Tribological Insensitivity of an Ultralow-Wear Poly(etheretherketone

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Tribological Insensitivity of an Ultra-Low Wear PEEKPTFE Polymer Blend to Changes in Environmental Moisture Diana R Haidar, Kazi Istiaque Alam, and David L. Burris J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12487 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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The Journal of Physical Chemistry

Tribological Insensitivity of an Ultra-Low Wear PEEK-PTFE Polymer Blend to Changes in Environmental Moisture Diana R. Haidar, K. Istiaque Alam, David L. Burris* 1

Department of Mechanical Engineering University of Delaware Newark, DE, United States

Abstract

The tribological properties of most high functioning tribological materials, including diamond, graphite, molybdenum disulfide, and polytetrafluoroethylene, depend strongly on environmental moisture. A particularly wear resistant alumina-PTFE, for example, loses its capacity for ultralow wear in dry environments because a moisture-dependent tribochemical degradation product is necessary to anchor and stabilize its protective transfer films. A recent study [Onodera et al., J. Phys. Chem. C, 2017, 121, 14589−14596] on a PEEK-PTFE composite suggested that the PEEK filler particles anchor PTFE transfer films to metallic surfaces via physical interactions that are, theoretically, insensitive to environmental moisture. This study tested the hypothesis that the physical nature of transfer film adhesion by PEEK-PTFE increases its wear tolerance to changes in environmental moisture. The optimal 20 wt% PEEK-PTFE composite exhibited the same ultra-low wear rates (8x10-8 ± 1x10-8 mm3/Nm) and low friction coefficients (0.18 ± 0.02) in dry nitrogen (0.05% RH) and humid air (30% RH). The results demonstrate that this unusually wear resistant solid lubricant material is also unusually insensitive to environmental moisture. Compared to the well-studied alumina-PTFE system, whose ultra-low wear rates correlate strongly to the prominence of carboxylate peaks in infrared (IR) spectra, carboxylate peaks were either greatly attenuated or absent in IR spectra of PEEK-PTFE following ultra-low wear sliding in both humid and dry environments. The results are consistent with the prediction from the Onodera group that the ultra-low wear rates of PEEK-PTFE can be retained in dry environments because the strong physical interactions between the PEEK filler and the counterface reduces or eliminates its dependence on water-dependent tribochemistry for transfer film adhesion. *corresponding author David L. Burris, Ph.D. Dept. of Mechanical Engineering University of Delaware [email protected]

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1.0 Introduction Solid lubricant materials are necessary in many mission critical aerospace applications where environmental extremes preclude the use of more traditional lubrication strategies.1–3 Satellites, for example, are loaded with tribological components including bushings, gears, gimbals, slip rings, and latches that must operate remotely, reliably, and with minimal friction in the extremes of an extraterrestrial environment.4 Ironically, the tribological properties of solid lubricants depend strongly on the same environmental considerations that motivate their use.1 While molybdenum disulfide, the gold standard for space applications, performs well under moisturefree environments, it degrades mechanically and tribologically when exposed to humid air;1,5,6 the Galileo high gain antenna failure has been attributed to the failure of an MoS2 coating following exposure to moist air during terrestrial testing or transport.7,8 All ~20 of the solid lubricant materials reviewed by Scharf and Prasad exhibited significant and detrimental tribological sensitivity to environmental moisture.1 One approach to solving this moisture-sensitivity problem is a composite material whose constituents perform well under different conditions. The resulting ‘chameleon’ coating ‘adapts’ to changing environments through sacrificial removal of the ‘unfavorable’ constituent and preferential expression of the ‘favorable’ constituent. Chameleon coatings, which were conceived and developed by the Air Force Research Laboratory, have proven to be environmentally tolerant even if not environmentally insensitive.9 Another approach uses a composite material whose constituents are expected to be insensitive to environmental moisture. Burris and Sawyer chose polytetrafluoroethylene (PTFE), a hydrophobic, chemically resistant, high temperature, and low friction polymer,10 as their starting point. While experimenting with alumina-filled PTFE solid lubricant composites, they discovered that one particular alumina particle reduced PTFE wear rates by two orders of magnitude compared to other comparable alumina-filled PTFE composites and nanocompoites.11,12 Krick et al. demonstrated that the unusual wear resistance of this ultra-low wear alumina-PTFE composite is due to a hybrid effect from the alumina, which consisted of nanoporous microparticles;13 these microparticles were large and strong enough to arrest subsurface damage, yet weak enough to disband into their constituent nanoparticles once reaching the sliding interface without significant abrasion to the protective transfer film. In effect, they possessed the benefits of nanoparticles and microparticles without the drawbacks of either. This ultra-low wear alumina-PTFE was selected, in addition to two chameleon coatings, one diamond-like carbon, and gold, for inclusion in an unprecedented tribology study of the most promising space lubricants in low earth orbit outside the International Space Station14. Unfortunately, subsequent analysis revealed excessive wear rates and an unusual blackening, rather than the typically-observed browning, of the wear track. Follow-up laboratory testing in dry nitrogen15 and high vacuum16 demonstrated that the absence of environmental water had caused the wear rate of this ‘inert’ solid lubricant to unexpectedly increase by ~100x.


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The first theoretical insight into the origins of increased wear rates in dry environments came from Onodera et al., who predicted that PTFE chain ends react with environmental water to form carboxyl groups, which help bond the transfer film to the substrate.17,18 Pitenis et al. used infrared spectroscopy to clarify the tribochemical details.19,20 They showed that PTFE chains were broken with surprising ease by Van der Waals attraction to the metallic counterface. These broken chains reacted with environmental oxygen and then water to form carboxylic acid, which formed a chelate salt with the metallic counterface. The loss of ambient moisture prevented this favorable tribochemical process, eliminated direct bonding of polymer chains to the counterface, destabilized the transfer film, and ultimately prevented the ultra-low wear rates expected of this otherwise promising solid lubricant.21 These studies highlight that fact that even the most chemically resistant materials can be chemically active under relatively gentle tribological conditions. Furthermore, the current body of work suggests that ultra-low wear PTFE requires favorable water-dependent tribochemistry to anchor an otherwise slippery PTFE transfer film. However, a recent study by Onodera et al. suggests another path toward environmental insensitivity of ultra-low wear PTFE-based solid lubricants.22 They showed that PEEK filler particles may preferentially anchor PTFE transfer films through strong adsorption to metallic counterfaces. We showed previously that PEEKPTFE blends exhibit ultra-low wear when tested in air because, like nanoporous alumina microparticles, PEEK particles are large enough to arrest subsurface cracks without damaging the protective transfer film.23 Together, the results of these two studies suggest that the stable transfer films and ultra-low wear rates previously observed for PEEK-PTFE may persist following the removal of environmental humidity. Here, we study PEEK-PTFE blends of varying composition in humid air and dry nitrogen to test the hypothesis that its ultra-low wear rates are less dependent on water-based tribochemistry and thus insensitive to changes in environmental moisture. 2.0 Experimental Methods 2.1 Materials The PEEK-PTFE composites used in this study were fabricated in the same manner as those reported previously.24 The PTFE powder used was Teflon™ 7C resin (30 µm reported diameter particles) from DuPont. The PEEK powder was 450PF molding resin (50 µm reported diameter particles) from Victrex. The prescribed percentage by weight of PEEK in PTFE were pre-massed from 0 to 100% PEEK before being combined. By volume, one part of the powder ensemble and two parts anhydrous ethanol were combined. Mixing proceeded using an ultrasonic horn with 460 W power applied for two out of every three seconds over five total minutes. This wet powder mixture was then dried under rough vacuum at 100 ˚C. The dried powder ensemble was cold compacted in a cylindrical mold at 100 MPa of pressure and then removed from the mold. This green part was heated in a furnace using a ramp to 365 ˚C at 120 ˚C/min, a 10 h hold at 365 ˚C, and a ramp down to room temperature at 120 ˚C/h.



2.2 Wear rate quantification Before testing, each specimen was machined into a rectangular pin of 6.4 mm x 6.4 mm contact area and 12.7 mm length. A flat counterface of 304 stainless steel (38 mm long x 25 mm wide) was prepared for testing by polishing and lapping to 30 nm ± 10 nm Ra. Prior to testing, the surface of each polymer pin was mounted in the linear reciprocating testing rig and preconditioned with 50 N of normal force (1.2 MPa pressure) against 600 grit SiC paper over 3 reciprocation cycles of 50.8 mm each to remove machining marks from the polymer surface and align the surfaces. Following this pre-conditioning step, the density and starting mass of each sample was determined by measuring the dimensions and mass with uncertainties of ±0.05 mm and 0.05 mg respectively.

Figure 1. Six-station pin-on-flat tribometer used for high-throughput wear rate measurements of polymer composite pins against 304 stainless steel counterfaces. The samples were tested using a normal force of 250 N (6.3 MPa), a sliding speed of 50.8 mm/s, and a reciprocating track length of 25.4 mm (50.8 mm per cycle) in dry nitrogen and humid air. Wear tests were conducted on the 6-station linear reciprocating pin-on-flat tribometer shown in Figure 1 at a normal force of 250 N (6.3 MPa), a sliding speed of 50.8 mm/s, and a wear track length of 25.4 mm (50.8 mm per cycle) in both dry nitrogen and humid air. To set up the test, the vertical stage was lowered to and locked at a fixed ‘testing’ location. Each of 6 samples was


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gripped with a cylindrical clamp, which was then inserted into a keyed holder until the sample made contact with the counterface at a tare load of (< 3 g) before tightening the clamp; this procedure ensured alignment of all samples at zero load and zero flexure deflection. The pneumatic pistons were then used to load the samples though a manually controlled pressure regulator at a common manifold. A single compression load cell (not shown) located between a sample holder and pneumatic applicator was used to set and measure the load throughout testing; the measured standard deviation was ~2% due to variations in air pressure, sample wear, and stiction of the seal in the piston/cylinder device. Deflections of the flexure (40 N/mm) with strain and wear consumed less than 5% of the applied load for unfilled PTFE and less than 1% of the applied load for all other materials, which were more resistant to elastic deformation, creep, and wear.23 Test were conducted at ~25 ˚C in a glovebox. ‘Dry nitrogen’ experiments were conducted with less than 10 ppm water using 99.9998% dry N2 gas with constant closed-cycle recirculation through an activated charcoal purifier.21 ‘Humid air’ experiments were conducted open to ambient laboratory air of ~30% RH. Independent but nominally identical samples were created for testing under the two environmental conditions. Dry tests were interrupted and samples were exposed to humid air periodically for mass loss measurements. Before reintroducing the sample into the clean glovebox, it was placed into an anti-chamber and subjected to five ‘pump-purge’ cycles designed to mitigate contamination; using this method, our humidity sensor was unable to resolve (to 1ppm) an increase in water contamination from test interruption. Samples were tested until either the total sliding distance reached 50 km or the accrued volume lost reached 10 mm3. For each sample, the steady state wear rate, k (mm3/Nm), was determined as the mean slope of volume lost versus the product of normal load and sliding distance for steady state conditions; the experimental uncertainty in the steady state wear rate was quantified as described by Sawyer and Burris.25 Because this instrument does not measure friction forces, limited follow-up friction coefficient measurements were made on a similar single-station tribometer identical to that used by Burris and Sawyer in the previous study of this composite material.23 2.3 Chemical analysis The infrared spectra of the worn and unworn surfaces were collected using a Bruker Tensor 27 Fourier-transform infrared (FTIR) spectrometer with a smart orbit attenuated total reflectance (ATR) accessory. These measurements were conducted as described previously.24 Each spectrum consisted of 64 scans co-averaged at 4 cm-1 spectral resolution. The spectra were collected by placing the polymer surface of interest (worn or unworn) into contact with the diamond ATR crystal. Prior to each measurement, the diamond was mechanically cleaned by the shearing motion of a cotton swab with acetone to remove any previously transferred material and then wiped further with a dry KimWipe cloth. 3.0 Results All of the wear volume measurements in the study are shown as functions of sliding distance in dry nitrogen and humid air in Figure 2. As expected, unfilled PTFE was the clear outlier,



wearing by up to 10 mm3 in as little as 100 m of sliding in both environments. Unfilled PEEK wore by 10 mm3 in just under 10 km of sliding, which reflects the fact that unfilled PEEK is ~100x more wear resistant than unfilled PTFE in both environments. The 20, 30, and 40wt% PEEK-PTFE samples lost between 1 and 3 mm3 in 50 km of total sliding distance in both environments, which demonstrates insensitivity of the wear response to environmental humidity and composition within this range.

Figure 2. Wear volume as a function of sliding distance for 0-100wt% PEEK-PTFE composites in (a) in dry nitrogen (1,000x regardless of environmental moisture; by contrast, the addition of 10wt% PTFE to PEEK had no significant wear reducing effect. The asymmetry of this curve is consistent with that reported previously by Burris and Sawyer.23 The minimum wear rate was observed at 20wt% PEEK-PTFE regardless of environment; this is the same optimal loading reported from independent studies by Burris and Sawyer23 and Onodera et al.22 after testing in air. At this optimal composition, steady state wear rates were comparable at 7 and 9 x 10-8 mm3/Nm in humid and dry conditions, respectively. Changes in environmental moisture had similarly insignificant effects on the steady state wear rates of 30 and 40wt% PEEK-PTFE composites. The 5 and 10% PEEK-PTFE composites had significantly lower wear rates in dry N2 (3 x 10-7 mm3/Nm) while 50% PEEK-PTFE had significantly lower wear rates in humid air.


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Figure 3. Wear rate as a function of filler loading for all samples tested in this study. The PTFEbased composites were filled with 0-100wt% PEEK in dry nitrogen (circles) and humid air (squares). Separate friction coefficient measurements were made for the optimal 20wt% PEEK-PTFE composite (results not shown). The composite had a stable friction coefficient of 0.18 ± 0.01 over 50 km in humid air; in dry nitrogen, a nominally identical but independent sample had a statistically indistinguishable friction coefficient of 0.19 ± 0.02. Both values are comparable to those reported by Onodera et al.22 at 20wt% in humid air and below those reported for unfilled PTFE (0.21) and PEEK (0.29). Friction coefficients were not measured for sub-optimal blends. The running surfaces and transfer films of 5-50wt% PEEK-PTFE following testing are shown in Figure 4. Sliding in air, especially for samples near 20wt%, produced the same brown discoloration found on the wear surface of ultra-low wear alumina-PTFE following testing in humid air. However, unlike alumina-PTFE, whose wear surface blackens during testing in dry environments,15,16 the wear surface of 20wt% PEEK-PTFE remained brown after testing in dry nitrogen. The intensity of this brown discoloration tended to decrease with deviation from optimal PEEK content.



Transfer film morphology, specifically thickness, depended more on environmental moisture than PEEK content. Transfer films formed in dry nitrogen were consistently thinner (0.05-0.1 µm) than those formed in humid air (0.3-1 µm); transfer films formed in humid air, especially near the optimal composition, were similar in coverage, thickness, and morphology to those formed by ultra-low wear alumina-PTFE in humid air.24,28,29 These results demonstrate that the wear resistance and transfer film stability of this material family are primarily driven by PEEK content and are largely insensitive to significant changes in environmental humidity and transfer film morphology.

Figure 4. Optical images of running surfaces on the polymer and transfer films on the counterface from 5-50wt% PEEK-PTFE following testing in dry and humid environments; images from humid environments are denoted by *. FTIR spectra are shown for each polymer running surface from 0-50wt% PEEK-PTFE following testing in humid air and dry nitrogen in Figure 5. The characteristic peaks for PTFE occur at 1150 and 1200 cm-1.20 The peaks at 1490, 1590, and 1650 cm-1 are attributable to PEEK. Spectra were scaled such that the most prominent peaks of the two constituents (1490 cm-1 for PEEK and 1150 cm-1 for PTFE) summed to 1. In each environment, the FTIR spectra demonstrate that PEEK content on the running surface tended to increase with the nominal PEEK content in the unworn composite. Interestingly, running surfaces from humid air testing exhibited consistently greater PEEK content than the corresponding sample testing in dry nitrogen. FTIR spectra are isolated for the optimal 20wt% PEEK-PTFE samples in Figure 6. The relative PEEK content is lowest for the unworn sample, which suggests that worn surfaces were PEEKrich due, presumably, to preferential removal of PTFE, a process common to most if not all PTFE-based composites.20,21,28,30 In humid air, PEEK enrichment was greater and the total worn volume was about twice that of the dry environment. Although growth in the 1650 cm-1 peak with sliding is consistent with the formation of carboxylate salts (1660 cm-1), the proportional growth of the 1590 and 1490 cm-1 peaks indicates that most of this sliding-induced growth at


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1650 cm-1 was attributable to PEEK rather than the tribochemical formation of carboxylates. Nonetheless, the shoulders at 1430 and 1660 cm-1 and the broad peak at 3390 cm-1 suggest that carboxylates did form during sliding, particularly in humid air, but to a limited extent when compared to the alumina-PTFE system.

Figure 5. Fourier-transform infrared (FTIR) spectra of polymer running films following wear testing in (A) dry nitrogen (

Tribological Insensitivity of an Ultralow-Wear Poly(etheretherketone

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