Effect of base oil polarity on the micropitting behaviour in rolling-sliding contacts

An automated micropitting test rig that uses the proven three-point contact configuration was applied to study the effect of base oil polarity on micropitting behaviour in rolling-sliding contacts. The tribological tests using one polar (ester) and one non-polar (polyalpha olefin [PAO]) base oil mixed with four different additives were first done in a micropitting rig to study the friction, wear, and micropitting performance. The tested specimen after tribological tests were examined by an optical microscope to study the micropitting on the surface. The X-ray photoelectron spectroscopy (XPS) was applied to obtain the related tribochemical information from the reaction layer. The results showed that higher wear and less micropits formed when testing with ester-based lubricants. This was related to the higher affinity to the steel surface of polar base oil molecules that can compete with additives to attach to the surface, resulting in a thinner reaction layer and shorter chain phosphates.