An analytical study of tribofilms generated by the interaction of ashless antiwear additives with ZDDP using XANES and nano-indentation

Abstract

Mixtures of zinc dialkyl dithiophosphates (ZDDP) and ashless fluorothiophosphates and thiophosphates were examined under tribological conditions. The tribofilms were studied using nano-indentation, electron microscopy and X-ray absorption near edge structure spectroscopy. With the addition of ashless fluorothiophosphates and thiophosphates, there is significant reduction in wear at identical phosphorous levels. Tribofilms formed with mixtures of ZDDP and ashless antiwear additives had stable pad-like structures, while those from ZDDP alone were more heterogeneous. When ZDDP alone is used, S L-edge indicates a preponderance of sulfates at the surface of the tribofilms, while in the presence of a mixture of antiwear additives a mixture of sulfates and sulfides was observed. In all the cases, the phosphates formed were short chain polyphosphates of Zn.