Wear reduction mechanisms within highly wear-resistant graphene- and other carbon-filled PTFE nanocomposites

Journal Pre-proof Wear reduction mechanisms within highly wear-resistant graphene- and other carbonfilled PTFE nanocomposites Suvrat Bhargava, Mary E. Makowiec, Thierry A. Blanchet PII:

S0043-1648(19)30313-8

DOI:

https://doi.org/10.1016/j.wear.2019.203163

Reference:

WEA 203163

To appear in:

Wear

Received Date: 18 February 2019 Revised Date:

8 September 2019

Accepted Date: 16 December 2019

Please cite this article as: S. Bhargava, M.E. Makowiec, T.A. Blanchet, Wear reduction mechanisms within highly wear-resistant graphene- and other carbon-filled PTFE nanocomposites, Wear (2020), doi: https://doi.org/10.1016/j.wear.2019.203163. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Wear reduction mechanisms within highly wear-resistant graphene- and other carbon-filled PTFE nanocomposites Suvrat Bhargavaa,1,∗, Mary E. Makowieca , Thierry A. Blancheta,∗ a Department

of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY, U.S.A.

Abstract In recent years, a few select carbonaceous fillers with nanometer-size dimensions, such as graphene platelets, activated carbon nanoparticles and carbon nanotubes, have been shown to reduce the wear rates of PTFE to levels approaching 10−7 mm3 /Nm and below. X-ray diffraction (XRD) and attenuated total reflectance (ATR) mode IR spectroscopy are used to show that these highly effective fillers provide wear resistance to PTFE through shared mechanisms. These mechanisms are also shown to operate in highly wear-resistant composites of PTFE with nanometer-sized particles of α-phase alumina. Addition of these highly effective fillers to PTFE results in a greater resemblance of the crystalline structure of PTFE at room temperature with a tougher and higher temperature phase. When slid against steel countersurfaces under ambient conditions, these fillers embedded within the PTFE matrix also enable the formation of robust transfer films through the formation of metal chelates. Keywords: PTFE, wear, fillers, graphene platelet, activated carbon, carbon nanotube

1. Introduction

[10] comprise a large fraction. Figure 1 shows the wear reductions achieved by the addition of some of Historically, micron-sized fillers have been used these carbonaceous fillers as a function of the loading. to reduce the high wear rate (approaching 10−3 Nanometer-sized particles of α-phase alumina [11, 12] mm3 /Nm) of PTFE by a couple orders-of-magnitude are the primary other reported nanofillers which have [1–3]. Such fillers often lose their ability to lower been shown to reduce the wear rates of PTFE to such the wear rate of PTFE when their size is reduced extents. to nanometer scale [4]. However, within the last fifteen years, a few select fillers with nanometer-sized Several wear reduction mechanisms have been prodimensions have been shown to be capable of re- posed to explain the wear resistance provided by ducing the wear rate of PTFE to levels approach- these fillers to PTFE. For instance, it has been hying down towards and now beyond 10−7 mm3 /Nm. pothesized that α-phase alumina nanoparticles could Of these known highly effective fillers, various forms provide high wear resistance to PTFE by stabilizing of carbonaceous fillers such as graphene platelets [5– the tougher and higher temperature phase-I of PTFE 7], carbon nanoparticles [8–10] and carbon nanotubes to temperatures lower than those expected from the equilibrium phase diagram [13]. Around room temperature, the molecular chains within the crystalline ∗ Corresponding authors: regions of PTFE could be arranged in a few different Email addresses: [email protected] (Suvrat forms or phases which undergo phase transitions near Bhargava), [email protected] (Thierry A. Blanchet) 1 Currently at TE Connectivity, Harrisburg, PA, U.S.A. 19 ◦ C and 30 ◦ C, at atmospheric pressure. Below Preprint submitted to Wear

December 21, 2019

19 ◦ C, crystalline regions of PTFE exist in phase-II, while between 19 ◦ C and 30 ◦ C they exist in phaseIV. At 30 ◦ C, phase-IV undergoes a first order phase transition into phase-I. Crack propagation in PTFE is considered to be strongly dependent on the temperature and, thus, the phase in which crystalline regions of PTFE are present [14]. Brown and Dattelbaum [15] reported that phase-II, which exists in equilibrium below 19 ◦ C at ambient pressure, has a lower fracture toughness than phase-IV (between 19 ◦ C and 30 ◦ C) and phase-I (above 30 ◦ C). This causes fracture to primarily occur through either microvoid coalescence or cleavage and results in brittle crack growth for phase-II. Fracture in phase-IV and phase-I, however, proceeds with the formation of fibrils which dissipate energy through localized plastic deformation and result in a more effective blunting of the crack tip. Brown and Dattelbaum [15] further showed that at high rates of crack growth, parallel to the direction of pressing of PTFE (during its press/sinter powder processing), phase-I offers higher fracture toughness than phase-IV. Nanoparticles of α-phase alumina were also proposed to assist in the formation of a thin and welladhered transfer film over steel countersurfaces [11, 12]. More recent studies [16–19] have in fact showed that the sliding wear of PTFE composites with nanometer-sized α-phase alumina particles could be accompanied with tribochemical reactions. Carboncarbon bonds in PTFE molecular chains could break due to the applied mechanical stresses, and the newly created terminal carbon atoms could react with atmospheric oxygen and water to form perfluorinated carboxylic acids. A nanometer-sized α-phase alumina particle present within the tribological interface could then act as a cross-link by bonding to several such perfluorinated carboxylic acids and effectively improve the toughness of the surrounding polymer matrix. Additionally, nanometer-sized α-phase alumina particles could gently clean the metallic countersurface of its surface films to reveal its more reactive metallic surface, and enable the formation of metal chelates between any perfluorinated carboxylic acids generated from tribochemical reactions and the metallic countersurface. This leads to the formation of a more stable and well-adhered transfer film which

Figure 1: Steady-state wear rates reported for various highly wear-resistant carbon-based (nanoparticles of activated carbon [9], graphene platelets [5, 6] and carbon nanotubes [10]) composites of PTFE as a function of the filler loading.

reduces the wear of the polymer nanocomposite even further. This work newly presented here is an attempt to systematically study the impact of the addition of the known highly effective carbonaceous fillers on the bulk PTFE using x-ray diffraction (XRD) and the composite’s tribological surface using attenuated total reflectance (ATR) mode IR spectroscopy. In addition, this work attempts to understand whether any of the wear reduction mechanisms operating within the highly wear-resistant PTFE nanocomposites with the carbonaceous fillers are shared with αphase alumina-filled PTFE nanocomposites. 2. Experimental Methods For all the polymer composites discussed in this work, granular “grade 7C” PTFE with an average particle size of about 28 µm was used as the matrix material. The mix/press/sinter powder processing technique used for producing cylindrical pucks of the PTFE composites has been described in detail elsewhere [4, 6, 9, 10, 20], including cold-pressing at 40 MPa for 10 minutes followed by free-sintering at 360 ◦ C for 3 hours with heating to and cooling from that 2

temperature at 100 ◦ C/hour. This work primarily investigates the wear reduction mechanisms operating in previously reported highly wear-resistant composites of PTFE with activated carbon nanoparticles (<50 nm) [9], graphene (nominal platelet thicknesses from 1.25 nm to 60 nm) [5, 6] and carbon nanotubes (∼9.5 nm diameter by 1.5 µm length with ∼8% carboxyl-functionalized surface) [10]. The test specimens were derived by milling the sintered pucks into ∼10 mm long square cylindrical pins which were flat-ended and had a square 4 mm×4 mm crosssection. The pins were worn against polished stainless steel (SS) 304 countersurfaces. The average surface roughness, measured using a stylus profilometer, of each of the SS 304 countersurface was within the desired range of 0.015 µm ≤ Ra ≤ 0.03 µm. This average countersurface roughness was earlier [21] shown to be small enough to prevent abrasion from dominating the more typical wear processes of PTFE, such as adhesive transfer wear and delamination wear, during dry sliding against typical engineering countersurfaces. Wear testing of all the samples discussed in this work was performed in earlier studies on a multistation wear tester under ambient air and room temperature conditions, and interrupted periodically to measure the mass loss from each pin [4, 6, 9, 10, 20]. Each pin was slid under 3.1 MPa contact pressure at 0.1 m/s speed with a 40 mm reciprocation stroke length until a steady-state was adopted of mass loss increasing linearly with the sliding distance, requiring up to several tens of kilometers of sliding distance and weeks of time to achieve in cases of highly wearresistant PTFE composites. In order to investigate the wear reduction mechanisms and the structural changes happening within the crystalline regions of the bulk PTFE matrix upon nano-filler addition, sintered PTFE composites were studied using a PANalytical X’Pert PRO MPD xray diffraction system. The measurements were performed at 20 ◦ C temperature using a copper Kα xray beam generated with 45 kV and 40 mA. The pre-selected 2θ range of 15◦ –50◦ was scanned with a step size of 0.026◦ . Additionally, the unworn and worn surfaces of the samples were analyzed using a

MIRacleTM single reflection horizontal attenuated total reflectance (ATR) accessory attached to a Varian 660-IR FTIR spectrometer with a high sensitivity cryogenic mercury cadmium telluride (MCT) detector. This accessory uses a ZnSe ATR crystal with a circular sampling area of 1.8 mm diameter. The spectra were obtained with an IR beam incident at an angle of 45◦ in the absorbance mode over a wavenumber range between 700–4000 cm−1 . 16 scans, each collected with a resolution of 2 cm−1 , comprise every ATR-IR spectrum presented here. 3. Results and Discussion 3.1. Probing the crystalline regions of bulk PTFE after the addition of highly effective fillers using x-ray diffraction (XRD) For PTFE, the peaks in the range of Bragg angle 35◦ <2θ <45◦ are known to be sensitive to its phase [23, 24]. As an example [22], unfilled PTFE was placed on a temperature-controlled stage and xray diffraction performed at temperatures of 45 ◦ C, 25 ◦ C and 15 ◦ C where phase-I, phase-IV and phaseII exist, respectively, at equilibrium. As seen in figure 2, for the higher-temperature phase-I that would exist at the 45 ◦ C temperature, only a single peak appears within the phase-sensitive 35◦ <2θ <45◦ range, near 36.6◦ . As temperature is dropped to 25 ◦ C with PTFE transitioning to phase-IV, two additional peaks near 37.1◦ and 41.4◦ newly appear. As temperature is dropped further to 15 ◦ C with PTFE now transitioning to phase-II, these new peaks near 37.1◦ and 41.4◦ continue to grow while the original single 36.6◦ peak of higher-temperature phase-I diminishes to the point of appearing more as a shoulder on a stronger 37.1◦ peak. As such, the area intensity enclosed by either the 37.1◦ or 41.4◦ peak ratioed to that of the 36.6◦ can be taken to represent the extent to which the higher-temperature phase-I is present (at lower ratio values) or absent (at higher ratio values). In figures 3 through 7, the strong peak near 18.1◦ is used for rescaling the diffractograms so that the height of this primary peak is the same across different diffractograms. This strong peak is associated with long-range ordering along the (100) lattice 3

planes in PTFE, and this peak is considered to be largely independent of the phase of PTFE [24]. The diffractograms for the various samples are also shifted vertically to allow for better visualization.

filled with these graphene platelets. Along with the strong PTFE peak near 18.1◦ some less intense PTFE peaks are observed near 31.6◦ , 36.6◦ , 37.1◦ , 41.4◦ and 49.2◦ which are associated with ordering along (110), (200), (107), (108) and (210) lattice planes, respectively. The composites with 8 nm and thicker graphene platelets also show a peak near 26.5◦ which is associated with the (002) lattice plane of graphite [25]. This peak disappears in the cases of the two thinnest (1.6 and 1.25 nm ) graphene platelets because there are very few atomic planes in the direction normal to the platelet planes. The inset in figure 3(a) provides a closer look at the diffractograms within the phase-sensitive region for PTFE. Despite the diffractograms being collected at 20 ◦ C, it appears that the addition of graphene platelets to PTFE results in raising the area intensity of the higher temperature (>30 ◦ C) phase-I peak near 36.6◦ with respect to those near 37.1◦ and 41.4◦ , as compared to that of unfilled PTFE. The addition of the three thickest graphene platelets—8 nm, 12 nm and 60 nm—seems to increase the area intensity of the peaks near 36.6◦ only modestly compared to that of unfilled PTFE, and the three graphene platelets appear to have a comparable effect on the area intensity of the 36.6◦ peak. This effect becomes more pronounced for the composites with the thinnest graphene platelets. The two thinnest graphene platelets (1.6 nm and 1.25 nm), in addition to increasing the area intensity of this 36.6◦ peak, seem to be able to lower the area intensities of the peaks near 37.1◦ and 41.4◦ . Furthermore, the 1.6 nm and 1.25 nm graphene platelets appear to have a comparable effect on the area intensities of the peaks. To analyze these phase-dependent differences quantitatively, the individual peaks in the phasesensitive region were fitted with Lorentzian functions. The ratios of the area intensities of these three peaks are quantitatively shown in figure 3(b), along with the wear rates reported earlier [6] for these composites. Error bars represent standard errors associated with the computation of the values. For unfilled PTFE, the peaks ∼37.1◦ and ∼41.4◦ are found to have more than twice the area intensity of the peak ∼36.6◦ . This indicates that the crystalline regions of PTFE have very little similarity with those present in

3.1.1. Effects of graphene platelet loading and thickness Typical diffractograms obtained at ambient room temperature of 20 ◦ C for sintered unfilled PTFE and various thickness graphene-filled PTFE composites at constant 4 wt% content, within 15◦ <2θ<50◦ , are presented in figure 3(a). Bhargava et. al. [6] had earlier reported the tribological performance of PTFE

Figure 2: Diffractograms of unfilled PTFE in phase-I, phaseIV and phase-II focusing on the peaks in the phase-sensitive region. The diffractograms were collected at 45 ◦ C, 25 ◦ C and 15 ◦ C by Kandanur [22].

4

the higher-temperature phase-I. In terms of the wear rates, unfilled PTFE wears at a rate far higher than any of the graphene-filled composites at 4 wt% loading. At 4 wt%, with the presence of thicker (8 nm, 12 nm and 60 nm) graphene platelets, the ratios of area intensities are found to reduce to about 1.5. This reduction aligns well with the reduction observed in terms of the wear rates where graphene platelets with

nominal thicknesses of 60 nm, 12 nm and 8 nm reduced the wear rates to similar levels with rates of about 7×10−6 mm3 /Nm. With a further reduction in the nominal thicknesses of the graphene platelets to under 2 nm, the ratios of the area intensities of the peaks reduce greatly to below 0.75. The two thinnest graphene platelets lowered the wear rates by another order of magnitude, and with a 4 wt% loading the wear rates were reported to be about 5×10−7 mm3 /Nm. Additionally, the closeness of the two peak ratios (∼37.1◦ /∼36.6◦ and ∼41.4◦ /∼36.6◦ ), for any given sample, reinforces the idea that phase-IV can be represented by either of the two peaks (∼37.1◦ and ∼41.4◦ ). These results show that the addition of graphene platelets, at 4 wt%, results in increased similarity of crystalline regions within the PTFE matrix with the tougher, higher-temperature phase-I. Additionally, the thinnest graphene platelets, which are already known to impart greater wear resistance than their thicker counterparts, perform better in increasing the similarity of the crystalline regions within the PTFE matrix with the tougher, higher-temperature phase-I. Figure 4(a) presents the diffractograms obtained at 20 ◦ C from unfilled PTFE and PTFE filled with constant thickness 1.25 nm graphene platelets at varying loadings of 0.32 wt%, 1.1 wt% and 4 wt%. The diffractograms show the peaks associated with PTFE. A closer look at the phase-sensitive region, as shown in the inset, reveals that the addition of 1.25 nm graphene to PTFE, even at a low loading of 0.32 wt%, results in diminished intensities of peaks near 37.1◦ and 41.4◦ with respect to that near 36.6◦ . Further, it appears that the area intensities of the peaks near 37.1◦ and 41.4◦ monotonically decrease, when compared with the area intensity of the peak near 36.6◦ , as the graphene platelet loading is increased from 0.32 wt% to 4 wt%. The ratios of area intensities of the peaks in the phase-sensitive region are presented in figure 4(b), along with the wear rates reported earlier by Bhargava et al. [6]. With the addition of just 0.32 wt% 1.25 nm graphene, the areas of the peaks ∼37.1◦ and ∼41.4◦ greatly diminish with respect to the area of the peak ∼36.6◦ and become very close to it. These ratios continue to decrease monotonically with in-

(a)

(b)

Figure 3: X-ray diffraction at 20 ◦ C of unfilled PTFE and PTFE filled with graphene platelets of various nominal thicknesses (60 nm, 12 nm, 8 nm, 1.6 nm and 1.25 nm) at 4 wt% loading: (a) Diffractograms over a broad 2θ range between 15◦ and 50◦ with an inset focusing on the phase-sensitive region and (b) ratios of area intensities of the peaks in the phasesensitive region. The wear rates reported earlier [6] are also shown.

5

creasing loading up to 4 wt% and approach values under 0.75. Again, the area intensities of the peaks near 37.1◦ and 41.4◦ remain quite similar to one another. For these composites too, the reported wear rates decrease with decreasing ratios of the area intensities of the peaks in the phase-sensitive region. These results show that the increasing loading of the 1.25 nm graphene results not only in increasing the wear resistance of the graphene-filled PTFE composite, but also the similarity of the crystalline regions

with the tougher, higher-temperature phase-I.

3.1.2. Does the addition of other highly effective carbonaceous nanofillers and α-phase alumina nanoparticles have a similar effect? Figure 5(a) presents the diffractograms for unfilled PTFE and PTFE filled with nanometer-sized (<50 nm) particles of activated carbon at varying 5 wt%, 10 wt% and 20 wt% loadings. Earlier, Kandanur et al. [9] showed that these activated carbon nanoparticles can increasingly impart extreme wear resistance to PTFE with increasing content, as summarized in figure 1. In addition to the peaks associated with (a) PTFE, the composites show a minor peak near 26.5◦ . Unlike the sharp and intense peak associated with graphitized carbon for greater platelet thicknesses in figure 3(a), this relatively weak peak, even at 20 wt% content, is associated with carbon that is instead much more amorphous. This observation is consistent with the description of the activated carbon particles provided by their commercial supplier. The inset provides a closer look at the diffractograms of the composites in the phase-sensitive region. Compared to unfilled PTFE, the 5 wt% nanocomposite shows a slightly bigger peak near 36.6◦ . With increasing loading, the area intensities of the peaks near 37.1◦ and 41.4◦ diminish relative to that near 36.6◦ . The (b) trend continues, and for the 20 wt% nanocomposite, the peak at 36.6◦ appears to be slightly bigger than the other two peaks in consideration. Ratios of area intensities of the peaks in the phasesensitive region are shown quantitatively in figure 5(b). The figure also shows the wear rates reported earlier [9] for the composites. For the 5 wt% and the 10 wt% composites, the areas of the peaks near 37.1◦ and 41.4◦ are found to be reduced to around 1.5 times the area of the peak near 36.6◦ . The composites were reported to demonstrate fairly similar wear rates of about 5×10−8 mm3 /Nm. When the loading of the activated carbon nanoparticles is inFigure 4: X-ray diffraction at 20 ◦ C of unfilled PTFE and creased to 20 wt%, the wear rate and the ratios of the PTFE filled with 1.25 nm graphene platelets at 0.32 wt%, 1.1 area intensities of the peaks reduce further, with wear wt% and 4 wt% loadings: (a) Diffractograms over a broad 2θ rate and the ratios approaching 1.5×10−8 mm3 /Nm range between 15◦ and 50◦ with an inset focusing on the phasesensitive region of PTFE and (b) ratios of area intensities of the and 0.5, respectively. These results indicate that peaks in the phase-sensitive region. The wear rates reported the crystalline regions within the PTFE matrix in earlier [6] are also shown. the composite bear increased resemblance with the 6

tougher, higher-temperature phase-I as the loading of nanometer-sized particles of activated carbon is increased. This increased resemblance is accompanied with a corresponding decrease in the wear rate offered by the composite. PTFE filled with carbon nanotubes were also analyzed in a similar way. The 0.8 wt% and 2 wt% composites were found to be wear-resistant earlier by Makowiec and Blanchet [10]. Figure 6(a) shows the diffractograms for unfilled PTFE and the 0.8 wt%

and 2 wt% composites. The peaks in the phasesensitive region indicate that the area intensities of the peaks near 37.1◦ and 41.4◦ decrease with respect to that of the 36.6◦ peak as the filler loading is increased. This trend is more quantitatively demonstrated in figure 6(b). Wear rates associated with the composites are also shown. For the wearresistant composites, the ratios of the area intensities are lowered to less than 1.5. This reduction in the ratios of area intensities is yet again accompanied by a near proportional decrease in the wear rates with rates approaching 10−6 mm3 /Nm. These observations at room temperature show that the ad(a) dition of carbon nanotubes too causes a greater resemblance of the crystalline regions to the tougher, higher-temperature phase-I, and this addition of carbon nanotubes results in imparting wear resistance to PTFE. Diffractograms obtained at 20 ◦ C from PTFE filled with nanometer-sized α-phase alumina particles at 0.8 wt% and 5 wt% are shown in figure 7(a). The α-phase alumina particles were sized between 27 nm and 43 nm and were earlier shown to also impart extreme wear resistance to PTFE [21]. In addition to showing the peaks associated with PTFE, the composites (especially, at 5 wt% loading) show several other relatively strong peaks located near 25.5◦ , (b) 35.1◦ , 37.7◦ and 43.3◦ . These peaks were matched to those listed for aluminum oxide in the powder diffraction database. Even with a relatively low loading of 0.8 wt%, the area intensities of the peaks near 37.1◦ and 41.4◦ decrease noticeably relative to those observed for unfilled PTFE. When the loading is increased to 5 wt%, the area intensities of the two peaks show a slight further decrease. These observations are quantified and presented in figure 7(b), along with the wear rates reported for these composites. For the two nanocomposites, the area intensities of the peaks near 37.1◦ and 41.4◦ are found to be only about 1.5 times of that near 36.6◦ . This reduc◦ Figure 5: X-ray diffraction at 20 C of unfilled PTFE and tion in the ratios of area intensities is yet again found PTFE filled with activated carbon nanoparticles at 5 wt%, 10 to be accompanied with a proportional reduction in wt% and 20 wt% loadings: (a) Diffractograms over a broad 2θ the wear rates, and nanometer-sized α-phase alumina range between 15◦ and 50◦ with an inset focusing on the phasesensitive region of PTFE and (b) ratios of area intensities of the particles also increase the similarity between the cryspeaks in the phase-sensitive region. The wear rates reported talline regions of the PTFE matrix in the composite earlier [9] are also shown. with the tougher, higher-temperature phase-I. 7

(a)

(a)

(b)

(b)

Figure 6: X-ray diffraction at 20 ◦ C of unfilled PTFE and PTFE filled with carbon nanotubes at 0.8 wt% and 2 wt% loadings: (a) Diffractograms over a broad 2θ range between 15◦ and 50◦ with an inset focusing on the phase-sensitive region of PTFE and (b) ratios of area intensities of the peaks in the phase-sensitive region. The wear rates reported earlier [10] are also shown.

Figure 7: X-ray diffraction at 20 ◦ C of unfilled PTFE and PTFE filled with nanometer-sized α-phase alumina particles at 0.8 wt% and 5 wt% loadings: (a) Diffractograms over a broad 2θ range between 15◦ and 50◦ with an inset focusing on the phase-sensitive region of PTFE and (b) ratios of area intensities of the peaks in the phase-sensitive region. The wear rates reported earlier [22] are also shown.

Although the exact mechanisms through which the known extremely effective nanofillers induce tougher phase-I-like arrangement of molecular chains within the crystalline regions of PTFE at temperatures lower than those otherwise suggested by the equilibrium phase diagram of PTFE remain unclear, the data presented in figures 3 through 7 show that they share this ability. As noted earlier [13, 14], the crack propagation in PTFE is considered to be strongly dependent on the arrangement of molecular

chains within the crystalline regions of PTFE, and any toughening of the crystalline regions which fibrillate more easily could reduce the sliding wear rate by impeding crack propagation and limiting the formation of wear debris. This increased similarity to phase-I can be qualitatively captured in terms of the ratios of area intensities of the peaks in the phasesensitive region. The fillers, however, appear to differ in their ability to induce phase-I-like behavior. For example, the 1.25 nm graphene platelets lowered the 8

ratios of area intensities towards 1.0 with as low as 0.32 wt% loading, whereas activated nanocarbons by 10 wt% loading have only reduced these area intensities towards a 1.5 ratio. Further, the reduction in area intensities achieved by the fillers does not directly correlate with the wear rates measured for the composites. For instance, when the ratios of the area intensities of the peaks are about 1.5, the wear rates of the composites of the fillers differ by as much as a few orders-of-magnitude. This may be expected with such a difference between the fillers possibly being explained by the hypothesis that fillers might have more than a single means to reduce composite wear rate. Fillers can modify the PTFE matrix such that its intrinsic wear resistance is increased (for example by increasing the similarity of the crystalline regions in the matrix with the tougher phase). Fillers may additionally contribute to the wear resistance of the composite utilizing their own intrinsic wear resistance through some rule-of-mixtures effect. Also, the behavior of the peaks, located in the phase-sensitive region, only indicates the state of PTFE. Fillers which effectively impart resistance to wear may also do so by other mechanisms, such as aiding the formation of a strongly adhered transfer film over the metallic countersurfaces.

3.2.1. Effects of graphene platelet loading and thickness

PTFE composites with 8 nm graphene platelets, at 0.32 wt%, 5 wt% and 10 wt%, were investigated to study the effect of graphene platelet loading on the tribological interface. ATR-IR spectra obtained from the unworn surfaces of these PTFE composites are shown in figure 8(a) along with the spectrum from unfilled PTFE. In addition to the previously described 1144 cm−1 band used for normalization, the spectra show another characteristic band for PTFE at 1198 cm−1 . The bands near 1198 cm−1 and 1144 cm−1 are associated with symmetric and asymmetric stretch of CF2 , respectively [26]. The spectra between 1900 cm−1 and 1400 cm−1 are found largely devoid of any intense bands. A broad minor band, near 3235 cm−1 , is visible in the spectra obtained from graphene-filled composites. This band may be attributed to the vibrations of the O-H groups [27] attached to graphene platelet surfaces. The intensities of the minor bands located near 2915 cm−1 and 2850 cm−1 appear to increase with increasing loading of the graphene platelets. They are attributed to C-H stretching vibrations [27] and appear in the spectra possibly because of defects in graphene platelets. In some spectra weak bands can also be seen around 2350 cm−1 and 1600 cm−1 , which have been attributed due to the presence of carbon dioxide and 3.2. Probing the role of highly effective fillers on the water vapor, respectively, in the path of the IR beam tribological interface using Attenuated Total Re- between the source and the detector [27, 28]. flectance (ATR-IR) Figure 8(b) shows the ATR-IR spectra obtained from worn pin surfaces of unfilled PTFE and the various 8 nm graphene-PTFE composites. The specFigures 8 through 11 present ATR-IR spectra tra from worn surfaces of composites with loadings obtained from unfilled PTFE and select example greater than or equal to 5 wt% show a number PTFE composites with graphene, CNT and activated of additional bands which are not prominently visnanocarbon fillers. Each of the presented spectra was ible in the spectra obtained from worn surfaces of scaled such that the amplitude of the characteristic the 0.32 wt% 8 nm graphene composite and unfilled PTFE band (due to CF2 stretching vibrations) near PTFE. Many of these bands are also largely absent 1144 cm−1 was equal for all the spectra within each in the ATR-IR spectra obtained from correspondsub-figure. This band was selected for normalizing ing unworn composite surfaces. Two strong bands the spectra as it has the greatest amplitude in the near 1660 cm−1 and 1585 cm−1 are seen with their spectra of unfilled PTFE. In addition, the spectra intensities monotonically increasing with increasing presented together on a graph were vertically shifted graphene loadings. The band near 1660 cm−1 can be for better visualization. attributed to the vibrations of carbonyl group in flu9

(a)

(a)

(b)

(b)

Figure 8: ATR-IR spectra obtained from (a) unworn and (b) worn surfaces of 8 nm graphene-filled PTFE composites with 0.32 wt%, 5 wt% and 10 wt% loadings.

Figure 9: ATR-IR spectra obtained from (a) unworn and (b) worn surfaces of 4 wt% graphene-filled PTFE composites with graphene platelets of different nominal thicknesses.

orinated carboxylic acids chelated to metals [18, 29] and also possibly includes absorption due to water [27]. The band near 1585 cm−1 can be attributed to asymmetric stretching of the acetate ion [27, 29]. A broad band with multiple peaks is observed for composites with more than 0.32 wt% graphene loading, near 1410 cm−1 . These peaks also seem to become more intense with increasing graphene loading. The symmetric stretching vibration of the carboxylate anion gives rise to this band [27, 29]. Additionally, a broad band near 3330 cm−1 emerges and its intensity appears to monotonically increase with increas-

ing graphene loading. This broad band is associated with O-H stretching in hydroxyl groups [27, 29]. PTFE composites with 1.25 nm, 8 nm, 12 nm and 60 nm graphene platelets at 4 wt% loading were analyzed to understand the effect of varying graphene platelet thickness on metal chelate formation. Figure 9(a) shows the ATR-IR spectra obtained from the unworn surfaces of these composites. Strong bands are again observed near 1198 cm−1 and 1144 cm−1 along with minor bands near 2915 cm−1 , 2850 cm−1 and 2350 cm−1 . ATR-IR spectra collected from worn pin surfaces of the various 4 wt% graphene-filled PTFE

10

composites are shown in figure 9(b). In addition to the bands associated with PTFE discussed previously, the spectrum from 1.25 nm graphene composite shows relatively intense and broad bands near 1660 cm−1 and 1410 cm−1 which are characteristic of the vibrations of the carbon-oxygen double bond in the carbonyl group. Such bands were also found by other researchers [18, 29] and their presence was attributed to tribochemistry leading to the formation of fluorinated carboxyl acids chelated to metallic countersurfaces. The band near 1585 cm−1 is linked to the stretching vibrations of the acetate ion. The bands around 1410 cm−1 were also observed in the spectra obtained from worn surfaces of 8 nm graphenePTFE composites in figure 8(b), with loading in excess of 0.32 wt%. Krick et al. [19] also observed a band with relatively low intensity at 1314 cm−1 in the ATR-IR spectrum obtained from the worn surface of highly wear-resistant (∼1×10−7 mm3 /Nm) PTFE composite with nanometer-sized particles of α-phase alumina. A broad band near 3205 cm−1 is also observed with another band near 3365 cm−1 appearing as a shoulder. A band of similar intensity can also be seen near 3050 cm−1 . Additionally, other bands are present near 1660 cm−1 , 1585 cm−1 and 1410 cm−1 . These bands are largely absent in the other spectra shown in figure 9(b). The broad band near 3365 cm−1 can be attributed to the alcoholic O-H stretching vibrations. In the spectra obtained from the worn surfaces of 5 wt% and 10 wt% 8 nm graphene-PTFE composites, this band appeared at a lower wavenumber of about 3330 cm−1 perhaps due of the different molecular structures of the generated alcohols/ethers. This band was also observed in the IR spectra obtained from the transfer films [17] and the wear debris [18] formed during the sliding of highly wear-resistant α-alumina-filled PTFE nanocomposites against stainless steel countersurfaces. The band near 3205 cm−1 can be attributed to the carboxylic acid O-H stretching vibrations and the band near 3050 cm−1 may be attributed to aromatic C-H stretching vibrations. These results suggest that chelate formation can be enhanced both by increasing the graphene content at fixed platelet thickness, and by decreasing platelet thickness at fixed graphene loading. An in11

creased chelate presence near the tribological interface would indicate increased tribochemical activity, possibly leading to a greater extent of cross-linking of the molecular chains with fluorinated carboxylic acid end groups through graphene platelets (which would improve the toughness of the surrounding polymer) as well as more robust transfer films, adhering better to the countersurface. In fact, this chelate formation may be proposed as proportional to the platelet surface area present per unit composite mass. A higher graphene platelet surface area per unit composite mass is expected to offer greater number of sites (through defects or the presence of chemically active groups) at which cross-linking could take place. Such a dependence of chelate formation on the surface area would also explain the clustering of wear rates of composites, with various graphene platelets and different filler loadings, around a single collapsed “mastercurve”, when instead plotted against the platelet surface area available per unit composite mass [6]. However, the presence of several bands, with varying intensities, between 1700 cm−1 and 1400 cm−1 makes the validation of such a hypothesis very challenging. 3.2.2. Are metal chelates also formed during the sliding wear of highly wear-resistant PTFE composites with activated nanocarbon particles and carbon nanotubes? The presence of metal chelates on worn surfaces of wear-resistant graphene-filled PTFE composites is interesting because metal chelates are also formed on the sliding interface of highly wear-resistant PTFE nanocomposites with α-alumina particles [18, 19]. If the formation of metal chelates could also be confirmed on worn surfaces of highly wear-resistant PTFE composites with activated carbon nanoparticles and functionalized carbon nanotubes, then this single wear resistance mechanism would be commonly shared among these few select nano-scaled fillers which are known to impart significant wear resistance to PTFE. ATR-IR spectra were collected from the unworn and worn surfaces of PTFE composites with activated carbon nanoparticles (<50 nm) at 0.12 wt%, 0.32 wt% and 0.8 wt%. Kandanur et al. [9] had earlier reported that the 0.8 wt% composite offered

a wear rate of about 2.8×10−7 mm3 /Nm. The 0.32 wt% composite wore at a rate which was about an order-of-magnitude higher than that measured for the 0.8 wt% composite. The 0.12 wt% composite existed near a threshold step in terms of the wear behavior with wear rates fluctuating intermittently between ∼10−4 mm3 /Nm and ∼10−6 mm3 /Nm. Figure 10(a) shows the ATR-IR spectra obtained from the unworn surfaces of these composites. In addition to the characteristic bands associated with PTFE, minor bands near 2915 cm−1 and 2850 cm−1 are again observed in spectra obtained from the 0.32 wt% and the 0.8 wt% composites. These bands, which are attributed to C-H stretching vibration, were also seen in the spectra obtained from graphene-filled PTFE composites. The band near 2350 cm−1 again simply results from varying CO2 of the environment along the beam path. Very weak bands can also be seen near 1550 cm−1 in the spectra obtained from the 0.32 wt% and the 0.8 wt% composite. These bands are associated with various forms of the carboxylate ion and likely arise due to moieties attached to the activated carbon nanoparticles. The ATR-IR spectra obtained from worn surfaces of the composites are presented in figure 10(b). These spectra have many similarities with the spectra obtained from worn surfaces of wear-resistant graphene-filled PTFE composites presented earlier. Spectra from the two highly wear-resistant composites (0.8 wt% and 0.32 wt%) show bands near 3365 cm−1 , 3205 cm−1 and 3050 cm−1 . These bands were also observed in the spectrum obtained from the worn surface of the 4 wt% 1.25 nm graphenePTFE composite. Additionally, the spectra show bands near 1650 cm−1 and 1410 cm−1 , which seem to become more intense with increasing loading of the activated carbon nanoparticles. These bands were also observed in spectra obtained from worn surfaces of the 4 wt% 1.25 nm graphene-PTFE composite and the 8 nm graphene-PTFE composites with 5 wt% or greater loading. The band near 1585 cm−1 is found to be relatively weak in comparison with the band near 1650 cm−1 . In the spectra obtained from wear-resistant graphene-filled PTFE composites, the intensity of this band was found to be comparable to the intensity of the band near

(a)

(b)

Figure 10: ATR-IR spectra obtained from (a) unworn and (b) worn surfaces of PTFE composites with activated nanocarbon particles at 0.12 wt%, 0.32 wt% and 0.8 wt% loadings.

1650 cm−1 . This possibly indicates structural differences between the chelates formed during the sliding wear of highly wear-resistant PTFE composites with graphene platelets and activated carbon nanoparticles. Surfaces of PTFE composites with carbon nanotubes were also analyzed in a similar manner using ATR-IR. Tribological characterization of these samples was earlier reported by Makowiec and Blanchet [10]. They found that the 5 wt% PTFE composite with carboxyl-functionalized carbon nanotubes showed a 2000-fold reduction compared to unfilled

12

PTFE, with wear rates approaching 10−7 mm3 /Nm. Figures 11(a) and (b) show the ATR-IR spectra obtained from unworn and worn surfaces, respectively of these carbon nanotube-filled PTFE composites. Spectra from unworn surfaces show characteristic bands associated with PTFE in addition to minor bands near 2915 cm−1 and 2850 cm−1 . These bands were also seen in the spectra from unworn surfaces of other composites presented earlier. Spectra from worn surface of only the 5 wt% composite show several bands near 1650 cm−1 associated with vibrations of the carboxylate ion. These bands

(a)

are largely absent in the spectra obtained from the worn surfaces of the composites with lower loadings of the carbon nanotubes. This serves as yet another example where the metal chelates are observed over the sliding interface of only the highly wear-resistant PTFE composites. These results show that each of the few select carbon-based nano-fillers known to impart extreme wear resistance to PTFE, enable tribochemical reaction pathways similar to those occurring during sliding wear of PTFE nanocomposites with α-phase alumina under ambient conditions. The carbonaceous nanoparticles bond to multiple perfluorinated carboxylic acids, forming as a result of the tribochemical reactions, and effectively serve as crosslinks which toughen the surrounding polymer matrix. Chelates, also formed during the tribochemical reactions, enable the formation of robust and welladhered transfer films over the metallic countersurfaces, which lower the wear rates further. 4. Conclusions Highly wear-resistant composites of PTFE with carbonaceous fillers such as graphene platelets, activated carbon nanoparticles and carbon nanotubes were probed using x-ray diffraction (XRD) and attenuated total reflectance (ATR) mode IR spectroscopy. Some wear reduction mechanisms activated by the these effective nanofillers were found to be shared across various forms. These mechanisms were also found to be operating within highly wear-resistant composites of PTFE with α-phase alumina nanoparticles. The key findings of this study are listed below.

(b)

Figure 11: ATR-IR spectra obtained from (a) unworn and (b) worn surfaces of PTFE composites with carbon nanotubes at 0.32 wt%, 0.8 wt%, 2 wt% and 5 wt% loadings.

13

1. When probed at room temperature of 20 ◦ C, XRD analysis showed that at a fixed filler loading the crystalline regions of the PTFE matrix in the composites with the two thinnest graphene platelets had the greatest resemblance to a tougher and higher-temperature phase of PTFE (phase-I). Crystalline regions of the PTFE matrix within the composites with thicker graphene platelets showed lesser resemblance with phaseI, but this resemblance was still greater in comparison with that possessed by unfilled PTFE. The resemblance to the tougher phase-I also

increased with increasing loading of graphene platelets at a given nominal platelet thickness, activated nanocarbon particles and similarly with an increasig loading of carboxylfunctionalized carbon nanotubes. This effect was also observed in composites of PTFE with α-phase alumina nanoparticles. 2. ATR-IR spectra showed that metal chelate formation on worn surfaces of only the highly wearresistant graphene-filled PTFE composites was enhanced both by increasing the graphene content at fixed nominal platelet thickness and by decreasing the platelet thickness at a fixed loading. Worn surfaces of highly wear-resistant PTFE filled with activated carbon nanoparticles or carbon nanotubes also showed more metal chelate formation at higher filler loadings. Under ambient conditions, the sliding wear of PTFE filled with α-phase alumina nanoparticles is already known to proceed with the formation of metal chelates. Thus, the studied carbonaceous fillers share this wear reduction mechanism with α-phase alumina nanoparticles. Acknowledgments Support during portions of these investigations from the National Science Foundation grant 1234641 is gratefully acknowledged. 5. References

[4] S. Bhargava, T. A. Blanchet, Unusually Effective Nanofiller a Contradiction of MicrofillerSpecific Mechanisms of PTFE Composite Wear Resistance?, ASME Journal of Tribology 138 (4) (2016) 042001–042001–8. [5] S. S. Kandanur, M. A. Rafiee, F. Yavari, M. Schrameyer, Z. Z. Yu, T. A. Blanchet, N. Koratkar, Suppression of wear in graphene polymer composites, Carbon 50 (9) (2012) 3178–3183. [6] S. Bhargava, N. Koratkar, T. A. Blanchet, Effect of platelet thickness on wear of graphenepolytetrafluoroethylene (PTFE) composites, Tribology Letters 59 (1) (2015) 1–12. [7] V. N. Aderikha, A. P. Krasnov, A. V. Naumkin, V. A. Shapovalov, Effects of ultrasound treatment of expanded graphite (EG) on the sliding friction, wear resistance, and related properties of PTFE-based composites containing EG, Wear 386-387 (2017) 63–71. [8] V. N. Aderikha, V. A. Shapovalov, Effect of filler surface properties on structure, mechanical and tribological behavior of PTFE-carbon black composites, Wear 268 (11-12) (2010) 1455–1464. [9] S. S. Kandanur, M. A. Schrameyer, K. F. Jung, M. E. Makowiec, S. Bhargava, T. A. Blanchet, Effect of activated carbon and various Other nanoparticle fillers on PTFE Wear, Tribology Transactions 57 (5) (2014) 821–830.

[1] J. K. Lancaster, The effect of carbon fibre rein- [10] M. E. Makowiec, T. A. Blanchet, Improved Wear Resistance of Nanotube- and other Carbon-filled forcement on the friction and wear of polymers, PTFE Composites, Wear 374-375 (2016) 77–85. British Journal of Applied Physics 1 (5) (1968) 549–560. [11] D. L. Burris, W. G. Sawyer, Improved wear resistance in alumina-PTFE nanocomposites with [2] K. Tanaka, S. Kawakami, Effect of varirregular shaped nanoparticles, Wear 260 (2006) ious fillers on the friction and wear of 915–918. polytetrafluoroethylene-based composites, Wear 79 (2) (1982) 221–234. [12] S. E. McElwain, T. A. Blanchet, L. S. Schadler, [3] T. A. Blanchet, F. E. Kennedy, Sliding wear W. G. Sawyer, Effect of particle size on the wear mechanism of polytetrafluoroethylene (PTFE) resistance of alumina-filled PTFE micro- and and PTFE composites, Wear 153 (1) (1992) 229– nanocomposites, Tribology Transactions 51 (3) 243. (2008) 247–253. 14

[13] L. Schadler, L. Brinson, W. Sawyer, Polymer nanocomposites: A small part of the story, JOM 59 (3) (2007) 53–60.

[22] S. S. Kandanur, The role of filler size & content and countersurface roughness in the wear resistance of alumina-PTFE nano-composites, Ph.D. thesis, Rensselaer Polytechnic Institute, 2010.

[14] J. A. Joyce, Fracture toughness evaluation of polytetrafluoroethylene, Polymer engineering and science 43 (10) (2003) 1702–1714.

[23] T. Fu, Z. Mo, P. Han, Y. Qi, S. Wu, D. Chen, Study on factors affecting room temperature transition of polytetrafluoroethylene, Chinese [15] E. N. Brown, D. M. Dattelbaum, The role of Journal of Polymer Science 4 (2) (1986) 170–179. crystalline phase on fracture and microstructure evolution of polytetrafluoroethylene (PTFE), [24] C. Marega, A. Marigo, V. Causin, V. KapePolymer 46 (9) (2005) 3056–3068. liouchko, E. D. Nicol, A. Sanguineti, Relation[16] B. A. Krick, J. J. Ewin, G. S. Blackman, C. P. Junk, W. G. Sawyer, Environmental dependence of ultra-low wear behavior of polytetrafluoroethylene (PTFE) and alumina composites suggests tribochemical mechanisms, Tribology [25] International 51 (0) (2012) 42–46. [17] A. A. Pitenis, K. L. Harris, C. P. Junk, G. S. Blackman, W. G. Sawyer, B. A. Krick, Ultralow wear PTFE and alumina composites: Its all about tribochemistry, Tribology Letters 57 (4) (2015) 1–8.

ship between the Size of the Latex Beads and the solid-solid Phase Transitions in Emulsion Polymerized Poly(tetrafluoroethylene), Macromolecules 37 (15) (2004) 5630–5637. H. B. Zhang, W. G. Zheng, Q. Yan, Y. Yang, J. W. Wang, Z. H. Lu, G. Y. Ji, Z. Z. Yu, Electrically conductive polyethylene terephthalate/graphene nanocomposites prepared by melt compounding, Polymer 51 (5) (2010) 1191 – 1196.

[26] H. W. Starkweather, R. C. Ferguson, D. B. [18] K. L. Harris, A. A. Pitenis, W. G. Sawyer, Chase, J. M. Minor, Infrared spectra of amorB. A. Krick, G. S. Blackman, D. J. Kasprzak, phous and crystalline poly(tetrafluoroethylene), C. P. Junk, PTFE Tribology and the Role of Macromolecules 18 (9) (1985) 1684–1686. Mechanochemistry in the Development of Protective Surface Films, Macromolecules 48 (11) [27] G. Socrates, Infrared and Raman characteristic group frequencies, John Wiley & Sons Ltd, (2015) 3739–3745. Chichester, England, third edn., 2001. [19] B. A. Krick, A. A. Pitenis, K. L. Harris, C. P. Junk, W. G. Sawyer, S. C. Brown, H. D. Rosen- [28] J. M. Chalmers, Mid-Infrared Spectroscopy: feld, D. J. Kasprzak, R. S. Johnson, C. D. Chan, Anomalies, Artifacts and Common Errors, John G. S. Blackman, Ultralow wear fluoropolymer Wiley & Sons, Ltd, 2006. composites: Nanoscale functionality from microscale fillers, Tribology International 95 (1) [29] M. Przedlacki, C. Kajdas, Tribochemistry of Fluorinated Fluids Hydroxyl Groups on Steel (2016) 245–255. and Aluminum Surfaces, Tribology Transactions [20] S. Bhargava, Tribological behavior of graphene49 (2) (2006) 202–214. filled polytetrafluoroethylene (PTFE) composites, Ph.D. thesis, Rensselaer Polytechnic Institute, 2017. [21] T. A. Blanchet, S. S. Kandanur, L. S. Schadler, Coupled effect of filler content and countersurface roughness on PTFE nanocomposite wear resistance, Tribology Letters 40 (1) (2010) 11–21. 15

• • •

PTFE nanocomposites with graphene, activated carbon and carbon nanotubes are characterized. Highly effective fillers share the ability to induce tougher & high-temperature phase-I of PTFE. Metal chelates are found on worn surfaces of only the highly wear resistant PTFE composites.