The effect of nitrogen on the mechanical properties of tetrahedral amorphous carbon films deposited with a filtered cathodic vacuum arc

The mechanical properties of both undoped and nitrogen containing tetrahedral amorphous carbon (ta-C and ta-C:N) films, such as hardness and E-modulus, are investigated. The mechanical properties are related to the film structures. The ta-C and ta-C:N films are prepared in a filtered cathodic vacuum arc process. Nanoindentation testing is used to measure the mechanical properties of these films. Since the thickness of films prepared for this study is <100 nm, a continuous stiffness technique is therefore used to determine the mechanical properties of these films. For the lightly doped ta-C:N films, the mechanical properties are competitive with those of undoped ta-C. However, with a further increase in the nitrogen content in the films, the mechanical properties of the ta-C:N films drop. The extent of decrease of the mechanical properties of ta-C:N films shows the dependence on the N₂ partial pressure or N ion energy. The reason for this may be due to the nitrogen doping efficiency. The excess nitrogen atoms contained in the films may promote the transformation from the sp³ bonding structure to sp² structure and further enlarge the size of sp² clusters, which causes the evident reduction of mechanical properties of the films.