A non-active molybdate ester (ME) was synthesized in a batch process. Its tribological performance and its synergistic effect with ZDDP in 5CST were evaluated using a four-ball machine, and the chemistry of tribofilms was analyzed with XANES. The results indicate that ME possesses excellent anti-wear and friction-reducing properties, not load-carrying capacity. Both ME and ZDDP show excellent synergistic tribological behavior in 5CST. According to the XANES results, the tribochemical films generated from ME alone are mainly composed of MoO3, and the tribochemical films generated from the oil blends containing ME and ZDDP consist mainly of MoS2, sulphate and polyphosphate.Highlights► A non-active molybdate (ME) was synthesized from cheap material in a batch process. ► ME possesses excellent anti-wear and frcition-reducing properties. ► ME can replace or partly replcace ZDDP in the case of anti-wear and friction-reducing. ► The relationship between tribofilms and tribological properties on containing-molybdenum compounds is better understood.

Two ashless dithiocarbamate derivatives, octyl 2-(dibutylcarmothioylthio) acetate (DDCO) and S-dodecyl 2-(dibutylcarbamothioylthio) ethanthioate (DDCS), were prepared. Thermal stabilities tests were conducted with a thermo-gravimetric analyzer (TGA). The tribological properties of each compound and their combinations with ZDDP in a mineral oil (HVI WH150) were evaluated using a four-ball tester. X-ray absorption near-edge structure (XANES) spectroscopy was used to characterize the chemical properties of tribofilms generated from DDCO, DDCS and their combinations with ZDDP. According to the TGA results, the synthesized compounds possess good thermal stability (initial decomposition temperatures are above 270 °C). It can be found that all the prepared compounds have better friction-reducing capacity than ZDDP, with anti-wear performance and extreme pressure property worse than ZDDP. However, their combinations with ZDDP perform better than ZDDP in tribological properties. The results of the XANES analyses indicate that the composition of the tribofilms from DDCO or DDCS is organic sulphide on the outer surface and pyrite with a little sulphite in the inner layer, which also suggests the –SC(=S)–N– part in additives structure plays key role in tribol-chemical behaviour. The XANES spectra of the combinations exhibit interestingly that the addition of DDCO or DDCS can increase the length of polyphosphate chain in the tribofilms.