Tribological analysis of nano-composite diamond-like carbon films deposited by unbalanced magnetron sputtering

Ti-containing nano-composite diamond-like carbon (DLC) coatings have been developed with improved tribological characteristics. These coatings were synthesized by sputtering of pyrolitic graphite and titanium targets using unbalanced magnetrons with pulsed plasma technology. Compared with the Ti-containing nano-composite DLC film, a graded Ti–C:H/TiCN/TiN film was also deposited by sputtering of titanium targets in reactive gases (nitrogen and C2H2) using the same deposition system. The Ti/C multi-layered DLC coating deposited at a substrate rotation speed of 5 rev./min depicted an amorphous structure with 2-nm periodic thicknesses, and gave satisfactory friction performance in the pin-on-disk tests with a wear rate of 1–3×10−17 m3/Nm and a friction coefficient of 0.09–0.1 against 100Cr6 steel and WC. The steady-state friction coefficient of nano-composite DLC films is lower than that of graded Ti–C:H DLC films (0.15–0.17). When sliding against soft copper ball, the friction coefficients of the DLC films were higher and increased with the sliding process from an initial friction coefficient of 0.15 with a promising wear behavior. Wear debris were primarily composed of copper mixed with surface impurities. SEM, EDX and XPS analyses depicted that the initial Ti contained C–O transfer films possess good adhesion between the sliding counterparts (100Cr6 and WC) and the coatings during the initial sliding stage. Micro-Raman spectra acquired from the nano-composite DLC coatings demonstrated a tendency of graphite formation of the transfer film. The readily transferred oxide-free graphite-like sp2 films form a lubricious layer, which possess low shear strengths under applied loads. The XPS analysis of DLC C1s core level also suggests that transfer film is sp2 dominated.