The influence of base oil polarity on the tribological performance of zinc dialkyl dithiophospate additives

ZDDP blended in two base oils with different polarities were tested in mixed rolling–sliding conditions. The results showed significant differences in friction and wear when the additive is blended in the different oils. When ZDDP is blended in a non-polar base oil, the additive molecules have a better access to the steel surface, leading to higher adsorption rates than when ZDDP is blended in a polar oil, resulting in the formation of thicker ZDDP-derived reaction layers and influencing tribological performance. These results indicate the importance of base oil polarity on the ability of ZDDP to reach the surface and the influence on tribological performance.