To access the evolving tribochemistry at the contacting asperities, a miniature pin-on-disc tribological apparatus was developed and combined with synchrotron X-ray Absorption Spectroscopy (XAS). The new apparatus makes it possible to study in-situ the transient decomposition reactions of various oil additives on different surfaces under a wide range of realistic operating conditions. The results suggest that the decomposition of ZDDP starts by forming intermediate sulphate species on the steel surface, which are readily reduced to sulphides of discontinuous clusters. The clusters can play different vital roles including binding the subsequently formed phosphate layers with the steel surface. Initially, the phosphate layers consist of short chains due to excess concentration of metal oxides on the steel surface. As the oxides’ concentration decreases in the subsequent layers, the short chains start to polymerise into longer ones. The polymerisation reaction appeared to follow first-order reaction kinetics with two distinctive phases. The first is a fast transient burst phase near the metal surface, whereas the second phase dominates the formation of the layers away from the metal surface and is characterized by slow kinetics.
To better understand the origin of the superior antiwear properties of the P-rich tribofilms, an Atomic Force Microscope (AFM) liquid cell was designed to form tribofilms in-situ while examining their textural and rheological properties over time. The obtained results indicate that the tribofilms behave as a molten glass with an average viscosity of 1×1012 Pa.s. This suggests that their superior antiwear properties originate from their intrinsic rheological properties that allow them to flow while formed, which was clear from their ability to maintain local order on the nanoscale through the motion, rearrangement and local reconfiguration of single and multiple patches of the formed tribofilm at the interface. This seems to effectively mitigate the smearing and wearing of the contacting asperities resulting in less wear. The findings of this study open future opportunities for quantitatively analysing the interfacial rheology and reaction kinetics governing a broad range of additives and substrates, which can help build mechanistic models capable of better predicting wear
