Effects of Tungsten Doping Contents on Tribological Behaviors of Tungsten-Doped Diamond-Like Carbon Coatings Lubricated by MoDTC

A systematic study was carried out to assess the influence of tungsten content on the tribological properties of tungsten-doped diamond-like carbon (W-DLC) coatings lubricated by molybdenum dithiocarbamate (MoDTC). Four W-DLC coatings with W doping content of 0.0 % (pure DLC), 3.08, 10.73 and 27.7 % were fabricated, respectively, following the microstructures, and mechanical properties of these W-DLC coatings were observed, and next, the friction and wear tests were conducted under boundary lubrication with MoDTC; finally, the valence states of tribofilms were identified by X-ray photoelectron spectroscope. The results showed that W-DLC coatings present lower coefficients of friction and higher wear rates under MoDTC lubrication comparing those under PAO lubrication. It also revealed that W content has little influence on anti-friction behaviors, while the wear rates decreased firstly and then increased with higher W content. It was proposed that the tribological properties of W-DLC coatings mainly depend on the ratio of hardness to Young's modulus (H/E), the degree of graphitization and the formation of tribofilms containing MoS2 and MoOx.