Experimental Observation of Zinc Dialkyl Dithiophosphate(ZDDP)-Induced Iron Sulphide Formation

Zinc dialkyl dithiophosphate (ZDDP) as a well-known anti-wear additive enhances the performance ofthe lubricant beyond its wear-protection action, through its anti-oxidant and Extreme Pressure (EP)functionality. In spite of over thirty years of research attempting to reveal the mechanism of action ofZDDP, there are still some uncertainties around the exact mechanisms of its action. This is especially thecase with the role of sulphide layer formed in the tribofilm and its impact on surface fatigue. Althoughiron sulphide on the substrate is hypothesised in literature to form as a separate layer, there has beenno concrete experimental observation on the distribution of the iron sulphide as a dispersed precipitate,distinct layer at the steel substrate or both. It remains to be clarified whether the iron sulphide layerhomogeneously covers the surface or locally forms at the surface. In the current study a cross sectionof the specimen after experiment was prepared and has been investigated with Transmission ElectronMicroscopy (TEM) and Energy-Dispersive X-ray (EDX) elemental analysis. A 5–10 nm iron sulphide layer isvisualised on the interface as a separate layer underneath the phosphate layer with an altered distributionof tribofilm elements near the crack site. The iron sulphide interface layer is more visible near the cracksite where the concentration of the sulphur is enhanced. Also, ZDDP elements were clearly detectedinside the crack with a varied relative concentration from the crack-mouth to the crack-tip. Sulphur ispresent inside the crack to a higher extent than in the bulk of the tribofilm.