Wear, Plasticity, and Rehybridization in Tetrahedral Amorphous Carbon

Wear in self-mated tetrahedral amorphous carbon (ta-C) films is studied by molecular dynamics and near-edge X-ray absorption fine structure spectroscopy. Both theory and experiment demonstrate the formation of a soft amorphous carbon (a-C) layer with increased sp² content, which grows faster than an a-C tribolayer found on self-mated diamond sliding under similar conditions. The faster \(\hbox{sp}^{3} \rightarrow\,\hbox{ sp}^{2}\) transition in ta-C is explained by easy breaking of prestressed bonds in a finite, nanoscale ta-C region, whereas diamond amorphization occurs at an atomically sharp interface. A detailed analysis of the underlying rehybridization mechanism reveals that the \(\hbox{sp}^{3}\, \rightarrow\hbox{ sp}^{2}\) transition is triggered by plasticity in the adjacent a-C. Rehybridization therefore occurs in a region that has not yet experienced plastic yield. The resulting soft a-C tribolayer is interpreted as a precursor to the experimentally observed wear.