The adverse effect of carbon in mild wear with ZDDP lubricant additive

Abstract

Over the years, investigators have extensively studied various aspects of cylinder bore polishing and different factors affecting this phenomenon have been identified: temperature, fuel sulphur, engine design, metallurgy and load. Moreover it is well known that bore polishing is largely caused by the formation of carbonaceous piston deposits and by carbon particles in suspension in the engine oil. But a survey of the literature has revealed that little information on the wear mechanisms is available and that there is a lack of high resolution analysis of materials formed during friction. In the work described here, wear tests were carried out with classical lubricant formulations (with ZDDP anti-wear additive), and the effects of several allotropic forms of carbon were compared. In parallel to wear experiments, local analysis was performed by the use of electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope to determine the carbon evolution in interfacial materials, leading to elucidation of their formation mechanism. Standard forms of carbon were analysed for basic comparisons, and the very high sensitivity of EELS to carbon and the good differentiation of carbon microstructures was confirmed. Carbon microstructure in the lubricant before testing was compared with that in the wear particles after testing. Main results were: pro-wear action of carbon in suspension in a lubricant; the absence of carbon in pure ZDDP wear particles; participation of carbon particles in the wear debris formation; structural disorganization of carbon during friction. Different hypotheses on carbon-induced wear mechanisms are examined.