Non-destructive depth analysis of acidic phosphate ester boundary layers by hard X-ray photoelectron spectroscopy

Energy-resolved HAXPES is proposed here as a method to analyze boundary layer formation on rough frictional surfaces, in which the vertical OLAP boundary layer structure containing organic components is non-destructively analyzed. In addition, HAXPES analysis was performed with high incident X-ray energies to obtain information about the entire boundary layer, and to distinguish the organic and inorganic layers for calculating the effective thickness of each. The results revealed that the vertical structure of the OLAP boundary layer consists of a surface layer of organic molecules, including esters and phosphates, and an inorganic iron phosphate layer at the substrate interface, with each component exhibiting a vertical concentration gradient. Furthermore, the effective thickness of the organic layer formed under the present sliding conditions was estimated to be about 50 to 80 nm, which became thicker and exhibited lower friction at increased running-in times, and is greater than the inorganic layer thickness of 10 to 15 nm. It is reasonable to attribute the observed low-friction and velocity-dependent properties to the thicker organic layer that preferentially supports the load during friction, demonstrating that this method is useful for determining the effects of boundary layers on tribological properties.