In this study, various zinc dialkyl dithiophosphate (ZDDP)-derived tribofilms were analyzed with X-ray photoelectron spectroscopy (XPS) to determine the relationship between tribofilm chemical composition and wear severity. Samples were generated on camshaft lobes from engine tests conducted at different oil temperatures, and wear depths were measured with a surface profilometer. Tribofilm specimens were analyzed by XPS to assess changes in film chemistry as a function of wear severity. Tribofilms present on all camshaft lobes were found to contain polyphosphate glass of various chain lengths. Long-chain polyphosphates were observed on low wear tracks, whereas short-chain polyphosphates were found on more severe wear tracks. Variations in polyphosphate chain length are also apparent at different wear depths along a low wear track. The presence of long-chain polyphosphates on low wear tracks was indicative of their superior antiwear properties, whereas the occurrence of short-chain polyphosphates on more pronounced wear tracks was indicative of their inferior antiwear properties.

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Reported is a new model describing deposit formation and ageing along the so-called universal mass deposition curve. The penn state micro-oxidation (PSMO) apparatus produced model deposits at varied test durations in order to study the ageing mechanism of thin film deposits from mineral base oil. Along the 'universal' mass deposition curve, mass deposition rates were correlated to film chemistry. Formed by concerted oxidation-polymerisation reactions, chemical group and elemental content analyses were performed by Fourier transformed infrared spectroscopy-attenuated total reflectance and energy dispersive spectroscopy, respectively. The results showed that deposit formation initiates as a film -uniform in composition. After an induction period, the film growth accelerates then slows and eventually falls off, with a decrease in mass often observed. These stages reflect underlying physical and chemical changes as the deposit progresses from an initial polymeric, lacquer-type deposit, towards a final dark, carbonaceous deposit, along the universal mass deposition curve. Carbonyl, hydroxyl and sp 3 hybridised carbon were identified as the main functional groups found across all deposit types albeit at different levels. Correlative behaviour was observed between changes in functional group infrared intensities and O-atom and C-atom content. These trends are interpreted as reflecting concerted deoxygenation and dehydrogenation processes, throughout the deposit, over the course of ageing, thus providing a foundation for the uniform layer model.

The Penn State Micro-Oxidation (PSMO) test was used in an inverse manner to pre-cover metallic pan surfaces with polymeric, transitioning and carbonaceous films. These pre-coated pans were then used as the initial test specimen/surface upon which fresh lubricant samples were aged. The effects of predeposits of varied ages were gauged against the baseline of a virgin metallic surface to decouple the lubricant-deposit system towards resolving their effect upon further deposit growth. From such data, a uniform deposition model describing deposit formation and aging was developed. Chemical characterisation of PSMO deposits by Fourier transform infrared-attenuated total reflectance and energy-dispersive X-ray spectroscope analyses provide additional supporting evidence of changes in chemical bonding (alkyl C-H and carbonyl C=O bond stretching vibrations) and composition (C-and O-atom content) as the deposits undergo deoxygenation and dehydrogenation reactions. Across different aged oils and films, the substitution tests show a declining activity towards mass deposition with film age, interpreted as decline in reactivity.