The influence of base oil polarity and slide-roll ratio on additive-derived reaction layer formation

Functional additives, particularly extreme-pressure and antiwear additives, in formulated oil will compete to adsorb and form a protective layer in tribological contacts. The thickness of the layer is determined by the equilibrium between the formation and removal processes. In this article, the interactions between additives and base oil molecules and operating conditions influence friction and wear are studied. One polar (ester oil) and one non-polar (poly-alpha-olefin) commercial base oil blended with zinc dialkyl dithiophosphates were studied. The tribological performance was evaluated using a ball-on-disc test rig under mixed rolling-sliding conditions in the boundary lubrication regime. An adapted in situ interferometry technique was used to monitor the additive-derived reaction layer formation. The properties of the additive-derived reaction layers were studied using surface analysis techniques, X-ray photoelectron spectroscopy and atomic force microscopy. A thicker layer was formed when the additive is blended in the non-polar oil. This observation suggests that base oil polarity determines the transport of additives to the surface, thereby controlling the maximum reaction layer thickness, friction and wear, as well as the morphology of the additive-derived reaction layer. However, the reaction layer chemical composition is not strongly influenced by the base oil polarity. Among the operating conditions, shear was identified as a fundamental parameter for the activation of additives on rubbing steel surfaces and the properties of the derived reaction layer.