Investigation of subsequent deposit growth on pre-existing lubricant deposits: a substitutional growth 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 pre-deposits 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.