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Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2015-05-31.

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Permanent Link: http://ufdc.ufl.edu/UFE0045457/00001

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Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2015-05-31.
Physical Description: Book
Language: english
Creator: Ewin, Jeffrey J
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2013

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 Jeffrey J Ewin.
Thesis: Thesis (M.S.)--University of Florida, 2013.
Local: Adviser: Sawyer, Wallace Gregory.
Electronic Access: INACCESSIBLE UNTIL 2015-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2013
System ID: UFE0045457:00001

Permanent Link: http://ufdc.ufl.edu/UFE0045457/00001

Material Information

Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2015-05-31.
Physical Description: Book
Language: english
Creator: Ewin, Jeffrey J
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2013

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 Jeffrey J Ewin.
Thesis: Thesis (M.S.)--University of Florida, 2013.
Local: Adviser: Sawyer, Wallace Gregory.
Electronic Access: INACCESSIBLE UNTIL 2015-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2013
System ID: UFE0045457:00001


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1 TRIBOLOGY OF POLYTETRAFLUOROETHYLENE AND ALUMINA COMPOSITES IN VARYING VACUUM PRESSURES By JEFFREY JOHN EWIN 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 2013

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2 2013 J effrey John E win

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3 To my parents M ark and Patti Ewin as well as my brother Tracy

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4 ACKNOWLEDGMENTS I would like to thank Professor W.G. Sawyer for his encouragement, mentorship, and teaching that helped me develop into a stronger engineer over the past three years of working in the tribology laboratory. I would also like to thank Professor Youping Chen and Professor Peter G. I fju for their teaching efforts throughout my time here at the University of Florida and also for serving on my graduate committee. I would like to thank Dr. Brandon A. Krick for his mentorship, collaboration, and friendship throughout my time at the Univer sity of Florida. I would also like to thank Angela A. Pitenis for her help in making all of the samples used in these experiments. I would also like to thank my DuPont collaborators, Dr. Chris topher P. Junk and Dr. Greg ory S. Blackman for their technical advice I would like to thank the Valspar Corporation for their financial support throughout my graduate studies I like to thank Juan M Ur u e a for his comic relief and support I would also like to thank all current tribology laboratory members for the daily support. I would also like to thank all past tribology laboratory members who made the work for this thesis possible. I would like to thank my parents Mark and Patti for their love and support. They always pushed me to be the best that I could be, and were always there to help me along the way. I would also like to thank my beautiful fianc e Rebecca Charles for her steadfast support in life and in my studies.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF FIGURES ................................ ................................ ................................ .......... 6 ABSTRACT ................................ ................................ ................................ ..................... 8 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 10 Tribology and Solid Lubricants ................................ ................................ ................ 10 PTFE ................................ ................................ ................................ ....................... 11 PTFE and various fillers ................................ ................................ .......................... 13 Prev ious Environmental Studies ................................ ................................ ............. 14 Objectives ................................ ................................ ................................ ............... 16 2 EXPERIMENTAL METHODS ................................ ................................ ................. 19 Sample Materials ................................ ................................ ................................ .... 19 Sample Preparation ................................ ................................ ................................ 20 Countersamples ................................ ................................ ................................ ...... 21 Wear Rate ................................ ................................ ................................ ............... 22 Overview of Vacuum Tribometer ................................ ................................ ............ 22 Uncertainty of Wear Rate and Friction Coefficient ................................ .................. 24 Vacuum Pressures ................................ ................................ ................................ 25 Stylus Profi lometry ................................ ................................ ................................ .. 26 3 EXPERIMENTAL RESULTS ................................ ................................ ................... 31 Wear Rate ................................ ................................ ................................ ............... 31 Friction Coefficie nt ................................ ................................ ................................ .. 31 Transfer Films and Wear Surfaces ................................ ................................ ......... 32 4 DISCUSSION ................................ ................................ ................................ ......... 41 PTFE ................................ ................................ ................................ ....................... 41 PTFE and Alumina Composi te ................................ ................................ ................ 42 5 CONCLUSIONS ................................ ................................ ................................ ..... 51 APPENDIX: CALCULATION OF MASS BASED WEAR RATE ................................ .... 53 LIST OF REFERENCES ................................ ................................ ............................... 54 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 61

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6 LIST OF FIGURES Figure page 1 1 Performance of various materials used as solid lubricants. ............................... 17 1 2 Results from Krick et al.. ................................ ................................ ..................... 18 1 3 X ray photoelectron spectrum from Krick et al. for the transfer films developed during sliding ................................ ................................ ..................... 18 2 1 A diagram of a polymer pin sliding on a stainless steel counterface. .............. 27 2 2 The linear tribometer is housed inside of a vacuum chamber ............................ 28 2 3 A comparison of various vacuum levels that occur due to altitude away from sea level. ................................ ................................ ................................ ........... 29 2 4 The linear tribometer is housed in a vacuum chamber. ................................ ...... 30 3 1 Wear rate of PTFE alumina composites and unfilled PTFE 7C plotted versus vacuum pressure.. ................................ ................................ .............................. 33 3 2 Friction coefficient of PTFE and PTFE alumina composites plotted versus vacuum pressure ................................ ................................ ............................... 34 3 3 Photographs of the worn polymer surface and the transfer film developed during sliding for PTFE 7C samples at varying vacuum levels. ....................... 35 3 4 Photographs of the worn polymer surface and the transfer film developed d uring sliding for PTFE alumina composite samples at varying vacuum levels. ................................ ................................ ................................ ................. 36 3 5 Representative profilometer scan of the transfer film developed on the stainless steel counterface during sliding for PTFE samples at the lower vacuum levels ................................ ................................ ................................ .... 37 3 6 Rep resentative profilometer scan of the transfer film developed on the stainless steel counterface during sliding for PTFE samples at the higher vacuum levels. ................................ ................................ ................................ .... 38 3 7 Representative profilometer scan of the transfer film developed on the stainless steel counterface during sliding for PTFE alumina composites at lower vacuum levels ................................ ................................ ......................... 39 3 8 Representative profilometer scan of the transfer film developed on the stainless steel counterface during sliding for PTFE alumina composites at higher vacuum levels ................................ ................................ ......................... 40

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7 4 1 Wear rate for PTFE alumina composites is plotted versus transfer film thickness. ................................ ................................ ................................ ........... 48 4 2 Friction coefficient for PTFE and PTFE alumina composites plotted versus transfer film thickness ................................ ................................ ........................ 49 4 3 Transfer film thickness of PTFE and PTFE alumina composites plotted versus vacuum pressure. ................................ ................................ ................... 50

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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 TRIBOLOGY OF POLYTETRAFLUOROETHYLENE AND ALUMINA COMPOSITES IN VARYING VACUUM PRESSURES By Jeffrey John Ewin May 2013 Chair: W.G. Sawyer Major: Mechanical Engineering Polytetrafluoroethylene (PTFE) is an excellent candidate material in solid lubricant applications due to its low friction, chemical inertness, high operating temperature, and vacuum compatibility. However, PTFE suffers from a high wear rate due to delamination wear. With the inclusion of a few weight percent of alpha phase alumi na particles, the wear rate of PTFE is drastically decreased by over four orders of magnitude. The filler not only shuts down large scale delamination and adds load support to the polymer matrix, but it also facilitates a tribochemical reaction with the s urrounding environment during sliding A linear reciprocat ing tribometer housed in a vacuum chamber was used to test PTFE and PTFE and alumina composites to determine the necessity of water in facilitating ultralow wear behavior The transfer film and run ning film developed during sliding at four different pressures of vacuum were analyzed to determine the dependence of water on the formation of transfer films. By varying the vacuum pressure from ambient pressure to a high vacuum pressure of 1x10 6 Torr, t he water content of the chamber was varied. From ambient pressure to 1x10 3 Torr, there was sufficient water in the environment to

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9 allow ultralow wear. Beyond this pressure, there was not enough water available in the chamber to form the chemical species necessary for ultralow wear. The formation of a robust, thin, brown transfer film generally accompanies ultralow wear in PTFE and alumina composites, which can only occur in environmental conditions with sufficient water content.

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10 CHAPTER 1 I NTRODUCTION Tribology and Solid Lubricants Tribology is the study of contacting materials in relative motion and their associated friction coefficient and the wear rate Traditionally, l ubricants such as oils and greases have been used to reduce the friction coefficient between two sliding interfaces Fluid lubricants drastically improve the tribological properties of materials because they form a fluid film to avoid intimate contact of moving components [ 1 7 ] The fluid film also reduces the friction coefficient which improves the efficiency of components that contain moving parts [ 1 7 ] The fluid lubricants require equipment (e.g. pumps, seals) to make sure that the lubricant stays in the region of contact. Addit ionally, the liquid lubricants may require a filtration system to remove debris particles and reduce contamination [ 8 10 ] Liquid lubricants have a limited thermal range of operation and can suffer from oxidation [ 11 14 ] Liquid lubricant s do not perform well in space either since there is insufficient gravity to keep the lubricant in contact, or to drive the liquid to a sump for a pumping system [ 15 16 ] With modern advances in materials development, s olid lubricants are emerging as viable options especially when fluid lubricants cannot be used. Solid lubricants have the ability to operate reliably in a variety of thermal environments as well as high vacuum applications [ 17 22 ] Solid lubricants have been implemented in a wide variety of applications including aircraft, satellites, bearings, seals, and orthopedic implants [ 23 30 ] Solid lubricants do not require pumps, seals, or filters to operate. Solid lubricants can be employed using a polymer or low shear strength metal as a coating on a metal substrate or as a solid component such as a bearing.

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11 In a typical solid lubricant tribological system there are two (or more) components in contact When these two components slide against each other, material is gradually removed from each component. This gradual removal of material in a sliding contact is known as wear. The rate at which a material changes volume due to wear is called the wear rate ; it is often the limiting factor for solid lubricants. Furthermore, the worn volume of materials is often liberated as wear debris which can cause problems in enclosed systems such as the human body [ 31 33 ] As higher performance mater ials are being designed to have better wear resistance, solid lubricants are becoming even more widely used. Figure 1 1 displays the wear rate and friction coefficient for various polymeric materials and composites that have been used as solid lubricants An ideal material would have a low friction coefficient and would never wear out. PTFE Po lytetrafluoroethylene (PTFE) is a well known solid lubricant since it was discovered to have the lowest coefficient of friction of any engineering polymer in the 1940s [ 34 37 ] PTFE consists of a carbon backbone that is fully terminated by fluorine atoms The fluorine bonds cause the molecule to have a helical sweep which creates a smooth outer surface. The C F bonds protect the polymer from chemical attacks [ 36 38 39 ] PTFE is vacuum compatible since the polymer has a low vapor pressure and exhibits low outgassing [ 34 36 39 42 ] PTFE has a high melt temperature for a polymer of around 352 C which makes it a great material for higher temperature applicat ions [ 34 37 39 43 ] PTFE has the lowest coefficient of friction of any engineering polymer [ 34 37] The smooth outer surface of the PTFE molecule allow for neighboring molecules to slip [ 17 40 44 45 ] The

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12 polymer is great at transfe rring from the bulk material to the mating surface that it is being slid against [ 46 51 ] The resulting layer that is transferred from the bulk polymer material to the mating surface is k nown as a transfer film Transfer films of PTFE ha ve been shown to form consistently on metals, as well as other materials during sliding. When the film on the counter surface was analyzed, the molecular surface was aligned in the direction of sliding [ 37 52 ] A study was performed by Gong [ 38 ] in which PTFE was slid on multiple metal surfaces. When metal counterfaces were analyzed using X Ray photoelectron spectroscopy (XPS), the chemical species of the transfer film of PTFE were the same for all the metals tested Gong theorized that since there were no new chemical species formed on the surface, the PTFE must shear below the sliding interface [ 38 ] During sliding, PTFE is transferred from the bulk material to the substrate and the friction decreases. After several cycles, the bulk material of PTFE is running on a film of PTFE deposited on the substrat e. The polymer metal interface is replaced with a polymer polymer interface. Low friction is associated with the creation of transfer films which has been widely studied [ 17 34 40 49 53 55 ] PTFE has a low resistance to wear, and typically has a wear rate on the order of 10 4 mm 3 / ( Nm ) (Fig ure 1 1) Pooley and Tabor [ 48 ] studied the high wear rate of the polymer and concluded that the smooth outer surface of the polymer allows for chains to slip past each other, and gives rise to its low wear resistance When sliding on a metal surfa ce, the adhesion of the polymer to the material is stronger than the subsurface shear forces that hold the polymer together [ 38 40 41 48 55 ] These forces cause cracks to form in a subsurface region of the bulk m aterial which create

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13 large chunks of material that crack from the surface [ 37 38 40 41 ] The dominant wear mode of PTFE is delamination [ 53 56 ] The typical transfer film formed has large clumps of wear debris along the film [ 45 55 57 ] The film is not chemically bonded to the surface, and can be easily removed. PTFE and various fillers Filler materials can be used to combine multiple optimal characteristics of the materials [ 41 56 58 70 ] PTFE can be added to a high friction polymer to reduce the friction coefficient [ 49 71 75 ] When harder materials are added to PTFE, the wear resistance improves [ 44 53 56 63 66 67 69 76 78 ] Many materials have been added to PTFE to try to improve the wear resistance [ 44 6 4 66 67 69 76 78 80 ] There are several theories pertaining to how filler materials help to abate wear of the bulk polymer. F iller materials can help to limit the size of the subsurface cracks created during sliding [ 53 66 69 ] Limiting the crack length decreases the size of particles that can be torn from the surface and reduces the wear of the bulk material. Hard and/or fibrous filler materials can add load carrying ability to the polymer to reduce crack growth [ 64 80 81 ] Tanaka proposed that fillers help prevent large scale destruction to the banded structure of PTFE [ 69 ] When improved wear resistance has been measured, the size of the wear debr is has been smaller than for unfilled PTFE [ 76 81 ] Early studies conducted by Tanaka found that there was a specific size and shape that the filler material had to be to improve wear resistance [ 69 ] The early view was that fillers should have a size on the order of microns, and be spherical or rod/fiber like in shape. This size of filler supported the theory that fillers limited crack propagation and added load support to the polymer [ 69 ] The wear rate of micron size filled particles is on the order of 10 5 to 10 6 mm 3 / ( Nm ) E arly studies concluded that nano sized fillers

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14 would not improve the wear resistance of PTFE because they were too small to mechanically halt large scale delamination and were transferred to the metal in the wear debris [ 69 ] Burris and Sawyer found that a particular alpha phase nano sized alumina powder in a PTFE matrix could achieve ultra low wear rates on the order of 10 7 mm 3 / ( Nm ) [ 77 82 ] Only a few weight percent of this nano size d alumina filler was necessary to decrease the wear rate of PTFE to this extent During sliding, t he polymer composite created a thin brown transfer film on the metal substrate which differs from the transfer film developed by pure PTFE. A thin tribofilm on the wear surface of the polymer is also developed during sliding [ 6 2 68 83 ] Through XPS, it was discovered that a new chemical species was found in t he transfer film which bec omes chemically bonded to the metal surface [ 84 ] The formation of a new chemical species during mechanical sliding has been termed tribochemistry. For unfilled PTFE 7C the polymer wear due to delamination and forms large patchy wear debris. When PTFE is filled with hard fillers the polymer has improved wear resistance. The f illers limit delamination and provide load support to the polymer matrix. To achieve ultralow wear, the delamination of the polymer is shut down, and a tribochemi cal reaction occurs at the sliding interface to form a thin transfer film. Previous Environmental Studies Several studies on the wear of materials [ 18 20 84 87 ] Burris studied the effect that cryogenic temperatures had on the tribological properties of PTFE [ 1 8 ] Burris found that the friction coefficient increased as the temperature decreased, and that the wear rate increased with decreasing temperature. Colbert studied the effect that water has on

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15 moly bdenum disulphide (MoS 2 ) by conducting tribological e xperiments in a vacuum environment [ 19 ] MoS 2 is known to be dependent on water, and the friction of the interf ace increases when water is available duri ng the sliding of the material. Krick and Sawyer constructed a pin on disc tribometer to test the tribological of solid lubricants in lower Earth orbit [ 20 ] Eight pin on disc tribometers were placed on the I nternational S pace S tation to conduct tribological experiments for a year. This was one of the first tribological experiments of solid lubricants to be conducted in a space environment. Krick et al studied the effect of the humidity of the environment on the wear rate of a PTFE alumina composite material [ 84 ] In an effort t o control environmental humidity, a n environment chamber with a linear reciprocating tribometer was filled with ultrahigh purity nitrogen (99.999%) gas. The environmental humidity was varied from 0.5% RH to around 90% RH. Figure 1 2 shows that the wear rate of the composite decreased with increasing humidity while the wear rate of PTFE was mostly unchanged Profilometry scans of the stainless steel substrates were made to understand the morphology and thickness of the transfer film in various environments When the transfer film for the 0.5% RH experiment was measured, the composite material abra ded the steel and did not form a robust, brown transfer film. Profilometry scans of transfer films of h igher humidity experiments showed a thin stable transfer film on the metal surface. As found in previous studies a brown stable transfer film has been synonymous with low wear materials [ 62, 68, 83 ] The transfer films were then examined using XPS to detect any chemical changes between pure PTFE and the PTFE alumina composite a fter sliding (Figure 1

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16 3) It was found that the PTFE alumina composite created a new chemical species during sliding. New peaks formed with a CHF bond and C CF 2 bond which suggests that the polymer was chemically reacting The study concluded that water is important for creat ing the necessary tribochemical reactions to achieve ult r a low wear. Objectives The remarkable link between wear rates and humidity of PTFE and alumina composites in previous studies has inspired tribological research in various vacuum pressures. The ap plications for this research are widely ranging from seals in vacuum assemblies, to bushings in aileron s of aircraft. In this study, PTFE wa s used as a matrix that wa s loaded with five weight percent alpha phase alumina. The polymer composite wa s tested on a linear reciprocator housed inside of a vacuum chamber that is capable of reaching a vacuum level of 5x10 6 T orr. Varying the level of vacuum varied the amount of atmospheric gases, including water oxygen and nitrogen that can react with the material of both the stainless steel counter sample, as well as the polymer composite. This work examine d the lowest possible pressures that the PTFE alumina composite would still achieve ultralow wear.

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17 Figure 1 1. Performance of various materials used as solid lubricants. An ideal material would never wear out and would have a friction coefficient of zero

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18 Figure 1 2. Results from Krick et al. There is a strong dependence between the humidity in the environment and the wear rate of PTFE alumina composites [ 84 ] Figure 1 3. X ray photoelectron spectrum from Krick et al. for the transfer films developed during sliding for (a) unfilled PTFE and (b) PTFE filled with 5 wt% alpha phase alumina on a stainless steel counter surface in a laboratory environment [ 84 ]

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19 CHAPTER 2 EXPERIMENTAL METHODS Sample Materials PTFE was used as the matrix material. Unfilled PTFE, which has a higher molecule weight compared to other fluoropolymers, was used as a control sample. PTFE control and PTFE composite samples were made from DuPont TM Teflon PTFE 7C resin (~35 m sized p articles). The PTFE matrix was filled with 5 weight percent of alpha phase alumina (~27 43 nm sized particles) and distributed by Nanostructured & Amorphous Materials Inc. The dispersion of the alumina particles into the matrix material is important to e nsure enough filler material will be present at the sliding interface. In an effort to obtain uniform dispersion, the sample powder was mixed using a Micronizer J et Mill (Sturtervant). The jet mill (fluid energy mill) is typically used in pharmaceutical applications to mix ingredients together for use in medications. The equipment uses compressed air to pulverize larger particle agglomerates into particles less than one micrometer in diameter. The air stream creates a vortex that carries the particles into a grinding chamber where high speed rotation causes finer particles to collide and break apart into increasingly finer particles. The larger particles are driven to the outside of the chamber by higher centrifugal forces and are ground by the walls o f the chamber as well as other larger particles, while fine particles move toward the center and exit the chamber to a collection container. The dry composite powder was prepared by first measuring 5 weight percent alumina powder and 95 weight percent PTFE on an Ohaus Adventurer model AR3130 balance with a resolution of 0.001 g. The powders were roughly mixed by

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20 hand to disperse the alumina throughout the PTFE resin. The mixture was then fed into the jet mill for one pass at a low feed rate to break dow n the larger PTFE agglomerates, as well as to better disperse the alumina into the PTFE resin. The jet milled mixture was then removed from the collection container, and fed through twice more to ensure a uniform dispersion of the alumina in the PTFE 7C m atrix. The unfilled PTFE powder was not processed by the jet mill. Sample Preparation The PTFE and PTFE composite samples were processed identically. Approximately 10 g of the sample material was packed into a cylindrical mold that was constructed of 440C stainless steel with the dimensions of 76.2 mm in length, 50.8 mm in diameter, and a 12.7 mm hole through the center. Two 12.7 mm diameter 440C plugs were used to compact the sample material from each end of the cylindrical mold. The mold and powder were placed in a hydraulic press, and a pressure of 100 MPa was applied to compress the m aterial. Since PTFE is not an injection moldable polymer, the samples were processed by compression molding. Once the resin was compacted, a pressure of 20 MPa was applied to the cylinder. The DuPont TM reported melt temperature for PTFE 7C resin is 35 2 C. Because the temperature of the sample was measured by means of a Type K thermocouple embedded about 19 mm into the side of the cylindrical mold, a higher temperature was used to ensure that the sample achieved a melted state. The cylinder was heate d from ambient temperature at 2 C per hour to 365 C. As the temperature increased, the pressure on the cylinder decreased to allow for thermal expansion of the polymer. The Omega thermal controller typically overshoots the set temperature by 5 C and s ettles at the set temperature of 365 C. The sample was held constant at 365

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21 C for one hour. The sample was then cooled at a rate of 2 C per hour to ambient temperature. The pressure is increased as the temperature is cooled to account for thermal con traction, and ensure that the polymer remains compacted to avoid the growth of voids in the matrix. The polymers were machined into a rectangular prism with dimensions of 6.35 mm x 6.35 mm x 12.7 mm for tribological testing. Each sample was cut using a mi lling machine to the specified dimensions. Figure 2 1 shows a diagram of the polymer pin sliding on the stainless steel countersample. The experimental surface has the dimensions of 6.35 mm x 6.35 mm. The experimental surface was polished using 800 grit sandpaper to an average surface roughness of 180 nm. The sample was sonicated in methanol for thirty minutes. The sample was allowed to dry in lab air for at least three hours to evaporate the alcohol prior to testing. Countersamples It is important to remember that a tribological system is composed of two materials, where both materials play important roles in the tribological performance. The polymers and polymer composites are slid against stainless steel countersamples. The countersample for this study was made of AISI 304 stainless steel finished to a lapped surface. Burris and Sawyer conducted studies to compare the wear rate of PTFE alumina composites run on several common surface finishes [ 83 ] The average surface roughness of the countersample was around 150 nm. The countersample was washed using soap and water to remove any oil from the machining processes. Methanol was applied to the washed surface to further clean the experimental surface.

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22 Wear Rate This work focuses on the wear or change in volume of the polymer composites. The wear rate is measured since wear is difficult to extrapolate from material properties, and an accurate measurement is achieved through experimentation. Wear rate can be computed in a variety of ways, but typically is in the dimensions of volume loss divided by the p roduct of the applied normal load and sliding distance [ 88 89 ] Wear rate is dependent on volume loss which allows materials over of a large density range to be compared. Since PTFE does not absorb water over time, mass based measurements are accurate. A mass based wear rate was computed and reported for the experimental samples. The volume of each sample was measured before the start of slid ing. The mass of the sample was determined by using a Mettler Toledo AX2055 balance. The density was calculated for the sample in g/mm 3 The mass of the sample composite was recorded prior to sliding, and after the total number of cycles was completed. Knowing the density of the sample, the volume loss could be calculated knowing the mass loss. The normal load and distance slid were recorded for the entire experiment. An overall wear rate was reported for each sample which does not exactly capture the steady state wear rate for the PTFE alumina composites. The wear rate is reported in units of mm 3 /(Nm). Overview of Vacuum Tribometer A linear reciprocating pin on flat tribometer was used to test the friction coefficient and wear rate of the polymer samples. A schematic of the tribometer is shown in Figure 2 2. The tribometer is housed in a vacuum compatible chamber so that the atmosp here that the sample is run in can be varied from ambient pressure to a

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23 pressure of 5x10 6 Torr. The normal force and the friction force are measured using a 6 axis load cell (AMTI MC2.5D 100). Since the friction coefficient cannot be directly measured, it is computed using the measured normal force and friction force. A double cantilever applies the normal load to the sample to limit the angular misalignments and deflections during sliding. The cantilever is mounted directly to the load cell to ensure that there is little interference in the load path. The cantilever is used to apply the normal load to the sample using a micrometer stage. The samples were tested at a sliding velocity of 50.8 mm/s, an applied normal load of 250 N, and a reciprocating length of 30.8 m m. The coefficient of friction fell within a range of 0.14 to 0.20 with a standard deviation of 0.01. Unfilled PTFE wore faster than the PTFE alumina composites. The unfilled PTFE samples could only survive 10,000 cycles before the length of the sample was too short to continue experimentation. Since the PTFE alumina composites would not wear out as quickly, these samples were subjected to more sliding cycles. The unfilled PTFE samples were slid for a total of 10,000 cycles, and the PTFE composite samp les were slid for a total of 100,000 cycles. To avoid contaminating the environment the mass of samples were measured before and after sliding experiments, without breaking the environment for intermittent measurements. An overall wear rate was calculated for the samples based on the difference betwe en the initial and final mass measurements. The normal load was applied using the displacement of the micrometer stage. As the sample experienced creep and wear, the load slowly decreased. The normal load was set using the micrometer stage before the va cuum pumps were turned on and

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24 before the samples started sliding. For the low vacuum experiments, the samples would experience an applied normal load for up to four hours before the friction experiment was started. To account for creep in the polymer, th e normal load was initially set to 258 N. The unfilled PTFE samples experienced high wear during the experiment, which corresponds to a decrease in the normal load from 250 N to around 175 N. This drift motivated an average normal load to be used in wear rate calculations. The PTFE alumina composites were low wearing, so the applied normal load did not vary as dramatically. The average normal load was also used to compute the wear rate for these samples. Uncertainty of Wear Rate and Friction Coefficient A detailed report of the uncertainty for the friction coefficient of a similar test appara tus was c ompleted by Schmitz et al. [ 90 ] Using the sensitivity of the load cell, and taking into account the misalignments of the instrument, the percent error in the friction coefficient of unfilled PTFE ( avg =0.14) was 2%. Since the standard deviation is larger than the calculated error, the standard deviation is shown as the error bars in the included figures. The uncertainty in the wear rate is largely influenced by the mass measurement. To lower the uncertainty of the measurement, a Mettler Toledo balance with a resolution of 0.02 mg was used to take mass measurements of the samples. The mass based wear rates were calculated using a Monte Carlo simu lation iterated 1,000 times [ 89 ]. The Monte Carlo simulation uses a random distribution to calculate the slope to the linear fit the two points of the wear rate 1,000 times as well as the uncertainty of the wear rate. The uncertainty was at least an order of magnitude smaller than the wear rate measurement. This uncertainty is used for the figures of wear rate. A further report

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25 of the uncertainty of the wear rate has been completed by B urris and Sawyer [ 89 ] a s well as Schmitz et al. [ 91 ] Vacuum Pressures Ambient pressure at sea level is 760 Torr. Low vacuum is below 760 Torr and extends to a pressure of 24 Torr which is the vapor pressure of water at 25 C. Medium vacuum ranges from 24 Torr to 1x10 3 Torr. High vacuum is in the range of 1x10 3 Torr to 1x10 9 Torr. Ultrahigh vacuum continues below a pressure of 1x10 9 Torr until eventually extremely high vacuum is reached. Outer space has a pressure that ranges from 1x10 6 Torr to 3x10 17 Torr. Figure 2 3 displays mechanisms operated at various pressures due to altitude. Pressure inside the atmosphere varies with altitude. The altitude that aircraft can operate at can be limited by altitude or ultimately the surrounding pressure. Airliners typically operate at an altitude of 34,000 feet which has a pressure of 179 Torr. Fighter jets and other military aircraft can operate at an altitude up to 70,000 feet which experience further reduced pressure of 34 Torr. A select few aircraft that are used for surveillance can fly at an altitude of 100,000 feet which operate in a medium vacuum level of 8 Torr. A medium vacuum level ext ends from 100,000 feet to the edge of the atmosphere to a pressure of 1x10 3 Torr. After the atmosphere, various satellites and space vehicles operate in high vacuum pressures. The tribometer is housed inside of a va cuum chamber shown in Figure 2 4 The chamber is connected to a scroll pump( Varion Scroll Vacuum Pump ), and a cryopump ( Cryo Torr 8 Cryopump ). The cryopump has a chilled airline that is supplied by a compressor that flows helium. The temperature of the pump head is held constant at a temper ature of 10 K. To achieve a vacuum pressure of 1x10 3 Torr, the scroll pump was

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26 used. The scroll pump was run for four hours to allow the pressure of the chamber to reach steady state. To achieve a vacuum pressure of 5x10 6 Torr, the cryopump was turned on after the scroll pump had achieved lower vacuum. The cryopump was run until the chamber reached a state of equilibrium. To reach a vacuum pressure of 5 Torr, the scroll pump was turned on to achieve a vacuum pressure of 1x10 1 T orr. The p ump was then turned off, and the leaking of air from the chamber equalized at a pressure of 5 Torr. The pressure was measured using an Ionivac ITR 90 pressure sensor. For all of the vacuum conditions, a steady state pressure was achieved before the tribo logical experiment was started. Stylus Profilometry The transfer films that formed on the stainless steel countersamples during sliding were characterized using a stylus profilometer (Veeco Dektak 8). A topographical scan was performed across the countersample to measure and map th e height of the transfer film. A stylus with a radius of 2.5 m was applied to the surface using a normal load of 5 mg and line spacing of 2.5 m. The height of the film that is reported is an average of the area that was measured.

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27 Figure 2 1. A diag ram of a polymer pin sliding on a stainless steel counterface. A stroke of 19 mm was used at a velocity of 50.8 mm/s. A transfer film usually develops as sliding cycles increase until the transfer film thickness achieves a steady state.

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28 Figure 2 2. The linear tribometer is housed inside of a vacuum chamber. The normal load is applied using a micrometer stage, and the sample is mounted to a double cantilever [ 19 ]

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29 Figure 2 3 A comparison of various vacuum levels that occur due to altitude away from sea level. Various equipment operates at differing altitudes.

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30 Figure 2 4 The linear tribometer is housed in a vacuum chamber. A scroll pump is connected to an inlet at the bottom of the chamber, and a cryopump is connected at the top.

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31 CHAPTER 3 EXPERIMENTAL RESULTS Wear Rate The wear rate versus vacuum pressure is plotted in Figure 3 1. Th e uncertainty bars shown for the wear rates are the uncertainties in the measurement after usi ng a Monte Carlo simulation [ 89 ]. The uncertainty in pressure corresponds to the range of vacuum pressures experienced by the sample during sliding. Unfilled PTFE was used as a control for this experiment. The polymer is vacuum compatible since it has a low vapor pressure and does not outgas. The wear rate remained relatively constant despite the wide range of vacuum pressures. The PTFE alumina comp osite samples retained a constant wear rate until a vacuum pressure of 3x10 3 Torr While operating at this pressure, the samples formed a thin stable transfer film that has been consistent with low wear. Below this vacuum pressure, the wear rate increas ed to 4x10 5 mm 3 /(Nm). Friction Coefficient A plot of friction coefficient versus vacuum pressure is shown in Figure 3 2. The friction coefficient for pure PTFE 7C appears to vary with changing vacuum pressure. The friction coefficient is lower at ambient conditions, and a t the lowest vacuum pressure of 4x10 6 Torr. The friction coefficient of PTFE alumina composites also varies with vacuum pressure. The friction is highest at ambient pressure and at a vacuum pressure of 4x10 6 Torr. At the intermediate pressures, the c oefficient of friction is lower than for the ambient pressure.

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32 Transfer Films and Wear Surfaces Images of both the transfer film developed on the 304 stainless steel countersamples and the matching worn polymer sample are shown in Figure 3 3 (unfilled PTFE) and Figure 3 4 (PTFE alumina composite). The optical images of these two tribological samples show a relatively even transfer film developed on the steel countersample s The transfer film allows for a polymer polymer contact during sliding which lowers the coefficient of friction. The transfer films were further analyzed using a stylus p rofilometer to measure the height of the transfer films developed for each experimental pressure. A 2.5 m stylus tip with an applied load of 5 mg was used to measure the transfer films after the friction experiments. The profilometry scans were conducte d at ambient pressure at a temperature of 25 C and 30 50% RH. The results of the profilomet ry scans for PTFE at low vacuum pressure s and high vacuum pressures are shown in F igure 3 5 and Figure 3 6 respectively The results of the profilometer scans for P TFE alumina composites at low vacuum pressure s and high vacuum pressures are shown in F igure 3 7 and Figure 3 8 respectively

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33 Figure 3 1 Wear rate of PTFE alumina composites and unfilled PTFE 7C plotted versus vacuum pressure. The normal load was 250N, a sliding speed of 50.8 mm/s and a stroke of 30.8 mm was used for the experiments. PTFE 7C shows no dependence on vacuum pressure while the PTFE alumina composites show a dependence on vac uum pressure.

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34 Figure 3 2 Friction coefficient of PTFE and PTFE alumina composites plotted versus vacuum pressure. The uncertainty in the coefficient of friction is the standard deviation of the measurement. The friction coefficient of PTFE and PTFE alumina composites show a dependence on pressure.

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35 Figure 3 3. Photographs of the worn polymer surface and the transfer film developed during sliding for PTFE 7C samples at varying vacuum levels. The sliding direction and transfer film are labeled in the photographs. The visible transfer film varies with the vacuum pressure. More transfer film appears to be formed on the steel counterface at a pressure of 760 torr and 1x10 6 torr. A running film is not developed on the worn surface of the polymer pins.

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36 Figure 3 4. Photographs of the worn polymer surface and the transfer film developed during sliding for PTFE alumina composite samples at varying vacuum levels. The sliding direc tion and transfer film are labeled in the photographs. A brown colored running film is developed on the polymer pins for pressure from ambient down to 6x10 3 torr. Tran sfer films are developed on the substrates at an ambient pressure down to a pressure o f 6x10 3 torr. When a thin transfer film is developed, the system achieves ultralow wear.

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37 Figure 3 5. Representative profilometer scan of the transfer film developed on the stainless steel counterface during sliding for PTFE samples at the lower vacuum levels A single line scan is plotted over the 3D representation of the transfer film. The pressure of (a) 760 torr forms a patchy transfer film with large flakey wear debris that is common for PTFE while for a pressure of (b) 3 torr, the developed tran sfer film becomes hard to measure.

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38 Figure 3 6. Representative profilometer scan of the transfer film developed on the stainless steel counterface during sliding for PTFE samples at the higher vacuum levels. For a pressure of (a) 9x10 3 torr the tra nsfer film has a low percentage of coverage than an ambient pressure. At a pressure of (b) 1x10 6 torr the transfer film has large flakey wear debris that is even larger than the film developed at ambient pressure.

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39 Figure 3 7. Representative profi lometer scan of the transfer film developed on the stainless steel counterface during sliding for PTFE alumina composites at lower vacuum levels. A thin stable transfer film is developed at (a) ambient pressure, and a thinner transfer film is developed at a pressure (b) 4 torr. Both samples achieve ultralow wear.

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40 Figure 3 8. Representative profilometer scan of the transfer film developed on the stainless steel counterface during sliding for PTFE alumina composites at higher vacuum levels. A thin transfer film is developed for (b) a pressure of 6x10 3 torr which is synonymous with ultralow wear. When the pressure is further reduced (b) to 5x 10 6 torr the transfer film becomes patchy and the wear rate increases.

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41 CHAPTER 4 DISCUSSION PTFE At ambient pressure unfilled PTFE formed large flakey wear debris on the countersample (Figure 3 3). The friction coefficient at ambient pressure was 0.14. The transfer film that was deposited on the stainless steel created a polymer polymer contact which gives rise to the low coefficient of friction of PTFE. For a pressure of 4x10 6 Torr the friction coefficient again is low and a transfer film is deposited on the countersample. The transfer film developed is not as flakey as the ambient pressure, and appears to be well oriented in the direction of sliding. A thicker transfer film reduces the friction coefficient for unfilled PTFE materials. For the two mi d range vacuum pressures in these experiments, a smaller portion of transfer film coverage is seen in Figure 3 3. During sliding, a transfer film may develop, but the transfer film does not adhere to the countersample at these two vacuum pressures. The t ransfer film may initially form on the surface only to wear away after subsequent sliding cycles. The friction coefficient is higher for these two samples since the contact at the sliding interface is more metal polymer than polymer polymer. For an ambien t pressure, the transfer film had an average height of 3.2 m (Figure 3 5). There is a large build up of wear debris on the side of the wear track. The profilometry scan reveals a thick, patchy transfer film that is formed during sliding which is common for PTFE sliding in ambient pressure conditions. The thick, patchy transfer film is associated with a low friction coefficient, and supports the theory that the polymer sample is sliding on the same polymer that is deposited on the counter surface.

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42 For t he vacuum pressure of 4 Torr, the thickness of the transfer film proved difficult to measure by profilometry. The large wear debris on the edge of contact is seen, but there is no clear evidence of a transfer film. The large wear debris forms because the sample has a high wear rate, and the delamination still occurs at the subsurface of the PTFE sample regardless of the vacuum pressure. The surprising result is that there is a very low amount of transfer film developed on the metal countersample. The fr iction coefficient is higher during sliding since the polymer is sliding on a metal surface, and the polymer polymer mechanism is not fully developed. Profilometry scans of the stainless steel countersample and worn PTFE sample in high vacuum conditions ar e shown in Figure 3 6. For a pressure of 3x10 3 Torr a transfer film formed in some regions, but did not fully cover the wear track. Since the transfer film had little robust coverage, the friction coefficient is higher than for samples of unfilled PTFE at ambient pressure conditions. The average hei ght of the developed transfer film is 1.7 m by profilometry scans. For the lowest vacuum pressure, 4x10 6 Torr, a visible transfer film developed on the metal countersample. The average height of the transfer film is 6.8 m, which is higher than the tran sfer film developed at an ambient pressure. The friction coefficient is similar to the ambient pressure due to the transfer film. Again the wear rate is not affected by the development of the transfer film. PTFE and Alumina Composite When examining the optical images for the PTFE alumina composite in Figure 3 4, there are clear dist inctions between transfer films developed on the countersample for various vacuum pressures. A thin, bronze colored transfer film is developed for samples that are low wear. For the vacuum pressure of 4x10 6 Torr, the wear rate is

PAGE 43

43 high and there is no bronze transfer film. Instead, a lighter colored transfer film is formed, and the wear rate is similar to a toughened PTFE matrix. The difference in the development of the transfer film at various vacuum pressures would suggest that the low wear of the PTFE alumina composites are dependent upon the water content in the environment. The friction coefficient for the PTFE alumina composites varies slightly with the vacuum pressure of the environment. By examining Figure 3 4, there is no clear evidence to explain the difference in the friction coefficient. There are added ceramic particles in the PTFE matrix that interact with the metal countersample and raise the friction coefficient beyond that of virgin PTFE. The pressures that yielded the highest fricti on coefficients were ambient pressure and 4x10 6 Torr. For the ambient pressure, a thin bro nze colored transfer film formed, and for a pressure of 4x10 6 Torr, there is a lighter colored patchy transfer film. The optical differences in the transfer film do not provide clear evidence to the rise in friction coefficient for these pressures over t he other vacuum pressure conditions The profilometry scans for the PTFE alumina composite countersurfaces in low vacuum conditions are shown in Figure 3 7. For the experiments conducted in ambient pressure there is a thin transfer film that is developed on the countersample. The transfer film is thin compared to that of the unfilled PTFE, and has an average thickness of 0.4 m. The samples that have a thin transfer film also have a low wear rate. The PTFE alumina composite experiment conducted in a vac uum pressure of 4 Torr developed the thinnest transfer film during sliding of the composite samples (Figure 3 7). The transfer films developed at this pressure were the thinnest for both composite

PAGE 44

44 and unfilled PTFE samples. The PTFE in both unfilled and filled composites was unable to adhere as well to the countersample for this vacuum environment compared to other vacuum pressure conditions The PTFE alumina composite sample still achieved ultralow wear which shows there was enough water in the system to exhibit ultralow wear behavior. It is an interesting result that the transfer film is the thinnest for this vacuum pressure. Figure 3 8 shows the transfer films developed on the stainless steel countersample and the worn PTFE alumin a composite sample for the high vacuum pressure environment. The transfer film developed for a vacuum pressure of 6x10 3 Torr is similar to the one developed in ambient pressure. A thin, stable transfer film is developed, and the sample had a wear rate o f 5x10 7 mm 3 /(Nm). The necessary chemical reactions were able to occur during sliding to achieve ultralow wear. The transfer film developed for the PTFE alumina composite during sliding for the sample run at 4x10 6 Torr differs from the previously discuss ed samples (Figure 3 8). The transfer film has large flakey wear debris that is smaller but similar to that formed by PTFE. A thin stable transfer film was not developed as in previous samples, and instead the PTFE is extracted from the matrix and deposi ted on the countersample and the wear rate increases to a value of 3x10 5 mm 3 /(Nm). This wear rate is similar to a toughened PTFE matrix. The filler acts to arrest crack propagation, and support more of the load in the polymer matrix. The vacuum pressure where there is insufficient water in the system to create a tribochemical reaction is in the range of 3x10 3 to 4x10 6 Torr. At adequate vacuum pressures, tribochemical reactions at the sliding interface form a

PAGE 45

45 protective transfer film on the metal counte rsample and tribofilm on the polymer sample, facilitating ultralow wear behavior of the PTFE and alumina composite. The wear rate of the PTFE alumina composites varies with the thickness of the transfer film developed on the stainless steel countersample ( Figure 4 1). The wear rate is proportional to the thickness of the transfer film to the third power of the thickness This phenomenon was also observed in other alumina filled PTFE wear experiments performed by Burris et al. [ 62 ] The wear rate increases as the thickness of the transfer film decreases. When a thin stable transfer film is deposited on the countersample the wear rate of the sample achieved ultralow wear. When the wear debris was larger the pressure was 4x10 6 Torr and the sample was higher wear. The friction coefficient varies with the transfer film thickness (Figure 4 2). Unfilled PTFE has a low friction coefficient when the tran sfer film height is higher (as compared to the thicknesses developed in these experiments). When there is a large transfer film, the contact of the sliding interfaces is polymer polymer. As the height of the transfer film decreases, the friction coeffici ent increases. Less transfer film acts at the interface of sliding which causes the contact to interact with possibly more metallic surfaces. For the friction coefficient of the PTFE alumina composites, an opposite relationship between transfer film thi ckness and the coefficient of friction exists (Figure 4 2). When the transfer film is thin, the friction is low. As the transfer film thickness increases, the friction correspondingly increases. The alumina particles that are dispersed in the polymer ma trix naturally increase the friction coefficient. When the

PAGE 46

46 transfer film is thicker, harder alumina particles are deposited on the surface which increases the friction coefficient of the mating contacts. The amount of transfer film that is deposited on th e surface varies with the vacuum pressure (Figure 4 3). The same trend exists for both unfilled PTFE and for the PTFE alumina composite which suggests that the vacuum pressure plays in role in the amount of transfer film that is deposited on the countersa mple. For both materials, the transfer film is relatively higher at an ambient pressure. When the samples are subjected to a vacuum pressure of 4 Torr during sliding, the thickness of the resulting transfer films is at a minimum. When the samples are te sted at increasingly lower vacuum pressures, the thickness of the transfer film increases even above the thickness developed at ambient pressure. The amount of water in the environment affects how much transfer film can deposit on the stainless steel coun tersurface. The results presented support the re search done by Krick et al. [ 84 ] as well as the work done by Burris and Sawyer [62 77, 82 ] The authors performed XPS on the countersample after sliding to examine the differences in sliding. Both authors found a new peak formed around 290 eV that suggests that the mechanical action of sliding produces a chemical reaction when PTFE bond is broken. When a fluorine atom is taken away from a carbon atom in mechanical scission due to sliding, the carbon termin ates itself again with the easiest molecule to bond with in its surrounding environment. The most abundant gas particles in ambient air are nitrogen and oxygen, and water is present from the humidity of the air. The vapor pressure of water is around 31 To rr at a temperature of 30 C. When the pressure is below 31 Torr, most of the gases in the vacuum chamber condense.

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47 Once the pressure is in the low mTorr range, the desorbing water makes up over 99% of the total gas load [ 92 ] At a vacuum p ressure of 3x10 3 Torr, most of the oxygen has been removed from the system and the remaining elements become nitrogen and water. Since the vacuum chamber is often exposed to ambient air, and the chamber is not baked out during pump down, the surfaces ins ide the chamber are covered in water molecules. The water is hard to pump out since it becomes absorbed in the surfaces, so water is still around in the system. At a low pressure of 5x10 6 Torr, water still exists in the system but in smaller quantities. At this low of a pressure there is not enough water to attach to the carbons when the fluorine is torn during sliding. Since most of the oxygen is not present at a pressure of 3x10 3 Torr, water is necessary for the PTFE alumina composites to achieve u ltralow wear. The water forms bonds with the carbon molecules as the fluorine is torn during sliding. Water is essential to allow for the production of a transfer film on the polymer surface, as well as the metal counter surface. The transfer film provi des protection to both surfaces in contact, and is the reason that ultralow wear is achieved. In the absence of water, at a low vacuum pressure, the PTFE alumina composites exhibit tribological performance similar to that of a toughened PTFE matrix.

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48 Figure 4 1 Wear rate for PTFE alumina composites is plotted versus transfer film thickness. The wear rate follows a power low relationship with thickness that is. The wear rate is proportional to the transfer film thickness cubed.

PAGE 49

49 Figure 4 2. Friction coefficient for PTFE and PTFE alumina composites plotted versus transfer film thickness. The friction coefficient varies with the t ransfer film thickness. For PTFE, as the transfer film increases, the friction coefficient decreases to a steady state value of 0.14. For PTFE alumina composites, the friction increases with increasing transfer film thickness. The hard alumina particles increase the friction coefficient over virgin PTFE.

PAGE 50

50 Figure 4 3. Transfer film thickness of PTFE and PTFE alumina composites plotted versus vacuum pressure. The thickness of both materials varies with vacuum pressure.

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51 CHAPTER 5 CONCLUSIONS When PTFE was filled with 5 weight percent of alpha phase alumina, the wear rate at ambient pressure was drastically different from unfilled PTFE. The inclusion of alumina nanoparticles not only limited crack propagations and toughened the PTFE matrix, bu t it also provided a mechanism for tribochemistry that is necessary for ultralow wear performance. By varying the pressure of the vacuum chamber, a difference was seen in the wear rate. From an ambient pressure down to a pressure of 3x10 3 Torr, the wear rate was relatively constant at a value around 1x10 7 mm 3 /(Nm). When the pressure was further lowered to a pressure of 4x10 6 Torr, the wear rate increased to 5x10 5 mm 3 /(Nm). Examining the transfer films show that when a thin stable transfer film is developed on the stainless steel countersample the sample achieves ultralow wear. Corresponding tribofilms are deposited on the sliding surface of the polymer. These thin tribofil ms act as a layer of protection and prevent large scale wear events. The transfer films form over a range of vacuum conditions from ambient pressure (760 Torr) to 3x10 3 Torr. At a lower pressure of 4x10 6 Torr, a patchy flakey transfer film is deposited on the countersample, and the wear resistance of the material decreases. At a high vacuum conditions, less water content was present in the environment to react with the PTFE alumina composite to form a transfer film that is necessary for ultralow wear pe rformance. The presence of moisture in the environment seems to be responsible for the tribochemical reaction necessary to achieve ultralow wear performance.

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52 The wear rate of PTFE was not greatly affected by varying the vacuum pressure conditions durin g experimentation. Varying the pressure did relate to some changes in the friction coefficient. Upon examining the transfer films developed during sliding, it is shown that the friction coefficient is lower when a transfer film is developed on the counte rsample. The thickness of the transfer film for both PTFE and PTFE alumina composites is dependent upon the vacuum pressure that the sample was subjected to during the experiment.

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53 APPENDIX A CALCULATION OF MASS BASED WEAR RATE Wear Rate mm 3 /(Nm) Mass loss of the sample (g) Density of the sample (g/mm 3 ) Volume loss of the sample (mm 3 ) Wear rate calculation with all measured quantities mm 3 /(Nm)

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61 BIOGRAPHICAL SKETCH Jeffrey J. Ewin was born in Florida to his proud parents. While living in Florida he developed a love for fishing, cycling, and science. During high school he refined his skills in fishing, cycling, and science. His intellectual pursuits led him to study mechanical engineering at the University of Florida in 2006. In his last year of his undergraduate degree, he worked in the University of Florida Tribology Laboratory under the mentorship of Professor W. G. Sawyer, where he was happily able to apply his p ractical knowledge of mechanical systems to solve interesting tribological problems in DuPont and Valspar samples and systems. This valuable research experience led him to pursue a graduate degree in mechanical engineering funded through his work in the Tr ibology Labor atory. Upon earning his degree in May of 2013, he plans to move somewhere near the water where he may continue to pursue his life long passions of fishing, cycling, and science.