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.