Lubricant-surface interactions of polymer-coated engine journal bearings

This work provides the methodology for the improved understanding of tribocontacts with tribolayers formed from the interactions of an engine oil and the surface of a journal bearing under boundary lubricating conditions. In particular, advanced surface characterisation combining X-ray photoelectron spectroscopy (XPS) and high resolution mass spectroscopy (MS) contributed to integrated knowledge on tribochemical and tribomechanical processes in the tribocontact studied. An engine connecting rod bearing lubricated with commercial OW-20 GF5 engine oil was evaluated as part of the tribosystem. Bearing material was a recently developed polymer-based overlay which increased in popularity in the automotive market owing to the durability and superior tribological properties. After engine bench tests and fatigue experiments, conventional oil analyses showed only minor changes regarding oil chemistry whereas MS revealed significant additive degradation, e.g., formation of organic phosphates from ZDDP and depletion of tris(2,4-di-tert-butylphenyl) phosphite additive. Regarding the fatigue experiments, surface analysis by XPS showed the formation of tribofilms with a thickness only in the range of a few nanometers and composed of P, S and Zn, i.e., mainly elements in ZDDP. This publication reports for the first time on surface characterisation by laser desorption ionisation MS applied to engine journal bearings: the presence of an organic phosphorus compound the origin of which is assumed from ZDDP could be revealed by this approach.