HAXPES Study on Chemical States of Reaction Films Formed on Metal Surfaces by Zinc Dialkyldithiophosphate and Molybdenum Dialkyldithiocarbamate

Abstract

The fuel-saving performance of automotive lubricants greatly depends on the composition and chemical 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 four kinds of reaction film on metal surfaces using lubricants formulated with ZnDTP only or with ZnDTP and MoDTC and then investigated the relationship between the lubricant formulations and the chain lengths of polyphosphates in the reaction films by angle-resolved HAXPES. The chain lengths of polyphosphates were estimated by the O1s spectra. We found that the polyphosphate chains in the reaction films formed by ZnDTP and MoDTC were shorter than those in the films formed by ZnDTP only, regardless of the length of the ZnDTP's alkyl group. We also found that films formed by ZnDTP with a shorter alkyl group had longer polyphosphate chains, regardless of whether MoDTC was also present.