Investigating binary oil additive systems containing P and S using X-ray absorption near-edge structure spectroscopy

Abstract

X-ray absorption near-edge structure (XANES) spectroscopy to monitor the chemical properties of tribochemical films generated from oil solutions comprised of two additives containing phosphorus and sulfur. Oil solutions containing diphenyl phosphate (DPP) or triphenyl phosphate (TPP), were blended with sulfurized ester (SE) and dithiocarbamate (DTC) extreme-pressure (EP) agents to generate tribochemical films under both antiwear (AW) and EP conditions. This study found that the chemistry of all tribofilms, was clearly dominated by the phosphate ester additives. XANES spectroscopy of the AW films revealed that in the presence of any S additive, phosphate esters were able to generate polyphosphate films, mainly comprised of an iron polyphosphate. S K-edge XANES spectra (probing the bulk) revealed that in the presence of either phosphate ester, sulfur was oxidized to sulfate. Noisy S L-edge XANES spectra (probing the surface) indicated that sulfur was mainly present in the bulk of the film. Thickness measurements of the AW films obtained directly from both P and S K-edge fluorescence yield (FY) spectra showed that the proportion of S was significantly reduced when compared to AW films generated by the S additives in the absence of either phosphate ester. In addition to the reduction of S present, a decrease in the amount of phosphorus was also apparent, indicative of competitive interactions at the surface between the additives. The reduction in phosphate in the tribofilms can be attributed to the reaction of either SE or DTC with steel during the heating of the oil solution prior to rubbing. Wear scar width (WSW) measurements revealed that wear protection is determined completely by the action of phosphorus and sulfur has little role, if any, in further protecting the substrate during AW conditions. XANES spectroscopy of the EP films containing DPP showed that DPP was able to generate a relatively thick phosphate EP film, in the presence of sulfur, leading to a decrease in the amount of sulfur present. Conversely, when TPP was used, only a thin phosphate film could form leading to an increased amount of sulfur present. Sulfur present, was in the form of FeS. The ability of DPP to react with the substrate at lower temperatures than TPP, thereby reducing the number of adsorption sites at the metallic surface, was responsible for the differences in the amount of S present.