Effect of surface morphology of diamond-like carbon coating on friction, wear behavior and tribo-chemical reactions under engine-oil lubricated condition

The tribological behaviors of diamond-like carbon (DLC) coatings sliding against steel lubricated with the low viscosity engine-oil containing MoDTC and ZnDTP additives are investigated. The “pre-structured DLC” coatings, which are deposited on the micro-shot peened rough surface substrates, show low friction coefficients and no significant wear, in comparison with the DLCs coated on the usual polished substrates. The surface structuring can have positive effects on the lubricated contact regime to promote MoS2 formation energetically in the additive-derived tribofilm, and inhibit an increasing DLC wear attributed to MoO3 reduction involving hydrogen evolution and transformation of DLC. We discuss that different tribo-chemical reactions occur, either positive or negative, depending on the surface morphology, hydrogen content and mechanical properties of DLC coatings.