Electrochemical effect on boundary lubrication of ZDDP additive blended in propylene carbonate / diethyl succinate

The objective of this study is to find appropriate surface potentials under which the ZDDP tribofim formation can be promoted efficiently with low ZDDP concentrations. Firstly, the boundary lubrication performance of ZrO2 ball and AISI 52100 steel plate in propylene carbonate (PC)/diethyl succinate (DES) blended with different ZDDP concentrations was investigated. With increase of ZDDP concentrations, the coefficient of friction and wear volume decreased and the tribofilm covered wider regions within the wear scars. Secondly, the boundary lubrication performance of the same tribo-system under different surface potentials was studied with a relative low ZDDP concentration. The coefficient of friction and wear decreased and the tribofilm-covered area increased along with the elevation of surface potentials, which is similar to the above mentioned concentration test results. Thirdly, the difference of ZDDP tribofilm chemical structures under different surface potentials were characterized by TEM, EDS and XPS. Finally, the mechanism of electrochemical effect on ZDDP tribofilm formation was discussed. It was considered that the effects of applied potentials on the adsorption of ZDDP additive and on activation energy of electrochemical reactions on the steel surface are the reasons for the observed electro-tribochemical experiment results.