Deposit formation from lubricant degradation: A uniform layer deposition model

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 sp³ 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.