Atomic-level and average stresses in diamond-like amorphous carbon films grown by computer simulation

Diamondlike amorphous carbon (DLC) is a transparent hard material approaching crystalline diamond in hardness and elastic modulus. The main issue restricting applications of this very promising material is a high compressive stress in DLC films after growth. I present recent results of atomic scale calculations of growth and mechanical properties of DLC films performed in collaboration with H.-U. Jaeger. The following topics are discussed:

- the nature of intrinsic stresses in DLC films;
- the dependence on the intrinsic stresses on the film deposition parameters;
- the effect of the intrinsic stresses on the second-order elastic constants of amorphous carbon.

The atomic models of DLC films are prepared by direct molecular dynamics simulation of ion-beam deposition. Similar to real as-deposited films, the simulated films have a high content of tetrahedrally coordinated atoms and large intrinsic compressive stresses in the region of steady-state growth. The stress in the region of steady-state growth is calculated as an average of atomic-level stresses derived from an empirical interatomic potential. The origin of the intrinsic stress in DLC films is discussed on the basis of the structural changes due to thermal annealing simulated by molecular dynamics method. We propose that the high stress in DLC films can be
related to local defects in amorphous carbon networks. The dependence of the second-order elastic contants of amorphous carbon on the intrinsic stress is examined by the method of homogeneous deformation. We nonlinear effects are shown to become appreciable at the stress values typical of as-deposited DLC films.