Wear of Hydrogenated DLC in MoDTC-Containing Oils

This paper describes a study of the effect on MoDTC-promoted a-C:H DLC wear of adding various surface-active additives used in engine lubricants, including ZDDP, an ashless EP additive, Ca detergents, dispersants, an OFM and a PAMA, to an MoDTC solution. Tribofilms formed on wear tracks on steel were analysed using SLIM, TEM, STEM-EDX, Raman spectroscopy and XPS. Relevant mechanisms by which these additives reduce the impact of MoDTC on DLC wear have also been suggested. DLC wear in PAO+Mo can be reduced by the presence of other surface-active additives in three ways. Firstly, asperity contact between DLC and steel can be mitigated by forming thick antiwear tribofilms. Secondly, other additives can increase the ratio of MoS₂:MoO₃, reducing the amount of wear-enhancing MoO₃ in the tribofilm. Thirdly, the amount of MoDTC tribofilm including MoO₃ can be reduced by the competitive adsorption of other surface-active additives. This study has practical implications for ways in which DLC surfaces can be protected by lubricant formulation.