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Tribological Behavior of an Expanded Polytetrafluoroethylene (PTFE) and Epoxy Coating: Measurements and Life Predictions

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Title:
Tribological Behavior of an Expanded Polytetrafluoroethylene (PTFE) and Epoxy Coating: Measurements and Life Predictions
Creator:
MCCOOK, NICOLE LEE ( Author, Primary )
Copyright Date:
2008

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Coefficient of friction ( jstor )
Composite materials ( jstor )
Contour lines ( jstor )
Fatigue ( jstor )
Friction ( jstor )
Moduli of elasticity ( jstor )
Shear stress ( jstor )
Sliding ( jstor )
Steels ( jstor )
Tribometers ( jstor )

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University of Florida
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University of Florida
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Copyright Nicole Lee McCook. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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5/31/2016
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658213802 ( OCLC )

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TRIBOLOGICAL BEHAVIOR OF AN EXPANDED POLYTETRAFLUOROET HYLENE (PTFE) AND EPOXY COATING: MEASUREMENTS AND LIFE PREDICTIONS By NICOLE LEE MCCOOK A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

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Copyright 2006 by NICOLE LEE MCCOOK

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iii ACKNOWLEDGMENTS I would first like to thank my mother, Ca thy, who always knows the right thing to say and always gives me excellent guidance. I would also like to thank James, who is always there to listen and brings a to n of joy and laughter into my life. I would like to thank my si sters, Cece and Billie, a nd my bother-in-law, Kawika, who are and have always been there for me and have given me enormous amounts of support and knowledge. I would like to acknowledge financial support for this work by W.L. Gore and Associates, Inc. In particular, the many helpful discussions with Jim Hanrahan and Betty Synder. Special thanks to my advisor, Dr. Greg Sawyer who has given me great opportunities, and to my other thesis co mmittee members, Dr. Tony Schmitz and Dr. Nam Ho Kim, for their guidance and work during my graduate studies. Thanks also to the Tribology Lab, in part icular Dave Burris and Jerry Bourne, for their hard work, advice, and additions to this thesis.

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iv TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES.............................................................................................................vi LIST OF FIGURES..........................................................................................................vii ABSTRACT.......................................................................................................................xi 1 INTRODUCTION........................................................................................................1 Polytetrafluoroethylene (PTFE)....................................................................................1 Expanded Polytetrafluor oethylene (ePTFE).................................................................2 Epoxy.......................................................................................................................... ..3 Delamination of Coatings.............................................................................................5 2 MATERIALS DESCRIPTION....................................................................................7 Composite Description.................................................................................................7 Composite Variability.................................................................................................10 3 EXPERIMENTAL APPARATUSES.........................................................................14 Tribological Testing Apparatuses...............................................................................14 Pin on Disk Tribometer.......................................................................................15 Linear Reciprocating Tribometer........................................................................16 Tribological Character ization Apparatuses................................................................18 Scanning White Light Interferometer (SWLI)....................................................19 Nano-Indentation Apparatus...............................................................................21 4 EXPERIMENTAL RESULTS...................................................................................24 PTFE and Epoxy Composite Coatings.......................................................................24 Experimental Conditions.....................................................................................24 Tribological Results.............................................................................................27 Characterization of Wear Track..........................................................................29 Transfer Film Thickness Characterization..................................................................33 Experimental Conditions.....................................................................................33 Tribological Results.............................................................................................34

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v Transfer Film Analysis........................................................................................35 Effects of Variations in Coating Properties on Tribological Response......................38 Experimental Conditions.....................................................................................39 Tribological Results.............................................................................................40 Variations in density of ePTF E film and coating thickness.........................40 Variations in weight percent of epoxy.........................................................42 5 DISSCUSSION...........................................................................................................46 Experimental Conditions............................................................................................46 Measurements and Modeling......................................................................................48 Tribological Results.............................................................................................48 Life Prediction Modeling....................................................................................48 Life Predictions of ePTFE and Epoxy Coatings..................................................56 6 CONCLUSIONS........................................................................................................59 APPENDIX: CONTOUR PLOTS OF VON MISES STRESSES....................................61 LIST OF REFERENCES...................................................................................................65 BIOGRAPHICAL SKETCH.............................................................................................68

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vi LIST OF TABLES Table page 2-1. Density, and uncompressed and comp ressed coating thickness of each of the twelve coatings shown in Figure 2-3........................................................................11 2-2. Weight percent, density, and uncomp ressed and compressed coating thickness of samples used in varying of epoxy weight percent study..........................................12 4-1. Experimental test matrix (load a nd speed). The wear track diameters were varied although tests were run the same number of revolutions. Low carbon steel pins with a radius of 2.4mm were used for all tests.........................................25 4-2. Initial central contact pressures P calculated using a circular Hertzian contact model (subscript H). The subsurface location of maximum shear stress calculated using the Hertzian contact analysis is given by ..................................26 4-3. Average wear rate, 63 10 /() KxmmNm, and friction coefficient, , for experiments run under the various matrix conditions. 5 repeat experiments under the 2N load and 1 m/s sliding sp eed were run on each material. The average values for wear rate and fricti on coefficient for all repeat experiments along with the standard deviat ion is given at the bottom of each chart along with the average number of cycles, n, for each experimental series...............................28

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vii LIST OF FIGURES Figure page 1-1. Wear rate (mm3/Nm) versus weight percen t for various PTFE matrix composites..................................................................................................................2 1-2. Scanning electron micrograph of an ePTFE film. The light regions are PTFE with the nodes being the more dense re gions and the fibrils are the strands connecting the dense nodes........................................................................................3 1-3. Wear rate (mm3/Nm) versus volume percent for various epoxy matrix composites..................................................................................................................4 2-1. 3-D pictorial of the ePTFE a nd epoxy composite coating with labeled characteristics and parameters....................................................................................8 2-2. Scanning electron microscopy images of the top view of the PTFE (light gray) and epoxy (dark gray) composites. Th e predominant nodal direction of the PTFE is indicated by the white open arrowheads....................................................10 2-3. Scanning electron micros copy images of the top view............................................11 3-1. Schematic of pin on disk tribometer.........................................................................16 3-2. Schematic of linear recipr ocating pin on disk tribometer.........................................18 3-3. Schematic of a vertical sca nning white light interferometer....................................20 3-4. Typical scan of a wear track sec tion and a 2-D slice taken through the wear track..........................................................................................................................21 3-5. Capacitive transducer assembly schematic...............................................................22 4-1. Friction coefficient traces versus time for one experiment under 2 N load and 1 m/s sliding speed......................................................................................................26 4-2. Tribological results for coatings high density, low density, skived PTFE and epoxy coatings. (a) friction coefficient ve rsus wt% epoxy, (b) wear rate versus wt.% epoxy. The experimental conditions are shown in the inset of the friction coefficient graph. The raw data is given in Table 2, the error bars are calculated

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viii from the standard deviation of the 5 repeat experiments at 2N load and 1.0 m/s sliding speed.............................................................................................................28 4-3. Scanning electron microscopy of the coatings. These experiments were run under 2N normal load and 1 m/s sliding sp eed. The upper images show the wear tracks through the coating where the slidi ng direction of the ball over the surface is parallel or transverse to the dir ection of predominant nodal orientation..............29 4-4. Nanoindentation loading schematic. Left: Cartoon of the indentation process. Right: Indentation programmed load profile ( N) versus time (s)..........................30 4-5. Load ( N) versus depth (nm) for neat epox y and skived PTFE. These curves were created on the nano-indenter an d 30 indents for each material were completed to get statistics on the modul us and hardness of each constituent material.....................................................................................................................31 4-6. Nano-indentation of 625 indents placed uniformly over a 120 m by 120 m area on the initial unworn su rfaces and a region inside the wear track were performed on coatings that were run under 2N normal load and 1 m/s sliding speed. These are the same surfaces shown in Figure 4-3........................................32 4-7. Tribological results for an ePTF E/epoxy coating on a linear reciprocating tribometer. a) Volume loss (mm3) versus the normal load (N) multiplied by the sliding distance (m). The slope of the regression line is the steady state wear rate. b) friction coefficient ve rsus sliding distance (km).........................................35 4-8. The applied normal load (mN) versus the indentation depth (n m). The left group is indents on the steel counter face w ith the right group being indents on the transfer film..............................................................................................................36 4-9. A plot of the transfer film thickne sses with a representative histogram of the distribution of the thicknesses. The light gray, dashed curves represent that of the steel indents and the bl ack curves are the indent s on the transfer film..............37 4-10. A cartoon of the steel counterface, the tr ansfer film (gray), and the path of the indents taken (top) and a re presentation of the transf er film thickness versus relative track position (bottom)................................................................................38 4-11. The average friction coefficient of the co atings prior to failure versus the product of the ePTFE density and the uncompressed thickness............................................41 4-12. Wear rate (mm3/Nm) versus the product of the ePTFE density (g/cm3) and the uncompressed thickness ( m). Black circles represent coatings that were run to failure and white circles are coatings that did not fail. The points are numbered 1-12 in order of ascending coating thickness...........................................................42

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ix 4-13. Friction coefficient versus epoxy wei ght percent. Open circles represent composite coatings made with 0.688g/cm3 density ePTFE film and the closed circles represent coatings made with the 0.572g/cm3 density ePTFE film..............43 4-14. Wear rate (mm3/Nm) versus epoxy weight percen t. Open circles represent composite coatings made with 0.688g/cm3 density ePTFE film and the closed circles represent coatings made with the 0.572g/cm3 density ePTFE film..............44 4-15. Coating thickness ( m) versus wear rate (mm3/Nm). The open circles indicate coatings that did not fail under cyclic lo ading and the closed circles indicate samples that were run to failure. Such coatings were run in excess on 20 million cycles........................................................................................................................45 5-1. A table of the functions and variable s used in the model along with the units in which they are described (left) and a sche matic of the coating with the variables used in this model (right).........................................................................................49 5-2. Cartoon of the failure modes of th e composite coating shown against number of passes. The first row shows interfacia l fatigue with minimal wear, the second row is interfacial fatigue and wear comb ined and the final row is wear with minimal fatigue........................................................................................................50 5-3. Contour plots of the subsurface shear stress based on finite element analysis. The black line indicates the interface between substrate and coating. The coatings varied in thickness from 30-360 m, and a 5N load and 3GPa elastic modulus were used in the analysis...........................................................................51 5-4. Plots of experimental data and model fits for the functions used in the cumulative damage model. ( a)Finite element results for the maximum shear stress at the interface versus the coati ng thickness. (b) The wear depth versus number of cycles for and in situ (L VDT) and ex situ (SWLI) measurements, along with the model fit to this data. The open points are samples that wear not run to failure.............................................................................................................53 5-5. Initial interfacial shear stress plotted versus the experi mental cycles at failure. A curve is fit to the samples failing less than 200,000 cycles and an endurance limit is set at 13MPa. The samples accumulating less than 200,000 cycles were assumed to experience only fatigue, th e second group experiences fatigue and wear and the run-out samples only ex perienced wear and did not fail....................54 5-6. Flow chart for numerical solution fo r life prediction based on coating thickness...55 5-7. Results of the numeric al solution for coating life based on the thickness of the coating. (a) Plot of the number of cycles to failure versus coating thickness for the model fit using the cumulative damage model and the experimental data. Curves for the expected life of coatings with wear in the ab sence of fatigue and for fatigue in the absence of wear are also given. (b)Comparison of wear rates of PTFE, epoxy, and the composite coat ing along with the fatigue curve...................57

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x A-1. Contour plots of the von Mises stresses for the 30 m coatings with various loads and elastic moduli.....................................................................................................61 A-2. Contour plots of the von Mises stresses for the 100 m coatings with various loads and elastic moduli...........................................................................................62 A-3. Contour plots of the von Mises stresses for the 300 m coatings with various loads and elastic moduli...........................................................................................63 A-4. Contour plots of the von Mises stresses for the 500 m coatings with various loads and elastic moduli...........................................................................................64

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xi ABSTRACT Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science TRIBOLOGICAL BEHAVIOR OF AN EXPANDED POLYTETRAFLUOROET HYLENE (PTFE) AND EPOXY COATING: MEASUREMENTS AND LIFE PREDICTIONS By Nicole Lee McCook May 2006 Chair: W. Gregory Sawyer Major Department: Mechanic al and Aerospace Engineering A composite coating of polytetr afluoroethylene and epoxy shows 1000x improvements, when compared to its constitu ents, in wear resistan ce and reduced friction coefficient under testing on a pin-on-disk tribometer. This coating is made by impregnating an expanded PTFE film with epoxy, which provides three unique functions: (1) the epoxy compartmentalizes the PTFE nodes, which is believed to reduce the wear of the PTFE, (2) the epoxy increases the mechani cal properties such as elastic modulus and hardness, and (3) the epoxy provides a rea dy interface to bond the films onto a wide variety of substrates easily and securely. To find an optimal makeup of the composite coatings several series of tribological e xperiments were conducted varying the PTFE density, coating thicknesses and epoxy wei ght percent. This study reports on the tribological results of these experiments. Sk ived PTFE films had wear rates on the order of K=10-3 mm3/(Nm) and friction coefficients around =0.2. The composite coatings had

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xii wear rates ranging from K=10-3 mm3/(Nm) to 10-8 mm3/(Nm) and friction coefficients ranging from =0.1-0.16. The neat epoxy films show ed significant scatter in the tribological measurements with wear-rates on the order of K=10-4 mm3/(Nm) and friction coefficients around =0.40. Nano-indentation mapping of the coatings and the transfer films suggests that the enhanced tribological be havior of these composites is believed to stem from the coatings’ ability to draw thin PTFE transfer films into the contact from the nodes of PTFE, which act like reservoirs. Solid lubricating coatings comprise a la rge segment of tribological materials and under a spherical contact the coa ting/substrate interface experien ces cyclic shear stress in the presence of wear, which can lead to delamination of the coatings from the substrate and premature failure. Using stress results from a finite element simulation of the static contact and experimental wear results; a numer ical analysis of the coupled failure modes was completed. This analysis provides a procedure for life predictions for a solid lubricating coating susceptible to fatigue failu res in concentrated ci rcular contacts based on thickness of the coating. The results of the modeling offer insight to designers on considerations in extending th e lives of coatings subject to cyclic stresses. The experimental data for cycles to failure vers us coating thickness fall closely to the fit for the cumulative damage model. This result al so shows that the coati ngs experience failure caused mostly by fatigue at the interface, wh ich greatly decreases their expected life when compared to only experiencing wear.

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1 CHAPTER 1 INTRODUCTION This work reports on the tribological pr operties and life pred ictions of a solid lubricating PTFE and epoxy composite that utilizes expanded PTFE coatings as the scaffolding and epoxy as reinforcement. This chapter discusses the background and review of literature for solid lubricati ng polymer composites consisting of the two constituents, PTFE and epoxy. A review of lite rature of the delamination of coatings is also discussed in this chapter. Polytetrafluoroethylene (PTFE) There is great enthusiasm for wear-resistan t, inert, environmentally insensitive, low friction, polymeric tribological coatings. Po lytetrafluoroethylene is an exceptional low friction polymeric material that is used in ma ny bearing applications as a solid lubricant. Unfortunately, PTFE suffers from poor wear re sistance, and is thus the subject of many tribological research projects in the area of composites [1]. Such research often incorporates hard filler particle s into the PTFE matrix such as glass fibers [2], copper [3], ceramics [4,5], carbon fibers [6], chopped carbon fi bers [7] and nano particles [8, 9] in an effort to enhance the wear resistance. Othe r research groups incorporate fillers that may act as additional solid lubricants such as bronze [10, 11], graphite [3], carbon nanotubes [12], molybdenum and tungsten disulfide [13], lead [14-16] , and boric-oxide [17] to improve wear resistance while retaining low friction coefficients. PTFE is also frequently used as filler in polymeric mate rials that have good m echanical properties but poor tribological properties, such as polyoxymethylene [18, 19], epoxy [20],

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2 polyetheretherketone [21], and polyimide [22] . Figure 1-1 shows an overview of weight percent versus wear rate of PTFE filled composites. The general trend of this data is that unfilled PTFE has very poor wear resistance in the 10-3 mm3/Nm range and with 1-30 weight percent filler up to a three orders of magnitude impr ovement in the wear rate is observed. In most of the give n examples wear rates reach 10-4-10-5 mm3/Nm with 12wt% filler and do not decrease drastically with the addition of larger weight percents of particles. Figure 1-1. Wear rate (mm3/Nm) versus weight percent for various PTFE matrix composites. Expanded Polytetrafluoroethylene (ePTFE) More specifically, the coatings in this work consist of expanded PTFE (ePTFE) as the matrix material which has unique propert ies when compared to fully dense skived PTFE. Expanded PTFE is porous with nodes of dense PTFE being interconnected to other nodes through a network of PTFE fibrils . Expanded PTFE films can have a wide range of engineered node shapes and sp acing, with porosities ranging from 5 to 90 percent. Additionally this node and fibril structure of expanded PTFE films provides increased strength to weight ratio and creep resistance as compared to fully dense PTFE films (2,200 grams/mm3). Expanded PTFE has previously been used in such

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3 applications as: breathable fabric, implantabl e medical devices, electronic cables and wire coatings, and gasket and filtration devices. To the author’s knowledge, this is the first use of ePTFE in a tribological application. A scanning electron micrograph of an ePTFE film is shown Figure 1-3. Figure 1-2. Scanning electron micrograph of an ePTFE film. The light regions are PTFE with the nodes being the more de nse regions and the fibrils are the strands connecting the dense nodes. Epoxy Recently, there have appeared publications looking at the tribological worthiness of nanocomposites made with epoxies, which in their neat form are a notoriously poor tribological performers. Silica [23], alumina [24], and silicon nitride [25] nano particles have all been incorporated into epoxie s with most additions improving both the coefficient of friction and wear resistance. There is also work that has incorporated fluorinated polyaryletherketone into epoxy to create a polymer/epoxy blend that, in the main, shows roughly 20% reductions in fricti on coefficient [26]. Recently, nanoscale

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4 fillers such as, TiO2 [27], TiO2 blends with PTFE and graphite [28], and SiO2 [29] have also been added to epoxy to increase wear resistance and lower friction coefficient in epoxy matrix composites. Chang et al. used nano-TiO2, PTFE, short carbon fiber, and graphite as fillers in an epoxy matrix, and wa s able to decrease the friction coefficient to =0.35 and decrease the wear rate 10x with 10 volume percent nano-TiO2 [27]. Shi et al. saw a decrease in friction coefficient, from =0.58 to =0.35, and 10x decrease in wear resistance with the addition of up to 2.0 volume percent of nanoscale Al2O3 to epoxy [24], and Wetzel et al. saw a 2x decrease in wear resistan ce with the addition of small volume percents of nanoAl2O3 [30]. Shi et al. also saw decreases in friction coefficient, from =0.7 to =0.32 with the addition of 2.25 volume percent of nano-Si3N4 [25]. Figure 1-2 shows the effect of the addition of filler s into an epoxy matrix. A decrease in wear rate is also observed in the epoxy matrix composites although not as significant as for the PTFE matrix composite s. The wear rates for the neat epoxy as shown below are in the low 10-4 mm3/Nm and high 10-5 mm3/Nm range and with the addition of up to 30 volume percent filler there is up to one orde r of magnitude improvement in wear rate. Figure 1-3. Wear rate (mm3/Nm) versus volume percent for various epoxy matrix composites.

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5 Delamination of Coatings Coatings comprise a large segment of tribological materials. The tribological properties of coatings are typically tested on pin-on-disk tribometers. Often, engineers are concerned with the life and/or abrasive we ar resistance of a coating. These machines have the ability to accumulate large numb ers of sliding cycles quickly with high precision, high rpm spindles, and they have th e ability to generate high contact stresses with light loads from a spherical pin. An e fficient test protocol is often employed to find the wear rate and coating life with one test. The coating is tested until failure occurs, and wear volume is calculated us ing coating thickness and pin ge ometry. This is a robust method for both assuming wear is the only mech anism of material removal. However, under a ball contact the coating/s ubstrate interface is usually the weak link and the cyclic shear stress at the interface can lead to delamination. The delamination theory of wear, which wa s described in detail by Nam Suh, can be applied to cyclic fatigue and crack gr owth problems for metals and polymers alike [31]. According to Suh, delami nation wear occurs in five step s. Briefly, the five steps are as follows: 1) when two sliding surfaces come into contact, asperities on the softer surface are deformed and the contact becomes an asperity-plane contact and each point on the softer surface experiences cyclic loading, 2) the aspe rities of the harder surface induce plastic shear deformation which accumu lates under cyclic loading, 3) subsurface deformation continues and cracks begin to nuc leate at some depth under the surface, 4) further cyclic loading causes cracks to exte nd, propagate, and join with other neighboring cracks, and 5) these cracks shear to the surface and wear debris will delaminate from the bulk material [32].

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6 The delamination theory can also be a pplied to coatings in a pin-on-disk experiment. The coatings experience a cycl ic shear stress at some distance under the contact which leads to crack propagation and delamination. This depth of this shear stress and crack nucleation is defined by the material properties and the contact geometry of the system in a concentrated circular contact. This fatigue induced delamination process proves to drastically shorten the expected life of the coatings, because in this configuration the depth of cr ack nucleation and propagation t ypically falls close to the interface between the coating a nd the substrate. Because of this the coatings tend to prematurely fail by crack coalescence and de lamination close to the bonded interface as opposed to only wearing through to the substrate.

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7 CHAPTER 2 MATERIALS DESCRIPTION The composite created in this study is an expanded PTFE and epoxy coating. This composite, which is processed as a thin coat ing, can then be relatively easily bonded to various substrates via the epoxy filler. Unlike most of the other PTFE composites, bearing components can be coated with th e composite coating as opposed to being machined from a compression molded composite billet. The expanded PTFE and epoxy network work synergistically creating a low friction and high wear resistance composite. This chapter describes the composites’ material parameters, characteristics, and variability. Composite Description The composite coatings had a wide range of tunability in thickness, density of the ePTFE film (0.304-0.904 g/cm3), and weight percents of ep oxy (19-53%). The thickness of the coating refers to the uncompresse d or post compression thickness of the ePTFE film with the epoxy combined, i. e. the composite coating. Th e density of the ePTFE film refers to the film prior to the addition of epoxy; it includes the dense PTFE nodes, fibrils and air in-between. The epoxy weight percen t is the amount of epoxy filler added to the ePTFE film. When referring to the ePTFE without epoxy the term film will be used and when referring to the ePTFE filled with epoxy, the term composite coating will be used. The expanded PTFE films were combined with an uncrosslinked epoxy and were then cured under pressure onto a carbon steel circular disk that had an av erage roughness of Ra=2.0 m. The weight percentages did not change under consolidation and curing,

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8 and porosity was not observed. All of the vari ations and properties of the coating are best described in a cartoon of the composite coatings shown in Figure 2-1. Figure 2-1. 3-D pictorial of the ePTF E and epoxy composite coating with labeled characteristics and parameters. Figure 2-1 shows a pictorial of the composite coating bonded to a steel substrate. The substrate is shown in gray, the PTFE pha se is shown in white and the epoxy phase in yellow. Several parameters and characteristics of the composite are labeled on the cartoon. Starting on the left side of the cartoon and continuing clockwise around each characteristic will be described in detail. The characteristic epoxy width describes the length of epoxy between neighboring nodes of PTFE. This is important because the epoxy is the high friction phase, as well as the reinforcing region of the composite. If this characteristic epoxy width is too large, hi gh friction and poor wear resistance will occur and if it is too small there may not be e nough epoxy to provide proper reinforcement of the PTFE. The next characteristic is the PTFE running film. These very thin films are

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9 drawn out of the PTFE nodes under sliding a nd will span some distance past the node partially covering the epoxy regions. These film s are believed to be the main factor in the low friction and high wear resistance resu lt. The next two characteristics are the PTFE node width and spacing. These are important becau se the PTFE nodes act as reservoirs for the lubricating pha se of the material. If the PTFE nodes are too small or far apart, then there will not be enough of the lo w friction phase to adequately span the entire epoxy region. The coating thickness and elastic modulus are also critical parameters of the coatings. The dependence on the thickness of the coatings will be described in further detail in the following chapters . The expanded portion of th e cartoon shows a view of the interface between the epoxy and PTFE nodes, wh ich increases the wear resistance by retarding crack propagation. The interconnecting PTFE fibrils are also in integral to this process. The subsurface cracks are due to the shear stre sses caused under sliding and will vary in depth based on contact geometry. This is where cracks initiate and coalesce causing wear or delamination. Th e last characteristic is the surface area fraction of PTFE at the interface (not labele d). Because these coatings are bonded to the substrate via the epoxy, if the PTFE su rface area is too large there will be a poor bond at the interface which causes premature failure and delamination. Scanning electron microscopy (SEM) images of representative resulting coatings are shown in Figure 2-2. The SEM images ar e top views of two coatings with different PTFE node density and differe nt weight percents of epoxy. These coatings were approximately 200 m in thickness with initial densities of 400 grams/mm3 (low density) and 950 grams/mm3 (high density), which co rresponds to a PTFE volume fraction of 0.18 and 0.43, respectively, and re sulted in a PTFE weight percent of 70% for

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10 the high density coating and 50% for the lo w density coating. The PTFE nodes are represented by the light gray regions and th e epoxy is represented by the dark gray regions. The PTFE fibrils interconnect th e nodes, and run though and across the epoxy regions. The SEM also shows a predominant no dal orientation which is believed to have an effect of the tribologica l properties of the coating. Figure 2-2. Scanning electron microscopy imag es of the top view of the PTFE (light gray) and epoxy (dark gray) composites. The predominant nodal direction of the PTFE is indicated by the white open arrowheads Composite Variability As stated previously, the composite th ickness, density of the ePTFE film, and weight percent of epoxy can be altered allowing for the coatings to be tuned for a specific applications. The following figure (Figure 23) shows scanning electron microscopy of top views of twelve coatings prepared for th is study with varying thicknesses and density of the PTFE film. The epoxy weight percent was 33% for each of the twelve coatings. The coatings are numbered from 1-12 in or der of ascending coating thickness. The uncompressed coating thickness ranges from 85 510 m which corresponds to 30360 m compressed coating thickness. Tabl e 2-1 lists the density and thickness

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11 (uncompressed and compressed) for each of th e twelve coatings. These twelve coatings were used to determine the effects of the total amount of PTFE av ailable on the friction and wear properties. Figure 2-3. Scanning electron micros copy images of the top view Table 2-1. Density, and uncompressed and co mpressed coating thic kness of each of the twelve coatings shown in Figure 2-3. Sample (g/cm3) ti( m) tf( m) 1 0.41 75 32 2 0.688 85 64 3 0.304 150 36 4 0.336 160 43 5 0.394 175 64 6 0.467 180 73 7 0.572 185 76 8 0.765 195 137 9 0.427 455 191

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12 Table 2-1. Continued. Sample (g/cm3) ti( m) tf( m) 10 0.495 455 203 11 0.625 465 254 12 0.904 510 343 A subset of these coatings was used in or der to test the effect of variations in weight percent of epoxy of the tribological properties. Two ePTFE films were chosen with different densities. Fo r each density of ePTFE film, the only assumed variation in the composite coating was epoxy weight percent. The purpose of this set of experiments was to determine the effects of epoxy weight percent of the tribologi cal properties of the coating. By setting the initial film th ickness, node spacing, and node width constant, i.e. keeping the density ePTFE film the same; the amount of PTFE availabl e is therefore also constant. With PTFE availability constant only variations in the affects of the compliance due to the amount of epoxy in the system should be observed. The same density ePTFE films as Sample 2 ( PTFE=0.688g/cm3) and Sample 7 ( PTFE=0.572g/cm3), shown in Figure 2-3, were used to create co mposites of different epoxy weight percents. For the 0.688g/cm3, three coatings were made with va rying loadings of 20, 33, 53 weight percent epoxy. The loading for the 0.572g/cm3density film were 19, 35, and 47 weight percent epoxy. Table 2-2 summarizes the variati ons (weight percent, density, uncompressed and compressed coating thickness) of the coatings used in the varying of epoxy weight percent study. Table 2-2. Weight percent, density, and uncompressed and compressed coating thickness of samples used in varying of epoxy weight percent study. sample wt% (g/cm3) ti( m) tf( m) 20.3 0.688 85 48 53 0.688 85 90 2 33 0.688 85 52

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13 Table 2-2. Continued sample wt% (g/cm3) ti( m) tf( m) 19.5 0.572 185 75 35 0.572 185 88 7 47 0.572 185 136

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14 CHAPTER 3 EXPERIMENTAL APPARATUSES Tribology is the study of inte racting surfaces in relati ve motion. The friction coefficient and wear rate are two of the measures of the tribological performance of a system that aim to explore the fundamental s of tribological systems. A tribological system is composed of three basic elements, (1) the structure-the types of materials in contact, the contact geometry and the filler particle dispersion, (2) the operating conditions-the gross motion, loads, stress, a nd duration, (3) the e nvironment and surface conditions-the surface chemical environment, topography, and ambient temperature. The friction coefficient, , is defined as the ratio of the normal force to the frictional force ( =Ff/Fn) when two bodies are in relativ e motion. The wear rate is defined as the volume of material removed fo r some energy input to a system. There are four basic modes of wear: adhesive, ab rasive, surface fatigue, and tribochemical. This chapter discusses the experimental apparatuses used to characterize the tribological properties of the expanded PTFE and epoxy coatings used in this study. Tribological Testing Apparatuses Tribological properties of materials are characterized on a variety of machines (tribometers) with a wide range of techniques. In tribology, it would be ideal to create a friction and wear test that exactly mimics the geometries, loads, and kinematics of the full scale application. It is impr actical to construct an apparatus for every application to meet the ideal criteria of such applications. Typical machines used to test the friction, wear, and other tribological characterization include: linear reci procating pin-on-flat, rotating

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15 pin-on-disk, block-on-ring, si mple thrust washer, simple bushing and crossed cylinder tribometers. Primarily for this study the pi n-on-disk and linear reci procating tribometers were used in characterizing fric tion coefficient, wear rates, an d the life of the coatings. A scanning white light interferom eter was used post test to examine wear tracks and a nanoindenter was utilized to determine wear track constituents and transfer film thicknesses. Pin on Disk Tribometer As stated previously, the tribological prope rties of coatings are typically tested on pin-on-disk tribometers. Thes e machines have the ability to accumulate large numbers of sliding cycles quickly with high precision, hi gh speed spindles, and they have the ability to generate high contact stress es with light loads from a spherical pin. The ePTFE and epoxy coatings in this study have a specific nodal orientation in which the lubricious phase dominates, shown in Figure 2-2. Us ing a rotating pin-on-disk tribometer, all possible orientations of the coating can be te sted and averaged at the same time in order to gain insight about the differences in tr ibological properties fo r all nodal directions. Figure 3-1 shows a schematic of the pin-ondisk tribometer used in this study, along with its features and expe rimental capabilities. The tr ibometer uses dead weight normal loading which is applied directly abov e the contact region. Friction forces are recorded through a load cell th at is located behind a lowfriction gimbal. Concentric circular wear tracks are positioned on the disk surface using a micrometer and stage. Wear tracks diameters can range from 0-35 mm. The stage also has vertical adjustment and houses the load cell, moment arm, and pi n assembly. The moment arm is held in a low friction gimbal with Y-axis and Z-axis ro tations and has a counter weight at the end for balance. The tribometer also consists of a high speed spindle which is capable of 30,000 rpm. The sample is directly mounted to the high speed spindle and can be up to

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16 50mm in diameter. The top of the sample hol der has a precision machined flat edge and when the top is compressed onto the sample, which sits atop a wave spring, the sample lays flush with this precision edge creating a nearly perfect level surface. This method of sample mounting is used to reduce vibrations which can lead to misleading friction coefficient data. The pin is typically a sphere of radius 2.4-3.175 mm. The tribometer is equipped with a linear variable displacem ent transducer, LVDT, (not shown) which measures in situ changes in wear depth. The friction force and LVDT data are recorded every second through a data acquisiti on system and a personal computer. Figure 3-1. Schematic of pin on disk tribometer. Linear Reciprocating Tribometer The linear reciprocating pi n-on-disk tribometer is commonly used in testing tribological properties of bulk polymers and polymer composites at slower speeds and lower contact pressures than the rotating pinon-disk tribometer. Th e contact region is a

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17 prescribed area with a flat on flat contact. This creates a situation where the contact area is not changing throughout a te st. In this study the linear reciprocating tribometer was used to create a transfer film on a flat count erface as opposed to a spherical pin. This allows for better characterization of the transf er film, because it is larger and planar as compared to the spherical pin. The linear reciprocating tribometer also allows for prescribed orientations of the coating to be tested to gain information on the differences in tribological properties sepa rately as opposed to averagi ng the orientation affects like with the rotating pin on disk tribometer. The tribometer shown schematically in Fi gure 3-2 creates a reciprocating sliding contact between the two surfaces of interest. This tribometer is located inside a softwalled clean room with conditioned air that has a relative humidity between 25-50%. A four-shaft pneumatic thruster creates the lo ading conditions of th e contact using a 61.2 mm bore pneumatic cylinder. The cylinder is no minally protected from transverse loads by four 12 mm diameter steel rods. An electro-p neumatic pressure regulator controls the force produced by the thruster. The pneumatic pressure output is controlled using a variable voltage input in combination with an active control lo op within the electropneumatic system. A linear positioning table is used to create the reciprocating motion between the stationary pin and counterface. The positioning system is composed of a table, ball screw, and stepper motor. S liding speeds up to 152 mm per second are possible. The force created by the thruster a nd friction force genera ted by the contact is monitored using a six-channel force transduc er. This load cell, which is mounted under the thruster, monitors forces created in the X, Y and Z-axes as well as the moments about these axes and reacts all the forces and moments on the pin. The transducer output

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18 voltages are recorded using a data acquisition system (500 Hz sampling rate) and personal computer. This tribom eter and an uncertainty analys is of the friction coefficient and wear rate are discussed in detail by Schmitz et al. [34, 35]. Figure 3-2. Schematic of linear r eciprocating pin on disk tribometer. Tribological Characterization Apparatuses The reduction in wear rate and friction coefficient occurs only when the balance between the rate at which lubr icant is being provided to th e surface versus the rate at which lubricant is being ejected from the c ontact is favorable. The balance between the lubricant supply/removal and the lubricous nature of the fillers are strongly affected by, volume fraction of filler, normal load, sliding speed, relative humidity, and environment. This balance is largely related to the form ation of a transfer film on the counterface material. In order to achieve the best balance, a thin, uniform layer of the lubricous filler should cover the interface. Tr ansfer layers in the wear track and on the counterface can be analyzed using a variety of techniques including atomic force microscopy, scanning

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19 white light interferometry (SWLI), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and nanoindentation. This study uses scanning whiter light interferometry and nano-indenta tion techniques to analyze the post test properties of the composites and the resulting transfer films in the wear track and on the counterface. Scanning White Light Interferometer (SWLI) White light interferometry is based on detecting the coherence peak created when two polychromatic wave interfere. Using wh ite light the surface on the sample is brought into focus. The white light travels through a series of lenses that collect and make the light parallel. Once the white light is made parallel, it is divided using a beam splitter in which a portion of the light is directed to wards the sample and the other portion is directed to a reference surface. The beam s that are reflected are then recombined forming an interferogram which is projected onto the CCD camera and analyzed by software. The intensity of the fringes is Gaussian and at a maximum at the focal plane. The system is translated vertically along the optic al axis bringing peaks and valleys into the focal plane. The heights of features relati ve to each other are determined based on the intervals of the intensity frames. The most probable intensity can be solved for each point and a map of the intensity of every point re lative to each other ca n be created. This map is representative of the sample surf ace showing differences in height along the surface. The desired magnification is se t by choosing a Mirau or Michelson type interferometer. Michelson interferometers typically have a range of 2-5x magnification where as a Mirau interferometer would be used for higher magnification from 10-100x. Figure 3-3 shows a schematic of a typical ve rtical scanning white light interferometer.

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20 Figure 3-3. Schematic of a vertical scanning white light interferometer. For this study the SWLI was used to determine the profile of the wear track following the experimental testing to gain in formation about the worn volume. Figure 34 shows a typical scan created using a SWLI. The bottom picture shows a 3-D representation of a wear track. A 2-D sli ce is taken through this data normal to the direction of sliding and from this data the wear track width and wear track depth can be determined. The 2-D data can also be analyzed on a PC to determine the total crosssectional area. This area is then used to determine the worn volume and therefore the wear rate.

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21 Figure 3-4. Typical scan of a wear track section and a 2-D slice taken through the wear track. Nano-Indentation Apparatus The nano-indenter was used in this study to determine the mechanical properties of the coatings preand post-expe riment. Using this apparatus, the reduced elastic modulus and hardness can be determined for the coatings. Mapping of small regions of the composite was also completed to show the compartmentalized nature of the material. Another use for the nano-indenter in this study is measuring transfer film thicknesses on the counterface. A schematic of the nano-indenter lateral transducer assembly and piezo group is shown in Figure 3-5 and described below. Loads and displacements are measured using a capacitive transducer. The capacitive transduc er assembly contains two parallel fixed plates with a parallel center pl ate supported by springs. An in denter tip is mounted to the center plate. To apply a load, a DC potential is applied between the lower plate and the center plate. An electrostatic attraction displaces the center pl ate towards the bottom

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22 plate. Based on the spring constant of the support springs, the voltage can be calibrated to a force. For displacement measurements, an AC signal is applied between the center plate and top plate, and an AC signal of equal magnitude 180 out of phase is applied to the lower plate and the center plate. When th e center plate is equidistant from the top and bottom plates, the net signal is zero. When the center plate is di splaced a voltage is recorded and calibrated to a distance. A similar transduc er is mounted to provide lateral displacements. Figure 3-5. Capacitive tran sducer assembly schematic The capacitive transducer assembly is mount ed to a three axis piezo tube scanner similar to those found in atomic force micr oscopes (AFM). The piezo tube scanner allows for precise tip placement and can be us ed in conjunction with the transducer as a scanning probe microscope. Using the tube scan ner to raster the indenter tip in contact with a surface, the trans ducer can collect z displacem ents and produce an AFM-like image. The tube scanner is mounted to a z-axis stepper motor which provides coarse translations for tip approach. An optical CCD microscope is also attached to the z-axis

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23 stepper motor. The optical microscope a llows for accurate sample location and test placement. The sample stage is attached to x-y stepper motors for translation. All systems are computer controlled. The equipm ent frame also contains an active vibration isolation device. The work ing components of the nano-in denter are enclosed in a thermal-acoustical isolation chamber with provisions for environmental control. Figure 3-6. Enlarged view of nano-indenter components.

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24 CHAPTER 4 EXPERIMENTAL RESULTS To find an optimal makeup of the composite coatings several seri es of tribological test matrices were created varying specifi c parameters and char acteristics of the composite. This chapter reports on the tribological results from each of these matrices. The matrices include: (1) initial high and low density composites, shown in Figure 2-2, versus the composite constituents, PTFE and epoxy, (2) variations of thickness and density of the ePTFE film, and (3) variations in epoxy weight percent. This chapter also includes nano-mechanical testing of the transf er film on the counterface and in the wear track. PTFE and Epoxy Composite Coatings The initial set of experiments was created to test the generic tribological properties of a new ePTFE and epoxy coating versus its constituents, ePTFE and epoxy; and hypothesize on the wear and friction mechan isms in a networked coating. Nanomechanical testing was also completed on the coatings inside and out side the wear track to determine the changes in coating properties before and after sliding. Experimental Conditions Experiments were run on a rotating pin-on-disk tribometer that is located in a softwalled cleanroom, within a conditioned labor atory environment. The relative humidity varied between 25% 50% during these tests. The experimental c onditions are shown in Table 4-1. Briefly, the matrix was a 2x2 in sliding speed (0. 25 m/s & 2.5 m/s) and normal load (1N & 3N) with a cen ter point (1 m/s & 2 N) that was repeated 5 times. The

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25 pin was a 2.4 mm radius low car bon steel sphere that was held stationary at the end of the loading arm. A schematic of the experimental apparatus is shown in Figure 3-1. During testing the normal load and sliding speed we re prescribed but the wear track diameters and the rotating speed of the disk were varied. Table 4-1. Experimental test matrix (load and speed). The wear track diameters were varied although tests were run the sa me number of revolutions. Low carbon steel pins with a radius of 2.4mm were used for all tests. 0.25 / Vms 2.5 / Vms 1nFN 2nFN 1.0 / Vms 3nFN Four different coatings were evaluated: skived PTFE (fully dense), high density composite, low density composite, and an unfilled epoxy. The top views of the high and low density coatings used in this matrix are shown in Figure 2-2. Because the normal load was controlled during this testing and each coating has a different elastic modulus the initial contact pressures varied from sample to sample. Table 4-2 provides estimates of the peak contact pressures fo r each of these coatings under the 3 different loads tested. The calculations used a Hertzian contact anal ysis for all the coatings, and additionally used an elastic foundation model for the hi gh and low density coatings, which were significantly thinner than the fully dense PTFE film and the epoxy sample. The depth of maximum subsurface shear is also indicated in Ta ble 4-2, and in all cases is less than this coating thickness.

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26 Table 4-2. Initial cen tral contact pressures P calculated using a circular Hertzian contact model (subscript H). The subsurface location of maximum shear stress calculated using the Hertzian contact analysis is given by skived PTFE 1.3 0.40 EGPa high density 2.9 0.40 EGPa low density 4.0 0.40 EGPa epoxy 4.6 0.40 EGPa 1nFN 43HPMPa 51 m 73HPMPa 39 m 91HPMPa 35 m 99HPMPa 33 m 2nFN 54HPMPa 64 m 92HPMPa 49 m 115HPMPa 44 m 129HPMPa 42 m 3nFN 62HPMPa 73 m 105HPMPa 56 m 131HPMPa 50 m 148HPMPa 48 m Friction coefficient was calculated a nd recorded during testing through a computer data acquisition system. Each sec ond an average value of the coefficient of friction along the wear track wa s calculated and recorded. A plot of the coefficient of friction versus time is shown in Figure 4-1 for the 4 different coatings evaluated. Figure 4-1. Friction coefficient traces versus time for one experiment under 2 N load and 1 m/s sliding speed.

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27 Wear tracks were examined post-test under optical microscopy and a scanning white-light interferometer. Wear volumes were calculated by measuring 4 locations along the wear track and numerically integrat ing the interferometry data to obtain the cross-sectional area. The average cross-sectional area was then multiplied by the circumference to obtain an estimate of the vol ume of the wear track. Because the wear track geometry includes both wear and creep damage gravimetric (mass loss) analysis was also used as a check for the volume calcula tion. Only in the case of the skived PTFE was creep significant and wear rate calculations used the gravimetric analysis for this case. Mass loss could not be resolved due to water uptake from the epoxy in the other coatings, and creep was negligible. This wa s also confirmed by applying a dead weight load of 3N on the pin sample resting agains t each coating at a sta tionary location for 8 hours: there were no dete ctible indentations. Tribological Results The experimental results are given in Table 4-3. The fr iction coefficients and the wear rates are plotted versus weight percent of epoxy in Figure 4-2. It is readily apparent that these coatings do not operate via a rule-of-mixtures, as both the wear rate and friction coefficient are lower than either of the cons tituents alone. The origin of the reduced friction is believed to be related to the lo w shear strength PTFE films being drawn out from nodes into a transfer film that covers the epoxy. Because th e elastic modulus of these films is increased by the addition of th e epoxy, the contact areas are likely reduced as compared to the full density PTFE.

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28 Figure 4-2. Tribological re sults for coatings high density, low density, skived PTFE and epoxy coatings. (a) friction coefficient versus wt% epoxy, (b) wear rate versus wt.% epoxy. The experimental conditions are shown in the inset of the friction coefficient graph. The raw data is given in Table 2, the error bars are calculated from the standard deviation of the 5 repeat experiments at 2N load and 1.0 m/s sliding speed. Table 4-3. Average wear rate, 6310 /()KxmmNm, and friction coefficient, , for experiments run under the various matrix conditions. 5 repeat experiments under the 2N load and 1 m/s sliding sp eed were run on each material. The average values for wear rate and fricti on coefficient for all repeat experiments along with the standard deviation is gi ven at the bottom of each chart along with the average number of cycles, n, for each experimental series. skived PTFE epoxy 0.25 / Vms 2.5 / Vms 0.25 / Vms 2.5 / Vms 1nFN 1,860 K 0.21 1,240 K 0.25 1nFN 120 K 0.128 179 K 0.664 3nFN 769 K 0.20 1,540 K 0.24 3nFN 101 K 0.479 218 K 0.401 988 K 108 ; 0.224 0.013 32,250 n 246 K 356 ; 0.408 0.098 92,500 n high density coating low density caoting 0.25 / Vms 2.5 / Vms 0.25 / Vms 2.5 / Vms 1nFN 1.75 K 0.161 7.11 K 0.167 1nFN 2.63 K 0.137 8.73 K 0.132 3nFN 6.10 K 0.163 2.30 K 0.154 3nFN 3.76 K 0.129 35.2 K 0.125 5.30 K 1.61 ; 0.167 0.004 100,000 n 4.55 K 1.76 ; 0.133 0.011 95,000 n

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29 To examine such a hypothesis scanning elec tron microscopy of the wear tracks was performed. Scans where the slid ing direction is parallel and tr ansverse to the direction of nodal orientation were taken from the same w ear track and is shown in Figure 4-3. Figure 4-3. Scanning electron microscopy of th e coatings. These experiments were run under 2N normal load and 1 m/s slidi ng speed. The upper images show the wear tracks through the coating where the sliding direction of the ball over the surface is parallel or transverse to the direction of predominant nodal orientation. Characterization of Wear Track Nano-mechanical testing was done on both th e unworn surfaces and on the transfer films within the wear tracks using a depth sensing inde ntation technique on the nanoindenter discussed previously and shown in Figure 3-3. The hardness for such tests is defined as the ratio of the maximum load di vided by the projected co ntact area (units of pressure). The depth sensing technique uses a calculated area based on the depth of indentation, where the tip area function was ge nerated by curve fitting a plot of residual projected area versus contact depth from indentions on a fused quartz standard with a manufacturer reported modulus of 72 GPa and Po isson’s ratio of 0.070. For this study a Berkovich diamond indenter with a total incl uded angle of 142.3, a half angle of 65.3,

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30 and a tip radius of 100 – 200 nm was used. Unloading force displacement curves were analyzed to determine reduced modulus and hardness using a method first described by Doerner and Nix [35] and later refined by O liver and Pharr [36]. The programmed load profile is shown in Figure 4-4. Briefly, a 400 N load is applied, then it is held for 2.5 seconds while the depth is monitored, and then the diamond is unloaded and the indentation image is taken. Figure 4-4. Nanoindenta tion loading schematic. Left: Cartoon of the indentation process. Right: Indentation programmed load profile ( N) versus time (s). Initially indents were taken on a skived PTFE coating and a neat epoxy puck to get an accurate load versus depth curve for each of the coatings constituents. For the skived PTFE and the epoxy samples 30 indents were pe rformed to gather statistics about the parent materials. These curves are shown in Figure 4-5. Once it was determined that the constituents could be easily identified from each other, automated indentations over a grid of 25 x 25 points with indention spacing of 5 m were run on original surfaces and inside wear tracks. For these experiments a load of 0.5mN was applied at 0.1mN/s, held for 5 seconds, and then unloaded at 0.1mN/s.

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31 Figure 4-5. Load ( N) versus depth (nm) for neat epoxy and skived PTFE. These curves were created on the nano-indent er and 30 indents for each material were completed to get statistics on the modulus and hardness of each constituent material. In Figure 4-6, maps of the hardness and m odulus (from the same samples as shown in Figure 4-3) are given. In these maps a proj ected contour plot is the floor and the 3-d surface map resides directly above. The aver age value is given in the upper left-hand side of each plot. For the indents perform ed on the fully dense PTFE surface and the epoxy surface the average values of the ha rdness were =48 MPa with standard deviation of =29 MPa for the PTFE and =205 MPa and =268 MPa for the epoxy. The average values of the reduced modulus of elasticity were =1.32 GPa with standard deviation of =0.55 GPa for the PTFE and =4.64 GPa and =2.68 GPa for the epoxy.

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32 Figure 4-6. Nano-indent ation of 625 indents placed uniformly over a 120 m by 120 m area on the initial unworn su rfaces and a region inside the wear track were performed on coatings that were run under 2N normal load and 1 m/s sliding speed. These are the same surfaces shown in Figure 4-3. The plot shown in Figure 4-6 captures the compartmentalized nature of these original composites with the dark blue regions consistent with the hardness and modulus values of PTFE. The average elastic modulus of the original coatings was 2.86GPa and 4.03GPa for the high density and low density composites respectively. Calculations of the composite elastic modulus using a linear rule-of-mixtures for the high density and low density composites are 2.76 and 3.47 resp ectively. This agreement between the expected values and the measured values is remarkable considering the small size of the sampled region. Inside the wear tracks both the hard ness and modulus are reduced, suggesting increased PTFE concentration. Using the lin ear rule-of-mixtures to solve for the PTFE volume fraction in the wear track gives 0.93 and 0.60 for the high density and low density composites respectively. This nano-mechan ical testing supports the earlier hypothesis

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33 that the transfer films are PTFE rich. Furt her, the nodal shape, si ze, spacing, and volume fraction likely play key roles in whether or not the transfer films are either nearly pure PTFE overlaying the epoxy or a mixture of PTFE and epoxy. Transfer Film Thickne ss Characterization To further investigate the tribological prope rties of the composite coating, a linear reciprocating tribometer was used to develop a transfer film and to determine the tribological properties under reciprocating sliding. This experiment allows for the transfer of material to occur on a flat surface as opposed to a spherical pin, which is very difficult to examine. The hypothesis is that thin, uniform transfer films are a governing factor in low wear materials. Using nano-m echanical testing (described previously) the thickness of the transfer film can be dete rmined and support hypot hesis that the thin, uniform transfer films create im proved tribological properties. Experimental Conditions This experiment was run on a linear reciproc ating tribometer that is located in a soft-walled cleanroom, within a conditioned laboratory environment. The relative humidity varied between 25% 50% during tests. The experimental conditions are sliding speed of 50.8mm/s and normal load 800N. The pin consisted of a composite ePTFE and epoxy coating bonded to a steel cylinder. The diameter of the pin was 12.7mm, which led to a 6MPa contact pressu re. The reciprocati ng stroke was 25.4mm and the test was run for 400,000 cycles (20km ). A schematic of the experimental apparatus is shown in Figure 3-2. Friction coefficient was calcu lated and recorded during testing through a computer data acquisition system. Each second an aver age value of the coefficient of friction along the wear track was calculated and recorded . The wear rate was determined by a

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34 gravimetric method. Experiments are interrupted periodi cally and mass loss measurements are converted to wear volume loss measurements as a function of the number of cycles of sliding. Previously, gravimetric methods were not used due to the small wear track to tota l surface area ratio. In the recipr ocating flat on flat configuration the wear track was the same size as the tota l surface area and theref ore larger mass losses were observed and the epoxy weight ga ins were considered negligible. Tribological Results Figure 4-7 shows the tribol ogical response of the ePTF E and 46 weight percent epoxy coating under reciprocating sliding. Th e wear rate was dete rmined to be 2.2x10-7 mm3/Nm and the friction coefficient was ave=0.12. Figure 4-6a shows a plot of the volume loss (mm3) versus the normal load (N) multip lied by the sliding distance (m). Each point on the plot is an interrupt ed mass measurement with its corresponding uncertainty. The uncertainty in normal load and sliding distance is assumed to be very small and is not shown on the plot. A least squares regression is drawn through the data points. The slope of this line repres ents the steady state wear rate (mm3/Nm). Figure 47b shows a plot of the friction coefficient versus sliding distance (km). The friction coefficients starts around =0.08 and experiences transient behavior for the first 2km of the experiment. Once the sample reaches steady state the friction coefficient is =0.12 for the remainder of the experiment. The transient behavior is attributed to the development of the transfer film. Once a smoot h, uniform, thin transfer film develops the friction coefficient reaches steady state behavior.

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35 Figure 4-7. Tribological results for an ePTFE/epoxy coa ting on a linear reciprocating tribometer. a) Volume loss (mm3) versus the normal load (N) multiplied by the sliding distance (m). The slope of the regression line is the steady state wear rate. b) friction coefficient versus sliding distance (km). Transfer Film Analysis After the completion of the linear reciproc ating experiment, the transfer film that developed on the steel counterface was examined with a nano-mechanical tester. A series of 20 indents were taken across the transfer f ilm that spanned from the steel, across the transfer film and back to the steel. This procedure will help determine transfer film thickness and how uniform the transfer film is across the entire wear track. As stated previously, the hypothesis is that a thin, uni form transfer film aids in favorable tribological properties. Figure 4-8 is a plot of the applied normal load (mN) versus the indentation depth (nm). The left group contains indents on the steel counterface with the right group consisting of indents on the transfer film. Th e loading curve for the steel indents is steep and reaches up to 150nm in depth. Comparativ ely the loading curve on the transfer film is much less steep indicating a much more co mpliant material. The indent depths reach up to 400 nm, indicating that the indenter indented completely through the transfer film.

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36 The unload curves are characteristic of steel for both sets for curves. The unload curves of the transfer film indents are represen tative of steel because by a depth of 300-400nm the indenter is through the transfer film and on the steel substrate. Figure 4-8. The applied normal load (mN) ve rsus the indentation depth (nm). The left group is indents on the steel counter f ace with the right group being indents on the transfer film To determine transfer film thickness from the data in Figure 4-8, the known steel indents are grouped together and the unload curves of all of the curves are lined up. The start of the load curve for the known steel indents is set as ze ro transfer film thickness. Each of the transfer film indents is measured off of this zero to determine the thickness of the transfer film at that point. Figure 4-9 shows a plot of the transfer film thicknesses. The light gray, dashed curves represent that of the steel indents and the black curves are the indents on the transfer film. As stated previously, the unload curves are lined up and the thickness of the transfer film is measured based on the initial part of the load curve. There is a representative histogram of the di stribution of the transfer film thickness across

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37 the wear track. It is bimodal with a norma l distribution about 190nm and another peak at 140nm. The average thickness was estimated to be around 180nm. This process is subjective and thicknesses are considered to be a close estimate of the true value of the thickness. Figure 4-9. A plot of the transf er film thicknesses with a representative histogram of the distribution of the thicknesses. The light gray, dashed curves represent that of the steel indents and the black curves are the indents on th e transfer film. Figure 4-10 (top) shows a cart oon of the steel counterface, the transfer film (gray), and the path of the indents taken. As stated before the indents were taken completely across the wear track. Figure 4-10 (bottom) s hows a representation of the transfer film thickness versus relative track position. The transfer f ilm thickness ranged from 140nm and to 204nm with an average thickness of 180nm . The transfer film is relatively thin and very uniform which supports the hypothesis that the thin, uniform transfer film plays an important role in the favorable tribological properties.

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38 Figure 4-10. A cartoon of the steel counterface, the transfer film (gray), and the path of the indents taken (top) and a represen tation of the transfer film thickness versus relative track position (bottom) Effects of Variations in Coating Properties on Tribological Response To further investigate the hypothesis that nodal size, spacing, and volume fraction play a role in the tribological response of th ese coatings, test matrices were designed to test the response of specifically tuned coatings. The first was a series of twelve coatings that had various densities of PTFE film and different initial film thicknesses. Scanning electron micrographs of these coatings are show n in Figure 2-3. As stated previously, all of these coating had 33 weight percent of epoxy. The second test matrix was to test the effects of a variation of wei ght percent epoxy. Two ePTFE films were chosen from the original twelve coatings and different weight percents of epoxy were added. The same density ePTFE films as 2 ( PTFE=0.688g/cm3) and 7 ( PTFE=0.572g/cm3) in Figure 2-3 were used to create composites of differe nt epoxy weight percents. For the 0.688g/cm3,

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39 three coatings were made with varying loadings of 20, 33, 53 weight percent epoxy. The loading for the 0.572g/cm3 density film were 19, 35, and 47 weight percent epoxy. Experimental Conditions Experiments were run on a rotating pin-on-disk tribometer that is located in a softwalled cleanroom, within a conditioned labor atory environment. The relative humidity varied between 25% 50% during these tests. The experimental conditions were a 5N normal load and a sliding speed of 1m/s. Th e pin was a 3.175 mm ra dius stainless steel sphere that was held stationa ry at the end of the loadi ng arm. A schematic of the experimental apparatus is shown in Figure 31. The wear track diameters were 10-20mm and the rotational speed was varied in order to maintain the 1m/s sliding speed. Each sample was tested until failure occurred. At coating failure, spikes occur in the friction coefficient data and exposed steel within the wear scar can be seen visually. Wear tracks were examined post-test under optical mi croscopy and scanning whitelight interferometry. Wear volumes were calculated by measuring 4 locations along the wear track and numerically in tegrating the interferometry data to obtain the crosssectional area. The average cross-sectional area is then multiplied by the circumference to obtain an estimate of the volume of the w ear track. For coatings that were run to failure, the thickness of the coating was determined by scanning white-light interferometry and the worn area was calcula ted from a geometrical relationship between the radius of the pin and the thickness of the coating and multiplied by the circumference of the wear track to obtain the worn volume.

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40 Tribological Results Variations in density of ePTFE film and coating thickness The coatings represented in the following plots vary in density of the ePTFE film and the thickness of the composite coating. They are numbered for their coating thickness(1-12) and correspond to the views in Figure 2-3 and the values in Table 2-1. The following plots are shown versus the produc t of the density of the ePTFE film and the initial uncompressed composite coating thic kness. This value represents the amount of PTFE available for lubrica tion in a given cross-section of a coating. This value is important in order to search for an optimum coating variation. The density of the ePTFE film sets the nodal spacing and node width whic h affects the characteristic length of the running films. The running films, as shown pr eviously, are integral to the synergistic tribological response of the coat ings. The hypothesis is that if the amount of PTFE in the coating is too large, the friction coefficient and wear rate will be increased due to the increased conformability; and if the amount of PTFE is to little, the wear rate will increase and the friction will increase due to insufficient coverage of the epoxy. Figure 4-11 shows the average friction coeffi cient of the coatings prior to failure versus the product of the eP TFE density and the uncompressed thickness. This plot shows that the friction coefficient for all twelve coatings was around =0.13, with variations in the average friction coefficient from 0.11-0.16. The standard deviation in friction coefficient is also plotted for each sample. The large deviation from the average friction coefficient was due to th e initial transient in friction co efficient. It is noted that there is a slight trend of in creasing friction coefficient with increasing PTFE availability.

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41 Figure 4-11. The average friction coefficient of the coatings prior to failure versus the product of the ePTFE density and the uncompressed thickness. Figure 4-12 shows the wear rate (mm3/Nm) versus the product of the ePTFE density (g/cm3) and the uncompressed thickness ( m). There is a strong trend of decreasing wear rate with increasing PTFE availability which spans five orders of magnitude. The least wear resistan t coating had a wear rate of 2x10-3 mm3/Nm and the most wear resistant coati ng had a wear rate of 2x10-8 mm3/Nm. Coatings 10-12 represented by the unfilled circles did not fail and ran in excess of 20 million cycles. The results from this experimental matrix suggest that even though there is a slight increase in friction coefficient with PTFE av ailability, there are great benefits in wear resistance with larger amounts of PTFE.

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42 Figure 4-12. Wear rate (mm3/Nm) versus the product of the ePTFE density (g/cm3) and the uncompressed thickness ( m). Black circles represent coatings that were run to failure and white circles are coatings that did not fail. The points are numbered 1-12 in orde r of ascending coating thickness. Variations in weight percent of epoxy To narrow down the strong contributors in wear resistance, two ePTFE films were chosen with different densities. For each density of ePTFE film, the only assumed variation in the composite coating was epoxy loading. By setti ng the initial film thickness, node spacing and node width cons tant for each set should rule out the dependence of the tribological properties on PTFE availability. In the following plots, the open circles will represent comp osite coatings made with 0.688g/cm3 density ePTFE film and the closed circles repres ent coatings made with the 0.572g/cm3 density ePTFE film. All the coatings us ed in this portion of the study were run to failure.

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43 Figure 4-13 shows a plot of the average fric tion coefficient prior to failure versus epoxy weight percent. The fric tion coefficient ranged from 0. 11-0.14, which is consistent with all previous results. There was no a pparent dependence on density of the ePTFE film or the epoxy weight percent in this data. The standard deviation for each test is also shown to describe the variabili ty in the friction coefficien t throughout a given test. The large deviation from the average friction coe fficient was due to the initial transient in friction coefficient. Figure 4-13. Friction coefficient versus epoxy weight percent. Op en circles represent composite coatings made with 0.688g/cm3 density ePTFE film and the closed circles represent coa tings made with the 0.572g/cm3 density ePTFE film. The wear rate versus epoxy weight percen t is shown in Figure 4-14 for each of the different weight percents of epoxy and densitie s of ePTFE coatings. There appears to be a strong trend of decreasing wear rate with increasing epoxy wei ght percent, w ith the less dense ePTFE film having an even lower wear rate for all epoxy weight percents than the

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44 more dense ePTFE film. The wear rates range 4 orders of magnitude from 3.5x10-6 mm3/Nm to 2x10-2 mm3/Nm for the low density 47wt% epoxy coating and the high density 20wt% epoxy coatings, respectively. Th is result indicates that with increasing amounts of epoxy that the wear resistance is im proving. This results strongly contradicts the results from the previous study. Figure 4-14. Wear rate (mm3/Nm) versus epoxy weight percen t. Open circles represent composite coatings made with 0.688g/cm3 density ePTFE film and the closed circles represent coa tings made with the 0.572g/cm3 density ePTFE film. Due to the results of the epoxy weight pe rcent study, further i nvestigation of the wear and failure factors needed to be exam ined. Looking back at Figure 4-12, there was a strong dependence on the amount of PTFE av ailable, but there was also a similar dependence on thickness. The wear rate m onotonically decreases w ith increase coating thickness. The same is true for the various epoxy weight percent samples. The thickness increases with increasing epoxy weight percen t and therefore the w ear rate monotonically decreases with increasing coating thickness. Due to the overwhelming dependence of

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45 wear rate on coating thickness and the cyclic nature of the pin-on-disk tribometer, is was hypothesized that the failure was due to a fa tigue induced delamination of the coatings, due to the cycle shear stresses, in conjunction wi th the wear properties of the material. Figure 4-15 shows a log-log plot of the wear rate of all of the samples versus the compressed coating thickness. Chapter 5 will show the modeling of the combined damage failure modes of these coatings as a function of th e coating thickness. Figure 4-15. Coating thickness ( m) versus wear rate (mm3/Nm). The open circles indicate coatings that did not fail under cyclic loading and the closed circles indicate samples that were run to failure. Such coatings were run in excess on 20 million cycles.

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46 C HAPTER 5 DISSCUSSION This chapter presents modeling of the be havior of the ePTFE and epoxy coatings described in the previous chapters. These co atings experience a fatigue–induced failure mode when tested on a pin-on-disk tribomet er. In coatings, the coating/substrate interface is usually the weak link and under a spherical contact the cyclic shear stress on the interface can lead to de lamination at the interface. Using finite elements and a numerical analysis of the c oupled failure modes a proced ure for life predictions in coatings susceptible to fatigue failures in He rtzian contacts is provided and offers insight to designers on considerations in extending the lives of coatings subject to cyclic stresses. Experimental Conditions Composite coatings used in this por tion of the study were comprised of approximately 30 weight percent epoxy with in expanded PTFE scaffolding. As described previously, the epoxy filler strength ened the composite, and directly bonded it to the substrate, while PTFE provided lo w shear running films at the tribological interface. Nano-indentation techniques previo usly discussed were used to calculate an average coating modulus of E = 3 GPa, with the st eel having a modulus of E = 200 GPa. Poisson’s ratios of 0.4 and 0.3 were used fo r the coating and steel, respectively. An experimental matrix varying the thickness of the coating from 30 m to 360 m was used to determine the sensitivity of the coati ng failure on coating thickness, as well as a method for estimating the stress-life, S–N, curve for the interface.

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47 Rotating pin-on-disk experi ments were completed with a sliding speed of v = 1m/s and a normal load of F = 5N. The pin counterbody was a 3.175 mm radius stainless steel ball. Each sample was tested until failure. At coating failure, spikes occur in the friction coefficient data and exposed steel within the wear scar can be seen visually. Scanning white light interferometry was used to measur e the thickness of the coating after failure. Using the thickness of the co ating and the radius of the pin a worn volume was calculated. Then the wear rate was de termined by dividing this volume by the dead weight normal load and the total sliding dist ance. A more detailed description of the material and test apparatus is gi ven in Chapter 2 and Chapter 3. Axisymmetric finite element analysis was used to obtain the stress state at the interface. The coating was discretized by 50 mesh, while the substrate was by 50 mesh. In order to improve the solution accura cy, smaller element size was used near the contact region. A surface-to-s urface contact modeling techni que that prevents contact elements and target elements from penetrati ng each other was employed in this model. In this contact–target strategy, the contact pressure was only calculated for nodes on the contact elements. Thus, contact elements are generated on the coating surface, while target elements on the ball. An augmented LaGrangian formulation was used to find the contact point locations and pressure. This technique possesses the st ability of the penalty method but imposes the impenetrability of the Lagrange multip lier method. The contact problem becomes nonlinear even if the structur e experiences a small deformation; thus, the structural equilibrium configuration is found by increm entally changing the applied load. In

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48 general, six or seven Newton–Raphson itera tions are required to find the converged configuration for each load step [37,38,39]. In finite element analysis, the traction stre sses and worn shape are neglected; i.e., a ball is assumed to contact with a flat surface, thus overestimating the stresses after large wear losses. A matrix varyi ng the normal load from 1 – 7 N, elastic modulus of the coating from 1 GPa, and the thic kness of the coating from 30 – 500 m is completed to provide more insight on the roles of each. Contour plots of the von Mises’ stress are shown in detail in the Appendix. The shear st resses at the interfaces were determined for coatings within the tribological experiment al matrix, with an elastic modulus of E = 3 GPa, a normal load of Fn = 5N, and varying in thickness from 30 – 360 m. Measurements and Modeling Tribological Results A log-log plot of wear rate versus coa ting thickness was shown in Figure 4-15. This wear rate assumes that wear was the only mode of material removal. The wear rate varies from 10 3 mm3/(Nm) to 10 8 mm3/(Nm) and decreases monotonically with increasing coating thickness. The strong correl ation between w ear rate or life and coating thickness suggests that mechanical effects we re contributing to the failure of these coatings. It should be noted that the thic kest four samples did not fail but only wore. Life Prediction Modeling Due to the strong correlation of decrea sing wear rate with increasing coating thickness, there was motivation to determine what the other causes of failure were and led to the modeling of the life of the coatin gs based on thickness. The following section will describe in detail the numerical simulati on of this model. Figure 5-1 describes the variables and functions used in this model.

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49 Figure 5-1. A table of the functions and va riables used in the m odel along with the units in which they are described (left) an d a schematic of the coating with the variables used in this model (right) According to Suh, N.P. , when two sliding surfaces come into contact, the softer material experiences cyclic st resses that lead to crack formation and coalescence at some distance below the contact. The joining of fatigue cracks cause sheets of material to delaminate from the bulk material [32]. For the coatings used in this study, the cracks initiate close to the bonded in terface and when the cracks coal esce the coating will fail by debonding from the interface instead of ejecting wear debris at the surfac e. This leads to the first failure mode which is cyclic fatigue due to the shear stress at the interface, in the absence of wear. The second failure mechanism is wear of the coati ngs. In the absence of cyclic fatigue, the coatings should wear at some rate leading to uniform material removal throughout the life of the coatings. The basis of the model used in this study is the combination of the fatigue–induced failure and the wear–induced failure. Figure 5-2 shows a pictorial illustrating the three failure modes of the coatings under sliding. The top series s hows a coating that is subject to large shear stresses close to the interface. The coating develops cracks that coalesce very early in its life and the

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50 coating fails very quickly. The second series shows a coating that initial has low enough shear stresses at the interface to begin th e wear process, but eventually reaches a thickness where the stresses are higher and cr acks begin to initiate and join leading to delamination of the coating. The third series shows a coating that does not experience cyclic fatigue and only experiences wear. Figure 5-2. Cartoon of the failure mode s of the composite co ating shown against number of passes. The first row shows interfacial fatigue with minimal wear, the second row is in terfacial fatigue and wear combined and the final row is wear with minimal fatigue. A hand calculation using Hertz’s formulas for a inch diameter steel ball ( = 0.3, E = 200 GPa), on a flat polymer surface ( = 0.4, E = 3 GPa), gives a depth of maximum subsurface shear stress of approximately 70 m, or the thickness at which wear rate becomes a strong function of thickness (Figur e 4-15). This simple calculation suggests that fatigue due to the shear stresses at the interface is governi ng the lives of these coatings under concentrated circular contacts. Finite element analyses were performed to obtain accurate stresses at the coating substr ate interface. Figure 53 shows contour plots

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51 of the subsurface shear stresses under the experimental conditions ( Fn=5N and Ecoating=3GPa). The maximum shear stress at the interface decreases with increasing thickness from 42MPa to 5MPa for the 30 m and 360 m coatings, respectively. Also, the discontinuity in elastic properties can be s een to result in a stress discontinuity at the interface. The contours are regions of consta nt shear stress with increasing stress as the color spectrum goes from dark blue to red. The Appendix shows the contour plots of the entire finite element analysis which includes a matrix varying the normal load (1 – 7 N), the elastic modulus of the coating (1 – 7 GPa), and the thickne ss of the coating (30– 500 m) in order to see the affects due to these variations on th e von Mises stresses. Figure 5-3. Contour plots of the subsurface shear stress based on finite element analysis. The black line indicates the interface between substrate and coating. The coatings varied in thickness from 30-360 m, and a 5N load and 3GPa elastic modulus were used in the analysis. It was hypothesized that the cyclic shear stress at the interface was the critical parameter in determining failure of these coatings, so these data were extracted from the

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52 finite elements results and are plotted in Fi gure 5-4a versus the coating thickness for each coating of with appropriate normal load and elastic modulus. A curve fit to this data gives a continuous function for interfacial stre ss as a function of th e distance between the interface and the point of contact, h , and is given in Eq1. 0.729(5.197*(/0.001))()56.9hhe (1) An in-situ LVDT trace and interrupted SWLI measurements of , or the wear depth, for several un-failed samples are plotte d as a function of reci procation cycles in Figure 5-4b. Initially, the pene tration rate is very high, lik ely a result of both the high wear rate transient period as well as the small initial area or contact. Upon bedding in the penetration rate becomes stable around 10 million cycles. The LVDT trace followed a coating of 200 m thickness from the start until failure. The coating failed around 20 million cycles. The curve fit for delta as a function of number of cycles is fit to the LVDT data before the coating failed and is extrapolated out when the coating begins to wear in linearly. The SWLI data, of several samples prior to failure, follows the same trends as the LVDT data, but the LVDT in cludes elastic deformation and creep. The curve fit applied to this data is given by, 68(/410)()(81013)(1)NNNe (2) As wear occurs, increases, h = t is decreased and the interfacial stress increases exponentially.

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53 010203040506070 0 10 20 30 25 15 5 model fit to failure (a) (b) SWLI (ex situ) LVDT (in situ) ( m)N (cycles x 106) 0100200300400500 0 5 10 15 20 25 30 35 40 45 50 xy (MPa)coating thickness ( m) Figure 5-4. Plots of experime ntal data and model fits fo r the functions used in the cumulative damage model. (a)Finite element results for the maximum shear stress at the interface versus the coati ng thickness. (b) The wear depth versus number of cycles for and in situ (L VDT) and ex situ (SWLI) measurements, along with the model fit to this data. The open points are samples that wear not run to failure. The original coating thickness can be in serted into Eq1 to obtain the initial maximum shear stress at the inte rface. This initial interfaci al shear stress is plotted versus the experimental cycles at failure in Figure 5-5. A confounded but conservative estimate of the S–N diagram can be obtained by assuming the samples having low cycles to failure did not incur wear . An arbitrary 200,000 cycles was chosen as the cutoff for neglecting wear. This data was curve fit and an endurance limit of 13 MPa is chosen based on the regression and the samples th at only incurred wear. The corresponding expression for the S–N diagram is given by Eq3, 88(2.510*)(2.510*)()(1.98ln()47)()13(1)NNNNee (3) and is valid for stresses gr eater than the endurance limit.

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54 Figure 5-5. Initial interfacial shear stress plotte d versus the experimental cycles at failure. A curve is fit to the samples failing le ss than 200,000 cycles and an endurance limit is set at 13MPa. The samples accumulating less than 200,000 cycles were assumed to experience only fa tigue, the second group experiences fatigue and wear and the run-out samples only experi enced wear and did not fail. Solving Eq3 for the number of cycles to failure as a function of shear stress enables the calculation of damage per cycle at a gi ven stress. A numerical cumulative damage scheme is used to calculate the wear depth and the accumulated damage over a given cyclic interval. The damage accumulation is calculated by dividing the number of cycles in an interval at the stress associated with the current thic kness by the number of cycles required for failure at that stress level. These accumulations are added to obtain the damage as described by Eq4. ()fN Damage N (4) When damage is equal to unity, the coating is determined to have failed. Equation 4 is solved numerically to obtain the number of cycles to failure for a coating of any

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55 thickness. This solution process is easily visual ized in the form of a flow chart; this flow chart is shown in Figure 5-6. The inputs to the model are the initial coating thickness, to, and the cyclic interval. Figure 5-6. Flow chart for numerical solution for lif e prediction based on coating thickness. The wear depth, , is first calculated at the initia l cycle number and then the change in thickness, h , can be calculated. Once the new thickness, h , is determined, the shear stress, , can be determined for that thickness. That shear stress, , is then checked to determine if it is above the endurance limit (in this case = 13 MPa). If the shear stress is less than the endurance limit, the number of cycles is incremented by the cycle interval, N . The values for , N , h , and are updated and will contin ue to be until the shear stress is above the endurance limit. This implies that the coatings are only experiencing wear and no fatigue until this endurance limit is met. Once the value of the shear stress reaches this limit the fatigue begins to enter into the life of the coatings. The number of

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56 cycles to failure, Nf, is determined for the calculated sh ear stress. The amount of damage at that cycle number is then calculated. On ce the sum of the incremental damage is equal to unity the coating is considered to have faile d. If the damage is not equal to unity, the number of cycles is incremented and th e process is repeated until the damage accumulates to unity. Life Predictions of ePTFE and Epoxy Coatings Figure 5-7a shows a plot of the number of cycles to failure versus coating thickness for the model fit using the cumulative damage model and the experimental data. Curves for the expected life of coatings that expe rience wear in the abse nce of fatigue and for fatigue in the absence of wear are also give n. The lives of the experimentally tested coatings all fall closely to the predicted life. Had the assumption that wear was negligible for coatings failing in fewer than 200,000 cy cles been in error, the cumulative model would fall below the data; i.e., the model woul d predict failure faster than the coatings failed in the experiment indicat ing that the S-N curve was in error, and would require a reverse solution of the current model to find the S-N diagram. This assumption is valid only because the wear rate of the material itself is very low. Wear only begins contributing where the model a nd fatigue lines diverge for 150 m coatings. The wear rate of the material is like ly in the range from K=1 8 mm3/Nm. A material with a wear rate of K=1 5 mm3/Nm, such as epoxy, would ha ve equal contributions from wear and fatigue, while a less wear resi stant material such as PTFE, K=1 3 mm3/Nm, would wear through before accumulating any fa tigue damage. Figure 5-7b shows this comparison of wear rates of PTFE, epoxy, and the composite coating.

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57 Figure 5-7. Results of the num erical solution for coating lif e based on the thickness of the coating. (a) Plot of the number of cycles to failure versus coating thickness for the model fit using the cumulative damage model and the experimental data. Curves for the expected life of coatings with wear in the absence of fatigue and for fatigue in the absence of wear are also given. (b)Comparison of wear rates of PTFE, epoxy, and the composite coating along with the fatigue curve. One conclusion that could be drawn from th e above analysis is that coating wear rates less than K=1 5 mm3/Nm does little to improve coat ing life. This is true only for this coating in this geometry for these loads. To the au thor’s knowledge, however, there are few applications where one would contac t a polymeric coating on a steel substrate with a steel ball. This phenomenon is more of an artifact of th e testing. There are applications where Hertzian type contacts ar e incurred by solid lubr icant coatings, such as a bushing with clearance. However, th ey are comparatively conformal, the maximum subsurface stresses are low and deep, and for practical purposes, fatigue is absent. A cursory investigation of this highly designed coati ng on a rotating pin-on-disk tribometer would misrepresent the performance of the material. This effect would be further exaggerated with a smaller radius ball. This chapter illustrates the potential error in tribological analysis of coatings on one of the most common test configuration in tribology. The methodologies presented shoul d be considered whenever dealing with

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58 wear resistant materials with potential fatigue stresses. These concepts can also be used as a design tool in non-conformal bearing systems.

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59 CHAPTER 6 CONCLUSIONS This manuscript reports on a polymer compos ite coating that is comprised of PTFE nodes and fibrils encapsulated by epoxy. In init ial experiments of a high density and low density coating versus the composite’s constitu ents, the friction coefficients were reduced over that of pure PTFE and the wear rates were reduced by over 100x. It is suggested that the mechanism for this reduction in both friction and wear is related to the PTFE being drawn out of the nodes and forming a transfer film over the epoxy regions. This conclusion was in agreement with the hypot hesis, and supported by nano-mechanical testing of the PTFE running films in the w ear track and on the transfer film on the counterface. The nano-mechanical testing in the wear track revealed that the epoxy regions were covered by mostly PTFE which led to reduced friction coefficient and wear rates. The nano-indentation on the counterface transfer film showed that the transfer films were thin and uniform which was also expected to lower friction and wear rates. Further experiments on varying the PTFE availability and epoxy weight percent showed that the friction coefficients are larg ely insensitive to any variation. The wear rates appeared to have a dependence on the am ount of PTFE availabl e in the matrix (2) showing increased wear resistan ce with increased amount of PTFE available. Then the wear rates appeared to have a strong dependence on the amou nt of epoxy in matrix (3) with increased wear resistan ce with increasing epoxy weight percents. The previous two results were in sharp contradiction with each other and therefore it was found the coatings when the data from both sets were collapsed together that the wear rate was dependent on

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60 coating thickness. Due to the overwhelmi ng dependence of wear rate on coating thickness and the cyclic nature of the pin-on-disk tribometer , is was hypothesized that the failure was due to a fatigue induced delamina tion of the coatings, due to the cycle shear stresses, in conjunction with the wear properties of the material. This hypothesis was demonstrated by creating a cumulative damage model including wear and cyclic fati gue using finite element analys is and a numerical solution. The failures of the films were found domina ted by cyclic fatigue caused by the shear stress at the interface. This was in agreemen t with the hypothesis but concluded to be an artifact of the pin-on-disk test having no implications for the life of coatings in typical bushing or thrust face applications. Despite the widespread use of pin-on-disk testing, and the typical casualness with which it is tr eated, careful consider ation is required or incorrect conclusions can be dr awn. As with many tests, the potential error in wear quantification arises due to the very high wear resistance of the material being tested.

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61 APPENDIX CONTOUR PLOTS OF VON MISES STRESSES This appendix includes contour plots of the von Mises stresses for the entire finite element analysis which includes a matrix vary ing the normal load (1 – 7 N), the elastic modulus of the coating (1 – 7 GPa), and the thickness of the coating (30 m) in order to see the affects due to these va riations on the von Mises stresses. Figure A-1. Contour plots of the von Mises stresses for the 30 m coatings with various loads and elastic moduli.

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62 Figure A-2. Contour plots of the von Mises stresses for the 100 m coatings with various loads and elastic moduli.

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63 Figure A-3. Contour plots of the von Mises stresses for the 300 m coatings with various loads and elastic moduli.

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64 Figure A-4. Contour plots of the von Mises stresses for the 500 m coatings with various loads and elastic moduli.

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68 BIOGRAPHICAL SKETCH Nicole Lee McCook was born January 3, 1982, in Inverness, Florida, to Sylvia and William McCook. She moved to Winter Park , Florida, at the age of one where she attended elementary, middle, and high sc hool, graduating from La ke Howell High School in 2000. Nicole began her education at th e University of Florida in August 2000 and graduated cum laude in mechanical and aer ospace engineering in April 2004. Nicole began research in the Tribology Laboratory in the summer of 2003 under the guidance of Dr. W.G. Sawyer. She will receive her mast er of science in mechanical and aerospace engineering in May 2006. After receiving her master’s degree she w ill continue research in the Tribology Laboratory working towards her PhD.