Evolution of tribofilms under lubrication conditions experienced in engine valve trains

A block on ring tribometer employing AISI E52100 steel components, was used to simulate cam and tappet tribology in order to evaluate the tribological performance and evolution (frictional performance, wear coefficients, surface chemistry species / quantification and film structure) of the tribofilms. Lubricants consisted of a mineral base oil containing zinc dialkyl dithiophosphate (ZDDP) plus a full dispersant-detergent-antioxidant package and an identical blend but with the addition of molybdenum dithiocarbamate (MoDTC). Energy Dispersive X-Ray analysis (EDX) and X-ray Photoelectron Spectroscopy (XPS) chemical analysis techniques have been employed, allowing the crucial link between testing duration and film properties to be evaluated. Factors such as the evolution of surface topography due to tribofilm formation and hence wear protection and friction control offered, are analysed using Atomic Force Microscopy (AFM). The contrasting frictional difference between the two lubricants is correlated to wear, surface roughness effects, structure and surface chemistry. The relationships between the running-in of an in service valve train system and the tribo-chemical parameters analysed in this tribometer based work are discussed. It is found that friction reduction observed is primarily a consequence of tribofilm shear strength changes and not nano-scale tribofilm surface topography, which are comparable in magnitude at different frictional coefficients. © 2004 Elsevier B.V. All rights reserved.