The effects of film structure and surface hydrogen on the properties of amorphous carbon films

Molecular dynamics simulations were used to examine the mechanical and tribological properties of amorphous-carbon thin films with and without surface hydrogen. The simulations showed that the three-dimensional structure, not just the sp(3)-to-sp(2) carbon ratio, is paramount in determining the mechanical properties of the films. Particular orientations of sp(2) ringlike structures create films with both high sp(2) content and large elastic constants. Films with graphite-like top layers parallel to the substrates have lower elastic constants than films with large amounts of sp(3)-hybridized carbon. The layered structure of the hydrogen-free films caused them to have novel mechanical behavior, which also influenced the shape of the friction versus load data. The atomic-scale structure of the film at the interface was of critical importance in determining the load at which tribochemical reactions (or wear) between the counterface and films were induced.