The Chemistry of Antiwear Films Generated by the Combination of ZDDP and MoDTC Examined by X-ray Absorption Spectroscopy

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Abstract
In order to obtain a better understanding of the decomposition mechanisms of antiwear and antioxidant engine oil additives such as zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC), tribochemical and thermally deposited films were prepared from oil solutions containing varying concentrations of these two additives. The chemistry of these films was studied by using X-ray absorption (XANES) spectroscopy and X-ray photoelectron (XPS). The antiwear properties were examined on a Plint rubbing machine by measuring the amount of wear as a function of time and the concentrations of the additives. The XANES and XPS spectra indicate that the dominant species from MoDTC in the antiwear films resembles that of a MoS2-like film, and in the thermal films, the dominant species are those of a sulphate and MoO3. The spectra also indicate that the phosphorus from ZDDP forms polyphosphate chains of varying lengths. The combined use of organo-Mo compounds and Zn alkyl dithiophosphates showed a synergistic effect by reducing both friction and wear.