Elastic Properties of Diamond-Like Amorphous Carbon Films Grown by Computer Simulation of Ion-Beam Deposition Process

Atomic-scale calculations were performed for the first time to investigate mechanical
properties of amorphous carbon films grown by a realistic simulation of ion-beam deposition process. The simulated films have a thickness of a few nanometers and reproduce the main structural features of real films, with the bulk content of sp3 bonded atoms varying from 35 to 95%, depending on the ion energy (E = 20-80 eV). Employing empirical interatomic potentials for carbon, the average bulk stresses as well as the atomic-level stress distributions were calculated and analysed. The bulk stresses were found to depend not only on the ion energy, but also on the film quality, in particular, on such structural inhomogeneities as local fluctuations of the sp3 fraction with the depth. The local variation of the bulk stress from the average value considerably increases as the local content of sp2 bonded atoms increases. Elastic constants of amorphous carbon films were also computed using the method of inner elastic constants, which allows for the stress dependence of elastic constants to be analysed. This investigation showed how the Young's modulus varies as a function of the lateral bulk stress in an amorphous film.