Mechanochemical functionality of graphene additives in ultralow wear polytetrafluoroethylene composites

Poly-tetrafluoroethylene (PTFE) is a well-known solid lubricant, but its poor wear resistance limits its tribological applications. Certain carbonaceous fillers reduce PTFE wear rate by up to 4000-fold with no sacrifice of the friction coefficient; whereas the responsible mechanisms remain unclear. We systematically studied the effects of three graphene fillers with different aggregate strengths (graphene nanoplates, few-layer graphene and single-layer graphene) on the wear resistance of graphene-PTFE composites. The results found (1) graphene of weak aggregate strength outperform that of strong aggregate strength by 100 times in wear reduction, (2) strong correlations between wear rate, tribofilm carboxylate signal intensity and filler-matrix area per unit composite mass. The carboxylate signals likely correspond to chemical bonding between mechanochemically degraded PTFE, graphene and the steel counterface within highly crosslinked and adherent transfer films and worn polymer surfaces. Results in this study support two independent (primary versus secondary) wear reduction hypotheses in literature which were further backed by DFT simulations of filler-matrix interactions, TEM/EDS analysis of wear debris, SEM/EDS images of the polymer surfaces, and environmental test results in a dry argon (Ar) atmosphere. The results of this paper are also of instructive value for the development of solid lubricants with highly controllable wear characteristics.