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1 SURFACE STUDIES OF DRY AND SOLID LUBRICANTS UNDER DIFFERENT ENVIRONMENTAL CONDITIONS By GREGORY JAMES DUDDER A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUI REMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010
2 2010 Gregory James Dudder
3 To who else Mom and Dad
4 ACKNOWLEDGMENTS I am indebted to many individuals over the course of my four years in Gainesville, F lorida, for w i thout whom this document concluding my time here would not be possible. I first am thankful f or the guidance ; scientific professional and pyrotechnic al offered by my advisor Scott Perry. His practice of thorough ly examining all aspects of a problem and double checking the details served as an exemplary scientific role model helped to sharpen my critical thinking skillset and provide d me with a new respect for the virtue of patience Thanks are also due to Brandon Krick and Gregory Sawyer for supplying the original pin on disc tribometer design, offering philosophical advice, evaluat ion critique of the in vacuo version of the apparatus and help r esolv ing issues during calibration and setup. Ira H ill w as also integral in the process authoring the data acquisition software and taking the time to assist with challenges that arose during the initial trials. Lastly, I once again thank my parents for their long distance support and encouragement to journey to new places, see new sites, and conquer new challe nges.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 13 ABSTRACT ................................ ................................ ................................ ................... 22 CHAPTER 1 APPLICATIONS OF DRY AND S OLID LUBRICATION ................................ .......... 24 Solid Lubricants ................................ ................................ ................................ ...... 24 Processing Methods for Solid Lubricants ................................ ................................ 27 Improving MoS 2 Performance with Additives ................................ .......................... 30 Characterization of Solid Lubricant Coatings ................................ .......................... 37 Vapor Phase Lubr ication ................................ ................................ ......................... 41 Direction ................................ ................................ ................................ .................. 44 2 THE IMPLEMENTATION OF AN IN VACUO PIN ON DISC TRIBOMETER AND SURFACE CHARACTERIZATION METHODOLOGY ................................ ............ 46 Experimental Philosophy ................................ ................................ ........................ 46 Design Philosophy ................................ ................................ ................................ .. 47 Sample Transfer S ystem ................................ ................................ .................. 47 Environmental Chamber ................................ ................................ ................... 48 In Vacuo Pin on Disc Tribometer ................................ ................................ ............ 49 Tribometer Arm ................................ ................................ ................................ 50 Piezo Actuator ................................ ................................ ................................ .. 51 Rotating Sample Stage ................................ ................................ .................... 51 Motor and Reducing Gearbox ................................ ................................ .......... 52 Mounting Block ................................ ................................ ................................ 52 Power and Signal Connections ................................ ................................ ........ 53 Pin on Disc Testing Parameters ................................ ................................ ....... 54 Characterization Methods ................................ ................................ ....................... 54 X ray Diffraction ................................ ................................ ................................ 54 X ray Photoelectron Spectroscopy ................................ ................................ ... 56 Atomic Force Microscopy ................................ ................................ ................. 58 Experimental Methodology ................................ ................................ ..................... 60 3 THE TRIBOLOGY OF TRADITIONAL SOLID LUBRICANTS ................................ 69
6 Traditional Solid Lubricants ................................ ................................ ..................... 69 Experimental ................................ ................................ ................................ .... 74 Results ................................ ................................ ................................ ............. 76 Conclusions ................................ ................................ ................................ ...... 80 4 T HE TRIBOLOGY OF MOS2 SB2O3 AU COMPOSITE COATINGS ..................... 85 Introduction ................................ ................................ ................................ ............. 85 Experimental ................................ ................................ ................................ ........... 87 Samples ................................ ................................ ................................ ........... 87 X ray Diffraction ................................ ................................ ................................ 87 Pin on Disc Tribometry ................................ ................................ ..................... 87 X ray Photoelectron Spectroscopy ................................ ................................ ... 88 Atomic Force Microscopy ................................ ................................ ................. 89 Results ................................ ................................ ................................ .................... 89 As Received Film Characterization ................................ ................................ .. 89 Pin on Disc Tribometry ................................ ................................ ..................... 91 X ray Photoelectron Spectroscopy Measurem ents ................................ .......... 91 Topography and Microtribometry Measurements ................................ ............. 94 Conclusions ................................ ................................ ................................ ............ 96 5 THE TRIBOLOGY OF MOS2 SB2O3 C COMPOSITE COATINGS ..................... 112 Introduction ................................ ................................ ................................ ........... 112 Experimental ................................ ................................ ................................ ......... 115 Samples ................................ ................................ ................................ ......... 115 X ray Diffraction ................................ ................................ .............................. 115 Tribometry ................................ ................................ ................................ ...... 115 X ray Photoelectron Spectroscopy ................................ ................................ 117 Atomic Force Microscopy ................................ ................................ ............... 118 Results ................................ ................................ ................................ .................. 118 As Received Film Characterization ................................ ................................ 118 Pin on Disc Measurements ................................ ................................ ............ 119 X ray Photoelectron Spectroscopy Measurements ................................ ........ 120 Topography and Microtribometry Measurements ................................ ........... 123 Discussion ................................ ................................ ................................ ............ 125 Conclusions ................................ ................................ ................................ .......... 128 6 THE TRIBOLOGY OF ALCOHOLS ON NATIVE SILICON OXIDE ....................... 142 Introduction ................................ ................................ ................................ ........... 142 Experimental ................................ ................................ ................................ ......... 145 Results ................................ ................................ ................................ .................. 147 Discussion ................................ ................................ ................................ ............ 150 Conclusions ................................ ................................ ................................ .......... 151 7 CONCLUSIONS ................................ ................................ ................................ ... 158
7 Solid Lubricants ................................ ................................ ................................ .... 158 Dry Lubricants ................................ ................................ ................................ ....... 161 In Vacuo Pin on Disc Tribometer ................................ ................................ .......... 161 APPENDI X A X RAY DIFFRACTION PEAK ANALYSES ................................ ........................... 164 B THERMODYNAMIC CALCULATIONS ................................ ................................ 1 66 C HERTZIAN CONTACT PRESSURE CALCULATIONS ................................ ......... 171 LIST OF REFERENCES ................................ ................................ ............................. 175 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 192
8 LIST OF TABLES Table page 4 1 The parameters of MoS 2 Sb 2 O 3 Au pin on disc tests ................................ ....... 101 5 1 The parameters of MoS 2 Sb 2 O 3 C pin on disc tests ................................ ......... 132 A 1 The m easured X r ay diffraction p eaks from the mixed 316 stainless steel /as received MoS 2 Sb 2 O 3 C film. ................................ ................................ ............ 164 A 2 The m easured X ray diffraction p eaks from the a s r eceived MoS 2 Sb 2 O 3 C film. ................................ ................................ ................................ ................... 165 B 1 Thermodynamic values for different species at 298.15 K elvin ......................... 167 C 1 Values used to calculate contact pressures for high and low contact pre ssure conditions. ................................ ................................ ................................ ........ 174
9 LIST OF FIGURES Figure page 1 1 Types of physical vapor deposition film architectures ................................ ......... 45 2 1 Schematic of an Omicron GhmB platen shown with a mounted sample. ........... 62 2 2 Schematic of the platen sample holder mounted to the end of the magnetically coupled tra nsfer arm rod. ................................ ............................... 62 2 3 Diagram of the vacuum complex with transfer arm system. ............................... 63 2 4 The symbol key for the vacuum diagram of th e environmental test chamber and dosing system. ................................ ................................ ............................. 64 2 5 The vacuum diagram of the environmental test chamber and dosing system .... 65 2 6 The finalized SolidWorks design for the in vacuo pin on disc tribometer. .......... 66 2 7 Schematic of the X ray diffraction process ................................ ......................... 67 2 8 Schematic of the X ray photoemission process of a neon atom. ........................ 67 2 9 Schematic of an atomic force microscope measurement ................................ ... 68 3 1 The crystal structure models of tr aditional solid lubricants ................................ 81 3 2 Average coefficient of friction versus cycles for highly oriented pyrolytic graphite pin on disc testing ................................ ................................ ................ 82 3 3 X ray photoelectron spectroscopy measurements of freshly cleaved highly oriented pyrolytic graphite, after 30 second exposure to ozone ......................... 83 3 4 X ray photoelectron spe ctroscopy measurements of freshly cleaved highly oriented pyrolytic graphite after 3 hours exposure to ozone ............................... 83 3 5 X ray photoelectron spectroscopy measurements of OFF wear track, ON wear tra ck, and the ON OFF wear track difference of highly oriented pyrolytic graphite ................................ ................................ ................................ .............. 84 4 1 Illustration of low angle X ray diffraction measurement of MoS 2 Sb 2 O 3 Au coating. ................................ ................................ ................................ ............... 98 4 2 X ray diffraction measurement of MoS 2 Sb 2 O 3 Au coating taken at a angle ................................ ................................ ................................ .................... 98 4 3 Atomic force microscopy topographical images of the as received MoS 2 Sb 2 O 3 Au coating ................................ ................................ ................................ 99
10 4 4 Atomic force m icroscopy topographical images of the low friction feature within the as received MoS 2 Sb 2 O 3 Au coating taken at ................................ .. 100 4 5 Atomic force microscopy topographical images of the void feature within the as received MoS 2 Sb 2 O 3 Au coating taken at ................................ ................... 101 4 6 Graph showing the average coefficient of friction versus cycles of MoS 2 Sb 2 O 3 Au coatings under different environmental conditions for 5 000 cycles clockwise. ................................ ................................ ................................ ......... 102 4 7 Illustrations of X ray photoelectron spectroscopy measurements taken of. ...... 102 4 8 X ray photoelectr on spectra of unworn MoS 2 Sb 2 O 3 Au ................................ ... 103 4 9 X ray photoelectron spectroscopy measurements of the change in the ratio from off the wear track region to on the wear track region of MoS 2 Sb 2 O 3 Au coatings. ................................ ................................ ................................ ........... 104 4 10 Optical images of atomic force microscope cantilever measurement positions on ................................ ................................ ................................ ..................... 104 4 11 Atomic force microscop y topographical images of the as received MoS 2 Sb 2 O 3 Au coating ................................ ................................ .............................. 105 4 12 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 Au coating produced by sliding under 760 T orr air (5 0% relative humidity) ................................ ................................ ................................ ......... 106 4 13 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 Au coating produced by sliding under 10 7 Torr vacuum ....................... 107 4 14 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 Au coating produced by sliding under 150 Torr oxygen ......................... 108 4 1 5 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 Au coating produced by sliding under 8 Torr water ............................... 109 4 16 Atomic force microscopy topographical images of variou s MoS 2 Sb 2 O 3 Au coatings ................................ ................................ ................................ ............ 110 4 17 Atomic force microscopy topographical images of MoS 2 Sb 2 O 3 Au coating ..... 111 5 1 Illustrations o f the low angle X ray diffraction measurements taken of ............. 130 5 2 X ray diffraction measurements from MoS 2 Sb 2 O 3 C coatings taken at a low angle. ................................ ................................ ................................ ............. 130 5 3 Atomic force microscopy topographical images of the as received MoS 2 Sb 2 O 3 C coating ................................ ................................ .............................. 131
11 5 4 Graph showing the average coefficient of friction versus cycles of MoS 2 Sb 2 O 3 C coatings under different environmental conditions for 300 cycles clockwise. ................................ ................................ ................................ ......... 132 5 5 Illustrations of X ray photoelectron spectroscopy measurements ..................... 133 5 6 X ray photoelectron spectra of unworn MoS 2 Sb 2 O 3 C ................................ .... 134 5 7 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 C coating produced by sliding under 760 Torr air (50% relative humidity) ................................ ................................ ................................ ........... 135 5 8 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 C coating produced by sliding under 10 7 Torr vacuum ......................... 136 5 9 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 C coating produced by sliding under 150 Torr oxygen .......................... 137 5 10 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 C coating produced by sliding under 8 Torr water. ................................ 138 5 11 Atomic force microscopy topographical im ages various MoS 2 Sb 2 O 3 C coatings ................................ ................................ ................................ ........... 139 5 12 Atomic force microscopy topographical images of MoS 2 Sb 2 O 3 C coating ....... 140 5 13 Schem atic of an atomic force microscope probe tip scanning over a rectangular Sb 2 O 3 particle imbedded on the surface of an as received MoS 2 Sb 2 O 3 C film. ................................ ................................ ................................ .... 140 5 14 X ray photoelectron spectroscopy m easurements of the change in the ratio from off the wear track region to the wear track region of MoS 2 Sb 2 O 3 C coatings. ................................ ................................ ................................ ........... 141 6 1 Graph showing the average coefficient of friction versus cycles of native oxide silicon (100) under different environmental conditions for 1000 cycles clockwise. ................................ ................................ ................................ ......... 153 6 2 Graph showing the average coefficient of friction versus cycles of native oxide silic on (100) between 1 Torr pentanol and vacuum ................................ 153 6 3 Graph showing the initial average coefficient of friction versus cycles of native oxide silicon (100) under different conditions ................................ ......... 154 6 4 The average coefficient of friction versus cycles for the decreasing pentanol test. ................................ ................................ ................................ ................... 154
12 6 5 X ray photoelectron spectroscopy measur ement of the carbon 1 s spectra for (bottom to top): adsorbed pentanol, the tribofilm produced in Torr pentanol, and the worn tribofilm under vacuum ................................ ................................ 155 6 6 X ray photoelectron spectroscopy mea surement of the silicon 2 p spectra for (bottom to top): adsorbed pentanol, the tribofilm produced in Torr pentanol, and the worn tribofilm under vacuum ................................ ................................ 156 6 7 X ray photoelectron spectroscopy measurement of the oxygen 1 s spectra for (bottom to top): adsorbed pentanol, the tribofilm produced in Torr pentanol, and the worn tribofilm under vacuum ................................ ................................ 157 C 1 Schematics of. ................................ ................................ ................................ .. 171
13 LIST OF ABBREVIATION S AES Auger electron spectroscopy, a surface sensitive spectroscopic technique that uses X rays or electrons to produce Auger electrons from a sample. The emitted electrons are collected as a function of their kin etic energy. Characteristic binding energies are back calculated to give elemental information. AFM Atomic force microscopy, a microscopy technique that uses a sharp probe mounted on a cantilever to detect van der Waals forces between the sample and probe tip. This technique is able to give atomic scale topographical, normal force, and lateral force information. AFRL Air Force Research Laboratory ASTM American Society for Testing and Materials BCC Body centered cubic, a crystal structure with a cubic basis and an additional atom located at the center of the cube. DFT Density functional theory, computer modeling technique that uses electron population densities to analyze chemical states EDS Energy dispersive spectroscopy, a characterization technique utilize d by scanning tunneling microscopes that uses high energy electrons with a constant energy to probe a sample, the scattered secondary electrons are analyzed as a function of energy. This provides elemental information relating to the sample. FCC Face cente red cubic, a crystal structure with a cubic basis and atoms located at the center of each side plane of the cube. FIB Focused ion beam, a machining technique that uses heavy ions to produce side cut samples for transmission electron microscopy. FTIR Fouri er transform infrared spectroscopy, a spectroscopic technique that uses infrared photons to probe a sample. Specific chemical formations absorb the energized photon, vibrate, and transmit the photon with a reduced energy. Photons are measured as a function of wavelength and chemically specific vibrations are indicated by a lack of intensity at correlated vibrational wavelength values. FWHM Full width at half maximum, the width of a peak defined by the value at half the maximum intensity of the peak.
14 HCP He xagonal close packed, a crystal structure with a hexagonal basis and three symmetrical atoms located within the plane horizontally bisecting the hexagon. LASER Light amplification stimulated emission ray LEED Low energy electron diffraction, a surface sens itive crystal structure characterization technique that uses low energy electrons to probe the sample. LFM Lateral force microscopy, a surface characterization technique that utilizes an atomic force microscope to measure and image the magnitude of the lat eral force signal during scanning. ISS International space station M1.6 Metric fastener thread size of 1. 6 mm diameter by 0.4 mm pitch M2 Metric fastener thre ad size of 2.0 mm diameter by 0.4 mm pitch MAIC Major analytical instrumentation center MISSE 7 M aterials international space station experiment 7, a series of tribological pin on disc experiments operated on the exterior of the international space station NASA National Aeronautics and Space Administration PVD Physical vapor deposition, a category o f processing techniques that is defined by the use of ions, photons, or electrons to impact target materials used as sources for thin film deposition. RPM Revolutions per minute SAM Self assembled monolayer SDP Sputter depth profiling, a characterization t echnique that uses inert, high energy ions to sputter a sample, removing the surface atoms for spectroscopic characterization in conjunction with either auger electron spectroscopy or X ray photoelectron spectroscopy. SEM Scanning electron microscopy, a mi croscopy technique that uses electrons to probe electronically conductive samples for microstructure and elemental information. STM Scanning tunneling microscopy, a microscopy technique that uses a conductive tip to probe the surface of a conductive sample The tip is passed over the surface and the electrical current magnitude between the tip and surface is collected. Atomistic topographical
15 analysis and some spectroscopic information is possible with this technique. TEM Tunneling electron microscopy, a mi croscopy technique that uses electrons to probe extremely thin samples for microstructure, elemental, and crystal structure information. TMD Transition metal dichalcogenide, a group of compounds which are stoichiometrically defined as being made of one tra nsition metal atom and two chalcogenide atoms. TOF SIMS Time of flight secondary ion mass spectrometry, a characterization technique which measures the mass of ejected components of a sample under ion bombardment. The mass is determined by measuring the ti me it takes a component to travel over a known distance with a constant excitation energy. UHV Ultra high vacuum, designated as pressures less than 10 9 mbar UV Ultra violet, a bandwidth of energized photons with wavelengths between 100 350 nm VPL Vapor ph ase lubrication/lubricant XPS X ray photoelectron spectroscopy, a spectroscopic characterization technique that uses X rays to excite core shell electrons from a sample. Emitted electrons are collected as a function of their kinetic energy. Characteristic binding energies are back calculated to give elemental and chemical quantitative information. XRD X ray diffraction, a crystal structure characterization technique that uses X rays to probe the sample.
16 Elements Compounds and Materials 304SS 304 stainles s steel, an austenitic steel alloy that contains, by weight %, 0.08% carbon, 19% chromium, 9% nickel, and 2.0% titanium 316SS 316 s tainless steel, an austenitic steel alloy that contains, by w eight %, 0.03% carbon, 17% chromium, 12% nickel, 2.5% molybdenum, and 2.0% magnesium Used as a substrate material 440C 440C s tainless steel, a martensitic stainless steel alloy that contains by w eigh t% : 0.7% carbon, 17% chromium, 0.75% Nickel, and 1.0% Molybdenum. Used as a pin material Ag Silver Al Aluminum Al 2 O 3 Alumi na, dialuminum trioxide Ar Argon Ar + Argon ion As Arsenic a n n type dopant in silicon wafers Au Gold BiTiO 3 Bismuth Titanate, a piezoelectric material BN Boron nitride B 2 O 3 Diboron trioxide C Carbon C Carbon chain m olecule with number of carbo n atoms Co Cobalt Cr Chromium D LC Diamond like c arbon, an allotrope material of carbon defined by the predominance of sp 3 hyb ridiz ed, covalent bonding between carbon atom s Fe Iron Fe 3 C Iron carbide, cementite
17 H Hydrogen H 2 Molecular Hydrogen (gas phase) H 2 O Wate r HOPG Highly oriented pyrolytic graphite, an allotrope material of carbon defined by the predominance of sp 2 hybridized, covalent bonding between carbon atoms and a planar (001) orientation. H 2 S Hydrogen sulfide H 2 SO 4 Hydrogen sulfate In Indium MoO 3 Molyb denum trioxide MoS 2 Molybdenum disulfide MoSe 2 Molybdenum diselenide MoTe 2 Mo l ybdenum ditelluride N 2 Molecular Nitrogen (gas phase) Nb Niobium NbS 2 Niobium disulfide O Oxygen O 2 Molecular oxygen (gas phase) Pb Lead PbO Lead Oxide PEEK Polyetheretherketone PFPE Perfluoropolyether PI Polyimide PTFE Polytetrafluoroethylene S Sulfur Sb Antimony
18 Sb 2 O 3 Diantimony trioxide Sb 2 S 3 Diantimony trisulfide Se Selenium Si Silicon SiC Silicon Carbide Si 3 N 4 Silicon Nitride SiO 2 Silica, silicon dioxide SO 2 Sulfite Ta Tanta lum TaSe 2 Tantalum diselenide Te Tellurium Ti Titanium Ti6Al4V Titanium alloy containing by weight %, 90% titanium, 6% aluminum, and 4% vanadium TiC Titanium carbide TiN Titanium nitride W Tungsten WC Tungsten carbide WS 2 Tungsten disulfide WSe 2 Tungsten d iselenide YSZ Yttria stabilized zirconia, an oxide material defined by the minority substitution of yttria into zirconium sites of zirconia. Zn Zinc Zr Zirconium ZrO Zirconia
19 Units of Measurement at% Atomic percentage Degrees Celsius, a unit of tempera ture equal to degrees Kelvin 273.15 Zero is defined by the temperature of the solid/liquid phase change of water under 1 atmosphere of pressure cm Centimeter, a unit of length equal to 10 2 meters eV Electron Volt, a unit of energy = 1.6 19 Joules g Gr am, a unit of mass equal to 1/12 th the mass of a mole of carbon GPa Giga Pascal, a unit of pressure equal to 10 9 Pascals = 10 9 N/m 2 J Joule, a unit of energy equal to 1 kgm 2 /s 2 K Kelvin, a unit of absolute temperature = degrees Celsius + 273.15 Zero is d efined by absence of any thermal vibration of a molecule. k g K ilogram a unit of mass equal to 10 3 grams kJ KiloJ oule, a unit of energy equal to 10 3 Joules = 10 3 kgm 2 /s 2 L Liter, a unit of volume equal to 1000 cm 3 = 10 3 m 3 m Meter, a unit of length defin ed in France m Micrometer, a unit of length equal to 10 6 meters mL Milliliter, a unit of volume equal to 1 cm 3 = 10 6 m 3 m m M illimeter, a unit of length equal to 10 3 meters mol Mole, a unit of quantity equal to 6.023 10 23 units MPa Mega P ascal, a unit of pressure e qual to 10 6 Pascals = 10 6 N/m 2 N Newton, a unit of force = 1 kg m/s 2 nm N anometer, a unit of length equal to 10 9 meters Pa Pascal, a unit of pressure = 1 N/m 2 = 1 kg/m s 2 RH % Relative humidity percentage s Second, a unit of time equal to how long it takes light to travel 299 792 458 meters in vacuum
20 W Watt, a unit of power equal to 1 J / s = 1 kg m 2 /s 3 wt% Weight percentage
21 Symbols Coefficient of frict ion, equal to the lateral force divided by the normal force experienced between two bodies during a tribological interaction Also known as the kinetic or sliding coefficient of friction. the later al strain divided by the longitudinal strain of a material under a tensile load. BE Binding energy of an electron orbital, equal to the amount of energy required to remove an electron from orbit to the vacuum. E Elastic modulus, a material property for the amount of force/unit area required to elastically strain a sample B by which diffr action within a crystalline solid takes place Incident angle of a directed X ray source to the plane of the sample surface Wavelength of a photon Plan c rate of a photon, 6.62610 34 Js = 6.62610 34 kgm 2 /s Frequency of a photon Work function, the amount of energy required to remove an electron from the top of the valence band to vacuum
22 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 SURFACE STUDIES OF DRY AND SOLID LUBRICANTS UNDER DIFFERENT ENVIRONMENTAL CONDITIONS By Gregory James Dudder December 2010 Chair: Scott S. Perry Major: Materials Science and Engineering Advanced lubrication schemes depend on the presence of specific solids at or the continuous delivery of a gas to the sliding interface to manage friction and wear and are known to have a stro ng environmental dependence. An in vacuo pin on disc tribometer was designed to allow controlled environmental testing of the solid lubricants in order to determine the role of atmospheric components on the ir frictional behavior Solid lubrication testing of highly oriented pyrolytic graphite, MoS 2 Sb 2 O 3 Au, and MoS 2 Sb 2 O 3 C films was carried out under environments of 760 Torr air (50% relative humidity), 150 Torr oxygen, 8 Torr water, 610 Torr nitrogen, and 10 7 Torr vacuum. Dry lubrication testing of the native oxide of silicon (100) surfaces was carried out under environments of 760 Torr air (50% relative humidity), 1 Torr pentanol, and 10 7 Torr vacuum. Pin on disc tribometry revealed a strong dependence of friction and wear as a f unction of sliding envi ronment. MoS 2 Sb 2 O 3 Au and MoS 2 Sb 2 O 3 C films were strongly affected by the presence of water molecules. Friction and wear were observed to increase in the presence of partial pressures of water when compared to vacuum, oxygen, and nitrogen
23 environmental testing. Spectroscopic analysis of the MoS 2 Sb 2 O 3 Au and MoS 2 Sb 2 O 3 C films showed a general trend of MoS 2 expression at the surface of low friction wear tracks. However, high friction could not be directly linked to the expression of a specific species within the wear track. X ray photoelectron spectroscopy of the tracks created under ambient and water environments yielding high friction showed no clear relationship between the two conditions, even though their frictional behavior was similar. As revea led by atomic force microscopy measurements, the microstructure of the wear tracks of MoS 2 Sb 2 O 3 Au films produced under vacuum were predominantly low friction MoS 2 The vacuum wear tracks of MoS 2 Sb 2 O 3 C films showed a mixed microstructure with both low f riction MoS 2 and moderate friction C. Ambient wear tracks for MoS 2 Sb 2 O 3 Au films contained a minor, higher friction constituent, identified as Au, in the presence of the major constituent, MoS 2 Ambient wear tracks for MoS 2 Sb 2 O 3 C films were more comple x, expressing a majority of higher friction Sb 2 O 3 and graphite constituents, with a reduced fraction of MoS 2 These micro tribometry measurements correlated well with those made by the pin on disc tribometer and X ray photoelectron spectroscopy characteriz ation. Dry or vapor phase lubrication methods employing 1 pentanol were effective in reducing the friction of silicon (100) compared to sliding under ambient or vacuum environments. A continuous supply of 1 pentanol served to lubricate silicon surfaces thr ough the formation of a tribochemical film which was composed primarily of ( CH 2 ) x species However, the presence of a tribofilm was not responsible for the lowered friction coefficient but it did enable the extreme wear protection previously reported fo r this lubrication technique.
24 CHAPTER 1 APPLICATIONS OF DRY AND SOLID LUBRICATION The use of liquid based lubricants is the most familiar type of lubrication method for many individuals. H owever, there are other schema s that can be employed depending upon the requirements of the application. This document focuses on two alternative lubrication methods. Solid lubrication i nvolves sliding contacts in which interfacial shear is observed to occur within layers of a solid, often one that has been added in the fo rm of a thin coating Solid lubricants are heavily utilized in aerospace applications due to the unique demands of a vacuum environment. Dry lubricants function quite differently than solid lubricants. The lubrica n t in this methodology resides in a gaseous or vapor phase in the area surrounding the sliding contact and serves to lubricate the contact upon condensation at the sliding interface. The consistent replenishment of th e lubricant is one of its attractive benefits. Dry lubrication is thought to be a viable candidate for micro electromechanical systems (MEMS) These advanced, microscopic devices possess extreme operating conditions in which traditional liquid and solid lubricants cannot perform effectively Solid Lubricants One such extreme operating c ondition is low earth orbit and extraterrestrial space. The design, construction, and operation of satellites, orbiters, and other extra terrestrial platforms may seem at present like a mature technolog y However, space technologies were born from the scie ntific and engineering surge that took place during World War II in the 20 th century. As is the case in many fields of technology, some of the greatest advances came from the most desperate of times. The creation of the V2 solid rocket, high altitude aircr aft, and trajectory calculation machines s erve d as the foundation for
25 the Space Race in the decades to follow. On October 4 th 1 957 Sputnik 1 was successfully launched 1 Th e placement of a man made object into space was an event had a profound effect on th e human paradigm and demonstrated the future ability to collect and distribute information from new, unique perspectives. As the size, duration, and complexity of extra terrestrial apparati grew, so did the push for better solid lubrication of the mechanic al components involved The need for solid phase lubrication of space vehicles arises from the primary environment of operation, a vacuum. Traditional liquid lubricants will simpl y vaporize 2 limiting any functionality as well as damaging sensitive systems and components nearby. As a field, aerospace solid lubricants can be divided into f ive distinct material groups: 1. Soft metals 2. Lamellar crystalline solids 3. Polymers 4. Diamond like carbon (DLC) 5. Composites The first category of solid lubricants en tails soft metals. Soft metals primarily have a face centered cubic ( FCC ) crystal structure In turn, they often exhibit energetic efficiency in sliding as a result of inherent slip systems 3 Metals that have been successfully tested and em ployed for aerospace applications include gold (Au) silver (Ag) lead (Pb) and indium (In) Although In has a body centered cubic ( BCC ) crystal structure 2,4,5 its low melting point and inherently low hardness render it a viable candidate system. In gene ral, the low shear strength of this collection of metals en ables them to be effective lubricants, however soft metals also experience relatively high wear rates at room temperature 6 This limits their function and mechanical applications and better perform ing materials have been identified for room temperature applications
26 However, at temperatures above 500 C metal lubricants offer the potential for lubrication due to their large atomic masses and related low vapor pressures. As a result they have been successfully used in applications that experience high temperatures where other lubricants would evaporate or undergo thermal degradation Soft metals possess an 4 Materials with a layered crystal structure constitute the second category and some are thought to be intrinsically lubricious 7,8 Their value as lubricant s is thought to derive from their atomic structure. Intrinsic lubricants may be endowed with low shearstrength between layers m acroscopically embodied by their ability to peel and delaminate easily. Their layered crystal structure creates highly anisotrop ic properties. S hearing transverse to the layers is much more difficult. This gives some materials within this class great mechanical and thermal stability. Graphite and molybdenum disulfide ( MoS 2 ) are examples of layered materials that have high melting p oints, >2000 5 T ransition metal dichalcogenides (T MD s) are stable up 450 C where they dissociate under high vacuum conditions of 10 8 to 10 9 Torr 9 The preferred film orientation is normal to the layer, exhibited in the (001) family 10 Graphite, talc mica, and molybdenite ( naturally occurring MoS 2 ) are all layered materials that are geological in origin and have historically been used pr ior to the age of space exploration 11 Examples of synthetically produced layered materials used for solid lubricat ion include tungsten diselenid e ( WSe 2 ) and boron nitride ( BN ) 12,13 The third category consists of polymeric materials such as polytetrafluoroethylene (PTFE) polyimides (PI), and polyetheretherketone (PEEK) The se materials have been shown to be effective for lubric ation in vacuum due to their relative inertness compared to other polymeric materials 2,14 However, they exhibit glass transition temperatures less
27 than 300 and thus are not suited for applications with thermal or direct solar (ultra violet ) exposure 2,5 The fourth category of solid lubricants is made up of materials belonging to the class of synthetic diamond like carbon (DLC) films. The se materials possess a majority of sp 3 hybridized bonding between carbon atoms, whereas graphite has sp 2 hybridized bonding 6 These films have an amorphous structure, with little long range order. They can be treated with hydrogenation for enhanced vacuum performance 15 Currently, a fifth c ategory of solid lubricants entails the incorporation of a nano filler into a vari ety of materials matrices and is being evaluated for cyclical operation between multiple extreme environmental conditions 16 These often contain more than one solid lubricant component depending upon th e intended operating parameters and are processed usin g complex, modern methods. Processing Methods for Solid Lubricants In order to understand the materials engineering and selection process behind modern aerospace solid lubricant coatings, it is important to follow the evolution of research into the capabi lities and the methods used to produce them. There are three main processing methods for coating mechanical parts: burnishing, bonding, and physical vapor deposition (PVD). Each has its own strengths and limitations. Burnishing is a physical, mechanical p rocess which serves to smear the solid lubricant over an extremely smooth substrate with a cloth or ball under a constant load 2,17,18 The resulting coating can be just a few microns thick and is inexpensive to create. The tribological performance of a bur nished coating is heavily sensitive to the type of substrate used and the surface roughness ; smoother is better 17 The adherence of a burnished MoS 2 film has been f ound to be dependent on the ability to form sulfide
28 compounds with the substrate. There are however several drawbacks. First, the coating thickness is not typically uniform over the substrate and thus leads to uneven, inconsistent performance between duplicate samples The second issue with burnishing is that the finished product has been found to have a poor service wear life. T his is often caused by features called blist ers, areas where the coating has not adhered well with the substrate. Over time these blisters act as delamination points which bring about failure and severely shorten the lif etime of the coatings. This shortfall has led to the development of bond ed coatings Bonded coatings utilize additional chemicals to create a mixture which can be applied to the part and then is either heat treated or left to stand, allowing the solvent to vaporize leaving the lubricant and bonding ag ent behind on the substrate 2,19 Adhesion can be enhanced by micro shot peening prior to bonding 20 The films can be applied by aerosol spraying, liquid submersion, or brushing. Bonded films are the thickest o f the three main methods, several microns, but operators have much better control over the As a result the finished products have demonstrated better long term tribological performance, while remaining cost effective. The major limitation to bonding is that the coatings can be too thick for some applications. Nominal thickness to optimize tribological performance is usually 1 2 and this cannot always be achieved with b onded coatings. Providing low friction performance with a thin coati ng is vital for applications that seek to minimize power consumption (deep space missions) and for those with operations requiring precise control over device manipulation or sensitivity to minute positional changes (tracking targets from orbit). In order to meet the requirements of these missions, a different coating process is necessary.
29 Until primarily using burnishing or bonding techniques. a third m ethod of applying tribological coatings was developed based on PVD technologies 4,21 Examples of PVD techniques include ion plating, ion sputtering, and pulsed laser deposition 22 24 E ach method bombards one or more source material targets positioned above the substrate with high energy particles: ions, electrons, or photons. The energetic interaction causes the ejection of the target material resulting in a flux of this material toward s the substrate. The application of a bias to the target and /or substra te can promote adhesion and nucleation of particulate s For ion beam assisted deposition, a flux of inert gas ions is directed at the substrate to enhance the interaction of the sputtered target material with the substrate. PVD techniques have the ability to produce thin films with precise control over crystallographic orientation and chemical composition. Gradient, multilayer and co sputtered film architectures can also be created. Figure 1 1 schematically represents these different film architectures. The characteristics of the deposited film a re sensitive to several processing variables includ ing pressure of the io n source gas within the chamber material target bias substrate temperature and substrate bias 25,26 As PVD technology has matured, MoS 2 soli d lubricant films exceeding the quality of those produced by other methods have been produced PVD methods however, are not without limitations While the film characteristics c an be optimized to give low friction, reliable, and longer term performance t han burnishing or bonded films, the process itself is limited by virtue of the need to utiliz e vacuum chambers for deposition. This makes it much more costly than burnishing and bonding and cannot be easily applied to all substrate materials. Copper and s ilver are
30 two examples of substrates which are not compatible with this process, since they react with sulfur to produce undesirable compounds. At times, such deleterious reactivity can be overcome by depositing an intermediate buffer layer (titanium or ch romium for MoS 2 films ) between the substrate and film 27,28 Buffer layers must adhere well to both the substrate and film. P arts are often cleaned by sputtering prior to film deposition. Substrates that are not electronically conductive can also be problem atic due to the inability to apply an electronic bias. Lastly, the application process is geometrically limited to only stationary material target sources. The physical orientation between the ostructure and thickness. Curved surfaces are difficult to coat uniformly because they are susceptible to shadowing effects Given the high operating cost of industrially manufactur ing solid lubricant films using PVD techniques, companies have continued to s eek new alternatives to reduce expense s, maintain quality, expand capa bility, and broaden the range of part sizes and shapes that can be coated. Improving MoS 2 P erformance with Additives While the ability to create high quality coatings continues to be refined, a n additional pathway to improve MoS 2 coatings h as simultaneously emerged and is currently being developed and tested. As mentioned previously, MoS 2 does not perform under a l l conditions. While its vacuum lubrication capability is exemplary, its p erformance in ambient, humid conditions is poor. Mixing MoS 2 with other c onstituents has been prac ticed in an effort to produce better overall performance 19 However, given e practice involved secular trade knowledge within the industry. Over this time period, v ery little research was published relating to the engineering of better solid lubricant coatings.
31 This change d with the publication of a paper in 1981 by Bernard Stupp who worked for Hohman Plating, a company involved in the aerospace solid lubrication industry 29 The research article was the first civilian based systematic study of additives to improve sputter deposited MoS 2 coatings. In the study, Stupp attempted to address a common problem of MoS 2 films produced by the sputter deposition method. These films contained nonequiaxed grains with high surface area and a lack of density within the microstructure. This caused them to be prone to oxidation and inconsistent pe rformance. To correct this, Stupp sought to increase the density of the films through co deposition of metal elements or alloys with the MoS 2 The results indicated that several of the additives ( chromium cobalt nickel gold and tantalum ) were beneficia l across all evaluation parameters. The films showed an increased wear lifetime, lowered friction, resistance to aging, and uniform friction c haracteristics. T he author chose nickel for an in depth study and determined the optimal amount of Ni to be 5 7 wt %. The corresponding microstructure showed an increase in density and grain size. A follow up by Spalvins on 5 wt% Au additions to MoS 2 demonstrated an improved tribological performance under vacuum conditions 30 TEM imaging of the fa ilure areas of MoS 2 Au films confirmed a denser microstructure with less wear debris. By the late stud ied the effects of a variety of additives. Additives for MoS 2 and other transition metal dichalcogenides ( TMD s) can be o rganized according to the ir tribological effect and role Soft metals such as Au 29 34 and Pb 35 38 have been shown to work well in densifying the microstructure and providing improved vacuum and ambient tribological performance. A w ide range of Au additions
32 from a few percent to over 90 at % Au in MoS 2 have been studied for different applications 34,39 The optimized composition has been isolated to be betwe en 7 at % and 15 at % Au 31 X ray diffraction ( XRD ) measurements have indicated that the presence of Au a lso had a positive influence on the amount of (002) oriented MoS 2 grains 31 Such an orientation decrease s the amount of reorientation that will occur during sliding, and reduce s the exposure of highly active edge sites. The physical size and mass of Au mak es it an ideal oxygen diffusion barrier as well, with h igher Au content s yielding better protection from sub surface oxidation. M ost modern tertiary composite coatings involving MoS 2 contain less than 10 at % Au 40 The addition of Pb has prove n to be effect ive in the field of high temperature applications. The tribological performance of pure MoS 2 coatings has been shown to suffer at operating temperatures greater than 300 C 41 43 An increase of MoO 3 composition within the film formed at these temperature s has been correlated with observed dec line in performance. Like Au, Pb is soft so it s h ears easily at room temperatu res ; but unlike Au, it oxidizes and becomes an oxygen g etter for the film This in turn allows MoS 2 to continue to provide lubrication through a higher temperature range than normal 23,35,36 As operational demands increase, the need for coatings capable of operating at high temperatures will continue to drive solid lubrication research. The second group of additives for TMDs are the intermetallics. Research into adding titanium ( Ti ) 33,38,44,45 chromium ( Cr ) 46 48 zirconium ( Zr ) 26 or nickel ( Ni ) 40,49 in to MoS 2 films has resulted in several patents and successful c ommercialization. Tribological measurements have shown these types of coatings to have dramatically longer ambient lifetimes than pure MoS 2 coatings. This is attributed to increased
33 mechanical hardness based upon indentation testing. Wear rates for these c oatings are at least one order of magnitude lower than and adhesion is improved over single component MoS 2 films Titanium has been suspected of playing the role of gettering oxygen atoms and thus protecting the MoS 2 from oxidation during ambient operation 50 however n ickel does not display a similar effect 49 X ray diffraction (XRD) of co sputtered metal and MoS 2 films has shown the microstructure to be nanocrystalline, or amorphous 26 With the incorporation of these metals, t he orientation of the MoS 2 rem ains basal, (002), but the grains themselves are not present or are extremely small, less than 50 nm. Therefore, the addition of these types of metals is thought to limit the growth of large, (002) oriented MoS 2 grains during deposition. L ike soft metal ad ditions, the optimal content composition for these metals has been found to be between 5 and 15 at% 51 There is a direct correlation between metal content, wear life, and mechanical hardness. The overloading of MoS 2 with metal has resulted in extremely bri ttle films which fail to lubricate properly. The third additive group can be characterized as metals and nonmetals which share the same 2H MoS 2 crystal structure as MoS 2 (W, Ta, Nb, Se, and Te) 7,9,52,53 These additives are intentionally chosen for their a bility to substitute with either the metal or the chalcogenide site Substitution is usually verified with XRD, measuring the change in lattice spacing with concentration. Metal substitutions with W 46,54 56 and Nb 57 59 have shown to reduc e wear and frictio n coefficients in humid, ambient and high temperature conditions. The addition of Se and Te has also shown potential 60,61 The incorporation of Te increased the lifetime of the coatings at room temperature as well as above 300 61 The responsible mechanism is thought to be the formation of double oxides involving molybdenum and substituted Te. While th e incorporation of Te extends
34 the operating temperature range, the formation of MoO 3 is not totally inhibited. The friction of the film in the presence of water vapor remains high. The joint substitution of metal and nonmetal sites is accomplished with co sputtering of a WSe 2 target with MoS 2 46,51,62 This produces a strained atomic structure which induces curvature in the layers 63 Overa l l, t hese films are mechanically harder, have lower ambient wear rates with little wear debris, exhibit lower coefficients of friction and have higher maximum operating temperatures However, these types of coatings still experience oxidation of m olybdenum in moisture rich, moderately high (>450 C) temperature operations In order to operate at even higher temperatures, oxides must be added E arly explorations of materials t o improve high temperature performance of MoS 2 considered oxides 64,12 The se materials we re primarily chosen for their thermal stability above 300 19 2 O 3 MoO 3 and PbO to be beneficial for MoS 2 tribological behavior 23,40,49,64 67 Several mechanisms were thought to be res ponsible for the enhanced performance of MoS 2 oxide films included mechanica l improvement in film toughness oxidation resistance inhibition of crack growth preferential oxidation of the additive formation of lubricious double oxides with molybdenum an d film densification. An important study on oxide additions to solid lubricants by P.W. Centers in 1987 sought to correlate mechanical and thermal properties of MoS 2 oxide films with tribological performance 64 The results of the study surprisingly showed a direct relationship with the wear volume of the film and the shear strength of the oxide. Mechanically stronger oxides with high hardness values exhibited the greatest wear during tribological testing in dry, high temperature conditions. Oxides such as Sb 2 O 3 were found to be effective as additives due to their low shear strength during tribological interactions. Use of harder, Al 2 O 3 produced a wear
35 rate several orders of magnitude higher. Thermal calculations of the increase in temperature caused by tri bological operation generated values just below the melting points of the soft oxides (>300 C). The softness of the oxide was believed to induce plastic deformation. However, since Sb 2 O 3 was mechanically harder than MoS 2 the layered material r emained the primary active lubricant with a persistence of the p referential orient at ion By deforming along with the MoS 2 the soft oxides do not easily fracture away from the film microstructure, causing the wear rates to be low. For PbO the case is different. It to o has low shearstrength, but it forms a lubricious double oxide compound, PbMoO 4 under tribological operation 23 This is similar to the effect seen in MoS 2 x Te x films 61 ZnO additions to WS 2 films have also improved high temperature performance This effe ct is thought to be due to the creation of ZnWO 4 68,69 Mechanically hard oxides such as Al 2 O 3 and yttria stabilized zirconia (YSZ) are used as additives for improving wear behavior of TMDs at temperatures higher than 300 C 70 73 The addition of other mech anically hard compounds including borides carbides and nitrides with MoS 2 ha s also been evaluated with the intention of improving wear rates 74 76 The last avenue to alleviate high friction behavior of MoS 2 in ambient conditions is to simply combine it w ith a lubricant that performs well under those conditions. The addition of graphite to MoS 2 is one of the oldest mixtures used within the community with accounts dating back to 8,77 The use of graphitic carbon in modern aerospace sputter deposition coatings became common 77 80 The incorporation of graphite with MoS 2 produc s the desired effect of increasing the ambient lifetime of MoS 2 based films. XRD me asurements of the samples offer an explanation for the increased li fetime of these films revealing intercalation of the graphite layers
36 with MoS 2 The reorientation of the layers un der tribomechanical interaction brings about an optimized structural support in terms of both film wear and oxidation. The active edge sites of graphite have been proposed to act as oxygen getter locations, allowing MoS 2 to remain nascent 77 By combining two solid lubricants together, the range of operating environments and lifetime of the films have been i ncreased. However, these films a re sti ll limited in their applications given the presence of atomic oxygen encountered in low earth orbit. The rapid oxidation of graphite and eventual debris formation has proved u nsatisfactory for widespread use 81,82 In light of this critical deficiency, trib ologists have turned toward s tertiary component films. Tertiary, MoS 2 based films have been developed in an effort to optimi ze the different synergistic mechanisms of additives. Among the first of the widespread tertiary films was MoS 2 Sb 2 O 3 Au 40 The oxid e was incorporated for its low shear strength and ability to form an amorphous phase and induce a nanocrystalline MoS 2 grain size. This enhanced the mechanical strength of the films, while decreasing the oxidation rate of MoS 2 and provided a physical suppo rt for oriented layers of MoS 2 to slide upon during tribological interactions 64,65,67 Au was added for its ability to increase the density of the film microstructure, leading to better long term performance and adherence to the substrate. Combining both o f these materials in minor constituencies with MoS 2 led to a superior coating. Variations on this type of coating eventually led to many commercial specifications which are still utilized today. Other early tertiary systems designed for ambient/vacuum oper ation included MoS 2 PbO graphite, MoS 2 PTFE graphite, and MoS 2 Sb 2 O 3 graphite 83,84 sputtered systems developed to improv e ambient/vacuum performance had matured. The next step was to create one
37 coating that could operate repeate dly between extreme environments including cycling between ambient pressures and vacuum, and performing well at high temperature. Thermal cycling experienced during orbital maneuvers is one of the most difficult operatin g requirements for any satellite. T herefore, the development of coatings which can withstand the 9 2 minut e periodic transitions from 4 0 C to 6 0 C for the expected lifetime of many years is critical for any orbital application 85 In g eneral withstanding 10,000 cycles is considered a benc h mark for validation of coating s 85,86 Additionally, vehicles whose missions include repetitive trips into and out of orbit require coatings that can withstand the extreme environmental cycling between ambient and vacuum conditions. To meet these objectiv es, researchers have sought to create a new class of lubricating film s that could maintain performance over a variety of operating environments. These new solid lubricant films were c omposites which contained more than one active solid lubricant component. The first adaptive coatings were developed by Air Force Research Laboratories ( AFRL ) in Dayton O hio 16 The WS 2 WC DLC coatings featured adaptability between vacuum and ambient operating conditions 16,87,88 Later generations of the coatings would replace the carbide component with oxides such as ZnO 68,69 YSZ 70 73 and Al 2 O 3 70,89 for mechanical hardness and add soft metals like Ag 90 and Au 70,73,89,90 for high temperature performance. Such materials remain at the forefront of solid lubricant development. C haracterization of Solid Lubricant Coatings The characterization of solid lubricant materials has spanned a number of experimental approaches. Within the tribology community there are several basic tools utilized to study, evaluate, and measure the perform ance of solid lubricant coatings. The first tests run after a new film has been synthesized involv e crystallographic and
38 microscopy characterization techniques. Determining the crystallography of the film is crucial to understand ing the atomic ordering of the film, what phases are present, their grain size, orientation, and possible texturing. For single crystal MoS 2 coatings, this information is invaluable as it relates the effect of film orientation to the overall expected performance and lifetime of the coating The crystallography of a thin film coating is often acquired using grazing incidence X ray diffraction 91 Use of a small incidence angle is necessary to enhance the signal collected from the film itself and minimize t he signal from the substrate a nd/or intermediate buffer la yer 35,56,92,93 In cases where information pertaining to surface crystallography is required, an even more sensitive technique can be employed, low energy electron diffraction (LEED) 94 97 The limiting factor in practicing LEED is that it requires a vacuum chamber to perform measurements Therefore all samples must be vacuum compatible and the analysis of polycrystalline, comp osite materials is limited without the incorporation of mechanically controlled rotating stages and dete ction equipment. In light of this, XRD remains the most commonly used technique for determining crystallographic information. topographical microstructure can be directly probed with microscopy techniques. Optical light microscopy and especiall y scanning electron microscopy (SEM) characterization are prevalent in many topical reports 30,40,98 These methods are able to provide details on the grain size, morphology, and distribution with in the microstructure and how they correlate with diffraction findings. Given the nanosc opic grain size and heterogeneous distribution of modern solid lubricant films, transmission electron microscopy (TEM) has played an increasingly integral role in understanding both the physical structure and tribolo gical roles o f the components 83,99 Analysis is
39 usually accomplished by sampling a cross section of the film prepared by a focused ion beam (FIB) technique. Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) are also important tools used to probe the atomic to micro scale nature of films 100 105 Spectroscopic information related to the film can be acquired by a variety of techniques. Among the most popular within the reviewed literature is energy dispersive spectroscopy (EDS) accomplished with the SE M 50,92,99,106,107 While EDS can give information relating to the elemental identi ty of a film, it is not sensitive to chemical s tate of the s pecies. Techniques providing greater surface sensitivity include Auger electron spectroscopy (AES) 94,108,109 X ra y photoelectron spectroscopy (XPS) 93,109 113 and Raman spectroscopy 23,35,65,84,105 AES is a semi quanti tative technique that while less sensitive to chemical bonding states, provides useful information regarding elemental composition of the near surface region XPS is highly sensitive to chemical state information of species found at or near the surface and t hus is best used for chemical species identification and quantification. Raman spectroscopy is a technique that can detect differences in chemical s pecies, but cannot be used universally as a result of selection rules and limited sensitivity Fourier transform infrared spectroscopy (FTIR) has also been used in studies to identify species present at tribological surfaces The physical quality of a sol id lubricant coating can be tested using a variety of metrics and instruments. The thickness of a PVD thin film is often characterized in situ with a quartz crystal microbalance, or post processing using light interferometry or AES/XPS sputter depth profil ing 61,80,114 Among the most important requirements for any high performance coating is the strength of the interface between the film and the substrate. The adhesion of the coating is commonly verified with a scratch adhesion
40 test, whereby a sharp tip is passed at a constant speed over the film while an increasing load is applied until the coating fails 50,54,57 This process is monitored through changes in acoustic emissions or the coefficient of friction. The mechanical hardness of the film is another imp ortant characteristic to measure. Macroscopic hardness measurements can be made with a Rockwell indentation test 88,115 More sensitive studies can be accomplished with Vickers hardness testing with a microscopic probe tip 106 These types of tests can also yield an elastic modulus value for the film, which can be in strumental in correlating and explaining how load, velocity, pressure, and thermal energetics affect the tribological performance and lifetime of the film. Evaluating and simulating the tribologi cal conditions of a real world sample can be accomplished by several methods There are two basic mechanical test configurations for tribometers. The first is a linear tribometer, which passes a contacting body (a spherical pin) across the sample in a 1 di mensional actuation 54 This test method is best used for bidirectional or reciprocal cycling, whereby the direction motion is reversed at the end of each pass. Extremely small length scale linear tests are referred to as fretting wear tests 10,116,117 Fret ting tests are used to simulate vibrational fatigue of the coating and not necessarily service operations. The second testing configuration is a pin on disc tribometer and typically features a stationary pin with a rotating sample 85,118 This method is bes t suited for unidirectional cycling tests, although reciprocating testing can be done by reversing the rotar y motion of the rotating sample; this requires precise knowledge of the rotation speed and control over the motor. A n additional type of tribometer features three or four balls in sliding/rolling contact with the surface in order to enhance the wear rate of the sample allowing for expedited analysis 108 Within the past 20 years, researchers have also developed a microscopic tribometry technique
41 called lateral force microscopy (LFM) 119 This characterization method utilizes an AFM cantilever to probe the surface on nanometer length scales As the tip is passed along the surface, the lateral and normal forces are simultaneously monitored by the deflectio n of an aligned laser reflect ed from the top of the cantilever towards a four quadrant photodetector. The result ing signal can be calibrated to the corresponding normal and lateral forces. One unique advantage of this technique is that it allows for both s patial and volumetric representation of the tribologic al sample. These attributes enable simplified w ear rate quantification in contrast with other macroscopic methods which often require a secondary characterization technique to determine the topographic al evolution of the wear tracks. However, LFM measurements must undergo standardized calibration practice s in order to provide quantitative lateral force data Within the literature there have been many proposed methods and a variety is still employed by r esearchers 120,121 This makes direct comparison of LFM data difficult between research groups. Additionally, the contact pressures experienced in these tests are high due to the exceedingly sm all contact area produced by the probe tip Furthermore tip wea r is a common phenomenon which takes place during measurements introducing difficulties to understanding the evolution of contact pressure over the course of a measurement and over the lifetime of the cantilever tip. Vapor Phase Lubrication The technolog y used to create the silicon microchip can also be directed to produce mechanical devices. This miniaturization of mechanical components to dimensions on the micron or shorter length scale has created an entirely new field of devices, m icro electromechanic al systems (MEMS) MEMS offer the ability to design and build increasingly capable, compact devices for a variety of applications ranging
42 from accelerometers used in smart phones to micro pumps and relays. 122 125 As the size of these systems decrease, the influence of length, area, and volume dependent properties are enhanced 122,126 The friction between two bodies is inversely proportional to the area of contact T herefore interfacial friction has been seen to dominate some systems with miniaturization. The backbone material behind the realization of MEMS is silicon, which has a passive, oxide layer approximately 2.7 nm thick 127 It is this surface that defines the tribological disadvantageous properties of many MEMS devices and calls for the development o f novel lubrication schemes. Among the well defined issues with these devices is the efficient movement between interlocking gears and other sliding mechanical components, which are prone to seizure (i.e. stiction) of interacting silicon dioxide surfaces 12 3,128,129 Therefore, a lubricant must be employed to allow MEMS to function. Choosing the correct lubricant for MEMS however has been a challenge to researchers. Many materials have been evaluated with respect to overcom ing the forces present. Traditional mechanically hard coatings such as Si 3 N 4 TiN, TiC, and DLC, used for their low wear h ave shown im proved cyclical lifetimes 124,130 However, these coatings are susceptible to water adsorption from ambient conditions which can lead to capillary condensat ion of significant water volumes and the resulting stiction of the moving components. Self assembled monolayers (SAMs) 128,131 134 and polymers such as perfluoropolyether (PFPE) 135,136 have been studied for their effectiveness in preventing water adsorption and have provided some improvement of tribological performance. However these lubricating systems suffer from high wear rates and the overall effective lifetimes still do not meet the requirements for many real world applications. A n additional approach i s needed
43 Such an approach is vapor phase lubrication (VPL) 125,127,137,138 This technique takes advantage of the benefits of liquid phase lubrication, namely the persist ent delivery of a tribologically active phase, in the absence of the formation of larg e capillary forces between the interacting components. Alcohols such as pentanol and ethanol, among others, have been shown to be effective long term lubricants for silicon based components, for they provide not only a low coefficient of friction but even more imp ortant an extremely low wear rate when compared with alternative methods. Alcohols are thought work well as a VPL due to their specific molecular structure, which feature s a hydroxyl group at one end. The hydroxyl, C OH, has a large adsorption pot ential with the oxide surface common to silicon based MEMS 139 In light of this, the role of alcohol based vapor phase lubrication involves two steps first dissociative adsorption onto the surface of the MEMS components and second, lubrication of the int eracting bodies 140 143 The application of alcohols as vapor phase lubricants for MEMS was proposed by Gellman in his study modeling vapor transport within micro electromechanical devices 125 Prior studies showed alkyl and alkoxyl monolayers to be effectiv e lubricants when chemically attached to pretreated silicon 144,145 Another report discussed and promoted the vapor phase lubrication of vanadium carbide (100) surfaces with short chain alcohols 146 In their study on the room temperature vapor phase lubric ation of native silicon oxides, Strawhecker et al investigated the effectiveness of n propanol molecules 138 Their work showed that effective wear protection was activated at 25% of the vaporization pressure of propanol. Measurements were taken with an AF M tip scanning the surface of a silicon wafer. Subsequent studies revealed not only that other short chain, linear alcohols such as ethanol (C2), butanol (C4), and pentanol (C5) were
44 effective as well, but also that a clear, proportional relationship was e stablished between the alkyl chain length and the resulting adhesion force. For example pe ntanol with its 5 carbon atoms i s a better vapor phase lubricant than propanol, which ha s 3 carbon atoms 143 In contrast, w ater molecules were reported to have the opposite effect. The presence of water molecules on the surface of silicon increased the friction and wear in a controlled, 50% RH environment compared to a dry, argon environment 139 This critical series of experiments showed that alcohol molecules could be used to effectively lubricate the primary materials used for MEMS devices. However, an understanding of how these alcohols function physically and chemically is not well developed Direction The remainder of this d issertation include s five main section s. The design, construction and incorporation of the in vacuo pin on disc tribometer within a vacuum analysis system and description of the characterization methods used in surface studies are covered in the following chapter. C hapter 3 present s measure men ts and descr iptions of the tribological performance of a traditional solid lubricant, graphite under different environmental conditions T h is will serve as a control case stud y on which subsequent investigations of composite coatings may be investigated in terms of their fundamental behavior as layered solid lubricants. The e ffects and roles of additional constituents added to MoS 2 films will be elucidated from pin on disc testing presented in Chapters 4 5. Lastly, Chapter 6 will discuss the possibility of incorporating vapor phase lubrication schemes into advanced micro electromechanical systems ( MEMS ) intended for use in satellites
45 A B C D Figure 1 1. Types of physical vapor deposition film architectures A) MoS 2 thin film B )gradient thin film C)multilayer thin film and D)c o sputtered thin film. The gray colored substrate is not to scale.
46 CHAPTER 2 THE IMPLEMENTATION O F AN IN VACUO PIN ON DISC TRIBOMETER AND SURFACE CHARACTERIZA TION METHODOLOGY Experimental Philosophy In order to study the complicated phenomena of lubrication transfer film formation, wear, and failure, surface sensitive characterization techniques must be utilized. Spectroscopic identification of surface species can be acqu ired via X ray photoelectron spectroscopy (XPS) W hen tribological measurements are conducted within the experimental vacu um chamber complex XPS measurements can be performed without contamination XPS collects quantitative information pertaining to the composition of surface species present o n an analyz ed sample 147 XPS require s an ultra high vacuum (UHV) to operate UHV conditions are necessary in order to preserve sample cleanliness and to allow the emitted electrons the mean free path needed t o reach the detector. The electron analyzer functions by pr ecisely measuring the kinetic energy of the emitted electrons. Any gas phase species present within the vacuum chamber pose s a collision risk with the trajectory of the electrons. The greatest challenge is not that the particles keep the electrons from phy sically reaching the detector, lower ing the overall count intensity but more importantly, retard ed causing measurement errors under moderate to high vacuum conditions Maintaining vacuum throughout the experimental proces s also helps to minimize sam ple contamination during analyse s and transfer T he abil ity to conduct experiments under vacuum conditions is paramount to the validity of any studies that are to involve surface sensitive phenomena such as the formation of trib ofilms, the environmental effect on chemistry and tribology of solid lubricants, and elucida ting the roles for the different constituents within the composite solid lubricant films Thus, the design, construction,
47 and operation of a n in vacuo tribometer mu st provide a mean s for isolated environmental testing, including a means to introduce both gas and liquid phase species as well as transfer of the sample under vacuum to the XPS analysis chamber Design Philosophy Sample Transfer System The implementation of the tribometer consisted of two simultaneous phases T he first phase was the design and construction of the environmental test chamber and vacuum sample transfer system to the XPS analysis chamber. The second phase was the construction of the tribomete r itself. The overall design philosophy focused on utilizing a standard Omicron Nanotechnology GhmB sample platen compatible with both the tribometer and analysis chamber stage (figure 2 1 ) The sample transfer system consist ed of three linear, magnetic all y coupled transfer arms to shuttle the sample platen between locations The longest arm move d the sample from the main analysis chamber to the load lock and to t he tribometer chamber entrance The chamber entrance was a spherical cube shaped junction with flanges allowing for the addition of a platen storage rack, or an ion sputter gun for sample cleaning. A second arm perpendicular to the first feature d a custom designed stainless steel stage to receive the platen and transfer it into the environmental tes ting chamber (figure 2 2 ) The 304 stainless steel sample stage was machined by T MR Engineering of Micanopy F lorida A third arm parallel to the first, took possession of the sample platen at the center of the environmental test chamber and insert ed the sample o nto the tribometer stage The platen was secured to the tribometer stage by the use to two tantalum Omicron Nanotechnology GhmB platen stage spring clips. The stage also served as the rotating portion of the tribometer Following tribological measu rements, t he sequence wa s
48 reversed for transfer from the tribometer to the analysis chamber. Figure 2 1 shows a schematic of the vacuum chamber complex and the sample transfer steps Environmental Chamber A stainless steel environmental chamber was u sed t o hous e the tribometer and to control the environment during testing The 8 inch diameter and 24 inch tall cylindrical chamber wa s kept under vacuum using a Leybold TURBOVAC 151 turbo molecular pump rated at a pumping speed of 150 L/s, backed by a rotary vane, mechanical pump A vapor trap and manual valve were positioned between the two pumps The top side of the chamber featured a window to view tribometer operation and sample transfer process. The front side of the chamber was connected to a n HVA 4.5 in ch pneumatic gate valve which could be closed using a pressurized gas feedline and electronically operated switch. When closed, the gate valve isolate d the chamber from the rest of the vacuum complex Due to the physical height of the existing sample tran sfer system for the XPS analysis chamber and the availability of in house vacuum componen t s the location of the environmental chamber was not directly above the preexisting through hole within the mounting table. Additionally, the bottom of the chamber wa s elevated several inches above the table. In order to connect the bottom of the chamber to the turbo molecular pump, a 4 inch stainless steel connection pipe line with 6 inch ConFlat flanges had to be designed and mounted The pipeline was built by Thermio nics Northwest Inc of Port Townsend, Washington according to the specifications supplied. A n HVA 6 inch manual gate valve located below the chamber could be closed above the turbo molecular pump to ensure isolation and complete control over the introductio n of species and environmental conditions. Pressure within the chamber could be
49 mon itored via a Convectron gauge (a mbient to 10 4 Torr) or a nude ion gauge (10 5 Torr to 10 11 Torr) Environmental conditioning of the sample co uld be accomplished through a inch inlet located in the center of the environmental chamber. A manual valve was attached to the exterior of the chamber and affixed to a inch stainless steel gas feedline Using this dosing system, the entire interior of the chamber including the t ribometer and sample are subjected to the same conditions. A second inlet system was assembled to direct a localized flux of gas through a 1/16 inch nozzle pointed towards the leading edge of the tribometer pin during clockwise rotation of the sample Th e minute fl ow was moderated through a pin hole doser The feedthrough to the chamber wa s located above the tribometer on a 4 d imensional manipulator (X Y Z and rotational) The inch stainless steel feed line was connected to a Nupro valve through which ga seous species c ould be introduced within a 4 way cross. The cross could be isolated using the two valves connected on opposite ends With this assembly, a liquid species could be vaporized from a 20 mL Chemglass inch glass ampule connected to the cro ss with Swagelok ultra Torr fittings and i ntroduced to the pin hole doser The diagram s of the environmental test chamber and dosing system are shown in figures 2 2 and 2 3 I n V acuo Pin on Disc Tribometer The archetypical design of the tribometer was bas ed on the MISSE 7 series of pin on disc tribometers transported to the International Space Station ( ISS ) and installed in Nov ember 2009 during the STS 129 mission 85 During the initial design and modification phase, several strict guidelines were establish ed that had to be adhered to. First, all components had to be vacuum compatible. Second, the overall size of the tribometer
50 had to fit within a preselected environmental chamber to accommodate sample transfer from the tribometer to the analysis chamber. La stly but most importantly, the tribometer had to be able to deliver reliable, replicable data. A final version of the tribometer is illustrated in figure 2 4 Tribometer Arm There we re five main componen ts to the tribometer. The first wa s the tribometer a rm itself. function wa s to house the pin and strain elastically along two axes with normal loads of 1 Newton or less. Its shape provide d a platform for two sets of strain gauges, one set to measure the strain resulting from the applied normal force and the other set t o monitor the resulting lateral forces during sample rotation At the head wa s a tapped void w here a clean, spherical ball could be firmly positioned with a set screw. Stainless steel, tungsten, molybdenum, and titanium wer e all studied as possible materials for the tribometer arm. All of the materials were vacuum compatible and had relatively high yield strengths. However the material with the best perform ance for such an application possesse s both a high yield strength and a low elastic modulus to allow for maximum strain sensitivity without deformation. Thus, titanium was selected due to its large r yield strength to elastic modulus ratio It was machined by SureTool Inc from Dayton, OH from the dimensions and specification s supplied to them. The strain gauges were supplied and mounted on to the tribometer arm by Sensing Systems Corporation of New Bedford, Massachusetts. The strain gauges measure d both the normal and lateral forces and were calibrated using a series of small masses suspended from the head of the tribometer.
51 Piezo Actuator The second component wa s the actuator for the tribometer arm. The actuator functioned as the device that applie d the normal load to the ball and sample. The main requirement was to provide a large vertical displacement o f the head away from the tribometer within the compact space. A commercial piezoelectric actuator from Cedrat, model APA 120S, equipped with the vacuum compatible option was chosen after dynamic strain simulations were run on several a vailable actuator models using S olid W orks computer aided drafting program. The results showed the APA 120S model to be the best choice considering size, displacement, and cost. The actuator functioned by applying a positive, direct current to expa nd a horizontal stack of BiTiO 3 blocks. This caused the actuator frame to contract vertically. The actuator frame wa s attached vertically with screws near the base of the tribometer arm. The head of the tribometer arm wa s lowered until the tip of the ball contacted the sample and the measured normal force signal from st rain gauges reached the desired value. Rotating Sample Stage The third component wa s the rot ating sample stage. It functioned as the platfo rm on which the sample platen sat and wa s the flat disc component for t he revolving pin on disc test. The disc made of titanium for material continuity and t wo t hrough holes were incorporated along the bottom to accommodate the guide prongs used during sample transf er operation. The under section included a tight, square bottom hole to accommodate the insertion of the D shaped drive shaft of the motor/gearbox assembly The shaft was secured to the flat face of the drift shaft with a set screw via a threaded thru hole The platen was secured using Omicron N anotechnology GhmB molybdenum spring clips and retention parts supplied by Omicron Nanotechnology GhmB
52 Nanotechnology. For ease of machining and flexibility in the use of the tribometer the spring clips were secured onto separate rectangular side pieces w hich could be removed from the rotating sample stage. The top of the side pieces had to allow clearance to pass underneath the tribometer arm during rotation of the sample stage All components of the sample stage were machined by SureTool Inc of Dayton, O hio from dimensions and specifications supplied to them. The sample stage wa s mounted overtop the fourth component, the motor assembly. Motor and Reducing Gearbox The motor assembly include d the motor and the reduction gearbox. Both of these were purchased from Maxon Motors of Fall River Massachusetts and featured v acuum compatibl e options The motor model wa s an EC 20 Flat that measure d 10 mm in height and could reach a top speed of 5000 rpm. The reducer gear head had a ratio of 84 : 1 allowing for a maximu m sample stage speed of 60 rpm. This reducer was locate d directly above the motor and secured with a set screw to the las t component, the mounting block Mounting Block The fifth component wa s the mounting block. The tribometer arm, actuator, and motor we re all directly attached to the mounting block with M2 stainless steel screws The 3 mm radius of the pin on disc track was defined by the position of the drive shaft through hole. The motor/gearbox assembly was attached with two M1.6 stainless screws. The overall length of the block could not exceed 9 cm to allow clearance for the operation of the second transfer arm in front of the tribometer when fully retracted. Furthermore, it could be no more than 5 cm tall in order to allow the prongs of the third t ransfer arm to fully engage The location of the rotating sample stage also had to be far
53 enough away from the front edge of the piezo to allow room for the prongs of the platen pincher head unit to fully engage. The block was machined by SureTool Inc of D ayton, O hio from dimensions and specifications supplied to them. The mounting block itself wa s connected to a four dimensional manipulator via a in ch diameter stainless steel rod and held in place with a set screw and flat bottom Allen head bolt A type K thermocouple was attached to the Allen head bolt to monitor the block temperature. Power and Signal Connections Power inputs and signal outputs for all of the components were wired through 10 pin, 4 pin, and Type K thermocouple UHV feedthroughs. All of the feedthroughs were located on a 4 D manipulator and use d vacuum compatible wiring to connect the leads to the respective instrumentation. The strain gauges were wired using two Wheatstone bridge configurations, one full bridge for the normal force and another full bridge for the lateral force. Each strain gauge ( force ) signal wa s relayed through a Sensotec in line amplifier to a National Instruments SCB 68 signal accumulation box then to a National Instruments 6221 data acquisition card. All strain gaug e signals we re routed through coaxial wires with the outer shields connected together to a comm on ground at the base of the 10 pin connector head. An additional comm o n ground was formed next to the signal accumulation box. A National Instruments Labview V8 .5 data acquisition program was written by Ira Hill and used for tribometer operation and data collection. The software allows the user to select the direction and speed of rotation, to control the applied force during operation, and to customize the exper iment time a nd data acquisition rate. Data wa s saved to a Microsoft Excel compatible file format and includes normal force, later al force, friction coefficient and time.
54 Pin on Disc Testing Parameters The main purpose of pin on disc testing is to measure b oth th e friction and wear performance under controlled tribological conditions The ASTM G 99 Standard gives a wide but definitive explanation of the operational parameters for pin on disc testing and the determination of mechanical properties from the mea sured data 148 The tribometer used in this work met the defined parameters, including applied normal force, wear track diameter, and ball size A review of relevant articles involving tribological studies of solid lubricants with a pin on disc tribometer a lso helped to determine a range of operational Hertzian contact pressures for the intended studies 10,11,65,149,150 Characterization Methods X ray Diffraction As stated earlier, X ray diffraction is commonly used to determine the crystallinity, or ientation and grain size of a deposited solid lubricant film 151,152 XRD operates by directing high energy X rays, usually Cu K 1 (wavelength of 1.541 ) towards a sample. Whereas XPS excitation is described with excitation energies, XRD is concerned with the wave length scale metrics. The wavelength of the X rays used in diffraction experimentation must be smaller than the smallest lattice constant that the operator wants to measure. The high energy X rays diffract coherently with the ordered structure of the sampl e to produce intensities located at specific trajectories from the sample corresponding to Bragg 153 (1 1) The integer ( n ) of the diffracting wavelength is usually set to be a value of 1. The wavelength of the X ray is termed ( ) The d istance between diffract i ng p lanes ( ) and
55 the diffracti on angle to the plane ( ) ar e va riables. Figure 2 5 illustrates the diffraction process. There are two ways to measure different diffraction angles. The first is with a stationary source and sample stage. The X ray detector is moved along the radi us of the center of the sample Anoth er configuration keeps both the source and detector stationary and the sample is rocked to allow measurement of different Bragg angles intensities. The measured intensit ies are plotted v ersu s 2 to portray the information. Peak positions are used to hel p identify the lattice spacing. From this basis, crystal structures, lattice constants, and lattice strain can be determined. Additional information related to the grain size of the diffraction so urces can be calculated using S c Formula 154 : (1 2) The diameter of the grain, ( the full width at half maximum value (FWH M ) of the diffraction peak ( ), and Bragg angle are all variables T he X ray wavelength ) is the only constant. D ue to the energy of the X rays, the sampling depth for XRD is more than 10 m. This is certainly more than enough to penetrate through modern thin films which can be just several microns thick. In order to ge nerate a better representation of the diffraction spectrum of these types of samples, lower incidence angles, 5 or less, are used in XRD measurements of thin films. While the sampling depth may still be greater than the thickness of the films, the proport ion of the signal originating from the intended material is much larger. A limitation of this method is that interaction area on the sample surface increases from a beam size of 1 to 2 mm to 8 to 12 mm 151 This necessitates a minimum sample size of approxi mately 1 cm for low angle XRD measurements. The low angle XRD measurements were performed in the
56 U niversity of F lorida I nstrument C enter (MAIC) by Dr. Valentin Craciun with a MRD system using a Cu K X rays. X ray Photoelectron Spectroscopy The surface sensitive technique of XPS was utilized to characterize the elemental and chemical species o f samples before and after tribological testing For this purpose, an Omicron Nanotechnology GhmB Al K (148 6.7 eV) monochr omatic X ray source was used in conjunction with an Omicron Nanotechnology GhmB EAC2000 Sphera hemispherical, 7 channel analyzer. The system featured two X ray anodes. The first was a small anode with a maximum operating power of 300 W. The second was a larger source with a maximum power of 400 W. Emitted X rays diffracted from a curved quartz mirror to deliver and focus only Al K X rays o n the surface of the sample. The platen was electrically connected to an isolated ground The sample stage was located at the end of the shaft of a 4D sample manipulat or The sample could be rotated a full 360 degrees around the axis of the shaft Measurements taken at different takeoff angles relative to the sample normal alter the sampling depth of the measured electrons. Higher takeoff angles lead to greater surface sensitivity. In general, most measurements were acquired at takeoff angles of 5 5 The emitted electron s were collected by the Sphera h emispherical a nalyzer using an electro static lens capable of focusing on specific areas of the sample and direct probing of photoemitted electron energies from microscopic locations on the sample surfa ce. The electrons are energetically filtered within twin hemispher es of the analyzer Applying a voltage across the hemispheres allows only electrons with a specific kinetic energy to reach the detector. Electrons with too much velocity will be collected o n the outer hemisphere, while electrons slower than the specific energy are collected on the inner hemisphere.
57 Varying the voltage of the hemispheres allows the operator to collect a spectrum of intensities across a range of kinetic energies. Figure 2 6 sc hematically represents the photoemission process. The binding energies of the electrons can be calculated using the measured kinetic energy of the electron ( ) the energy of the excitation source ( ) and the calibrated work function of the instrume nt ( ) 155 (1 3 ) The Sphera 2000 a nalyzer was calibrated with a sputter cleaned, 99.99% pure (111) silver sample from Mateck setting the measured silver 3 d 5/2 peak binding energy to 368.30 eV. However, the measured spectrum from a n el ectrically insulating sample may not represent the correct binding energy. Consideration must be given to electrical charging effects at the surface. For samples with poor electrical conduction, the ability to replenish the surface with new electrons after photoemission becomes d ifficult during the course of the entire measurement or even from the start Samples suffering from surface charging include o xides, nitrides, polymers, and other poor conductors To correct this issue, charge neutralization of the sample surface is necessary. Charge neutralization uses additional sources of charged particles such as electrons and ions to electrically neutralize the sample surface during measurements. The initial surface charg e for a non conductive sample is positiv e E lectrons from an electron gun can be used to neutrali ze the positive charge caused by the depletion of surface electrons. Over time, the flux of incoming electrons from the electron gun (1.5 to 3 .0 e V) to the surface will overco mpensate the emission of electrons caused by X ray excitation. This leads to an excess negative charge at the surface. In turn, this can be addressed through an additional charge neutralization process This entails the introduction of inert ions such as Ar + at low kinetic energi es (20 to 100 eV) to
58 counterbalance the negative charge while limiting damage to the surface The proper charge neutralization operation includes the simultaneous interaction of X rays, electrons, and ions at the surface of a sample. While the peak bindin g energies can be properly measured for an insulati ng sample, the FWHM of the component peaks will increase due to the presence of char ged particles around the sample. The charged particles will both expedit e and retard the velocity of emitted electrons. T he analysis chamber is equipped with both a 1 5 e V CN 10 electron gun and a 5 keV PHI FIG 5CE ion sputter gun to electrically neutralize the surface of nonconductive samples during measurements A camera was mounted on the exterior of the analysis chambe r to provide a video feed of the analysis area. The operator can then use the video image to determine which part of the sample is being probed. The sample analysis area was projected on the screen and the spatial limits for the largest, and middle sized a pertures were traced on to clear plastic films that were place on the screen. Atomic Force Microscopy The characterization technique employed for both topographical and micro tribological measurement was the atomic force microscop e Atomic force microscopy (A FM ) is a technique that monitors interfacial forces to detect a tomic positions. The probe commonly used is a Si 3 N 4 cantilever with an atomically sharp tip. As the probe tip descends closer to the surface, the tip undergoes first attractive then repulsiv e forces as the electron densities of the tip and sample interact Upon physical contact, the strong repul sion between the electron clouds of the atoms of the tip and sample produce a de flect ion of the cantilever. T ip deflection can be detected by directin g a light amplification stimulated emission ray ( LASER ) to wards the reflective top of the
59 cantilever The reflected beam then travels towards a four quadrant photodetector figure 2 7 The spot intensity is adjusted t o be located at the center of detector prior to beginning any measurement With this setup, the physical deflection of the probe tip generate s a voltage signal proportional to motions of the cantilever and the corresponding movement of the spot on the four quadrant photodetector. The position o f the cantilever tip is controlled with the use of piezo electrics. The design of AFM centers on the control of p iezo displacements so as to map out the deflection of the tip/cantilever as a function of location within the X Y plane parallel to the sample. Two common practices in measur ing the topography of a sample with AFM are the constant contact mode and the tapping mode. Constant contact imagining scans the surface of a sample so as to produce a constant normal deflection of the cantilever The topogr aphy of the sample is represented in the piezovoltage used to maintain this condition This method can also be used to detect the lateral deflection of the cantilever. The frictional forces acting on the probe tip are measured via the lateral motion of the spot across the photodetector quadrants Tapping mode imaging oscillat es a cantilever tip above the surface, near its resonance frequency to produce the greatest controllable deflection amplitude possible. When the tip is brought close to the surface, t he initial attractive forces acting on the probe tip are detected through changes in the amplitude and phase of the modulated signal This allows the topography of the sample to be imaged wi thout the risk of physical dama ge to the surface caused by the hig h contact pressures associated with direct contact imaging Lateral force microscopy (LFM) is accomplished by calibrating the lateral deflection signal to an equivalent lateral force. This allows the detection of lateral or
60 friction forces as a function of position across the sample surface. While image quality may be affected by the topographic features on the surface, the approach allows the potential differentiation of chemical species on the surface. The AFM used in this series of studies was an Asylum Research model MFP 3D. Constant contact mode imaging was employed to allow simultaneous collection of both topographical and frictional force data. T he cantilever s used were all Si 3 N 4 triangular shaped sharp tip probe s from Veeco with a stated normal stif fness of 0.58 N/m. The normal force response was c alibrated using a thermal oscillation test and t he lateral force response was calibrated using the w edge method founded by Carpick 120 Experimental Method ology There were several pin on disc experimental v ariables that were determined to be important to understanding the performance of solid lubricants. Foremost among them were the environmental conditions. Tests were conducted u nder ambient, 760 Torr air at 50% RH, 10 7 Torr vacuum, and partial pressures o f the three main atmospheric components: 150 Torr oxygen, 8 Torr water vapor and 610 Torr nitrogen Hu midity is known to be tribologically disadvantageous for MoS 2 Under high vacuum and dry conditions, MoS 2 has exhibit ed low friction, long life behavior, hence its utilization in space applications. However when humidity rises above 50% RH the tribological performance of MoS 2 deteriorates rapidly. Other experimental variables included the b all material, pin load, rotational speed, direction, and the numbe r of cycles These were kept constant for the individual studies of a specific lubricant system After pin on disc testing, the samples were transferred under vacuum to the XPS analysis chamber. Core level photoelectron scans were taken of the element al co nstituent s of the films as well as of oxygen and carbon. To compl e ment the macroscopic tribological measurements
61 a set of atomic force microscopy scans using sharp Si 3 N 4 tips were conducted. Topographical information was collected from scans ranging in si ze from 1 m to 40 m. Simultaneous friction force maps were measured from the scans 1 m to 1 0 m in size From this characterization process, a set of carefully designed, complementary studies we re able to identify the species dictating the tribological resp onse in a number of s olid lubricant s systems
62 Figure 2 1: Schematic of an Omicron GhmB platen shown with a mounted sample. Figure 2 2: Schematic of the platen sample holder mounted to the end of the magnetically coupled transfer arm rod.
63 Figure 2 3 Diagram of the vacuum complex with transfer arm system
64 Figure 2 4 The symbol key for the vacuum diagram of the environmental test chamber and dosing system.
65 Figure 2 5 The vacuum diagram of the environmental test chamber and dosing system
66 Figure 2 6 The finalized SolidWorks design for the in vacuo pin on disc tribometer. The tribometer arm is artificially colored gold. The platen is shown with a 14x10 mm sample mounted on top of it. Tantalum strips are drawn to portray how the sample wa s secured.
6 7 Figure 2 7 Schematic of the X ray diffraction process. law are labeled. Figure 2 8 Schematic of the X ray photoemission process of a neon atom
68 Figure 2 9 Schematic of an atomic force microscope measure ment
69 CHAPTER 3 THE TRIBOLOGY OF TRA DITIONAL SOLID LUBRI CANTS Traditional Solid Lubricants In order to evaluate and understand the performance of modern solid lubricant coatings and the respective roles of their constituents, it is beneficial to study the material s on which many of the composites are based. From chapter 1, lamellar solids are just one gr oup of solid lubricants. Within this group there are two common solid lubricants, graphite and MoS 2 Figure 3 1 shows a model of the ir lamellar crystal stru ctures The first illustration is of highly oriented pyroly tic graphite (HOPG) f ig ure 3 1a. As can be seen graphite has a lamellar crystal structure with hexagonal symmetry 156 Th e stacking of the layers is an A B A configuration in the sense that there is a lateral shift of the structure in alternating layers nt ra layer carbon to carbon bonding is characterized by sp 2 hybridization 157,158 Inter layer bonding is controlled by much weaker van der Waals interactions This leads to highly anisot ropic properties, explai ning the ease of cleavage of the layers. While graphite mechanically exhibits a low shear strength it has a high thermal stability as indicated by its melting temperature of 3000 C 5 Although widely debated for decades, it is not general accepted that g molecules 159 Boron nitride (BN) has a similar crystal structure and has also been historically studied alongside graphite 12,13,160,161 The second traditional sol id lubricant studied in later chapters is MoS 2 based ( figure 3 1b ) It too features a layered structure but its layers are made up of trigonal prismatic bond ing between Mo and S 7,52 The tr i layer of MoS 2 consist s of S Mo S sequences spread along the X Y p lane. The primarily covalent bonding between the
70 molybdenum and sulfur atoms satisfies all of the valence requirements between the sulfur and molybdenum atoms. The stacking sequence of the layers follows an A B A configuration, termed 2H MoS 2 Like graphit e, this leaves only van der Waals forces interacting between layers of MoS 2 creating a low energy system of planes along which shear easily occurs. MoS 2 has a high melting point of 2800 C 5 I ts tribological performance is limited to specific environments. The performance of MoS 2 is known to be adversely affected under humid, ambient conditions. It has been reported that f riction dramatically increases when the partial pressure of water i s 50% 162 The deterioration of tribological performance is accompanied by a n increase in the oxidation of the molybdenum to form MoO 3 The formation of MoO 3 is thought to act as an abrasive, aiding t he wear of the MoS 2 lubricating film. As a result, MoS 2 is best suited for vacuum applications and inert conditions 109 Other compounds that possess similar layered, hexagonal structure include MoSe 2 MoTe 2 WS 2 WSe 2 NbSe 2 TaS 2 and TaSe 2 52,53,163 165 Graphite is an inexpensive, common lubricant used in man y industrial applications. Given that MoS 2 was not commercially viable at the time, early aeronautical aircraft manufacturers chose graphite as the primary lubricant. However, it proved to be ineffective in the low moisture conditions of the high altitudes 159,166,167 Engines and propellers were especially prone to failure as illustrated by the almost 600 fuselages left War II 168 Experimental rformed best in a water rich, ambient environment, but was vulnerable to out gassing in the absence of water vapor, especially in vacuum 159 MoS 2 displayed a complementary behavior, whereby it was optimal as a lubricant under vacuum and dry conditions, but negatively affected by the
71 moisture content in ambient 169 To overcome the issue, a common practice was to mix MoS 2 with graphite and other elements/compounds in order to improve the overall performance across a wide range of operating conditions 19,170 Solid lubricants are a class of materials often characterized by their lamellar crystal structure. While one lubricant may have a similar atomic structure as another, their performance can vary dramatically depending upon both the compound and environment. Graphite is a solid lubricant commonly used for a variety of terrestrial applications given its low friction and wear behavior under ambient conditions. However, its performance in space environments is known to be extremely poor. Coatings used for space applications are instead often based upon molybdenum disulfide, MoS 2 In commercial coatings, additives are also incorporated to enhance ambient lubricity, wear resistance, and product lifetime. These complex, composite coatings are constantly under refine ment. A current generation of coatings incorporates Sb 2 O 3 as well as graphite into MoS 2 deposits. Such composites are intended to perform well in both ambient and vacuum environments due to the presence of graphite and MoS 2 Under ambient conditions, graph ite is thought to dominate the tribological interactions. As conditions change from ambient to vacuum, the role of solid lubricant transitions from graphite to MoS 2 Extensive tribological research has shown that MoS 2 Sb 2 O 3 C composites achieve a low coeff icient of friction and wear rate under ambient conditions. When in vacuum, its performance is similar to other MoS 2 based coatings with a low coefficient of friction, <0.05, and a long lifetime. However, actual space conditions also include 5 eV atomic oxy gen. This species is thought to lead to the deterioration and eventual failure of the coatings due to degrad at ive reactivity of the lubricant constituents
72 In order to evaluate the role of graphite in MoS 2 Sb 2 O 3 C coatings, it is necessary to establish a f undamental understanding of how environmental species interact and affect the tribological performance of graphite itself under the specific conditions being employed in subsequent measurements. I nteractions with oxygen and water are of particular interest It is well documented that the presence of water is b eneficial and necessary for lubricious tribological interactions 159,158 The case of oxygen is less clear. T he presence of oxygen has been reported to have both a positive and a negative effect on perf ormance 158,171 One study found cycling between dry air and dry N 2 that the coefficient of friction of graphite is similar between the environments. Inert gas es such as nitrogen and argon have little effect on friction as their concentrations are changed 161 Vacuum environments, representing an absenc e of atmospheric components, have been shown to promote high friction and high wear in graphitic samples 159 Based on results such as these there are several hypothesized causes for the low friction behavior of graphite under ambient conditions. The first and oldest theory, is that the anisotropic nature of the crystal structure lends itself to preferential shearing parallel to the planes 172 This had to be modified to account for the environmental sensitivi ty of graphite The modified theory stated that water acted as a vapor phase lubricant of the graphite surface, forming a monolayer of water under ambient conditions and lowering the free energy of the interacting surfaces 166,173 Another theory suggested that s hearing is accommodated by intercalation of small molecular species such as water and oxygen 161 The intercalation is thought to lead to an expansion of the distance between carbon layers, thus lowering the energy required for shearing to take place. Evidence of intercalation has yet to be
73 verified in graphite samples and there is ample room to question the validity of the intercalation theory 174 The second major theory suggests that wear and surface energetics play a major role in graphite tribolog y 166 The basal plane of graphite has a low surface energy which minimizes frictional behavio r upon motion 175 However, given th at m echanical deformation occurs upon sliding, edge sites become prevalent enough to dominate the tribological behavior 174 The theory states that low friction arises due to passivation of the dangling bonds along the edge planes via hydrogen and oxygen atoms bonding to carbon edge sites 176 This would also describe the opposite behavior observed in vacuum environments whereby an absence of these passivating species results in high friction and wear. Experimental support for this theory is more prevalent. Numerous studies note the difference between humid, ambient conditions and vacuum tribological behavior 159,161,177,178 Addition ally, oxygen, water, carbon dioxide, and atomic hydrogen have all been shown to lower the coefficient of friction when present. Indirect support for a surface energy theory also comes from the success at creating amorphous carbon and even diamond like carb on ( DLC ) films which maintain low coefficients of friction in vacuum when hydrogenated 179 181 The fact that both amorphous carbon and DLC coatings perform poorly in vacuum but well in air underscores the potential similarity in mechanisms with graphite. T he last support for the surface energy passivation theory stems from a study that found no increase in basal plane spacing for graphite when measured in vacuum, ambient air, or water saturated air 182 The c urrent understanding holds that surface energetics are the main controller in determining graphite tribological performance 158,171,177 Our series of tests involving controlled environments of ambient component species and ozone with highly oriented pyrolytic graphite (HOPG) will
74 attempt to further verif y or refut e the edge passivation theory as well as to offer insight to why failure occurs for solid lubricant coatings containing carbon while in space but not during in vacuo testing on earth. Experimental The sample used for tribological and spectroscopic analysis was a n in house 5mm by 5 mm piece of HOPG The tests were always performed on a fresh ly cleaved surface, produced via the tape peel method. The sample was secured to an Omicron Nanotechnology GhmB platen via spot welded tantalum strips. The in vacu o pin on disc tribometer was based on a design by Brandon Krick and Gregory Sawyer 85 with several modifications to allow it to fit into the environmental chamber and to allow sample insertion/extraction, figure 2 4 As described in chapter 2, t he tribomete r arm, rotating disc, and body are all made from titanium. Titanium is not only vacuum compatible, but also has a relatively high yield stress to elastic modulus ratio. This gives the tribometer arm the ability to experience greater forces without deformat ion, thus ensuring a larger range of accurate measurements. The rotating disc has a sample stage designed to accommodate the Omicron Nanotechnology GhmB platen, allowing for transport of the sample t o other chambers for analysis and/or treatments. The trib ometer was attached to a three dimensional manipulator for maneuverability during the sample transfer process and operation. The pin was a 3.175 mm diameter, Si 3 N 4 sphere purchased from McMaster Carr. Gas was inlet to the chamber via a feed through attache d to the environmental chamber. Nitrogen and oxygen gases were purchased from Airgas. T he water was deionized to an electrical resistance greater than 18 MOhms and stored in a glass vial conne cted through a stainless steel tube connected to the manipulator and with a 1/16 inch nozzle. The water sample was cleared
75 of atmospheric contaminants via multiple freeze, pump, and thaw cycles prior to introduction. The testing load was 0.5 N, as calibrated by a set of known masses sus pended from the arm in both normal and lateral orientations. The sample was rotated at a speed of 5 rpm, in a clockwise direction for at least 300 cycles. The Hertzian mean contact pressure was calculated to be 240 MPa assuming a sphere on flat contact geo metry The property values assumed for the Si 3 N 4 odulus of 310 GPa and a 36.5 GPa and 0.25. The resulting wear tracks were measured to have a diamet er of 7 mm and be 40 m wide. In order to generate wear tracks with sufficient width for spectroscopic measurements, a flattened pin head was employed. It was created by sanding a flat onto a 3 mm diameter, stainless steel ball with 1500 grit sandpaper for several thousand cycles. The raw normal and lateral signals were recorded as a function of time every 0.05 seconds. Th ese were then then averaged every 1.00 second and saved to a summary output file. The resulting coefficient of friction v ersu s cycles gr aphs show an average of the summary data file for every cycle, 12 seconds. All samples experience a run in period As a result, al l samples were run for at least 300 cycles in the given environment in order to establish a steady state behavior The environ mental chamber was equipped with a turbo molecular pump with a backing mechanical pump. The base chamber pressure was less than 5x10 8 Torr. Species were introduced after the chamber was isolated with a mechanical gate valve. The sample was tested in envir onments isolating each atmospheric component of interest, at partial pressure s corresponding to those of
76 ambient conditions : air (760 Torr), vacuum (1x10 7 Torr), nitrogen (6 10 Torr), oxygen (150 Torr), and water (8 Torr). In a separate stud y HOPG sample s were treated with ozone (O 3 ) from an Ozone Solutions ozone generator Ultra high purity (UHP) oxygen was used as a feedstock. The generated ozone was discharge d within a fume hood or the stainless steel environmental chamber through the dosing l ine nozzle (760 Torr) For spectroscopic analysis of the wear track area, the sample was run under a constant ozone flux for 900 cycles. All measurements were taken at room temperature 298K Samples were analyzed before and after tribological testing usin g an Omicron Nanotechnology GhmB Al K monochromatic ( 1486.7 eV ) XPS source operated at 300 W, together with a 7 channel Sphera hemispherical analyzer. The base pressure of the XPS chamber was 1x10 10 Torr. The energy analyzer was calibrated to an Ag 3 d 5/2 binding energy peak of 368.0 eV. The analysis region had a high magnification diameter of 630 m. The samples were oriented at a takeoff angle of 55 defined as the angle between the analyzer axis and surface normal. XPS spectra were taken of the C 1 s an d O 1 s energy regions All core shell analys e s were conducted at a 0.05 eV step size, 1.0 second integration time, and 20 eV pass energy. XPS peaks were fit following a Shirley background subtraction method 183 and assuming a 90% Gaussian 10% Lorentzian sha pe. Elemental concentrations were calculated from normalized peak areas using published atomic sensitivity factors 184 Results The pin on disc testing of HOPG in different environments showed diverse responses as illustrated in the data of figure 3 2 The ambient performance of HOPG was confirmed to have low friction, low wear characteristics, fig ure 3 2a. There was little
77 visible dusting on the sample after running for 300 cycles. The average coefficient of friction, between cycles 100 and 200 was calcu lated to be 0.070. This is in contrast to vacuum performance which was confirmed to be extremely poor, fig ure 3 2b. The average coefficient of friction was not only larger, 0.165, but the recorded f riction curve itself was erratic and increasing throughout the t est indicative of wear taking place during the course of the measurements Wear was further verified through the presence of loose debris, or dust, near the wear track on the sample. Testing in pure, dry nitrogen showed a smaller average coefficient of fr iction (0.084) than vacuum, but slightly larger than ambient, fig ure 3 2c The standard deviation was also larger than ambient testing. Upon visual inspection, there was some dust near the wear track. The tribology of HOPG in oxygen was measured to behave similarly to that of ambient. The average coefficient of friction was 0.070, with little to no dusting of the sample fig ure 3 2d This low friction, low wear behavior in an oxygen environment correlates with previous studies. The last ambient environment al component test concerned water. HOPG in a water saturated environment showed low friction, low wear performance on par with both ambient and oxygen conditions. The average coefficient of friction for water was 0.075, fig ure 3 2e Again, the signal varia nce was small and the friction value fairly constant throughout the test. This behavior is typical of HOPG low friction sliding. The theory of edge plane passivation would suggest that oxygen and water help to minimize friction by reducing the surface en ergy of HOPG. Given that our testing showed both oxygen and water to be beneficial in bring ing about low friction, low wear performance, it is of interest to understand the role of a more reactive species of oxygen in influencing the tribological propertie s of graphite. These studies would also help to
78 understand how atomic oxygen plays a role in space tribological conditions. If indeed oxygen is an important passivator for HOPG, then exposure to ozone during ambient testing should yield information about h ow reactivity and oxygen in general affect tribological performance. The c oefficient of friction v er s us ozone rich conditions is shown in fig ure 3 2f While the average coefficient of friction in ozone rich conditions is 0.078, the overall effect of having ozone interactions with HOPG yields a fairly low friction, low wear outcome. This graph would suggest that oxygen as an element and the reactivity of a species is an important factor in enabling low friction behavior for HOPG XPS measurements of HOPG w ere taken before and after 30 seconds of ozone exposure and over 3 hours of ozone exposure Figure 3 3 shows the resulting carbon 1 s and oxygen 1 s spectra. Freshly cleaved HOPG yields a carbon 1 s peak at 284.10 eV. This arises from the inert, basal C C bon d 184,185 Asymmetry of the peak is noticeable and corresponds well with published graphite spectra 157,186 The source of the asymmetry is unique to graphitic carbon and is believed to be caused by higher energy edge site photoemission. A broad feature arou nd 291 bonding of the sp 2 hybridized C C atomic bond 157 There was no significant signal from the oxygen 1 s spectrum. Upon a brief, 30 second exposure to ozone, the carbon 1 s spectrum remains unchanged, however the oxygen 1 s spectrum shows a two component formation. The larger component was fit at 531.60 eV and the smaller at 533.05 eV. These are thought to be due to the dissociation of ozone at the carbon surface to create C=O and C O bonds 187 Given that there is no significant diff erence between the carbon 1 s spectra of freshly cleaved HOPG and briefly exposed HOPG, the C O species are thought to be too few to be readily discernable given the inherently
79 small cross section of the carbon atom A greater difference is found upon prol onged ozone exposure. A peak located at 288.20 eV is associated with C=O 187,188 XPS m easurements taken of prolonged ozone exposed HOPG at larger and smaller takeoff angles show the area percentage of the O 1 s area to the C 1 s area to increase with the ta keoff angle. The percentage of O 1 s ranges from 3.5 at % for 30 degrees and increases to 8.0 at % at a takeoff angle of 75 degrees. Therefore, oxygen is seen to be relegated to the surface of ozone exposed, unworn HOPG. Figure 3 4 shows both the C 1 s and O 1 s spec tra of ozone exposure HOPG at takeoff angles of 30, 55, and 75 degrees. In a separate experiment, XPS characterization of the wear tracks from operation under a flux of ozone for 900 cycles was done both insid e and outside of the wear track (figure 3 5 ) Compositional analysis of the wear track shows it to contain less oxygen than the unworn area, 7 at % to 9 at % oxygen. The calculated oxygen composition of the unworn, ozone exposed HOPG is increased further at a 75 takeoff angle, 10 at % to 15 at % oxygen. Figure 3 5 portrays the carbon 1 s and oxygen 1 s difference spectra of the wear track and the unworn area. The first set of experiments verifies the interaction of ozone species with the surface of HOPG. The accumu lation of C O and C=O species onto the nascent HOPG surface Wear tracks produced under a flux of ozone still show the presence of a C=O species. This indicates that while there is less oxygen within the wear track than outside of it, the presence of oxyge n does not automatically lead to increased wear. D usting of the graphite surface wa s confirmed by the presence of visible debris on the surface primarily by sliding under 10 7 Torr vacuum and was less pronounced under 610 Torr nitrogen
80 Conclusions The co ntrast between ambient and vacuum sliding over HOPG was dramatic. Sliding in humid air exhibited a smooth, low friction behavior, while under vacuum, both the friction signal and deviation increased, indicative of wear The re was no environmental effect of sliding under partial pressures of the reactive atmospheric species, oxygen and water In fact, it was only under an environment of 610 Torr nitrogen, that could induce some wearing of the sample surface, however the friction signal remained relatively lo w, 0.086. This suggests that low friction behavior of HOPG is not inherently linked with available oxyg en or water molecules. Wear however is strongly affected by the absence of reactive species. T he presence of a highly reactive o zone species on the near surface of the wear track does not produce significant wear or a higher coefficient of friction. is essential for continued low friction behavior. This correlates with previous studies that in dicated that oxygen acted a s an edge passivating species for gra phitic samples 82,158,161,173,189 The presence of a highly reactive species such as ozone produces favorable bonding of oxygen with exposed carbon dangling bonds throughout the tribological interaction. The loss of oxygen throughout the wear process would explain the difference in oxygen content between the wear track and off of it. T here is not an appreciable chemical difference between ins ide and outside the wear track, meaning that the presence of oxygen within the wear track does not adversely affect th e tribological performance of HOPG films. The oxygen 1 s spectrum presents a similar story. There is only a reduction in the intensity of the spectrum inside the wear track than outside.
81 A B Figure 3 1 The crystal structure models of traditional solid lubricants A) graphite with four atomic layers, and B) MoS 2 showing four molecular layers. Molybdenum atoms are colored purple and sulfur atoms are yellow.
82 A B C D E F Fig ure 3 2 Average c oefficient of friction v ersu s cycles for highly oriented pyrolytic g raphite pin on disc testing in A ) Air 50% RH B)V acuum, C)Nitrogen, D)Oxygen, E)Water, and F ) Ozone exposure environments
83 A B Fig ure 3 3 X ray photoelectron spectroscopy measurements of freshly cleaved highly oriented pyrolytic graphite after 30 se cond exposure to ozone, and after 3 hours of exposure to ozone A ) Binding energy adjusted and area normalized c arbon 1s spectrum B ) Binding energy adjusted o xygen 1 s spectrum A B Fig ure 3 4. X ray photoelectron spectroscopy measurements of freshly cl eaved highly oriented pyrolytic graphite after 3 hours exposure to ozone at takeoff angles of 30, 55, and 75 degrees of the A ) carbon 1 s spectrum and B ) oxygen 1 s spectrum
84 A B Fig ure 3 5 X ray photoelectron spectroscopy measurements of OFF wear trac k, ON wear track, and the ON OFF wear track difference of highly oriented pyrolytic graphite run in ozone for 900 cycles A ) Carbon 1 s spectrum B ) Oxygen 1 s spectrum
85 CHAPTER 4 THE TRIBOLOGY OF M O S 2 S B 2 O 3 A U COMPOSITE COATINGS Introduction In the world of solid lubricants, molybdenum disulfide (MoS 2 ) is primarily used in dry, water free environmental applications. Its lubricity is thought to derive from its layered crystal structure containing S Mo S int ra layer bonds. However the performance of MoS 2 bas ed lubricants suffers when water is introduced to the operating environment. To solve this problem, additives such as Au 29,30 Ti 44 WSe 2 51,63 PbO 23 Sb 2 O 3 67 and many other elements/compounds have been incorporated into deposited films, aiming to address a range of issues in film performance Based upon more than two decades of research, investigators have developed several mechanisms thought to be responsible for the improvement of ambient MoS 2 based coating lifetimes. For example, it has been hypothesiz ed that s oft metals, such as Au, assist by densifying the microstructure of the film and enhancing the favorable (002) orientation of MoS 2 29,30 Inter metallic elements such as Ti are thought to enhance the hardness of the material and decrease the oxidati on rate of Mo by serving as the more thermodynamically reactive metal 26 Th e incorporation of Ti into MoS 2 has resulted in a lower wear rate compared to pure MoS 2 Substitutional additives such as W and WSe 2 are thought to produce localized strain within t he layer, leading to decreased oxidation and preferential orientation 63 The incorporation of oxides like PbO and Sb 2 O 3 has been shown to play several roles. PbO has been shown to densify the microstructure as well as to re act with Mo at higher temperature s and stabilize the film 23,66,84 Sb 2 O 3 is thought to be a mechanically stabilizing phase existing just beneath the uppermost MoS 2 layers 65
86 Today, composite films comprised of MoS 2 Sb 2 O 3 and Au are widely used for lubricating mechanical components in sp ace. While new coatings are currently under development, there still remain many unanswered questions regarding the mechanisms governing the performance of MoS 2 Sb 2 O 3 Au films over a wide range of environments. In a recent paper, Scharf et al 99 used focus ed ion beam techniques (FIB) and transmission electron spectroscopy (TEM) to show that the particle size and concentration of gold increase s at the surface as a result of tribological interactions. This effect was most evident with sliding under humid, amb ient conditions. Testing under dry nitrogen conditions resulted in the formation of larger gold nanoparticles near the surface, with a (002) oriented MoS 2 phase present at the sliding interface. Testing under ambient, 50% RH, resulted in more severe agglom eration of gold. The wear track surface was found to consist of both a crystalline, gold phase with a (111) orientation and a (002) oriented MoS 2 phase. The transfer film on the ball was solely (002) MoS 2 The presence of crystalline gold at the surface wa s credited for the resulting increase in friction, leading to a calculated shear strength value nearly doubled for the film under ambient than dry nitrogen conditions. There was little difference in the wear rates for the two environments over both short a nd long term testing. The present study seeks to examine the effect of exposure to environmental components on the tribological properties of MoS 2 Sb 2 O 3 Au commercial coatings. The study has investigated the change in interfacial properties (mechanical, c hemical and morphological) that occur during sliding and attempts to verify the role of MoS 2 Sb 2 O 3 and Au within such active interfaces.
87 Experimental Samples MoS 2 Sb 2 O 3 Au samples were magnetron sputtered and burnished onto (100) silicon sections by Hohm an Plating Inc of Dayton, Ohio. The thickness of the samples was 1 dimensions were 14 mm by 10 mm allowing direct mounting onto Omicron Nanotechnology GhmB platens f or use with both the p in on disc tribometer and sample transfer to the XPS analysis chamber. X ray D iffraction the assistance of Dr. Valentin Craciun using their MRD system with a Cu K 1 X rays. Measurements were taken at 5 incidence to increase the surface sensitivity of film analysis. The measurement was taken with a step size of 0.02. The temperature was ambient, 25 C. Pin on D isc T ribometry The pin on disc tribometer features a rotating sample stage holder attached directly to a drive shaft of a Maxon vacuum compatible motor with an 84:1 reducer gearbox. The tribometer arm is equipped with strain gauges to measure normal and lateral forces during operation and was calibrated usin g standardized masses. The ball was made from 440C steel. A flat was produced on the ball by polishing with increasingly fine sandpaper, up to a maximum of 1500 grit, in order to increase the contact area. The diameter of the flattened section was approxim ately 1mm, in turn producing the wider wear track needed for subsequent analysis. Films were run in each experimental condition twice for 5000 cycles each time Table 4 1 shows the
88 parameters of the pin on disc tests. The width of each wear track varied wi th the environmental condition, with the largest being at least 800 m via optical microscopy. Normal and tangential force signals were collected every 50 milliseconds; this data was averaged at 1 second intervals and saved to a summary data file. The summ ary data was then averaged again at 12 second intervals to generate an average friction coefficient for each cycle; these were plotted as the average coefficient of friction per cycle in figure 4 5 The coefficient of friction reported for a specific envir onmental condition represents the average of cycles 1000 5000. Uncertainty in measured friction coefficient is better than 0.002, which is substantially less than the variation of the measured friction coefficient 85 X ray P hotoelectron S pectroscopy An Omi cron Nanotechnology GhmB Al K (1486.7 eV) monochromatic XPS source was used with an EAC2000 Sphera hemispherical, 7 channel analyzer. Calibrated with a 99.99% pure silver sample to measure a silver 3 d 5/2 peak at 368.30 eV, the number 3 aperture gave an el ectron collection area of 700 m in diameter. All XPS core spectra were taken using a step size of 0.05 eV, and a pass energy of 20 eV The elemental cores measured were molybdenum 3 d sulfur 2 p antimony 3 d oxygen 1 s carbon 1 s and gold 4 f The antimony 3 d and oxygen 1 s core spectra occur within the same binding energy range and thus were measured together. Sensitivity factor s for quantitative analysis assumed a 90 degree relationship between the incident X rays and the electron analyzer. 184 All 4 f peak splits were fixed to a 3:4 area ratio, 3 d peak splits to a 3:2 area ratio, and 2 p peak splits to a 2:1 area ratio. The ful l width at half maximum (FWHM) of peaks corresponding to specific species was constrained to be the same throughout the series of meas urements. All peaks were fitted with a 90% Gaussian,
89 10% Lorentzian shape. A fitting algorithm, minimizing the squared sum of the difference between the artificial peaks and the measured spectrum, was used to finalize the overall peak fit Fitting was comp lete when the value of the standard deviation of the residual spectrum was less than 2.0. A tomic F orce M icroscopy A n Asylum Research MFP 3D operated in constant con tact mode to analyze b oth the as received films as well as the wear tracks of air and vacuu m. The approach employed a Si 3 N 4 cantilever / tip assembly possessing a normal spring constant of 0.61 N/m The size and distribution of components within the film surfaces were analyzed through the detection of lateral forces, according to previously descr ibed procedures. Results As Received Film Characterization The as received films were characterized for both crystallography and microstructure. The illustration of the XRD analysis testing is presented in figure 4 1. The results of the low angle analysis are given in figure 4 2. The measurement shows no d ramatic peaks associated with a cry stalline sample The broad intensity formation around 15 could originate from MoS 2 but overall, the profile is more representative of an overall amorphous microstructure and matches well with other XRD measurem ents of similar coatings containing antimony oxide constituents 49,99 Also of note is the absence of any peaks originating from the substrate. This suggests that depth of analysis was shallower than the film thickness Another symmetrical scan about the normal axis of the coating reinforced the amorphous microstructure with a similar scan profile.
90 Figure s 4 3 to 4 5 are images taken with the AFM of the as received film at different scan sizes and of different features within the microstructure. The first set of images was taken from 20 m to 1 m figure 4 3 The microstructure is made fr om sub micron sized, agglomerated features dispersed throughout the film. A majority of the accurately represented by these images. The surface roughness as measured for the 20 33 nm. Roughness decreased slightly to 25 nm in the 2 size scan. There were two noteworthy features located within the microstructure. The first was single smooth, micron sized feature that exh ibited a lower friction behavior than its sur roundings figure 4 4. The images were taken at 10 m, 5 m and 2 m scan sizes. The left column features the topographical images and shows the relative heights of the surface with increasing magnification The feature is difficult to res olve in the topo graphical imagining method but can be identified by its straight polygonal shape, size and relatively smooth surface Looking at the right column highlights the feature very well figure 4 4d to 4 4 f. The right column is of images taken simultaneously wit h the ones on the right, but these were captured using the lateral force microscopy method. The low intensity regions are indicative of a measured relatively low er friction force interacting on the scanning AFM tip. Due to the constituency of the as receiv ed film, this feature is assumed to be MoS 2 Au and Sb 2 O 3 are n ot expected to display such a low value friction signal based upon previous measurements and experience The last set of AFM images were taken of a void in the microstructure, figure 4 5. The t opographical image of the void is best represented in the first image, figure 4 5 a. The scan sizes were taken from 40 m, 20 m, and 10 m. The magnitude of the
91 vertical change between the side walls of the void and the surface of the microstructure affects the intensities displayed to the left and right of the void as the tip scans across the feature. Pin on Disc Tribom etry Pin on dis c testing of the MoS 2 Sb 2 O 3 Au coatings showed a strong dependence on e nvironment ( f igure 4 6 ) The average coefficient of friction measured in ambient air of 50% relative humidity was 0.156 from cycle 1000 to 5000. In contrast, t he average coefficient of friction within the same range of cycles in vacuum was 0.054. Oxygen had of friction of 0.068. However, it was the presence of water that dramatically increased friction between the pin and the film resulting in an average coefficient of friction similar to that in air, 0.132. These results, measured under well characterized and carefully defined environments, serve to confirm the relationship between the water in the atmosphere and poor tribological performance of MoS 2 based films. X ray P hotoelectron S pectroscopy Measurements I llustration s showing the sample analysis configuration used in the XPS measurements of MoS 2 Sb 2 O 3 Au coatings on and off of t he wear track are shown in figure 4 7. Characteristic XPS spectra for the as received film and the wear track in air are shown in f igure 4 8 The left hand set was taken from outside the wear track while the right hand set was measured from within the wear track. The first row contains the Mo 3 d spectrum. Splitting of the Mo 3 d 5/2 and 3 d 3/2 peaks was set to have an energy diff erence of 3.1 eV 184 MoS 2 peaks were identified at binding energies of 229.0 eV and 232.1 eV 184,190 MoO 3 peaks were assigned at bind ing energies of 232.2 eV and 235.3eV 41 The remaining minor portion of the spectrum is said to be sulfur deficient
92 MoS 2 x at 230.3 eV and 233.4 eV, corresponding to results of sputtering studies of MoS 2 which showed a linear relationship between the oxid ation state of molybdenum and its binding energy 113 The convoluted S 2 p spectrum arose from a number of species; for each, a 2 p split o f 1.18 eV 184 In fitting the spectra, peaks assigned to MoS 2 species we re restricted to a 2:1 area ratio Sulfur peaks a ssociated with this assignment appear at 161.7 eV and 162.9 eV 110,184 For fitting purpose, a 1.33:1 area ratio was set for the sulfur deficient MoS 2 species, with slightly higher binding energies of 163.0 eV and 164.2 eV 113 The species was not thought to be a reduced form of Sb 2 O 3 due to the unfit area within the S 2 p spectrum and the absence of a n intensity in the O 1 s spectrum The Sb 3 d and O 1 s spectra are inherently overlapped and were the most technically challenging to assign. Sb 3 d peaks at 530.7 eV and 540.0 eV were assigned with a peak splitting of 9.34 eV to Sb 2 O 3 species 184,191,192 Although this is higher than 530.0 eV, often listed value in literature, this gave a consistent fit throughout the series of measurements The O 1 s peak from both t he MoO 3 and Sb 2 O 3 were assigned to an intensity at 530.7 eV, consistent with the little difference between the species reported in literature 184,193 Also in this spectral region, intensity at 531.7 eV and 532.4 eV were attributed to C=O and C O species 188 ,194 The C 1 s spectrum is not shown, but the fit of O 1 s at these energies was strictly guided by the stoichiometric relations to the C 1 s and agreed with the establis hed Au 4f values at 84.2 eV and 87.9 eV 184 Interfacial sliding in air was observed to result in compositional changes, marked by an increase in MoS 2 at the surface of the wear track. In all experimental conditions, the solid lubricant was found to be more prevalent in the wear tracks than in the as
93 received films themselves. This is most evident after operation under vacuum. Figure 4 9 displays the changes in relative ratios of film constituents, derived from the quantitative fits of the XPS spectra, perfo rmed in accordance with the procedures outlined above. Unique to vacuum operation, the Sb 3 d spectrum confirms the formation of metallic Sb, likely as a result of the mechanical deformation of the Sb 2 O 3 particles in an oxygen free environment. It is hypoth esized that as tribological interactions continue under this environment, oxide particles lose oxygen atoms to the film or more likely, to the low pressure surroundings. Measurements taken from the film itself and an area containing the wear track produced while sliding under an oxygen environment show a n expression of MoS 2 at the surface. Both the MoS 2 : MoO 3 and MoS 2 :Sb 2 O 3 have positive changes of 20.71% and 11.51% in value T he change of MoS 2 :Au and S : Au values showed a trend similar to sliding under vacuu m with a positive change of in the ratios of 6.04% and 4.45% The se results suggest that the amount of sulfur relative to molybdenum in MoS 2 decreases while sliding under vacuum and oxygen. Given that MoS 2 : MoO 3 was shown to increase at the surface, the o xygen of the environment could be substituting with vacant sulfur sites during sliding However, the oxygen itself does not react completely with MoS 2 but instead produces MoS 2 x O x species This effect has been seen before by Lince 93,195 who determined that some oxygen substitution into the MoS 2 lattice aided in reducing the friction of sputtered MoS 2 films. Sliding in partial pressures of water produced high friction, similar to the values obtained while sliding in 50% RH air. The spectroscopic measure ments of wear track produced under 8 Torr water r eveal s a negative ratio change of MoS 2 :MoO 3 This can
94 be interpreted as being indicative of either extensive oxidation of MoS 2 to MoO 3 within the wear track or as a decrease in the overall MoS 2 surface expre ssion in favor of another species. This is the only environmental condition that yields an increase of MoO 3 at the surface. Thermodynamically, the oxidation of MoS 2 to MoO 3 is non spontaneous with water as a reactant. This process is correlated with a redu ction in the MoS 2 to Sb 2 O 3 ratio. The constituent ratios involving Au reveal a decrease for all three ratios, MoS 2 :Au, S:Au, and Sb 2 O 3 :Au, indicating that Au does not appear to significantly contribute to the tribological response under humid conditions. T herefore, sliding under water saturated conditions is viewed to lead to the oxidation of the solid lubricant, MoS 2 producing a higher friction than observed in either vacuum or oxygen environments. According to the findings of Scharf et al 99 gold i s bel ieved to crystallize at the surface of MoS 2 Sb 2 O 3 Au films under ambient conditions From the results presented here, it is noted that MoS 2 :Au, S:Au and Sb 2 O 3 :Au all decreased with respect to their presence in the as received film, after sliding under amb ient conditions. For sliding under vacuum conditions, a decrease in the Sb 2 O 3 :Au ratio potential results from breakdown of oxide particles during testing. Unlike for ambient conditions, this is observed in the absence of decreases in other ratios involving gold. It is noted that the ~ 1 at% of Au in the near surface region renders the assignment of compositional differences difficult Topography and Microtribometry Measurements Optical photographs of the atomic force microscope ( AFM ) tip over the analysis ar ea of the film are shown in figure 4 10. AFM contact mode images of the MoS 2 Sb 2 O 3 Au coatings indicated the films to be comprised of micron sized clusters of particles
95 appearing as a collection of nodules across the surface ( figure 4 11 ) T opographical im ages are shown for the wear tracks produced under 760 Torr air (figure 4 12), 10 7 Torr vacuum (figure 4 13), 150 Torr oxygen (figure 4 14), and 8 Torr water vapor (figure 4 15) environments. The wear tracks produced under air, vacuum, and oxygen environme nts show a densified microstructur e. Sliding under water generate d a surface top ography characterized by micron sized pores. Fig ure 4 1 6 display s the topographical and lateral force maps for the as received film, the wear track of a sample run in air, and the wear track created in a vacuum environment. Additional images of the wear tracks produced by sliding under oxygen and water are shown in figure 4 1 7 The oxygen wear track had a uniform friction force map with an average value of 60 nN, while the water produced wear track had a lower average friction force value of 27 nN For the water surface the average friction was dominated by the large areas across the smooth film surface while areas within pores exhibited higher values The roughness value of the as received MoS 2 Sb 2 O 3 Au coating was high, 30.4 nm for the 20 m sca n size. The roughness values decreased within the wear tracks. The roughness was highest in wear tracks produced in 8 Torr water, (21.7 nm) and the lowest value attained under vacuum (16.1 nm). When measured in air, the as deposited film surface exhibited an average lateral force signal approximately three times greater than that within the wear tracks produced in air. Across the surface of this wear track, individual components are distinguished through localized regions of higher ( ~ 2X) friction. Similar l ateral force measurement of the wear track formed in vacuum revealed the surface to consist of regions of low friction decorated with isolated points of high friction. The regions of higher intensity friction within the ambient wear track are
96 consistent wi th an enhanced presence of Sb 2 O 3 or more likely, Au at the surface, given that the XPS data suggest ambient wear track composition changes in favor of Au over Sb 2 O 3 Because the AFM measurement were conducted in air, the wear tracks generated under ambien t and vacuum conditions do not exhibit large differences in frictional character as seen in the pin on disc measurement. These results are consistent with prior AFM investigations of compositionally pure MoS 2 interfaces. Instead, the present AFM results ar e seen to primarily confirm the restructuring of the MoS 2 Sb 2 O 3 Au composite film surface to consist of flattened terraces decorated by regions of higher friction Conclusions Environmental testing under ambie nt and vacuum conditions, as well as under par tial pressures of oxygen and water was found to produce differences in the tribological behavior, composition, and microstructure of MoS 2 Sb 2 O 3 Au films. As with most MoS 2 based films, vacuum conditions p roduced the most favorable tribological performance. There, a low coefficient of friction correlates with an enhancement of the MoS 2 composition within the wear track area. In contrast to the vacuum results, sliding under ambient 50% RH conditions produced interfaces exhibiting high friction, accompanied by a decrease in Sb 2 O 3 composition and possible decomposition to elemental Sb within the wear track. The microstructure of the wear tracks w ere smooth, yet displayed the potential contribution of high friction components across the film surface. Tes ting in w ater saturated conditions yielded a higher coefficient of friction but little additional information was taken from the spectroscopic measurement in terms of compositional alterations Oxygen was found to play a very limited role in tribological behavior and no significant composition changes were measured. At the root of the
97 performance of this composite film coating, it appears that the expression of gold at the surface during ambient operation serves to define the ultimate tribological behavior for the material under these conditions. Fu r thermore, the systematic testing in well controlled environments allows water to be identified as the primary species responsible for this surface compositional change, although the exact mechanism by which this transfor mation occurs is yet to be resolved.
98 Figure 4 1 Illustration of low angle X ray diffraction measurement of MoS 2 Sb 2 O 3 Au coating. The red colored region indicate s the approximate area of sampling. The incident angle for this measurement was 5 Fig ure 4 2 X ray diffraction measurement of MoS 2 Sb 2 O 3 Au coating taken at a angle
99 A B C D Figure 4 3 Atomic force microscopy topographical images of the as received MoS 2 Sb 2 O 3 Au coating taken at A)20 m scan size, B)10 m scan size C) 5 m scan size and D)2 m scan size. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt. I ntensity scales are set equal to each other for ease of comparison.
100 A D B E C F Figure 4 4 Atomic fo rce microscopy topographical images of the low friction feature within the as received MoS 2 Sb 2 O 3 Au coating taken at A)10 m B)5 m C)2 m and the concurrent lateral force signal maps taken at D)10 m E)5 m, and F)2 m. Later al force magnitude scales wit h intensity. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt. I ntensity scales are set equal to each other for ease of comparison.
101 A B C Figure 4 5 Atomic force microscopy topographical images of the void feature within the as received MoS 2 Sb 2 O 3 Au coating taken at A)40 m B)20 m C)10 m. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt. I ntensity scales are set equal to each other for ease of comparison. Table 4 1: The parameters of MoS 2 Sb 2 O 3 Au pin on disc tests Operating Para meter Value Normal load 1.0 N Disc rotation speed 5 rpm Cycles 5000 Wear track diameter 7 mm Linear sliding speed 1.9 mm/s Contact pressure 1 MPa Temperature 25 C Environment 760 Torr air at 50% RH 150 Torr oxygen 8 Torr water 10 7 Torr vacu um
102 Figure 4 6 Graph showing the average coefficient of friction v ersu s cycles of MoS 2 Sb 2 O 3 Au coatings under different environmental conditions for 5000 cycles clockwise A B Figure 4 7 Illustrations of X ray photoelectron spectroscopy measur ements taken of A)As received MoS 2 Sb 2 O 3 Au coating and B)Wear track produced under different conditions. The red colored regions indicate the approximate area of sampling. All measurements were taken at a take off angle of 55
103 Figure 4 8. X ray p hoto electron s pectra of unworn MoS 2 Sb 2 O 3 Au A) Molybdenum 3 d spectrum B) Sulfur 2 p s pectrum C) Antimony 3 d and Oxygen 1 s s pectra D) Au 4 f s pectrum and MoS 2 Sb 2 O 3 C wear track under air ( 50% RH ) E) Molybdenum 3 d s pectrum F) Sulfur 2 p s pectrum G) Antimony 3 d an d Oxygen 1 s s pectra and H) Au 4 f s pectrum
104 Figure 4 9 X ray photoelectron spectroscopy measurements of the change in the ratio from off the wear track region to on the wear track region of MoS 2 Sb 2 O 3 Au coatings. A B Figure 4 10 Opti cal images of atomic force microscope cantilever measurement positions on A ) as received MoS 2 Sb 2 O 3 Au co ating and B ) the wear track produced in air.
105 A C B D Figure 4 1 1 Atomic force microscopy topographical images of the as received MoS 2 Sb 2 O 3 Au coating taken at A) 10 m scan size B ) 2 m scan size, and of the wear track produced in air 50% RH taken at C ) 10 m scan size D ) 2 m scan size. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt
106 A B C D Figure 4 12. Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 Au coating produced by s liding under 760 Torr air (50% relative humidity ) taken at A)20 m scan size B)10 m scan size, C)5 m scan size, and D)2 m scan size. All images were taken usi ng a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt.
107 A B C D Figure 4 13. Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 Au coating produced by sliding under 10 7 Torr vacuum taken at A)20 m scan s ize B)10 m scan size, C)5 m scan size, and D)2 m scan size. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt.
108 A B C D Figure 4 14. Atomic force microscopy topographical images of wear track of the M oS 2 Sb 2 O 3 Au coating produced by sliding under 150 Torr oxygen taken at A)20 m scan size B)10 m scan size, C)5 m scan size, and D)2 m scan size. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt.
109 A B C D Figure 4 15. Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 Au coating produced by sliding under 8 Torr water taken at A)20 m scan size B)10 m scan size, C)5 m scan size, and D)2 m scan size. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt.
110 Figure 4 1 6 Atomic force microscopy topographical images of various MoS 2 Sb 2 O 3 Au coatings A)As received MoS 2 Sb 2 O 3 Au coating, B)Wear track created under ambient 50% RH, C)Wear tra ck created under 10 7 Torr vacuum and concurrent lateral force maps of D)As received MoS 2 Sb 2 O 3 C coating, E)Wear track created under ambient 50% RH, and F)Wear track created under 10 7 Torr vacuum Intensity scales have been set constant for both topograp hical and lateral force maps. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt
111 A B C D Figure 4 17. Atomic force microscopy topographical images of MoS 2 Sb 2 O 3 Au coating A) wear track of the pro duced by sliding under 150 Torr oxygen, B) wear track produced by sliding under 8 Torr water and concurrent lateral force maps of C) wear track of the produced by sliding under 150 Torr oxygen, and D) wear track produced by sliding under 8 Torr water. Inte nsity scales have been set constant for only the lateral force maps. All images were taken using a 0.42 N/m sharp cantilever in constant contact mode of 1 Volt
112 CHAPTER 5 THE TRIBOLOGY OF MOS 2 SB 2 O 3 C COMPOSITE COATINGS Introduction As an essential compo nent to the deployment and operation of increasingly complex space apparati, solid lubricants are constantly asked to deliver reliable, low friction performance over a variety of environmental conditions 2,6 Molybdenum disulfide (MoS 2 ) is a common industri al solid lubricant with a layered crystal structure that has been relied upon for many decades 7,8 The drawback to using MoS 2 is its sensitivity to environmental conditions 169,22,162 Operation under humid, ambient conditions has been found to lead to high friction, wear, and premature failure 149 In the past, materials such as gold 29,30 titanium 44 lead 35 carbon 196,77,19 tungsten diselenide (WSe 2 ) 51 and antimony trioxide (Sb 2 O 3 ) 64,65 have been evaluated as additives in an effort to address this problem. continuing into the following decades aimed at explaining its inherent low friction behavior, the source of environmentally driven high friction, and substrate optimization 1 97,170,198,13,17,9 The 1980's and 1990's saw an explosion of research into additives for MoS 2 their effectiveness, and the theory of their functions. In the last decade, the focus of solid lubricant design expanded to the development of adaptive films t o provide low friction across a wider range of operating conditions as well as cyclical applications between different extreme environments. This goal was pursued through the incorporation of a second lubricating constituent, creating ternary, even quatern ary component films 6,16,70,87,199 202 MoS 2 Sb 2 O 3 C is one such adaptive coating, combining the proven vacuum tribological performance of MoS 2 with
113 graphitic carbon, commonly used for ambient, lubrication applications 159,83 While there are only a few repo rts on this particular class of coating, they are generally thought to function by preferential constituent migration towards the active interface, with the top constituent becoming the primary lubricant. Secondary migration has also been observed and is t hought to play a mechanical role supporting the active lubricant layer and providing a barrier to crack propagation, which can lead to wear and premature failure Prior reports of the tribology of MoS 2 Sb 2 O 3 C coatings are extremely limited 83,203,85 Zabi nski et al 83 ., utilized Auger, Raman, and energy dispersive spectroscopy (EDS), SEM, and TEM to investigate the wear track of this coating produced under different environmental conditions. Tribological testing of the coatings demonstrated high friction be havior for an ambient, humid environment, and low friction for both vacuum and dry nitrogen environments. Hamilton, et al 203 have also demonstrated low friction, low wear performance for these same coatings under dry nitrogen conditions. The Zabinski stud y reported evidence that the constituents present in the wear track change depending upon the operating environment. MoS 2 was measured to be the main component of tracks worn in vacuum and dry nitrogen conditions. The wear track in air was dominated by gra phitic carbon and Sb 2 O 3 according to AES measurements. Raman spectroscopy measurements did not detect a Sb 2 O 3 signal. Tunneling electron micrographs and energy dispersive spectroscopy show carbon to be concentrated both at the tribological interface as wel l as surrounding the larger Sb 2 O 3 particles immediately below the interface. Environmental cycling between dry nitrogen and ambient conditions yielded wear tracks with distinct Raman spect r a. The evidence pointed towards MoS 2 acting as the primary lubrican t in a dry
114 nitrogen environment, while graphite was the tribologically active constituent in the humid, ambient environment. Sb 2 O 3 was acknowledged as playing several critical roles: reorienting the active lubricating component, providing a mechanically ha rd surface to support and isolate a thin layer of the active lubricant, preventing crack growth, and serving as an oxidation barrier. It has also been shown to affect the microstructure of MoS 2 based films, reducing intercolumnar porosit y 99 While the aut hors correlate wear track constituents under ambient, dry nitrogen, and vacuum conditions, there is a lack of testing of an oxygen rich, water free environment. Such an inclusion would isolate whether the presence of water molecules is solely responsible f or the degraded performance of the films, or whether it is a combination effect of oxygen and water acting together in an ambient, humid environment which brings about expedited surface crack formation and coating failure. Crack initiation and development is also a general topic which remains to be completely described. Understanding the location and process of subsurface micro crack formation and growth would benefit the eng ineering of future coatings. The objective of the present study is to correlate tr ibological performance with environmental, chemical, and microstructural changes in order to advance the model of constituent migration to the active interface of MoS 2 Sb 2 O 3 C under different environmental conditions. The study employs complementary charac terization techniques, XPS and AFM, to investigate and quantify the performance of MoS 2 Sb 2 O 3 C coatings and the precise chemical nature and microstructure of the wear track produced by sliding under controlled conditions. The coatings have been evaluated in 10 7 Torr vacuum, in the presence of air (50% RH), and in the presence of the individual components of this ambient condition: 150 Torr oxygen, 8 Torr water The studies were
115 performed with a custom in vacuo pin on dis c tribometer, located in an isolate d environmental chamber and interfaced with a monochromatic XPS system, thus avoiding the contamination of wear surfaces between tribological and spectroscopic tests. The design of the tribometer was based upon a series of space tribometers used as part of the MISSE 7 study 85 Microscopic analysis conducted with AFM provided topographical, microstructural and localized friction behavior information directly from the wear track. Experimental Samples MoS 2 Sb 2 O 3 C coatings 4 to 5 m thickness were deposited using a proprietary method onto (100) silicon and 316 stainless steel coupons by Tribologix Inc. of Dayton, Ohio. The silicon substrates 14 mm by 10 mm were directly mounted to Omicron Nanotechnology GhmB platens, allowing for use within both the pin on disc tribometer and Omicron Nanotechnology GhmB XPS. X ray Diffraction the assistance of Dr. Valentin Craciun using their MRD system with Cu K 1 X rays. Measure ments were taken at 5 incidence for the SS316 run and 3 incidence for the film analysis. Both measurements were taken with a step size of 0.02. The temperature was ambient, 25 C. Tribometry All measurements were made using a pin on dis c tribometer hou sed in an environmental chamber connected to the XPS system. The tribometer featured a rotating sample holder, which secured platens on which the MoS 2 Sb 2 O 3 C coated
116 substrates mounted ; these platens could be easily removed from the sample holders and plac ed directly into the XPS without ever opening the environment to and poisoning the sample with laboratory air. The tribometer arm is equipped with strain gauges to measure normal and lateral forces during operation and was calibrated using standardized mas ses. The ball was made from 440C steel. A flat was produced on the ball by polishing with increasingly fine sandpaper, up to a maximum of 1500 grit, in order to increase the contact area. The diameter of the flattened section was approximately 1 mm, in tur n producing the wider wear track needed for subsequent analysis. For each environmental condition, three tests of at least 300 cycles were performed in neighboring locations on the same sample in order to generate a large wear area to ensure that the majo rity of XPS data collected corresponded to regions of wear. Although there existed as much as 100 m between the wear tracks, the wear tracks composed approximately half of the analyzed area. The width of each wear track was measured to be at least 100 m via a camera mounted above the AFM stage. Normal and tangential force signals were collected every 50 milliseconds; this data was averaged at 1 second intervals and saved to a summary data file. The summary data was then averaged again at 12 second interv als to generate an average friction coefficient for each cycle; these were plotted as the average coefficient of friction per cycle. The coefficient of friction reported for a specific environmental condition represents the average of cycles 100 300. Uncer tainty in measured friction coefficient is better than 0.002, which is substantially less than the variation of the measured friction coefficient.
117 X ray P hotoelectron S pectroscopy An Omicron Nanotechnology GhmB Al K (1486.7eV) monochromatic XPS source was used with an EAC2000 Sphera hemispherical, 7 channel analyzer. Calibrated with a 99.99% pure silver sample to measure a silver 3 d 5/2 peak at 368.30 eV, a circular aperture produced an electron collection area 700 m in diameter XPS spectra were collected using a 0.05 eV step size and pass energy of 20 eV The spectra collected were from the molybdenum 3 d sulfur 2 p antimony 3 d oxygen 1 s and carbon 1 s core regions. The antimony 3 d and oxygen 1 s core spectra occur within the same binding energy range and thus were measured together. Sensitivity factors for quantitative analysis were taken from the PHI XPS Handbook and assumed a 90 degree relations hip between the incident X rays and the electron analyzer 184 3 d peak splits were fixed to a 3:2 areal ratio a nd 2 p peak splits were fixed to a 2:1 ratio. The fit of the oxygen intensity to a multiple of species was guided by stoichiometric relationships of the compounds found to contain oxygen. The Mo 3 d peak splits were fixed at 3.1 eV between the 3 d 5/2 and 3 d 3/ 2 components. S 2 p peaks were fixed at 1.18 eV between the 2 p 3/2 and 2 p 1/2 while Sb 3d peak splits were fixed at 9.34 eV separation values according to literature 184 All peaks within a core region were constrained to the have the same FWHM value and were fit with a 90% Gaussian, 10% Lorentzian shape. A fitting algorithm, minimizing the squared sum of the difference between the artificial peaks and the measured spectrum, was used to finalize the overall peak fit spectrum. Fitting was complete when the valu e of the standard deviation of the residual spectrum was less than 2.0. For example, the standard deviation of 1.77 in figure 5 6 a is comparable to an unfit area of 0.35 at% of the overall film composition.
118 A tomic F orce M icroscopy An Asylum Research MFP 3 D AFM was operated in a constant contact mode, analyzing both the as received films as well as wear tracks produced in air and vacuum. The Si 3 N 4 cantilever was measured to have a normal stiffness of 0.61 N/m via thermal harmonic analysis, and the lateral f orce sensitivity was calculated using the wedge method 120 Constituent grain size and distribution over the film surfaces were analyzed via lateral force detection The lateral signals were converted to maps of the lateral force acting on the cantilever du ring translation across the coating surface. The mean friction forces were calculated by taking the difference between the trace and retrace signals and dividing by two. Constituents within the microstructure displayed distinctive coefficients of friction, allowing the delineation of one constituent from another. Results As Received Film Characterization The process of XRD measurement is illustrated in figure 5 1. Measurements were taken of both the as received film, and of a region where a majority of sam ple was from the SS316 substrate. Figure 5 2 shows the XRD analysis of the as received MoS 2 Sb 2 O 3 C coating showed the presence of ordered, crystalline constituents. The measured spectrum showed a strong, primary peak at a 2 value of 14.41 corresponding for (002) MoS 2 7,91 Diffraction signals at 27.73 and 32.13 from Sb 2 O 3 can also be identified as (222) Sb 2 O 3 and (400) Sb 2 O 3 204 The signal resulting from the substrate can be identified at 43.65 44.23 and 50.77 Cry stal size analysis using 2 grains of 17 4 nm and 13 7 nm for (222) Sb 2 O 3 Carbon was too weak and convoluted to attain a FWHM value with high confidence.
119 AFM topographical images of the as rec eived MoS 2 Sb 2 O 3 C coatings are presented in figure 5 3. The ima ges were taken at increasingly smaller scan sizes from 40 m 20 m 10 m 5 m 2 m and 1 m The overall microstructure seen in the larger scan size show s a prevalence of particle cluster formations with great variation in sizes and topography. Smaller scan sizes revealed submicron sized particles w ithin the microstructure itself. The particles have a measured w idth of 50 to 65 nm and heights from 6 to 10 nm. The actual width of the particles will b e smaller than measured, given the interaction of the surface with a n atomic force microscope probe tip exhibiting a fin ite radius of curvature at the end of the tip The first point of interaction between the probe tip and the particle will take place on the side of the probe tip, not direct ly at its apex This interaction serves to enlarge the measured width signal from the particle, generating an image that is slightly magnifie d. Figure 5 13 shows a n illustration of a n atomic force microscopy measurement using a hemispherical probe tip with a 20 nm radius scanning over a 20 nm by 6.7 nm rectangular shaped part icle. Assuming these dimensions, the initial interaction takes place 14.9 nm away from the edge of the particle. The measured particles had siz es ranging from 40 nm to 60 nm which corresponds to physical particle sizes of 10 nm to 30 nm The se values correlate well with the calculated size of the Sb 2 O 3 particles from the X ray diffraction peaks Pin on Disc Measurements Tribological pin on dis c testing revealed a strong init ial environmental dependence for the MoS 2 Sb 2 O 3 C films. Figure 5 4 displays coefficients of friction v ersu s cycle for the four environmental conditions. In less than 50 cycles, trends were established for high friction and low friction regimes. Low fricti on was measured under oxygen and vacuum environments. The lowest coefficient of friction was measured in vacuum, with
120 a value of 0.06. High er friction was measured under ambient and water conditions, with friction coefficients around 0.15. For both ambient and water conditions, the average coefficient per cycle increases immediately after the start of testing. Based on these results, water is identified as the species responsible for high friction behavior. The effect of the environment can also be noticed under vacuum and oxygen conditions, whereby the average coefficient decreases from the test initiation. Extended sliding for both ambient and vacuum conditions was conducted for over 5000 cycles. The coefficient of friction for air remained constant and st able, while the coefficient of friction for the film in vacuum continued to decrease until reaching a steady state value less than 0.03. X ray P hotoelectron S pectroscopy Measurements The illustration of the wear tracks and the XPS measurements of the sampl es are shown in figure 5 5. Spectroscopic analysis of the films both on and off the wear tracks revealed the following chemical species portrayed in figure 5 6 For an unworn film, MoS2 peaks were detected in the Mo 3 d spectrum figure 5 6a. They were iden tified at binding energies of 229.0 eV and 232.1 eV. Likewise, in figure 5 6b, the S 2 p spectrum had resolvable peaks at 161.8 eV and 163.0 eV corresponding to MoS 2 Both the molybdenum and sulfur peaks agreed with established literature values for MoS 2 190 ,41,116,110 Additional p eaks evident in figure 5 6a at bind ing energies of 232.3 eV and 235.4 eV were assigned to MoO 3 190,116,205 A second sulfur species found primarily on unworn surfaces was strongly exhibited in the vacuum sample. The species was iden tified as a sulfate with binding energies of 168.7 eV and 169.9 eV. This correlated well with a strong O 1 s signal around 532.3 eV having a corresponding 4:1 inte n sity ratio (figure 5 6c). The measured C 1 s spectrum shown in figure 5 6d consisted of a
121 majo rity of C C bonding, as indicated by the intensity at 284.2 eV and 285.1 eV. The existence of two peaks is attributed to sp 2 and sp 3 hybridized bonding or edge defects (C H bonding) 157,186 The absence of a corresponding feature at 291.0 eV suggested that the carbon present in the film ha d either minor long range graphitic order, or more likely wa s amorphous having no long range crystallinity 157,206,185 The smallest peak at 286.0 eV was assigned to C O bonding. Figure 5 6c shows the overlapping Sb 3 d and O 1 s spectra. In this convoluted region, Sb 2 O 3 peaks resulting from the 3 d 5/2 and 3 d 3/2 of the antimony atom appear at 530.3 eV and 539.6 eV. The oxygen intensity arising from Sb 2 O 3 as well from MoO 3 is assigned to a value of 530.3 eV Although this is sl ightly higher than the 530.0eV value often listed in literature, this provides a consisten t fit throughout t he series of data sets 184,191,192,207,193 The O 1s intensity at a binding energy of 532.3 eV was attributed to C O and SO 4 species 194 while intens ity at 533.0 eV is assigned as arising from H 2 O 193 Many of the same features are observed in the spectra obtained from the wear track generated in a vacuum environment ( f ig ure 5 6e to 5 6 h). In addition, the vacuum worn wear track was found to contain C= O and O C=O bonding due to surface contamination, figure 5 6 h. Here, C=O bonding is located at 287.6 eV while evidence for O C=O species is observed at 288.8 eV. The O 1s spectrum was correspondingly fit ( f ig ure 5 6 g) with two signals, one for O C=O and th e other for the O=C O component. The O C=O bond is expected to be at a higher binding energy than the O=C O. Values of 533.9 eV for O C= O and 531.7eV for O =C O have been reported in the literature. C=O bonding is assigned intensity at 531.7 eV 184,188 Elsew here for the vacuum worn surface, the only appreciable chemical alteration in constituent species appears in the reduction of antimony trioxide to metallic antimony. This species was detected only
122 following tribological measurements under vacuum conditions It is notable that the strength of this signal decreased with time after tribological testing was finished, indicating the reactivity of the species, even in vacuum. No other reactions or decomposition of the composite constituents were detected. Further analysis of these data reveals t he greatest composition change upon wear in vacuum to be the increase in MoS 2 intensity, c oinciding with a decrease in carbon intensity The finding is borne out in surface concentrations, expressed in atomic percentages fo llowing normalization with relevant elemental sensitivity factors. The molecular composition of the near surface region of the as received film (f ig ures 5 6 a to 5 6 d) is 67. 3% graphit ic carbon, 10. 6 % MoS 2 7.3 % Sb 2 O 3 8 .1 % C O, 3.2% SO 4 2.0% H 2 O, and 1.5% MoO 3 The composition of the near surface region of the vacuum wear track is 47. 5% graphit ic carbon 1 3 5 % MoS 2 8. 0 % Sb 2 O 3 with 2 4 1 % C O x 4. 2 % SO 4 and 2. 3 % MoO 3 Similar tribological measurements and spectroscopic analyses were performed on the resul ting wear tracks generated under ambient (50% RH) environments and under partial pressures of oxygen and water (data not shown). The composition of the wear track in ambient operation was 61.9% graphite 13.14% MoS 2 11.1% Sb 2 O 3 7.5% C O, 3.0% SO 4 1.8% Mo O 3 and 1.5% H 2 O. Initial differences between the wear track compositions of the ambient and vacuum wear tracks show the near surface regions of the ambient wear track to have a higher graphite and Sb 2 O 3 content while vacuum wear tracks contain have more M oS 2 XPS characterization of the wear tracks generated under an oxygen partial pressure environment shows Mo 3 d peaks of MoS 2 located at 229.1 eV and 232.2 eV. Mo 3 d peaks of MoO 3 peaks were at 232.3 eV and 235.4 eV. The S 2 p spectrum has MoS 2 peaks at 162 .0 eV and 163.1 eV. There is a slight signal attributed to an SO 4
123 species at 168.1 eV and 169.3 eV. The C 1 s spectrum is comprised of three carbon species: C C (284.3 eV), C H (285.2 eV), and C O (286.2 eV). The Sb 3 d spectrum is assigned with intensity arising f rom Sb 2 O 3 with peaks at 530.3 eV and 539.6 eV. These are the same values as those fit with the ambient sample. Finally, the O 1 s spectrum was assigned as having contributions from Sb 2 O 3 and MoO 3 peaks at 530.3 eV and C O at 531.7 eV. The final composition for the oxygen wear track was calculated as 60.2% graphitic carbon, 16.9% MoS 2 11.7% Sb 2 O 3 5.3% C O, 3.0% MoO 3 and 3.0% SO 4 Tribological testing of the film in partial pressures of water resulted in spectral features si milar to those observed above. On ly differences in the relative presence of the different species present were observed. The computed quantitative composition for the water sample was 58.4% graphitic carbon, 16.3% MoS 2 11.5% Sb 2 O 3 8.0% C O, 2.2% MoO 3 2.1% SO 4 and 1.5% H 2 O. For the wea r tracks generated in partial pressures of water and oxygen, a higher MoS 2 composition than the ambient wear track is detected in the near surface region. Topography and Microtribometry Measurements Topographical images are shown for the wear tracks produ ced under 760 Torr air (figure 5 7), 10 7 Torr vacuum (figure 5 8), 150 Torr oxygen (figure 5 9), and 8 Torr water vapor (figure 5 10) environments. Fig ure 5 16 displays the topographical and lateral force maps for the as received film, the wear track of a sample run in air, and the wear track created in a vacuum environment. Microstructural characterization via AFM, figure 5 11 indicated that wear of the coating surface also produced a reduction in surface roughness for wear tracks in both air and vacuum. Roughness values measured as RMS over representative 2 m by 2 m areas were 18 nm for the as received film, 9 nm for the wear track in air, and 6 nm for the wear track in vacuum. The as received film
124 surface expressed circular particles less than 100 nm in size, while the rest of the morphology was relatively smooth. A portion of the wear track resulting from operation in air is shown in f ig ure 5 11 b, with evidence of plowing seen in the streaking from left to right. Even with the existence of this topogr aphy, the roughness has been reduced to approximately half that of the as received film, suggesting the tribological interaction flattens the composite microstructure even in the mo st severe of the environments t ested. This effect is clearly evident in the image taken from the wear track in vacuum; figure 5 11 c contains very few distinguishing features across the field of view neither the low frequency of surface particles present in figure 5 11 a nor the plowing scars in figure 5 11 b are evident across the wear track produced in vacuum The corresponding lateral force images collected in the identical regions as the topography presented in figures 5 11 a to 5 11 c provide additional insight into the dynamic nature of the coating surface. Although all three fr iction images were collected in an ambient air environment, differences in the film character are still apparent. Dark regions indicate areas of low friction, whereas light regions represent higher friction. The brightness of these images has been adjusted to allow a direct comparison of the mean friction experienced during the sliding of the Si 3 N 4 tip across the surface. Figure 5 11 d displays the spatially resolved friction of the as received film, revealing a localized contrast in intensity caused by the composite nature of the film. In figure 5 11 e, measured from the wear track in air, the mean friction of the wear surface is clearly seen to be higher In addition, friction is seen to be largely independent of the surface topography (plowing marks) and th e size of the features exhibiting the highest friction increases. Figure 5 11 f depicts the lateral force measured within the wear track generated by sliding under vacuum. Here, regions of low friction are prevalent and
125 numerous compared to the wear track i n air, with a general reduction in mean friction also observed. Additional lateral force images of the wear tracks produced by sliding under oxygen and water are shown in figure 5 12 The lateral force maps of the wear tracks produced under both oxygen and water environmental sliding show the clear presence of three constituents. The microstructure of the oxygen wear track figure 5 12c, features a distribution of low, moderate, and high friction force regions. In contrast, figure 5 12d shows the microstruc ture of the wear track produced under water displays larger areas of high friction forces and finer reduced overall areas of low friction The low f riction regions could represent areas of MoS 2 while m oderate friction regions could be graphi tic carbon, and high friction regions could be the Sb 2 O 3 or oxidized MoO 3 Discussion Together, the XPS and AFM results obtained from wear surfaces generated through tribological measureme nt performed under carefully controlled environmental conditions reflect specific changes in surface composition as a function of sliding environment In order to fully evaluate the effect of operating in different environmental conditions, ratios of the n ormalized XPS integrated intensities for each species were calculated for regions both within and outside of the wear track. The change in the ratios from the film to the wear track gives an analysis method for determining which species are prevalent in th e film and which species preferentially remain within the wear track. The direct comparison of worn and unworn regions exposed to the same environmental conditions provides the opportunity to separate tribologically induced changes from those potentially o ccurring thro ugh adsorption and/or reaction. Fig ure 5 13 shows the change in the ratio s of MoS 2 :Sb 2 O 3 MoS 2 :C, S:C, and MoS 2 :MoO 3 and Sb 2 O 3 :C for
126 each environmental condition. The expressed uncertainty of the ratios represents the sum of errors in the spec tral fits of the two species being compared. In comparing the collection of ratios between the various environments, t he greatest change in surface composition is seen to occur when sliding occurs under vacuum conditions, where MoS 2 :C increases by more th an 75% Likewise, Sb 2 O 3 :C increases by more than 75% while the MoS 2 :Sb 2 O 3 ratio experiences a positive change, suggesting an overall migration of both MoS 2 and Sb 2 O 3 during tribological interactions under vacuum with MoS 2 being the most prefer red From th is analysis, it can be concluded that the presence of MoS 2 heavily contributes to the low friction performance under vacuum. It is worth noting however, that the carbon ratios could be influenced by removal of adventitious carbon from the surface during th e initial sliding cycles. For operation in ambient air, the picture is different. There is a strong decrease in MoS 2 :Sb 2 O 3 and an increase in the Sb 2 O 3 :C ratio. The positive MoS 2 :C and S:C ratios may again be partially influenced by the removal of adventi ti ous carbon during tribological testing. Given this, the XPS data suggests that the surface of the wear track in air is influenced by interactions involving Sb 2 O 3 There is still a large percentage of the overall wear track composition that is carbon, 57% but only Sb 2 O 3 is observed to increase in prevalence a t the surface under these conditions This could be an additional cause of the increase in friction measured both in macroscopic and microscopic measurements of the wear track surface. Operation in ox ygen seems to have very little effect on the surface composition, with the most notable change occurring in the modest decrease of the MoS 2 :MoO 3 ratio. This suggests that tribological interactions under this environment may be controlled by carbon species Evaluation in partial pressures of w ater yields few similarities with air (50% RH),
127 however oxygen environmental testing does. For both air and oxygen conditions, the ratio of MoS 2 :Sb 2 O 3 decreases This is coupled with an increase in the Sb 2 O 3 :C ratio Co ncerning the oxidation of MoS 2 only water conditions produced a net decrease in the MoS 2 :MoO 3 ratio, suggesting that while the oxidation of MoS 2 is not strongly exhibited in these composite films aqueous species are the most likely to bring about oxidati on of the coating Friction, spectroscopic, and microscopic data are all crucial in describing the tribological interactions of MoS 2 Sb 2 O 3 C in different environments. Pin on dis c testing reinforces the sensitivity of MoS 2 based lubricants to water and sug gests that it is the presence of water molecules and only water molecules that cause s structural reorganization of the coating surface responsible for the high er friction behavior observed under ambient conditions. Spectroscopic data and lateral force maps present similar trends. The composite nature of the film is disrupted and species are presented at the surface to relieve the energetics at play. For vacuum conditions the dramatic increase in MoS 2 :C provides evidence of MoS 2 being the primary lubricant at the surface controlling tribological behavior marked by low friction performance. A secondary constituent Sb 2 O 3 can also be detected as being at the surface via XPS The increase in Sb 2 O 3 in both vacuum and ambient conditions agrees with previous studi es of MoS 2 Sb 2 O 3 which identified Sb 2 O 3 as a con stituent acting synergistically with MoS 2 Previous testing of MoS 2 Sb 2 O 3 films found enrichment of only MoS 2 at the surface of both the film and tip of the pin. However, TEM micrographs and Raman spectrosco py have indicated the location of Sb 2 O 3 directly beneath the lubricating MoS 2 surface 99
128 Regarding the effect of operating in ambient conditions, XPS measurements show an increase of Sb 2 O 3 at the expense of MoS 2 at the surface. Lateral force mapping indica tes a change in surface constituent s through an increase in friction Topographical features do not appear to play a role in the increased friction due to the fact that the wear tracks are difficult to resolve in the lateral force map compared to the as re ceived film. Upon closer inspection of figure 5 7e the smearing of particles, creating high friction regions in the direction of the sliding lends support to them being Sb 2 O 3 as this correlates with the increase in Sb 2 O 3 :C seen in the XPS measurements. As stated earlier, carbon remains the majority constituent in the near surface region of the ambient wear track and its role in either controlling or moderating the friction and wear at the surface cannot be ruled out. However, the tribological data presents a clear role for water in MoS 2 based lubricants. The negative effects of operating in ambient conditions (high friction performance) can be directly related to the presence of an aqueous species in the atmosphere. Conclusions Composite, solid lubricant Mo S 2 Sb 2 O 3 C films exhibit an environmental dependence which affects friction properties surface composition, and microstructure within the wear track. In the present study, t wo types of friction behavior have been identified. Testing in vacuum or partial pressures of oxygen environments yield s a low friction behavior, while sliding in ambient (50% RH) conditions or partial pressures of water results in a high friction behavior In this systematic study, w ater has been determined to be the species responsib le for detrimental tribological performance Importantly, partial pressures of oxygen are shown to have an inconsequential effect on the coefficient of friction with values remaining near the lowest obtained in vacuum
129 Sliding on these film surfaces under vacuum serves to preferentially bring MoS 2 to the surface, producing a low friction exceedingly smooth wear track. Testing under ambient conditions produces a higher friction surface with increased Sb 2 O 3 concentration and lowered MoS 2 composition. Evalua tion of wear tracks produced by sliding in partial pressures of oxygen and water reveals only modest changes in near surface composition, yet a marked difference in tribological response, with high friction produced in the presence of water vapor, but not in the presence of molecular oxygen.
130 A B Figure 5 1 I llustration s of the low angle X ray diffraction measurements taken of A)316 st ainless s t eel substrate and MoS 2 Sb 2 O 3 C coating and of the B) A s received MoS 2 Sb 2 O 3 C coating. The red colored regi ons indicate the approximate area of sampling. The incident angle s used in the measurements were 5 and 3 Figure 5 2 X ray diffraction measurements from MoS 2 Sb 2 O 3 C coating s taken at a low angle The black colored measurement was taken over an area of mixed coating coverage and the purple colored measurement was taken over an area of complete a real coating coverage.
131 A B C D E F Figure 5 3 Atomic force microscopy topographical images of the as received MoS 2 Sb 2 O 3 C coating taken at A)40 m scan size, B)20 m scan size, C)10 m scan size, D)5 m scan size, E)2 m scan size, and F )1 m scan size. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt.
132 Table 5 1: The parameters of MoS 2 Sb 2 O 3 C pin on disc tests Operating Parameter Value Normal load 1.0 N Disc rotation speed 5 rpm Cycles 300 W ear track diameter 7 mm Linear sliding speed 1.9 mm/s Contact pressure 1 MPa Temperature 25 C Environment 760 Torr air at 50% RH 150 Torr oxygen 8 Torr water 10 7 Torr vacuum Figure 5 4 Graph showing the average coe ff icient of friction v ers u s cycles of M oS 2 Sb 2 O 3 C coatings under different environmental conditions for 300 cycles clockwise
133 A B Figure 5 5 Illustrations of X ray photoelectron spectroscopy measurements taken of A)As received MoS 2 Sb 2 O 3 C coating and B)Wear tracks produce d under different conditions. The red colored regions indicate the approximate area of sampling. All measurements were taken at a take off angle of 55
134 Figure 5 6 X ray p hotoelectron s pectra of unworn MoS 2 Sb 2 O 3 C A) Molybdenum 3 d spectrum B) Sulfur 2 p s pectrum C) Antimony 3 d and Oxygen 1 s s pectra D) Carbon 1 s s pectrum and MoS 2 Sb 2 O 3 C wear track under v acuum E) Molybdenum 3 d s pectrum F) Sulfur 2 p s pectrum G) Antimony 3 d and Oxygen 1 s s pectra and H) Carbon 1 s s pectrum
135 A B C D Figure 5 7. At omic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 C coating produced by sliding under 760 Torr air (50% relative humidity ) taken at A)20 m scan size B)10 m scan size, C)5 m scan size, and D)2 m scan size. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt.
136 A B C D Figure 5 8. Atomic force microscopy topographical images of wear tra ck of the MoS 2 Sb 2 O 3 C coating produced by sliding under 10 7 Torr vacuum taken at A)10 m scan size B)5 m scan size, C)2 m scan size, and D)1 m scan size. All images were taken using a 0.58 N/m sharp cantilever in constant contact mode of 1 Volt.
137 A B C D Figure 5 9 Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 C coating produced by sliding under 150 Torr oxygen taken at A)20 m scan size B)10 m scan size, C)5 m scan size, and D)2 m scan size. All images w ere taken using a 0.42 N/m sharp cantilever in constant contact mode of 1 Volt.
138 A B C D Figure 5 10. Atomic force microscopy topographical images of wear track of the MoS 2 Sb 2 O 3 C coating produced by sliding under 8 Torr water taken at A)20 m scan size B)10 m scan size, C)5 m scan size, and D)2 m scan size. All images were taken using a 0.42 N/m sharp cantilever in constant contact mode of 1 Volt.
139 Figure 5 11 Atomic force microscopy topographical images of various MoS 2 Sb 2 O 3 C coatings A)As received MoS 2 Sb 2 O 3 C coating, B)Wear track created under ambient 50% RH, C)Wear track created under 10 7 Torr vacuum and concurrent lateral force maps of D)As received MoS 2 Sb 2 O 3 C coating, E)Wear track created under ambient 50% RH, and F)Wear track created under 10 7 Torr vacuum.
140 A B C D Figure 5 12. Atomic force microscopy topographical images of MoS 2 Sb 2 O 3 C coating A) wear track of the produced by sliding under 150 Torr oxygen, B) wear track produced by sliding under 8 Torr water and concurrent lateral force maps of C) wear track of the produced by sliding under 150 Torr oxygen, and D) wear track produced by sliding under 8 Torr water. Intensity scales have been set constant for only the lateral force maps. All images were taken us ing a 0.42 N/m sharp cantilever in constant contact mode of 1 Volt Figure 5 13: Schematic of an atomic force microscope probe tip scanning over a Sb 2 O 3 particle imbedded on the surface of an as received MoS 2 Sb 2 O 3 C film. Here t he diameter of the hemispherical probe is taken to be 40 m. The particle is 6.7 nm in height and 20 nm wide. The measurement of the feature produces an imaged particle that is 49.9 nm wide.
141 Figure 5 1 4 X ray photoelectron spectroscopy measurements of the change in the ratio from off the wear track region to the wear track region of MoS 2 Sb 2 O 3 C coatings.
142 CHAPTER 6 THE TRIBOLOGY OF ALC OHOLS ON NATIVE SILI CON OXIDE Introduction M icro electromechanical systems (MEMS) electronic devices their ability to perform t he amazing tasks that we ask of them. Technologies such as p ress ure sensors, stability control of cameras, miniaturization of antennae and optical switches owe their functionality to these microscopic machines However due to their size, they suffer from i ncreased stiction and thus require lubrication to operate in air. Many techniques have been attempted to lubricate MEMS, but few have had as much success as dry lubrica tion also known as vapor phase lubrication. R ecent literature pertaining to the vapor phase lubrication (VPL) of micro electromechanical systems has focused on understanding how 1 pentanol species function as a lubricant. Adsorption of the pentanol molecules onto silicon is the first primary step in the process. The adsorption behavior has been shown to establish a monolayer of surface coverage by 10% of the saturation pressure, with a reduced activation of multilayer formation. Additional adsorption of pentanol molecules only occur at pressures greater than 80% of the saturation value. Anal ysis of the pentanol absorbed onto Si with attenuated total reflection infrared (ATR IR) spectroscopy from 2600 3700 cm 1 detected vibrational peaks corresponding to CH 2 CH 3 and OH bonding. Characterization of the sliding tracks with microinfrared (micro IR) spectroscopy was limited to only the C H x and SiO x regions (2700 to 3000 cm 1 and 800 to 1400 cm 1 ), but did not show a significant change in species intensity during low wear conditions. It is noted by the authors that the micro FTIR technique is no t sensitive enough to detect monolayer species on a surface. Therefore the absence of a signal
143 from the pentanol wear track is not a final statement on the matter. However, when high wear behavior was exhibited (low pentanol partial pressure and/or high co ntact load) the micro FTIR technique was able to clearly measure an effect. In the worn areas, the SiO x signal increased, as well as the C H x species intensities. There was an enhancement of a feature in the 800 to 900 cm 1 region that was not specifically discussed or identified. A search of literature found this same feature to be present in IR spectra taken from hydrolyzed silica, indicating possible Si OH bonding within the wear track. Tribological testing with a MEMS device showed the lubricating effec t of the alcohol vapor to be reversible. Removal of pentanol from the gas phase causes a gradual reduction in performance of an interface previously run in in pentanol. The rate of reduced performance wa s inversely proportional to the prior operating time A lack of sufficient pentanol vapor leads to device failure through stiction. Upon the reintroduction of pentanol, the device is able to operate immediately as if it were new once more. This suggested that the primary mechanism responsible for low frictio n was the presence of vaporized pentanol. The tribofilm formed during operation was though to play a secondary role, dampening the friction response when there was an absence of nearby pentanol vapor molecules. An explanation for the wear behavior has been provided in a density functional theory (DFT) study modeling both the protective role of alcohols and the exacerbat ing role of water. Alkoxide species (Si O R) produced by the reaction of alcohols with the silicon oxide surface were found to have a higher Si O bond energy than hydroxide species (Si O H). Additionally, the chain length of the reacting alcohol was found to correlate with the cleavage energy. The theoretical energy required to break Si O bonds increased with the number of carbon atoms of the alcohol. This
144 statement was based upon the calculations for methanol and propanol species ; Time of flight, secondary ion mass spectrometry (TOF SIMS) analysis of the silicon su rface inside and outside of the sliding track under a high partial pressure showed the strong creation of alkyl species within the track area with Si and SiH also present. Outside of the sliding area, masses assigned to Si, SiCH 3 SiOH, and Si(CH 3 ) 3 were d etected. There were no Si O, Si C fragments measured according to their masses within the low wear sliding track. Later studies investigating the wear behavior of this system showed the presence of Si O and Si C bonding occurred only under poor performing conditions causing wear. However, one issue with any SIMS analysis is the reconstruction of the original test sample from the measured mass distribution of sputtered parts. Preferential sputtering of a sample produces individual components that may not be quantitatively or even chemically representative of the original sample. For example, both atomic silicon and CO species have a molar mass of 28 g/mol. Discerning the elemental makeup of a signal centered around 28 g/mol becomes difficult to accomplish, le t alone quantify the ratio of the species present. The correlation of these results using an additional characterization method would add clarity to the precise nature of both the adsorbed species and the tribofilm which is created during operation in the presence of alcohols. Barnette et al 140 i ndicated that the formation of a tribofilm was not responsible for low friction behavior. However, the exact chemical state and role of the film itself remains an open question. It is surmised from computational and mass spectrometry that the creation of silicon, short chain hydrocarbon bonding may be constitute the final product of the tribofilm, but the spectroscopic verification has yet to be presented. Additionally, the role of the tribofilm in preventing wear co uld be
145 further explored by studying the silicon oxide presence on the sliding surface in both high and low wear conditions. The overarching issues to be addressed in this study are the exact chemical nature of tribofilm, the necessity of a tribofilm for lo w friction, low wear performance, and the corr elation of the detected surface species with current theories regarding VPL of silicon ox ides via alcohol adsorption Experimental The silicon samples were prepared by cleaving (100) s ilicon wafers to create 14 mm by 10 mm rectangular pieces. The silicon sample was mounted onto an Omicron Nanotechnology GhmB platen and secured with spot welded tantalum strips Cleaning was accomplished by exposure for 1 minute to an oxygen plasma The sample was immediately load ed into the tribometer stage and the vacuum chamber was closed and pumped down to high vacuum. Pentanol was purchased from Sigma Aldrich (1 The pentanol was transferred into a glass ampule within a glove bag filled with nitro gen gas. The ampule was then attached to a gas manifold with inch stainless steel tubing leading to a leak valve attached to the environmental chamber. The pentanol was purified through a series of freeze, pump, and thaw cycles to remove contaminants The pin on dis c tribometer allows measurement of frictional forces of the sample as described in the previous chapters. Each experimental condition was tested for 1000 cycles. The stage was set at a vertical position to give a normal load of 1.0 N. The spe ed of rotation was 5 rpm. The estimated contact pressure was small, around 1 MPa. The test produced a wear track 7 mm in diameter. The width of the wear track was difficult to estimate during low wear testing, but operation in air produced a wear track app roximately 1 mm in width. Tests were conducted at room temperature, 25C, and
146 the environmental conditions were: 760 Torr air at 50% RH, 10 7 Torr vacuum, and 1 Torr pentanol A second set of experiments were conducted to elicit the role of the vapor and tribofilm. The first test exposed a clean silicon surface to 1 Torr pentanol for 10 minutes and then started tribological testing under vacuum conditions. The second test cycled between 1 Torr pentanol and vacuum conditions. The third test started at 1 Tor r pentanol, then decreased the pressure in a step wise manner until the pressure was less than 10 5 Torr. An Omicron Nanotechnology GhmB Al K (1486.7 eV) monochromatic XPS source was used with an EAC2000 Sphera he mispherical, 7 channel analyzer, employing an aperture that produced an electron collection area of 700 m in diameter. All XPS core spectra were taken using a step size of 0.05 eV, pass energy of 20 eV, for 0.2 sec onds and swept 5 times to give a reduced noise, average spectrum. The elemental co res measured were silicon 2 p carbon 1 s and oxygen 1 s Sensitivity factors for quantitative analysis were obtained from the PHI XPS Handbook assuming a 90 degree relationship between the incident X rays and the electron analyzer. The integrated areas of t he silicon 2 p peak s were set at ratios of 1:2 and a separation of 0.6 eV 208,209 Oxygen peaks were restricted to their calculated stoichiometric area values based upon their component elemental spectra fittings. Each peak within a specific spectrum was co nstrained to the have the same FWHM value. All peaks were treated with a 90% Gaussian, 10% Lorentzian shape. A fitting algorithm, minimizing the squared sum of the difference between the fit peaks and the measured spectrum, was used to finalize the overall peak fit spectrum. Fitting was complete when the value of the standard deviation of the residual spectrum was less than 2.0.
147 Results Pin on disc tests were conducted at room temperature under 3 environmental conditions: 1 Torr pentanol, air, and vacuum. The summary graph shown in figure 6 1 portrays the remarkable lubricating effect of pentanol and the deleterious effect of operation in ambient conditions compared to vacuum. The run conducted under 1 Torr pe ntanol displayed a low friction and extremely l ow wear behavior. The relative absence of variance in the friction signal under stead y state conditions suggests that there is little detrimental wear occurring during the entirety of the test. The average steady state coefficient of friction was measured to be 0.17. Operation under ambient conditions showed poor performance, highlighted by a large signal variance. The coefficient of friction leveled off at around 0.6. The onset of visible, catastrophic wear took less than 20 cycles. Vacuum testing of silic on produced a gradually increasing friction response, reaching a near constant value around 0.47. The variance in the friction signal was similar to ambient testing, indicating wear. This was visually confirmed with the creation of a wear track, although i t was thinner than the one produced in air. A second set of experiments was performed to elicit the two mechanisms involved in VPL, adsorption of the species and the creation of a tribofilm. This test was designed to understand which step of the process de termines the final low friction, low wear behavior in these systems ( figure 6 2 ) Additionally these tests help to show the role of the tribofilm and whether it is a necessary for low friction performance. Figure 6 3 shows for the following scenarios: the first 100 cycles in vacuum, the first 100 cycles for Si exposed to 1 Torr pentanol for 10 minutes, the following 100 cycles in 1 Torr pentanol, the f ollowing 200 cycles in 1 Torr pentanol, the f ollowing 300 cycles in 1 Torr pentanol, and sliding under 1 To rr pentanol.
148 The decreasing pentanol pressure test had an initially low coefficient of friction from the 1 Torr pentanol A environmental sensitivity to partial pressure was immediately apparent upon decreasing the pressure to 0.5 Torr ( figure 6 4 ) The ma gnitude of the friction response was stabilized once the pressure reached 2 x 10 2 Torr. The room temperature vapor pressure of pentanol is stated to be 3 Torr, therefore the initial test conditions were greater than 10%, of the saturation pressure of the species the stated minimum threshold needed to produce a monolayer of pentanol 142,210 An increase in the both the value and deviation of the average coefficient of friction was initiated when the measured pentanol partial pressure passed below 10 5 Torr. This behavior was reversed when the pressure was increased to 1 Torr pentanol The friction signal became smoothed and decreased to a steady state value similar to the starting value X ray photoelectron spectroscopy characterization of the samples revea led room temperature adsor ption of pentanol onto an oxygen plasma cleaned (100) silicon sample after exposure to 1 Torr pentanol for 3 hours. Additionally, a tribochemical reaction was measured taking place at the near surface under 1 Torr pentanol, and we ar of the tribofilm during low pressure operation. This process was seen clearest in the carbon 1s spectrum, figure 6 5 The spectrum representing adsorbed pentanol was fit with two peaks corresponding to CH 2 (285.5 eV) and C OH (287.3 eV) species. The e lemental composition of the unworn, adsorbed pentanol on silicon surface was measured to be 37.0% silicon, 48.9% oxygen, and 14.1% carbon. The area ratio of the absorbed carbon chain species to the alkyl species was 2.861 These peak assignments are consis tent with historical alcohol adsorption studies 208,211 213 XPS measurements of the wear tracks produced via sliding under 1 Torr pentanol showed
149 the elemental composition to be 32.3% silicon, 43.1% oxygen, and 24.6 % carbon. This is telling, with an increa se of carbon content by more than 10 at%. Chemical species analysis of the C 1 s spectrum identified the species responsible. The intensity of the CH 2 species was higher after wear raising the CH 2 :C OH value to 4.895. This number is greater than the stoich iometric integer for pentanol, 4 and signifies an enhancement of CH 2 bonding. In addition, a new intensity was detected at 289.7 eV. This binding energy is thought to originate from a tribochemical C =O species 188,211,214 Lastly, under a vacuum environment the friction response was increased and exhibited extensive wear. The elemental composition was altered with 26.2% silicon, 37.4% oxygen, and 36.4% carbon. This suggests that primary wear induces the formation of new CO x species at the expense of existin g SiO 2 and SiO 4 species. This is verified by the C 1 s spectrum which beca me convoluted, expressing an additional species due to wear, C O 188,214 There was no evidence of silicon carbon bonding in any of the testing, indicated by the absence of a photoel ectron intensity around 283 eV 213,215 This suggests that any carbon silicon interaction involves an oxygen intermediary atom during low friction, low wear conditions. Wearing of the tribofilm degenerates the (CH 2 ) x and C OH species to form CO x species. Th e silicon 2 p spectra from figure 6 6 showed that the native silicon oxide surfaces entails bulk Si (99.3 eV) and SiO 2 species (103.7 eV and 102.8 eV) for all samples. The adsorbed pentanol showed no significant presence of Si OH species. Si OH species were detected in the sliding track under 1 Torr pentanol, as revealed by an increase in a feature at 104.5 eV. The intensity of the Si OH peak decreased upon sliding under vacuum. Thus, the Si OH species appears to be associated exclusively with the tribofilm formed during low friction sliding. Additionally, the peak area of SiO 2 : SiO 4 increases
150 within the wear track during sliding under 1 Torr pentanol and further upon wear in vacuum. This confirms tha t sliding alters the SiO 4 surface. Figure 6 7 shows the oxyg en 1s spectra for the three measurements. The adsorbed pentanol surface showed a majority of Si O bonds associated with SiO 2 species at 533.1 eV 209,213,216,217 C OH bonds from the pentanol were assigned to a smaller, binding energy of 531 8 eV 188,212,213 The tribofilm produced under 1 Torr pentanol had an O 1 s spectrum that showed the presence of 2 new species assigned to C O (532.5 eV ) and Si OH (534.8 eV) 188,217 These were previously indicated in the discussion of the Si 2 p and C 1 s spectra measurements. P eaks associated with SiO 2 (533.4 eV), C OH (532.8 eV), and SiO 4 species were also fit according to previous identities. The top spectrum was measured from the silicon sample run in 1 Torr pentanol and then vacuum in an effort to wear down the tribofilm. Th e most striking feature of th e O 1 s spectrum is the asymmetry to the lower binding energy side of the main peak. This was fit with C OH(532.3 eV), C=O(531.1 eV), and C O(532.3 eV) species 188 SiO 2 (533.0 eV) and Si OH (534 .8 eV) species were fit as well. D iscussion The reported tribological response of alcohol vapors on silicon oxide surfaces was reproduced, with native silicon oxide surfaces displaying low friction and very little wear at near pentanol saturation conditions. Under vacuum conditions, the si licon friction signal was higher and produced a visible wear track. Additional tribological testing supports the theory that maintaining a constant supply of pentanol is necessary for the system to exhibit low friction behavior 140,210 However, the resulti ng tribofilm shows some protective behavior even when the pressure is dropped below the lubricating threshold. The buildup of a tribofilm is evidenced by the number of cycles for the
151 coefficient of friction to reach a value of approximately 0.25 after run in under 1 Torr pentanol. The se results suggest that the tribofilm functions as a buffer between the counterface and the silicon surface during sliding, enabling extremely low wear of the silicon surface itself. The function of the vapor phase lubricant is to provide low friction operation, which is critical for MEMS devices in removing stiction and seizure. Spectroscopic testing was able to identify species formed due to tribological interactions, mainly ( CH 2 ) x species with correlating Si OH species format ion. It is surmised that the increased bonding of a carbon atoms with oxygen at the surface of silicon enhances the adhesion of the tribofilm to the oxide atoms. The breakdown of these bonds into C=O and C O components as well as the increase in C C bondin g under vacuum wear conditions indicate that the tribofilm could form as a result of the initial sta ges of wear. This appears to match the micro FTIR and ToF SIMS data from Barnette et al where presumed (CH 2 ) x species were shown to increase within the sli ding area, during wear conditions 140 The eventual physical removal of the native silicon oxide surface occurs only after the tribofilm can no longer offer tribomechanical protection. This is shown by the decrease in oxygen signal and increase in the eleme ntal silicon to silicon oxide ratio while sliding under vacuum Conclusions Pentanol is an established vapor phase lubricant for MEMS applications. Tribological testing has shown that the low friction behavior is only exhibited under high partial pres sures of pentanol. The tribofilm was identified as a (CH 2 ) x species bonding with the silicon oxide surface. The presence of a tribofilm or adsorbed monolayer of pentanol does not produce the same frictional response as 1 Torr pentanol. However, the amount of tribofilm present on the surface does affect the amount of cycling it takes
152 to degrade the tribofilm into surface C=O and C O species and increase the coefficient of friction to an equivalent value of bare vacuum sliding. Operation in ambient conditions yielded the highest coefficient of friction and the greatest amount of wear debris pres ent at the surface.
153 Figure 6 1 Graph showing the average coefficient of friction v ersu s cycles of native oxide silicon (100) under different environmental conditio ns for 1000 cycles clockwise. Figure 6 2 Graph showing the average coefficient of friction v ersu s cycles of native oxide s ilicon (100) between 1 Torr pentanol and vacuum. Runs under 1 Torr pentanol were 100, 200, and 300 cycles clockwise. Runs under v acuum were kept constant at 100 cycles clockwise
154 Figure 6 3 Graph showing the initial average coefficient of friction v ersu s cycles of native oxide silicon (100) under different conditions Figure 6 4. The average coefficient of friction v ersu s cyc les for the decreasing pentanol test
155 Fig ure 6 5 X ray photoelectron spectroscopy measurement of the carbon 1 s spectra for ( bottom to top ) : adsorbed pentanol, the tribofilm produced in Torr pentanol, and the worn tribofilm under vacuum
156 Fig ure 6 6 X r ay photoelectron spectroscopy measurement of the silicon 2 p spectra for ( bottom to top ) : adsorbed pentanol, the tribofilm produced in Torr pentanol, and the worn tribofilm under vacuum
157 Fig ure 6 7 X ray photoelectron spectroscopy measurement of the oxyg en 1 s spectra for ( bottom to top ) : adsorbed pentanol, the tribofilm produced in Torr pentanol, and the worn tribofilm under vacuum
158 CHAPTER 7 CONCLUSIONS Solid Lubricants Aerospace applications require solid lubricants to coat the surfaces of components used for mechanical operations such as deploy ment of solar panels for power, communication equipment, positioning control systems and many other functionalit ies. Over the decades many materials have been tested and used as solid lubricant coatings. MoS 2 ba sed coatings are the current industry standard for satellites. These materials are defined by their superior vacuum environmental performance, but their functionality is limited under ambie nt operation. Proof testing of only 10 to 100 cycles can lead to pr emature wear of the micron thin coatings. While research for better solid lubricant films has been an active area of research for more than 40 years, an understanding of the fundamentals responsible for the performance of modern, nanocomposite coatings rem ains an underdeveloped field. The growth of surface sensitive characterization techniques has given an opportunity for scientists and researchers to effectively probe the complex processes thought to govern the performance of these materials. The approach of the surface studies detailed in this d issertation combined the tribological performance testing of a reliable, vacuum compatible tribometer design with an ultra high vacuum analysis system. X ray photoelectron spectroscopy (XPS) is capable of measuring the chemical changes taking place on the top atomic layers of the resulting wear tracks produced under controlled test environments. The incorporation of an environmental test chamber and sample transfer system with the vacuum complex gave confidence of s urface integrity during the transfer process from tribometer to
159 analysis chamber. Precise con trol over the tribological test environment was crucial for conducting studies on the effect of atmospheric species on friction performance. Characterizing the sur face microstructure with an atomic force microscope (AFM) added a method to verify spectroscopic measurements and provided information pertaining to the microstructure and topographical environmental dependence. Tribological testing took place under enviro nments of 760 Torr air (50% relative humidity) 150 Torr oxygen, 8 Torr water vapor 610 Torr nitrogen, and 10 7 Torr vacuum The materials tested consisted of a standard solid lubricant ( highly oriented pyrolytic graphite ), a current generation solid lubr icant coating ( MoS 2 Sb 2 O 3 Au ) and an experimental coating ( MoS 2 Sb 2 O 3 C ) Combining a set of complementary surface characterization techniques with the ability to control environmental conditions provides a n opportunity to study and deduce the fundamental chemical reactions and surface expression taking place during tribological interaction s The ultimate goal was to provide new, reliable, and insightful commentary about the complex relationship between film performance and the possible surface mechanisms taking place in a variety of operating conditions Experimentally, from this series of surface studies, the environment of operation was found to have a dramatic effect on the tribological behavior, surface species composition, and microstructure for both dry and solid lubricating systems. For graphite, the presence of oxygen molecules, or water vapor was necessary for low friction, low wear sliding performance. The average coefficient of friction measured under these conditions was 0.07. The results agree and support a lubrication theory for graphite, whereby, the atmospheric molecules are thought to passivate edge defect sites created while sliding. Sliding under vacuum yields a higher average coefficient of friction, 0.165, and extensive wear debris comp ared to ambient sliding
160 MoS 2 Sb 2 O 3 Au films displayed a n environmental dependence different from graphite These films were characterized as possessing a low sliding coefficient of friction under vacuum and a higher friction under ambient, 0.056 compared to 0.156 respectively Water vapor led directly to the high friction behavior exhibited under ambient testing. Oxygen species d id not significantly increase the coefficient of friction for these types of films. Evaluation of the microstructure reveals fil m densification and smoothing of the sliding area. XPS and l ateral force microscopy indicated an increase of Au concentration within a matrix of MoS 2 at the surface during ambient testing. V acuum sliding showed a uniform, lower friction surface consisting of primarily MoS 2 Sliding under vacuum was facilitated by MoS 2 The mechanism responsible for the ambient expression of Au is not yet fully understood and requires further attention. In the case of coatings containing both MoS 2 and graphite constituents, the environmental dependence of tribological behavior was dominated by MoS 2 These films performed best under vacuum with a measured average coefficient of friction of 0.06. A higher coefficient of friction for MoS 2 Sb 2 O 3 C films was measured under both ambient and 8 Torr water conditions, 0.15. Spectroscopic analyses showed there to be a correlation between high friction sliding and the MoS 2 :MoO 3 ratio. A negative change in the ratio was recorded within the wear tracks of films exhibiting the highest fri ction behavior. Therefore, the production of MoO 3 at the expense of MoS 2 is thought to produce the increased coefficient of friction. Topographic c haracterization of both MoS 2 based films reveal ed a smooth, low friction surface within the wear tracks forme d under vacuum conditions. Smearing of the Sb 2 O 3 constituent was observed in lateral force images, clearly indicating the role of film restructuring in development of environmentally dependent tribological properties.
161 Dry Lubricants Vapor phase lubrica tio n such as alcohol based molecules were foun d to be beneficial for both the sliding friction and wear of native silicon oxide surfaces. Under 1 Torr pentanol, the average coefficient of friction was 0.17. Ambient and vacuum testing resulted in a higher fri ction and the formation of visible wear debris. The average coefficient of friction for vacuum sliding leveled off at 0.47. In air it was even higher, 0.6 0 The threshold for successful lubrication by this approach was found to exist at pressures greater t han 10% of the saturation vapor pressure. The tribochemical formation of ( C H 2 ) x species on the silicon oxide surface was linked to low friction sliding and resistance to wear. The destruction of the (CH 2 ) x and C OH bonds reduces the wear protection of the dry lubricants and was evidenced by the increase in C=O and C O bonds. In Vacuo Pin on Disc Tribometer The in vacuo pin on disc tribometer was proven to be effective in measuring the friction characteristics of a variety of lubricating regimes and mater ials. The environmental chamber and dosing system allow ed the capability to introduce both gaseous and volatilized liquid phase species to the chamber and control their partial pressures. The unique tribometer sample holder design and transfer arm system p ermitted sample transfer from the tribometer to the XPS analysis chamber under vacuum with a high degree of precision. However, there were some issues that came up during the course of the experiments. They des erve mention so as to help improve the next ge neration of testing apparati Th e larges t critical experimental issue dealt with the width of the wear track. Sliding under a 3.175 mm diameter 304SS sphere produces a wear track width of
162 approximately 50 This correlates well with Hertzian contact calculations assuming a sphere on flat contact with a normal load of 1 N, th e sphere has a 210 s modulus of 238 GPa and a Poiss This is much smaller than the diameter of the sampling area of the XPS, 700 solve this issue was to produce successive, multiple wear tracks on the sample under the same environmental conditions and for t he same duration. This r equired the production of 5 to 10 wear tracks that were extremely close together, expanding the time to complete one sample measurement, and the net time to complete an entire set of measurements. Because the wear track diameter cou ld not be changed, this also meant that the wear tracks overlapped, thus introducing issues of material transfer between tracks. An additional problem of utilizing this method was control over the location of the wear tracks to be separate from each other but have a range within the 700 area. Clearly an alternative approach was necessary. In order to produce wider wear tracks, t he contacting ball was polished to produce a flat section, approximately 1 mm in diameter. This increased width of the wear track, effectively decreas ed t he overall number of wear tracks that had to be made The drawback to using a flattened pin is that it cannot be removed from its position once fabricated. The pin must be reused for every successive test an d is difficult to quantify the exact contact area during sliding due to possible misalignment between the flat section of the pin and the sample surface. Future pin on disc designs should take this limitation into account. Several ideas to resolve the wear track width issue could involve the continued use of a flattened head or the use of a standard, spherical ball in conjunction with a micrometer XY stage mounted to the tribometer arm to control the
16 3 position of the pin relative to the center of rotation o f the sample stage. This approach would effectively allow the in situ adjustment of the wear track diameter. Another possibility could be the design of a custom stainless steel flat head pin with a threaded body to allow removal and reuse. This plan would enable the user to efficiently image the bottom of the pin without having to separate the entire tribometer arm apparatus from the electrical interconnects, which are extremely sensitive to any adjustment or repositioning. A second issue that came up duri ng the tribometry studies was the short circuiting of the piezo actuator. The failure of this device in the latter portion of the studies was attributed to the repeated, long term operation of the tribometer under water vapor near saturation Any piezo ele ctric material is going to be sensitive to operating in such conditions at high voltages. Therefore, the pi ezo electric actuator in future tribometers intended for environmental testing should balance the design needs for vacuum compatibility, size, displa cement, and cost. Lastly, future environmental testing of wear would be aided with the capability for the data acquisition program to allow cyclical reversal of the motor. The current program gives the user the ability to manually reverse the direction of rotation, which limits the types of tests that can be conducted. A software platform to design bidirectional experimental sets would give enhanced capability to the in vacuo pin on disc tribometer and add a new dimensionality to aid tribological envir onmental investigations
164 APPENDIX A X RAY D IFFRACTION PEAK ANALYSES While the X ray diffraction (XRD) measurements of the MoS 2 Sb 2 O 3 Au films did not produce distinct peaks associated with crystalline phases, the as received MoS 2 Sb 2 O 3 C films did. The p roper identification of the individual peaks provides important information relating to the overall crystallinity of the film crystal structure and orientation of the constituents lattice dimensions, and grain size. All of these can play roles in the ove rall performance of solid lubricating films. The use of a grazing incidence angle in these measurements was necessary to maximize the measured s ignal originating from the film while minimizing the signal from the substrate. The incidence angle used in the measurements ranged from 3 to 5 degrees. Table A 1 presents the positions of the peaks measured the film supported on a 316SS substrate (figure 5 2 ), their assigned identities, and literature values used as references for identification. Table A 2 presents the same information pertaining to the measured as received MoS 2 Sb 2 O 3 C film (figure 5 2) Table A 1 The m easured X ray diffraction p eaks from the m ixed 316 stainless steel / as r eceived MoS 2 Sb 2 O 3 C film. Peak Identity Reference 204,218,219 14.37 MoS 2 ( 002) 14.41 27.61 Sb 2 O 3 (222) 27.71 43.65 Fe (111) 43.5 44.75 martensite (110) 44.6 50.77 Fe (200) 50.5
165 Table A 2. The m easured X ray diffraction p eaks from the as r eceived MoS 2 Sb 2 O 3 C film. Peak Identity Reference 204,218,220,219 14.41 M oS 2 (002) 14.41 26.43 C (004) 26.63 27.73 Sb 2 O 3 (222) 27.71 28.87 MoS 2 (004) 29.05 32.09 Sb 2 O 3 (400) 32.10 35.07 Sb 2 O 3 (331) 35.07 39.59 MoS 2 (103) 39.59 43.69 Fe (111) 43.5 44.09 MoS 2 (006) 44.20 46.11 Sb 2 O 3 (440) 36.05 49.85 MoS 2 (105) 49.85 50.73 Fe (200) 50.5 54.51 Sb 2 O 3 (622) 54.59 57.23 Sb 2 O 3 (444) 57.23
166 APPENDIX B THERMODYNAMIC CALCUL ATIONS The role of therm odynamics is to describe the conditions of state for a given system. Calculating the change in the Gibbs free energy for a s et of reactions gives a direct method to determine whether the proposed process will take place spontaneously, or whether the reaction requires external energy to create the product species. A net negative change in the Gibbs free energy is said to be spon tan eous A reaction that increases the Gibbs free energy is termed nonspontaneous and endothermic The calculations can also indicate which reaction pathway is most likely to proceed in a complex system The reaction with the greatest reduction of the Gibb s free energy is the thermodynamically most likely process For this series of studies, the oxidation of the solid lubricant coatings is a mechanism suspected of reducing performance. The proposed chemical reactions focus on the oxidation process of MoS 2 t o MoO 3 and graphitic carbon to CO 2 The reactions provide a basis for both the environment where oxidation is most likely to take place, and which film constituent is mostly likely to oxidize in the given environment. Three environments were analyzed, oxyg en rich, water rich, and air. For the oxidation of MoS 2 to MoO 3 there are three different reaction pathways, one for each environment. Likewise, the same can be proposed for C to CO 2 Table B 1 presents the thermodynamic values used to calculate the react ions at room temperature.
167 Table B 1: Thermodynamic values for different species at 298.15 K elvin Species Enthalpy kJ/mol Entropy J/mol K Gibbs Free Energy kJ/mol MoS 2 ( s ) 276.14 62.57 294.79 MoO 3 ( s ) 745.17 77.78 768.37 C ( s ) 0 5. 70 5. 6 0 O 2 ( g ) 0 205.07 61.16 H 2 O ( g ) 241.83 188.84 298.13 H 2 ( g ) 0 130.68 38.96 H 2 S ( g ) 20.5 0 205.77 81.85 SO 2 ( g ) 296.84 248.2 1 370.85 H 2 SO 4 ( g ) 735.13 298.78 824.22 CO 2 ( g ) 393.52 213.79 457.27 To calculate the Gibbs free energy for a sp ecies G: G = H T S (B 1) The change in Gibbs free energy for a reaction is given by: G = products reactants (B 2) The three proposed oxidation reactions for MoS 2 to MoO 3 in oxygen rich, water rich, and humid air environments are : 2MoS 2 ( s ) + 7O 2 ( g ) 2MoO 3 ( s ) + 4SO 2 ( g ) (B 3) MoS 2 ( s ) + 3H 2 O( g ) MoO 3 ( s ) + 2H 2 S( g ) + H 2 ( g ) (B 4) 2MoS 2 ( s ) + 9O 2 ( g ) + 4H 2 O( g ) 2MoO 3 ( s ) + 4H 2 SO 4 ( g ) (B 5) For an oxygen rich environment the change in Gibbs free energy for the oxidation of MoS 2 is given by: G B 3 = (2 768.37 + 370.85) 61.16) = 2002.4 kJ/mol (B 6) G B 3 (per mole MoO 3 ) = 1001.2 kJ/mole MoO 3 (B 7) The negative Gibbs free energy value of reacting MoS 2 with O 2 to produce MoO 3 i ndicates that the oxidation of Mo S 2 in an oxy gen rich environment is spontaneous and
168 very possible. For a water rich environment, the change in Gibbs free energy for the oxidation of MoS 2 is given by: G B 4 = ( 81.85 + 38.96) ( 298.13) = 218.17 kJ/mol (B 8) G B 4 (per mole MoO 3 ) = 218.17 kJ/mole MoO 3 (B 9) The positive Gibbs free energy value of reacting MoS 2 with H 2 O to produce MoO 3 suggests that the oxidation of MoS 2 in a water rich environment is nonspontaneous and therefore not likely without addition al input e nergy For a humid, air environment, the change in Gibbs free energy for the oxidation of MoS 2 i s given by: G B 5 = ( 768.37 + 4 824.22 ) ( 294.79 + 61.16 + 4 298.13 ) = 2501 kJ/mol (B 10 ) G B 5 (per mole MoO 3 ) = 1250.5 kJ/mole MoO 3 (B 11 ) The negative Gibbs free energy value of reacting MoS 2 with O 2 and H 2 O to produce MoO 3 sugges ts that the oxidation of MoS 2 in a humid, air environment is spontaneous Just as important is the calculation for the specific decrease of Gibbs free energy brought about by the formation of one mole of MoO 3 The value of B 11 is less than B 9, which show s that oxygen and water molecules act in a synergistic relationship to bring about oxidation. Humid air is t he most likely test environment under which oxidation of MoS 2 will take place at room temperature. To summarize the calculations for the oxidation o f MoS 2 to MoO 3 o nly oxygen and humid air conditions are predicted to allow oxidation Oxidation in a water rich environment is not thermodynamically favored for MoS 2 The three proposed oxidation reactions for C to CO 2 in oxygen rich, water rich, and humi d air environments are:
169 C ( s ) + O 2 ( g ) C O 2 ( g ) (B 12 ) C ( s ) + 2H 2 O( g ) C O 2 ( s ) + 2H 2 ( g ) (B 13 ) 2 C( s ) + O 2 ( g ) + 2 H 2 O( g ) 2 C O 2 ( s ) + 4H 2 ( g ) (B 14 ) For an oxygen rich environment, the change in Gibbs free energy for the oxidation of graphite is given by: G B 12 = ( 457.27 ) ( 5.60 + 61.16) = 401 71 kJ/mol (B 15 ) G B 12 (per mole C O 2 ) = 401.71 kJ/mole C O 2 (B 16 ) The negative value of reacting C with O 2 to produce CO 2 suggests reaction of carbon and oxygen gas to produce carbon dioxide gas indicates that the oxidation of graphite in an oxygen rich environment is spontaneous and possible. For a water rich environment, the change in Gibbs free energy for the oxidation of graphite is given by: G B 13 = ( 457.27 38.96 ) 298.13) = 55.46 kJ/mol (B 17 ) G B 13 (per mole C O 2 ) 55.46 kJ/mole C O 2 (B 18 ) The positive value of reacting C with H 2 O to produce CO 2 suggests that the oxidation of graphite in a water rich environment is nonspontaneous and therefore not likely to occur without additional energy input For a humid, air environment, the change in Gibbs free energy for the oxidation of graphite is given by: G B 14 = ( 457.27 61.16) ( 457.27 + 38.96) = 346.24 kJ/mol (B 1 9 ) G B 14 (per mole C O 2 ) = 173.12 kJ/mole C O 2 (B 20 ) The negative Gibbs free energy value of reacting C with O 2 and H 2 O to produce CO 2 suggests a spontaneous process in ambient conditions is possible. Looking at the set of calculations for the oxidation of graphite, the most likely pathway is going to occur un der
170 an oxygen rich or humid, ambient environment. Like MoS 2 graphite oxidation is not lik ely to occur under a water rich environment. The most likely scenario for graphite oxidation is sliding under oxygen. Given the entire set of calculations, the negative tribological effects associated with the formation of MoO 3 c ould be present within the wear tracks formed under air (50% RH) or 150 Torr oxygen. However, the lubricating effe cts attributed to the oxidation of graphite edge sites are also possible under those conditions. Ultimately, the calculations show that the oxidation of MoS 2 is favored over graphite for both environments and therefore MoS 2 to MoO 3 is the probable reaction to take place.
171 APPENDIX C HERTZIAN CONTACT PRE SSURE CALCULATIONS The contact pressure of the pin acting on the revolving flat surface has a dominating e ffect on the f riction and wear response of dry and solid lubricants. Contact pressures determine the area of contact, which can lead to wider wear tracks. Additionally, increasing contact pressures play a role in either decreasing the coefficient of friction, as is the case for MoS 2 or increasing the coefficient of friction as in the case for silicon in air sliding. Lastly, the contact pressure calculations are a p ortion of the mathematical process used to estimat e the temperature experienced at the contact interface du ring sliding. Here to understand the possible temperature effects on the sliding surface composition, the range of contact pressures experienced during testing is calculated. Figure C 1 shows the two boundary conditions for mechanical loading. The first s hows a high pressure condition, where there is a 3.175 mm diameter 3 04 SS sphere on flat contact under 1 N load ( figure C 1a ) The illustration on the right, figure C 1b portrays the low pressure condition of a flattened, 1mm diameter, 3 04 SS flat on flat co ntact under 1 N load. The loading condition values are given in table C 1. A B Figure C 1: Schematics of A) Sphere on flat contact and B) Flat on flat contact modeled after figure 7.9 in Engineering Tribology 3 rd Edition 221
172 There are t hree key values which are important to determine the contact pressures between two bodies : the radius of the contact area (a), average contact pressure (p average ), and maximum contact pressure (p max imum ) These can be written as: a = (3 W R /E ) 1/3 (C 1) p average = W/( a 2 ) (C 2) p maximum = 3 W/(2 a 2 ) ( C 3) The first value to calculate is the reduced radius of curvature for a s phere on flat contact R : 1/R = 2 /R x (C 4) 1/R x = 1/R A + 1/R B (C 5) Sol ving for C 5: 1/R x = 1/0.0015875 + 1/ = 629.9212 1/m (C 6) Thus, C 4 becomes: 1/R = 2 629.9212 = 1259.84252 1/m (C 7) R = 1/ 1259.84252 = 0.00079375 m or 7.9375 10 4 m (C 8) The m aterial properties are assumed to be 3 04 SS (Body A) and MoS 2 (Body B). 1/E = 1/2 [(1 A 2 )/E A + (1 B 2 )/E B ] (C 9) Using the values from table C 1/E = 1/2[(1 0.3 2 )/(21010 9 ) + (1 0.27 2 )/(23810 9 )] = 4.1110 12 1 / Pa (C 10) E = 1/(4.1110 12 ) = 2.4310 9 Pa = 243 GPa (C 11)
173 From th e calculated value s of C 8 and C 11, the radius of the area of contact can be found using equation C 1 : a = (3 W R /E ) 1/3 (C 1) a = [(3 1 7.9375 10 4 / ( 24310 9 ) ] 1/3 = 2.7 696 10 5 m 2 (C 12) Now that the contact radius is known, the average and maximum pressures can be found using equations C 2 and C 3: p average = W/( a 2 ) (C 2) p average = 1 / [ ( 2.69624 10 5 ) 2 ] = 4.38 10 8 Pa = 438 M Pa (C 13) p maximum = 3 W/(2 a 2 ) (C 3) p maximum = 3 1 /(2 ( 2.69624 10 5 ) 2 ] = 6.57 10 8 Pa = 657 M Pa (C 3) The largest value for the Hertzian contact pressure is 657 M Pa for a sphere on flat contact. The low contact pressure condition is a simpler calculation. For this model, the area of contact is assumed to be the area of the flattened head. This can be calculated using the area of a circle: a = r 2 (C 14) Given that the estimated diameter of the flattened head is 1 mm, equation C 14 becomes: a = 0.00 0 5 2 = 7 85 10 7 m 2 (C 15) Assuming that that the pressure is uniformly distributed over the entirety of the flat contact, the average pressure during sliding can be written as: p av erage = W/a (C 16) p average = 1/(3.1410 6 ) = 1.2710 6 Pa = 1.27 MPa (C 17)
174 Thus, the low contact pressure condition has a n estimated value of 1.27 MPa. The range of the calculated contact pressures span s almost three orders of magnitude, from 1.27 MPa to 657 M Pa. Table C 1: Values used to calculate contact pressures for high and low contact pressure conditions. Property High Pressure Condition Low Pressure Condition R A 1.5875 mm R B E A 210 GPa A 0.3 E B 238 GPa B 0.27 W 1 N 1 N r 0.5 mm
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192 BIOGRAPHICAL SKETCH Gregory James Dudder was born to two metallurgists in Richland, Washington As a child growing up he had ample opportu nity to explore the world around him. Spending several years as a grade schooler in the Washington, DC area furthered his appreciation for history, travel, and enhanced his natural curiosity. In 2002 h e graduated from Hanford High School. When it came time to choose a co llege, the choice was easy. As a lifelong fan of the University of Washington, he enrolled there in the autumn pursuant on a degree in materials engineering Graduating with his B achelor of S cience in m aterials science and e ngineering, t he decision to atte nd graduate school was buoyed by his mentor, Dean Paxton, who recommended the University of Florida. Looking at a map, Gregory decided it was time to try out a new area of the country and departed Seattle, Washington for Gainesville, Florida. This move in hindsight would be fortuitous given the spate of national championships won by the University of Florida since decision was made in the early spring of 2006 Entering graduate school, h e was trained in the new discipline of surface scien ce by his a dvisor Scott Perry studying the interactions on material surfaces and between interfaces at an atomic level He earned his M aster of S cience in m aterials s cience and e ngineering in the spring of 2008 O utside of research, Gregory will miss the football on B arbeque and the sun shining warm almost year round. He received his D octor of Philosophy in materials science and engineering from the University of Florida in the fall of 2010 and intends to pursue a career that furthers his interests and takes him close r to home, wherever that may be. Go Gators!