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.