A particular alumina-PTFE nanocomposite has distinguished itself with unusually large wear reductions at trace filler loadings. Recent studies have shown that the formation of carboxylic acid end groups in humid environments is a critical part of the wear reduction mechanism. This finding has significant implications for the utility of the material for space and high temperature applications. In this paper, wear rate, morphology, composition and chemistry of the wear surfaces were characterized as a function of environmental composition and surface temperature to better understand the environmental limitations of this solid lubricating system and the associated wear resistance mechanisms. The following results were found: (1) ultra-low wear rates were retained with increasing interface temperature up to 100°C, (2) carboxylates were

The tribological properties of most high functioning tribological materials, including diamond, graphite, molybdenum disulfide, and polytetrafluoroethylene, depend strongly on environmental moisture. A particularly wear-resistant alumina−polytetrafluoroethylene (PTFE), for example, loses its capacity for ultralow wear in dry environments because a moisture-dependent tribochemical degradation product is necessary to anchor and stabilize its protective transfer films. A recent study [Onodera et al., J. Phys. Chem. C, 2017, 121, 14589−14596] on a PEEK−PTFE composite suggested that the poly(etheretherketone) (PEEK) filler particles anchor PTFE transfer films to metallic surfaces via physical interactions that are, theoretically, insensitive to environmental moisture. This study tested the hypothesis that the physical nature of transfer film adhesion by PEEK−PTFE increases its wear tolerance to changes in environmental moisture. The optimal 20 wt % PEEK−PTFE composite exhibited the same ultralow-wear rates (8 × 10 −8 ± 1 × 10 −8 mm 3 / Nm) and low friction coefficients (0.18 ± 0.02) in dry nitrogen (0.05% RH) and humid air (30% RH). The results demonstrate that this unusually wear-resistant solid lubricant material is also unusually insensitive to environmental moisture. Compared to the well-studied alumina−PTFE system, whose ultralow-wear rates correlate strongly to the prominence of carboxylate peaks in infrared (IR) spectra, carboxylate peaks were either greatly attenuated or absent in the IR spectra of PEEK−PTFE following ultralow-wear sliding in both humid and dry environments. The results are consistent with the prediction from the Onodera group that the ultralow-wear rates of PEEK−PTFE can be retained in dry environments because the strong physical interactions between the PEEK filler and the counterface reduce or eliminate its dependence on water-dependent tribochemistry for transfer film adhesion.

Reduced wear rates of filled polymeric tribomaterials consistently accompany improvements in the appearance of the transfer film, a protective layer of debris that adheres to the counterface. As a result, wear reductions are often attributed to the favorable effects of the filler on transfer film quality. However, the cause-effect relationship between fillers, transfer film quality, and polymer wear performance remains uncertain due, in part, to a lack of standard metrics for assessing transfer film quality. Methods for quantifying transfer film thickness and area fraction have been proposed previously; although some studies show strong correlations between these parameters and wear rate, others have demonstrated a lack of general applicability. In a more recent study, it was proposed that the characteristic size of the gaps in the transfer film (freespace length) may better reflect visual differences in transfer film quality and more directly relate to debris size and wear rate. In this paper, a representative collection of common tribological polymers and composites were subjected to wear testing and transfer film topology characterization to better generalize the link between polymer wear performance and transfer film topology (thickness, coverage, and domain size-scales). The free-space length provided the best correlation with steady state wear rates of the tribopolymers in this study; furthermore, the trendline from this study proved to be the best independent predictor of wear rates from previous studies. The results suggest that, among the metrics considered, the free-space length provides the best independent measure of transfer film quality in the context of polymer wear.

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