Ultralow wear fluoropolymer composites: Nanoscale functionality from microscale fillers

Ultralow wear fluoropolymer composites: Nanoscale functionality from microscale fillers

Author’s Accepted Manuscript Ultralow Wear Fluoropolymer Composites: Nanoscale Functionality from Microscale Fillers Brandon A. Krick, Angela A. Piten...

2MB Sizes 27 Downloads 93 Views

Author’s Accepted Manuscript Ultralow Wear Fluoropolymer Composites: Nanoscale Functionality from Microscale Fillers Brandon A. Krick, Angela A. Pitenis, Kathryn L. Harris, Christopher P. Junk, W. Gregory Sawyer, Scott C. Brown, H. David Rosenfeld, Daniel J. Kasprzak, Ross S. Johnson, Christopher D. Chan, Gregory S. Blackman

PII: DOI: Reference:

www.elsevier.com/locate/jtri

S0301-679X(15)00451-X http://dx.doi.org/10.1016/j.triboint.2015.10.002 JTRI3876

To appear in: Tribiology International Received date: 25 June 2015 Revised date: 2 October 2015 Accepted date: 5 October 2015 Cite this article as: Brandon A. Krick, Angela A. Pitenis, Kathryn L. Harris, Christopher P. Junk, W. Gregory Sawyer, Scott C. Brown, H. David Rosenfeld, Daniel J. Kasprzak, Ross S. Johnson, Christopher D. Chan and Gregory S. Blackman, Ultralow Wear Fluoropolymer Composites: Nanoscale Functionality from Microscale Fillers, Tribiology International, http://dx.doi.org/10.1016/j.triboint.2015.10.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Krick et al. Ultralow Wear Fluoropolymer Composites: Nanoscale Functionality from Microscale Fillers Brandon A. Krick1, Angela A. Pitenis2, Kathryn L. Harris3, Christopher P. Junk4, W. Gregory Sawyer2,3, Scott C. Brown4, H. David Rosenfeld4, Daniel J. Kasprzak4, Ross S. Johnson4, Christopher D. Chan4 and Gregory S. Blackman4 1

Department of Mechanical Engineering and Mechanics Lehigh University Bethlehem, PA 18015 2

Department of Mechanical and Aerospace Engineering University of Florida Gainesville, FL 32611 3

Department of Materials Science and Engineering University of Florida Gainesville, FL 32611 4

DuPont Central Research and Development 200 Powder Mill Road, PO Box 8352 Wilmington, DE 19803 Abstract Polytetrafluoroethylene (PTFE) filled with certain alumina additives has wear rates over four orders of magnitude lower than unfilled PTFE. The mechanisms for this wear reduction have remained a mystery. In this work, we use a combination of techniques to show that porous, nanostructured alumina microfillers (not nanofillers) are critical for this wear reduction. The microscale alumina particles break during sliding into nanoscale fragments. X-ray microtomography, transmission electron microscopy and infrared spectroscopy reveal nanoscale alumina fragments accumulated in the tribofilms. Tribochemically generated carboxylate endgroups bond to metal species in the transfer film and to alumina fragments in the surface of the polymer composite. These mechanically reinforced tribofilms create robust sliding surfaces and lead to a dramatic reduction in wear. Keywords Polytetrafluoroethylene, aluminum oxide, alumina, nanocomposite, microcomposite, composite, ultralow wear, tribochemistry, tribology, mechanochemistry Corresponding Author Gregory S. Blackman DuPont Central Research and Development 200 Powder Mill Road, PO Box 8352 Wilmington, DE 19803 phone: (302) 695-2415 email: [email protected]

page 1

Krick et al. 1. Introduction The wear and frictional dissipation mechanisms of polytetrafluoroethylene (PTFE) have been of scientific and practical interest for over half a century since the accidental discovery of PTFE polymer and its remarkably low coefficient of friction [1-4]. One disadvantage of using unfilled PTFE for tribological applications is its high wear rate [1-4]. Usually PTFE is compounded with various fillers to improve its wear performance. Many different additives (e.g., glass fibers, various microparticles, carbon fibers, other polymers) reduce its wear rate by one or more orders of magnitude [5-7], while the PTFE matrix provides a low friction coefficient. A dramatic reduction in wear of PTFE composites has been achieved by mixing granular PTFE 7C with 1-5 vol.% alumina, to produce wear rates of 1x10-7 mm3/(Nm) or lower [5-24]. The first paper documenting this ultralow wear behavior in PTFE /alumina composites evaluated the wear of PTFE filled with three different alumina fillers, distinguished by the manufacturer designated particle sizes of “44 nm”, “80 nm” (Alumina C in Table 1) and “500 nm” (Alumina A in Table 1) [17, 1924]. Alumina C achieved steady state wear rates around 8x10-7 mm3/(Nm), which is 100 times lower than the other composites. The authors observed no correlation between wear and particle size, but in a later paper determined that the 44 nm particles were spherical delta-gamma phase alumina, while the other particles were alpha-phase [23]. McElwain et al. and Blanchet et al. later reported that the wear rate of PTFE with 2.9 vol. % (~ 5 wt. %) alpha-phase alumina nanoparticles was two orders of magnitude lower than with “microparticles” [19, 21] (Figure 1). In this reference the authors were relying on the supplier designated particle size.

Figure 1 Wear rate (mm3/(Nm)) as a function of supplier-designated particle size. The present study describes a more accurate determination of particle size, but the figure gives an historical context. Blue data points are from the present study, and red from the previous literature [21].

page 2

Krick et al. In the early development of PTFE / alumina composites for tribological applications, this remarkable reduction in wear prompted many studies of the mysterious mechanisms responsible for the ultralow wear behavior. These mechanisms are complex and result from several interrelated phenomena. It has been hypothesized by several groups that the addition of inorganic particles somehow interrupts or inhibits the subsurface crack propagation of PTFE, which normally produces large fluffy wear debris [19]. There is also a hypothesis that successful additives change the crystalline nature of the PTFE in favor of a tougher crystal form, although so far there is no definitive evidence for such a mechanism[6, 25]. Another proposal was that the “banded structure” of PTFE (as observed in TEM micrographs), was disrupted and accounted for some of the changes in wear performance [8, 26]. To date, there is no compelling evidence for any of these proposed mechanisms, despite their persistence in the literature. We now have evidence that tribochemical reactions occur during the initial wear process and lead to the formation of a thin robust transfer film and running film. This unique tribochemistry, in combination with the appropriate alumina filler, leads to the exceptionally low wear rates. In this paper, we show that the special alpha-phase alumina that leads to ultralow wear is actually composed of porous 1-10 µm agglomerates that break up during the initial stages of wear and react with the tribochemical species formed from the broken PTFE chains. 2. Materials and Sample Preparation 2.1 Alumina powders Several alumina powders were purchased for use as filler material in granular Teflon® PTFE 7C in this study. The selected powders include samples from the same suppliers as previously reported in the PTFE/alumina composite literature, as well one new particle source (Alumina B). Alumina A: Alfa Aesar alpha-phase alumina powder (Stock #42573, 99.95%) with a supplier-specified approximate particle size of 0.35 to 0.49 µm (no method given, and previously referred to as 0.5 µm αalumina [19, 23]). Published wear measurements on a “microcomposite” of this material in PTFE gave wear rates about 10 times lower than unfilled PTFE [19, 21]. This material was verified to contain alphaphase alumina (corundum) with a crystallite size of > 100 nm by powder X-ray diffraction. Alumina B: Almatis calcined alpha-phase alumina powder (grade A 16 SG, 99.8%) with a supplierspecified typical d50 particle size of 0.5 µm (Cilas laser particles sizer). This powder was selected because it was in the size range of interest for ultralow wear additives to PTFE. Alumina C: Alfa Aesar alpha-phase alumina powder (Stock #44652, 99%) with a supplier-specified approximate particle size of 60 nm (no method given), discussed in detail by Burris and Sawyer [19, 21, 23]. This powder was selected because it has been reported to produce ultralow wear PTFE composites [17]. This material was verified to contain alpha-phase alumina (corundum) with a crystallite size of > 100 nm by powder X-ray diffraction. Alumina D: Alfa Aesar alpha-phase alumina powder (Stock #44653, 99%) with a supplier-specified approximate particle size of 27-43 nm (no method given). This powder was selected because it has been reported to produce ultralow wear PTFE composites [17, 19-24, 27-34]. This material was verified to contain alpha-phase alumina (corundum) with a crystallite size of > 100 nm by powder X-ray diffraction. Alumina E: Nanostructured and Amorphous Materials, Inc. alpha-phase alumina powder (Stock # 1015WW, 99.5%) with a supplier-specified approximate particle size of 27-43 nm (no method given). This powder was selected because it has been reported to produce ultralow wear PTFE composites [17,

page 3

Krick et al. 19-21, 23]. The manufacturer states this material is mostly alpha-phase with 5-10% gamma. Interestingly, this filler was not added to PTFE composites until much later than Alumina C and D. Aluminas A, C and D are no longer commercially available, so this material was selected as an alternative solely on the supplier-designated particle size, based on the belief that nanoparticles were necessary for achieving ultralow wear rates [29, 30, 35, 36]. 2.2 Composite Preparation PTFE / alumina composites used to generate the wear data in Table 1 were made using DuPont Teflon ® PTFE 7C resin (~35 µm particle size) and the various alumina powders dispersed at 5.0 wt % (~2.9 vol. %) relative to PTFE 7C. The two solid materials were sonicated in isopropyl alcohol. After solvent removal, the powdered samples were consolidated into billets using a hydraulic press, and sintered at 380 °C. Molded samples were machined and sanded to an average roughness of 100 nm. The PTFE / alumina composite used in Figure 6 was made using DuPont Teflon ® PTFE 7C resin (~35 µm particle size) and Alumina B dispersed at 7.0 wt % relative to PTFE 7C. The two solid materials were mixed in a jacket-cooled (- 70 oC) Waring blender, consolidated into a billet using a hydraulic press, and sintered at 380 oC. The molded sample was then cut with a water-jet into a 10 mm x 10 mm x 6 mm wear test sample. 2.3 Countersamples The PTFE composites used in Table 1 were slid against 304 stainless steel rectangular flat plates (38 mm x 25 mm x 3.7 mm). The metal samples were finished with a lapping process which resulted in an average roughness of approximately 150 nm. These countersamples were similar to those used in previous studies of PTFE composites [19]. A new metal countersample was used for each wear test experiment, and was first washed with soap and water, rinsed with methanol, wiped with a low-lint laboratory wipe and allowed to dry for ~20 min. prior to wear testing. 3. Experimental Methods 3.1 Alumina Characterization Particle size by static light scattering (SLS) Dispersions of each sample were prepared as follows: 0.1 g of each aluminum oxide was added to 50 mL of twice deionized water (Millipore, MilliQ Plus) with 0.4 g/L tetrapotassium pyrophosphate (TKPP) into a clean 100 mL Nalgene® high density polyethylene (HDPE) bottle. The sample was vortexed for 1 minute and then transferred to a jacketed 100 mL glass beaker connected to a circulation bath set to 5 o C. The samples were magnetically stirred and sonicated in the jacketed beaker for a total process time of ten minutes using a 20 kHz QSonica, LLC. (Newtown, CT) Q700 sonication system equipped with a ½ inch sapphire tipped probe. The beaker sonication settings were adjusted to deliver 61 watts to water following the procedure of Taurozzi et al [37]. Sonication proceeded in two minute intervals with one minute pauses between processing to prevent excessive temperature deviations. Particle size distribution measurements were conducted on a Microtrac Inc. (Montgomeryville, PA) X100 laser diffraction instrument equipped with an automated small volume recirculator module (ASVR). Samples were analyzed after sonication in 0.4 g/L TKPP in filtered deionized water. All samples were analyzed using a refractive index of 1.7 for alumina utilizing the assumptions for a transparent and irregular material. A refractive index of water (1.33) was used for the fluid phase. Instrument operability

page 4

Krick et al. was verified by utilizing a titanium dioxide standard reference material (SRM 8988; NIST, Gaithersburg, MD) as a control. The average of three analyses is reported. Alumina surface area by Brunauer, Emmett and Teller method (BET) Nitrogen adsorption/desorption measurements were performed at -195.9 o C on a Micromeritics ® ASAP model 2400/2405 porosimeter. Samples were degassed at 150 oC overnight prior to data collection. Surface area measurements utilized a five-point adsorption isotherm collected over 0.05 to 0.20 P/P0 and analyzed via the BET method [38]. P is the pressure of the gas above the sample (generally at liquid nitrogen temperature); P0 is the ideal gas pressure at the temperature of the sample being measured (typically around 760 Torr). Particle size/shape by scanning electron microscopy (SEM) The samples were first added to ethanol at approximately 0.01 wt. %, and then bath sonicated for 20 minutes. Immediately afterwards, 2 drops of the solution were placed onto an highly ordered pyrolytic graphite (HOPG) surface and the solvent was allowed to evaporate under ambient conditions. The samples were then imaged using an FEI Quanta 600F operating at an accelerating voltage of 8.5 keV and pressure of 0.5 to 0.8 Torr (depending on sample) to reduce charging and drifting. Images were collected with an acquisition time of 93 seconds for normal images of 1024 x 943 pixels, and 372 seconds for high resolution images of 2048 x 1887 pixels. High resolution images were further processed by image analysis to determine size of the alumina particles. Additional sample preparation was done using the same particle suspension used in the Static Light Scattering experiments described above. The suspension was spin coated onto single crystalline silicon wafers or HOPG graphite for Scanning Electron Microscopy determination of the particle size and shape. The imaging was the same as described above. 3.2 Tribology Experiments A linear reciprocating tribometer was used for all wear experiments in Table 1 [7]. The polymer sample (6.3 x 6.3 x 12.7 mm) was mounted to a six-channel load cell which measured the normal and tangential forces. The polished 6.3 x 6.3 mm face of the polymer sample was pressed against the steel countersample in a flat-on-flat configuration. Normal load of ~ 250 N was applied to the ~40 mm2 interface to yield a nominal contact pressure of ~ 6.25 MPa. The load was applied and controlled by a pneumatic cylinder and thruster. The countersample was driven directly by a motorized linear ball-screw stage, which reciprocated with a stroke of 25.4 mm at a rate of 50.8 mm/s. These tribological sliding conditions were chosen to be the same as previous studies [7, 39, 40] to simplify comparisons between studies. Wear of the polymer was monitored using two methods. The wear displacement of the pin was monitored during sliding with a linear variable differential transformer (LVDT); this measurement captures both deformation of the polymer pin (elastic, plastic and creep) as well as linear wear of the polymer sample. The second and more sensitive wear measurement is achieved by intermittent mass measurements of the polymer using a Mettler Toledo scale with a resolution of 10 µg. A wear volume of the polymer, Vlost, was calculated using the change in mass, mlost, and the density of the composite. Wear rates are calculated by dividing the volume lost during sliding, in mm3, by the normal force, FN, times the

page 5

Krick et al. sliding distance, d, in meters, resulting in a wear rate, k, in units of mm3/(Nm), as shown in (Eq. 1 and Fig. 1) [7, 17, 20, 23, 41-43]. An interesting characteristic of these ultra-low wear polymers is that the initial wear rates range from 10-5 to 10-6 mm3/(Nm), but as the experiment progresses the wear rate drops steadily to as low as 5x10-8 mm3/(Nm) or sometimes lower. Steady-state wear rates are calculated from near the end of the wear experiment and are reported in Table 1. Associated uncertainties were calculated using a Monte Carlo technique [39, 40, 43, 44]; uncertainties in wear rates are typically lower than the variation of the wear rates and are often smaller than the data points shown on the plots.

(1) A separate test configuration was used to measure wear rate and friction coefficients as a function of pressure and velocity, and to determine the PxV values where catastrophic failure occurred (failure denoted by rapid increase in wear measured by thickness or height loss change). The TS-01D TriboSpectrometer™ (Tribis Engineering, Shelby Twp., MI, USA) used a 4140 stainless steel disk (35.5 cm in diameter, 2.9 cm thick, hardened to Rc 40-45 and Blanchard polished to RMS roughness of 80-100 nm) and a flat 10 x 10 x 6 mm polymer composite sample in a unidirectional rotary flat-on-flat configuration with a wear track radius of 105 mm. The pressure and velocity were independently varied in a matrix to determine the wear rates and friction coefficients at various stages, and eventually the P and V at which the part failed. Experiments typically started at low PxV (P = 1.0 MPa, V = 0.66 m/s as break-in conditions to ensure conformal contact) and proceeded to higher PxV values in increments until the amount of sample wear or creep exceeded a certain user specified limit. In the experiment described herein, a PTFE 7C / 7.0 wt. % Alumina B sample was tested up to a PxV at which the sample failed (P = 10 MPa, V = 0.66 m/s). Failure for this sample involved a large increase in wear rate along with a multimillimeter sized piece of the polymer running surface being stripped away. The worn test specimen subsequently provided a convenient sample to probe the tribochemical reactions with ATR-IR in regions with well-developed running film and a nearby area with the same processing and tribological history with no visible running film (see Figure 5). 3.3 Polymer wear surface characterization X-ray microtomography X-ray microtomography (XMT) data were collected using a Skyscan 1172 microtomograph. Specimens were prepared by cutting a 1.5 x 1.5 x 10 mm stick from a wear tested sample using a diamond wafering saw, such that the wear surface was one of the long faces. Adhesive tape was applied to the face opposite the wear surface to facilitate identification of the wear surface in the reconstructed X-ray images. The prepared specimens were mounted and imaged individually, with the long axis vertical and aligned with the axis of sample rotation. Flat-field-corrected, median-filtered radiographs were obtained with a tungsten anode source operated at 40 keV and 250 mA with no soft X-ray filtering. Five frame averaged images were obtained over a 180° range of sample rotation with a 0.3° step between images, with an image pixel size of 0.74 mm for an effective image resolution of about 1 mm. Reconstruction of an image stack from the radiographs was performed using NRecon software with ring artifact correction set at 10, Gaussian filtering of 3, and post alignment adjusted to minimize image artifacts. Data were reconstructed in a region of interest that included the wear surface and the space just above it, but

page 6

Krick et al. excluded any cut surfaces resulting from sample preparation. Three dimensional images were rendered using Volume Graphics, VGStudio Max 2.2.5.76503. False color and opacity were mapped based on voxel values as appropriate to obtain images showing either polymer and inorganic filler, or filler only. The rendering software also provided size distribution information on filler aggregates larger than the resolution limit. ATR-IR The surface of the worn polymer composite was analyzed by attenuated total reflectance infrared (ATRIR) using a Golden Gate (Specac) horizontal diamond ATR unit. Spectra were collected with pressure applied from the overhead clamping device, and corrected for the ATR effect (depth of penetration versus wavenumber) to closely resemble transmission spectra. SEM of the running film The running film and some of the bulk material was first removed from the polymer composite by careful cutting with a razor blade. The specimen was adhered to a standard 12.5 mm aluminum stub with double-sided carbon tape, then coated with 2 nm of osmium using a Filgen OPC80T Osmium Plasma Coater. The images were produced using a JEOL 7600F high resolution SEM at 5 keV at high vacuum. Images were collected with an acquisition time of 112 seconds at a resolution of 1280 x 1024 pixels. For the cross section of the running film, the aforementioned sample and razor blade were immersed in liquid nitrogen. Both were removed, and the sample was immediately cut. The cross section surface was then adhered to a standard 12.5 mm aluminum stub with double-sided carbon tape and coated with 2 nm of osmium. The sample was also imaged using the aforementioned JEOL 7600F high resolution SEM at the same conditions. TEM of the running film The running film plus some of the bulk material was embedded in a two-part epoxy, Buehler EpoxiCure® 2, at a 10:1 epoxy:hardener ratio. The sample was then cut into ethanol using a diamond knife in a Leica Cryo-Ultramicrotome set at 50 nm slices and -90°C. The samples were then floated on water at room temperature and collected on holey carbon Cu grids. They were then imaged in a JEOL 2000FX TEM operating at 200 keV using a Gatan Orius CCD camera with a 4s exposure time. 3.4 Transfer film topography characterization Stylus profilometry (KLA Tencor P16) with a 2 µm radius probe and 1 mg load was used to map transfer film topography. Fifty 10 mm line scans were acquired over a 150 µm section in the central region of the transfer film. A section of the bare stainless steel surface on each side of the transfer film was used to flatten the profiles. The average thickness was calculated from the difference between the baseline and the center of the 2 x 10 mm section described above. 4. Results and Discussion 4.1 Wear of various alumina filled PTFE composites

page 7

Krick et al. All of the alpha-phase alumina samples used in this study and in the previous literature are included in Table 1. They are arranged in order of their median particle size (d50) as measured by static light scattering. The measured steady state wear rates (linear reciprocating test) at a pressure of 6.25 MPa and a velocity of 50.8 mm/s are also included in the table. Figure 1 compares the current wear rate results with previous literature values [21] as a function of vendor supplied particle size. Present results for PTFE filled with Alumina A, C, and D agree with the prior study. A seemingly anomalous data point arises from the PTFE composite filled with Alumina B, which exhibits the ultralow wear rate of 1.4x10-7 mm3/(Nm), even though the vendor-reported particle size is identical to that reported for Alumina A (around 500 nm). In order to reconcile these data, a much more detailed investigation into the nature of these particles was required. 4.2 Particle Size Analysis In previous publications the wear results for composite materials made with the different alpha phase alumina particles were plotted as a function of the vendor supplied particle size, showing what appeared to be a clear boundary between ultralow wear (< 1x10-7 mm3/(Nm)) and moderate to poor wear (> 1x10-5 mm3/(Nm)) with a large transition in wear rate appearing above about 100 nm in particle size (Figure 1 [19, 21]). The particle size that was reported in each of these papers was provided by the vendor (see Table 1) and does not represent the actual particle size. The vendor does not indicate how they measure the particle size. We suspect, based on our own BET measurements, that this approximate particle size was based on a calculation from a BET surface area measurement. BET is the simplest method for determining particle size because it does not require sophisticated instrumentation or timeconsuming microscopy; but it is often misleading. The calculation of an average particle size from a BET measurement (Equation 2) assumes that the particles are spherical, dense, and monodisperse. D(nm)=6000/(SA(m2⁄g)×ρ(g⁄cm3))

(2)

where D is diameter; SA is BET surface area; ρ is density However, most of the alpha-phase alumina particles used in these studies are irregular in shape and are porous, thus fail to satisfy the assumptions for the BET particle size calculation as seen both in the SEM micrographs and the static light scattering results shown in Figure 2. The SEM micrograph in Figure 2c shows a typical field of view of Alumina E. This particular alumina has a vendor-reported approximate size of 27-43 nm. The inset at the bottom left of Figure 2d contains two tiny white dots that represent what a 27 and 43 nm particle look like on the same scale. In Figure 2d we begin to understand the reason why the BET calculated particle size is so very different from the actual particle size for Alumina E. The alumina particles are very lacy or porous with extremely small pores which create a surface area and leads to a gross underestimation of the actual particle size. It is time consuming and impractical, but not impossible, to obtain statistical particle size and shape information from a series of micrographs, but it is much easier to use a technique like static light scattering which will measure the average particle size and distribution in a dilute suspension of the particles. The median particle size from light scattering is included in Table 1 and the particle size distributions from which the median particle size were calculated are in Figure 2e. Alumina E is the largest of the particles with a median particle size of 3.950 µm, i.e., over 100 times larger than its BET particle size as calculated from our surface area measurement.

page 8

Krick et al. The particles in Table 1 are listed in increasing order by median particle size as determined by static light scattering. The median sizes range from 0.267 µm for Alumina A to 3.950 µm for Alumina E. Also included in the table are both the measured BET surface areas and the calculated particle sizes using Equation 2. Note the large difference between the BET calculated particle size and the median particle size from light scattering. Alumina A has the closest correspondence between the two measurements, which is explained by the micrographs in Figure 2a and 2b. The magnification of Figures 2a and 2c are identical and it is clear that Alumina A particles are much smaller than Alumina E particles. The inset in Figure 2b shows what a 350 nm particle (vendor-specified particle size) would look like to scale. The individual particles of Alumina A are irregular in shape, but not porous, therefore the surface area measured by BET for Alumina A is very close to static light scattering median particle size (Table 1). This agreement between the two techniques suggests that Alumina A particles are fully dense. Alumina E is at the other extreme with a factor of over 100 between the BET calculated particle size and the median particle size measured by static light scattering. The large difference is typical of particles like those in Figures 2c-2d with large internal and external surface area. As the median size of the particles in Table 1 increases from Alumina A to E, the calculated particle size from BET generally decreases. The particle size distribution in Figure 2e continuously shifts to higher particle size consistent with the increase in the median. For example Alumina A has only a single relatively narrow peak that extends from 0.1 to a little less than 1 µm and the median (d50) is 0.267 µm. Alumina B shows three distinct peaks and the distribution extends out to almost 10 µm. It is possible that the smaller size peaks in the distribution are primary particles and the larger ones are the large porous aggregates, but proving it would require additional detailed analysis that is beyond the scope of this paper. The combination of light scattering, BET surface area measurement, and extensive microscopic examination of the particles with SEM and TEM shows that the surface area and porosity are increasing from top to bottom of the table. It is often suggested that one of the special characteristics of nanoparticles is the large surface area (catalytic activity for example depends on surface area). However, in this case the internal porosity is not accessible by the PTFE polymer chains in the bulk of the part. This polymer has a very high molecular weight (20 million) and the PTFE chains themselves are unreactive, until tribological contact occurs. Despite the high surface area of some of the alumina particles there is no evidence of interaction with the polymer and no reinforcement or unexpected change in mechanical properties detected in the bulk of the part. Chemical bonding of PTFE chain ends and reinforcement only occurs after the tribochemical changes in the running film surface formed in the early stages of wear [30].

page 9

Krick et al.

Figure 2: SEM micrographs of Alumina A (a and b) and Alumina E (c and d). The inset in (b) shows what a 350 nm particle would look like at this magnification. The inset in (d) shows what a 27 and 43 nm particle would look like at this magnification. The particles were coated on smooth featureless single crystal silicon or graphite substrates to simplify image analysis for particle size and shape. (e) Particle size distributions from static light scattering.

page 10

Krick et al. 4.3 Tribofilm analysis In almost all of the exceptionally low wear rate (< 10-7 mm3/(Nm)) samples (Alumina particles B, C, D, E) tested against a 304 stainless steel countersurface, both contact surfaces eventually developed a distinctly bronze-colored polymeric tribofilm easily seen by the naked eye. For the unfilled fluoropolymer control samples (K~ 4 x 10-4 mm3/(Nm)) and other filled composites that exhibited poorer wear performance (alumina A, K ~ 2.6 x 10-5 mm3/(Nm)), the transfer film (sliding-induced deposited polymeric tribofilm on the surface of the steel) was often patchy and translucent, and easily pushed aside by the moving test specimen. The fluoropolymer sample wear surface in these cases maintained its original color, and often showed deformation caused by excessive creep. The polymeric transfer film from alumina A was constantly removed from the metal, and dark grey or black wear debris decorated the edges of the wear path. Stylus profilometry was used to examine the steel surfaces after the wear experiments. Figure 3 is a comparison of the average profile of the steel surface for Aluminas A ( K ~ 2.6 x 10-5 mm3/(Nm)) and E (K ~ 5.7 x 10-8 mm3/(Nm)) over a 10 mm x 0.15 µm region in the center of the nominally 6.4 mm wide wear area. Alumina E formed a uniform, robust and adherent transfer film with an average thickness of 1.07 µm after over 56,000 meters of sliding. Alumina A, in stark contrast, created 20 µm deep gouges in the stainless steel surface, thus producing the dark grey or black wear debris. The wear experiment for Alumina A had to be halted after only 5,600 meters of sliding due to the excessive wear.

page 11

Krick et al.

Figure 3. Stylus profilometry of steel countersurface after the completion of a wear experiment in the linear reciprocating wear test. The traces are averages of 50 individual scan lines near the center of the wear area. The Alumina A composite (gray line) causes significant damage to the steel surface and was stopped after only 5,600 m sliding distance. The Alumina E composite (black line) produces a uniform bronze transfer film 1.07 µm thick after over 56,000 m of sliding. At first glance, the transfer film topography results seemed to contradict the SLS particle size measurements. Intuition would suggest that the larger the particle size would be more damaging to the steel countersamples. However, Alumina A (d50 = 0.267 µm) composites resulted in significant abrasion of the steel countersample, with wear scars much deeper than the average measured particle size. Alumina E had the largest particle size by static light scattering (d50 = 3.95 µm), yet its composite with PTFE had the lowest wear rate. This sample produced transfer films that were significantly thinner than the average alumina particle size. And still, a recent XPS study revealed a significant (~ 1-5 atomic %) alumina concentration at the surface of the transfer film [30]. Now, if we consider the new finding that the larger particles are porous, we can hypothesize that the porous particles are friable and break up due to mechanical stresses at the tribosurface, thus ameliorating the damage to the metal countersurface and the tribofilm. Hard, inorganic fillers in PTFE have been shown to accumulate at the polymer wear surface while simultaneously reducing the wear of PTFE and increasing the wear of the steel surface through abrasion [45]. X-ray microtomography images of the near surface region of the PTFE / Alumina E polymer composite showed evidence of the accumulation of smaller alumina particles at the wear interface, as well as micrometer-sized particles in the bulk (Figure 4). This observation further supports the mechanical-stress-induced reduction of particle size during the wear experiment. The accumulation of alumina particles within the tribofilms also presents a new mechanism for mechanical reinforcement of the polymer wear surface. Pitenis et al. reported that the PTFE C-C bonds can be broken by mechanical stresses during sliding (tribochemistry) [30]. The terminal carbon atoms created by this tribochemical event react with environmental species (O2 and H2O), resulting in perfluorinated carboxylic acid end groups that can chelate to the surface of metals and metal oxides [30]. The nano-scale alumina fragments that accumulate within the tribofilms in the present study can bond directly to the tribochemically-generated Rf-COOH endgroups. More than one carboxylateterminated PTFE filament chelated to the same nano-scale alumina fragment could result in effective crosslinking of the PTFE near the wear surface. This chemical interaction between matrix polymer and filler would further improve the toughness and creep resistance of the composite. This hypotheses is also consistent with the significant increase in hardness and modulus of the wear surface of the PTFE / alumina composite with extended sliding distance as reported by Krick et al. [28].

page 12

Krick et al.

Figure 4: 3D X-ray microtomography of side view of PTFE 7C / 5.0 wt. % Alumina E composite after completion of the wear experiment. The top portion of the figure shows an accumulation of alumina in the running film. Additional direct evidence for accumulation of alumina at the wear surface and for refinement / reduction in size of the alumina particles in successful ultralow wear composites is shown in the TEM micrographs in Figure 5. Five micrometers and further away from the running film surface, the TEM images are consistent with the X-ray microtomography results in which both techniques give evidence of occasional well-distributed microscale filler particles. A distinctly different region is observed, though, within the top few micrometers of the worn polymer surface. This near-surface region shows fine scale features that energy dispersive spectroscopy (EDS) in both the TEM and SEM indicate are alumina. This observation is consistent with the microtomography and X-ray photoelectron spectroscopy results, but the electron micrographs show the scale of the alumina particles present in the running film.

page 13

Krick et al.

Figure 5: Transmission Electron Micrograph (TEM) images of the running film of an Alumina E composite after 56,000 m of sliding. The running film is several micrometers thick and the alumina near the actual tribosurface is smaller than in the bulk of the part. The dramatic run-in behavior of this PTFE and alumina system has been well documented [5-24], often reporting initial wear rates as high as 2 x 10-5 mm3/Nm that transition to wear rates less than 1x107 mm3/Nm. This has been tied to the development of tribofilms as well as morphological and tribochemical changes to those tribofilms. The new evidence of alumina accumulating at the surface of

page 14

Krick et al. the polymer composite, as measured by X-ray microtomography (Figure 4) and TEM (Figure 5), further supports the role of run-in in generating a compositionally and structurally altered wear surface. Krick et. al. previously assessed the development of this “running-film” formed on the wear surface of the polymer and documented that its formation was directly linked to observed transition in wear rate (dropping 4 orders of magnitude with respect to PTFE 7C). Through nanoindentation, they also found that the films mechanical properties (modulus and hardness) increased with sliding. This is consistent with alumina accumulation and reinforcement of the wear surface. Remarkably, the accumulated alumina does not leave severe scratches in the surface of the steel. This is because of two primary reasons: a transfer film protects the metal surface [ref] and the alumina that is accumulated on the polymer wear surface has actually been broken down into nano-scale fragments (Figure 5), such that it cannot generate the pressures to penetrate the film and scratch the steel. Previous researchers have examined the worn surface of the PTFE / alumina composite parts with scanning electron microscopy and described the surface as consisting of “mudflat” cracking [19, 21, 22, 46]. A note of caution is warranted because this structure is merely a consequence of electron beam induced damage due to the high energy electron beams used in the SEM coupled with the electron poor nature of the fluoropolymer. In fact, a correlation has been observed between increasing exposure time and worsening of the “mudflat” cracks [47]i. The fibrils that are observed beneath the surface exposed by the cracks are similar to the fibrillation that Kitamura et al. observed in stretched PTFE [48]. This suggests that the running film shrinks under electron beam damage but remains mostly cohesive. Since the surface is well entangled with the underlying PTFE, the shrinking running film stretches the bulk PTFE and with extended exposure the shrinking pulls the bulk PTFE further causing more fibrillation. This “mudflat” cracks is a very different behavior as the PTFE composite away from the running film shows different decomposition behavior, more similar to bulk PTFE. Since the running film is showing electron beam degradation behavior different than the expected chain scission, we must conclude the running film has a different chemistry than bulk PTFE. X-ray microtomography coupled with ATR-IR spectroscopy on the wear surface of the worn polymer support both the tribochemical changes and the alumina reinforcement hypotheses for wear reduction mechanism in this material system tested on the Tribis pin-on-disk instrument (Figure 6). Teflon® PTFE 7C filled with 5.0 wt. % Alumina B was slid against a stainless steel sample to develop the tribofilms, resulting in ultralow wear rates. After the films were formed, the pressure was increased until “failure” of the composite, as indicated by both an increase in wear rates and friction coefficient, at which point the experiment was stopped. The wear surface of the composite is pictured in Figure 6a, revealing a bronze-colored running film (associated with low wear) on the left half, and a white surface where the running film had been ripped away on the right half. This sample enabled simultaneous analysis of a low-wearing region and a region that suffered wear-induced failure. High resolution X-ray microtomography through both regions of the part revealed micro scale fillers in the bulk and accumulation of finer scale alumina at the very surface of the bronze-colored running film (Figure 6b, note the resolution of microtomography under these conditions is slightly less than 1 µm per voxel). An ATR-IR spectrum of this bronze-colored running film (Figure 6c) showed the normal pair of CF2 peaks at 1206 and 1152 cm-1, as well as two new broad C=O peaks associated with tribochemically-generated perfluorinated carboxylate end groups at ~ 1650 and 1430 cm-1 (Figure 6c) [30, 49]. These carboxylate endgroups chelate to metals within the tribosystem [30] with the hypothesized structure shown in figure 6c. In agreement with the buildup of near-surface Al203 observed by microtomography and SEM is the concomitant increase in absorption in the metal oxide region; the IR peaks at 2920, 2851, and the broad rise below 1000 cm-1 of the ATR-IR spectrum of region 1 (Figure 6c) match very closely the published IR spectrum of Alumina B powder found in [36].

page 15

Krick et al. This significant increase is seen only in region 1 (Figure 6c), but not in the failed region 2 (Figure 6d) of the running film, or in an unworn surface of the polymer composite (Figure 6e). 4.4 What makes a particular alumina filler ultralow wear? - A conceptual framework It has long been a mystery why certain inorganic particles when blended into PTFE lead to four orders of magnitude improvement in wear performance over neat PTFE and others lead to only a modest 1-2 orders of magnitude reduction in wear. The mechanisms for ultralow wear are complex and in this paper we focus on the nature of the alumina particles themselves. “Good” alumina particles are not nanosized as the vendor-supplied approximate particle size would lead us to believe. The “best” alumina particles are 0.3 to 10 µm in size (as determined by SLS) with multiple peaks in the particle size distribution. However, if those particles were micrometers in size and fully dense they would be very abrasive and destroy the countersurface eliminating any chance for the formation of a stable transfer film. In fact, dense alpha-phase alumina particles above 10 µm in size do precisely that; leading to dark grey or black wear debris and obvious extreme metal substrate abrasion [19]. To date, the best ultralow wear alumina particles are non-spherical porous hard agglomerates, or aggregates of smaller nano-sized particles. Based on the similarity in morphology, we suspect that they are formed using a sol-gel process to produce boehmite alumina particles (AlO(OH)) [50]. When the small boehmite particles are heated to above 1,000 °C, they successively lose water and are converted to alpha-phase alumina, the most thermodynamically stable phase. Along with the chemical changes, the surface area drops rapidly from over 300 m2/g for boehmite to less than 50 m2/g for alpha-phase alumina [50-52]. We believe that the resulting sintered alpha-phase alumina is then ground back down to produce the micrometer-sized particles (with nanoparticle-like surface areas) used in the present and previous studies. The original size of the boehmite particles is preserved in the structure of the porous aggregates, and results in the BET surface area measurements are shown in Table 1. The porous alpha-phase alumina particles retain their multimicron size during the formation of the PTFE composites, as shown in the microtomography and TEM images of the bulk parts. The shear forces available during the various methods of blending alumina with polymer are insufficient to break up the aggregates. However, during the wear experiments, these aggregates are exposed to the surface of the steel. The mechanical stresses on the captured alumina particles leads to the fracture and refinement of the alumina. Four independent analytical methods: TEM, ATR-IR, X-ray microtomography, and XPS all show the increase in concentration of alumina at or near the running film surface. Additionally, the TEM images suggest that the particles are broken into smaller, possibly even nano-sized, particles. In the early stages of the wear process, broken alumina particles are in intimate contact with the metal countersample, where they clean the surface and allow chemical bonding of the fluoropolymer Rf-COOH chain ends to occur [30]. As sliding continues, the smaller alumina fragments are less abrasive and do not severely damage the steel countersurface, thus allowing the formation of the stable polymeric transfer film. Bonding of the carboxylic acid ends to the surface of the clean metal aids in the production of the transfer film, which protects the steel from any subsequent damage. Bonding of the carboxylic acid ends to the external surface of the alumina essentially crosslinks and binds the running film together. This enhancement of mechanical properties in the near-surface region serves to eliminate the fracture and creep failure modes of the PTFE composite part under certain P and V testing conditions. Eventually, when the testing pressure and/or velocity are increased enough, the composite becomes unstable and large portions of the running film and transfer film fail as seen in Figures 6a and b.

page 16

Krick et al. Fully dense particles such as Alumina A cannot break up as easily during the wear test, and therefore are able to cause significant abrasive damage of the metal substrate and transfer films under the large local contact pressures, leading to orders of magnitude worse wear rate. There appears to be an optimum in the combination of particle size and porosity (or friability) which leads to stable ultralow wear behavior. Non-spherical dense particles as small as Alumina A (0.267 µm), when dispersed in PTFE, have been shown to cause metal countersurface damage against 304 stainless steel and yield mediocre wear rates (2.6E-5 mm3/Nm, Table 1). Dense microparticles larger than 10 µm also caused metal countersurface abrasion and poor wear performance. Only porous, hard, friable, non-spherical particles in the 0.3-10 µm particle size range (as measured by SLS) were shown to be ultralow wear additives to PTFE. We believe these particles cleaned the metal surface during the early stages of the wear test, thereby allowing the tribochemically-created reactive fluoropolymer chain ends to both bind to the metal countersurface and act as crosslink sites in the running film to reduce fracture and creep [28]. The resulting wear couple contained robust and tribochemically-modified fluoropolymer composite on both sides, which protected the metal countersurface and the polymer composite part from wear damage. It was only in these unique cases that the wear rates were observed to drop to near zero, thereby enabling the parts to slide for kilometers with minimal loss of material.

page 17

Krick et al.

Figure 6: X-ray Microtomography and ATR-IR of PTFE 7C / 7.0 wt% Alumina B composite, worn to pressure-induced failure. a) Image of polymer wear surface revealing running film (region 1: left) and failed section (region 2: right). b) Microtomography of worn surface, false color of dense (alumina) domains corresponds to domain volume. c) IR spectra of region 1 revealing system tribochemistry. d) IR spectra of region 2. e) IR spectra of unworn polymer surface.

page 18

Krick et al. 5. Conclusions The complete mechanism of the 4 order of magnitude wear reduction observed by filling granular PTFE with certain types of alumina is still not entirely understood. Our experience with this material has taught us that it is a complex tribosystem, controlled by multi-length and force scale physical, mechanical and chemical interactions that are dependent on sliding environment, history, material pairing, sliding parameters (velocity, pressure), surface roughness and other system parameters. While a complete mechanism is likely more complicated than the presented mechanistic observations, in this paper we have shown direct experimental evidence backed by thorough characterization, linking multiple mechanistic hypotheses that govern what makes some fillers produce ultralow wear composites. From direct experimental observations, we gather conclusions about the requirements and mechanistic roles of the alumina filler: Direct observations: 1. Alumina particles for ultralow wear PTFE composites are porous and micro in size, not nanoparticles like originally presumed. Previous reports used supplier-designated particle size, which we have shown are likely based on BET surface area measurements. Complimentary SEM and SLS measurements actually reveal that these particles are ~1-10 µm average particle size with significant porosity. While micro in external particle dimension, these mesoporous-like filler materials are still “nano” in surface area, and have nanometer-scaled features. 2. In general, wear rate decreased with increasing BET area to average particle size ratios for the particles evaluated. This is directly shown with experimental wear results coupled with detailed alumina SLS and BET analysis. 3. Ultralow wear alumina composites result in thin, uniform, robustly-adhered polymeric transfer films while the higher-wearing composites result in patchy transfer films and significant metal countersample abrasion. These films are significantly thinner than the primary alumina particle size. This is supported directly by transfer film topography and is consistent with the literature. 4. Alumina accumulates at the sliding interface and is reduced in size compared to the bulk composite. Microtomography, XPS, TEM and ATR-IR confirm the increased concentration of submicrometer scale alumina at the wear surface in ultralow wearing systems. 5. New endgroup chemistry is directly linked with wear reduction. Tribochemical formation of carboxylic acid endgroups within the transfer film and running film occur in ultralow wear PTFE / alumina systems and disappear beyond the pressure-induced wear limits of the system. Mechanistic implications (requirements and mechanistic roles of the filler) 1. Multi-scale fillers necessary for wear reductions. Micrometer-scaled particles reduce subsurface delamination of PTFE composites in the bulk to near surface region [28, 30], while nanometerscale alumina particle fragments (see mechanistic implication 2) stabilize and reinforce tribofilms (see mechanistic implication 4) 2. Porosity of alumina results in mechanically friable alumina. These friable particles can be broken into sub-micrometer particles at the sliding interface through tribologically-induced shear

page 19

Krick et al. stresses. As the BET surface area to SLS-measured particle diameter ratio increases, the particle porosity and presumably friability also increase. 3. Porous particles are at the delicate threshold of abrasiveness. They are capable of removing surface contaminants and oxides from the metal countersamples (to enable chelation of polymer endgroups to metal countersample) while neither abrading the metal countersample nor removing the transfer film after > 50 km sliding distance. The gradual reduction in particle size in the near-surface region can reduce its effective abrasiveness. 4. Sub-micron scale particle fragments accumulate within the wear surface (running film) of the polymer composite and in the transfer film on the steel and serve to mechanically and chemically reinforce the polymer. Multiple (>2) tribochemically-generated carboxylic acid endgroups in the PTFE can bond to alumina fragments within the tribofilms and effectively act as crosslinks. Acknowledgments The authors would like to thank the many collaborators for their thoughtful comments and insight, including Alan Allgeier, Heidi Burch, Joseph Galperin, Gary Halliday, Tim Krizan, Karl Lehman, and Lei Zhang from DuPont.

page 20

Krick et al. Tables and Captions: Table 1: Summary of all alpha-phase alumina materials used in this work and in previous literature. The order in the table is based on the median or d50 particle size from light scattering. The vendor specified particle size is included for comparison. Wear rate reported is the steady state wear rate for DuPont PTFE 7C filled with 5 wt. % alumina filler measured on the linear reciprocating tribometer at 6.25 MPa and 50.8 mm/s (see materials and methods).

page 21

Krick et al. References

[1] M.M. Renfrew and E.E. Lewis, Polytetrafluoroethylene - heat-resistant, chemically inert plastic. Industrial and Engineering Chemistry. 38, 870-877 (1946). [2] K.V. Shooter and D. Tabor, The frictional properties of plastics. Proceedings of the Physical Society. Section B. 65, 661 (1952). [3] R.F. King and D. Tabor, The effect of temperature on the mechanical properties and the friction of plastics. Proceedings of the Physical Society of London Section B. 66, 728736 (1953). [4] D.G. Flom and N.T. Porile, Friction of teflon sliding on teflon. Journal of Applied Physics. 26, 1088-1092 (1955). [5] D.L. Gong, Q.J. Xue, and H.L. Wang, Study of the wear of filled polytetrafluoroethylene. Wear. 134, 283-295 (1989). [6] T.A. Blanchet and F.E. Kennedy, Sliding wear mechanism of polytetrafluoroethylene (PTFE) and PTFE composites. Wear. 153, 229-243 (1992). [7] W.G. Sawyer, K.D. Freudenberg, P. Bhimaraj, and L.S. Schadler, A study on the friction and wear behavior of PTFE filled with alumina nanoparticles. Wear. 254, 573-580 (2003). [8] K. Tanaka and S. Kawakami, Effect of various fillers on the friction and wear of polytetrafluoroethylene-based composites. Wear. 79, 221-234 (1982). [9] S. Bahadur and D. Tabor, The wear of filled polytetrafluoroethylene. Wear. 98, 1-13 (1984). [10] D.L. Gong, B. Zhang, Q.J. Xue, and H.L. Wang, Effect of tribochemical reaction of polytetrafluoroethylene transferred film with substrates on its wear behavior. Wear. 137, 267-273 (1990). [11] Z.Z. Zhang, Q.J. Xue, W.M. Liu, and W.C. Shen, Friction and wear characteristics of metal sulfides and graphite-filled PTFE composites under dry and oil-lubricated conditions. Journal of Applied Polymer Science. 72, 751-761 (1999). [12] F. Li, F.Y. Yan, L.G. Yu, and W.M. Liu, The tribological behaviors of copper-coated graphite filled PTFE composites. Wear. 237, 33-38 (2000). [13] F. Li, K.A. Hu, J.L. Li, and B.Y. Zhao, The friction and wear characteristics of nanometer ZnO filled polytetrafluoroethylene. Wear. 249, 877-882 (2001). [14] J. Khedkar, I. Negulescu, and E.I. Meletis, Sliding wear behavior of PTFE composites. Wear. 252, 361-369 (2002). [15] W.X. Chen, F. Li, G. Han, J.B. Xia, L.Y. Wang, J.P. Tu, and Z.D. Xu, Tribological behavior of carbon-nanotube-filled PTFE composites. Tribology Letters. 15, 275-278 (2003). [16] N.V. Klaas, K. Marcus, and C. Kellock, The tribological behaviour of glass filled polytetrafluoroethylene. Tribology International. 38, 824-833 (2005). [17] D.L. Burris and W.G. Sawyer, Improved wear resistance in alumina-PTFE nanocomposites with irregular shaped nanoparticles. Wear. 260, 915-918 (2006). [18] S.Q. Lai, L. Yue, T.S. Li, and Z.M. Hu, The friction and wear properties of polytetrafluoroethylene filled with ultrafine diamond. Wear. 260, 462-468 (2006). [19] S.E. Mcelwain, T.A. Blanchet, L.S. Schadler, and W.G. Sawyer, Effect of particle size on the wear resistance of alumina-filled PTFE micro- and nanocomposites. Tribology Transactions. 51, 247-253 (2008). [20] D.L. Burris, S. Zhao, R. Duncan, J. Lowitz, S.S. Perry, L.S. Schadler, and W.G. Sawyer, A route to wear resistant PTFE via trace loadings of functionalized nanofillers. Wear. 267, 653-660 (2009). page 22

Krick et al. [21]

[22]

[23] [24]

[25]

[26] [27]

[28]

[29]

[30]

[31] [32] [33] [34] [35] [36]

[37]

T.A. Blanchet, S.S. Kandanur, and L.S. Schadler, Coupled effect of filler content and countersurface roughness on PTFE nanocomposite wear resistance. Tribology Letters. 40, 11-21 (2010). D.L. Burris, B. Boesl, G.R. Bourne, and W.G. Sawyer, Polymeric nanocomposites for tribological applications. Macromolecular Materials and Engineering. 292, 387-402 (2007). D.L. Burris and W.G. Sawyer, Tribological sensitivity of PTFE/alumina nanocomposites to a range of traditional surface finishes. Tribology Transactions. 48, 147-153 (2005). B.A. Krick, J.J. Ewin, G.S. Blackman, C.P. Junk, and W. Gregory Sawyer, Environmental dependence of ultra-low wear behavior of polytetrafluoroethylene (PTFE) and alumina composites suggests tribochemical mechanisms. Tribology International. 51, 42-46 (2012). T.A. Blanchet, F.E. Kennedy, and D.T. Jayne, XPS analysis of the effect of fillers on PTFE transfer film development in sliding contacts. Tribology Transactions. 36, 535-544 (1993). K. Tanaka, Y. Uchiyama, and S. Toyooka, The mechanism of wear of polytetrafluoroethylene. Wear. 23, 153-172 (1973). W.G. Sawyer, B.A. Krick, G.S. Blackman, and C.P. Junk, Articles having low coefficients of friction, methods of making the same, and methods of use, U.S. Patent 2012. B.A. Krick, J.J. Ewin, and E.J. McCumiskey, Tribofilm Formation and Run-In Behavior in Ultra-Low-Wearing Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites. Tribology Transactions. 1058-1065 (2014). A. Pitenis, J. Ewin, K. Harris, W.G. Sawyer, and B. Krick, In Vacuo Tribological Behavior of Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites: The Importance of Water for Ultralow Wear. Tribology Letters. 53, 189-197 (2014). A. Pitenis, K. Harris, C. Junk, G. Blackman, W.G. Sawyer, and B. Krick, Ultralow Wear PTFE and Alumina Composites: It is All About Tribochemistry. Tribology Letters. 57, 18 (2015). C.P. Junk, P.G. Bekiarian, and M.D. Wetzel, Slurry technique for producing fluoropolymer composites, U.S. Patent 2012. G.S. Blackman, C.P. Junk, W.G. Sawyer, B.A. Krick, and M.D. Wetzel, Low-wear fluoropolymer composites, U.S. Patent 2012. J. Urueña, A. Pitenis, K. Harris, and W.G. Sawyer, Evolution and Wear of Fluoropolymer Transfer Films. Tribology Letters. 57, 1-8 (2015). W.G. Sawyer, N. Argibay, D.L. Burris, and B.A. Krick, Mechanistic Studies in Friction and Wear of Bulk Materials. Annual Review of Materials Research. 44, 395-427 (2014). J. Ye, H.S. Khare, and D.L. Burris, Transfer film evolution and its role in promoting ultra-low wear of a PTFE nanocomposite. Wear. 297, 1095-1102 (2013). M. Faghihi and A. Shojaei, Properties of alumina nanoparticle-filled nitrile-butadienerubber/phenolic-resin blend prepared by melt mixing. Polymer Composites. 30, 12901298 (2009). J.S. Taurozzi, V.A. Hackley, and M.R. Wiesner, Ultrasonic dispersion of nanoparticles for environmental, health and safety assessment – issues and recommendations. Nanotoxicology. 5, 711-729 (2011).

page 23

Krick et al. [38] [39]

[40]

[41] [42] [43] [44]

[45] [46]

[47]

[48]

[49] [50]

[51] [52]

S. Brunauer, P.H. Emmett, and E. Teller, Adsorption of Gases in Multimolecular Layers. Journal of the American Chemical Society. 60, 309-319 (1938). T.L. Schmitz, J.E. Action, D.L. Burris, J.C. Ziegert, and W.G. Sawyer, Wear-rate uncertainty analysis. Journal of Tribology-Transactions of the Asme. 126, 802-808 (2004). T.L. Schmitz, J.E. Action, J.C. Ziegert, and W.G. Sawyer, The difficulty of measuring low friction: Uncertainty analysis for friction coefficient measurements. Journal of Tribology-Transactions of the Asme. 127, 673-678 (2005). D.L. Burris and W.G. Sawyer, Tribological behavior of PEEK components with compositionally graded PEEK/PTFE surfaces. Wear. 262, 220-224 (2007). D.L. Burris, Investigation of the tribological behavior of polytetrafluoroethylene at cryogenic temperatures. Tribology & Lubrication Technology. 64, 40-50 (2008). D.L. Burris and W.G. Sawyer, Measurement uncertainties in wear rates. Tribology Letters. 36, 81-87 (2009). J.F. Archard and W. Hirst, The Wear of Metals under Unlubricated Conditions. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences. 236, 397-410 (1956). V.N. Aderikha and V.A. Shapovalov, Tribological behavior of polytetrafluoroethylenesilica composites. Journal of Friction and Wear. 32, 124-132 (2011). S.S. Kandanur, M.A. Rafiee, F. Yavari, M. Schrameyer, Z.-Z. Yu, T.A. Blanchet, and N. Koratkar, Suppression of wear in graphene polymer composites. Carbon. 50, 3178-3183 (2012). U. Lappan, U. Geißler, and K. Lunkwitz, Changes in the chemical structure of polytetrafluoroethylene induced by electron beam irradiation in the molten state. RADIATION PHYSICS AND CHEMISTRY. 59, 317-322 (2000). T. Kitamura, S. Okabe, M. Tanigaki, K.-I. Kurumada, M. Ohshima, and S.-I. Kanazawa, Morphology change in polytetrafluoroethylene (PTFE), porous membrane caused by heat treatment. Polymer Engineering & Science. 40, 809-817 (2000). M. Przedlacki and C. Kajdas, Tribochemistry of fluorinated fluids hydroxyl groups on steel and aluminum surfaces. Tribology Transactions. 49, 202-214 (2006). M. Nguefack, A.F. Popa, S. Rossignol, and C. Kappenstein, Preparation of alumina through a sol-gel process. Synthesis, characterization, thermal evolution and model of intermediate boehmite. Physical Chemistry Chemical Physics. 5, 4279-4289 (2003). C.J. Brinker and G.W. Scherer, Sol-gel science: the physics and chemistry of sol-gel processing. Academic press, (2013). R.A. Shelleman, G.L. Messing, and M. Kumagai, Alpha alumina transformation in seeded boehmite gels. Journal of Non-Crystalline Solids. 82, 277-285 (1986).

page 24

Krick et al.

Highlights:     

PTFE/alumina composites wear rates are 4 orders of magnitude lower than unfilled PTFE Low wear behavior derived from nanostructured alumina microfillers Nanostructured alumina microfillers become nanoscale fragments at the interface Alumina fragments covalently bond to tribochemically altered PTFE Tribofilms consist of alumina accumulation and tribochemically altered PTFE

page 25