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