Wear in tetrahedral amorphous carbon induced by sp3/sp2 phase transition

Tetrahedral amorphous carbon (ta-C) coatings have attracted high scientific attention in recent years due to their outstanding low friction coefficients in lubricated environments and their superior resistance to abrasive as well as adhesive wear. Consequently, they are highly suitable as protective coatings in automotive components such as combustion engines to reduce wear and friction. Nevertheless, ta-C abrade under extreme conditions and an atomistic understanding of the wear processes in ta-C is crucial for optimizing its wear resistance. The present study employs atomic-scale simulations to investigate the basic principles of wear between hydrogen- free tetrahedral-amorphous carbon (ta-C) films, which are modeled state-of-the-art by an improved version of the well-known Brenner bond-order potential. During tribological contact, these mainly sp3 hybridized films tend to form an amorphous carbon (a-C) tribomaterial mainly consisting of carbon atoms in sp2 configuration. Furthermore, the observed tribolayer grows faster than the a-C between diamond surfaces under similar conditions, which can be explained by easy breaking of weak bonds in a finite ta-C region triggered by plasticity in the adjacent a-C while the diamond/a-C transition occurs at an atomically sharp interface. The difference in the a-C tribofilm formation between ta-C and diamond is in agreement with near-edge x-ray absorption fine structure spectroscopy (NEXAFS) studies published recently.