The influence of viscosity on the friction in lubricated DLC contacts at various sliding velocities

Diamond-like-carbon (DLC) coatings are one of the promising types of protective coatings for various mechanical applications, including those under lubricated conditions. In recent years the research focus of lubricated DLC contacts has been primarily on the chemical aspects of lubrication, while the physical properties of the oils and the other physical lubrication aspects were to a large extent neglected. In this work we analyse the friction behaviour of self-mated DLC/DLC contacts that use various viscosity grades of polyalphaolefin (PAO) base oils, i.e., 18, 30 and 46 mm2/s, at a range of velocities from 0.04 to 0.41 m/s. For a comparison, we also investigated some steel/steel contacts. At low velocities (up to 0.08 m/s) we observed almost no effect of the viscosity for the DLC surfaces, suggesting a predominant influence of the DLC “solid–solid” contacts on the friction. However, relatively strong, physically adsorbed oil-film layers were present on the DLC surfaces, which were able to prevent the coatings from wearing out under low-speed conditions, that otherwise occur under dry conditions. In the high-velocity region (above 0.17 m/s), the viscosity has no effect on the steel contacts, but higher-viscosity oils tended to reduce the DLC friction, suggesting that for DLC surfaces, higher viscosities are required for the same film thickness and “quality” of lubrication than for steel surfaces. This is due to the poorer wetting and oil-adsorption properties of DLC compared to steel. However, at the same time, this effect leads to reduced friction in DLC contacts, which was about 15% lower for all the investigated sliding velocities.