Antiwear Properties and Tribochemical Action Mechanisms of Three S-Containing Triazine Derivatives as Additives in Rapeseed Oil

The antiwear properties of three S-containing triazine derivatives with different active groups, referred to as HOT, HOMA, and TOMA, as additives in rapeseed oil (RSO) were evaluated using a four-ball friction and wear tester. The morphologies of the worn surfaces of the steel balls were observed using a scanning electron microscope (SEM). The tribochemical action mechanisms of the S-containing triazine derivatives in rapeseed oil were investigated by means of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES). It was found that all three S-containing triazine compounds added in the rapeseed oil at a proper concentration showed rather good antiwear behavior. XPS and XANES analytical results showed that the tribofilms from the worn surfaces of the steel balls lubricated with RSO-1.2 wt% HOMA and the discs lubricated with RSO-3.0 wt% HOMA were mainly composed of some adsorbed N-, O-, and S-containing organic compounds, along with appreciable oxidized S products such as sulphite and sulphate. In contrast, the films from RSO-1.2 wt% TOMA showed a large concentration of FeS and FeS2, with a smaller amount of oxidized S products. The large difference in the chemical compositions of the tribofilms of RSO-HOMA and RSO-TOMA could be closely related to the differences in their active element content and bond types.