Isolating the tribochemical and mechanical effects of nanofillers on PTFE wear

Abstract

Ultra-low-wear PTFE nanocomposites rely heavily on water-dependent tribochemistry, which reinforces surfaces by anchoring tribochemically-modified chains to nanofillers and the countersurface. In a recent study, we showed that trace nanofillers (0.1 wt%) reduced the wear rates of an already low wear PEEK-PTFE blend by 40-fold with minimal tribochemistry. Interestingly, wear rates increased by 2-fold at 5 wt% nanofillers despite increased tribochemical accumulation. This observation raises questions about the tribochemical and mechanical roles of nanofillers in this material system, particularly at the surface. This paper aimed to isolate these effects. Specifically, we varied environmental humidity to promote or inhibit favorable tribochemical accumulation while maintaining subsurface stability via PEEK reinforcement. When we discouraged tribochemical reinforcement using a dry environment, high loadings of nanofillers had severely detrimental effects on wear rates and tribofilm stability. For example, the addition of 5 wt% nano-alumina to 5 wt% PEEK-PTFE increased wear rates by > 100-fold in the dry environment. By contrast, the addition of trace amounts (0.1 wt%) of nano-alumina had no detrimental effect on wear rate (10−7 mm3/Nm) or tribofilm stability. These results suggest that the mechanical effects of nanofillers were primarily destabilizing rather than stabilizing and that these effects increased with filler loading. In humid environments, however, these adverse effects of nanofillers, particularly at loadings >1 wt%, were offset by the favorable competing effect of tribochemical accumulation. Trace nanofiller loadings (∼0.1 wt%) optimized surface reinforcement at both environmental extremes because they provided the tribochemical benefits of the nanofillers while minimizing their mechanical costs.