Interfacial Gradient and Its Role in Ultralow Wear Sliding

A particular nanosized alumina (α-Al2O3) filler reduces the wear rate of polytetrafluoroethylene (PTFE) by nearly 4 orders of magnitude in ambient environments through the formation of stable interfacial tribofilms. One key to the unusual success of this system is the tribochemical degradation of the polymer and subsequent formation of carboxylate salts, which directly bond the PTFE to both the alumina nanofiller and the metallic counterface. However, previous studies have shown that the exceptional wear resistance of these cross-linked, stable, and well-anchored surfaces vanishes when slid against surfaces of slightly higher surface energy. In this paper, we elucidate the effects of these interfacial gradients within the native ultralow wear composite-on-transfer film system using interrupted wear tests and intermittent surface analysis. As anticipated, the transition from high wear to ultralow wear was accompanied by small adherent debris, tribochemical formation of carboxylates, increased surface energy, and increased adhesion. Interestingly, we observed significant differences on either side of the interface during low wear sliding; compared to the running films on the composite surface, the transfer films on the counterface exhibited consistently greater tribochemical degradation, surface energy, and adhesion to a model alumina probe. This interfacial gradient, we propose, is a necessary feature of the ultralow wear system and functions by setting the direction and driving force for transfer wear. In this case, the interfacial gradient stabilizes the transfer film and minimizes the driving force for running film wear.