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Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2012-08-31.

Permanent Link: http://ufdc.ufl.edu/UFE0041897/00001

Material Information

Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2012-08-31.
Physical Description: Book
Language: english
Creator: Keith, James
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: Mechanical and Aerospace Engineering -- Dissertations, Academic -- UF
Genre: Mechanical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Statement of Responsibility: by James Keith.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Sawyer, Wallace G.
Electronic Access: INACCESSIBLE UNTIL 2012-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041897:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041897/00001

Material Information

Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2012-08-31.
Physical Description: Book
Language: english
Creator: Keith, James
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: Mechanical and Aerospace Engineering -- Dissertations, Academic -- UF
Genre: Mechanical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Statement of Responsibility: by James Keith.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Sawyer, Wallace G.
Electronic Access: INACCESSIBLE UNTIL 2012-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041897:00001


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1 DESIGN OF A PIN ON DISK TRIBOMETER WITH IN SITU OPTICAL PROFILOMETRY By JAMES HARLEY KEITH A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2010

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2 2010 James Harley Keith

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3 DEDICATION Heres to you Jesus one solid dude.

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4 ACKNOWLEDGEMENTS I would like to thank Dr. Richard Schulz, Granddaddy for taking me fishing for the first time and being a lifelong idol. He was the original mystery meat ch ef. I would like to thank Dr. Sarah Schulz, SaSa, for her wisdom, patience and financial support through college. I would like to thank John and Carrie Keith for their continuing support Thank you for putting up with me and my mess in the driveway. I would like to thank Mike Braddock for his guidance and enthusiasm. I would like to thank Misti Keith for her love and support while I lost sleep over the stresses of this work. I would like to thank Dr. Greg Saw yer for the adventures over the past three years. He made me a better engineer and taught me to push life to the limits at every opportunity. Granddaddy would have approved I would like to thank all of the members of the University of Florida Tribology Lab, current and past for their efforts. Without their hard work to build upon, my success would not have been possible. I would especially li ke to thank Ira Hill for helping me automate the tribometer in this work.

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5 TABLE OF CONTENTS page ACKNOWLEDGEMENTS ............................................................................................... 4 LIST OF TABLES ............................................................................................................ 6 LIST OF FIGURES .......................................................................................................... 7 ABSTRACT ..................................................................................................................... 8 CHAPTER 1 INTRODUCTION ...................................................................................................... 9 2 REVIEW OF LITERATURE .................................................................................... 10 Optical Profilometry ............................................................................................. 10 Wear Rate and Coefficient of Friction .................................................................. 11 In Situ Wear Monitoring ....................................................................................... 11 3 PIN ON DISK APPARATUS ................................................................................... 13 Tribometer ........................................................................................................... 13 Linear Positioning Stages .............................................................................. 13 Cantilever ...................................................................................................... 14 Capacitance Probes ...................................................................................... 14 Rotary Positioning Stage ............................................................................... 14 Data Acquisition and User Interface .............................................................. 15 Force Measurement ...................................................................................... 15 Optical Profiler ..................................................................................................... 15 4 MEASUREMENT UNCERTAINTY ......................................................................... 17 Error Source Analysis .......................................................................................... 17 Mathematical Analysis ......................................................................................... 19 5 TEST RESUSLTS AND DISCUSSION ................................................................... 22 6 CONCLUSION ........................................................................................................ 25 APPENDIX LIST OF REFERENCES ............................................................................................... 26 BIOGRAPHICAL SKETCH ............................................................................................ 27

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6 LIST OF TABLES Table page 4 1 Measurement uncertainty analysis results ............................................................. 21

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7 LIST OF FIGURES Figure page 2 1 Typical microscope based whitelight interferometer. [1] ...................................... 10 3 1 EZ RoD pin ondisk tribometer with in situ optical profilometry ............................. 13 4 1 Capacitance probe target alignment error. a) Target with angular misalignment independent of displacement. b) Displacement dependent angular misalignment. ..................................................................................................... 17 4 2 Component errors of horizontal X axis, EBX: pitch error motion of X, EAX: Roll error motion of X. ................................................................................................ 18 4 3 Schematic of the pinondisk contact with EYY error. EYY: Position error of the Y axis that offsets the pin from center. ............................................................... 19 5 1 Optical profilometer images of Au 95 wt %, Cu 5 wt % disk before and after sliding contact. The images in the left column are height maps and images in th e right column are intensity maps of the a) & b) surface before test, c) & d) after 15 cycles of sliding, e) & f) after 750 cycles of sliding. ............................... 22 5 2 Ultra nanocrystalline diamond coated sphere on Au 95 wt %, Cu 5 wt % disk. a) Normal and friction force versus cycle number, b) resultant coefficient of friction versus cycle number. .............................................................................. 23

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8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science DESIGN OF A PIN ON DISK TRIBOMETER WITH IN SITU OPTICAL PROFILOMETRY By James H. Keith August 2010 Chair: W. G. Sawyer Major: Mechanical Engineering A pin on disk t ribometer with in situ optical profilometry was constructed. The tribometer is capable of loads up to 200 mN, 25 revolutions per minute and angular positioning resolution of 50 rad. An ultranano crystalline diamond coated sphere was run on a 95 wt % Au and 5 wt. % Cu disk Friction results and in situ wear track images were presented. Measurement uncertainty analysis was performed for loads typical of the test in this work and was shown to be less than 5%.

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9 CHAPTER 1 INTRODUCT IO N Tribology is the study of interacting surfaces in relative motion and encompass es the fields of friction, lubrication and wear. To investigate the reaction of different surfaces in contact and the governing mechanisms of friction, lubrication and wear, many test apparatuses have been conceived. These tribometers have taken many fo rms ; a common arrangement is the pinondisk w here a stationary pin is imposed on a rotating disk. Typically, the apparatus will be instrumented with force or deflection sensors to meter the friction force. Depending on the loading scheme, the normal force may be instrumented as well. This work des cribes a pin ondisk tribometer with in situ optical profilometry designed for the University of Florida Tribology Lab and analyses its ability to accurately measure normal and friction forces.

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10 CHAPTER 2 REVIEW OF L ITERATURE Optical Profilometry Optical profilometry is a method of non contact surface height measurement commonly used to quantify surface roughness. A type of optical profilometry of particular interest to this work is interference microscopy. Interference microscopy combines an interferometer and microscope into one instrument for surface measurement. The objectives are either Michelson, Mirau or Linnik (all being modifications of the Michelson interferometer)[ 1 ]. Figure 2 1 depicts the light path of a Mirau interferometer similar to the profilometer used in this work. The objective is vertically scanned to induce interference fringes on the surface. The fringes are recorded by the charge coupled d evice and are used to generate a surface height map. Figure 2 1 Typical microscope based whitelight interferometer [1]

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11 Wear Rate and Coefficient of Friction Two bodies in contact moving w ith r espect to one another may generate wear debris. J. F. Archard developed the following relationship that aid s in the prediction of the rate of material loss to wear (equation 1) mKsP W p (1) Wh ere W is the worn volume, K a dimensionless constant related to the probability of a wear particle, s is the sliding distance, P is the applied load and pm is the flow pressure of the softer material [ 2 ]. A convenient modification of Archards equation is WKsP (2) w here K is a dimensional constant and the relation does not require knowledge of material flow pressure. The coefficient of friction, denoted as is the ratio of the friction force on two bodies in contact and the normal force applied to them as seen in equation 3 f nF F ( 3 ) In Situ Wear Monitoring Wear mechanisms can be complex involving solid mechanic, thermodynamic and chemical phenomenon, among others, and also be transient. For example, a lubricious film could fatigue and delaminate from the substrate after a million loading cycles but show negligible wear up until that point. To label that scenario with one wear rate could be misrepresentative. Measuring volume loss in situ can highlight wear trends that could otherwise be lost in ex situ volume loss analysis. Bares et al studied copper oncopper sliding

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12 electrical contacts in a humid copper environment. The tribometer was designed for linear reciprocation with in situ scanning white light interferometry. W ear rates were associated with different current densities by measuring volume loss in situ [3]

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13 CHAPTER 3 PIN ON DISK APPARATUS Tribometer The tribometer in this work is a symphony of instruments that are responsible for the loading of two surfaces under prescribed motion to generate friction forces and wear debris while metering the loads relative motion and wear track evolution. Figure 3 1. EZ RoD pin ondisk tribometer with in situ optical profilometry Linear Positioning Stages Pin load and track radius are adjusted by two manual, linear ball bearing stages manufactured by OptoSigma (model number 1230715). The stages are mounted orthogonal to one another and the sample surface. In this configuration, pin position is independently adjustable along the X and Z axes depicted in F igure 3.1. The stages are adjusted by manipulation of side mounted micrometers. The micrometers have verniers every 10m of linear translation and position can be i nterpolat ed to approximately 3m between the verniers.

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14 Cantilever The pin is mounted on a flexible member that is referred to as a cantilever. T he cantilever selection process suffers from a solid mechanic duality. It must be stiff whilst compliant. Real surfaces are not flat or smooth and a s the disk rotates the cantilever must be compliant enough to follow the disks surface but stiff enou g h to impart the desired load onto the pin. For this work, the stiffest cantilever available from CSM instruments was called into service. Cantilever number ST 15 has a 2.367 mN/m stiffness in the normal direction and 5.217 mN/m stiffness in the fricti on direction. Capacitance Probes Lion Precision Eli te 3 mm capacitance probes measure the displacement of the cantilever. They explicitly measure the capacitance between the probe and the target. The change in dielectric constant of the atmosphere is neglected, so the only change in capacitance is from the change in distance between the probe and the target. The range of the capacitance probes used on this tribometer is 100m. Combined with the ST 15 cantilever, this tribometer can impart loads up to 200mN Rotary Positioning Stage The disk is fixed to a Physik Instrumente (PI) M 060PD rotary positioning stage that i s driven by a PI C 863 Mercury Servo Controller. The stage is driven by a pre loaded worm gear reduced direct current servo motor and has a maximum velocity of 90 /sec and a resolution of 50 rad. The Wyko NT9100 optical profilometer as configured has approximately 50 mm between the work table and the objective focal point so a criterion for stage selection was stage height The M 060 series has a height of 29 mm leaving 19 mm of vertical clearance for the sample and sample holder.

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15 Data Acquisition and User Interface A National Instruments PCI 6221 16 bit data acquisition card was use with LabVIEW to record and organize the voltages from the capacitance probes. Virtual instruments for LabVIEW that aided in the motion commands for the rotary positioning stage were provided by Physic Instrumente. The data acquisition card is set to sample the normal and friction force channels at a rate of 1 kHz. In unison, the angular position of the stage is requested by means of the Physic Instrumente position query virtual instrument. The 1kHz data is averaged every second and stored in a cell next to the angular position of that data acquisiti on cycle. Due to the overhead of the file save, screen update and other peripheral operations, the code saves at a frequency of just over 10 Hz. Force Measurement Friction and normal force are measured via the equipment listed in the previous subsection headings. The forces are calculated by the relations nCLnCPnFCCV ( 4 ) fCLfCPfFCCV ( 5 ) where the subscript n indicates the normal direction and the subscript f indicates the friction direction. CCL is the cantilever constant, CCP is the capacitance probe constant, and V is the change in the output voltage. Optical Profiler For surface profile characterization, the tribometer is affixed to the twoaxis linear motion table of a Wyko NT9100 Optical Profiler. By coherence scanning interferometry the NT9100 produces threedimensional surface maps of the object under test with sub -

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16 nanometer vertical resolution [ 1 ]. The NT9100 used in this work is equipped with a 20x Mirau objective and fieldof view (FOV) multipliers of 0.55x, 1x and 2x. The test in this work was performed under the 0.55x FOV multiplier resulting in a n 11x magnification. The measurement array i s 640 x 480 pixels resulting in an analysis area of 437 m x 583 m.

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17 CHAPTER 4 MEASUREMENT UNCERTAI NTY No measurement is without error and the first step towards precision is admitting that there is a problem. The purpose of this chapter is to quantify the error in the force measurements taken by the pin ondisk tribometer. Error Source Analysis The tribometer has errors that stem from the electrical system and the mechanical system For this analysis, the electrical system includes the capacitance probes and the data acquisition card and the mechanical system includes the cantilever and cantilever alignment with respect to the disk sur face. Figure 4 1 Capacitance probe target alignment error. a) Target with angular misalignment independent of displacement. b) Displacement dependent angular misalignment. The Lion Precision capacitance probes used in this work are capable of measuring distances with a resolution of 15 nm. That resolution is attainable over the 100 m

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18 range of the probes only if the target translates in an ideal manner. If the target surface is not normal to the measuring face of the capacitance probe as in Figure 4 1 a the output signal will have an offset suggesting that the probe target is closer than it appears The tribometer uses a target that is affixed to a cantilever that displaces in an arc. The arc motion imposes a displacement dependent zero offset as depicted in Figure 4 1 b Figure 4 2 Component errors of horizontal X axis, EBX: pitch error motion of X, EAX: Roll error motion of X. The data acquisition card from National Instruments, used for this tribometer, records analog voltages and stores them as digital voltages in increments of 1/2n where n is the number of bits. The card is capable of 16bit yielding a voltage increment of approximately 15 V. Manufacturing errors cause misalignment of the m easurement axes with respect to th e force vectors of interest. The significant contributors of measurement error from

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19 mechanical misalignment were determined to be EA X ( roll error motion of X ) E BX (pitch error motion of X) and EYY (positioning error) [4]. The first two are depicted in F igure 42 and the latter is depicted in Figure 43. Figure 4 3 Schematic of the pinondisk contact with EYY error. EYY: Position error of the Y axis that offsets the pin from center Mathematical Analysis The root sum square method is used for uncertainty analysis of measurements of the contact forces in the normal and friction directions. The root sum square method is as follows : 1 2 22 2 12 12...Rn nRRR uuuu xxx ( 6 )

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20 Simply explained, the uncertainty in R is the square root of the sum of the squares of the partial derivative of R with respect to each variable in R multiplied by the uncertainty in the variable. Applying equation 6 to equation 3 4 and 5 yields : 1 2 2 2nfFF nfuuu FF ( 7 ) 1 222 2n CLn CPn nFCCV CLn CPn nuuuu CCV ( 8 ) 1 222 2f CLf CPf fFCCV CLf CPf fuuuu CCV ( 9 ) With these equations, the uncertainty in precision was calculated for the normal force, fricti on force and coefficient of friction. A nominal value of 50 mN was considered for the normal force and a nominal value of 15 mN was considered for the friction force. The normal force is typical for the test mentioned in this work and the friction force is a wors t case friction value from said test. A h igh value of friction force would maximize the capacitance probe error, the largest contributor of measurement error. To appreciate the total uncertainty, bias errors are added to the precision errors with the same root sum square method and are represented as: 1 22 2 TPBuuu (1 0 ) where the bias errors generate from the geometric errors depicted in figure 4 2 and 43.

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21 The total calculated uncertainty for the friction force measurement, normal force measurement and coefficient of friction calculation are presented in Table 41 below. Table 4 1. Measurement uncertainty analysis results Measurand Nominal Value Uncertainty Fn 50 mN 1.12 Ff 15 mN 0.30 0.3 0.0 1 As tab ulated, all of the calculated uncertainties are under 5 %. The largest contributor to uncertainty is the cantilever stiffness coefficient provided by CSM instruments.

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22 CHAPTER 5 TEST RESUSLTS AND DI SCUSSION A wear test was run on the pinondisk tribometer The pin was loaded against the disk at a nominal load of 50 mN and slid relative to the disk at a velocity of 30 degr ees per second or 5 revolutions per minute. After every 360 degrees of revolution, the stage was stopped to image the wear track. This was performed for 750 cycles. Figure 51 shows height map and intensity map images. Images c) and d) are after 15 cyc les of sliding when the first wear scar appears. Substantial plastic deformation can be seen in images e) and f). Figure 5 1 Optical profilometer images of Au 95 wt %, Cu 5 wt % disk before and after sliding contact The images in the left column are height maps and images in the right column are intensity maps of the a ) & b) surface before test, c) & d) after 15 cycles of sliding, e) & f) after 750 cycles of sliding.

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23 The normal force and friction force are plotted against cycle number in Figure 51 a and the resultant coefficient of friction is plotted in F igure 51b. Figure 5 2 Ultra nano crystalline diamond coated sphere on Au 95 wt %, Cu 5 wt % disk. a ) Normal and friction force versus cycle number, b ) resultant coefficient of friction versus cycle number. As mentioned in Chapter 2, the tribometer is configured to save angularly resolved friction forces. The data plots in Figure 52 are from measurements taken at a unique angular location of 180 degrees of stage rotation. This location was chosen as it is under the objective of the profilometer when the stage is at rest. In Figure 52 a the normal force is plotted with respect to the cycle number. In the first 100 cycles of the test, the load ramps from just under 50 mN to approximately 55 mN. It seems that the fluctuation in load is a product of thermal expansion F riction in the worm gear and resistance in the motor are likely sources of heating To verify th e hypothesis of transient temperatures the stage was fitted with a thermocouple in subsequent friction tests After starting a frictio n experiment, the temperature displayed on the thermocouple reader would raise about 5 C. In test force fluctuation was minimized in subsequent tests by commanding a stage rotation of approximately 80

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24 revolutions at 25 rpm before the friction test was begun. This quickly raised the temperature of the stage to the steady state operating temperature that 5 rpm would produce. Further rotation of the stage at 25 rpm would overheat the stage causing the normal force to decrease during the test until the stea dy state temperature was reached. The step increase in force at approximately 700 revolutions in Figure 52 a was due to the building air conditioning shutting down for Christmas break. As mentioned in Chapter 2, the sensitivity of adjustment for the manu al stages is 50 m per revolution, allowing for an accurate linear movement of approximately 3 m by interpolating between verniers. However, the smallest move between stick slip intervals of the micrometer head seems to be on the order of 1 m. For this work, the cantilever stiffness was 2.376 mN/ m. Thus, the smallest single adjustment possible was in excess of 2 mN. After multiple attempts, the sta ge could usually be adjusted to a desired force value within 1 mN.

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25 CHAPTER 6 CONCLUSION A pin on disk tribometer with in situ optical profilometry was built for the University of Florida Tribology Lab. The uncertainty was calculated. T he applied normal force has shown to drift approximately 1 m per C in temperature change of the rotary stage. Although this effect can be partially mitigated by bringing the rotary stage up to temperature before the test, a more precise fix would be to add an active normal force stage with feedback The tribometer would, of course, need to be renamed from EZ RoD to HardRoD.

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26 LIST OF REFERENCES [1] A.G. Olszak, J. Schmit, M.G Heaton, Interferometry: Technology and Appl ications (2001) Veeco Metrology Group. Retrieved Dec. 24, 2009 from http://www.veeco.com [2] J.F. Archard, W. Hirst, The wear of metal s under unlubricated conditions, Proc. Royal Soc. of London. Ser. A, Mathematical and Physical Sciences, 236 (1956) 397410. [3] J.A. Bares, N. Argibay, N. Manutler, G.J. Dudder, S.S. Perry, G.R Bourne, W.G. Sawyer High current density copper on copper sliding electrical contacts at low sliding velocities, Wear 267(2009) 417424. [4] H. Schwenke, W. Knapp, H. Haitjema, A. Weckenmann, R. Schmitt, F. Delbressine, Geometric error measurement and compensation of machines An update, CIRP Annals Manufacturing Technology 57 (2008) 660675.

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27 BIOGRAPHICAL SKETCH The author is a lifelong Florida resident. His interests in mechanical engineering were first realized in high school when he was working on trucks and Jeeps in his parents driveway. In pursuit of a mechanical engineering degree, h e received an A ssociate of A rts from Okaloosa Walton Community College in Niceville, Florida in 2003. He received a B achelor of S cience in mechanical engineering from the University of Florida in 2007. He married the love of his life, now Misti A. Keith, that following summer.