Characterization of anti-wear films formed from fresh and aged engine oils

Modern engine oils are formulated with zinc dialkyl-dithiophosphate (ZDDP) compounds to protect moving metal components from wear. As emissions regulations for automobiles become more stringent, lower concentrations of these additives are being considered in future oil formulations to extend the life of the catalyst after-treatment systems. However, any formulation changes need to be carefully tested to ensure there is no degradation in friction and wear performance. While evaluating engine oils in a prior study, we found that the valve-train wear improved markedly as the oil aged. Changes in tribological performance were linked to differences in surface film chemistry on the valve tappet components. In the current study, we examine the friction and wear of fresh and aged engine oils in controlled laboratory tests using a simulated valve-train apparatus, ball-on-flat, and cylinder-on-flat test rigs. Surface films that form during sliding were characterized at different length scales and depth distributions using reflection infrared spectroscopy, Raman spectroscopy, and high-vacuum surface analysis. The types of species present, such as phosphates, were different between fresh and aged oil. Carbonate and sulfide were present in films formed from fresh oil, but were usually absent from aged oil. Wear control may be related to a competition between carbonate and phosphate species. Oxidized-oil-derived entities are present on aged oil coupons and appear to improve friction performance. Thus, differences in film composition are used to explain performance differences of fresh and aged oils and the results from the different test rigs. (C) 2007 Elsevier B.V. All rights reserved.