Thermally induced evolution in non-hydrogenated and hydrogenated amorphous carbon films: A molecular dynamics research

Amorphous carbons have received significant attention due to their excellent tribological properties. In this paper, molecular dynamics (MD) simulations were performed to study the thermally induced structural evolutions and hybridization transitions in non-hydrogenated (a-C) and hydrogenated (a-C:H) amorphous carbon films. The results indicated that the amorphous carbon gradually transforms into a graphitic carbon network under thermal effect. The hybridization transition is characterized by a three-stage process: (i) elongation of the existing bonds, (ii) breakage of one existing bond or formation of one bond, and (iii) relaxation. For low-density a-C film, the formation of void defects can be observed within high-temperature regions. With increasing densities, the diffusivity of a-C films decreases significantly, indicative of better thermal stability. Also, a glass- transition process can be observed in a-C films at around 3200 K, showing no obvious correlation with film densities. For a-C:H films, the incorporation of low-mass hydrogen atoms induces a highly diffusive amorphous network, which would deteriorate the thermal stability. With increasing hydrogen contents, both glass-transition temperatures and activation energies decrease. Also, as shown by the lower potential barriers and bond stretching distances, hybridization transitions are prone to occur in a-C:H films with high hydrogen contents.