Abstract: The effect of overbased calciumphenate detergents on the chemical composition of tribofilms fromzinc dialkyldithiophosphate (ZDDP) under boundary lubrication has been studied using P, S, Ca, and C X-ray absorption near edge structure (XANES) spectroscopy. Boundary lubrication was achieved by a pin-on-disk reciprocating wear machine. Calciumphenate detergents, in both high and low overbased forms, were used in combination with ZDDP. C K-edge XANES showed that when higher overbased detergents were used the micelles containing calcium carbonate broke down and calcium carbonate was deposited in the tribofilm. Direct evidence for the formation of calcium phosphate from ZDDP in combination with detergents was provided by XANES spectroscopy at the P-, K- and L-edges. Zn2+ ions were replaced by Ca2+ in the polyphosphate structure of tribofilms forming short chain polyphosphates. The effects of these chemical changes were correlated to the tribological performance of ZDDP/detergents systems.

Key words: ZDDP, Detergent, Tribofilm, XANES