X-ray absorption near-edge structure analysis of the chemical environment of zinc in the tribological film formed by zinc dialkyl dithiophosphate decomposition on steel

Calculation of X-ray absorption near-edge structure (XANES) has been used to investigate the chemical nature of zinc in thermal films formed on the surfaces of steel coupons by the additive zinc dialkyl dithiophosphate (ZDDP) in oil solution. Carbon K, oxygen K, and zinc L edge XANES spectra were used to characterize the thermal films and a model film formed by addition of cumene hydroperoxide (CHP) to the ZDDP. Thermal films formed by heating at up to 100 °C for 5 h showed little change from unreacted ZDDP. At 160 °C for 5 h, the ZDDP underwent thermal solution decomposition with similarities to the oxidation induced by CHP. The Zn L edge XANES spectra were compared to calculated spectra of the model surface species ZnO and ZnS in the wurtzite structure and ZnS in the zinc blende structure. Agreement between experiment and simulation is found for the 160 °C thermal film with a roughly two monolayer film of ZnO and for oxidation by CHP with a three to four monolayer film of ZnS.