Behavior and interaction of boundary lubricating additives on steel and DLC-coated steel surfaces

Abstract Tribofilm formation and frictional performance of lubricant additives — oleamide, ZDDP and their mixture formulated in PAO — on steel and diamond-like carbon (DLC)-coated steel surfaces was studied under severe boundary lubrication conditions. The binary additive system reduced friction to a greater extent compared to the individual additives for all the surfaces. Chemical composition analysis of the tribofilms using surface spectroscopic techniques (ToF-SIMS and EDX) revealed that the formation of tribo-reactive ZDDP film on steel surface is suppressed in the presence of oleamide. In turn, this results in lower friction. Frictional performance of the additives on diamond DLC and graphite DLC-coated steel surfaces was evaluated and correlated with their wear performance. Analysis of tribofilm thickness and chemical composition revealed that the additives did not form a cohesive film on diamond DLC surface. On the contrary, graphite DLC coating completely worn out from the substrate when additives were present singly in the formulation. However, the additives in combination prevented delamination of graphite DLC coating.