X-ray absorption spectroscopy of tribofilms produced from zinc dialkyl dithiophosphates on Al–Si alloys

Abstract

X-ray absorption near edge structure (XANES) analysis has been used to characterize the chemistry of tribologically derived phosphorus films from a mineral base oil with a zinc dialkyl dithiophosphate (ZDDP) additive. These species were detected in the wear scars found on both counter-surfaces from a reciprocating boundary contact between different aluminum–silicon alloys and either the same alloy or with a steel counter-surface. Two different Al–Si alloys were used which had significantly different silicon contents, 0.7% (aluminum 6061) and 7% (aluminum 319), respectively. Couples between A319/A319, A319/52100 steel, 52100 steel/A319, A6061/A6061, A6061/52100 steel, and 52100 steel/A6061 were investigated. The phosphorus K- and L-edge spectra show that with the onset of wear of the aluminum matrix an aluminum phosphide species is found in the wear scar. Furthermore, unreacted-ZDDP, a linkage isomer, and polyphosphate species were detected in many of the wear scars. The spectra showed little chemical-variation with increase in rubbing time for each series studied. Excluding the 52100 steel/Al alloy couples, it was found that for the various A319 couples the A319/52100 steel performed the best. For the A6061 couples, the A6061/A6061 couple was the best. In all cases the modes of wear are strongly dependent on the yield strength of the materials used.