing done in this study was performed on a Hysitron

Triboindenter. The Triboindenter is a commercially available nanodisplacement

mechanical testing system whose primary function is nanoindentation (Figure 3-3).






x-y stage motor





isolation cover





piezo controller

transducer controller

stepper motor
S. controller






Figure 3-3. Hysitron Triboindenter with upper thermal-acoustic isolation cover removed
to show interior details.









Loads, displacements, and measurements are performed by a capacitive transducer

(Figure 3-4). The capacitive transducer assembly contains two parallel fixed plates with

a parallel center plate supported by springs. An indenter tip is mounted to the center

plate. To apply a load a DC potential is applied between the lower plate and the center

plate. An electrostatic attraction displaces the center plate towards the bottom plate.

Based on the spring constant of the support springs, the voltage can be calibrated to a

force. For displacement measurements, an AC signal is applied between the center plate

and top plate, and an AC signal of equal magnitude 1800 out of phase is applied to the

lower plate and the center plate. When the center plate is equidistant from the top and

bottom plates, the net signal is zero. When the center plate is displaced a voltage is

recorded and calibrated to a distance. A similar transducer is mounted to provide lateral

displacements.

lateral transducer
1. /assembly







top plate' j
center plate
bottom plate



Figure 3-4. Capacitive transducer assembly schematic

The capacitive transducer assembly is mounted to a three axis piezo tube scanner

similar to those found in AFMs. The piezo tube scanner allows for precise tip placement

and can be used in conjunction with the transducer as a scanning probe microscope.









Using the tube scanner to raster the indenter tip in contact with a surface, the transducer

can collect z displacements and produce an AFM-like image. The tube scanner is

mounted to a z-axis stepper motor which provides coarse translations for tip approach.

An optical CCD microscope is also attached to the z-axis stepper motor. The optical

microscope allows for accurate sample location and test placement. The sample stage is

attached to x-y stepper motors for translation (Figure 3-5). All systems are computer

controlled.


z-axis stepper
motor


CCD optical
microscope


Figure 3-5. Enlarged view of Triboindenter components

The components are mounted on granite to dampen vibrational effects. The equipment

frame also contains an active vibration isolation device. The working components of the

Triboindenter are enclosed in a thermal-acoustical isolation chamber with provisions for

environmental control.


capacitive transducer
assembly

indenter tip


L U









CSM Microtribometer

The CSM Microtribometer is a commercially available tribological testing device

that operates on scales orders of magnitude larger than the Triboindenter, but well below

those of conventional test equipment (Table 3-1). This tribometer is available with a

linear reciprocating stage or an interchangeable rotating stage to mimic both

reciprocating and pin-on-disc tribometers. Rough stage translations are handled by

stepper motors and fine translations and reciprocations are performed by piezos. A dual

flexure both applies normal load and reacts to frictional forces between the pin and the

counterface. Mirrors are mounted on this flexure in the horizontal and vertical directions.

Optical light intensity sensors are positioned at a distance from the mirrors to read the

magnitude of deflection of the flexure in the frictional and normal load directions (Figure

3-6).


test assembly


housing
horizontal 0
se r Y carrier block --

vertical counterface dual flexure
sensor -- mounting S reciprocating
set -A-' stage



vertical stiffness horizontal stiffness

v.a, 0. 0
JlZ


Figure 3-6. Schematic of CSM Microtribometer [22].









Table 3-1. Comparison of equipment loads and displacements

Conventional
Reciprocating Hysitron
Tribometer Triboindenter
maximum normal
load 1.6 kN 10 mN
maximum lateral
load 1.6 kN 2 mN
maximum normal
displacement 50 mm 5 [tm
maximum lateral
displacement 0.2 m 15 [tm


C SM


CSM
Microtribometer


1N


1N


0.1 mm


0.6 mm














CHAPTER 4
EXPERIMENTAL PROCEDURES

Hysitron Triboindenter Procedures

Tip Preparation

For the most part, tips in indentation testing are not considered to be a parameter

involved in a test. Typically tips are chosen so that they are significantly harder than the

material being tested so that it can be assumed that they experience no plastic

deformation. Diamond is the material of choice for indentation testing because it is the

hardest material and its elastic properties are well characterized so that any elastic

deformation experienced by the tip can be back calculated from the test data. Diamond

tips are expensive costing approximately $2000 each. For indentation testing, this cost is

not a huge factor because the tips will last for an exceptionally long time. In hardness

testing, a sharp tip is often desirable to reach high contact pressures and induce plastic

deformation.

In wear testing, the two materials in intimate contact are both considered. Often

wear tests consist of self mated materials, gold on gold for example, or materials from a

specific application, e.g., a bearing on a race. It is often found that both materials

experience some change. In the case of a hard pin (or tip) moving across a softer flat,

transfer of the softer material to the hard pin is of interest. Observing this material

transfer in cross section is desirable, but would require destruction of the pin. Due to the

high cost of diamond tips, it is not practical to section them. It is not uncommon for the

pin to experience wear along with the surface. For diamond tips, wear of the tip should









be negligible and no transfer film of gold on to the diamond tip has been observed in the

scanning electron microscope (SEM). Self mated materials could be tested by using

coating techniques on the tip, but again, the high tip cost makes this prohibitive. An

alternative to high cost diamond tips is needed.

A crucial parameter in wear testing is the contact pressure. Contact pressure is

used to target desired wear modes, and to replicate applications. The factors involved in

contact pressure are normal load and contact area. The normal load can be controlled by

the Triboindenter, and the contact area varies with the tip geometry. In contrast to

hardness testing, wear tests are often performed with much lower contact pressures

requiring a blunt rather than sharp tip. By selecting tips of various radii, a wide range of

contact pressure can be achieved.

For the reasons mentioned above, tips of relatively large radii are produced in

addition to the sharp tips available from the manufacturer. Sapphire spheres of 100 [tm to

1 mm are commercially available for a nominal cost of approximately $10.00 each. The

original equipment manufacturer's (OEM) tip holder is machined on a lathe with a

conical cut such that when a sapphire sphere is place in the tip holder, it will self-center

(Figure 4-1). A small amount of cyanoacrylate based adhesive is used to attach the

sapphire sphere to the OEM tip holder. The adhesive is allowed to cure overnight. The

transducer piezo assembly is then removed from the Triboindenter for easier access.

Once the tip has sufficiently cured, it is threaded on to the capacitive transducer with a

special torque limiting wrench.









= 1 gm


R = 250 gm















OEM diamond large radius sapphire tip
indenter tip

Figure 4-1. Illustration of an OEM diamond indenter tip and assembly of a large radius
sapphire tip

Sample Mounting

The Triboindenter has a stage that contains six positions which are magnetized by

rare earth magnets. Samples mounted on atomic force microscope (AFM) disks can be

placed on the stage and will be sufficiently held in place for indentation testing. In

preliminary tests, it was determined that this mounting system may not be effective for

tests involving lateral translational forces. Therefore, samples are held in position with

an alcohol soluble adhesive in addition to the magnetic forces. Once the samples are

mounted and the stage replaced on the stage motor, the machine is powered on. The

samples are imaged and focused on using the Triboindenter's optical charge coupled

device (CCD) camera, and a safety area is generated by plotting the outline of each

sample and recording the points. This will define an area of safe travel for the indenter

tip.









Electrostatic Force Constant and Zero Volt Gap Calibration

The electrostatic force constant is a conversion used by the Triboindenter to convert

an applied voltage to a force. This constant is determined by the area of the plates and

the distance between them squared. Because the center plate is suspended by springs,

and different mass tips can be used, the plate gap spacing can change. The calibration

procedure is as follows:

* A load of 600 uN is set in the load function editor. This load will displace the tip
through a large range of motion.
* Perform advanced z-axis calibration is selected from the drop down menu.
* The plate is moved through its range of motion and a plot is generated of the
electrostatic force calibration based on the spring constant and applied voltage
versus displacement.
* The curve is fit to a quadratic and the equilibrium position of the plate is recorded
and tared to zero and the calibration is complete.

Tip Optics Offset Calibration

All tests on the Triboindenter are set up on the sample using the optical CCD

microscope camera. The computer then translates that area to the indenter tip to perform

the test. Therefore a calibration procedure is needed to record the differences in the x, y,

and z positions of the center of the optical focus and the indenter tip. The standard

procedure developed by the OEM relies on residual indents large enough to image

optically and works well in indentation applications with sharp, small radius tips. For

tribological applications a blunt radius tip is often desired, so an alternate method had to

be developed.

Standard tip optics offset procedure

As mentioned above, this optics offset procedure involves optically imaging

residual indents. A relatively soft, polished sample is required for this procedure. A

single crystal aluminum sample with [001] out-of-plane orientation is included with the









Triboindenter. This sample is mounted on the stage. The open loop load function editor

is set with a maximum load of 10 mN. This load with a sharp tip of less than 5 [m radius

will leave a residual indent that can be easily imaged with the optical CCD microscope.

A safety zone is defined around the aluminum sample, and an area free of indents is

chosen to perform the calibration. The optics offset procedure is chosen from the

software menu, and the machine automatically translates the tip to the approximate

position above the center of the optical focus. The user is prompted to use the z height

control to lower the indenter tip to within 1 mm of the sample surface. Once the user

does so, an automated pattern of seven indents in the shape of an H is performed on the

sample at the prescribed load (Figure 4-2). The machine then translates the sample back

to the optical microscope, and the user is prompted to set the microscope reticule over the

center indent. The computer then records this difference in the x, y, and z positions of the

center of optical focus and the indenter tip. Once this procedure is completed, an eighth

indent is added to the pattern so not to confuse it with another pattern in a later tip optics

offset calibration.













0=

x1k M. Blad Lel 5 I
"U* i""" ___~ mm~l.... a K....H


Figure 4-2. Optical CCD microscope image of residual H-pattern indents in Al (001)
sample









Modified tip optics offset procedure

This procedure was developed for large radius tips where the residual indentation

left by the maximum load would not be resolvable in the optical microscope. By

calculating the depth 6 of a residual impression left by a spherical indenter of radius R,

one can see how indenters of sharp radii less than 5 tm leaves an impression that is

resolvable by optical microscopy, but impressions from indenters of blunt radii greater

than 100 [tm cannot be resolved optically. Hardness was defined earlier as the normal

load divided by the projected area of the residual impression. Figure 4-3 shows a

schematic of a spherical indenter on a flat surface and the cross sectional geometry to

solve for 6, the depth of impression, where Fn is the normal load, 2a is the diameter of the

residual impression, 'a' is the radius of residual impression, and R is the radius of the

indenter tip.

F.













Figure 4-3. Representation of spherical indenter on flat surface and cross sectional
geometry to solve for depth of indentation

The hardness equation can be rearranged as follows:

F F F/ (4-1)
A ra2 H7i









For the relatively soft aluminum sample, H = 500 MPa. With the machine set at

the maximum load of 10 mN, solving for 'a' yields a = 2.5 [tm. Using geometry from

Figure 4-3, the following can be written:

= R- R-a2 (4-2)

For tips of radius R = 5 [tm, the depth of residual impression is 670 nm, which is

within the limits of resolution in the optical microscope. Considering a tip of radius R =

100 [tm, the depth of the residual impression is 30 nm which cannot be resolved in an

optical microscope.

Fortunately, the Triboindenter has a scanning probe imaging feature which works

much like an AFM in that the tip can be held in contact with a surface at a low load and

the piezo tube scanner can raster the tip over the surface. At each point, or pixel, a z-

height is recorded along with the x and y position of the tip. This data is used to generate

an image by applying contrast values to the z-height values. This scanning feature has a

resolution limit below 1 nm in the z direction because the capacitive transducer is used to

record the z-height.

The following method was developed to work around the resolution limits of the

optical microscope. This method uses a low load which can be beneficial in fields

outside of tribology. If one were to design a delicate functional tip that could not survive

the high load used in the standard optic offset procedure, yet still needed accurate tip

positioning, the modified tip optics offset method could be employed.

Since a scanning probe technique would be used to calibrate the tip optics offset, a

sample was designed and produced to contain features that would be easily imaged and

located in both the optical CCD microscope and the scanning probe microscope feature









of the Triboindenter. A silicon wafer was chosen because of its highly polished surface,

and its compatibility with the focused ion beam milling technique. The wafer was

cleaved into a 5 mm x 5 mm section and mounted on a 12 mm AFM disk with a

cyanoacrylate based adhesive. This specimen will now be referred to as the modified tip

optics offset sample and is shown in Figure 4-4. The following features were carefully

chosen and patterned on the wafer using an FEI DB-235 focused ion beam (FIB) SEM.

* series of concentric circles
* 200 ptm outer diameter
* 20 pm radial spacing
* 40 ptm inner diameter
* two sets of orthogonal crosshairs rotated 450 from each other
* 1 ptm patterned line width











40- m ----4gm




-- 200 pm -
Figure 4-4. SEM image of the patterned modified tip optics offset sample

The series of concentric circles were chosen because it is easy to determine, from

an arc, the direction of travel required to head towards the center. A pattern size of 200

pm outer diameter was chosen because it has been observed that from a tip change the tip

optics offset rarely varied by more than 100 ptm and a 200 ptm feature is easily located in

an optical microscope. When the scanning imaging function of the Triboindenter is









started, a rastering pattern of 40 am x 40 [m is a typical scan size, therefore a radial

spacing of 20 [m will ensure that at least one line is caught in the initial scan. The inner

diameter of 40 [m was chosen because it could be imaged in the initial scan size once

located. The two sets of orthogonal crosshairs rotated 450 from each other were chosen

because if only one set was present, and one of the lines were aligned closely with the

rastering direction, that line would not be imaged. The two sets ensure at least 3 of the 4

lines will be imaged. The reason for the 1 [m line width is two-fold. First, a line of 1

[m in width can be resolved in an optical microscope. For the second reason, we return

to Figure 4-3 and Equation 4-2. Equation 4-2 can be rearranged and solved for 'a' to

give:

a = R2 -(R -)2 (4-3)

From the geometry of Figure 4-3, 2a will be the minimum width of the line spacing

required for a given R and 6. The largest radius tip of interest in this study is R = 250

[m. Hysitron publishes the z axis resolution limit to be 0.04 nm with a noise load floor

of 0.2nm. A value of 6 = 0.5 nm should be within the resolution of the equipment. Using

these values in Equation 4-4 gives a value for a = 0.5 [m, making the line width 2a = 1

km.

If a FIB is not available, a similar tip optics offset sample could be produced using

a soft polished sample such as the Al (001) and a sharp indenter tip. Using the automated

methods feature of the Triboindenter, one could set up a similar pattern to that described

above and run the pattern with a sharp tip, high load, and a soft sample. Once the sample

is produced, the sharp tip can be exchange for a blunt tip and the modified tip optics

offset procedure could be followed.







Once the modified tip optics offset sample is produced and mounted on the stage
with a safety limits area defined, the blunt tip installed on the capacitive transducer, and
the electrostatic force calibration and zero volt gap calibration completed, the modified
tip optics offset procedure is initiated. The sample's concentric rings are located and
centered to the video window reticule using stage translations (Figure 4-5).
O Ilm









Fi e. [4 Ii Blt e i s
uiae 5d l 1 t l o Im

Figure 4-5. Modified tip optics offset sample centered in the video window
From the main positioning window, tip optics offset is selected. Then new 'single indent'
is selected from the window seen in Figure 4-6.

I Cl rHa in. II Ics '" 'o'" d I "'''' O" '11 I "" '""" I I Co, I
New'H' Pattern New 'Single Indent' Default X & Y Offset Previous Indent Cancel



4f

Figure 4-6. At this point, new single indent is selected
In the load function editor, a low load of 2 kN or less is programmed. By setting a low
load, the machine will go through the standard procedure of performing an indent, but no










damage to the sample or the tip will occur. The machine will translate the sample so that

the tip is over the sample and the user will be prompted by the window in Figure 4-7.

Calibrating the Optics
Z SAFETY IS DISABLED TAKE CARE!!
Use the green + Z button to move the tip down
to approximately 1 millimeter above the sample.
Click OK when Finished
I OK | [ Cancel ]

-z Col .-
Medium-
S Fine-
+ Z IUltra Fine-


Figure 4-7. Manual height adjustment window

From this window the indenter tip is lowered to approximately 1 mm from the

sample surface. The machine then slowly approaches the sample surface until contact is

made, and the low load indent is executed. The sample is then translated back to the

optical CCD microscope and the window in Figure 4-8 pops up.


Z SAFETY IS DISABLED TAKE CARE !!!
Use the Red +/- X and Blue +/- Y buttons to locate the indent pattern.
OR
Click the"Auto Search" button to locate the indent pattern.
Once the indent pattern is located. Put the cross hairs over the single dot or
if"H indent calibration pattern,the dot between the 2 legs of theH" pattern.
Click Ok when Finsihed
[~~~OK~ ~~] [~ Cancel ~


Figure 4-8. Search option window

At this point, no translation is made even though the machine prompts the user to

locate the pattern. This set is used only to set the z-axis offset. The x and y offset is set










in following steps. The 'ok' button is pressed without any x or y translation. Now from

the Triboscan toolbar, the imaging mode is selected. The windows in Figure 4-9 will

open. Initial scan parameters of 50-80 [m scan size, 2 kN set point, and 1 Hz scan rate

are entered. Topography and gradient imaging windows are selected.


We ConMO
-g'Lte EH 4aiD4F- p E31 9 2 [a Z =1
I I|Topography Forwar radlent IForward



r-










R sit* "o.
o70.000 Ana. 4 70 000 YIm


















Tip approach is selected and once the Triboindenter has indicated that the tip is in

contact with the surface, the scanning is started. Once one complete raster is finished,

some of the tip optics offset sample should be visible. Now the stage controls are used to

translate the sample towards the center of the circles. It is critical to use only the stage

controls and not the x-y offset to translate the sample. The x-y offset is a bias applied to

the piezo tube scanner which will give an inaccurate tip optics offset calibration if used


aL__I-


I --


1~It


I









here. Determining the direction for translation is easily identified by moving radially

inward from the patterned arcs. For translation it is easiest to think of the sample being

stationary and the reticule moves in the direction indicated on the x-y translate arrows.

Relative direction lengths can be entered and are convenient for 'homing-in' on the

center. As the center is approached, the raster size is reduced which effectively zooms in

on the image. Once the center of the sample is located on the imaging reticule, the tip is

withdrawn from the sample and the sample is translated automatically back to the optics

window. Now the amount of tip optics offset can be visualized in the video window

(Figure 4-10).











.Medium-





S Reticue
I cr oats.
XScae mm] 1 1 1 BlackLevel
Ys[ate Drun [11 1 Zoom Factor rCx M -


Figure 4-10. Actual tip optics offset seen in the video window

No stage translations are made at this point. Again calibrate optics is selected from

the drop down menu, and the window in Figure 4-6 appears. New single indent is

selected again and a low load is entered in the load function editor window. The sample









is automatically translated beneath the indenter tip, and the window in Figure 4-7 appears

again. The tip is manually lowered to within 1 mm of the surface and the machine

performs the low load indent. Upon completion, the sample is translated back to the

optical CCD microscope and the auto search window seen in Figure 4-8 reappears. Now

stage translations are used to center the tip optics offset sample pattern to the center of the

video window reticule, and the 'ok' button is pressed in the auto search window. The

calibration is now complete and can be checked by returning to the scanning imaging

mode to verify the pattern is at the center of the imaging reticule. This entire procedure

was completed with no load greater than 2 kN.

Quick Approach

Now that all calibrations are complete and the samples are mounted with safety

zones defined, an area of interest is located using grid lines etched by the FIB. A "quick

approach" is performed which involves setting a load threshold and approaching the

sample surface. Once the load threshold, called the "set point" is reached, the

Triboindenter records the z height as the sample height. This procedure is performed for

each sample safety area.

Tip Area Function Calibration

With depth sensing indentation machines, load and displacement are measured,

however, an area of residual impression is necessary for analysis of modulus and

hardness as can be seen in Equation 4-1. From the analysis above, it is shown that a

residual impression cannot be imaged or measured optically. It would be convenient to

be able to extract area calculations based on displacement data. Therefore a tip area

function relating the residual area to the depth of indentation is needed. For micron and

larger scale indentation depths, ideal tip geometry can be assumed and an area function









can be generated from simple geometric relationships. The ideal area function for an

indenter with pyramidal Berkovich geometry is A=24.5*hc2 where he is the contact depth

and A is the residual area (see Data Analysis section for in-depth explanation). At the

nanometer level, the tip of a pyramidal indenter is rounded, therefore the assumption that

the three faces of a pyramid intersect at a point is invalid, and the ideal area function

breaks down and greatly underestimates the residual area based on the depth. A method

has been devised to generate a tip area function by creating a series of indents in a

standard of known elastic modulus. In this section, the mechanics of generating a tip area

function are explained. For a detailed mathematical description of the procedure, refer to

the Data Analysis section.

A fused quartz standard is loaded on the stage of the Triboindenter. A series of

approximately 100 indents are made targeting a depth range to cover the series of

experiments to be run. The series is run and the data is saved. A power law curve fit is

performed on the unloading portion of the curves. A tangent line to the power law curve

is used to find the stiffness from which an area is calculated. The calculated area versus

displacement for all the indents is plotted, and a curve fit is performed to generate an area

versus displacement function. This function is saved for this tip and displacement range.

A new function must be generated for each different tip and the functions must be

verified periodically on a standard to ensure the tip geometry has not changed.

Machine Compliance Calibration

In a depth sensing indentation device, the desired measurement is the position of

the tip relative to the sample surface. This is determined by displacement, however

measured displacements include compliance of the system along with the tip and the

sample.









There are several factors that can contribute to the system compliance such as:

* Indenter tip-the indenter tips are handmade from various materials and adhesives.
* Sample mounting-if the sample mounting procedure varies in materials, the
compliance will vary.
* Transducer mounting-the transducer must be mounted the same way each time
and should be level and properly seated.

Therefore it is necessary to determine the system compliance and remove it from the

measurements. This section only explains the procedure for the compliance calibration.

The mathematics are explained in detail in the Data Analysis section.

Once the other procedures have been completed, the transducer calibration constant

is set so that the machine compliance is 0.00. A set of 15 indents are made with loads

varying from 5 mN to 10 mN. Curves from the data are ploted and the multiple file

analysis is executed. A plot is generated of the inverse of the stiffness versus the inverse

of the square of the load. A linear fit is generated and the machine compliance in nm/mN

is equal to the y-intercept times 1000. This number is entered for machine compliance.

A new machine compliance is generated for each tip and any time the conditions

mentioned above might change.

Testing Modes

The Triboindenter has three basic modes of operation which are illustrated in

Figure 4-11. The static indentation mode brings the tip into contact with the surface,

applies a normal load, and records load and displacement. The scratch mode brings the

tip into contact, applies a normal load, translates the tip laterally, and records normal

load, normal displacement, lateral load, and lateral displacement. The rastering wear test

mode applies a normal load, and uses the piezo tube scanner to raster the tip over an area.

No quantitative data except the prescribed normal load is collected in this test.










indentation mode







scratch mode







rastering wear mode







Figure 4-11. Schematic illustration of Triboindenter testing modes.

Depth sensing indentation

Depth sensing indentation tests yield data in the form of loads and displacements

which can be analyzed to obtain material properties such as elastic modulus and hardness

as described in the data analysis section. Once the calibrations have been completed,

load versus displacement versus time functions are generated in the load function editor.

The area of interest is located, and the test is initiated. Indentation tests can be operated

in load control, displacement control, or open loop where a DC bias voltage is set based

on the electrostatic force calibration to approximate a maximum load. Once the test is

started, the tip approaches the surface and makes contact at the set point. The load is

increased and load, displacement, and time are recorded. A general 'rule of thumb' has

been established for testing thin films on substrates which suggests maximum indentation









depth should not exceed 10% of the film thickness to avoid a contribution from the

substrate properties.

Rastering wear test

For data analysis, fiducial marks are placed using the indentation mode prior to

running the test. An automated pattern of eight indents surrounds the test area (Figure 4-

12). An atomic force microscope AFM scan of this area with the fiducial marks is

necessary prior to testing. In this test, a normal load is prescribed in the wear function

editor along with an area to raster over and a scan rate. The number of raster lines is set

at 256. The area of interest is selected in the optical CCD microscope. Once the test is

started, the tip approaches and contacts the surface, applies the desired load, and the

piezo rasters the tip over the prescribed area for a given number of cycles.


Figure 4-12. SEM image of a rastering wear test showing fiducial indentation marks in
circles.









Reciprocating Wear Test

The scratch test mode is modified to replicate a reciprocating linear tribometer.

This mode was originally designed to place a normal load on the tip, and using the 2D

transducer, translate the tip to produce a scratch. The test mode is often employed for

testing relative coating adhesion. By using this method with a low normal load, and

programming multiple passes in the function editor, a nanodisplacement scale

reciprocating tribometer is emulated.

CSM Microtribometer

Tip Preparation

Tips on the CSM Microtribometer were mounted on the cantilever flexure using a

cyanoacrylate based adhesive [22] (Figure 4-13). The tips are commercially available

BK7 optical glass lenses with a semi-spherical geometry.

1. flexure
2. vertical
displacement sensor
3. mirrors
4. horizontal
displacement sensor
5. pin
6. sample
7. reciprocating stage
8. LVDT

Figure 4-13. Schematic of the CSM Microtribometer.

Sample Mounting

The sample is mounted on a SEM microscope stub using a cyanoacrylate based

adhesive. The SEM stub is fixed to the linear reciprocating stage. Stepper motors are

used for coarse positioning of test area with the aid of a portable microscope.









Reciprocating Wear Test

The Microtribometer has a normal load range of 0.5 mN 1 N. This range of

applied normal loads is achieved by having three different types of dual flexures, with

varying stiffness values. Low range normal loading (0.5-100 mN) is achieved with a

glass flexure, middle range loading (50 mN-300 mN) is achieved with a stainless steel

flexure, and high range loading (200 mN-1N) is achieved with a thicker stainless steel

flexure. The appropriate flexure for the desired load range is selected, and the testing

parameters of normal load, track length, sliding speed, and sampling rate are entered in

the computer. The machine records lateral load and lateral position. From this data,

friction coefficient and sliding speed can be calculated.














CHAPTER 5
DATA ANALYSIS

Indentation Testing

Indentation testing is a convenient, relatively non-destructive method to determine

mechanical properties such as hardness and elastic modulus. Hardness is defined as:

H=P/A (5-1)

where P is the normal load and A is the projected residual area in the direction of the

load. Microhardness tests are typically performed by applying a known load and

calculating the area of the residual indent from measurements taken in an optical

microscope. Nanoindentation has been defined as tests involving normal displacements

on the order of nanometers [23]. Residual indents from tests at this scale have

dimensions smaller than the resolution limits of optical microscopy and would require

electron microscopy for measurements. Since electron microscopy is costly and time

consuming, requiring a subsequent characterization of the sample, post-indentation,

methods have been developed to calculate the residual area based on a measured depth of

indentation.

The Oliver-Pharr Method

The most widely accepted method for depth sensing nanoindentation analysis

involves a model first proposed by Doerner and Nix and later refined by Oliver and Pharr

[24, 25]. This analysis is often referred to as the Oliver and Pharr Method. Once load

and displacement data has been collected and plotted as shown in Figure 5-1, a power law

curve fit of the form:










P = A(h-h)"'


(5-2)


surface profile after


under load


loading


unloading,


possible
range for
h,


h for =0.72

displacement, h

Figure 5-1. Schematic of load versus displacement curve along with a cross section of an
indented surface. The range of E is indicated for various indenter geometries.

where P is the load, h is the depth and hf is the depth of the residual impression, is fit to

the unloading portion of the data. Sneddon derived the following equation to describe the









load as a function of displacement for an infinitely rigid cylindrical flat punch into an

elastic half-space:

4Ga,
P = h (5-3)
1-v

where P is the load, G is the shear modulus, a is the radius of the cylinder, h is the depth

of penetration, and v is the Poisson ratio [26]. By substituting:

E
G (5-4)
2(1+ v)

for G, Equation 5-3 becomes:

2Ea
P= h (5-5)
(1 v2)

Rearranging the equation for the area of contact of the indenter to:


a (5-6)


and substituting, Equation 5-5 becomes:


P= 2 A E h (5-7)
(1-v2)

and differentiating with respect to h yields:

dP 2 -A E
dh (1-v2) (5-8)

where dP/dh is, by definition, the stiffness.

One must now consider the elastic properties of the indenter. Defining the modulus

of the system as the reduced modulus Er yields:

1 (1- 2) (1- 2)(5
-(5-9)
y FS F









where Es and Vs are the Young's modulus and Poisson ratio for the indented surface and

Ei and vi are the Young's modulus and Poisson ratio for the indenter. Substituting into

Equation 5-8 yields:

dP- 2 J E (5-10)
dh J

Solving for Er gives:

.1 S (5-11)
Er = S--S
2,I

Oliver, Pharr, and Brotzen have shown that this equation holds true for any

axisymmetric indenter, and will extend to pyramidal indenters with a correction factor, s,

for different geometries [27]. A function to describe contact area from displacement

along with Equation 5-11 gives a convenient method to measure the reduced modulus

from data shown in Figure 5-1. The Poisson ratio of the material of interest is often

unknown, therefore reduced modulus rather than Young's modulus is often reported.

For a Berkovich indenter of ideal geometry, the relationship between contact area

and contact depth can be calculated from simple geometry:

A = 24.5 hc2 (5-12)

However, due to the inability to produce a perfectly sharp indenter, some radius is

expected at the tip apex. For large indents, this radius is negligible, but for nano-

displacements, it cannot be neglected. An area function to relate contact area as a

function of contact depth can be generated by indenting on a fused quartz sample of

know elastic modulus, E = 72 GPa. A series of indentation tests of increasing

displacement over the desired range are run on the standard sample. Load versus









displacement plots are generated to find the stiffness as described above. By solving for

he from the relationship in Figure 5-1:

S= hP max (5-13)
S
hc hmax m


and Equation 5-10 solved for area and plotting A, versus he, a curve fit is generated in the

form:

A,(h) = Cohc2 +Clh +_C2h +C3hl +C4h0 + C5,h (5-14)

Indentation testing data can now be analyzed from eqn. 5-1 and 5-11 to obtain hardness

and reduced modulus. The Oliver and Pharr method includes the following assumptions:

* Material does not pile-up outside the indent.
* The unloading behaves as an isotropic elastic half-space.
* The tip area function is well approximated over the range of displacements.


Compliance Method (Stone's Method)

When designing mechanical testing equipment, engineers choose stiff materials to

reduce the effect of machine compliance upon sample data, however, machine

compliance cannot be ignored. Compliance is defined as the inverse of stiffness.

Compliance of the entire system, CT, can be written as:

T =C C, (5-15)

where Cm is the compliance of the load frame (machine) and all other compliances except

the sample, and Cs is the compliance of the sample. The load frame compliance includes

sample mounting, tip, frame, and stage. The following method is suggested by Hysitron

to determine the compliance of the machine [28]. The inverse of Equation 5-10 can be

substituted in Equation 5-15 for Cs to give:










(5-16)
Cr =C +
2E, /-A


solving Equation 5-1 for area and substituting yields:

(5-17)
C, = C- + -
c m2E4


Assuming hardness is constant at large indentations, a series of large indents can be run

and plotting the inverse of the total measured compliance versus 1/Pmax will give y-

intercept equal to Cm. This method can be run on various mounting techniques and

different tips to determine the compliance for any given testing conditions. Figure 5-2

shows a plot of the total compliance measured versus the inverse of the square root of the

maximum load for various large loads on a quartz standard, an aluminum standard, and a

(001) silicon wafer.

25-
+ silicon (001)
m quartz
20- o aluminum (001)





S10





00O0 0002 0.004 0.006 008 0010 0.012 0014
1


Figure 5-2. Measured compliance versus square root of maximum load for a load range
of 5mN to 10mN on quartz, Al (001), and Si (001).

A least squares fit line is generated to determine Cm. Both the quartz standard and the

silicon sample show good agreement in the machine compliance of 2.6 nm/mN. The









aluminum sample shows a slightly higher compliance of 3.0 nm/mN which may be

attributed to differences in sample mounting.

A similar yet alternative method to analyze machine compliance and extract

hardness and modulus information has been developed by Stone et al. [29]. Stone et al.

begin by defining a material compliance parameter:

J 2 H (5-18)
J0 = PmC
0 maxCs E2
r

where Pmax is the maximum load, Cs is the compliance of the sample, H is the hardness of

the sample, and Er is the reduced modulus. Note that the parameter Jo is independent of

the tip geometry. This parameter was originally developed by Joslin and Oliver [30] who

noted that by substituting for the definition of hardness from Equation 5-1 and stiffness

from Equation 5-11, one could obtain:

H 4 P (5-19)
E2 7r S2

Stone's parameter is missing the 7t/4 term from Joslin and Oliver's analysis. This 27/4

term arises from the geometry in Sneddon's [26] analysis of the cylindrical punch, which

Pharr et al. have shown to apply to any axis symmetric indenter [27]. Stone incorporates

a geometrical factor in his Er term which is adjusted depending on tip geometry to

account for the 7t/4 term. The indenter used in this study is modeled as an axisymmetric

indenter and no geometrical term is incorporated in the reduced modulus; therefore a

parameter will be substituted in Stone's analysis to better fit this study. This parameter,

Ko, is defined as:


K p 2 H (5-20)
S mas 4 E2
p









By taking Equation 5-15 and multiplying by the square root of Pmax yields:

C, [pV =C c p +C p- (5-21)

and substituting for the material parameter:

Cr P- = Cm -iC + i (5-22)

Now plotting the CTPmax2 versus Pmax1 yields a line with a slope of the machine

compliance and an intercept of Ko2 provided the material shows no indentation size

effect. This analysis is useful in that the material parameter can be verified for the

calibration standard and material data can be extracted without knowing the indenter tip

geometry. Just as in the Oliver and Pharr analysis, the unloading is assumed to behave as

an isotropic elastic half-space.

Modified Winkler Method

The Oliver and Pharr model works well on materials that do not exhibit significant

pile-up. However, their model greatly overestimates hardness and modulus in materials

that exhibit pile-up due to the underestimation of the contact area [31-33]. This

overestimation of properties is amplified in soft thin films on hard substrates due to

excessive pile-up. The compliance method of Stone et. al. extracts a material parameter

that requires knowledge of either the hardness or the modulus to determine the other.

The Stone model also assumes no indentation size effect.

An alternate model is developed based on the Winkler model of a bed of

independent springs to account for material pile-up. Figure 5-3 shows schematic

representations of indent cross sections from the Oliver-Pharr model, the modified

Winkler model, and the output from an actual indent in a material exhibiting pile-up.












..- o- ~cross section outline
Oliver-Pharr
-.- modified Winkler

Figure 5-3. Schematic illustration of indent cross sections from Oliver-Pharr and
Modified Winkler compared to an actual indent.

It is assumed that each differential element is constrained orthogonal to the loading

direction due to surrounding material. The justifications for this assumption are the

contact area radius is much larger than the film thickness and the Poisson ratio of gold is

0.42 which is nearly incompressible. The indenter tip is assumed to have axisymmetric

geometry and is modeled based on a curve fit to a 4th order polynomial from micrographs

(see Chap. 6). To account for pile-up and plastic deformation neglected in the Oliver and

Pharr model, each discretized element is examined to determine the onset of plastic

deformation based on the hardness. Upon plastic deformation, the volume deformed is

transferred to the next differential element. Once the maximum displacement is reached,

elastic unloading is assumed. This model is developed to attempt curve fits with load

versus displacement data collected from the nanoindenter.

The first assumption of constraint due to the surrounding material and the indenter

tip/displacement scale results in no elastic strain in the directions orthogonal to the

loading direction such that Exx=yy=0. From this assumption, and effective modulus can

be calculated from:

{} = [S] {c} (5-23)

where s is the strain, S is the compliance matrix, and a is the stress. Expanding yields:






59


1 -v -v
0 0 0
E E E
EEE
-v 1 -v
El 0 0 0 (
x E E E
EEE
Ey V V 1 0 0 0
Ez E EE Ez
yz 000 1 0 0
r7 G r
1 (5-24)
Yy 0 0 0 0 0 J r (5-24)
G

0 0 0 00
G

where E is the Young's modulus. Solving for the stiffness and assuming no shear

coupling yields:

E(1- v) -Ev -Ev(5-25)
1
C= S-' 1 -Ev E(1- v) -Ev
-Ev -Ev E(1-v)

The assumption of no strain in the x and y directions:

(0 (5-26)
{e}= 0


can be substituted into:

{0} = [C] C) (5-27)

which yields:

E(1-v) (5-28)
(1 + v)(1 2v)
(1+v)(l-2v) z


The coefficient:








E(1-v) (5-29)
(1+ v)(1-2v)

is Lame's constant and can be thought of as an 'effective modulus.'

Each element of the model film is described in cylindrical coordinates as illustrated

in Figure 5-3. For the illustration, the tip geometry is shown as spherical; however, for

calculations, the 4th order polynomial fit from micrographs of the tip characterization in

Chapter 6 is used.

s dA = s dO ds
Sd0^--C dA = 2n s ds



~ds

+max


max x tan-' -D,


d=D -R- 1-cos sin-' R


d R

Do

Figure 5-3. Schematic illustration of a differential element described with cylindrical
coordinates and tip displacement based on a spherical tip geometry.
A MATLAB code is developed to increment tip displacement and calculate

differential element elastic-plastic response sequentially. Figure 5-4 shows the variable

designations for the loading and unloading response.















LL L
mmn f






pre-load post-load

Figure 5-4. Differential element illustration prior to (left) and after load (right) with
variable notation indicated.

At each displacement increment, the stress on each element is calculated by:

(5-30)
LO

where a is the stress, h is the effective modulus, 6 is the incremented displacement, and

Lo is the film thickness. The incremental load is calculated by:

dF = odA (5-31)

If o=H, the hardness, the amount of volume from that incremental displacement over the

differential element is transferred to the next element in the outward radial direction. The

force is summed and the total displacement is compared to the maximum displacement.

The displacement is incremented until the maximum displacement is reached, upon

which, the unloading cycle begins. The loading code is represented in the flow chart in

Figure 5.5.









Loading Initialize to depth to zero


Figure 5-5. Flow chart diagram of loading code.

The unloading is assumed to be elastic. Each element begins compressed to the

point Lmin in Figure 5-4. The material will only recover to the point Lf. Figure 5-6 shows

a flow chart of the elastic unloading code.









Unloading


Figure 5-6. Flow chart of unloading code.

Rastering Wear Test

In the rastering wear test mode, no data is collected during the test by the

Triboindenter. The input normal load is the only quantitative information available

without subsequent analysis. Often in wear testing, a wear rate is reported to quantify the

material performance. The wear rate, K, is the volume of material removed per normal

load per distance of sliding. The normal load is the input parameter of the test, and the

distance of sliding can be calculated by the square root of the area rastered over times










256, the number of raster lines, times the number of passes over the raster area. The

volume can be calculated in the following manner:

* Fiducial marks are placed around the area to be worn prior to testing.

* An area larger than the test area is scanned in the AFM to include the fiducial
marks.

* The desired rastering test area is run for a given load and number of passes.

* The larger area including the fiducial marks is again scanned with the AFM.

* The digitized data from the final scan is aligned to the data from the initial scan
using the fiducial marks.

* The initial surface is then subtracted from the final surface.

* The data below the zero surface plane is integrated to calculate a volume of
material removed.

This volume can now be used in the wear rate calculation. The subtracted data can be

plotted to reveal wear morphology as shown in Figure 5-7.

--- ..

r-

50

40

30

20 nm
10


-10
18
16 -20
14
12 1-30
108 16 18
gm 6 12 4
4 10
2 4 6 gm
2

Figure 5-7. Rastering wear test data plot showing a trenched area on the right of the test
area and a pile-up area on the left.









Triboindenter Reciprocating Wear Test

The reciprocating wear test mode, or scratch mode, is a normal load control and

lateral displacement control test. The Triboindenter collects normal displacement, and

lateral force data during the experiment. The normal displacement data is subject to

errors from sample tilt and thermal drift. To compensate for sample tilt, a low load pre-

scan of the wear track is collected and this displacement data is subtracted from the data

collected during the test. In the indentation mode, there is a provision to monitor thermal

drift for a selected amount of time, and then subtract the recorded drift from the

displacement data. A typical indentation test is run over a short period of time. The drift

monitor can be set to record drift over a similar amount of time and wait until the drift

reaches some minimum threshold, or until the drift is sufficiently linear. In the

reciprocating wear mode, this is not practical due to the length of time that the test is run.

Therefore, the displacement of the test will be verified by cross sections taken from wear

tracks and measured in SEM or TEM and adjusted accordingly.

Lateral force data is also subject to errors due to sample tilt. To remove tilt error

from the lateral force data, the initial low load pre-scan friction data is subtracted from

the data. During the reciprocating tests, material piles up at the ends of the tracks, and

near the endpoints of reciprocation, displacement is increased due to smaller initial

contact areas. The frictional data is filtered to examine sliding conditions excluding these

transient areas (Figure 5-8). The friction force is the average per sliding pass and can

then be examined versus sliding cycle.
















z 20





4-1
-20-


-40


-60
0.0 2.0 4.0
sliding position (lim)

Figure 5-8. Data from a 10-cycle reciprocating wear test showing the transient areas near
the ends of the reciprocation and the area over which the friction force data is
analyzed.

CSM Microtribometer

Frictional force data is collected and analyzed in a similar manner to the

reciprocating test in the Hysitron Triboindenter. An extensive development of data

collection and analysis was presented by Dickrell [22].














CHAPTER 6
CHARACTERIZATION

Tip Characterization

The indenter tip used for the majority of testing in this study is a single crystal

diamond. Diamond is a convenient material due to its high hardness and resistance to

wear. In addition, it is well known that gold does not wet carbon which will be beneficial

to avoid material transfer from the film to the indenter tip. Due to high costs and limited

available geometries, indenter tips of various materials may be sought out. The following

methods can be applied to most tip materials. Tip geometry is critical in analysis of

indentation and wear testing. Properties such as elastic modulus, hardness, shear stress,

and friction coefficient are all calculated from contact areas based on tip geometry.

Inaccuracies in tip geometry will cause significant errors in contact area calculations

which will propagate to the above mentioned properties. Methods for determining tip

geometry include: the indentation method, the scanning method, and electron

microscopy imaging.

Indentation Method

This method, developed by Oliver and Pharr [25], involves creating a series of

indents of various depths on a standard sample of known elastic modulus, typically fused

quartz. By generating a plot of area versus contact depth as described in Chapter 5, a

function in the form of Equation 5-14 can be generated to describe the contact area as a

function of contact depth. This method has proven reliable for samples that exhibit high

hardness with little plastic deformation; however, it greatly underestimates the contact









area in samples that exhibit pile-up which leads to errors in hardness and modulus [32].

This method is also lacking in three-dimensional information which is necessary in

calculating properties derived from lateral translation of the tip.

Scanning Method

The scanning method is an adaptation of scanning tunneling microscopy (STM)

and atomic force microscopy (AFM) techniques that involve scanning the tip over a

sample with well characterized features or features of much smaller radii than the tip [34-

36]. By scanning a feature with a radius much smaller than the tip, the image produced is

representative of the tip (Figure 6-1).

indenter tip
Ii
---- -- f
_\ \ \ I







scan path

sharp asperity
scan direction

Figure 6-1. A schematic representation of a large radius tip scanning a small radius
asperity.

An FEI DB235 dual beam focused ion beam (FIB) microscope (described later in greater

detail) was used to produce a sharp pyramidal asperity with a radius of less than 100 nm.

A silicon wafer was mounted in the FIB and the normal to the sample was tilted to 200

with respect to the ion beam. A trench was milled, and the sample was rotated 120 about

the ion beam. Another trench was milled and the sample was again rotated 1200. This









procedure was repeated at progressively lower beam currents to sharpen the asperity

radius (Figure 6-2).























Figure 6-2. Sharp asperity milled on silicon to scan indenter tip.

The sharp asperity sample is mounted on the Triboindenter and the desired tip is

installed on the transducer. The Triboindenter is set to imaging control which places the

transducer into load feedback mode. An imaging set point load of 1 pN is selected.

Using the piezo tube scanner, the tip rasters over the sharp asperity feature maintaining

the 1 pN load and recording x, y, and z positional data. The x, y, and z data is entered in

scanning imaging software to produce the image seen in Figure 6-3.

This procedure is attractive because it can be run just prior to and upon completion

of testing to verify tip geometry. Because it is an in-situ method, removal of the tip for

characterization is not required. This method suffers from longevity of the sharp asperity










feature. The feature should be made from the hardest, toughest material available. A

diamond sample of the sharp asperity would be optimum.


height (nm)
12.06.
-372.77
-757.60
-1142.42


2.40
3.60


4,`~
7,


height (nm)
38.70
-354.86
-748.41
-1141.97 1


Figure 6-3. Scanning image produced with a sharp asperity by rastering first in the x
direction, and then in the y direction.









Transmission Electron Microscopy Method

A direct measure of the tip geometry is the best method for characterization.

Imaging the tip in a transmission electron microscope provides a high resolution profile

of the geometry. A fixture to mount the indenter tip in a TEM sample holder was

fabricated and is seen in Figure 6-4. The indenter tip is fixed in a polymeric mounting

device with a blind threaded hole as received from the equipment manufacturer. The

polymeric fixture is glued into the TEM mounting fixture, using a conductive adhesive.

The mounting fixture is attached to the TEM sample holder with countersunk set screws.



indenter
tip mounting
fixture






Figure 6-4. Schematic drawing of TEM 200 CX sample holder with tip holder fixture
and indenter tip mounted.

The normal loading axis of the indenter tip is oriented such that it is parallel with the

sample holder tilt axis. The sample holder is inserted into a JEOL 200CX TEM and the

tip is imaged at 50 kX and 200 kV. The sample is adjusted so the normal loading axis

and sample tilt axis are at the eucentric height. The indenter tip can be rotated from 600

counter-clockwise (CCW) to 600 clockwise (CW) to produce profile images through 1200

of rotation. A series of images are collected at 400 CCW, 0, and 400 CW, the images are

digitized and points are plotted along the profile. The data is fit to a curve to approximate

an axisymmetric geometry to describe the tip shape, see Figure 6-5.














0.5 cgm

400 ccw


0 40 cw


0 200 400 600 800 1000
a (nm)

Figure 6-5. The upper images are taken at 400 CCW, 0, and 400 CW respectively. The
lower image shows data points plotted along the profile and subsequently
curve fit to a 4th order polynomial.

This method can also provide crystallographic orientation information in addition

to tip geometry. By entering diffraction mode with the beam focused on the indenter tip,

a Kikuchi map image is produced. The crystallographic planes that are orthogonal to the

tilt axis remain parallel to the electron beam and therefore diffract through the range of

tilt. Since the normal loading axis of the indenter tip is coincident with the tilt axis the

planes that remain diffracting through the tilt, or the planes indexed to correspond to the

Kikuchi band upon which the tilt occurs, correspond to the planes normal to the loading

axis. One degree of freedom remains to describe the crystallographic orientation of the

indenter tip. By noting the tilt angle at zone axes along the Kikuchi band, the









crystallographic orientation of the tip is characterized, see Figure 6-6. Figure 6-7 shows a

schematic representation of the crystallographic orientations of the indenter tip and the

direction of reciprocation in the wear tests.

Once the crystallographic orientation of the indenter tip is characterized, elastic

properties of the tip can be calculated from the stiffness matrix. Single crystals are often

elastically anisotropic and therefore the Young's modulus and Poisson ratio are

orientation dependent [37].





2*ccw 26Scc 7*ccw m\ cw 38*cw

1]400)







Figure 6-6. Schematic representation of the Kikuchi map observed in diffraction mode
indexed such that the (100) plane is orthogonal to the tip loading axis. The
major zone axes are labeled with the sample holder tilt angle readings.

[010o





sliding direction


[0011


Figure 6-7. Schematic representation of a top down view of the single crystal diamond
indenter tip with crystallographic orientations and sliding direction indicated.
Note the loading axis of the indenter corresponds to the [100] direction as
indexed in the image.










The orientation dependent Young's modulus of a cubic single crystal is given by:

1 1 (6-1)
= i 2[(S- S12)- S44 (/2m2 +2n2 +/ 2n2)
E 2

where Euvw is the Young's modulus for the direction of interest; Si1, S12, and S44 are the

coefficients of the compliance matrix; and 1, m, and n, are the direction cosines of the

[uvw] with respect to the unit cell axes [38] Equation 6-1 contains the stiffness matrix

for diamond [39]


1079 124 124 0 0 0
124 1079 124 0 0 0
C 124 124 1079 0 0 0 G
S 0 0 0 578 0 0
0 0 0 0 578 0
0 0 0 0 0 578


which can be converted to the compliance matix by:


0.949 -0.0979 -0.0979 0 0 0
-0.0979 0.949 -0.0979 0 0 0

S. C 1 -0.0979 -0.0979 0.949 0 0 0 -1
U U 0 0 0 1.73 0 0
0 0 0 0 1.73 0
0 0 0 0 0 1.73/


From figure 6-7, the loading orientation of the indenter tip is characterized and therefore

the Young's modulus in the loading direction is calculated at 1053 GPa.

X-ray Diffraction

X-ray diffraction is used to characterize the grain size and the texture of the gold

films. A Philips APD 3720 diffractometer with a Cu Ka (X=1.54178 A) x-ray source is

used to scan the samples. Figure 6-8 shows a schematic drawing of the diffractometer.




















X-ray Source
Sample

Bragg's law representation










Figure 6-8. Schematic drawing of an x-ray diffractometer with a representation of
Bragg's law.

The x-ray source is in a fixed position, and the sample is rotated through 0. The x-ray

detector is rotated 2 for every 1 of the sample and is therefore rotated 20 with respect to

the x-ray source. A plot is generated containing the number of counts recorded by the

detector versus 20. From Bragg's law:

nA = 2d sin (6-2)

where n is an integer (1, 2, 3....), X is the wavelength of the radiation, d is the spacing of

the diffracting planes, and 0 is the Bragg angle, the spacing of the diffracting planes can

be determined. Samples are scanned from 300 to 1300 20 and data is collected to

determine crystallographic texture and to choose a family of planes to analyze for grain

size approximation.









Crystallographic Texture

Often in processing a certain crystallographic orientation develops due to

conditions. These conditions may be due to mechanical processing, thermal gradients

upon solidification, or preferred growth orientation. For thin films, the latter is often the

case. Since most single crystals exhibit anisotropic mechanical properties, it tends to

reason that polycrystalline samples with a preferred orientation will also exhibit similar

anisotropy.

Analysis of crystallographic texture by x-ray diffraction can range from

exceptionally simplistic when only one out-of-plane orientation exists, to rather complex

as the orientation becomes more random. For the simple case when only one out-of-

plane orientation exists, Bragg's law will only be satisfied for planes parallel to the

surface and they will be the only intensities that show up in the 20 scan. To analyze the

texture of a more random sample, one must consider the integrated intensities of the

various peaks that show up in the scan. This is not a simple direct comparison because

the intensity is a function of the diffracting planes and the Bragg angle through the

structure factor, multiplicity factor, and the Lorentz-polarization factor [40]. A simple

method to approximate the sample texture is obtained in the JCPDS files. Each file

contains space group, lattice parameter, planar spacings, and relative peak intensities for

perfect powder samples. A perfect powder sample is equiaxed, or randomly oriented,

therefore by comparing relative peak intensities collected from a sample of interest with

those of the JCPDS file, one can draw conclusions about the texture.

Samples were scanned from 300 to 1350 20 with a step size of 0.02. Figure 6-9

shows 20 scans for the six different gold coatings used in the test matrix. All six show a

strong (111) texture with no other significant orientations.











EBE 20
silicon (100)



gold
(111)

gold
(222)

0 40 60 800 10 120 140
20


EBE 200
500,000.

400,000 gold silicon (100)
(111)
< 300,000

200,000

100,000 gold
(222)
0 -
20 40 60 80 100 120 140
20



EBE400
1,400,000 g
gold (111)

=i 1,000,000

600,000
gold
200,000 (222)
0
20 40 60 80 100 120 140
20


10,000

8,000



4,000

2,000

0
2


IBS 100
silicon (100)


gold
(111)


gold
(222)
) 40 60 80 100 120 140
20


IBS 300
100,000
silicon (100)
80,000

60,000

40,000 gold
(111)
20,000 gold
(222)
20 40 60 80 100 120 140
20



EBE 500
1,200,000-
I rnnn n] gold (111)


800,000
600,000
400,000
silicon gold
2,00,00 (100) (222)

20 40 60 80 100 120 140
20


Figure 6-9. Scans of the six different gold films shown with indexed peaks.


Equiaxed polycrystalline gold has a Young's modulus of 78.5 GPa [41], whereas


single crystal gold is elastically anisotropic with the <111> orientation being the stiffest.


It is reasonable to assume that the textured film will exhibit the elastic properties of the


single crystal rather than those of an equiaxed polycrystalline sample, therefore the


Young's modulus is calculated to be EAu= 117 GPa using Equation 6-1 and the


stiffness matrix for gold:


10.000



oooo
8,000

4,000-

2,000-

0
2










190 42.3 42.3 0 0 0
42.3 190 42.3 0 0 0
42.3 42.3 190 0 0 0 GP
U 0 0 0 161 0 0
0 0 0 0 161 0
S0 0 0 0 0 161)

Grain Size

The grain size of a metal affects the yield strength by the Hall-Petch relationship

[37]:

1
o = o,+ kD 2 (6-3)


where Gy is the yield strength, D is the grain size, and Go and k are material parameters.

To elucidate the plastic response of a material, the grain size should be well

characterized.

Scherrer first showed that decreasing particle size resulted in slight deviations

from Bragg's law in diffraction due to incomplete destructive interference, which results

in peak broadening [42]. Peak broadening can be due to particle size, or grain size, and

residual strain, and is usually a combination of the two. Warren and Averbach devised a

method to represent diffraction peaks as Fourier series. They showed that by comparing

families of peaks, (111) and (222) for example, coefficients due to strain and grain size

could be separated because the strain coefficient is dependent upon the index of the

planes, whereas the grain size coefficient is not. These calculations have been

incorporated into software packages for stain and grain size analysis.

To collect data for the grain size and strain measurements, the diffractometer was

set to scan 2.50 at 38.20 20 and 2.50 at 81.70 20 which correspond to the (111) and











(222) respectively. To increase resolution, the step size was set at 0.050. The data was

entered in the Philips Line Profile software along with scans from standard samples with

similar d-spacing, and the Warren-Averbach grain size and strain analysis was

performed. Figure 6-10 shows results of the film grain size and strain with respect to the

film thickness. The grain size is increasing with film thickness up to 45 nm grains in

films over 300 nm thick. All films show 0.1% compressive strain except the sample

designated EBE20 with a film thickness of 27 nm and 0.22% compressive strain.

50 0.25
45. m a
. 40 02




20 0.1 U
1S0
'S






10 0.05
s
0 0
0 100 200 300 400 500 600 0 100 200 300 400 500 600
thickness (nm) thickness (nm)

Figure 6-10. Plots of grain size and strain versus gold film thickness.

Scanning Electron Microscopy

Samples were imaged to examine wear tracks and determine film failure in an FEI

DB235 field emission gun (FEG) microscope by first locating the test area from the grid

lines that were milled prior to testing. Once the area was located, magnification was

increased and the sample was oriented so that the direction of wear was along the

horizontal, or x-axis. The sample was then tilted about that axis 52 to provide better

perspective. Test areas were imaged at 20-25 kX magnification using a secondary

electron detector, 30 [tm aperture, 5 keV accelerating voltage and number 2 spot size.

Some samples displayed beam sensitivity, so image quality was sacrificed to protect the









sample from repeated rastering with the beam. Each wear test from the matrix was

imaged and examined for coating damage and breakdown.

Focused Ion Beam Milling

A Dual Beam FEI focused ion beam (FIB) DB235 microscope was used for grid

line etching as mentioned earlier. This microscope is equipped with a field emission gun

(FEG) high resolution SEM column situated vertical. Mounted 52 with respect to the

electron column is a liquid metal ion source (LMIS) FIB column using gallium for the

ion source. The microscope has a gas injection system (GIS) which allows platinum

deposition by heating the organic-metallic compound methylcyclopentadienyl (trimethyl)

platinum to 313 K and then opening the injection system in the microscope chamber.

The low pressure of the microscope chamber (- 105 Torr) causes the compound to out

gas and adsorb on the sample surface. Scanning with the ion beam reduces the

compound leaving a platinum layer on the surface. The area of platinum deposition can

be controlled by the rastering area. The microscope is also equipped with an Omniprobe

Autoprobe 200 which provides in-situ mechanical micro-manipulation.

Transmission Electron Microscopy Sample Thinning

Samples for TEM examination must be at least less than 200 nm thick for most

materials. For HRTEM, samples must be typically less than 100 nm thick. Producing

cross sectional TEM samples of a uniform thin film using conventional techniques is

challenging. Attempting to produce site specific cross sectional TEM samples with

conventional techniques of micron sized features would require a degree of luck that

would be better suited for playing the Florida Lottery. Fortunately, the FIB microscope

has the capability of locating micron sized sites and milling thin cross sections from

them.








Once the wear tests were finished, samples were selected for TEM examination.

On those samples, the wear tests were located using the grids lines. The wear test was

oriented such that the cross section would be orthogonal to the sample surface, and either

orthogonal or parallel to the wear direction. For cross sections that were parallel to the

wear direction, indicator marks were etched for final thinning process (Figure 6-11).

Fiducial 'x' marks were milled for automated milling.


Figure 6-11. SEM micrograph of wear track in center of image to be prepared with cross
sectional TEM parallel to the wear direction. Both indicator and fiducial
marks milled by ion beam are indicated with arrows.
Platinum was deposited 2 um x 15 um x 1 um high to protect the area of interest while

milling. Trenches approximately 15 atm x 5 tm were milled at 5 nA beam current to 6

tm deep at the base and sloped out to about 1 tm deep on both sides of the desired cross

section area. Progressively lower beam currents were then used to remove more material

and reduce ion damage. Once the sample is thinned to approximately 1 atm thick, the

stage is tilted and the sample is undercut with only small tabs of material keeping the


lom









cross section connected to the bulk. The sample is then tilted back and the final

automated polishing steps are performed at 300 pA current to thin the sample to

approximately 300 nm. Once the automated script is complete, the thinning is controlled

manually at 30 pA to thin the sample to less than 100 nm thick. The cross section is then

cut free with the 30 pA beam current, and the sample is removed from the FIB. Figure 6-

12 illustrates the steps.

a) b) c)









d) e) f)










Figure 6-12. FIB cross section sample preparation. A) area of interest is located and
fiducial marks are milled. B) platinum is deposited. C) front trench is
milled. D) back trench is milled. E) cross section is undercut. F) cross
section is cut free.

The sample is then transferred to an optical microscope equipped with hydraulic

micromanipulators. Glass rods are heated and pulled in tension to form sharp radius tips

of approximately 1 am radius. The rods are attached to the hydraulic micromanipulators

for precise movement. The cross section area is imaged with the optical microscope and

the glass rod is brought into contact with the cross section. The cross section is attached

to the rod by static forces and it is lifted out of the sample trench. The sample is removed









and replaced with a carbon film coated copper TEM grid. The static attraction between

the carbon film and the cross section is greater than the attraction between the cross

section and the glass rod so the cross section can be deposited on the carbon film. The

process is illustrated in Figure 6-13. The cross section is now ready for TEM

examination.



















Figure 6-13. Cross section is removed from the sample trench with a glass rod attached
to a hydraulic micromanipulator and then deposited on a carbon film coated
copper TEM grid.

Slice and View Serial Sectioning

The FEI DB235 FIB microscope is capable of milling with the ion beam, followed

by imaging with the electron beam. Using these techniques, a series of images can be

produced similar to tomography. From these images, movies can be assembled to

produce a view of 'moving' through the bulk of a sample and with the aid of computer

software, three dimensional reconstructions of the sectioned sample can be generated.

There are alternate methods for producing a series of slice and view images. The first









described will be referred to as the 'fixed sample method.' The second technique will be

referred to as the 'rotated sample method.'

Fixed sample method

In this method, the sample is not rotated once the sectioning process has begun.

The sample is oriented orthogonal to the ion beam or 52 from the electron beam. A

sloped trench is milled to clear material so the sectioning area can be imaged with the

electron beam (Figure 6-14). Some bulk material from around the area of interest can be

removed prior to sectioning to aid in imaging.


E-em Spo IIit VV u


Figure 6-14. SEM image of trenched area in preparation of fixed sample method slice
and view serial sectioning procedure.

A series of ion beam slices, each followed by an electron beam image, is recorded. The

fixed sample method can be automated using scripting software which requires initial set

up but then runs to completion without user interaction.