HAXPES Study on the Chemical States of Reaction Films Formed on Metal Surfaces by Zinc Dialkyl Dithiophosphate

The fuel-saving performance of automotive lubricants greatly depends on the composition and structure of the reaction films that form on metal surfaces through chemical reactions between the additives in the lubricant and the metals. In this study, we prepared reaction films on metal surfaces using two lubricants formulated with different base oils of different degrees of polarity and an anti-wear additive commonly used in automotive engine oils, and examined the chemical state of the films using HAXPES. It was observed that polyphosphates found in the reaction film formed with the lubricant formulated with the polar base oil had shorter chains than those in the reaction film formed with the lubricant formulated with the non-polar base oil. In the latter reaction film, polyphosphates with longer chains may concentrate near the film surface. By contrast, in the former reaction film the polyphosphates may have chains of mostly the same length. [DOI: 10.1380/ejssnt.2018.396]