Ultra-low friction of tetrahedral amorphous diamond-like carbon (ta-C DLC) under boundary lubrication in poly alpha-olefin (PAO) with additives

Diamond-like carbon (DLC) coatings have increased demand as protective hard coating for mechanical components due to their exceptional mechanical and tribological performance. They provide ultra-low friction, excellent anti-wear properties and adhesive protection. In recent years, researches focus on the use of DLC coatings under boundary lubricated contacts to increase fuel efficiency of automotive by controlling the friction and to reduce the emission of harmful elements. Commercially available fully formulated oils are designed for ferrous surfaces and contain various additives to enhance the base oils, but interaction between DLC surfaces and those formulated oils is not yet fully understood. In this study we investigated frictional properties of ta-C DLC coating when lubricated with a base oil poly alpha-olefin (PAO) and PAO containing friction modifier glycerol mono-oleate (GMO), anti-wear additives ZnDTP and mixture of GMO+ZnDTP to understand the mechanism of ultra-low friction. The results exhibit that ta-C give ultra-low friction in pure PAO for DLC/steel and DLC/DLC tribopair. GMO additivated PAO provide smooth run-in period for transition to ultra-low friction regime and also enhance the durability of coating. ZnDTP behave differently depending on the presence of ferrous surfaces on the contact. It forms pad-like wear protective tribofilm both on ta-C and steel surfaces for DLC/steel contact, while it form thin white layer on ta-C surfaces for DLC/DLC contact. Using GMO and ZnDTP together does not show any synergistic correlation for steel/steel, DLC/steel and DLC/DLC combinations.