Friction and wear characteristics of acidic phosphate ester boundary layers analyzed by near-edge X-ray absorption fine structure

During friction at extremely low surface pressures, interesting phenomenon occurs in oleyl acid phosphate (OLAP) the friction coefficient exhibits a positive gradient in the velocity range of boundary lubrication, to mixed lubrication transitions, and the friction coefficient decreases as the running-in time increases. This phenomenon is presumed to be due to the action of the boundary layer,. therefore Therefore, in this study, we analyzed friction surfaces with containing residual friction test oil still present using by near edge X-ray absorption fine structure. The results were then combined with first-principles calculations, to investigate the chemical state of the boundary layer in a state close comparable to that of sliding. As a result, The results inferred it was considered that the iron salt of a phosphate ester and the coordination structure of an iron-centered phosphate ester were generated by OLAP-added containing oil and aggregated near the interface with the base material during friction. Furthermore, a boundary friction model that considers non-Newtonian characteristics was applied to an experimentally obtained friction diagram to verify the effect of the boundary layer on the friction characteristics. The maximum effective viscosity calculated from a function obtained by fitting the friction diagram was approximately 3000 Pa·s, which was equivalent to that of common greases. These results indicated that the characteristic frictional properties of OLAP are due to the action of its grease-like organic boundary layer.