Ultralow Wear PTFE-Based Polymer Composites—The Role of Water

Jul 9, 2019 - The role of water in the tribochemical mechanisms of ultralow wear polytetrafluoroethylene (PTFE) composites was investigated by studyin...
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Article Cite This: Macromolecules 2019, 52, 5268−5277

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Ultralow Wear PTFE-Based Polymer CompositesThe Role of Water and Tribochemistry Kasey L. Campbell,† Mark A. Sidebottom,§ Cooper C. Atkinson,‡ Tomas F. Babuska,‡ Claudia A. Kolanovic,‡ Brian J. Boulden,∥ Christopher P. Junk,† and Brandon A. Krick*,‡ Department of Materials Science and Engineering and ‡Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, Pennsylvania 18015, United States § Mechanical and Manufacturing Engineering Department, Miami University, Oxford, Ohio 45056, United States ∥ Boulden Company, Conshohocken, Pennsylvania 19428, United States Downloaded via BUFFALO STATE on July 26, 2019 at 06:44:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: The role of water in the tribochemical mechanisms of ultralow wear polytetrafluoroethylene (PTFE) composites was investigated by studying 10 and 20 wt % polyether ether ketone (PEEK)-filled and 5 wt % αAl2O3-filled PTFE composites. These composites were run against stainless-steel substrates in humidity, water, and dry nitrogen environments. The results showed that the wear behavior of both composites was significantly affected by the sliding environment. Both composites achieved remarkably low wear rates in humidity because of tribochemically generated carboxylate end groups that anchored the polymer transfer films to the steel substrate. In nitrogen, PTFE−PEEK outperformed PTFE−αAl2O3 because of polar carbonyl groups on PEEK, which increased the surface energy of PEEK, aiding it in adhering to the substrate and resulting in a transfer film. Both composites in water exhibited high wear. The water oversaturated the functional groups at the end of the polymer chains and prevented the formation of a transfer film. chanics of filler materials,28,39,44,45 countersurface roughness,37,46 polymer morphology,17,35,44 transfer film formation,13,30,35,37,38,41 composite dispersion,17 and a range of other sliding parameters such as load,47 velocity,7 and contact pressure.47 Through studies varying the sliding environment (humid air, dry air, and vacuum)29,30,36 as well as IR spectroscopy of tribofilms, tribochemistry was identified as a major player in the difference between ultralow wear PTFE− alumina composites28,30,36,37,39,40 and other marginally wearresistant reinforced PTFE systems.7,8,19,23,27,48,49 Under standard conditions (e.g., humid air, moderate contact pressure of ∼6 MPa, and moderate sliding velocity of ∼50 mm/s) PTFE−αAl2O3 exhibits ultralow wear (K on the order of or less than 10−7 mm3 N−1 m−1). However, its wear rate increases by orders of magnitude under high vacuum (K ≈ 1.5 × 10−5 mm3 N−1 m−1)36 and other environments without water vapor,29,30,36 limiting the possible applications and performance of this remarkable material. The role of the alumina filler in wear reduction is attributed to both the micron and nanometer scale character of the alumina oxide particles: microscale fillers reinforce PTFE to reduce subsurface delamination of the PTFE matrix, which would otherwise result in large flakey wear debris.6−8,11,37,45 However, hard micron-scale fillers can abrade the metal counter-sample

1. INTRODUCTION Polytetrafluoroethylene (PTFE), a homopolymer of tetrafluoroethylene, has been used in a wide array of applications ranging from the automotive and aeronautics industries to biomedical materials.1 More specifically, in the tribological community, this polymer has received notoriety because of its low friction ( 10−4 mm3 N−1 m−1) as a tribological material in its unfilled state.6−12 Fillers such as graphite, glass, and bronze have been added to unfilled PTFE to reduce its high wear from 1 to 3 orders of magnitude.13−18 However, not all fillers have an equal impact on the wear mitigation of PTFE-based composites. Wear mitigation can range from a few percent up to multiple orders of magnitude.6−8,15,16,19−27 Certain α-Al2O3 (α-alumina) fillers notably reduce the wear rate of unfilled PTFE up to 4 orders of magnitude (reported as low as 5 × 10 − 8 mm 3 N - 1 m−1).6,13,17,20,28−33 Similarly, the addition of 20 wt % polyether ether ketone (PEEK) to PTFE has been shown to reduce the wear rate by more than 5 orders of magnitude (K as low as 8.5 × 10−9 mm3 N-1 m−1)approximately 1000 times lower than unfilled PEEK.34 1.1. PTFE and Alumina. The wear mechanism responsible for the ultralow wear of PTFE and alumina has been thoroughly studied.13,17,26,30,31,33,35−43 The primary mechanisms include multiple length, force, and time-scale interactions that are physical, mechanical, and chemical in nature, including tribochemistry,30,36,40 filler reinforcement,26,33,41,44 nanome© 2019 American Chemical Society

Received: February 13, 2019 Revised: May 31, 2019 Published: July 9, 2019 5268

DOI: 10.1021/acs.macromol.9b00316 Macromolecules 2019, 52, 5268−5277

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Macromolecules and the protective film (tribofilm) on the polymer and countersample (transfer film) formed during sliding. It was demonstrated that the alumina particles must be porous (and thus friable) to promote the breakdown and accumulation of the filler within the tribofilms at the sliding surface,45 which mechanically reinforce the sliding layer that protects the counter-sample from abrasion.39 Friction-induced shear stresses can break C−C bonds in the PTFE backbone; the carbon radicals generated react with environmental water and oxygen, forming new carboxylate end groups.28,40,43 These carboxyl end groups chelate to the metallic counter-sample and form a robust transfer film.40,45,50 These end groups also chelate to filler particle fragments on the wear surface of the polymer to create a reinforced sliding layer.39,45 Computational models by Onodera et al. agree with the experimental-derived hypothesis that carboxylic acid end groups adhere to the metal counter-surface, in turn creating an anchored transfer film.38,42,43,51−53 1.2. PTFE and PEEK. Burris and Sawyer found wear rates of PTFE−PEEK composites to be as low as 2.3 × 10−9 mm3 N-1 m−1, among the lowest wear rates ever reported for a polymer composite.34 Within this experiment there was significant variation in the wear rate of the composite (more than an order of magnitude), mostly attributed to variations in the microstructure of the composite caused by processing variants. The first major hypothesis was that a favorable microstructure was an interpenetrating network of PTFE and PEEK as well as fibrillation of the PTFE caused by processing with a jet mill. Many groups have had difficulty recreating comparable materials (and tribological performance) since this initial report. Recent results from Onodera et al. maintain that the wear mechanism for these composites are governed primarily by physical rather than chemical interactions where the PEEK fillers anchor PTFE fibrils onto the metallic counter-sample.52 Similarly, results from Haidar et al. compared the wear of PTFE−PEEK composites in humid air and dry nitrogen and reported them to be ∼8 × 10−8 to 1 × 10−7 mm3 N−1 m−1 for 20 wt % PEEK in PTFE, that is approaching 2 orders of magnitude higher than the original ultralow wear PTFE−PEEK reported by Burris and Sawyer. Haidar et al. found the wear of PTFE−PEEK to be insensitive to its sliding environment (humidity).54 This is exciting from a materials tribology application (e.g., pumps, compressors, seals) perspective because it suggests that tribochemistry is not the dominant factor for producing the ultralow wear rates observed for these types of composites. 1.3. PTFE-Based Materials in the Aqueous Environment. Although the tribological behavior of PTFE−PEEK composites have been studied in humid air, vacuum, and nitrogen, there has not been extensive study of these composites submerged in water, an environment particularly relevant to many applications where water is used as a coolant in rolling mill bearings, or used as a working fluid in pumps.55 Early studies have examined the tribological behavior of PTFE and its composites in water,55−57 and have concluded that the presence of water reduces the coefficient of friction to an extent and can potentially increase the wear rate. Tanaka et al. found that the wear of unfilled semicrystalline polymers, including PTFE, is significantly greater in aqueous environment. This has been attributed to the inability of the polymer to form a transfer film.55,56 Watanabe et al. examined the friction and wear of PTFE composites with different fillers [i.e., glass fiber, carbon fiber, graphite, bronze powder, and glass fiber-(molybdenum sulfide) MoS2] in a variety of environments (i.e., pure water, salt

water solution, and solutions with varying pH values) and found that PTFE−carbon fiber composites had the highest wear rate in water because of the counter-sample surface being changed (i.e., roughened) and abraded by the agglomeration and adherence of glass fibers, which resulted in PTFE being unable to disband and form a protective layer on the surface.57 However, the wear rate showed no increase when MoS2 was added as a third constituent to the PTFE−glass fiber composite and when the countersample surface was treated with gold because it interrupted the transfer of glass particles. More recent studies such as Chen et al. focused more on the processing and mechanical contributions’ synergy of composite constituents and their tribological performance submerged in water.58 1.4. Objectives. This paper assessed the role of tribochemistry of two venerated ultralow wear PTFE-based composites: PTFE−alumina and PTFE−PEEK. The specific aims were to demonstrate how sliding environments humid air, submerged deionized water, and dry nitrogen affect the tribological properties of these ultralow wear PTFE composites, ultimately providing insight into their complex and multiscale wear mechanisms. The central hypothesis of this work is that ultralow wear of PTFE−alumina and PTFE−PEEK composites are dependent on the sliding environment and rely on tribochemical mechanisms. This hypothesis was evaluated using tribological experiments, small molecule infrared experiments, and chemical analysis of the tribofilms. There are two primary questions this paper attempts to address: 1. What is the role of tribochemistry of PTFE−alumina and PTFE−PEEK composites in the submerged water environment? Industrially relevant, fundamental studies on these remarkably wear-resistant materials have not been performed in the submerged environment. In the first study on the environmental sensitivity of PTFE−alumina composites, Krick et al. casually mentioned that PTFE and alumina were not wear-resistant in the submerged water environment; however, this was before the tribochemical mechanistic framework28,36,40 was proposed for PTFE−alumina composite systems. Furthermore, the wear of PEEK and PTFE in the submerged environment has not been reported. 2. Is the wear of PTFE−PEEK composites sensitive to humidity? Recently, Haidar et al.54 found the wear of PTFE−PEEK composites to be comparable in humid air and dry nitrogen environments, agreeing with the computational results from Onodera et al.52 Both of these studies agreed that the physical interactions between the PTFE−PEEK composites and the metal counter-sample were the primary mechanisms that stabilized the transfer film, resulting in an ultralow wear composite. However, experiments completed by Haidar et al. revealed slight differences in the IR spectra of the PTFE−PEEK when slid in humid air versus dry airspecifically around 1685 cm−1. This research aimed to (a) determine the origin of this chemical difference through small molecule IR experiments and (b) test its importance in the tribology of PTFE−PEEK composites through environmentally controlled tribology experiments and spectroscopy of tribofilms. 5269

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2. MATERIALS AND SAMPLE PREPARATION

K = V /(Fn × d)

PTFE−PEEK (at 10, 20 wt % PEEK) and PTFE−α-Al2O3 (at 5 wt % αAl2O3) composites were studied. The composites were made using PTFE (Chemours Teflon PTFE 7C resin), PEEK (VICTREX PEEK 450PF resin), and αAl2O3 (Nanostructured and Amorphous Materials, stock # 1015WW). These materials were consistent with prior studies of ultralow wear PTFE−α-Al 2 O 3 and PTFE−PEEK composites.6,26,28,29,39,46 Dry mixtures were prepared by weighing constituent material powders on a laboratory scale (OHAUS SPX123 Scout Portable Balance) and mixed using a stainless-steel spatula. The hand-mixed, dry mixtures (∼10 g) were then dispersed in isopropyl alcohol until well saturated (∼120 mL), followed by sonication using an ultrasonic horn (Branson Ultrasonic Corporation, SFX550, Danbury, CT, SFX550). Sonication was performed once for 5 min continuously at an amplitude of 40%. After sonication, the solvent was removed by drying the mixture in an oven at 75 °C. The dried powder was cold compressed into a stainless-steel cylindrical mold (12.7 mm diameter, ∼38 mm long) using a hydraulic press (Carver Press model #: 3853-9) with an applied pressure of ∼55 MPa. Once the samples were compressed, they were wrapped in aluminum foil and sintered. The sintering profile included heating at a rate of 240 °C/h to a target set point of 370 °C, then soaking at 370 °C for 30 min. The samples were then cooled down to 294 °C at a rate of 60 °C/h then held at 294 °C for 25 min, and then cooled to room temperature at a rate of 240 °C/h. Samples were then machined into rectangular prisms with dimensions of 6.3 × 6.3 × 12.7 mm (Figure 1). A wet polishing

(1)

where V is the worn volume (or volume lost because of wear) in mm3 as measured by mass loss divided by the composites’ density, Fn is the normal force in newtons, and d is distance in meters. Ktotal (total wear rate) considers the volume lost over the entire experiment (typically 500k cycles unless the test is stopped because of excessive wear). Kincremental (incremental wear rate) considers the change in volume over a single mass interval experiment. Steady-state wear rates Kss were calculated using a Monte Carlo analysis, as outlined in Schmitz et al.,59 fitting the last 300,000 sliding cycles (of 500,000 total cycles) for experiments that went to completion or the last three mass points for experiments that were ended prematurely because of excessive wear. Incremental wear rates were based on the difference in volume over one of the massing intervals and calculated by eq 1. Cycle-based friction coefficients were measured by averaging the middle 50% of the forward and reverse data in the reciprocating stroke.53 Reported average friction coefficients are weighted averages over the entire set of experiments, such that each cycle is weighted the same. Sliding experiments were performed in three environments. Humid air experiments were performed in the climate-controlled lab air environment, which ranged from 50 to 60% RH. The water submerged experiments were performed on the same tribometer in lab air; however, the steel counter-sample and sample were submerged in deionized water in an ultrahigh molecular weight polyethylene reservoir. The nitrogen experiments were performed on the same tribometer in a glovebox (MBraun Labmaster Pro SP) with a nitrogen environment consisting of O2 < 0.5 ppm, H2O, 0.5 ppm. Unlike prior environmentally controlled experiments for PTFE-based composites,29,30,36,54 the balance was in the glovebox, where mass measurements were taken to prevent contamination of the sample with humid air. Contamination, even brief and intermittent, could significantly alter the tribochemistry of the system. 3.2. Infrared Spectroscopy. A Thermo Fisher Scientific iS10 apparatus was used to collect spectra of the bulk (untested) and the running film (worn surface) of each polymer composite. All spectra consisted of 64 scans with a spectral resolution of 4 cm−1. The spectra were collected by placing the polymer surface in contact with a zinc selenide attenuated total reflectance (ATR) crystal. Prior to each measurement, the zinc selenide was cleaned using a cotton swab blotted with ethanol to remove all material that may have been left behind. A background scan was collected before every sample. Infrared spectra of the transfer films were collected with a Harrick SplitPea ATR microscope interfaced to a PerkinElmer Spectrum One Fourier transform infrared spectrometer. The accessory was employed in reflectance mode in which the internal reflection element was removed. A standard deuterium triglycine sulfate detector was employed. Spectra collected with this device represent the average of 32 individual scans possessing a spectral resolution of 4 cm−1. Baseline subtraction was performed for each spectrum using the PerkinElmer Spectrum Software’s baseline subtraction feature. Transmission infrared spectra were collected of carbonyl iron (Fe) powder (Sigma-Aldrich, lot #MKBZ6877V) and perfluoro heptanoic acid (CF 3 (CF 2 ) 5 CO 2 H) (SynQuest Laboratories, Inc, lot # 00012193)Fe/CF3(CF2)5CO2H. The iron/acid sample was made by weighing out ∼2 g of carbonyl iron powder into a flask with a stir bar. Approximately 200 mg of (CF3(CF2)5CO2H) was weighed and diluted with ∼10 mL isopropanol (IPA) and then stirred. All constituents were measured under nitrogen in a glove box. Once the acid/IPA mixture was diluted, it was transferred to the flask with the iron carbonyl powder and stirred for ∼3 min until consistent. Once stirring was complete, the IPA was evaporated from the mixture. Evaporation took ∼1.5 h. The iron/acid was then ground with potassium bromide (KBr) using a mortar and pestle and formed into a pellet using a manual bolt press. The KBr powered was dried for 24 h in an oven at 110 °C before it was used. Pellet composition was 90:10 [iron/acid: (Fe/KBr)]. Samples were surveyed using a Nicolet Magna 560 FT-IR spectrometer (Thermo Fisher Scientific, Unity Lab Services Madison, WI). Background scans were collected using an empty gas cell used to mount the (CF3(CF2)5CO2H)−Fe/CF3(CF2)5CO2H sample. The

Figure 1. Linear reciprocating tribometer loading geometry. wheel with 800 grit SiC sandpaper was used to polish each of the samples. All specimens were sonicated in a methanol bath at room temperature for 30 min to ensure that there were no contaminants on the samples from the machining and polishing processes. The samples were removed from the methanol bath and allowed to dry for a minimum of 4 h in ambient air before testing. The counter-sample (Figure 1) was a 304L stainless-steel coupon with a lapped surface finish (Metal Samples Company-Alabama Specialty Products Inc, Munford, AL) (38 mm × 25 mm × 3.7 mm) with an average roughness, Ra ≈ 0.15 μm. Each counter-sample was prepared by washing with Alconox soap and tap water followed by a methanol rinse. All sample counter-samples were left to dry in laboratory air for at least 30 min prior to testing.

3. EXPERIMENTAL METHODS 3.1. Tribology Experiments. A linear reciprocating tribometer was used to perform reciprocating flat-on-flat wear experiments (Figure 1) as described in the literature to perform all wear and friction tests.59 Wear experiments were performed and massed at intervals of increasing number of sliding cycles: 1k, 1k, 1k, 1k, 5k, 10k, 10k, 10k, 10k, 50k, 100k, 100k, and 200k cycles, for a total of 500,000 cycles. Samples were massed (using a laboratory scaleMettler Toledo XS205DU 0.00001 g resolution) after each of the 14 experiments to measure wear. Samples with excessive wear were stopped prior to 500,000 cycles. A normal load of 250 N with a corresponding applied pressure of ∼6.2 MPa, a sliding speed of 50 mm/s, and a sliding stoke of 25 mm were used for all experiments comparable to other papers on PTFE−alumina and PTFE−PEEK.13,26,28,30,36,39,40,46,60 Wear volume, V, was measured by dividing the intermittent mass loss of the polymeric wear samples by the measured density of each polymer composite. Wear rate, K (mm3 N−1 m−1), was quantified as 5270

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Macromolecules sample was left in the bolt press holder for ease of use. Spectra were converted to absorbance for comparison and the end group regions consisting of carboxylate salts (1600−1800 cm−1) were examined and the orientation of the carboxylate salt coordination was determined. 3.3. Stylus Profilometry. Stylus profilometry scans were performed on a KLA D-500 stylus profilometer (Milpitas, CA). Scans were performed in three different locations using a scan length of 9 mm across each sample. A scan speed of 0.1 mm/s and a stylus force of 614 μN was used for all scans. Sample tilt within each scan was corrected for using the stylus profilometer manufacturer’s software package.

4. RESULTS AND DISCUSSION Wear volume and wear rates of both PTFE−αAl2O3 and PTFE− PEEK composites showed significant dependence on the sliding environment (Figure 2a). Generally, the PTFE composites, both PTFE−αAl2O3 and PTFE−PEEK, had low steady-state wear rates in humid air (K < 10−7 mm3 N−1 m−1; Figure 2b, Table 1); remarkably low in PTFE−PEEK (K ≈ 10 −8 mm 3 N −1 m−1; Figure 2b, Table 1). The composites had the highest wear rates in submerged deionized water (>10−5 mm3 N−1 m−1). The wear rates in nitrogen were in between those of air and submerged water for both cases. However, PTFE−PEEK still had ultralow wear in nitrogen (K ≈ 1−2 × 10−7 mm3 N−1 m−1; Figure 2b, Table 1). The friction coefficients were affected by environment, albeit less significantly in Figure 2c; these will be discussed below on a case-by-case basis. 4.1. Wear, Friction, and Tribochemistry of PTFE−αAl2O3. The importance of water in tribochemical mechanisms for the ultralow wear performance of the PTFE−αAl2O3 system has been specifically linked to tribochemical chain-scission of the C−C bond of the PTFE backbone. This subsequent reaction with oxygen and environmental water (in the form of humidity) forms new carboxyl end groups that bond tribofilms to the metallic substrates as well as bond PTFE to the alumina filler in the surface of the composite.28,30,40,45 Surprisingly, submersion in deionized water does not enable these tribochemical processes to suppress the wear, resulting in extremely high wear rates for PTFE−αAl2O3 (steady-state wear rate of ∼6.1 × 10−4 mm3 N−1 m−1; Figure 2b); this is 10,000 times higher than when the test is performed in humid air (K ≈ 6.7 × 10−8 mm3 N−1 m−1) and 100 times higher than in dry nitrogen (K ≈ 6.0 × 10−6 mm3 N−1 m−1). In fact, the wear of PTFE−αAl2O3 in submerged deionized water is higher than unfilled PTFE in any environment (Figure 3). The friction coefficient of PTFE and PTFE−αAl2O3 in submerged deionized water (0.10 & 0.16, respectively) environments was significantly lower than in other environments (>0.13 & >0.21, respectively). The ATR−IR spectra of the PTFE−αAl2O3 wear surfaces in each environment and unworn control highlighted the differences in the tribochemical processes for sliding in air versus submerged in deionized water versus dry nitrogen (Figure 4a). The dominant peaks shown in each spectrum at 1216 and 1159 cm−1 correspond to backbone −CF2− units in PTFE. Peaks at 1665 and 1434 cm−1 were present only in the sample slid in air, resulting in ultralow wear rates. These peaks are associated with tribochemically generated carboxylic salts and are consistent with ultralow wear PTFE material systems.28,29,36,39,40 It has been demonstrated that the tribochemical breaking of C−C bonds in the PTFE backbone occurs in non-ultralow wear PTFE systems, and even unfilled PTFE and perfluoroalkoxy polymer (a random copolymer of TFE and a few mol % of perfluoroalkyl vinyl ether).61,62 It is hypothesized that tribochemically generated carboxyl/carboxylic acid end groups are not unique to ultralow wear PTFE; they are expected to form

Figure 2. (a) Volume loss vs sliding distance for representative PTFE−αAl2O3 (5 wt % αAl2O3) and PTFE−PEEK (20 wt % PEEK) composites in humid air, submerged deionized water, and dry nitrogen sliding environments. (b) Steady-state wear rates as calculated by Monte Carlo technique59 for PTFE−αAl2O3 (5 wt % αAl2O3) and PTFE−PEEK (both 10 and 20 wt % PEEK) composites as well as unfilled polymers (both PEEK and PTFE) in humid air, submerged deionized water, and dry nitrogen sliding environments. (c) Average friction coefficient for PTFE−αAl2O3 (5 wt % αAl2O3) and PTFE− PEEK (20 wt % PEEK) composites in humid air, submerged deionized water, and dry nitrogen sliding environments. All uncertainty bars considered variation within a single experiment and do not represent repeat experiments.

in higher wearing PTFE composites and unfilled PTFE but only accumulate measurably (via IR) in the tribofilms of the ultralow wear composites. This occurs because conditions for ultralow wear PTFE promoted extended shearing cycles on the same region of the polymer, which created progressively more carboxylic acid end groups that bond to nearby metallic surfaces to form protective films. These films minimize the debris generated and ejected during sliding because they adhered to the sliding contact surface, greatly increasing their accumulation and measurability. Carboxyl groups of PTFE fragments bonded to nanoscale alumina filler in the tribofilms (on both the wear 5271

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Table 1. Summary of Steady-State Wear Rates and Uncertainty for 10 and 20 wt % PEEK−PTFE and 5 wt % PTFE−Al2O3 Composites in Humid Air, Deionized Water, and Dry Nitrogen PEEK−PTFE

composition (wt %)

environment

μaverage

KMonte Carlo (mm3 N−1 m−1)

Kuncertainty

10 wt % PEEK

humid air deionized water dry nitrogen humid air deionized water dry nitrogen humid air deionized water dry nitrogen humid air deionized water dry nitrogen humid air deionized water dry nitrogen

0.15 0.09 0.13 0.17 0.09 0.09 0.21 0.16 0.22 0.13 0.10 0.18 N/A 0.19 N/A

2.2 × 10−8 5.2 × 10−5 2.1 × 10−7 1.1 × 10−8 4.2 × 10−5 1.7 × 10−7 6.7 × 10−8 6.1 × 10−4 6.1 × 10−6 5.1 × 10−4 1.5 × 10−4 4.9 × 10−4 5.0 × 10−6 8.9 × 10−5 N/A

1.4 × 10−8 3.2 × 10−6 3.2 × 10−8 7.8 × 10−9 5.2 × 10−6 2.3 × 10−8 1.1 × 10−8 8.1 × 10−5 7.4 × 10−7 6.6 × 10−5 2.6 × 10−5 4.7 × 10−5 1.0 × 10−6 1.7 × 10−5 N/A

20 wt % PEEK

PTFE−αAl2O3

unfilled PTFE

unfilled PEEK

5 wt % αAl2O3

for the ultralow wear performance. This process could be structural, chemical, and/or mechanical in nature; however, the dependence on the sliding environment suggested a strong chemical dependence. In humid air, the incremental wear rate dipped to as low as ∼8 × 10−9 mm3/(N m) for 20 wt % PEEK (after only 500k cycles; Figure 5), recovering wear rates comparable to those previously reported by Burris and Sawyer for the first time since the initial report on ultralow wear PTFE− PEEK composites.34 Similar to previous results by Haidar et al., PTFE−PEEK composites exhibited ultralow wear in both humid air and dry nitrogen;54 the present findings differ from Haidar et al. in that PTFE−PEEK composites were not insensitive to environmental moisture as the steady-state wear rate of PTFE−PEEK slid in air was more than an order of magnitude lower (17×; Figure 5b) than in dry nitrogen for 20 wt % PEEK (Kair ≈ 1.1 × 10−8, Knitrogen ≈ 1.7 × 10−7 mm3/N m) and about an order of magnitude lower for 10 wt % PEEK (Kair ≈ 2.2 × 10−8, Knitrogen ≈ 2.13 × 10−7 mm3/N m) (Table 1). This is visualized by the wear volume versus Fnd plot, where the magnitude of the slope of the three-point fit is equal to the steady-state wear rate (Figure 5b). The ATR−IR spectrum of the 20 wt % PEEK composites worn in all environments and an unworn control is shown in Figure 6a. The ATR−IR spectra of 20 wt % PEEK were highlighted because it exhibited the lowest steady-state wear rate in this study and previous studies by other authors.34,52,54 As previously stated, these composites achieved ultralow wear in both humid air and dry nitrogen environments. However, a shoulder peak appeared in the IR spectra of the PTFE−PEEK composite slid in air at around ∼1685−1725 cm−1. This peak was assigned to asymmetric stretching of the carboxyl ligand. It was nonexistent in the wear surfaces of the PTFE−PEEK composite when slid in nitrogen (O2 < 0.5 ppm, H2O < 0.5 ppm), submerged deionized water, or unworn. This shoulder appeared in the spectra of Haidar et al.54 However, its importance was difficult to assess because of several factors: (1) carbonyl groups of PEEK overlap and confound the ability to distinguish tribochemically generated carboxylate groups, (2) all experiments were exposed to humid air before performing IR experiments. Thus, the reaction of tribochemically generated species with water was possible after sliding experiments were completed and (3) the relative magnitude of the peak was very

Figure 3. Incremental wear vs sliding distance for PTFE−αAl2O3 composites in humid air, submerged deionized water, and dry nitrogen. Unfilled PTFE was also shown for comparison purposes.

surface of the polymer and transfer film), further reinforcing this wear-resistant system.45 Carboxyl groups are imperative to the adhesion of PTFE tribofilms (both transfer films on the steel and tribofilms on the wear surface of the polymer). The present submerged experiment further demonstrates this hypothesisin the submerged case, water was present and carboxyl end groups could form. However, hydrogen bonding between water and the tribochemically generated carboxyl group prevented the end groups from chelating and forming well-adhered transfer films on the metallic surfaces. PTFE−αAl2O3 produced thin, welladhered brown transfer films when slid in air, but had substantially thicker, patchy transfer films when worn submerged in water (Figure 4b,c). The large, patchy morphology of the transfer film that formed in submerged water suggested a greater dependence on mechanical adhesion mechanisms, such as mechanical interlocking of polymer conforming to the rough features of the steel. The high contact angle between PTFE and water may have driven agglomeration of PTFE wear debris into larger accumulations to reduce the overall interface area between PTFE and water. 4.2. Wear, Friction, and Tribochemistry of PTFE−PEEK. For PTFE−PEEK composites, the incremental wear rate decreased with increasing sliding distance (Figure 5). This is similar to PTFE−αAl2O3 (Figure 3) and is also known as “runin”. This suggests that some transient processes were responsible 5272

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Figure 4. (a) ATR−IR spectra of PTFE−5 wt % αAl2O3 composites in humid air, submerged in deionized water, and dry nitrogen as well as an unworn control. (b) Polymer wear surfaces of PTFE−5 wt % αAl2O3 (top) and transfer films on 304L stainless-steel counter-samples formed in the submerged water, dry nitrogen, and humid air environments. (c) Height profile of the transfer films formed on steel in the humid air, dry nitrogen, and submerged water environments; acquired with a stylus profilometer.

the tribochemistry of the ultralow wear PTFE−PEEK composite. Reflectance IR of the transfer films formed on the steel counter-samples also shows significant differences between PTFE−PEEK slid in humid air versus nitrogen (Figure 6c); specifically, shoulders appear at 1685 and 1449 cm−1 for the transfer film formed in air. These are consistent with carboxylate metal salts of both monodentate (1685 cm−1) and bridging (1449 cm−1) configurations, which were also shown in the smallmolecule transmission IR experiments. This is consistent with the hypothesis that tribochemically generated carboxyl end groups stabilize well-adhered transfer films and serve to improve the wear performance of PTFE-based composites. IR and wear experiments suggest that the bond between a tribochemically generated carboxylic acid and steel is only formed when sliding in humid air. This correlates with the ∼10 times better wear rate in humid air over dry nitrogen. The fact that PTFE−PEEK composites are still ultralow wear in dry nitrogen suggests that there are other mechanistic pathways for transfer film adhesion in this system. The ether, carbonyl, and hydrocarbon components of PEEK all interact more favorably with steel than PTFE does, forming adhesion pathways that negate the need for forming a new tribochemically generated carboxyl end groups.52,54 This seemingly dismissed the need for tribochemical reaction with humid air to achieve ultralow wear rates, although reactions with the trace 0.5 ppm water cannot be ruled out completely. Finally, we cannot confirm or rule out the possibility of chain scission of PEEK and its role in film adhesion; although, a tribologically cleaved PEEK backbone would enable higher mobility and, possibly, adhesion of PEEK functional groups to the steel counter-sample.

small, as was the nature of the relative ratio of end groups to bulk polymer. To differentiate peak locations of PEEK functional groups from the tribochemically generated carboxylate−metal salts previously associated with ultralow wear PTFE composites, an IR model compound was made. Iron microparticles (∼10 μm diameter) were mixed with perfluoroheptanoic acid (CF3(CF2)5CO2H) as an analog to steel interacting with the tribochemically generated carboxylate end group. The transmission IR spectra of the perfluoroheptanoic acid/iron mixture revealed peaks associated with multiple acid−steel chelation configurations (Figure 6b). A similar perfluorinated compound, perfluorooctanoic acid C7F15COOH, had been used in other experiments to determine and assess the wear mechanism of PTFE−αAl2O3 composites.40 The Fe−C6F13COOH IR model compound showed an absorbance at 1685 cm−1, which is associated with the asymmetric stretch of the iron carboxylate in a monodentate configuration.40,63 Another notable peak at 1667 cm−1 denoted an asymmetric stretch of the iron carboxylate in both a bidentate and a bridging conformation as well as a symmetrical stretch at 1449 cm−1 denoting a bridging interaction.64 Additional peaks worth highlighting are asymmetrical stretches in CF2 at 1350, 1240, and 1190 cm−1. Other stretches include an asymmetrical stretch in CF2 and CF3 at 1204 cm−1 as well as a symmetrical stretch at 1149 cm−1. The IR absorption peak at 1685−1725 cm−1 on the PTFE−PEEK composite wear surface from the humid air environment (Figure 6a) corresponds with adsorption of the monodentate COO− to iron in the perfluoroheptanoic acid/iron experiments (Figure 6b); this is the key difference (aside from wear rate) that suggested significant evidence of environmental sensitivity on 5273

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Figure 5. (a) Incremental wear rate vs sliding distance for 20 wt % PEEK composites in all environments. (b) Worn volume (V) vs sliding distance times normal force (Fn × d) for 20 wt % PEEK composites in humid air and nitrogen; fits were from Monte Carlo analysis of the last three data points. Slope of the linear fits was the steady-state wear rate or Kss. PTFE−20 wt % PEEK was selected as the focal point, as it has the lowest wear rate in this study and previous studies.34,52,54

In the submerged deionized water environment, Kwater ≈ 5.2 × 10−5 and 4.2 × 10−5 mm3/(N m) for 10 and 20 wt %, respectively; this was more than 3 orders of magnitude higher than in air and 2 orders of magnitude higher than in nitrogen. As with the PTFE−αAl2O3 case, this was likely because hydrogen bonding to excess water prevents chelation of a thin, robust transfer film. More specifically, it was assumed that during sliding carboxyl groups can still form (O2 and H2O still present); however, a hydrogen bond network around the carboxyl chain end prevented interaction with the metal surface. This hydrogen bond network may have also prevented PEEK functional groups (with otherwise significant work of adhesion52) from forming a transfer film on the steel. Interestingly, the relative ratio of PEEK to PTFE was much higher in the IR of the water-worn sample, suggesting that PTFE preferentially wears away (Figure 6a). It was surprising to the authors how poorly these otherwise wear resistant PTFE-based polymers perform in submerged water sliding environment, especially considering PTFE composites (including PTFE−PEEK) are ubiquitous in sliding water sealing applications. 4.3. Mechanistic Hypothesis for PTFE−PEEK Composites. Numerous mechanistic hypotheses have been proposed for these ultralow wear PTFE-based systems (i.e., PTFE− alumina and PTFE−PEEK), all of which have merit and contribute to the complex, multi force, length, and time scale chemical, physical, mechanical, and material phenomena responsible for the uniquely low wear rates.26,29,30,36,40,45,50,60

Figure 6. (a) Attenuated Fourier transform infrared spectrum of PTFE−20 wt % PEEK-worn surfaces in humid air, dry nitrogen, and submerged deionized water environments as well as an unworn control (b) transmission infrared spectrum of perfluoroheptanoic acid and iron disks. (c) Reflectance IR of transfer films on steel formed by PTFE−20 wt % PEEK sliding in the humid air and dry nitrogen environments.

Combining relevant aspects from this work and prior publications, we propose a mechanism pertaining specifically to the relationship between tribochemistry, sliding environment, polymer composition, and wear (neglecting very important mechanical aspects). We postulate that: • Mechanically induced shear stresses caused chain scission of the C−C bonds in the backbone of PTFE during wear. These broken bonds formed radicals that can react with environmental oxygen and moisture to form carboxylic acid end groups (Figure 7a). • These end groups then chelated to the surface of the metallic counter-sample in one of the three possible configurations, forming a well-adhered transfer film in humid air (Figure 7b). 5274

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Figure 7. Mechanistic hypotheses for the tribochemistry of ultralow wear PTFE composites. (a) Schematic of shear-induced chain scission of C−C bonds in PTFE and tribochemically generated end groups. (b) Schematic of transfer film adhesion involving various chelation configurations of tribochemically generated carboxylate end groups and the steel counter-sample. (c) Schematic of transfer film adhesion involving PEEK functional groups to steel. (d) Schematic of water impeding transfer film adhesion.

• In dry nitrogen, the polar carbonyl groups of PEEK anchored polymer transfer films to the metallic countersample (Figure 7c). • In submerged water, hydrogen bonding prevented the adherence of the tribochemically generated carboxylate end groups or PEEK functional groups and prevents the formation of a robust transfer film (Figure 7d).

and the significantly lower wear rate of PTFE−PEEK in air as compared to nitrogen, PTFE−PEEK composites were not found to be insensitive to the sliding environment. In nitrogen, the tribochemically generated species that were observed in the ATR−IR in humid air were absent. However, the wear rate was on the order of 10−7 mm3/N m. This was supported by the absence of the required metal complex formed in the humid air environment, which suggests that the ultralow wear behavior of these composites in PTFE−PEEK was primarily the result of polar carbonyl groups in the PEEK backbone. These carbonyl groups assisted in adhering the transfer film to the counter-sample. In the submerged environment, no tribochemical peak was observed in the ATR−IR spectrum. In nitrogen, PTFE−αAl2O3 performed significantly worse than PTFE−PEEK composites. This poor performance may be attributed to the limited carboxylate end groups generated during sliding, whereas PTFE−PEEK was inherently loaded with functional groups that could adhere the transfer film to the metallic counter-sample. In submerged environments, the wear rates for both PTFE−αAl2O3 and PTFE−PEEK were highest and transfer films are poorly adhered. This was because water impeded transfer film adhesion by saturating the functional groups (tribochemical carboxylates or functional groups of PEEK) with hydrogen bounds, which prevented direct bonding with the metal counter-sample. Overall, the wear mechanism of these composites is complex with obvious mechanical, physical, and chemical contributions. More studies are needed to understand the complex interdependence of the multiscale mechanical, physical, and chemical phenomena that result in the significant reductions in wear by filling PTFE with alumina or PEEK.

5. CONCLUSIONS Tribological experiments were performed on both PTFE−αAl2O3 and PTFE−PEEK composites in a range of sliding environments (humid air, dry nitrogen, and water) to probe the role of tribochemistry in these ultralow wear composite systems. Wear behavior was significantly affected by the nature of water in the sliding environment. Evidence that PTFE− PEEK-based composites are tribochemically dependent on their sliding environment has been reported and supported by tribological experiments and IR spectroscopy. Wear of PEEK− PTFE composites was affected by environmental water, resulting in remarkably low wear for humid (K < 10−8 mm3/N m), higher, yet still ultralow, wear for dry nitrogen (K ≈ 2 × 10−7 mm3/N m) and surprisingly high wear submerged in water (K ≈ 4 × 10−5 mm3/N m). IR spectra of tribofilms showed that chelation of tribochemically generated carboxylate end groups to the counter-sample is consistent with the lowest measured wear rates in humid air. Carboxylate peaks diminished in tribofilms slid in nitrogen and water, where wear performance is worse than in air. Spectral overlap of peaks from the carbonyl groups in PEEK and possible tribochemically generated carboxyl groups complicates interpretation of the PEEK−PTFE tribofilm IR data. Because of this, a small molecule analog of perfluoroheptanoic acid/iron microparticles was synthesized and analyzed to give an authentic control sample that showed IR peaks indicative of chelated iron perfluorocarboxylates. This IR spectrum contained a shoulder at ∼1685 cm−1 in the humid-airgenerated PTFE−PEEK transfer film, which was not present in the spectra of other PTFE−PEEK tribofilms. This suggested that tribochemically generated carboxylate end groups chelated to iron are the major difference in the chemistry of the tribofilms generated in various environments. Considering this IR result

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (610) 758-5772. Fax: (610) 758-6224. ORCID

Brandon A. Krick: 0000-0003-3191-5433 Notes

The authors declare no competing financial interest. 5275

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant 1463141 (Support for Krick, Sidebottom, Atkinson). This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant #1452783 (Campbell) and 1842163 (Babuska). We also acknowledge support from the Pennsylvania Infrastructure Technology Alliance (PITA), grants PIT-18-18 and PIT-16-16. The authors would like to thank Wendy Breyer and Gabrielle Esposito of Lehigh University for training on the IR spectroscopy instrument as well as Greg Blackman, Heidi Burch, Holly Salerno from DuPont Company and student members of the Lehigh Tribology Laboratory for comments, feedback, and insight. We also appreciate thoughtful discussions with David Burris (University of Delaware).

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