Chemical and mechanical analysis of tribofilms from fully formulated oils Part 1 – Films on 52100 steel

The authors report, for the first time, a comprehensive chemical and mechanical characterisation of antiwear films prepared from a fully formulated oil that is commercially available. Wear increases substantially when using the fully formulated oil compared to using ZDDP alone. X-ray absorption near edge structure (XANES) spectroscopy at the P K- and L-edges, S K-edge, Mo L-edge, B K-edge, Ca L-edge, O K-edge and Fe and Zn L-edges permits chemical characterisation of the major elements in the thin films. Ca phosphates, ZnS and MoS₂ are the main P and S species formed, contrary to previous studies involving only ZDDPs, whereby Zn phosphates are the dominant species. These findings can be accounted for by using the hard and soft acid and bases (HSAB) principle. Small amounts of CaCO₃ are present, but no B was detected, implying it does not become incorporated into the film. Atomic force microscopy (AFM) reveals continuous pads with a relatively uniform indentation moduli (125 ± 10 GPa), separated by trenches that are essentially comprised of uncovered steel substrate.