Interactions of Diamond-Like Carbon Coatings with Fully Formulated Engine Oils

Abstract

Diamond-like carbon (DLC) coatings have been known for their low friction and high wear resistance for many years, but only in recent years have these coatings been used in mass-produced engines due to significant improvement in coating deposition techniques resulting in good adhesion on substrate materials. The interactions of these coatings with engine oils have been a focus for further research in recent years. In this investigation, the interactions of engine oil additives with five different DLC coatings containing two different doping elements were explored using a laboratory reciprocating rig under boundary lubrication condition. Two different contact geometries were investigated: a piston ring-on-liner and cylinder-on-flat to simulate piston ring and cylinder bore and cam and tappet applications. It was observed that DLC-coated piston rings offered lower friction coefficients and lower wear of the liner compared to commonly used Mo-NiCr-coated piston rings. However, the wear of the DLC-coated rings varied. The presence of lubricant additive–derived films could be observed on the wear surfaces of three of the DLC-coated surfaces irrespective of their doping element. In addition, the thickness of the lubricant additive–derived films on DLC-coated surfaces (a) was lower than that formed on ferrous materials and (b) depended on engine oil formulations.