Tribological properties of nc-TiC/a-C:H coatings prepared by magnetron sputtering at low and high ion bombardment of the growing film

Abstract

Two series of nc-TiC/a-C:H coatings were deposited by a hybrid PVD–PECVD process of titanium sputtering in argon/acetylene atmosphere at two configurations of magnetic field resulting in different impinging ion fluxes on the growing film. The composition of the coatings was varied by changing the acetylene gas flow during the depositions. Tribological tests were performed under conditions of emulating dry machining using 100Cr6 steel ball and silicon nitride ball as sliding counterparts. High temperature tribo-tests at 300°C and 500°C were performed with silicon nitride ball counterpart to examine the thermal stability of the coatings deposited at 320°C. Special attention was paid to design coatings with optimal chemical composition for high hardness. The coefficient of friction (CoF) and wear as a function of C/Ti are presented. It is observed that in the range of 1<C/Ti<2 the CoF is largely independent of the ion flux during the deposition and is ~0.2–0.3. The CoF then decreases with increasing carbon content up to a certain limit. Highest carbon-containing coating shows an increased CoF and wear. The coatings became strongly oxidized after the high temperature test. The CoF for coatings in the high hardness region is stable around 0.3 for the whole test at 300°C, the CoF at 500°C was stable at ~0.2 for the first half of the test, and then the coating failed. The coating in the wear tracks was mostly delaminated.