Mechanochemistry of Zinc Dialkyldithiophosphate on Steel Surfaces under Elastohydrodynamic Lubrication Conditions

Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig capable of applying very high loads has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear str...