Effects of detergent on the chemistry of tribofilms from ZDDP: studied by X-ray absorption spectroscopy and XPS

Abstract

The interaction of overbased metallic detergents with zinc dialkyldithiophosphate (ZDDP) on the formation of tribofilms under boundary lubrication has been studied. The condition of boundary lubrication was achieved by a pin-on-disk reciprocating wear machine. The chemical composition of tribofilms was studied using P, S, Ca and C X-ray absorption near edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS). Calcium phenate and salicylate detergents, in high and low overbased forms, were used in combination with ZDDP. The C K-edge XANES and XPS analyses showed that when high-overbased detergents are used, micelles containing calcium carbonate break down and calcium carbonate is deposited in the tribofilm. Direct evidence for the formation of calcium phosphate from ZDDP in combination with the detergents, is provided by XANES spectroscopy at the P K-and L-edges. It was found that Ca2+ ions are replaced for Zn2+ in the polyphosphate structure of tribofilms, forming short chain polyphosphates. The effects of these chemical changes have been correlated with the tribological performance of ZDDP/detergents systems. The XANES study at C K-edge, and S L- and K-edge indicated that the Ca salicylate micelle is not as easily disrupted as that of the calcium phenate.