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, oleyl acid phosphate (OLAP) has an interesting phenomenon: The friction coefficient has 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, we analyzed friction surfaces with friction test oil still present via a 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 to that of sliding. As a result, the iron salt of a phosphate ester and the coordination structure of an iron-centered phosphate ester were generated by OLAP-added 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 3,000 Pa·s, which was equivalent to that of common grease. These results indicated that the characteristic frictional properties of the OLAP are due to the action of its grease-like organic boundary layer.