Ti/Al co-doping induced residual stress reduction and bond structure evolution of amorphous carbon films: An experimental and ab initio study

Abstract

Amorphous carbon (a-C) films co-doped by two metals exhibit a desirable combination of mechanical and tribological properties for wider applications. Nevertheless, the structural evolution of metal co-doped a-C films from the atomic and electronic scales is a critical pre-requisite to illustrate the intrinsic mechanism of residual stress reduction. Herein, we first fabricated the Ti/Al co-doped a-C films with different concentrations by a hybrid ion beam system. When the co-doped Ti/Al concentrations were <Ti3.9 at.%/Al4.4 at.%, the Ti and Al atoms were dissolved in the a-C matrix without forming a carbide phase; with the Ti/Al concentrations further increased beyond of Ti3.9 at.%/Al4.4 at.%, the crystalline titanium carbide nano-particles appeared, while Al existed in the form of aluminum oxidation state in the films. The residual stress, hardness and elastic modulus of films were strongly dependent on the chemical state of the co-doped Ti/Al atoms, which decreased first and then increased with Ti/Al concentrations. Ab initio calculations revealed that the residual stress reduction in a-C films caused by Ti/Al co-doping was mainly attributed to the significant relaxation of distorted CC bond lengths and the formation of a weak covalent interaction for TiC bonds and the ionic interaction for AlC bonds.