Film thickness effects on mechanical and tribological properties of nitrogenated diamond-like carbon films

Nanoindentation, nanoscratch and ball-on-disk tests were used to determine the film thickness effect on the mechanical and tribological properties of nitrogenated diamond-like carbon (CNx) films, which were deposited on Si (100) substrates by an electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MP-CVD) system. Except for the film with a thickness of 20 nm, there existed peak values of hardness for all films thicker than 44 nm. When the thickness ranged from 44 to 235 nm, the peak values of hardness and the corresponding indentation depth increased along with increasing film thickness. The results of scratch resistance tests showed that the critical loads of the fracture were independent of thickness for thinner films, however, they increased rapidly with increasing thickness for thicker films. Ball-on-disk sliding tests indicated that the friction coefficient decreased with increasing thickness for thinner films, while there was no obvious thickness dependence of the friction coefficient for relatively thicker films. The formation of transferred layer and graphitization of the films during sliding resulted in the decrease of friction coefficients at early stages of sliding.