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.

Research highlights
► Metal dialkyl dithiophosphate compounds (MDTPs) are used in lubricating oils due to their multifunctional performance, zinc dialkyl dithiophosphates (ZDDP) being the most widely used.
► However, several studies have proved ZDDP to have detrimental effects on wear under certain operating conditions, and to enhance friction when the system is operating in mixed and boundary lubrication regimes.
► To identify the plausible mechanisms that govern the detrimental effect of those additives on tribological performance, it is necessary to identify the parameters affecting the additive-derived layer formation and the tribological properties of this layer, as it is directly related to the additive reactivity towards the surface.
► This study addresses the effect of base oils on the performance of lubricant additives under mixed rolling–sliding conditions present in bearing applications.