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Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-05-31.

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Permanent Link: http://ufdc.ufl.edu/UFE0043902/00001

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Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-05-31.
Physical Description: Book
Language: english
Creator: Krick, Brandon Alexander
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

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Subjects / Keywords: Mechanical and Aerospace Engineering -- Dissertations, Academic -- UF
Genre: Mechanical Engineering thesis, Ph.D.
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theses   ( marcgt )
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Statement of Responsibility: by Brandon Alexander Krick.
Thesis: Thesis (Ph.D.)--University of Florida, 2012.
Local: Adviser: Sawyer, Wallace G.
Electronic Access: INACCESSIBLE UNTIL 2014-05-31

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Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0043902:00001

Permanent Link: http://ufdc.ufl.edu/UFE0043902/00001

Material Information

Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-05-31.
Physical Description: Book
Language: english
Creator: Krick, Brandon Alexander
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

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

Notes

Statement of Responsibility: by Brandon Alexander Krick.
Thesis: Thesis (Ph.D.)--University of Florida, 2012.
Local: Adviser: Sawyer, Wallace G.
Electronic Access: INACCESSIBLE UNTIL 2014-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0043902:00001


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1 EXPLORING THE ULTRA LOW WEAR BEHAVIOR OF POLYTETRAFLUOROETHYLENE AND ALUMINA COMPOSITES By BRANDON ALEXANDER KRICK A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2012

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2 2012 Brandon Alexander Krick

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3 To D anny Lee Krick

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4 ACKNOWLEDGMENTS First off, while these acknowledgments are long, they by no means show my appreciation for the many people who have helped me achieve this goal in my life. I would like to start by thanking DuPont and the Air Force Office of Scientific Research f or financial support. I thank the extended DuPont team for their supply of materials and analysis and characterization techniques. I thank Greg Blackman and Chris Junk of DuPont for their intellectual and personal support in our endeavors of understanding these systems; leaps and bounds were made on t his research with their sup port mainly through long (but never long enough), enthusiastic phone calls and conversations I thank John Jones of the Air Force Research Laboratory for his tireless contributions on the low earth orbit space tribometer. This research would not have bee n possible without the many members of the Tribology Laboratory at the University of Florida, both past and present I acknowledge lab m embers before me who have laid the groundwork by building instruments and performing experiments, thus paving the way for my research. I also thank lab m embers who have overlapped with me who were willing to assist on a daily basis. Specific thanks are owed to UF Tribology Lab members Jeff Ewin Kellon Marchman and Kathryn Harris for their contributions in this work. I also thank many of the excellent faculty of the University of Florida for their technical contributions and discussions, including Dr. Hahn and Dr. Perry. I thank my extended family, including my grandparents, aunts, uncles, cousins, and in laws, who I now know must be the closest and most supportive family anyone could have. I thank my sister, Courtney, for always pushing me to be better, whether she knew it or not I thank my parents, Tom and Debbie, for working tirelessly to make sure I had every oppor tunity in life and always encouraging me to be my very best at

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5 every stage in life I thank m y mother for her endless support and always making sure I had or could get anything I need or want. I thank m y father for encourag ing me to think critically and q uesti on things; he somehow always has the answer to my questions, no matter what the subject. Not one thought, sentence, word nor letter in this document would be possible nor matter to me without the love and support of my wife, Kaley Her selfless supp ort, above all others, has made it possible for me to achieve this goal in my life. Finally I thank my graduate advisor, Greg Sawyer who has been a mentor throughout my graduate studies, and has always diligently work ed to put his students first, giving them every opportunity to excel. He has taught me by example w hat it means to be a scholar, mentor, teacher and a leader.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ .......... 10 LIST OF FIGURES ................................ ................................ ................................ ........ 11 ABSTRACT ................................ ................................ ................................ ................... 15 CHAPTER 1 INTRODUCTI ON ................................ ................................ ................................ .... 17 Tribology ................................ ................................ ................................ ................. 17 Materials Tribology in Mechanical Design ................................ ............................... 18 Solid Lubricants ................................ ................................ ................................ 19 Motivation: Development of Low Friction, Ultra L ow Wear Materials for Dry Sliding ................................ ................................ ................................ ........... 21 2 THEORETICAL BACKGROUND AND MOTIVATION FOR STUDYING ULTRA LOW WEAR POLYTETRAFLUOROETHYLENE SYSTEMS ................................ .. 24 Polytetrafluoroethylene ................................ ................................ ........................... 2 4 Polytetrafluoroethylene as a Tribological Material ................................ .................. 25 Polytetrafluoroethylene Composites ................................ ................................ ....... 27 Conventional Filled Polytetrafluoroethylene Composites ................................ 29 Ultra L ow 2 O 3 Nanocomposites ............ 32 Research Motivation and Objectives ................................ ................................ ...... 34 3 HYPOTHESIS FOR CHEMI CAL MECHANISM FOR ULTRA LOW WEAR BEHAVIOR ................................ ................................ ................................ ............. 36 4 MATERIALS, METHODS AND EXPERIMENTAL DESIGN ................................ .... 41 Polymer Composite Materials and Preparation ................................ ....................... 41 Materials ................................ ................................ ................................ ........... 41 Composite Preparation ................................ ................................ ..................... 43 Dispersion techniques ................................ ................................ ................ 44 Compre ssion molding ................................ ................................ ................ 44 Analysis Methods ................................ ................................ ................................ .... 44 Tribometers ................................ ................................ ................................ ............. 45 Tribometry: Fundamental Measurements in Tribology ................................ ..... 45 Friction c oefficient m easurements ................................ ............................. 46 Wear rate m easurements ................................ ................................ ........... 47 Linear Reciprocating Tribometer ................................ ................................ ...... 48

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7 Environmental Tribometers ................................ ................................ .............. 52 Va cuum tribometer ................................ ................................ ..................... 52 Environmentally controlled tribometer ................................ ........................ 53 Space tribometer ................................ ................................ ........................ 54 In Situ Tribometers ................................ ................................ ........................... 57 Optical in situ microtribometer ................................ ................................ .... 57 In situ Surface Plasmon Resonance ................................ .......................... 59 Experimental Design ................................ ................................ ............................... 62 Survey of Filler Materials ................................ ................................ .................. 62 Characterization of Filler Materials ................................ ................................ ... 63 Environmental Studies ................................ ................................ ..................... 64 In vacuo ................................ ................................ ................................ ..... 64 Humidity and oxygen ................................ ................................ ................. 65 Ethanol ................................ ................................ ................................ ....... 66 Low earth orbit spac e ................................ ................................ ................. 67 Composite Run In and Running Film Studies ................................ ................... 68 Transfer Film and Running Film Analysis ................................ ......................... 69 Exploring Tribochemical Chain Scission in PTFE: Can Bonds Be Broken During Sliding? ................................ ................................ .............................. 70 5 EXPLORING AND CHARACTERIZING FILLERS IN PTFE ................................ ... 72 Wear Tests for Various F illers ................................ ................................ ................. 72 Characterization of Fillers ................................ ................................ ....................... 79 The Alumina Filler ................................ ................................ ............................ 79 TEM and SE M micrographs of filler materials ................................ ............ 80 Size analysis by light scattering and SEM ................................ ................. 87 Chemical analysis by x ray photoelectron spectroscopy ............................ 91 Titania Fillers ................................ ................................ ................................ .... 91 Mineralogical Fillers ................................ ................................ .......................... 92 6 ENVIRONMENTAL STUDIES ................................ ................................ ................ 98 Vacuum ................................ ................................ ................................ ................... 98 Relative Humidity and Oxygen ................................ ................................ .............. 101 Ethanol ................................ ................................ ................................ .................. 103 Low Earth Orbit Spac e ................................ ................................ .......................... 104 7 COMPOSITE RUN IN AND RUNNING FILM DEVELOPMENT ........................... 109 8 TRANSFER FILM AND RUNNING FILM ANALYSIS ................................ ............ 115 Transfer Film Appearance, Thickness and Morphology ................................ ........ 115 Environmental Dependence ................................ ................................ ........... 117 Transfer Film Adhesion and Robustness: Scratch Test ................................ 121 Transfer Film and Running Film Chemistry ................................ ........................... 123 XPS ................................ ................................ ................................ ................ 123

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8 Infrared Absorption ................................ ................................ ......................... 125 Time of Flight Secondary Ion Mass Spectroscopy (TOF SIMS) ..................... 125 9 TRIBOCHEMICAL CHAIN SCISSION AN UNEXOECTED ROUTE TO ULTRA LOW WEAR SYSTEMS ................................ ................................ ........... 126 SPR/SERS Molecular Wear of PTFE ................................ .......................... 126 Optical I n Situ Tribometer ................................ ................................ ............... 128 A Model Based on Intermolecular Forces ................................ ....................... 129 10 DISCUSSION ................................ ................................ ................................ ....... 133 Formation of Mechanisms from Experimental Results ................................ .......... 133 Wear Survey and Characterization of Filler Materials ................................ .... 133 Identifying general trends ................................ ................................ ......... 133 Alumina ................................ ................................ ................................ .... 136 Titania ................................ ................................ ................................ ...... 138 Mineralogical filler s ................................ ................................ .................. 138 Wear mechanism: requirements of the filler ................................ ............. 139 Environmental Studies ................................ ................................ ................... 141 Vacuum ................................ ................................ ................................ .... 141 Nitrogen ................................ ................................ ................................ ... 142 Humidity ................................ ................................ ................................ ... 142 Oxygen ................................ ................................ ................................ .... 143 Ethanol ................................ ................................ ................................ ..... 143 Low earth orbit: ram ................................ ................................ ................. 144 Low ea rth orbit: wake ................................ ................................ ............... 144 Wear mechanism: requirements of the environment ................................ 145 Composite Run In: Formation of Transfer Films and Running Films .............. 145 Filler concentration effect on run in ................................ .......................... 145 Run in and friction ................................ ................................ .................... 147 Running film and transfer film dynamics ................................ .................. 148 Robust transfer films ................................ ................................ ................ 149 Wear mechanism: interactions between the running film and the transfer film ................................ ................................ ........................... 149 Tribochemistry ................................ ................................ ................................ 150 Formation of a new chemical species in the running films and transfer films ................................ ................................ ................................ ...... 150 Tribological bond breaking ................................ ................................ ....... 151 Wear mechanism: the tribofilms and a chemical mechanism ................... 151 Proposed Mechanism Summary ................................ ................................ .... 152 Applying the Mechanism: New Successes ................................ ........................... 154 Selecting Alternative Fillers ................................ ................................ ............ 154 Other Matrices Filled with Alumina and Similar Fillers ................................ .... 154 Polymer Blends ................................ ................................ .............................. 154 11 CONCLUSIONS ................................ ................................ ................................ ... 156

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9 LIST OF REFERENCES ................................ ................................ ............................. 157 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 165

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10 LIST OF TABLES Table page 2 1 Advantages and Disadvantages of PTFE as a tribological material. .................. 25 4 1 List of materials used as fillers in composites ................................ ..................... 41 4 2 List of tribological composites used in comparative experiments. ...................... 43 4 3 Summary of analysis tools ................................ ................................ .................. 45 5 1 Friction coefficient and wear rate results for PTFE and its composites with various fillers ................................ ................................ ................................ ....... 72 6 1 Wear rate and friction for PTFE and PTFE/alumina composites in various mixtures of nitrogen, water and oxygen. ................................ .......................... 102 6 2 Wear and friction of PTFE and alumina composites in nitrogen, humid nitrogen and ethanol doped nitrogen. ................................ ............................... 104

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11 LIST OF FIGURES Figure page 2 1 Structure of PTFE.. ................................ ................................ ............................. 24 2 2 Variations of wear rate and coefficient of friction with speed at temperatures ................................ ................................ 26 2 3 Subsurface crack propagation and delamination wear in PTFE ......................... 27 2 4 Wear rate vs. friction coefficient for tribological composites of PTFE and a several other tribological engineering polymers ................................ .................. 28 2 5 Shutting down the subsurface crack propagation and delamination wear mechanism in PTFE ................................ ................................ ........................... 29 2 6 Wear rate vs. filler volume percent for PTFE matrix composites. ....................... 30 2 7 Wear volume vs. sliding distance for PTFE and some composites. ................... 32 2 8 Wear rate vs. filler volume percent for PTFE matrix composites. ....................... 33 2 9 Illustrating wear improvements made by alumina filled PTFE. ........................... 34 3 1 Carbon XPS spectrum for transfer films of PTFE and PTFE/alumina composites. ................................ ................................ ................................ ...... 37 3 2 An opportunity for degradation, reactivity and tribochemistry as a route to low wear polymer composites. ................................ ................................ .................. 38 3 3 A proposed chemical mechanism for the formation of COOH groups in PTFE. 39 4 1 Schematic of friction coefficient and wear rate ................................ ................... 47 4 2 Linear reciprocating tribometer. ................................ ................................ .......... 49 4 3 F riction coefficient measurement ................................ ................................ ........ 50 4 4 Calculating wear rate from mass loss measurements. ................................ ....... 51 4 5 Linear reciprocating vacuum tribometer ................................ ............................. 53 4 6 Linear reciprocating environmentally controlled tribom eter. ............................... 54 4 7 Space tribometers ................................ ................................ .............................. 55 4 8 Low earth orbit environment as experienced by MISSE ................................ ..... 56

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12 4 9 Optical in situ microtribometer schematic. ................................ .......................... 58 4 10 In situ surface plasmon resonance experiments u tilizing th e Kretschmann configuration ................................ ................................ ................................ ....... 60 5 1 Wear rate versus friction coefficient for PTFE composites filled with various filler materials ................................ ................................ ................................ ..... 74 5 2 Wear rates of PTFE composites filled with of various fillers. .............................. 75 5 3 PTFE and SiO2 composites ................................ ................................ ............... 77 5 4 Scanning electron micrographs of wear ribbon from PTFE/SiO2 composites .... 78 5 5 mina ................................ ... 80 5 6 ............................ 81 5 7 ............................ 82 5 8 ............................ 83 5 9 Transmission electron micr ............................ 84 5 10 ................................ 85 5 11 ................................ 85 5 12 ................................ .. 86 5 13 ................................ .. 86 5 15 Scanning electron micrograph ................................ 87 5 16 mina. ................................ ................................ ......................... 88 5 17 alumina. ................................ .......................... 89 5 18 measured by the SEM. ................................ ................................ ....................... 90 5 19 ................................ ................... 91 5 20 Transmission electron micrographs of various types of titania filler materials. ... 92 5 21 Sca nning electron micrograph of pyrophyllite powder ................................ ........ 93

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13 5 22 Scanning electron micrograph of talc powder ................................ ..................... 94 5 23 Scanning electron micrograph of dolomite powder ................................ ............. 95 5 24 Scanning electron micrograph of kaolin powder ................................ ................. 96 5 25 Scanning electron micrograph of kyanite powder ................................ ............... 97 6 1 PTFE and alumina composited in vacuum vs air. ................................ ............... 99 6 2 Evaluating the wear resistance of a transfer film. ................................ ............. 100 6 3 Wear rate and friction for PTFE and PTFE/alumina composites in various mixtures of nitrogen, water and oxygen ................................ ............................ 102 6 4 Preliminary space tribometer results ................................ ................................ 105 6 5 PTFE/ alumina ram post flight pictures ................................ ............................. 106 6 6 PTFE/ alumina wake post flight pictures. ................................ .......................... 107 6 7 Raman spectroscopy for PTFE/ alumina samples post flight ........................... 108 7 1 Total volume lost as a function of sliding cycle for 2, 5 and 8 weight percent ................. 110 7 2 alumina composites on steel. ................................ ................................ ........... 111 7 3 alumina composites on steel. ................................ ................................ ........... 112 7 4 alumina composites on steel. ................................ ................................ ........... 113 7 5 Total wear rate vs. computes running film coverage for the PTFE and 8 ......................... 114 8 1 ................................ ............. 116 8 2 Transfer film topography. ................................ ................................ .................. 116 8 3 nitrogen envi ronments with water contents of 69 %RH and 0.6% RH .............. 118 8 4 Scanning electron micrographs of transfer films generated by PTFE/ 5wt% ......................... 119 8 5 ................... 120

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14 8 6 Transfer film image and topography for unfilled PTFE in nitrogen enviro nments with water contents of 76 %RH and 0.8% RH ............................ 121 8 7 SEM micrographs of scratches performed on transfer films ............................. 122 8 8 ................................ ...... 124 8 9 IR absorption for transfer films ................................ ................................ .......... 125 9 1 SPR reflectivity intensity vs. sliding cycle ................................ ......................... 127 9 2 Surface enhanced Raman spectrum for PTFE transfer film ............................. 128 9 3 In situ optical microscopy of PTFE sliding on glass ................................ .......... 129 9 4 Mechanical chain scission of PTFE when slid against glass: a force balance approach ................................ ................................ ................................ .......... 132 10 1 PTFE composites filled with 5 wt% of various fillers ................................ ......... 134 10 2 Hypothesized PTFE wear regimes as observed by composite studies ............ 135 10 3 Micrographs of a) fumed silica and b) nickel nanostrands ................................ 141 10 4 % fumed silica ................................ ................................ ................................ .. 146 10 5 Visualization of alumina filler in PTFE composites ................................ ........... 147 10 6 A proposed chemical mechanism for the formation of COOH groups in PTFE 152

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15 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy EXPLORING THE ULTRA LOW WEAR BEHAVIOR OF POLYTETRAFLUOROETHYLENE AND ALUMINA COMPOSITES By Brandon Alexander Krick May 2012 C hair: W. Gregory Sawyer Major: Mechanical Engineering P olytetrafluoroethylene (PTFE) is a chemically inert, low outgassing, high melting point polymer that has the lowest friction coefficient of any bulk polymer ; however, its high wear rate limits its use as a bulk tribological material. Composites of P TFE and alpha phased alumina particles produce extraordinarily low wear rates that can be more than four orders of magnitude less than the wear rates of virgin PTFE. This reduction in wear cannot be explained by traditional mechanical mechanisms. The goal of this research is to understand the mechanism responsible for the ultra low wear behavior of these co mposites. An understanding of the mechanisms will allow for the design of smarter, ultra low wear materials. Several experimental routes were taken to explore the system. Numerous fillers of various chemical identities, phases, shapes and sizes were add ed to PTFE to observe their effects on the wear properties of PTFE and compare them to understand what makes an ultra low wear filler material Characterization was performed on the various fillers to determine why some produce ultra low wear composites w ith PTFE and others do not. Running film and t ransfer film thickness, morphology and chemistry were

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16 studi ed to provide valuable insight in to the behavior of these composites T hese studies prompted the hypothesis of a role of tribochemistry in generating low wear. T he effects of environmental oxygen and humidity on the tribological performance of PTFE/alumina composite and PTFE samples were studied. It was found that wear rate of PTFE/alumina composites is dependent on the humidity of the environment T h is dependence further suggests that a tribochemical mechanism is responsible for the ultra low wear behavior of these PTFE/alumina composites. Further understanding of these mechanisms promoted the design of low f r iction, ultra low wear solid lubricant systems, which are becoming increasingly important as tribological components are a premium for designs for efficiency and sustainability. Compo sites designed provide lower wear and friction than the highest end commercially available polymers and polymer composites.

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17 CHAPTER 1 INTRODUCTION Tribology Tribology, derived from the Greek tribos meaning materials in rela tive motion. the staggering amount of money wasted due to the friction and wear of materials [ 1 ] Although his report was the first formal report on the economic impacts of friction and wear, friction and wear are important phenomena that have always challenged mankind. For example, the invention of the wheel is a direct response to the need to overcome friction. L eonardo Da Vinci recorded early observations on the proportionality of friction and normal force L ater, Amontons and Coulomb laid out the fundamental rules of friction. Reduction of friction coefficient by a single percent could save countless dollars in energy costs. W ear of materials is becoming increasingly more important and could easily have as much of a functional and economic impact as friction For example, in many industrial applications, components become worn and must be replaced T hese replace ments may be costly due to expensive components, labor and the down time of the equipment while the part is being replaced. Mechanical engineers are trained to design around structural, thermal and even environmental failure criterion so it is no surprise that many systems are sufficiently designed in these areas. The lack of knowledge on properly selecting materials for and designing tribological components combines with an underwhelming availability of low wear materials ultimately r esults in wear causin g of end of useful life and often catastrophic failure for many systems. In fact, to paraphrase my advisor, wear is so commonly the end of useful life for a design

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18 or product that synonymous with an acceptable end of useful life of a product. The present global trend for efficiency, sustainability and the desire to push designs be faster and stronger, last longer and operate in extreme conditions has pushed the fundamental need for material development, specifically in the field of tr ibology, giving rise to the subfield of materials tribology. Materials Tribology in Mechanical Design Most mechanical assemblies have one or more solid interface s that can be critical to the function of the assembly W hen there is a relative motion between these solids friction and wear occur For most systems, it is desirable to have low friction and low wear to make these systems either work better, require less energy, or last longer. Some systems require high friction ( i.e. tires) and some even high we ar ( i.e. crayons), however most efforts are focused on developing low wear and low friction applications. In design of a mechanical system, the properties of the materials selected by the engineer are as imperative as the design itself Often, design engi neers will use reference charts or other published and non published data to select a material for application specific properties As an engineer tries to optimize a design, accuracy of these properties become of crucial importance. This can be challengin g when designing tribological components as friction coefficients and wear rates are not simply material properties. Friction and wear behavior of materials depend on many parameters and conditions, including: m aterial pairing c ontact geometry a pplied normal load c ontact pressures r elative s liding speed m aterial surface topography and roughness

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19 e nvironment t emperature c hemical interactions s liding direction (unidirectional, reciprocating, random, etc.) These system specific variations in tribological properties require scientists to study material pairings in very specific experimental conditions to provide reliable data. Even worse, many mechanical assemblies have to work in multiple environments such as a watercraft that must operate in air and subme rged, or spacecraft components that must operate in both terrestrial and the various space environments [ 2 ] Some components must provide reliable wear and friction against multiple counter materials. Solid L ubricants Although extremely low friction coefficients and wear ra tes can be achieved with fluids and greases fluids and greases are often undesirable and sometimes unacceptable in many systems [ 3 ] F luid lubricants often have a narrow temperature range in which they can operate. Fluid lubricants require seals and filtration systems to keep proper function. In some cases they require specific sliding speeds to generate hydrodynamic lubrication. Greases are also affected by their operating environment including temperature and pressure. Fluids and greases can easily migrate or get contaminated. I n th e cases where a fluid or grease lubricant is no longer ideal, solid lubricants are an excellent solution. Solid lubricants provide many added benefits including: a b road operating temperature range n ew environmental capabilities (use in vacuum and other environments) t he l ubricant remains in the contact and is self replenishing s olid l ubricant can be applied as a coating

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20 Though a solid lubricant can offer many advantageous properties, they do have one major downfall that scientists and engineers must accept or try to ov ercome: solid lubricants wear. This wear manifests as design problems in that the lubricant ha s a finite lifetime and generate s wear debris. For obvious reasons it is desirable to increase the lifetime of a tribological component to as long as possible (o r at least until other parts of the system will fail first). T he generation of wear debris is a less obvious problem, but in many systems, such as biological implants, the wear debris composition and morphology is more important tha n the wear rates and fri ction coefficients. Many materials have been used as solid lubricants [ 3 ] Low shear strength metals and thin metal lic film s such as gol d silver and lead have been used as solid lubricants [ 4 ] These materials are inert and act as a sacrificial component. Lamellar solids such as Graphite, MoS 2 WS 2 talc, and boric acid are pop ular as extremely low friction solid lubricants; low interaction energy between lamellar layers allow these materials to shear easily [ 5 ] These materials often exhibit extreme dependence on environmental const ituents such as water [ 6 ] D iamond li ke carbon (DLC) coatings show extremely low friction and wear especially in dry and vacuum conditions [ 7 ] However tem perature and oxidati on are limit ing factors for conventional Some a attempt to alleviate these concerns by adding dopants Polymers and polymeric composites are frequent candidates for solid lubrication systems [ 8 11 ] Common tribological polymer matrices include polyether ether ketone (PEEK), pol yimide (PI), polyamide imide (PAI), Polyethylene and Polytetrafluoroethylene (PTFE). PEEK, PI, and PAI have desirable mechanical and

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21 thermal properties with moderate wear behaviors but have higher friction coefficient PTFE and similar fluoropolymers have low friction coefficient s but suffer from poor wear. U sing volume fractions of fillers, this wear can be reduced [ 9 12 47 ] Motivation: D eve lopment of L ow F riction, Ultra L ow W ear M aterials for D ry S liding Traditional solid lubricants for dry sliding each have their benefits and limiting properties. Engineers are pushing designs to last longer, be more efficient, and operate in more extreme environments. This means that solid lubricants must have lower wear rates, lower friction coefficients and be robust enough to operate in chemical physical and thermal extremes. Although commonly overlooked, in many cases i t is important for materials on both sides of the interface to be low wearing Som etimes, in low friction systems, when one material is low wear it is at the expense of another material. Dry solid lubrication systems that form transfer films can often prevent this problem and promote low friction and low wear of both entities in the con tact. To date t here is no universal answer to the material needs for solid lubricants but there are good and even great materials for specific applications or specific environmen ts. There are a few obvious paths for developing new low friction, low wear materials for applications in various environments and mechanical systems : f undamental research to find a new material set (exploring minerals, ceramics, polymers and inorganic carbon based materials to name a few) d eveloping composi te materials to: o r educe wear of low friction materials o r educe friction of low wear materials o r educe friction and wear of a system o r educe environmental sensitivity to friction and wear

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22 e xplor e thin coatings, films and composites e xplor e the unique propertie s of nanomaterials in reducing wear Polymer composites show promise as candidates for environmentally insensitive solid materials for dry sliding components. Polymers have unique properties and show the ability to blend these unique properties with other polymers, ceramics, metals and other materials. The synergistic combination of properties give engineers the ability to design materials for their applications. PTFE is a promising component for these systems as it is chemically inert, has a low friction c oefficient and its wear properties can be easily modified by the inclusion of fillers. system [ 45 ] This system showed ultra low wear behavior of both the polymer and the counter material which has been steel, titanium, glass, and Si, among others. It produce d wear rates of less than 1x10 7 mm 3 /Nm compared to 4 1x10 3 mm 3 /Nm while the friction coefficient remains low between 0.18 and 0.26 In some cases, the wear was reduced to as low as 3x10 9 mm 3 /Nm, which is approaching a practically ze ro wear system 1 The goal of my research is to understand this PTFE and alumina composite system in order to design more composites that share these properties and provide the engineering and scientific community with a route to ultra low wear material sy stems that can be intelligently designed for various applications. The path to achieving this goal is through: 1 B.A. Krick, J.J. Ewin, G.S. Blackman, C.P. Junk and W.G. Sawyer, Unpublished PTFE and composites, (2011).

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23 c haracterizing the friction and wear properties of this composite in various environments e xploring similar fillers to determine a commonality for fillers that produce low wear PTFE composites c haracterizing filler size, shape and chemical identity c haracterizing the transfer film and running film morphology, thickness, adhesion and chemistry u nderstand the physical, chemical and o ther mechanisms of this ultra low wear system and experimentally test proposed mechanism s t est new composites and filler materials based on our understanding of the mechanism s

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24 CHAPTER 2 THEORETICAL BACKGROU ND AND MOTIVATION FO R STUDYING ULTRA LOW WEAR POLYTETRAFLUORO ETHYLENE SYSTEMS T his section provide s a brief review of PTFE as a tribological material and discuss es the development of the low wear PTFE composite systems and ultimately the ultra low wear PTF S ome theories behind the friction and wear of PTFE and PTFE composite systems are also reviewed Finally preliminary observations and past experimental results that guide the experimental paths are discussed This section include s published and unpublished research performed by myself and others that have worked on these systems. It is by no means intended to be an exhaustive document on the friction and wear of PTFE. Polytetrafluoroethylene Polytetrafluoroethy lene is a unique and fascinating material that has been closely studied b y researchers since its accidental discovery in 1938 [ 12 19 21 24 27 30 33 35 37 38 48 59 ] It is chemi cally inert, vacuum compatible, stable at high temperatures and has a low friction coefficient [ 48 49 51 53 54 ] PTFE consists of a fully fluorinated single bonded carbon backbone ( Fig. 2 1) T he C F bonds are very strong and serve to protect the polymer from chemical attac k The molecular weight of PTFE can be more than 30,000,00 0 The structure of PTFE leads to its numerous advantages and disadvantages ( Table 2 1). Figure 2 1 Structure of PTFE. PTFE is a polymer consisting of a fully fluorinated carbon backbone [ 51 53 ]

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25 Table 2 1. Advantages and Disadvantages of PTFE as a tribological material. Advantages chemical inertness Disadvantag es poor wear resistance high melt temperature > 300C wear debris is often large and plate like low friction coefficient <0.2 transfer films are easily removed from counter samples vacuum compatible difficult to bond as a coating non abrasive cannot be injection molded quiet operation Polytetrafluoroethylene as a T ribological M aterial In the early 19 4 0s PTFE was reported as having a friction coefficient of 0.1, the lowest friction coefficient of any bulk polymer [ 49 ] In the 19 50s, this extremely low friction prompted a spark of interest among many researchers in the tribology and polymer communities [ 50 53 60 62 ] I n these studies f riction coefficients were reported between .04 and 0.4 It w as hypothesized that PTFE makes a thin, molecularly aligned transfer film on the counter surface that it slides against [ 63 ] As a r esult, the relative sliding occur s at an interface between PTFE and the PTFE transfer film, which produces a low, consistent friction coefficient for PTFE when paired with a large variety of countersurface materials. Pooley and Tabor found that the frictio n coefficient of PTFE was low when sliding in the direction of the oriented PTFE molecules, however, increased significantly when sliding perpendicular to the molecular orientation [ 55 64 ] F rom this observation they conclude d that the low friction in PTFE is caused by fully fluorine encased rigid chains sliding over one another PTFE does suffer from one major downfall: a high wear rate averaging a round much as 1x10 4 to 1x10 3 mm 3 /Nm under common engineering sliding conditions [ 1 2 18 63 ] This high wear rate limits the use of PTFE as a bulk solid lubricant. The high

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26 wear rate and the wear mechanisms of PTFE also lead s to large, flakey wear debris, which can also be very disadvantageous. Low wear rates have been observed in PTFE at low contact pressures and sliding speeds [ 12 63 ] H owever, under typical engineering sliding speeds and loads, wear rates are often very high at as much as 1x10 3 mm 3 /Nm. Tanaka mapped out the wear of PTFE as a function of temperature and sliding speed (Figure 2 2). This transition i n wear rate suggests a threshold between multiple wear mechanisms Tanaka proposed that the banded structure [ 53 54 ] of PTFE is responsible for this large scale, flakey wear debris and compared it to a deck of cards At lower sliding speeds and contact pressures, the wear of PTFE is extremely low. Figure 2 2 [ 12 ] Reprinted with permission from K. Tanaka, Y. Uchiyama, S. Toyooka, Mechanism of wear of polytetrafluoroethylene, Wear 23 (1973) 153 172. Elsevier Limited, Oxford

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27 Studies by Blanchet and Kennedy suggest a different source of the flakey wear debris. They suggest that a t higher sliding speeds, a de lamination wear is initiated due to the kinetic friction coefficient reaching a threshold [ 21 ] At this threshold, subsurface shearing of the polymer becomes preferential as the shear stress at the internal interfaces of the PTFE become lower than the shear stress at the interface. This promo tes subsurface crack propagation joining, and delamination wear [ 65 ] as outlined in Figure 2 3 Figure 2 3 Subsurface crack propagation and delamination wear in PTFE as described by Blan chet and Kennedy [ 21 ] Polytetrafluoroethylene Composites PTFE is used as both a matrix and filler in composites for tribological applications [ 9 12 47 ] ( Figure 2 4) B oth PTFE filled composites and PTFE matrix composites have

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28 extreme success stories with wear rates less than 1x10 7 mm 3 /Nm [ 37 38 40 45 ] (approximately three to four orders of magnitude less than bulk PTFE). As a filler, the effect of reducing wear [ 38 ] In contrast, when fillers are added to PTFE as a matrix to reduce the wear, there is often an increase in friction coefficient [ 45 ] Figure 2 4 provides an overview of wear and friction for various tribological polymers, including PTFE and several PTFE composites Figur e 2 4. Wear rate vs. friction coefficient for t ribological composites of PTFE and a several other tribological engineering polymers from several sources. A PTFE 2 B PI [ 11 ] C PAI [ 11 ] D PEEK [ 11 ] E UHMWPE [ 11 ] F PET [ 11 ] G PFA 1 H Rulon Gold 1 I Rulon Maroon 2 J SP211 2 K PTFE 2 O 3 1 L PTFE Al 2 O 3 1 M Torlon 4275 2 N Torlon 4301 3 O Torlon 4630 2 P Victrex PEEK/PTFE 1 Q PTFE ZnO [ 30 ] R PTFE carbon nanotube [ 32 ] 1 B.A. Krick, J.J. Ewin, G.S. Blackman, C.P. Junk and W.G. Sawyer, Unpublished PTFE and composites, (2011). 2 A. Bennett and W.G. Sawyer, Unpublished polymer wear testing, (2011).

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29 Conventional F illed P olytetrafluoroethylene C omposites Almost everything imaginable has been included as a filler material in PTFE M ost fillers serve to reduce wear while increasing friction. Fillers serve to reduce the wear by typically one or two orders of magnitude S everal hypothesis exist as to why fillers serve to reduce the wear of PTFE [ 13 16 21 30 32 33 45 56 66 68 ] Preferen tial load support [ 14 69 ] and arresting crack propagation [ 21 67 ] a re among the two most popular theories Fillers arrest subsurface cracks and prevent the PTFE from delaminating at the surface ( Figure 2 5) Figure 2 5 Shutting down the subsurface crack propagation and delamination wear mechanism in PTFE. Cracks are arrested by various filler sizes and geometries including 1) microparticles, 2) fibers, 3) nanoparticles and 4) Composite networks of filler and PTFE matrix. Traditionally hard micro and nano particles and fibers /networks of glass, polymers and other mate rials and metals are used as fillers in PTFE. Blanchet and Kennedy proposed that fillers serve to shut down the delamination wear of PTFE [ 23 ] Through experiments with high density and low density poly ethylene, Briscoe proposed that

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30 fillers promote the generation of thin, well adhered transfer films that reduce wear by slowing the removal and subsequent replacement of transfer films [ 13 ] Figure 2 6 shows a brief review of several filled PTFE composites wear rate as a function of filler material and volume percent ; optimum loadings at grea ter than 10% suggest reinforcement and crack arresting mechanisms for the reduction in wear. Figure 2 6 Wear rate vs. filler volume percent for PTFE matrix composites. The fil lers include ZnO [ 30 ] carbon nanotubes (CNT) [ 32 ] Al 2 O 3 [ 33 ] and irradiated FEP [ 25 ] It should be noted that the source of Al 2 O 3 filler in this experiment is not the same as the Al 2 O 3 filler in the ultra low wear composites.

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31 With these filled PTFE composites, the wear rates can be commonly reduced to around 1x10 5 mm 3 /Nm (and as low as 1x10 6 mm 3 /Nm). In systems where the wear rate is close to the wear rate of PTFE at slow sliding conditions, the primary function of the filler is to provide load support [ 14 ] and arrest subsurface cracks and halt delamination wear [ 23 ] As filler volume concentrations increase, it is possible that the sliding is no longer occurring between the PTFE and the countersample, but is occurring between the filler and the countersample, the filler and the tribofilm, the sample and the tribofilm, the sample and the filler or some dynamic combination of these possible material interactions For nano particle filled systems that produce low wear at low filler percent Li et al agreed with the theory of Briscoe that thi n, robust, we ll adhered transfer films bond to the counter sample and protec t the sample from further wear [ 13 30 ] Others suggested that nanoparticles prevent the crystalline structure of the PTFE from being destroyed during sliding [ 32 ] Tanaka however, found that nanoparticles that are too small are no longer able to adequately prevent the large scale destruction of the band ed structure of PTFE to successfully slow down the wear of PTFE [ 14 ] Many PTFE composites show some appreciable run in behavior in which the initial wear rate is higher than that of the steady state wear rate ( Figure 2 7). This run in suggests there is an initial transient process occurring that leads to lower wear systems. It could be that enough of the polymer must be worn away to generate some critical amount of filler at the surface. It could also be t he formation of a well adhered protective transfer film that the polymer slides over the transfer film.

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32 Figure 2 7 Wear volume vs. sliding distance for PTFE and some composites. Samples include PTFE 3 PTFE filled with single walled carbon nanotubes [ 46 ] and PTFE filled with Al 2 O 3 3 Ultra L ow Wear P 2 O 3 N anocomposites Some filler materials provide a remarkable reduction in wear ( over 1000x increase in wear resistance) at very low volume percent (less th an 2 ) Alpha phased alumina is one of those fillers. Figure 2 8 compares the wear rate o f PTFE composites with this new 3 B.A. Krick and W.G. Sawyer, Unpublished ptfe and composites, (2010).

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33 filler at various loading fractions to the composites shown in Figure 2 6 At low loading fractions it is better than some of the other composites by several orders of magnitude. Figure 2 8 Wear rate vs. filler volume percent for PTFE matrix composites. The fillers include ZnO [ 30 ] carbon nanotubes (CNT) [ 32 ] Al 2 O 3 [ 33 ] and irradiated FEP [ 25 ] At this low wear rate and filler content traditional fracture toughening and preferential loading theories for wear reduction are no longer sufficient B urris, Sawyer, Blanchet and Schadler have spent many years studying these systems [ 37 44 45 47 ] In their most recent publicati on, Burris et al. suggest that the low wear behavior of these systems is caused by a unique morphology of the PTFE that resists wear [ 45 ] They

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34 phase alumina that induce a favorable response by the PTFE which manifests into modified morphology These claims are supported by observations from atomic force microscopy results of the polymer composite surfaces. While this is an important observation, it is not enough to adequately understand these systems. Research Motivation and O bj ectives The wear reduction provided by low weight percent fillers is substantial and can drastically change the industry of polymer solid lubricants. Consider the time it would take to wear 1 mm of height off of a 6.4mm by 6.4mm squar e cross section sample ( Figure 2 9) under reciprocating sliding experiments of 25.4 mm stroke, 50.8 mm/s sliding speed and a 250 N normal load. Unfilled PTFE would slide for approximately 2.4 hours before wearing 1mm of height A PTFE and 2.5 volume percent would survive the sliding for approximately a year and a half before wearing 1mm of total height Figure 2 9 Illustrating wear improvements made by alumina filled PTFE. This figure compares the time required to wear away 1mm of height from a standard s

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35 A change this great with this small amount of filler in any mechanical property is astonishing and promotes the desi re to fully understand the system. In any composite research, a 40% improvement is a success, but these systems are approaching 10,000 to even 100,000 times improvements in wear; these composite systems are approaching virtually zero wear. To date, there i s a lot that the scientific community does not understa nd about these systems H owever, my research aims to clear up the unknowns and provide a better understanding of and a mechanism for these low wear systems. A better understanding could lead to further improvements in friction and wear of materials and ultimately give a route to design ing polymer composite system s based on desired property set s

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36 CHAPTER 3 HYPOTHESIS FOR CHEMI CAL MECHANISM FOR ULTRA LOW WEAR BEHAVIOR Studies suggest that the ultra low wear PTFE and alpha alumina composites are dynamic systems The alpha alumina filler appears to serve several key functions. First of all, the filler must shut down the gross wear mechanism of PTFE; this i s achieved by adding virtually any filler of adequate size to PTFE. The filler serves to provide p referential load support [ 14 69 ] and arrest crack propagation [ 21 67 ] in the PTFE. While this is a widely accepted mechanism in the r eduction of wear in PTFE, the filler must serve additional functions in reducing wear in the ultra low wear composites Burris et al. [ 11 ] suggested t hat the alumina filler serves to modify the crystallinity of PTFE stabilizing phase I PTFE morphology, which has been shown to perform better tribologically than other phases of PTFE. Throughout many publications and presentations, Burris and Sawyer point to the importance of chemistry in the ultra low wear systems. In h is dissertation [ 70 ] Burris presents XPS data that shows a new tribochemical species in the transfer film of ultra low wear materials He suggested that the new 288 eV peak in the C 1s spectrum was consistent with defluorinated PTFE ( Figure 3 1) Burris mentioned several possibilities for the chemi cal identity o f this degraded or defluorinated chemical species in the transfer film, including carbon carbon double bonds, a conjugated carbon backbone, terminal and branched CF 3 groups, cross linked carbon structures, COF and COOH. Experiments performed in my research suggest that carboxylic acids (COOH) could be the source of the 288 eV peak in the XPS and also be an important formation in the mechanisms of ultra low wear PTFE and alumina composites.

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37 Figure 3 1. Carbon XPS spectrum for transfer films of PTFE and PTFE/alumina composites. R esults from Burris and Perry on PTFE and low wear PTFE composite transfer film. A new tribochemical species was observed. Burris found that many low wear PTFE materials, including PTFE that is filled with alumina, irradiated PTFE and composites that include PTFE as a filler material [ 70 ] T his work by Burris suggests that exploration of chemical degradation and reactivity of PTFE is an obvious direction toward the

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38 under standing of the mechanisms responsible for the ultra low wear of these systems ( Figure 3 2 ) Figure 3 2 An opportunity for degradation, reactivity and tribochemistry as a route to low wear polymer composites. Burris presented an experiment in h i s dissertation where he measured percent of oxygen content in the transfer film of low wear PTFE alumina composites [ 70 ] His results showed that a higher percentage of oxygen is associated with higher friction coefficients. He al so show ed that an increase in the percentage of oxygen is associated with the formation of the thin, robust transfer films consistent with low wear PTFE composites [ 70 ] It is possible that the increase in oxygen is actually an increase in carboxylates terminating end groups of the PTFE. It was first pointed out by Junk et al. that these

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39 carboxylates can chelate to metal counte rsurfaces 1 ( Figure 3 3) Chelation of PTFE to the countersurface creates a robust transfer film that protects both the polymer and the countersample. This mechanism can be environmentally dependent as it requires a source of oxygen or hydroxyl ions to form the carboxyl group. It also requires either the breaking of carbon carbon bonds or partial defluorination of the PTFE to give sites to form these carboxyl groups. Figure 3 3 A proposed chemical mechanism for the formation of COOH groups in the PTFE th at can chelate to a countersurface to form a robust transfer film in ultra low wear PTFE composites 1 It is my opinion that a bimodal (or more) distribution of filler particle size is most successful at producing ultra low wear composites. A filler materia l on the order of 1 5 m can successfully shut down the mechanica l gross wear mechanisms of PTFE. T he chemical identity of this filler is not extremely important, as long as it does not heavily degrade the polymer matrix. Secondly, a filler material on the order of 30 200 nm serves to promote the tribochemistry responsible for producing th e ultra low wear transfer film. T he chemical nature of this filler could be important as it is responsible for promoting the formation of carboxylates. A third, smaller pr imary particle size fumed silica filler may 1 C.P. Junk, B.A. Krick, J.J. Ewin, G.S. Blackman and W.G. Sawyer, Unpublished PTFE and composites, (2011).

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40 further reduce the wear of the system by promoting partial defluorination and crosslinking.

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41 CHAPTER 4 MATERIALS, METHODS A ND EXPERIMENTAL DESI GN Polymer Composite Materials and Preparation Materials Composites for all experiments were made at the Univ ersity of Florida or DuPont C entral Research & D evelopment. Table 4 1 presents a list of filler materials that w ere used in making composites accompanied by abbreviations and designated name s used through out this document The original ultra low wear composite producing filler is Unless otherwise stated, PTFE/alumina composites contain this alumina. Table 4 1. List of materials used as fillers in composites Type Abbreviation Designated name Source Claimed particle size (nm) Alumina T1 Al 2 O 3 Type 1 alpha alumina Alfa Aesar alpha alumina Stock # 44652 80 T2 Al 2 O 3 Type 2 alpha alumina Nanostructured and Amorphous Materials Inc. alpha alumina NA T3 Al 2 O 3 Type 3 alpha alumina Alfa Aesar alpha alumina Stock # 42573 500 T4 Al 2 O 3 Type 4 alpha alumina Alfa Aesar alpha alumina Stock # 44653 40 T5 Al 2 O 3 Type 5 alpha alumina Pace Technologies alpha alumina 50 T6 Al 2 O 3 Type 6 alpha alumina Almatis A16 SG alpha alumina NA T7 Al 2 O 3 Type 7 alpha alumina Nanostructured and Amorphous Materials Inc. alpha alumina #1040LQS 200 T8 Al 2 O 3 Type 8 alpha alumina Sasol alpha alumina 300 T9 Al 2 O 3 Type 9 alpha alumina Pace Technologies alumina ALR 0103 01 300 T10 Al 2 O 3 Type 10 alpha alumina Pace Technologies alumina ALR 0110 01 1000 T11 Al 2 O 3 Type 11 alpha alumina Pace Technologies alumina ALR 0150 01 5000 Al 2 O 3 delta gamma alumina Nanophase delta gamma alumina 44 Titania T1 TiO 2 Type 1 titania DLS 210 135 T2 TiO 2 Type 2 titania TiPure R103 230 T3 TiO 2 Type 3 titania Sachtleben RM 130F 100 T4 TiO 2 Type 4 titania Kemira M262 20 T5 TiO 2 Type 5 titania Kemira L530 30 T6 TiO 2 Type 6 titania Tayca MT 500HD 30

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42 Table 4 1. Continued Type Abbreviation Designated name Source Claimed particle size (nm) T7 TiO 2 Type 7 titania Evonik P 25 TiO 2 NA T8 TiO 2 Type 8 titania TiO2 uncoated Rutile DuPont NA T9 TiO 2 Type 9 titania Rutile Ceramic Grade Minnesota Clay Co. NA Abrasives CBN Cubic Boron Nitride Borazon BMP 1 <500 Diamond Diamond nanopowder Mypodiamond synthetic monocrystalline diamond powder 250 500 Minerals Pyrophyllite Pyrophyllite <40% pyrophyllite with impurities of quartz (50 60%), mica and kaolin from Minnesota Clay Co. NA Mullite Mullite 75 85% mullite with impurities of SiO 2 kyanite and cristobalite from Minnesota Clay Co. Talc Talc Talc (many impurities) from Minnesota Clay Co. NA Dolomite Dolomite Dolomite with <1% quartz and <1% tremolite from Minnesota Clay Co. NA Kaolin Kaolin clay Kaolin clay NA Kyanite Kyanite 85 95% kyanite with impurities of quartz, titania and cristobalite from Minnesota Clay Co. NA Bentonite Bentonite clay United Nuclear bentonite clay NA Carbonates Li 2 CO 3 Lithium Carbonate Minnesota Clay Co. NA Na 2 CO 3 Sodium Carbonate United Nuclear NA BaCO 3 Barium Carbonate Minnesota Clay Co. NA Nanofillers Fe Iron nanoparticles American Elements Fe 40nm spherical average particle size 40 100 SiO 2 Fumed Silica Cab o sil M50 agglomerates of spherical 5 10nm primary particles 5 10 Ni nanostrand Nickel nanostrands Conductive composites NA DuPont 7C PTFE r esin was used as the matrix for all PTFE. PTFE 7C is 30 m.

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43 Commercially available tribological polymer composites were used for several comparative friction and wear experiments These wear and friction results were inclu ded in the comparative chart in Figure 2 4 Th ese polymers are summarized in T able 4 2 Table 4 2 List of tribological composites used in comparative experiments in cluding an abbreviated name used when referring to the polymer composite and the manufactur er of the polymer composite. Abbreviation Polymer name Manufacturer Rulon AR Rulon maroon Saint Gobain Rulon J Rulon gold Saint Gobain SP211 DuPont Vespel SP211 DuPont Torlon 4275 Torlon 4275 polyamide imide Solvay Advanced Polymers Torlon 4301 Torlon 4301 polyamide imide Solvay Advanced Polymers Torlon 4630 Torlon 4630 polyamide imide Solvay Advanced Polymers PEEK/PTFE Victrex PEEK and PTFE Victrex In friction and wear experiments, the polymers and polymer composites are slid against a countersample. Typically, unless stated otherwise, this countersample is a 304L stainless steel sample with a lapped surface finish (average ). Composite P reparation Several routes were used for the creation of the polymer composites tested. In general the constituents are mixed on a mass percent basis (typically 5 percent filler by mass) and molded. Molding is performed by compression molding or free sintering for PTFE composites. Compression molding consists of consolidating the composite and t hen thermally curing the composite (heating to melt the polymer matrix and then cooling to room temperature ). There were several different dispersion and compression molding techniques used for the preparation of the PTFE composites.

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44 Dispersion t echniques For the various filled PTFE composites, d ry powders were massed out in desired ratios (typically 5 wt % filler a s a standard study unless otherwise specified ). Dispersion was achieved in various ways: jetmilling p owders mixed by grinding action with the use of a jetmill [ 33 ] IPA powders mixed by dispersing them in isopropyl alcohol with the assist of agitation by an ultrasonicating horn. After dispersion the isopropyl alcohol is allowed to evaporate a vacuum oven Compression m olding Following dispersion powder mixtures are consolidated in a cylindrical mold with a hydraulic press at hydrostatic pressures of much greater than 50 MPa After this, powders are thermally cured in one of two ways : Method 1: Composite is kept in press under light pre ssure and the sample is heated at 2C/min to 380C, which is above the melting point of PTFE. The composite is then held at the melting point for three hours before being ramped down to room temperature at 2C/min. Samples made t his way will be ref erred to molded Method 2 : The sample is heated at 2C/min ute to 380C where it is held for 30 minutes before cooling down at a rate of 1C/minute. This is the method done by all PTFE composites produced at DuPont Central Research & Development f or this research Analysis Methods Many analysis techniques were utilized in this research. A brief summary of analysis techniques that were used for character ization is provided in T able 4 3 Some techni ques will be discussed in more detail.

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45 Table 4 3 Summary of analysis tools Abbreviation Technique Information derived Tribometry V arious tribometers used for different desired application and in situ capabilities F riction and wear data of a material set at a particular set of parameters XPS x ray photoelectron spectroscopy Chemical species identity SEM Scanning electron microscope Micrograph of the sample TEM Transmission electron microscopy Micrograph of thin specimen Raman Raman Spectroscopy Vibrational spectra for material ID SERS Surface enhanced Raman spectroscopy Vibrational spectra for material ID with enhanced signal of up to 106 times improvements SPR Surface plasmon resonance spectroscopy Molecular adsorption/desorption and material transfer Stylus profilometry Stylus profilometry Topography FTIR Fourier transform infrared spectroscopy Chemical ID from absorption vibrational spectrum Scratch CSM micro scratch tester Relative scratch resistance and adhesion strength of the transfer film Tribometers Tribomet ry: Fundamental Measurements in Tribology Tribometry is defined as the measurement of friction and wear of tribological systems as performed by a tribometer. Tribometers are used tribological performance and, ultimately, its worth as a solid lubricant. It is also very important to point out that measurement practices and standards vary for different research groups. Direct comparison of results between various research groups should be considered carefully as friction coefficient and wear rates can be affected by the methods of whic h the measurements are made and the many

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46 parameters and conditions of sliding experiments For this reason, it is best t o make arguments of trends unless all of the materials, parameters and conditions are matched exactly, and even then, it may be best fo r a researcher to recreate an experiment to evaluate the material in their instruments for the best comparison. Friction c oefficient m easurements friction force, F f is directly proportional to the normal load, F N ( Figure 4 1a). A friction coefficient, c an then be defined by E quation 4 1: ( 4 1) Friction coefficients are measured with a tribometer. M ost modern tribometers apply a known normal load by mass or a measured normal load by an external loading system with a force transducer to measure the normal load. Friction forces are measured, and a friction coefficient can be determined. Schmitz et al. and Burris and Sawyer address the dif ficulties of measuring friction coefficient and the uncertainties associated with it [ 71 72 ] They conclude that misalignments with the transducer and the frictional and norm al forces can produce errors, but these errors can be compensated by finding the instrument related uncertainty of coefficient of friction and by performing sliding reversals to average out the error associated with the misalignment.

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47 Figure 4 1 Schemati c of a) friction coefficient and b) wear rate Wear r ate m easurements An accepted metric for reporting material wear was developed much later than that for friction coefficient. Archard and Holm suggested that the total volume of material removed during sliding (the wear volume), V, is proportional to the real area of contact multiplied by the sliding distance by the unit less proportionality constant known as a wear factor, K [ 73 75 ] T his wear factor can be a property of the material set, sliding conditions, surface topography and environment among other things. Th e wear factor can be ma nipulated to calculate the often more convenient and more physically direct specific wear rate [ 3 ] (also known as a dimensional wear rate [ 76 ] ), k, commonly measured in units of mm 3 /Nm ( Figure 4 1 b). The specific wear rate is simply the wear volume divided by the product of the normal load and the sliding dis tance, d, as shown in E quation 4 2.

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48 ( 4 2) There are numerous methods for making volume loss measurements used to compute wear rates. These measurements can be made directly, or volume losses can be inferred based on: d imensional changes in the material including height changes t opographical measurements of a wear made by a profilometer such as: o o ptical measurements (microscope, SEM, etc.) o i nterferometric measurement (interferometer) o s tylus profilometry o a tomic force microscopy i nterrupted mass measurements i nferred measurements based on the running through of a coating by observation of an increased friction coefficie nt Interrupted mass measurements on a linear reciprocating tribometer were used for wear rates reported in this research. Using the material density, a volume loss can be calculated by a change in mass. W ear rates are easily calculated by E quation 4 3. ( 4 3 ) Schmitz et al. Colbert et al. and Burris and Sawyer provide modern uncertainty analyses to several of the methods used in this research to find wear rates [ 71 77 78 ] Linear R eciprocating T ribometer The l inear reciprocating tribometer wa s used to perform a bulk of the tribological tests for this work (F igure 4 2 ) The tribometer is extensively discussed by Sawyer and Schmitz et al [ 33 71 79 ] Briefly, t he polymer sample was mounted to a 6 channel load cell which measures the normal and tangential forces. The polymer sample was loaded against the counter sample (normally 304L stainless steel unless otherwi se noted) with a normal load of 250 N ( as in Figure 2 9 above ). N ormal load was applied and

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49 controlled by a pneumatic cylinder. Utilizing reversals, reported friction coefficients have an uncertainty of less than 0.005 [ 71 ] T his uncertainty is less than the fluctuating component of frict ion coefficient which is a dynamic variation in the friction coefficient In this flat on flat configuration the nominal average contact pressure is approximately 6.3 MPa, which is 20 50% of the yield strength of PTFE. The counter sample was mounted dire ctly to a linear ball screw stage, which reciprocated with a stroke of 25.4mm at a rate of 50.8 mm/s. Tribological sliding conditions were chosen to be the same as previous studies [ 33 34 37 43 45 59 77 ] to match engineering c onditions and for easy comparison. Figure 4 2 Linear reciprocating tribometer.

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50 Normal and friction forces are measured through the 6 channel load cell as sliding reciprocations occur. During each sliding cycle, a frict ion loop is measured ( Figure 4 3 a). T his friction loop is periodically saved. More importantly, the friction loop is used to compute an average friction coefficient for that cycle along with a standard deviation in friction coefficie nt ( Figure 4 3 b). Figure 4 3 Friction coefficient measurement: a) single sliding cycle with reversal and b) average friction data. Average values and standard deviations of normal force, friction force, friction coefficien t, linear p in wear, temperature and humidity are all measured on a cycle by cycle basis and saved periodically. This produces data of statistical significance, in

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51 which observation of friction coefficients and wear volumes can be made as a function of sliding distanc e. Tests often r un for over one million cycles, which takes several weeks Int errupted mass measurements are taken to calculate wear measurements. For polymers like PTFE, the dimensional properties ca n change over time due to creep. M ass measurements provi de much more reliable measurements of wear volume and wear rates. It is also common to observe a brief run in period for PTFE composites in which the composites wear appreciably more than the steady state wear R eported values of wear are the steady state wear rates which are measured after the initial run in period as shown in Figure 4 4 Through both uncertainty analysis by both propagation of uncertainty and a Monte Carlo simulation developed by Burris and Sawyer [ 77 ] the uncertainty in wear rates for a representative experiment producing a wear rate of 1x10 7 mm 3 /Nm is typically 1 5 x10 9 mm 3 /Nm, which is smaller than the data points on the plots. For this reason, uncertainty is not reported. Figure 4 4 Calculating wear rate from mass loss measurements: a) interrupted mass measurements are used to plot b) volume lost vs. normal force times sliding distance which is used to compute a steady state wear rate.

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52 For all linear reciprocating tests, m olded sampl es were machined into 6.3mm x 6.3mm x 12.7mm pins. One 6.3mm x 6.3 mm end of the pin was polished flat with 800 grit silicon carbide polishing paper to a roughness of 0 0nm Samples were then sonicat ed in methanol for 30 minutes. A fter sonication, sam ples were dried for three hours in labora to ry to ensure evaporation of methanol before recording an initial mass measurement. Metal c ountersamples are 304 L stainless steel rectangular flat plates (38mm x 25mm x 3.7mm) with an average roughness f or most experiments T he effects of counter sample roughness on the wear properties of this material have been studied and further information regarding this surface are discussed by Burris and Sawyer [ 34 ] Counter samples were washed with soap and water, followed by me thanol. New samples and counter sam ples were used for each sliding experiment Environmental T ribometers E nvironmental constituents can play a large role in the friction and wear of these tribological systems. It is desirable to test these materials in a variety of environments to prove the limitations of the material set and understand more about the composite. Due to the dynamic nature of friction and wear, it is important to consider the many environments that a solid lubricant may be used in and test in conditions as close as possible to that environment. The following tribometers are each designed to test the wear and friction of these polymers in various environments. Vacuum t ribometer A linear reciprocating vacuum tribometer [ 78 80 ] was used to evaluate PTFE/alumina composites at 6x10 6 torr ( Figure 4 5 ) Aside from the vacuum environment, all other testing parameters including normal force, sliding speed, sliding

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53 stroke, sample geometry and loading configurations were kept the same in this test as the linear reciprocating experiments Figure 4 5 Linear reciprocating vacuum tribometer. Environmentally c ontrolled t ribometer A linear reciprocating tribometer similar to those previously described by Sawyer and Schmitz [ 3 3 71 79 ] was designed and built inside of an environmentally controlled glove box ( Figure 4 6 ). The glove box can be regulated to as low as 5 ppm of O 2 and 0.5 % relative humidity under a nitrogen backfill at 23 28 C. The glove box can be doped with environmental species including water at as high as 90% RH. Aside from the environment, all other testing p arameters in this test remained the same as the linear rec iprocating experiments

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54 Figure 4 6 Linear reciprocati ng environmentally controlled tribometer. Space t ribometer PTFE based materials have shown promise for use in low earth orbit because of their ability to endure large thermal ranges, vacuum conditions, resist chemical attack, and have a friction coefficien t that is reliably low in many environments [ 2 81 ] An opportunity was presented to build tribometers ( Figure 4 7 a and b) that would be exposed directly to the low earth orbit environment on board the Materials on the International Space Station Experiments (MISSE) series number 7 [ 2 ] MISSE 7 was a

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55 group of experim ents mounted to the International Space Station (ISS) and exposed directly to the low Earth orbit (LEO) environment. Figure 4 7 Space tribometers: a) single tribometer schematic and b) set of four tribometers for the ram or wake face of the International Space Station

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56 Figure 4 8 Low earth orbit environment as experienced by MISSE: a) and b) show the radiation spectrum observed in space as contrasted to the radiation felt by earth. c) Highlights the environments observed by the two sets of tribometers T he experiments were exposed to ultrahigh vacuum, temperature fluctuations of 40 to 60C, ultraviolet radiation, and atomic oxy gen ( Figure 4 8 ). PTFE/ 10wt% type 1 alpha alumina composites were selected to run on both the ram (direction of station travel) and wake (opposite the direction of travel) face of the experiments. The experiments are oriented in such a way that the ram ex periments are exposed to a high flux of atomic oxygen, while the wake experiments are generally in ultrahigh vacuum as

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57 summarized in F igure 4 8 and by Krick and Sawyer [ 2 ] Other samples tested on orbit are summarized by Krick and Sawyer [ 2 ] The s amples returned to E arth in June 2011 were collected in July 2011 and are currently being analyzed In S itu T ribometers Surfac es in intimate contact are difficult to analyze because of the inaccessibility at the interface I n situ techniques are a powerful tool used by materials tribologists to study the interaction between surfaces d uring contact and sliding at an otherwise in accessi ble interface [ 82 83 ] There are various p athways for in situ analysis of a surface. In situ spectroscopies such as Raman spectroscopy have bee n used to analyze the chemical nature of the interactions by examining the wear surface or transfer films during sliding or just after it exits the conta ct withou t changing the environment [ 84 85 ] In situ electron microscopy is increasingly popular; Murrash and Varenberg used a scanning electron microscope to analyze the interaction at the interface from the side [ 86 ] Marks showed a liquid like transfer of gold by in situ transmission electron microscope experiments [ 87 ] The state of the art techniques for in situ tribology was recently re viewed by Sawyer and Wahl [ 82 83 ] but briefly in situ techniques provide access to observe the tribological interactions du ring sliding. In situ techniques will be used in these studies to help answer questions that traditional macroscopic and microscopic friction and wear tests cannot. Optical i n situ micro tribometer An in situ optical micro tribometer has been developed to explore the contact between two solids in intimate contact during loading and sliding ( Figure 4 9 ) [ 88 ]

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58 F igure 4 9 Optical in situ microtribometer schematic. a) Schematic of optical in situ microtribometer: the sample (1) is slid against a transparent counter sample (2). The sample is mounted directly to a calibrated cantilevered force transducer flexure (3) Capacitance probes (4 and 5) measure the displacement of a target (6) mounted on the cantilevered flexure; with the calibration of the flexure, these displacements provide the normal and friction forces. A microscope objective (7) mounted directly beneat h the transparent counter sample held by a sample holder (8). b) Schematic of optical pathway. A monochromatic coherent light source passes through the microscope objective and up through the transparent counter sample. The light is reflected off of the su rfaces of the sample and counter sample back through the microscope objective and ultimately to a CCD camera. In the image there is a 0th order destructive interface representing contact and higher order fringes surrounding contact An interferometric optical analysis can be used to measure and observe contact size, contact geometry, near contact topography, tribofilm formation, tribofilm motion, tribofilm thickness, wear debris formation and wear debris morphology. The optical arrangement is in such a way that a 0 th order interference fringe highlights the real contact area of contact, while near contact regions are height mapped with higher order

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59 This optical technique is coupled with a microtribometer capab le of me asuring normal and friction forces ranging from 50 m to upwards of 2N I mages are synced with force and position measu rements. This instrument is an ideal tool to explore: C ontact mechanics of polymers Development of wear debris and morphology Transfer fi lm and third body sliding In s itu Surface Plasmon R esonance Surface plasmon resonance (SPR) is a technique that can detect molecular adsorption/desorption events on a thin metallic sensing layer deposited on a quartz prism. S urface plasmon resonance occu rs when energy from an incident photon is transferred into the conduction band of electrons of a metal, thereby propagating itself as a fluctuating electron density wave that propagates parallel and along the boundary of a metal and a dielectric medium [ 89 92 ] Any slight perturbati on to the surface of metal can alter the surface plasmon; this makes surface plasmon resonance a technique which is very sensitive in detecting adsorbed materials on the metal surface, includi ng the detection of mono layers [ 89 90 ] The Kretschmann configuration [ 93 ] is the setup used for SPR in our in situ experiments. In this configuration, a thin metal film is deposited on the bottom surface of a prism. The prism makes the coupling of a surface plasmon possible by slowing and changing the wavefunction of an incident photon. The wavefunction can be directly altered by changing the incident angle of the photon through the prism. The plasmon attenuation of the reflected ray. At this inst ance, a majority of the light energy is coupled

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60 into the surface plasmon, greatly reducing the reflected intensity. From here, if any material is absorbed to the metal sensing layer then the wavefunction at which resonance occurs is altered and an increase in reflected intensity of the photon is observed. This allows for a measurement of reflectance intensity versus incident angle to be made indicating changes due to absorbed material between the surface of the metal and the dielectric medium. Figure 4 1 0 In situ surface plasmon resonance experiments utilizing the Kretschmann configuration [ 93 ]

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61 Alternatively, one may expand the incident beam to cover a large surface area and then tune the incident ang le to plasmon resonance, and an image may be recorded of the reflected beam. Any changes to the sensing surface are detected by changes in intensity of the reflected image as the surface plasmon is disrupted by adsorbed analyte [ 94 ] As described above, since the surface plasmon is so highly dependent on the interface, measureable differences in reflectance will occur due to the smallest perturbations to the boundary, allowing for very sensitive surface measurements. The SPR method w as implemented into an in situ tribometer to monitor transfer from solid lubricant materials to a gold coated quartz prism ( Figure 4 10) 1 As solid lubricant material is transferred from the polymeric pin to the gold an increase in i ntensity is observed in the measured reflected beam at a fixed incident. A linear reciprocating pin on flat tribometer was used to perform the in situ friction and wear experiments. The tribometer was designed around the gold coated prism (the counterface) and mounted on a rotat ional stage. The rotational stage is used to adjust the angle of incident (and reflected) light to tune the initial plasmon effect in the system. The sample ( i.e. PTFE, UHMWPE or graphite) was mounted to a bi axial cantilever force transducer [ 2 ] The sample was loaded into contact with the gold surface of the prism with a manual micrometer and slid against it with a linear motor. The beam was approximately 2 m m in diameter and was centered on the transfer film at the midpoint of the sliding cycle. A 632 nm laser with power of 540 mW is transmitted through one leg of the 90 degree gold coated prism at the hypotenuse (gold coated) face. The reflected intensity 1 B.A. Krick, D.W. Hahn and W.G. Sawyer, Unpublished surface plasmon resonance spectroscopy (SPR) and surface enhanced raman spectroscopy (SERS), (2011).

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62 of this light is measured through the other leg of the prism with an optical power meter. In the initial setup of each experiment, t he angle of incidence is adjusted such that the reflected power is a minimum and the surface plasmon r esonance effect is a maximum After this, the polymeric sample is loaded against the gold surface o f the prism and slid. A value of reflected intensity is recorded a fter each sliding cycle (one forward and reverse motion). An increase in this intensity corresponds to a change in the adsorbates on the gold sensing layer of the prism, which in this case w ould be material transfer. Experimental D esign Experiments were designed to understand three fundamental ques tions: 1) What makes 2 O 3 composite system s have ultra low wear? 2) Can other materials reproduce the reduction in wear observed by this system? 3) How can we use this knowledge to design better composit es with specific functionality? These questions are complex and are by no means completely answered by this research With every answer there are undoubtedly more questions. Results of each of the following experiment sets are reported in their respective chapters following C hapter 4 Survey of Filler M aterials In an effort to understand what fillers would work in producing ultra low wear PTFE composites, we filled PTFE with m any filler materials including different types of alumina and titania, abrasive fillers such as cubic boron nitride and diamond nanopowder, mineralogical powders carbonates and other nanofillers ( Table 4 1). Each sample was mixed at 5 percent filler by w eight unless otherwise noted, in order to evaluate the materials on a comparative basis.

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63 All samples were tested on the linear reciprocator tribometer ( Figure 4 2). Experiments were performed with a normal load of 250 N, a sliding speed of 50.8 mm/s and a reciprocating stroke of 25.4 mm. All experiments were performed in the same c leanroom e nvironment under the same loading and sliding configurations. All samp les, unless noted, were slid against 304L stainless steel counterfaces with a lapped finish, a standard sample in our lab. For each sample, tests of 1,000, 10,000, 100,000 and 1,000,000 cycles were planned. The sample s were massed between each test. F or some of the higher wearing materials tests were stopped after the 100,000 cycle or the 10,000 cycle tests due to the excessive wear of the sample. Wear rates reported are the steady state wear rates, typically observed during the last and longest test of each experiment. Fri ction coefficients reported are also steady state friction coefficients. Discovering additional fillers that produce ultra low wear composites could lead to understanding the mechanism s for ultra low wear PTFE composite systems These experiments allow us to potentially identify new filler materials for ultra low wear and at the very least rule out certa in fillers. Furthermore, this type 1 alumina is no longer manufactured so an alternative filler material is desired to prevent losing this technology. Ch aracterization o f F iller Material s As we performed experiments on various alumina fillers it became obvious that not all alumina fillers are equal, or even what the manufacturer claimed they were For this reason, understanding what is unique about the ty pe 1 al umina and other fillers that produced ultra low wear composites is critical in understanding these materials and developing a mechanism

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64 Characteriz ation of size and shape was performed on the alumina and other fillers by S EM, T EM, and static light scattering. Statistical size and shape analysis was performed by Greg Blackman at DuPont. Chemical analysis was performed through XPS by Blackman. Characterization of the filler materials is important ; what makes composites of these fillers ultra low wear could be dependent on distribution of particle size, aspec t ratio, shape, surface area, hardness chemistry or a number of other possibilities Furthermore, type 1 alumina is n o longer ma nufactured ; there may have been a unique characteristic a bout t his f iller that made it und esirable for other applications, but ideal for this application. Environmental S tudies It is important to evaluate the tribological performance of these com posites in various environments. For one, it is important to understand what environments these polymers will work in to determine any limitations as it is used in applications. Secondly, any environmental effects on the performance of this composite may be useful in evaluating the proposed mechanism for its ultra low wear behavior The hypothesized mechanism involves tribochemistry that relies on environmental constituents to produce ultra low wear behavior. In v acuo alumina composites were evaluat ed in the v acuum environment. For these experiments, samples were tested at 6x10 6 torr in the vacuum tribometers ( Figure 4 5). The standard laboratory samples and loading and sliding parameters from the linear reciprocating tribometers were used for in vacuo t esting.

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65 Also, in vacuo tribological testing was performed to evaluate the wear resistance of running films and transfer films that are produced when PTFE and alumina composites have achieved ultra low wear. The transfer film and running films were d laboratory air environment (~21 23C and 35 55% RH) until ultra low wear was achieved. After the transfer films were developed, the vacuum chamber was sealed around the experimen ts and the pressure was reduced to less than 6x10 6 torr. The samples were then slid in their exact orientation to evaluate if the presence of the transfer film and running film would continue to promote the low wear behavior. Humidity and o xygen The impa cts of the presence of water and oxygen in the environment on the tribological behavior of the composites were observed in the environmental tribometer ( Figure 4 6). Utilizing an environmental enclosure the water and oxygen content could be controlled in t he presence of nitrogen. Environments tested include nitrogen with various humidity levels and nitrogen plus oxygen mixtures with various humidity levels. To produce environments of low oxygen contents, the chamber was first backfilled with nitrogen and an air scrubber was utilized to reduce water concentrations to less than 0.5 %RH (<100PPM H 2 O) and the oxygen to below 50 PPM (often stay as low as 5 PPM for dry nitrogen tests). After the environment was clean, the humidity was controlled by doping the envi ronment with water. To produce environments of nitrogen, oxygen and water mixes, the chamber was first backfilled with a dry air gas mixture (80% nitrogen and 20% oxygen). The air scrubber could not be used in this case due to the presence of oxygen. The purging took several days until the water content was as low as 2.5 %RH (~500 PPM H 2 O).

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66 Water content was then controlled by poisoning the environment with water vapor. For both cases the humidity was measured for the duration of the experiments. For the trace oxygen experiments, oxygen content was monitored. Wear tests were performed in the various environments for unfilled PTFE and composite Each sample was massed before and after wear testing and wear rates were calculated based on the total change in mass. Since incremental masses were not taken throughout the experiment, these wear rates are the total wear rates (single point wear rates) instead of the more commonly reported, and often lower, steady state wear rates. The single point w ear rates include the volume loss during the run in process, while the steady state wear rate simply reports what the wear is after run in of the sampl e. For this reason, these values are excellent to compare with each other, but should not be compared to wear values measured on the linear reciprocating tribometers that were used to produce most of the wear data in this research. Ethanol evaluated in the environmental tribometers in an environment consisting of nitrogen and ethanol. The environments were prepared as before by first purging with nitrogen and scrubbing the environment to remove oxygen and water. The environment was then doped with ethanol by pouring 100% biological g rade ethanol into a large steel pan. The surface area of the ethanol liquid was large to promote more rapid evaporation and a quick equilibrium concentration of ethanol. The sample was tested with a 250 N normal lo ad and 50.8 mm/s sliding speed, consistent with all wear tests The measurement was also a single point wear rate measurement and is comparable to other environmental tests.

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67 Low e arth o rbit s pace There is a fundamental need to develop and certify new materials for tribological applications in the space environment [ 2 59 ] PTFE could an excellent option due to its consistently low friction coefficient against a variety of surfaces in a variety of environment s [ 59 ] The primary limitation of PTFE as a space lubricant is its high wear rat composites, design engineers will have an entirely new design regime. experiments. The samples were expo sed to the low earth orbit space environment for approximately 18 months. The duration of testing and the number of sliding cycles done by the tribometers is still unknown at this time due to data corruptio n on the memory cards, h owever friction data is a vailable for some of the on orbit experiments. Loading and sliding conditions and configurations for these experiments were very different from our typical wear tests. was a 440 C steel ball (3 composite overmolded on to a steel disk. Testing parameters include a normal load of approximately 1 N and sliding speed of approximately 13.2 mm/s (14 RPM at 9 mm radius). he cleanroom environment prior to launch for 70 minutes of testing ( 55,440 mm or 980 cycles). This run in simulates the type of usage actual flight har dware would have before launch. S atellites and other space machinery are tested numerous times in the cle anroom en vironment before launch to ensure they work properly. Lubricants for these systems must work in both the terrestrial environment and the space environment.

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68 Experiments in the space environment have both fundamental and practical implications Stu dies on the effects of the space environment on these materials are fundamentally important. They may lead to further understanding on the mechanisms of which these materials achieve ultra low wear. T he evaluation of the wear performance of these materials as solid lubricant material candidates for space applications is also of practical importance. Composite Run I n and Running F ilm S tudies PTFE composites, as with many other solid lubricants, undergo a run in period in which the initial wear rates are grea ter than the steady state wear rates. For PTFE and alumina composites, the run in wear rates are as much as four orders of magnitude higher than the final steady state wear rates. Experiments were designed to observe the run na composites by interrupting sliding wear tests on the linear reciprocating tribometers for frequent mass measurements. This was done for three different filler weight percent composite run in. A picture of the running surface of the polymer was taken each time the sliding was interrupted for a mass measurement; t his allowed for optical analysis of the running film developed during sliding. Mass measurements and images were taken after sliding 1, 2, 3, 4, 5, 10, 20, 40, 50 60, 70, 80, 90, 100, 200, 300, 400, 500, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000, 11000, 12000, 13000, 14000, 15000, 20000, 25000, 50000, 75000, 100000 and 200000 cy cles. Images of the running films were processed using Matlab to compute a relative running film coverage to keep track of running film development.

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69 Transfer F ilm and Running Film A nalysis A transfer film is the material that is transferred from the sol id lubricant material to the surface of the countersample. Transfer films are ext remely important in PTFE based, and many other, solid lubricants. Transfer films are thought to be responsible for the friction characteristics of PTFE and are likely equally as important in the wear characteristics and performance of PTFE based composites. There are distinct differences in transfer films of unfilled PTFE, filled PTFE, and ultra low wear filled PTFE systems. These differences are likely to be critical in the we ar properties of the PTFE and may point toward a mechanism for ultra low wear systems. Transfer films of PTFE and PTFE/alumina composites were generated on the linear reciprocating tribometer and environmental tribometer. These transfer films were studied with a stylus profilometer to characterize the thickness and morphology of the transfer film. Scratch tests were performed on transfer films generated from a PTFE and a PTFE/alumina composite to evaluate and compare the ro bustness of transfer films. Scratch tests were performed on the CSM microscratch tester. A 1.6 mm radius silicon nitride spherical tip was used to scratch the transfer films generated on the linear reciprocating tribometer. The scratch started at an extern ally applied normal load of 0.05N and was ramped to 30N over a 5 mm scratch. The scratches were analyzed using the scanning electron microscope. Transfer films of unfilled PTFE and PTFE/alumina composites were also studied using x ray photoelectron spectro scopy, infrared absorption spectroscopy and time of flight secondary ion mass spectroscopy to attempt to determine the chemical identity of constituents in the transfer film and the nature of any chemical changes or new

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70 chemical species that may be observe d in the transfer films XPS was also performed on the running films on the surface of the worn polymer composite. XPS IR absorption and ToF SIMS were done at DuPont Exploring T ribochemical Chain S cission in PTFE : Can Bonds B e Broken During S liding? X ray photoelectron spectroscopy shows evidence of a new chemical species in the tribofilm of ultra low wear PTFE composites [ 70 ] Understanding what the species is and how it is formed is crucial to the understanding of low wear PTFE composites. It is important to determine if this is a cause or result of the low wear behavior of the composites. Chemical analysis was performed to de termine the presence of a new species that was generated tribochemically. Further experiments were designed to evaluate the feasibility of breaking carbon carbon bonds in the backbone of the PTFE polymer chain to fa cilitate this chemistry. Mechanochemical chain scission of rubbers and polymers was observed as early as the 1950s [ 95 97 ] When the polymer chain is broken, a radical is created that will be quickly terminated by nearby species. This would mean the creation of new end groups and may be responsi ble for the generation of the new chemical species found in the transfer films and running films of low wear PTFE systems. We have performed experiments and created a simple model to evaluate the possibility of chain scission in PTFE based systems. Surface plasmon resonance has extreme sensitivty and has been used in detection of monolayers [ 89 90 ] and cell cell and cell substrate dynamics and adhesion [ 98 ] ; it should be an ex c ellent tool to measure wear of PTFE at a molecular scale. In situ surface plasmon resonance experiments were per formed to observe the possibility of

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71 molecular wear in PTFE systems. It is hypothesized that PTFE will transfer to a counterface after one cycle of sliding. Surface plasmon resonance can detect subtle material transfer from the polymer to a gold coated pri sm and can confirm if PTFE is transferred after one cycle of sliding. Surface enhanced Raman spectroscopy (SERS) is ideally suited to determine the chemical species of the deposited transfer film on the gold coated quartz prisms ; t he surface enhanced effec t can amplify the Raman signal by 5 or 6 orders of magnitude over conventional Raman spectroscopy, such that the vibrational spectra of a thin transfer film can be measured when it normally could not with conventional Raman spectroscopy. SPR experiments wi th PTFE were compared with ultra high molecular weight polyethylene, which should act as a negative control by not transferring to the countersample during sliding. The in situ optical microtribometer was used to evaluate the wear of PTFE at the microscopi c level. A PTFE pin was slid against a transparent glass sample. Wear events were observed in situ with a microscope focusing through the glass countersample at the contact region between the PTFE and the glass. Finally, a model for molecular wear was deve loped based on a Hamaker analysis. The attractive forces between the PTFE and glass and the force required to create a chain scission event by yielding PTFE were equated to solve for the limiting geometry of a PTFE wear fibril that could produce chain scis sion during sliding. The feasibility of this geometry and event occurring were then evaluated.

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72 CHAPTER 5 EXPLORING AND CHARAC TERIZING FILLERS IN PTFE Wear T ests for V arious F illers As a n attempt to help explore and bound ultra low wear PTFE matrix systems, we filled PTFE with various filler materials at the same loading and tested the materials to match the performance of th longer produced. There is an immediate need to find an al ternative to this filler for the continuation of the development and use of the remarkable composite. Materials of many types were tested as filler material candidates Extensive testing of PTFE composites were performed at the same filler loadings (5 wei ght percent) at the same sliding conditions (50.8 mm/s sliding speed, 25.4 mm stroke and 6.5 MPa contact pressure). Results are summarized in Table 5 1. Table 5 1 Friction coefficient and wear rate results for PTFE and its composites with various fillers at 5 weight percent unless otherwise stated. It a lso includes the dispersion technique either IPA: dispersion in isopropyl alcohol by ultrasonication or jetmill, dispersion in a jetmill. T he molding technique is ether DuPont compression molded or UF compr ession molded as described in the experimental methods. It also includes the symbol that corresponds to the sample results in Figure 5 1. Filler Material friction coefficient wear rate ( mm3/Nm) Dispersion technique Molded by symbol Unfilled 0.13 4.30E 04 IPA DuPont A 2 % T1 A l 2 O 3 0.21 1.10E 07 IPA DuPont B 8 % T1 A l 2 O 3 0.23 4.80E 08 IPA DuPont C T1 A l 2 O 3 0.20 6.80E 08 Jetmill UF D T1 A l 2 O 3 0.22 4.70E 08 IPA DuPont E T1 A l 2 O 3 0.24 6.10E 08 IPA DuPont F T1 A l 2 O 3 0.20 3.60E 08 IPA DuPont G 5 % T1 A l 2 O 3 and 5 % SiO2 0.24 2.90E 08 IPA DuPont H T2 A l 2 O 3 0.19 1.60E 05 IPA DuPont I T3 A l 2 O 3 0.19 2.60E 05 IPA DuPont J T4 A l 2 O 3 0.18 2.00E 07 IPA DuPont K

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73 Table 5 1. Continued. Filler Material friction coefficient wear rate ( mm3/Nm) Dispersion technique Molded by symbol T5 A l 2 O 3 0.19 9.70E 06 IPA DuPont L T6 A l 2 O 3 0.21 1.40E 07 IPA DuPont M T6 A l 2 O 3 0.18 2.70E 07 IPA DuPont N T6 A l 2 O 3 0.18 1.10E 07 IPA DuPont O T7 A l 2 O 3 0.17 1.10E 05 IPA DuPont P T8 A l 2 O 3 0.20 4.00E 05 IPA DuPont Q T9 A l 2 O 3 0.20 4.90E 05 IPA DuPont R T10 A l 2 O 3 0.18 4.00E 05 IPA DuPont S T11 A l 2 O 3 0.21 1.40E 07 IPA DuPont T A l 2 O 3 0.17 3.90E 06 IPA DuPont U T1 TiO 2 0.23 1.40E 05 IPA DuPont V T2 TiO 2 0.22 3.10E 05 IPA DuPont W T3 TiO 2 0.18 1.50E 04 IPA DuPont X T4 TiO 2 0.21 3.00E 05 IPA DuPont Y T5 TiO 2 0.20 2.00E 05 IPA DuPont Z T6 TiO 2 0.21 7.00E 05 IPA DuPont a T7 TiO 2 0.18 2.80E 04 IPA DuPont b T8 TiO 2 0.23 1.10E 07 IPA DuPont c T9 TiO 2 0.28 3.10E 07 IPA UF d CBN 0.20 4.50E 05 IPA DuPont e Diamond 0.18 1.20E 05 IPA DuPont f Mullite 0.22 2.90E 07 IPA UF g Pyrophyllite 0.21 1.20E 07 IPA UF h Talc 0.17 3.70E 05 IPA UF i Dolomite 0.31 9.30E 08 IPA UF j Kaolin 0.17 1.00E 04 IPA UF k Kyanite 0.32 1.00E 06 IPA UF l Bentonite 0.17 6.10E 06 IPA UF m Li 2 CO 3 0.18 8.90E 05 IPA UF n Na 2 CO 3 0.15 1.70E 04 IPA UF o BaCO 3 0.18 4.10E 05 IPA UF p Fe 0.18 3.00E 05 Jetmill UF q SiO 2 0.14 2.00E 04 Jetmill UF r Ni nanostrands 0.20 6.00E 06 IPA UF s 5 % T1 A l 2 O 3 +5 t% Ni nanostrands 0.22 3.60E 08 Jetmill UF t

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74 An excellent way of comparing tribological polymers is to plot them on a scatter plot of wear rate in log scale ( Figure 5 1). Figure 5 1 Wear rate versus friction coefficient for PTFE composites filled with various filler materials Experiments were performed on a reciprocating tribometer sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless s teel sample at 250 N and 50.8mm See Table 5 1 for a legend of composite fillers. Figure 5 1 shows how the inclusion of fillers in PTFE can signif icantly alter the wear rate of PTFE by several orders of magnitude. All fillers tested cause an increase in friction coefficient. These experiments show clusters of composites with similar wear rates with large gaps between. These clusters are separated b y several orders of

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75 magnitude in wear rate. To date, more than eight fillers that produce ultra low wear PTFE composites have been discovered This is further visualized in Figure 5 2. Figure 5 2 Wear rates of PTFE composites filled with of various fill ers. Experiments were performed on a reciprocating tribometer sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless s teel sample at 250 N and 50.8mm

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76 Eleven different sources to determine if the wear reduction is simply caused by that particular phase of alumina. One alumina powder was used to evaluate another phase of alumina. The alumina fillers came in various shapes and sizes and ultimately four alumina fillers pr oduced the ultra lo w wear behavio r (type 1, type 4, type 6 and type 11) Many different sources of titania fillers were also studied. Titania is a common nanoparticle used in many industries, including as a pigmentation component in inks and dyes. Titania fillers are rea dily available as a nanopowder, which was desired to as a cause of the ultra low wear behavior Titania fillers allowed us to evaluate the possibility that another oxide could produce the wear behavior Overall, nine different sources of TiO 2 were added as fillers to PTFE composites and ultra low wear was achieved on two of them (types 8 and 9) Alumina powder is commonly used as an abrasive for polishing applications. Cubic boron nitride and nano diamond powders ( non oxide abrasive nanopowders ) were evaluated to test the hypothesis that a nano abrasive was responsible for creating ultra low wear by cleaning the advantageous carbons and oxides from the stee l creating a site for the PTFE to react. Neither sample produced ultra low wear composites when mixed with the PTFE at 5 weight percent. This negative result does not however confirm or deny the requirement of abrasiveness in the filler, as the CBN and dia mond powders may be missing something else that other fillers have. The new tribochemical species in the tra nsfer film of these composites, first observed by Burris [ 70 ] If degraded PTFE is

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77 inherently wear resistant, then the inclusion of reactive fillers to degrade the PTFE could provide a built in degradation mechanism that does not requ ire sliding. This may also confirm that defluorination or chain scission is the route to ultra low wear behavior. Iron and fumed silica nanopowders were added to PTFE to test the hypothesis that reactivity and defluorination within the composite were respo nsible for the ultra low wear behavior. Neither sample produced desirable wear rates ; however the fumed silica significantly altered the wear mechanism of PTFE. Instead of producing the typical flakey wear debris associated with PTFE, the PTFE/fumed silic a composite extruded a long ribbon like feature from the surface of the polymer sample that remained attached to the polymer when slid against steel ( Figure 5 3 ) The toughening provided by the fumed silica was thought to be a desirable property when combined with the alumina fillers, as it would help form a more persistent transfer film. To test this hypothesis percent fumed silica were tested in the linear reciprocating tribometer The composites resulted with a wear rate of 2.9 x10 8 mm 3 /Nm, t he lowest wearing composite produced by adding a small weight percent of fillers to PTFE. Figure 5 3 PTFE and SiO 2 composites: a and b ) Image of ribbon like wear debris extendi ng from sample.

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78 Figure 5 4 Scanning electron micrographs of wear ribbon from PTFE/SiO2 composites at a) 44x, b) 430x, c) 1500x and d) 3000x. The success of the alumina and silica composites led me to exploring several mineralogical samples that were sim ilar to mixtures of aluminas and silicates. Pyrophyllite, talc, dolomite, kaolin, kyanite, and bentonite were all tested at 5 weight percent in PTFE Pyrophyllite ( Al 2 Si 4 O 10 (OH) 2 ) mullite ( 3Al 2 O 3 2SiO 2 ) a nd dolomite ( CaMg(CO 3 ) 2 ) produced composites with steady state wear rates in the ultra low wear

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79 regime. K yanit e (Al 2 O 3 SiO 2 ) filled PTFE composites were temporarily in the ultra low wear regime at one point with a wear rate of 3.5 x 10 7 mm 3 /Nm, however this increased to around 1x10 6 mm 3 /Nm Talc ( Mg 3 Si 4 O 10 (OH) 2 ), kaolin (Al 2 Si 2 O 5 (OH) 4 ) and bentonite (hydrated aluminosilicate of Na, Ca, Mg and Fe) did not produce ultra low wear Mineralogical samples were poor purity and require further evaluation and characterization to be full y understood. The success of dolomite prompted studying more metal carbonates. Lithium carbonate (Li 2 CO 3 ), sodium carbonate (Na 2 CO 3 ) and barium carbonate (BaCO 3 ) were each tested at 5 weight percent in PTFE. No carbonates tested produc ed ultra low wear. In unpublished work, Vail filled PTFE with 5 weight percent nickel nanostrands to produce conductive PTFE samples 1 ; these composites were not ultra low wear, so we filled PTFE with 5 alumina to produce composites that were conductive and ultra low wear (k~ 3.6 x 10 8 mm 3 /Nm ). Characterization of F illers The Alumina F iller The filler s that repeatedly generate the ultra low wear composites w ere Alfa Aesar alpha phase alumina powder with supplier specified 80nm and 40nm average particle size (types 1 and 4 in my research) discussed further by Burris and Sawyer [ 37 ] Other fillers have generated ultra low wear composites and have gotten close to the wear ra te achieved by composites of the type 1 and 4 Alpha Aesar filler material. This 1 J.R. Vail, B.A. Krick and W.G. Sawyer, Unpublished ptfe and ni composites, (2010)

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80 filler is very unique, in that it is irregular in size and shape. Characterization of this filler and other fillers is vital to understanding what is required in a filler to pr oduce ultra low wear PTFE composites. TEM and SEM m icrographs of f iller m aterials Transmission electron micrographs were taken for as Nanophase alumina (Figure 5 5) as well as type 1 ( Figure 5 6 ), type 2 ( Figure 5 7 ), type 3 ( Figure 5 8 ), and type 4 ( Fi gure 5 9 Type 1 and 3 aluminas are very similar in the TEM studies. They are both irregular in shape and comparable in size. In fact, type 3 was produced by the same manufacturer as type 1 and was recommended as an adequate replacement. However, type 1 and type 3 produced dramatically different wear properties in PTFE composites. Types 2 and alumina were very different than type 1 and 4. Figure 5 5 Transmission electron micrograph of phase alumina. a and b) 250 nm window size. C and d) 50nm window size. TEM done by our collaborators at DuPont.

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81 Figure 5 6 Transmission electron micrograph of type 1 a d) 1.25 m window size E and f) 50nm window size. TEM done by our collaborators at DuPont.

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82 Figure 5 7 Transm a d) 1.25 m window size. e and f) 50nm window size. TEM done by our collaborators at DuPont.

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83 Figure 5 8 a d) 1.25 m window size. e and f) 50nm window size. TEM done by our collaborators at DuPont.

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84 Figure 5 9 a d) 1.25 m window size. e and f) 50nm window size. TEM done by our collaborators at DuPont.

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85 SEM micrographs w ere taken f or type 1 ( Figure 5 10 ), type 3 ( Figure 5 1 1 ), type 6 ( Figure 5 1 2 ), type 9 ( Figure 5 1 3 ), type 10 ( Figure 5 1 4 ) and type 11 ( Figure 5 1 5 alumina. Types 1, 3 and 6 had similar sizes and structures, with type 6 appearing more jagged than the other types. Type 11 alumina had a large range of particle sizes, with the smaller sized particles being similar to those observed in some of the other alumina powders. Type 3 alumina has no apparent difference from type 1 alumina from these SEM studies as well as the previous TEM studies. Figure 5 10 Note scale bar of a) 5 m and b) 1 m. SEM done by our collaborators at DuPont. Figure 5 11 Note scale bar of a) 5 m and b) 1 m. SEM done by our collaborators at DuPont.

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86 Figure 5 12 Note scale bar of 2 m. SEM done by our collaborators at DuPont. Figure 5 13 Scanning electron micrograph Note scale bar of 5 m SEM done by our collaborators at DuPont. Figure 5 14 Note scale bar of 20 m SEM done by our collaborators at DuPont.

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87 Figure 5 15 Scanning Note scale bar of 50 m. SEM done by our collaborators at DuPont. Size a nalysis by l ight s cattering and SEM We performed a characterization of the size distribution of the alumina powders What w e found was that the filler sizes are often far from what the manufacturer claims and are often far from uniform in distribution. Size distributions were found by using light

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88 scattering and by using SEM micrographs and image processing techniques to generate size dist ributions. Figure 5 1 6 shows the volume percent of filler as a function of particles size of the Type 1 and type 3 fillers are from the same source and are reported to be comparable by the manuf acturer. It should 7 ) was ultra low (k~2.6x10 5 ) was not ultra low wear at five weight percent filler. Aside from particle size distribution and wear properties, all other experiments found no differences between Figure 5 1 6

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89 Figure 5 17 shows the volume percent of filler as a function of particles size of the All three of these fillers are polishing grade fillers from the same manufacturer. It should be noted that types 9 (k~5x10 5 ) and 10 (k~4x10 5 ) ultra low wear while type 11 (k~1x10 7 ) was ultra low wear at five weight percent filler. Figure 5 1 7 ina.

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90 Further size characterization was done using SEM micrographs and processing the images to count the number of particles in each size range over a given sample space. Figure 5 1 8 shows an arbitrary particle count as a function of particle size for type Both type 1 and 6 alumina produced ultra low wear composites, but type 6 alumina (k~ 1x10 7 mm 3 /Nm ) never produced as low of wear when used as a filler as type 1 alumina (k<6x10 8 mm 3 /Nm) at five weight percent loading. Figure 5 1 8 measured by the SEM.

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91 Chemical analysis by x ray p hotoelectron s pectroscopy Type 1 and type 3 be comparable by the manufacturer. H owever type 1 produces ultra low wear while type 3 does not. XPS was performed to determine possible chemical differences between these fillers. No noticeable difference wa s obser ved in the aluminum ( Figure 5 1 9 a) or oxygen ( Figure 5 1 9 b) spectrums, leaving particle size distribution to be the only measured difference in these fillers. Figure 5 1 9 Oxygen spectrum. Titania Fillers In an effort to contrast titania fillers that worked to produce ultra low wear to titania fillers that did not produce ultra low wear T EM micrographs were taken for type 1 titania ( Figure 5 20 a ), type 2 titania ( Figure 5 20 b ), type 3 titania ( Figure 5 20 c ) and type 8 titania ( Figure 5 20 d) Type 8 TiO 2 was the only filler of these 4 to produce ultra low wear ; i particles.

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92 Figure 5 20 Transmission electron micrographs of vari ous types of titania filler materials. a) type 1 titania, b) type 2 titania, c) type 3 titania and d) type 8 titania. Please note change in scales in micrographs. TEM done by our collaborators at DuPont. Mineralogical F illers In an effort to contrast miner alogical fillers that worked to produce ultra low wear to those that did not produce ultra low wear, low resolution SEM micrographs were ta ken for pyrophyllite ( Figure 5 2 1 ), talc ( Figure 5 2 2 ), dolomite ( Figure 5 2 3 ), kaolin ( Figure 5 2 4 ) and kyanite ( Figure 5 2 5 ). Much larger particles are observed i n these mineralogical powders. Additionally, each of the working fillers has a large distribution of particle sizes. The pyrophyllite and dolomite filler, which produced ultra low wear behavior in PTFE comp osites, and the kyanite filler, which was found to periodically produce ultra

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93 low wear, had very large, jagged particles dispersed among very small particles. These working fillers are far from nano fillers. Figure 5 21 Scanning electron micrograph of p yrophyllite powder. Note scale bar of a) 100 m, b) 50 m and c) 20m.

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94 Figure 5 2 2 Scanning electron micrograph of talc powder. Note scale bar of a) 250 m, b) 100 m and c) 50m.

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95 Figure 5 2 3 Scanning electron micrograph of dolomite powder. Note scale bar of a) 250 m, b) 100 m and c) 50m.

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96 Figure 5 2 4 Scanning electron micrograph of kaolin powder. Note scale bar of a) 250 m, b) 100 m and c) 50m.

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97 Figure 5 2 5 Scanning electron micrograph of kyanite powder. Note scale bar of a) 125 m an d b) 50 m

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9 8 CHAPTER 6 ENVIRONMENTAL STUDIE S composites were performed in several environments, including vacuum, various atmospheric gasses including nitrogen, oxygen and water, an environment with a substantial pressure of ethanol and even the low earth orbit environment. Environmental tests serve two primary purposes: 1) to evaluate the performance of the solid lubricant in an environment in which it may be applied in and 2) to evaluate the importance of environmental constituents as a potential contributor to the ultra low wear mechanism to better understand these composites. Vacuum The vacuum environment is important in many characterization techniques, manufacturing app lications ( i.e. microelectromechanical systems and coatings) and it is a dominant characteristic of many space environments. There is need to develop l ow wearing and low friction lubricants for the vacuum environment to push vacuum technologies to new limi ts. PTFE filled with 5 wt. % alumina composites were tested using the vacuum linear reciprocator tribometer (Figure 4 5). Tests were performed both in vacuum and in the laboratory air environment by leaving the vacuum chamber open Results show a distinct increase in wear rate when the samples were slid in vacuum as opposed to the typical laboratory air environment (Figure 6 1). vacuum are typically between 1 and 5x10 5 mm 3 /Nm. For the reported experiment, the steady stat e wear rate of the PTFE and alumina composite in vacuum was 3x10 5 mm 3 /Nm and the wear rate in 35% RH laboratory air was 5x10 8 mm 3 /Nm. That is a

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99 change by almost three orders of magnitude. IN fact, the wear rate and wear behavior of PTFE and type 1 alumi na in vacuum is comparable to the non ultra low wear composites of PTFE in air. Figure 6 1 PTFE and alumina composited in vacuum vs air. Comparison of wear volume vs. sliding distance times normal force for PTFE composites filled with at 5 weight percent when slid in the vacuum environment (6x10 6 torr) vs. the laboratory air environment (760 torr) The slope of the line fit between these points is the wear rate in mm 3 /Nm. Tests were performed with normal load of 250 N, reciprocating cy cle length of 25.4 mm and sliding speeds of 50.8 mm/s. When tested in the laboratory environment, the PTFE and type 1 alumina composites produced ultra low wear rates accompanied by t hin, uniform, brown transfer films on the stainless steel countersamples and a brown running film on the running surface of the polymer sample. These transfer films were no longer observed when this

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100 material was tested in vacuum; instead, black wear debris with streaky and patchy black transfer film and running film were observ ed. The following experiment was designed t o test the inherent wear resistance of the brown running film and transfer film that is consistent with the ultra low wear behavior of these systems. A PTFE /alumina composite was slid against the 304L stainless s teel counterface in the humid laboratory air environment for 4000 m to produce the brown running and transfer films. After this, the vacuum chamber was reduced in pressure to 6x10 6 tor r and the sliding was resumed. Wear rates returned to values similar to the wear rates that are observed in standard vacuum tests Figure 6 2. Figure 6 2 Evaluating the wear resistance of a transfer film. Results of sliding PTFE/alumina composites in air to achieve ultra low wear followed by changing the environment to vacuum (6x10 6 torr) at 1x10 6 N*m of sliding distance*normal load (4000m of sliding) shown by the solid blue line. Results for wear testing in just vacuum and just lab air are shown as dashed lines. Tests were performed with normal load of 250 N, reciprocating cycle length of 25.4 mm and sliding speeds of 50.8 mm/s.

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101 When the ultra low wear rate increased significantly to a wear rate that is comparable to t he wear rates observed in the traditional filled PTFE composites. This increase suggests that the generation of the thin, robust transfer film indicative of ultra low wear systems is environmentally sensitive. The environmental sensitivity of the composite system prompted the ne ed to explore the effects of the chemical constituents of laboratory air on ultra low wear composites. Relative Humidity and O xygen The importance of environmental constituents was made apparent by the results from the tests comparin g the vacuum environment with the laboratory air environment. Primary constituents of the laboratory air environment include nitrogen, oxygen, and water vapor. To determine the importance of environmental water and oxygen, tribology experiments were performed on the environmental linear reciprocating tribometer ( Figure 4 6). nitrogen, nitrogen and various concentrations of water (measured by a hygrometer), nitrogen and 20 percent oxygen, an d nitrogen with 20 percent oxygen and various concentrations of water. Wear rates as a function of measured relative humidity are summarized in Figure 6 3. Results are summarized in Table 6 1. Transfer film morphology and thickness are explored further in the tra nsfer film analysis section of C hapter 8

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102 Figure 6 3 Wear rate and friction for PTFE and PTFE/alumina composites in various mixtures of nitrogen, water and oxygen. a) Single point w ear rates and b) steady state friction coefficient for PTFE and PTFE/alumina composites slid against stainless steel in nitrogen and 80% nitrogen/ 20% oxygen background gasses with various concentrations of water (%RH ) PTFE samples were slid for 500 m while PTFE/alumina composites were slid for 5 0 00 m with normal load of 250 N, cycle length of 25.4 mm and sliding speeds of 50.8 mm/s Table 6 1 Wear rate and friction for PTFE and PTFE/alumina composites in various mixtures of nitrogen, water and oxygen. Single point wear rates and steady state friction coeff icient for PTFE and PTFE/alumina composites slid against stainless steel in nitrogen and 80% nitrogen/ 20% oxygen background gasses with various concentrations of water ( % RH ). PTFE samples were slid for 500 m while PTFE/alumina composites were slid for 5,0 00 m with normal load of 250 N, cycle length of 25.4 mm and sliding speeds of 50.8 mm/s. Material Environment Humidity k (mm 3 /Nm) PTFE/ Alumina N 2 0.5 0.17 1.40E 06 0.6 0.15 7.40E 06 2.7 0.12 4.60E 07 10 .0 0.13 1.30E 07 30 .0 0.19 2.60E 07 69 .0 0.17 1.00E 07 78 .0 0.2 0 1.00E 07 N 2 + O 2 (80/20) 2.5 0.19 4.40E 06 35 .0 0.2 0 5.60E 07 68 .0 0.16 2.40E 07 PTFE N 2 0.8 0.14 5.80E 04 76 .0 0.11 4.60E 04 N 2 + O 2 (80/20) 3.6 0.16 6.30E 04 35 .0 0.12 4.30E 04 87 .0 0.14 4.90E 04

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103 Ethanol The results from the studies on the effects of the environmental presence of suggest the importance of water as a key to producing ultra low wear in the system. In an effo wear behavior of this composite, the composite was tested in an environment consisting of nitrogen and ethanol. Ethanol is a polar molecule with a hydroxyl group, similar to water. Like water, its vapor pressure is sufficient to produce a substantial percentage of gaseous ethanol when a container of ethanol is opened in the nitrogen environment and allowed to reach equilibrium. Using the environmental tribometer, s ingle point wear rates were observed for ree environments for comparison: nitrogen, nitrogen doped with water and nitrogen doped with ethanol. Doping was achieved by pouring the water or ethanol into a pan wi th a large surface area in the clean nitrogen environment. The environment was allowed to come to equilibrium over a 24 hour period before running tests. Each sample was loaded to 250 N and slid a total sliding distance of approximately 6000 m. It was obse rved that the wear rate for the ethanol environment was comparable to the dry nitrogen environment and was much higher than the wear rate observed in a humid environment Table 6 2 Again, it should be noted that these are single point wear rates, so their values will be higher than the steady state wear rates measured on the linear reciprocating tribometer wear experiments.

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104 Table 6 2 Wear and friction of PTFE and alumina composites in nitrogen, humid nitrogen and ethanol doped nitrogen. Samples were slid against 304 stainless steel with normal load s of 250 N an d sliding speeds of 50.8 mm/s for a distance of 6 ,000 m. Environment single point wear rate (mm3/Nm) friction coefficient N2 6x10 6 0.17 N2 and water 8x10 7 0.2 0 N2 and ethanol 5x10 6 0.1 0 Low E arth O rbit Space Preliminary results on friction of PTFE/alumina composites in the low earth orbit space environment report a steady friction coefficient ( Figure 6 4). At this point, wear rate and chemical degradation data has not been processed It is hypothesized that the wear of these composites will be better than PTFE. The most optimistic hypothesis is that the atomic oxygen in space can promote ultra low wear behavior by forming the carboxylic acids that are thought to be the source of the ultra low wear. Preliminary observations show that a running film exists on both the ram (atomic oxygen) and wake (high vacuum) experiments ( Figure s 6 5a and 6 6a respe ctively). On the ram side, a transfer film is observed on the ball that the composite was slid against ( Figure 6 5b). On this ball is also a lot of fine wear debris. The wake side however had transfer film on the ball ( Figure 6 6b). It had larger, more f lakey wear debris ( Figure 6 6b) similar to the wear debris observed in the test in which the composite was run in in air, and then allowed to continue to slide in vacuum ( Figure 6 2) Raman spectroscopy was performed on the PTFE/alumina composites after re turning from space. Micro Raman spectra were acquired both in and out of the wear

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105 track, where the running film is deposited, for both the ram (Figure 6 7a) and wake (Figure 6 7b) samples. Figure 6 4 Preliminary space tribometer results. Each data point represents an average value from seven clockwise rotations and seven counterclockwise rotations. Error bars shown are the standard deviation in friction coefficient for each test. Uncertainty is much less than the standard deviation.

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106 Figure 6 5 PTFE/ alumina ram post flight pictures: a) PTFE/ alumina composite mounted in a steel disc and b) steel ball that slid against the composite.

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107 Figure 6 6 PTFE/ alumina wake post flight pictures: a) PTFE/ alumina composite mounted in a steel disc and b) steel ball that slid against the composite.

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108 Figure 6 7 Raman spectroscopy for PTFE/ alumina samples post flight both in the wear track (blue) and out of wear track (orange) ran on the low earth orbit space tribometer a) ram and b) wake face.

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109 CHAPTER 7 COMPOSITE RUN IN AND RUNNING FILM DEVELOPMENT Ultra low wear PTFE and alumina composites exhibit a transient behavior in the early sliding cycles of long t erm wear tests. This period starts with high initial wear rates followed by a logarithmic decay in wear rate. The run alumina composites were studied with filler concentrations of 2, 5 and 8 weight percent. Composites were s lid against 304L stainless steel (lapped finish) in the linear reciprocating tribometer at a normal load of 250 N, sliding speed of 50.8mm/s and stroke length of 25.4mm. Samples were massed much more frequently than typical wear tests to observe the transi ent behavior. Figure 7 1 shows the total volume lost by the polymer (calculated by mass and density measurements) as a function of sliding cycle Each time the sample was removed from the tribometer to be massed, an image was taken of the running surface of the polymer. Figure 7 2 7 3, and 7 4 chronologically show the development of the running film for the 8 5 and 2 weight percent s In cycle 5 of the 8 weight per cent composite a small amount of steel is brought into the running film and it later becomes the dark spot in the upper right corner of the running films. The two weight percent alumina and PTFE composite initially began to run in and it appeared to be fo llowing the trend of reducing wear, but after around 5000 cycles the wear continued at a steady pace until around 15,000 cycles. This region of high wear is characterized by lower friction coefficients tha n the 5 and 8 weight percent composites. Also, Figure 7 4 sho w s that the running film has poor coverage during these cycles; t here w ere noticeable regions of the composite that looked similar to unfilled PTFE. This

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110 could be simply a function of dispersion of the filler or distribution of particle the v arious particle sizes in the filler. Image processing of the running films was performed to quantify the relative coverage of the running film. Utilizing M atlab images were converted to gray scale and the average color intensity was computed over the are a of the polymer surface for each image. The intensity averages were normalized by the average intensity of the first Finally, the total wear rate at each the time of each image acquisit ion is plotted as a function of the instantaneous running film coverage on a semi log plot ( Figure 7 5 ). Figure 7 1 Total volume lost as a function of sliding cycle for 2, 5 and 8 weight percent less steel at 250 N normal load, 50.8 mm/s sliding speed and 25.4 stroke length. Plotted on a) semi log, b) linear, and c) log log scales for visualization.

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111 Figure 7 2 alumina composi tes on steel.

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112 Figure 7 3 alumina composites on steel.

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113 Figure 7 4 alumina composites on steel.

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114 Figure 7 5 Total wear rate vs. computes running film coverage for the PTFE and 8

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115 CHAPTER 8 TRANSFER FILM AND RU NNING FILM ANALYSIS One of the most obvious differences in the wear of ultra low we ar PTFE composites is the differences in color of the transfer film on the surface of the countersample, the running film on the surface of the polymer and the wear debris generated during sliding Transfer films, running films and wear debris are essentia lly signatures of each composite. Observations of morphology, thickness and chemistry of the transfer films and running films provide evidence of mechanical and chemical mechanisms responsible for ultra low wear PTFE composites. Transfer Film Appearance, T hickness and M orphology Basic observations show that PTFE samples are typically white and make a flakey, translucent white transfer film; ultra low wear PTFE and alumina composites produce a brown transfer film with a minimal quantity of small wear debris when slid against steel The brown transfer fil ms are observed after one cycle of sliding and persist throughout the duration of the experiment for ultra low wear PTFE composites. The brown transfer film that is produced by the ultra low wear composite is thinner and less rough than that of PTFE and conventional PTFE composites. It also provides much better coverage of the counter surface. The transfer films, pictured in Figure 8 1, were mapped using a stylus profilometer. After mapping, the transfer films were burned off and the surface of the countersample was mapped using the same profilometer in the same orientation. The height maps of the countersurface were subtracted from the height maps of the transfer to produce the background subtracted transfer fi lm thick n ess ( Figure 8 2).

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116 Figure 8 1 2 O 3 Transfer films were generated by sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless steel sample Figure 8 2 Transfer film topography for a) PTFE a 2 O 3 Transfer films were generated by sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless steel sample at 250 N and 50.8mm/s over a distance of 500m 2 O 3

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117 Environmental D ependence Transfer films from the environ mental tests were all mapped by the stylus profilometer. Images and transfer film height profiles of the transfer films generated by nitrogen are shown in Figure 8 3 In the humid environment (69% RH) very thin, uniform transfer films were formed across the entire stroke of the experiments ( Figure 8 3a) ; these films appear to act as a protective boundary between the composite and the counterface This is contrasted by the non u n iform transfer films produced in the environment with low concentrations of water (0.6% RH); surface topography of these films show transfer film above and below the initial height of the steel ( Figure 8 3b), implying that the composite wore into th e steel counterface. Transfer films of PTFE/alumina composites on steel for several environmental tests were observed in the scanning electron microscope (Figure 8 4). Transfer films generated in humid environments show consistent coverage (Figure 8 4a an d 8 4b). However the transfer film morphology differed slightly between samples tested in humid air vs. humid nitrogen. Transfer films produced in h u mid air ha ve larger regions of transfer films with larger regions of exposed steel than the transfer films produced in the humid nitrogen environment. At higher magnitudes, the large regions of transfer film observed in the humid air experiments (Figure 8 cracks observed by Burris et al. in the running films on the surface of the polymer composite [ 11 ] ; this suggests that these regions are segments of the running films that have detached from the polymer surface and deposi ted on the transfer film. Transfer films produced in the nitrogen environment with trace water and oxygen provided much less coverage (Figure 8 4c). The transfer film region from samples that

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118 were slid in laboratory air (35 %RH) followed by vacuum (<6x10 6 torr) had poor (to know) transfer film coverage with flakey wear debris scattered throughout (Figure 8 4d). Figure 8 3 Transfer film image and topography for PTFE 2 O 3 composites in nitrogen environments with water contents of a) 69 %RH and b) 0.6% RH. Transfer films were generated by sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless steel sample at 250 N and 50.8mm/s over a distance of 3,000 m

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119 Figure 8 4 Scanning electron micrographs of transfer films generated by PTFE/ 5wt% 2 O 3 composites for several environmental experiments: a) 80 %RH water in nitrogen (80%)/ oxygen (20%), b) 80% RH water with <100 PPM oxygen in nitrogen and c) nitrogen w ith <1 %RH water and <100 PPM oxygen steel sample at 250 N and 50.8mm/s over a distance of 3,000 m. d) Transfer film generated by first sliding in laboratory air (35 %RH) to achieve ultra low wear running film and transfer film, then changing the environme nt to vacuum ( Figure 6 2).

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120 Figure 8 5 2 O 3 sliding in 80 %RH water in nitrogen (80%)/ oxygen (20%). Transfer film generated by sliding against steel sample at 250 N and 50.8mm/s over a distance of 3,000 m. Images and transfer film height profiles of the transfer films generated by PTFE in humid nitrogen and dry nitrogen are shown in Figure 8 6 In the humid environment (76% RH), thick transfer films with moderat e coverage were formed (figure 8 6 a); small amounts of brown wear debris hint of the reactivity seen in ulralow wear systems, though these materials were not ultra low wear. This is contrasted by the very thick, patchy, non u n iform transfer films produced in the environment with low concentrations of water (0.8% RH); surface topography of these films show patchy regions of thick transfer film ( Figure 8 6 b) There is no evide nce of PTFE wearing the steel in either environment

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121 Figure 8 6 Transfer film image and topography for unfilled PTFE in nitrogen environments with water contents of a) 76 %RH and b) 0.8% RH. Transfer films were generated by sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless steel sample at 250 N and 50.8mm/s over a distance of approximately 600 m. Transfer F ilm A dhesion and Robustness: Scratch T est Scra t ch tests were performed on the transfer films generated by sliding unfilled PTFE and PTFE/alumina composites on steel. These tests were p erformed to evaluate t he inherent toughness of the tr a n sfer films and the quality of the transfer film adhesion to the steel countersample. Transfer films were scratched with a 1.6 mm radius silicon

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122 nitride ball. Loads were ramped from 0.05 N to 30 N over a 5 mm scratch. The loading rate was 6 N/mm (0.5 N/s) and the sliding speed was 5 mm/minute. Scanning electron micrographs of the scratches on unfilled PTFE and PTFE/alumnia composite are shown in Figure 8 7. These contact pressures perform obvious deformation to the steel countersample. The PTFE transfer film is pushed out of the way, as observed in the SEM micrograph. In contrast, the well adhered PTFE/alumina transfer film is smeared by the scratch, but remains adhered. Figure 8 7 SEM micrograp hs of scratches performed on transfer films of unfilled PTFE (left) and PTFE/alumina composites (right). Scratches were performed from left to right. Transfer films were generated by sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless stee l sample at 250 N and 50.8mm/s over 2 O 3

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123 Transfer Film and Running Film C hemistry Differences in the transfer films beyond the morphological and color differences have been even more compelling. Ch emical changes in the transfer film have been consistent with ultra low wear PTFE composites. Knowing the chemical identity of the new tribochemical species is critical in understanding these systems. XPS Preliminary x ray photoelectron spectroscopy (XPS) suggested a degraded PTFE existed in these transfer films [ 70 ] identified in the C1s spec trum near binding energies of 288 290 eV, in the transfer films in the ultra low wear composite transfer film ( Figure 8 8 ). However, XPS of the running fi lms on the surface of the polymers shows the new chemical species is only present in the running film of the ultra low wear composite and is not present on the wear surface of the unfilled PTFE. This new species has almost become a unique identifier among all ultra low wear PTFE systems, including the PTFE/alumina systems, the PEEK/PTFE systems, and the low wear irradiated PTFE. Additionally in Figure 8 8 is XPS performed on an unworn surface of the PTFE/ alumina composite and the unfilled PTFE. These surfa ces were prepared by exposing an internal surface of the polymer or composite by cutting the sample with a razor blade. XPS of the PTFE/alumina composite and the unfilled PTFE were consistent with XPS of unfilled PTFE, showing no chemical alterations of th e PTFE in the unworn PTFE/ alumina composite.

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124 Figure 8 8 2 O 3 and b) PTFE. Transfer films were generated by sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless steel sample at 250 N and 50.8mm/s over a distance of 500m for 2 O 3 Spectra are for the transfer film (blue and red), the running film on the polymer (green) and an unworn surface of the polymer prepared with a razor blade (black)

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125 Infrared A bsorption Further a nalysis of the transfer film was performed by IR absorption. These experiments identified very different chemical species in the transfer films generated by Figure 8 9 ). Significant peaks were observed in the transfer film of the composite at approximately 3338 cm 1 1660 cm 1 1360 cm 1 and 1317 cm 1 Peaks consistent with PTFE and metal oxides were observed in both transfer films. Figure 8 9 IR absorption for transfer films of PTFE (blue) a composites (orange). Time of Flight S econd ary Ion Mass S pectroscopy (TOF SIMS) TOF SIMS was performed on the transfer film, running film an d unworn surface of the polymer. Results showed the presence of AlF 3 FeF 3 FeF 4 and partial ly degraded No metal fluorides or degraded PTFE signature were detected in an unworn section of the composite.

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126 CHAPTER 9 TRIBOCHEMICAL CHAIN SCISSION AN UNEX P ECTED ROUTE TO ULTRA LOW WEAR SYSTEMS The proposed mechanism and evidence of a new tribochemical species suggests that reactions occu r during sliding. For this to happen, the PTFE chain must either defluorinate or carbon carbon bonds of the backbone must b reak. Experiments in this section are designed to explore the feasibility of carbon carbon bonds break ing during sliding. SPR/SERS Molecular W ear of PTFE In situ s urface plasmon resonance experiments can provide evidence of molecular wear of PTFE. Slidin g experiments were performed for PTFE and ultra high molecular weight polyethylene ( UHMWPE ) slid against a gold coated quartz prism at a linear velocity of 1 mm/s and an externally applied normal force of 1 N 1 F or one experiment, PTFE was heated to approx imately 100 C to increase the amount of transferred material Figure 9 1 shows the reflected laser power normalized to the incident laser power as a function of sliding cycle. The arbitrary intensity of the SPR measurement, is equal to the measured intensity, offset by the initial intensity before sliding, all normalized by the incident laser intensity, as shown in Eq. 9 1. ( 9 1) For both PTFE samples, a strong increase in reflectance is observed, followed by a slow reduction. This corresponds to an increase in adsorbate material on the gold sensing layer of the quartz prism, which should be interpreted as molecular trans fer 1 B.A. Krick, D.W. Hahn and W.G. Sawyer, Unpublished surface plasmon resonance spectroscopy (SPR) and surface enhanced raman spectroscopy (SER S), (2011).

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127 from the PTFE to the quartz prism. As expected, minimal increase in signal was observed in the UHMWPE experiments implying minimal material transfer during this experiment. This helped to confirm that the change in intensity is caused by surface plasm on resonance and validates the testing methods. Figure 9 1. SPR reflectivity intensity vs. sliding cycle for PTFE heated to 100 C, unheated PTFE and UHMWPE. Left is linear scale and right is semi log scale. Surface enhanced Raman spectroscopy was performed on each transfer film that was produced on the gold coated quartz prisms in the in situ SPR experiments For the PTFE and heated PTFE transfer film, a spectrum was observed that is consistent with a PTFE reference ( Figure 9 2), confirming the tra nsfer of PTFE To demonstrate the power of this technique, the SERS spectrum of the PTFE transfer film is contraste d with the non surface enhanced Raman spectrum of a PTFE transfer film gener ated by sliding PTFE on quartz without a gold layer No signal o utside of the noise was observed for the SERS of the UHMWPE transfer film. This is consistent with the minimal change in signal observed by the SPR,

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128 implying that there was minimal transfer of UHMWPE; this is our negative control for this experimental tech nique. Figure 9 2. Surface enhanced Raman spectrum for PTFE transfer film on the gold coated quartz prism generated after 30 sliding cycles during in situ SPR experiments (blue) contrasted with the non surface enhanced spectra of PTFE transfer film on a quartz counterface (red). The reference Raman scattering for bulk PTFE is shown as a black dashed line. Optical I n S itu T ribometer The optical in situ microtribometer is useful at observing the wear mechanism of PTFE at a larger scale than the in situ SPR experiments and is a complementary

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129 technique to the SPR/SERS experiments. PTFE was slid against glass in the optical in situ micro tribometer. Figure 9 3 shows a sequence of images collected during sliding. From the images it was observed that a sheari ng event can produce a transfer filament ( Figure 9 3c) of PTFE to the glass. The PTFE sample will slide over the PTFE filament for some time without moving the filament ( Figure 9 3 d g) After several cycles of sliding, the filament may be picked up and mov ed from its initial transfer location to another location as shown in Figure 9 3h Figure 9 3 In situ optical microscopy of PTFE sliding on glass: a) static contact, b sliding c) static, with wear filament in contact, d) and e) sliding over wear filament, f) and g) wear filament moved during sliding and h) wear filament moved to outside of static contact The dark area is the contact between PTFE and glass. It is surrounded by interference fringes caused by the interference of reflections from the surface of the glass and from the surface of the PTFE in the near contact region. A Model Based on Intermolecular F orces Tribological chain scission of PTFE can be quantified by a simple model in which molecular segments and chains of the PTFE separate f rom the bulk when attractive

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130 forces between the PTFE and the counterface are great enough to overcome the force required to break off that segment or chain. For simplicity, consider a fibril of PTFE of radius, sliding again st a gold countersurface (Fig. 9 4a); t he attractive force between the PTFE fibril and the metallic countersurface, can be calculated as a function of the fibril radius, contact length, Hamaker constant be tween PTFE and the countersurface, and the separation distance between the contacting fibril and the countersurface, through a Hamaker analysis of a cylinder on flat as seen in Eq. 9 2. ( 9 2) The force required to break the fibril of PTFE is simply (Eq. 9 3): ( 9 3) The fibril will break away from the bulk when the attractive force becomes greater than the force required to break a fibril. For an extremely conservative case, we can consider the force attractive force to be the adhesive force times the friction coefficient By setting the forces equal to each other we can solve for the critical length at which a fib ril of a given radius and separation distance will break off (Eq. 9 4). ( 9 4) To apply numbers to the simplified Hamaker based model for molecular wear of PTFE, we consider the case of PTFE slidin g against a silver counterface a s a Hamaker constant for PTFE and silver wa s readily available The Hamaker constant for this system has been studied and an average value of 13.7 [ 99 ] is used for these

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131 experiments. We have measured the macroscopic friction coefficient of PTFE to range from 0.05 to 0.2. H owever, the friction coefficient was measured to be 0.14 for the in situ SPR experiment The failure criterion is based on the ultimate strength of the PTFE filament; in a conservative and limiting c ase we can consider the ultimate strength of expanded PTFE. Expanded PTFE is highly aligned PTFE and has a ultimate strength of approximately 500 MPa, which is twenty times that of PTFE making for an extremely conservative estimate. Finally, we must consid er the separation distance; it is difficult to assign a separation distance for out idealistic geometry, so we consider the cases of 1, 3 and 10 The results for the critical contact length required for tribomechanical chain scission of a PTFE filament w hen slid against a silver surface is plotted as a function of fibril radius in Figure 9 4 b for the parameters above and three separation distances. These plots are the extremely conservative scenario; as the idealistic system approaches a real system, eve rything scales in favor for tribological chain scission of PTFE bonds and molecular wear of the PTFE. The ultimate strength will be lower and the forces higher. Additionally, in real systems there will not be a cylindrical filament sliding; there will be a complex geometry in contact with the countersurface connected to the bulk through chain entanglement and tie molecules.

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132 Figure 9 4 Mechanical chain scission of PTFE when slid against glass: a force balance approach. By considering the attractive force between the PTFE and a countersurface and balancing that force with the force required to break a PTFE filament, a critical filament length can be found as a function of filament radius. a) Schematic of PTFE filament and b) critical length vs. radius for three fibril separation values (1, 3 and 10 ) for the conservative case of expanded PTFE on silver.

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133 CHAPTER 10 DISCUSSION Formation of Mechanisms from Experimental R esults Wear S urvey and C haracterization of F iller M aterials Identifying g eneral t rends Wear results from the broad filler studies spanned over four orders of magnitude. Throughout this span there are generalized clusters of fillers with comparable wear rates, with each cluster separated by large g aps as illustrated in Figure 10 1. These separations su ggest the possibility of three, or more, different wear regimes, with each regime being linked to a possible wear mechanism that must be shut down to produce ultra low wearing PTFE composites Figure 10 2 illustrates the three wear regimes hypothesized, although intermediary regimes are likely to exist in these systems. The drop from the first wear regime (PTFE) to the second may be explained by the PTFE literature that suggests the fillers serve to arrest subsurface cracks and prevent delamination wear. This mechanism has been widely explored as discussed in the background of this work. In some cases, nanofiller s alone are not sufficient to produce this drop in wear; this may be caused by the fact that the filers are too small to arrest the crack propagation in PTFE This wear regime is characterized by larger wear debris, poor coverage of transfer films and ofte n scratching of the countersample. The second drop in wear to the third regime is previously unexplained. The composites in this third regime are consistent with the development of thin, well adhered transfer films on the counterface and running films on the surface of the polymer. These films contain a new carbon species as observed by XPS and IR which is attributed to carboxylic acids and possibly other end groups terminating PTFE molecules in the

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134 transfer film and running film This is linked to all ult ra low wear systems observed in this study. This regime marks what I consider to be ultra low wear PTFE composites, consistent with minimal wear debris and thin, well covered tribofilms with the presence of carboxylic acids as identified by IR analysis. Figure 10 1 PTFE composites filled with 5 wt% of various fillers. Experiments were performed on a reciprocating tribometer sliding a polymer pin of 6.4 x 6.4 x 12.7 mm against a 304 stainless steel sample at 250 N and 50.8mm/s over a distance of 500m for 2 O 3

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135 Figure 10 2 Hypothesized PTFE wear regimes as observed by composite studies

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136 Alumina and one alumina powder. Of these fillers, four produced ultra low wear rates: type 1, 4, 6 and 11 with type 1 alumina being the highest performing material. Characterization of the alumina powders revealed that the alumina powders that generated ultra low wear PTFE composites all have a very broad distribution of filler sizes spanning from less than 100nm to greater than 1 m. They also are all irregular in shape when observed with microscopy techniques. Types 1, 3 and 4 alumina powders are all from the same supplier. Types 1 and 4 are older stock powders that are no longer in production. Both type 1 and 4 alumina powders have claimed sizes of much broader range than the specified size (80nm and 40nm respectively). Type 3 alumina is reported by the supplier to be the equivalent of type 1. Microscopy and XPS characterization show that it is very similar, yet type 3 is different from type 1 in two main ways: 1) type 3 does not produce ultra low wear when filled at 5wt% in PTFE and 2) has a much tighter size distribution. Speculation and research leads us to believe that the type 1 alumina (an older filler produced when nanopowders were emerging) was removed from production in favor of the higher quality controlled type 3 alumina Type 9, 10 and 11 alumina were three grades of alumina from the same supplier classified by particle size for polishing applications. Types 9 (k~5x10 5 mm 3 /Nm) and 10 (k~4x10 5 mm 3 ultra low wear while type 11 (k~1x10 7 mm 3 /Nm) was ultra low wear at five weight percent filler. Even more surprising, type 11 was the largest grade of the filler, specified by the manufacturer at 5 m while types 9 and 10

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137 were specified by the manufacturer at 1 m and 300 nm respectfully Further investigation of the powders by static light scattering showed that the specified size was the average particle size as measured by scattering. Each powder did however have a distribution of particle sizes. The type 11 alumina powder (like the type 1 before it) had a distribution of powders with peaks around 100 nm a nd 5 m. This further reinforces the importance of size distribution. The alumina produced a PTFE composite that had a wear rate of around 4x10 6 mm 3 /Nm. While this was not in the designated ultra low wear regime, it was significantly lower than the wear rate of PTFE and many of the PTFE composites. This alumina powder was unique among the alumina powders for several reasons. First, and most obvious, it was a different phase of alumina. Secondly, the particle shape was spherical when examined by TEM. Particle sizes of this filler as measured by TEM ranged from less than 5nm to around 150 nm, with no evidence of a larger phase. An interesting observation of the composites is that they produced a brown running and transfer film during sliding that is consistent with the ultra low wear composites; however the film s were far less stable than the running films observed in the ultra low wear composites. Perhaps the la rger particles in the ultra low wear fillers help to anchor the tribochemically generated running films and transfer films. The larger, irregularly shaped particles in these fillers may also abate the wear mechanism of PTFE that produces the fluffy/flakey wear debris observed in PTFE and non ultra low wear PTFE composites. Types 9 and 10 alumina also produce a hint of brown in the wear debris, running film and transfer film.

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138 Titania Nine different titania powders were tested as fillers in PTFE composites, y ielding only two ultra low wear composites. In fact, it was the last two titania powders tested, type 8 and type 9 titania that yielded ultra low wear PTFE composites. T his can make one begin to think about the possibility of other fillers that have been d eemed ineffective after trying just one variety of it, when the experimenter actually just used the wrong variety of the filler The titania fillers tested are largely uncharacterized. An SEM micrograph of type 8 titania suggests that it shares a similar s hape characteristic to the type 1 alumina. Mineralogical f illers The results from PTFE and alumina and PTFE, alumina and silica composites provided a hint at what is required in ultra low wear PTFE composites. Aluminum silicates and other minerals appear t o have the components of what is required for ultra low wear. Of the mineralogical filled PTFE composites tested, p yrophyllite ( Al 2 Si 4 O 10 (OH) 2 ), mullite ( 3Al 2 O 3 2SiO 2 ) and dolomite ( CaMg(CO 3 ) 2 ) produced composites with steady state wear rates in the ultra low wear regime; while kyanite (Al 2 O 3 SiO 2 ) filled PTFE composites were temporarily in the ultra low wear regime at 3.5 x 10 7 mm 3 /Nm before increasing to around 1 x 10 6 mm 3 /Nm Talc ( Mg 3 Si 4 O 10 (OH) 2 ), kaolin (Al 2 Si 2 O 5 (OH) 4 ) and bentonite (hydrated aluminosilicate of Na, Ca, Mg and Fe) all ( surprisingly ) did not produce ultra low wear composites Mineralogical samples were poor purity and require further evaluation and characterization to be fully understood. Preliminary SEM results show a distributi on of particle sizes for the working fillers, with much larger particles than were observed in the alumina fillers. The composites of mullite and kyanite abraded the steel counterf ace and steel.

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139 The microgra phs of kyanite show very large, >100m jagged p articles that may be responsible for the transition out of the ultra low wear regime by scratching the transfer film off of the steel. These large particles are observed in the pyrophyllite however pyrophyllite is softer than the steel and may be broken do wn into smaller particles in the transfer film during sliding ; this creates particles of the smaller phase in the transfer film, where they are needed and may eliminate the need for specialized nanofillers. Furthermore the se mineralogical powders could mak e for extremely economical composites, as they are mined and require no special synth esis (nanoparticles), costing fractions of a dollar by the pound. These mined samples have natural impurities that may be beneficial to these composites. Further character ization and experiments on these fillers is needed to fully take advantage of them in PTFE composites. Wear m echanism: r equirements of the f iller The broad filler survey studies shows that fillers that produce ultra low wear are not chemically limited to alumina. In fact, fillers with seemingly identical chemical identities do not necessarily provide the same ab ility to produce ultra low wear. This nearly identical as measured by XPS as shown in Figure 5 19 This does not mean that the chemical identity is not important, but it implies that there is more to creating ultra low wear PTFE composites than simply including a filler of a specific chemical identity. Size an d shape characterization of the alumina suggests the importance filler size and possibly shape on the wear properties of the composites The filler size characterization showed that the ultra low wear producing fillers reported to be ually distributions of nanoparticles and microparticles. This

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140 distribution seems to be extremely important in generating low wear. Perhaps a bimodal (or more) distribution of filler particle size is most successful at producing ultra low wear composites. I suggest that a larger filler material on the order of 1 5 m (or more) may function to successfully shut down the mechanical gross wear mechanisms of PTFE; the chemical identity of this filler is not necessarily important, as long as it does not heavily d egrade the polymer matrix. Secondly, a filler material on the order of 30 200 nm could serve to promote the tribochemistry responsible for producing the ultra low wear transfer film; the chemical nature of this filler could be important as it is responsibl e for promoting the formation of carboxylates, however more research would be required to make this prove this. This smaller phase may be generated during sliding if the filler is friable or softer than the counterface. The size distribution of the filler particle seems to be extremely important. Further experimentation and filler characterization is required to determine the optimal distribution of filler size. Mixing of several alumina fillers at different ratios may provide an even better wear rate than Furthermore, evidence from two ternary composites (PTFE/Al2O3/SiO2 and PTFE/Al2O3/Ni nanostrands) suggest that a third, smaller primary particle size filler may further reduce the wear. This was achieved with a fumed s ilica filler which may further reduce the wear of the system by promoting partial defluorination and crosslinking. This was also achieved using nickel nanostrands; the nanostrands could promote chemistry and crosslinking as well. Another possible reason for the additional reduction in wear yielded by including the fumed silica and nickel nanostrands in the PTFE/alumina

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141 composites is the structure of the fillers. They both have small primary particle sizes and exist in agglomerations of large aspect rati os and irregular networks ( Figure 10 3 ). Figure 10 3 Micrographs of a) fumed silica and b) nickel nanostrands Environmental S tudies Vacuum Vacuum experiments show that some environmental constituents are required to generate ultra low wear in these PTFE composites. The wear of the composite in vacuum is comparable to the wear rates of filled PTFE systems ( Figure 10 1). This is further evidence that the tribochemical mechanism is responsible for the substantial reduction in wear in these composites. The e xperiment in which a sample was slid in the laboratory humid air environment then transitioned to a vacuum environment showed that the presence of the brown, thin transfer film is not enough to make ultra low wear. This suggests that transfer film and run ning films are dominated by a dynamic balance between formation in the transfer and running films and wear of these films.

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142 Nitrogen The wear rate slightly lower than the wear rate in vacuum. Thi s is likely because of the residual water (and oxygen) in the envir onment. This is also evident in the SEM micrograph of the transfer film ( Figure 8 4c); this micrograph shows evidence of the transfer film generated in the humid environment, yet it has very poor coverage. This could be caused by trace amounts of water in the environment and residual water adsorbed on the surface of the polymer and t he steel counter sample The presence of large fillers (1 5 m) in the composite seem to be necessary for shutting down the flakey wear of PTFE in any environment, but can be problematic if the r obust running films and transfer films are not generated In the nitrogen (and vacuum) environment, the typical thin, brown, uniform transfer films were not formed and the steel counterface was scratched. Humidity The humid environment is the only environment confirmed to promote ultra low wear behavior in these composites. This promotes the idea of the tribochemical mechanism and is consistent with the proposed mechanism involving chain scission of PTFE filaments that are then terminated by oxygen and hydroxyl ions derived from the constituents of the environment Unlike the vacuum and nitrogen environments, the humid environment facilitates formation a thin, uniform, protective transfer film when PTFE and alumina composites are slid against steel. No scratches were observed in the counterfaces in humid environme nts. This suggests the transfer film formed in the humid environment pro tects the steel and polymer which were

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143 shown to be large enough and abrasi ve enough to scratch the steel when slid in the v acuum and nitrogen environments Oxygen The effects of oxygen on the wear behavior of PTFE and alumina composites are still unclear. In the few experiments performed, wear of PTFE and alumina composites was greater when oxygen was present at 20% then when i t was present in trace amounts, <100 PPM It may still be required for generation of the ultra low wear behavior of the PTFE/ alumina composites. SEM micrographs of the transfer films suggest the running films may generate and wear slightly faster in environments with nitrogen, water and 20% oxygen then environments with nitrogen, water and trace oxygen More experiments in these environments are required to understand these differences with statistical significance. Further experiments in environments void of oxygen but with nitrogen and water could verify the importance of oxygen in the generation of the tribochemical species observed in ultra low wear composites. Ethanol Environments of nitrogen and ethanol with trace amounts of water and oxygen failed to produce ultra low wear. It was hypothesized that the ethanol, like water, could be a source of oxygen or hydroxyl ions. Instead, this experiment produced a steady state friction coefficient of 0.1, the lowest ever measured on this composite at these loads and sliding velocities T here s hould still be tribological breaking of the carbon carbon backbones during sliding O nly now, instead of end groups with hydroxyl groups there are end groups with ethoxyl. This could result in a weaker interaction between the end groups on the PTFE and the countersurface, allowing for higher wear rates and lower friction.

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144 Low e arth o rbit: r am Although complete wear and friction data is unavailable for the experiments due to some difficulties with memory card corruption, there was evidenc e of a running film and a transfer film on t he PTFE/ alumina experiments performed in the ram environment. The primary constituent of this environment is atomic oxygen, suggesting the possibility of atomic oxygen participating in a mechanism for generating ultra low wear rates. There are also differences in the Raman spectroscopy results for the running film compared to the bulk polymer. This could also suggest tribochemical alterations of the polymer in the running films. Visual inspection of the sample su ggests that they held up for the 18 months they were exposed to the harsh ram environment. Low e arth o rbit: w ake The wake samples wore differently than the ram samples. Again, with corrupt data, it is difficult to tell how many cycles were run on the ram a nd wake experiments, or even argue if the ram and wake experiments ran the same number of cycles. But, from what we can tell, the PTFE and alumina composites tested in the wake environment on board MISSE 7 produced a running film on the polymer surface, bu t no transfer film on the steel ball. This is typical with experiments of this material in vacuum. Also, there was no significant difference in the Raman spectra of this sample in and out of the wear track. If these samples were to wear as vacuum samples, then wear rates could be in comparable to the in vacuo wear test results. While this may not be ultra low wear as categorized by this research, it is still more than 10 times better than unfilled PTFE and is an excellent candidate as a solid lubricant in s pace applications. Visual inspection of this sample suggests that it held up to the wake environment better than the ram environment for the 18 months they were on orbit.

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145 Wear m echanism: r equirements of the e nvironment Experiments in vacuum and the environmental tribometer showed that the environment in which sliding occurs is extremely important to for ultra low wear PTFE composites. The environment al results suggest that water, and possibly other species, is key in pro du cing the ultra low wear behavior. This suggests there is a chemical interaction responsible for producing ultra low wear. These results show that the effects of the filler are not simply mechanical. Composite Run I n: Formation of Transfer Films and Runni ng F ilms The tribochemically generated transfer films and running films form Decreased wear rates and increased friction were observed as sliding continued and transfer and running films developed. This run in process exists in all filled PTFE composites of this study; run in is often responsible for a large percent of the total wear of the polymer, even though run in can occur in less than one percent of the total sliding. Filler dispers ion quality and volume percent can reduce this run in period, reducing the overall wear of the polymer composite. This run in period could be almost more important than the overall wear rate f or many applications, particularly if it is to be used in commer cial applications Filler c oncentration e ffect on run i n The results of testing 2, 5, and 8 weight percent alumina filler in PTFE with frequent wear measurements in the transient regime showed that an increased filler percent (in this 2 to 8 % range ) corre sponds to faster run in. This may be as simple as a critical number, volume or area of filler particle s that must accumulate at or near the sliding interface to produce ultra low wear TEM micrographs suggest an accumulation

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146 were slid on steel for one million cycles at 250 N normal load and 50.8 mm/s ( Figure 10 4). Figure 10 4 TEM of transfer film weight % fumed silica. Transfer film was generated by sliding 1,000,000 cycles on 304L stainless steel at 250 N and 50.8 mm/s. Transfer film was scrapes off in water where it was floated on to a TEM grid. Exa mple silica particles highlighted in orange and alumina in blue. TEM performed by DuPont. and 8 weight percent is shown in Figure 10 5 These simulations are for random measured by light scattering. Particle coordinates were generated in Matlab and visualized using CrystalMaker. The density of filler material to PTFE is quite surprising. This visualization makes it more obvious why it may take longer to run in a two weight percent alumina filled PTFE composite as compared with an eight weight percent

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147 alumina filled PTFE composite, just by the statistics of wearing to a critical amount of fillers. It could also help visualize a composite s ability to arrest crack propagation as a function of filler percent. This may also suggest that at very low filler concentrations, dispersion is somewhat important for the run in and wear characteristics of the composite. Figure 10 5 Visualization of alumina filler in PTFE composites. Gray spheres represent alumina filler of various sizes in the 10 m x 10 m x 10 m PTFE cube. Filler size distribution based on light scattering measurements of type Run i n and f riction The run in studies also show the strong correlation between developing the running film with an increased friction coefficient and decreased wear. Burris also pointed out that the higher friction was associated with an increase in the percent of oxygen measured in the transfer film [ 70 ] Based on my experimental results, I

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148 associate this incr ease in friction with an increase in carboxylic acid and carboxylates terminating PTFE chains. These end groups chelate to the countersurface and filler particles and provide an overall increase in attractive forces at the interface, causing the increase i n friction coefficient. In some cases, this running film can become too thick and friction coefficients exceed 0.3. While the increase in friction is a drawback, it is greatly outweighed by the added benefit of extreme wear reduction. These carboxylic acid s form carboxylate salts by chelating to the surface of the countersample. This forms a robust, protective layer on the transfer film. Furthermore, this carboxylate appears to exist in the running film, possibly bonding to the filler material that is accum ulated at the surface, protecting th e surface of the p olymer. Running f ilm and transfer f ilm d ynamics Transfer film formation and, ultimately, the low wear behavior of this composite appear to be a dynamic process. For the best composites, transfer and run ning film s are formed quickly and reach equilibrium to where they do Running film better coverage of the running film is associated with lower wear rates and shorter run in times. These experiments also show that at low loading percent (i e 2 wt %) filler, poor dispersion can cause wear of the running films. In fact, these results seem to suggest that the filler is responsible for holding the running film to the polymer. Perhaps the distribution of larger filler particles help to keep the running film on the on the polymer, while the smaller f illers facilitate running film and transfer film formation. This could explain why many of the alumina fillers that have a primarily small particle size

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149 with no substanti al portion of larger particles, including alumina, produce a brown running film that is easily removed. There is a scenario in which the running film can become too thick; i n th ese systems the running film consisting of degraded PTFE, on the surface of the polymer can be worn away in a large wear event causing a single dramatic jump in wear and depositing a thick patch into the transfer film and exposing an unaltered portion of the composite (which appears white) This thick patch of dislodged material is usually released at the reversal, where t here is a large shearing event, which was also observed in unfilled PTFE in the optical in situ tribometer. If it is not transferred at the reversal sight, it is often pushed to either end of the stroke on reciprocating tests resulting in a thin region of transfer film in the middle of the stroke and thicker regions at the end of the stroke. Robust t ransfer f ilms The transfer film of ultr a low wear PTFE/ alumina composite on steel was shown to be inherently tough and well adhered to the countersurface by scratch tests. This is contrasted the transfer films of unfilled PTFE on steel that are easily pushed around and scratched off. While we have shown that the transfer film generated by ultra low wear PTFE/ alumina composites can be worn off in vacuum, it is still much tougher and better adhered to the countersample. This highlights the importance of a well adhered, tough transfer fi lms. Wear m ec hanism: i nteractions b etween the r un ning f ilm and the t ransfer f ilm It has long been suggested that PTFE systems transfer readily and most long term sliding of PTFE results in PTFE sliding on a PTFE transfer film. In ultra low wear PTFE/ alumi na composites, these transfer films are very well adhered to the countersurface.

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150 This is thought to be facilitated by the smaller phase filler particles that promote transfer film generation and larger filler particles that disrupt large wear events in the PTFE. Unique in the ultra low wear PTFE/ alumina, and titania and others fillers, is the formation of a well covered, robust running film. When PTFE/ alumina composites are slid against steel, they very quickly make these transfer and running films; the s liding interface is then between the transfer film and running film. The running film and transfer film greatly slow the wear process by protecting the polymer and counterface and limiting the interaction between th e polymer and the counterface. T his could decrease the chain scission events and slow the wear Tribo chemistry Formation of a n ew c hemical s pecies in the running f ilms and t ransfer f ilms XPS and IR spectroscopy shows evidence of a new chemical species in the tribofilm of ultra low wear PTFE composites. This chemical change could be simply the degradation of PTFE as it is allowed to persist for a longer time in the contact for ultra low wear composites allowing more mechanical energy to go into it. However, a more interesting possib ility is that it could be a route to the generation of robust, well adhered, protective transfer films that provide low friction and low wear operation of these systems. XPS and IR results suggest the presence of a new chemical species in the running film and transfer film of ultra low wear PTFE/ alumina composites. XPS results suggest this degradation is present in the transfer films of both unfilled PT FE and PTFE/ alumina composites. H owever, IR results suggest the chemistry is only observed in the transf er film o f the PTFE/ alumina composite. XPS also shows this degradation is present in the PTFE/ alumina running films and not the unfilled PTFE. IR results suggest that this tribochemical species could be carboxylic acid and carboxylate groups

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151 indicated by absorption at 3338 and 1660 cm 1 terminations of PTFE. Furthermore, IR absorption at 1425 and 1360 cm 1 suggests that these carboxylic acid terminations could form a metal salt with the countersurface; this could account for the robust adhesion of the tra nsfer films to the countersurface as observed by scratch tests. It is important to determine whether these new tribochemical species are simply the byproduct of ultra low wear or the cause. They could be a result of the ultra low wear material that is hea vily degraded because it is slid against for a long time. Humidity dependence suggests that this is unlikely and that the tribochemical species are at least partially responsible for the ultra low wear behavior of these PTFE composites Tribological bond b reaking In situ surface plasmon resonance results and analytical calculations based on Hamaker attraction suggest the possibility of chain scission of the carbon carbon bonds in PTFE when PTFE is slid against a metal. This is not a new concept, even in PTF E systems [ 100 101 ] Each chain scission event leaves a free radical that must be terminated. Environmental experiments suggest that when this site is termi nated by constituents from water, ultra low wear can occur. Wear m echanism: the t ribofilms and a c hemical m echanism A new tribochemical species is generated by chain scission events of the carbon carbon backbone; these sites are then terminated by constitu ents of the environment to generate carboxylate end groups These end groups chelate to the countersurface for m ing carboxylate salts with the metal countersample; these interactions enhance adhesion of the transfer film to the countersurface. This terminat ion should occur when sliding PTFE an d ultra low wear PTFE composites. H owever filled composites facilitate

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152 the accumulation of carboxylic salts, possibly by slowing the wear of the PTFE. These carboxylates are also present in the running films, where the y could serve to either replenish the transfer film or form a robust running film, anchored by micro and nanofillers. A possible chemical mechanism is shown in Figure 10 6. Figure 10 6. A proposed chemical mechanism for the formation of COOH groups in the PTFE that can chelate to the countersurface to form a robust transfer film in ultra low wear PTFE composites 1 Proposed Mechanism S ummary It seems that ultra low The filler, which consists of a large range of particle sizes, may serve the following functions: s hut down the large scale wear in PTFE through toughe ning mechanisms g ently exfoliate advantageous carbon and oxides from the countersample a ccumulate at the interface to facilitate transfer film and running film toughness a nchor the running film to the polymer and the transfer film to the countersurface f ac ilitate chain scission of PTFE with smaller phase particles by possibly: o acting as catalyst to promot e defluorination and subsequent chain scission or other reactivity o serving as an anchor to promote mechanical breaking of C C bonds 1 C.P. Junk, B.A. Krick, J.J. Ewin, G.S. Blackman and W.G. Sawyer, Unpublished PTFE and composites, (2011).

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153 o grinding PTFE to mechan ically break C C bonds f acilitate accumulation of carboxylic acid end groups b e the perfect size distribution or hardness to toughen PTFE and facilitate tribochemistry but not scratch the transfer film l arge phase may need to be friable, brittle or soft en ough to break up tribologically and be incorporated in transfer film, but still strong enough to toughen the matrix and prevent subsurface cracking and delamination wear h igh aspect ratio and irregularly shaped particles may be more successful For the ultr a low wear to be achieved, chemistry must be present that would allow carboxylate termination of PTFE upon chain scission of the PTFE This is achieved most readily by slidin g in an environment with water and possibly some amount of required oxygen This may work in other environments, such as low earth orbit, where there is atomic oxygen present that may facilitate this chemistry. It may also be achieved by including the chemistry in the composite by using hydrated fillers or PTFE that is already terminat ed by carboxylic acid. The transfer film and running film form and wear dynamically during sliding. After formation of running and transfer films, the sliding interface will continue to be between the running and transfer film until one or both is worn off These films serve to protect the countersample and the polymer. The transfer film is chelated to the countersample through carboxylate salts between carboxylate terminations of the PTFE and the metal countersamples. Interaction between the running film a nd transfer film are more attractive than the interaction between PTFE and PTFE, producing a slight increase in friction This increase in friction coefficient is outweighed by the substantial reduction in wear observed when the transfer film is slid again st the running film.

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154 Applying the Mechanism: New S uccesses Selecting Alternative F illers As shown in C hapter 5, several new fillers were discovered that produce ultra low wear in PTFE. Mineralogical fillers produced very cost effective ultra low wear compo sites. Additionally, several types of titania were discovered to produce ultra low wear. Finally, new sources of alumina were found to produce ultra low wear composites. Other Matrices Filled with Alumina and Similar F illers We also filled perfluoroalkoxy (PFA), another fluoropolymer with type 1 and type 6 alumina and achieved ultra low wear behavior 2 Wear rate of PFA was reduced from 3x10 4 to 1x10 7 mm 3 /Nm by inclu ding five weight percent alumina fillers. PFA has the added benefit of being injection moldable, so is a great alternative to PTFE composites for mass producing more complex parts. Polymer B lends The tribochemical mechanism also helped us develop another system in which PTFE is a filler material in polymer matrices 3 By including PTFE with functionalized endgroups as a filler in a tough polymer matrix, we were able to design composites with better mechanical properties and lower wear than the PTFE and type composites. By including 20 weight percent functionalized PTFE in matrices of nylon, ultra high molecular weight polyethylene, and polyether ether ketone, we have produced composites with wear rates of as low as 2x10 8 mm 3 /Nm in the laboratory environm ent. More impressively, the inclusion of the functionalized endgroups on the 2 G.S. Blackman, C.P. Junk, B.A. Krick and W.G. Sawyer, Unpublished PFA composites, (2011). 3 B.A. Krick, J.J. Ewin and W.G. Sawyer, Un published PTFE filled polymer blends, (2011).

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155 PTFE (or possibly the toughness of the matrix filler combination) allowed these composites to achieve ultra low wear (k~ 4x10 7 mm 3 /Nm) in the dry nitrogen environment.

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156 CHAPTER 11 CONCLUSIONS The inclus ion of a few percent by weight the wear rate of PTFE by more than 10,000 times. Extensive studies were performed to evaluate the ultra low wear behavior of PTFE composites to answer th e following questions: 1) What makes 2 O 3 composite system s have ultra low wear? All alumina fillers do not produce ultra low wear composites with PTFE, the shape and size distribution of the alumina is likely important in creating ultra low wear composites The environment in which PTFE a nd alumina composites operate can alter the transfer film and running film generated during sliding and ultimately reduce (or improve) the wear rate of the system by as much as three orders of magnitude New c hemical species are observed in the transfer films and running films generated during the sliding of ultra low wear composites. These are thought to be tribochemically produced and are possibly linked to a mechanism responsible for the reduction in wear of ultra low There is a dynamic balance in the formation and the wear of the protective running and transfer films 2) Can other filler materials reproduce the reduction in wear observed by this system? Fillers that produ ce ultra low wear behavior in PTFE are not limited to alumina 3) How can we use this knowledge to design better composit es with specific functionality? T he following designed composites have lower friction and lower wear than the state of the art commerciall y available polymers and polymer composi tes: o PTFE filled with various fillers (alumina, titania and low cost minerals) o Composites of other fluoropolymers such as PFA o P olymer blends that are ultra low wear and friction in many environments

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165 BIOGRAPHICAL SKETCH Brandon Alexander Krick was born in Tallahassee, Florida in 1985. He was raised in Tallahassee, where he graduated from Lawton Chiles High School in 2003. He began is studies at the University of Florida in the fall semes ter of 2003. While at the University of Florida, he raced mountain bikes with the club team, competing in four national championship events. engineering 2007, and followed that with a brief internship at Alstom Turbine Technologies. After his internship, he began his graduate studies in mechanical engineering at the University of Florida. He joined the University of Florida Tribology Laboratory, where he studied under his advisor, Professor W. Gregory S awyer. Here he worked studied many topics, including low earth space tribology, ultra low wear composites and interactions and mechanics of soft materials. He received his Doctorate of Philosophy in m echanical e ngineering in May of 2012. Brandon has goals of not only continuing research in the field of tribology, but interfacing his experience of tribology with other fields, including biology, energy and sustainability.