Combination of ashless antiwear additives with metallic detergents : interactions with neutral and overbased calcium sulfonates

Abstract The interactions of neutral and overbased calcium sulfonate detergents with ashless thiophosphate oil additives under boundary lubrication were studied. The ashless additives used were neutral and acidic dialkyldithiophosphate (DTPs) and neutral triaryl monothiophosphate (MTP). This study uses three surface analytical tools to provide elemental and chemical information at the surface and in the bulk of the derived tribochemical films. The elemental composition of the tribofilms was studied using X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray analysis (EDX) (combined with scanning electron microscopy (SEM)). Corresponding P and S X-ray absorption near edge structure (XANES) spectroscopy was also used to provide further insight into the chemical composition of the films. SEM images of the derived tribofilms revealed that each film had distinct topographical features associated with it. XPS and EDX revealed that when oil is blended with calcium sulfonate, considerable amount of calcium is incorporated into all the tribochemical films. The phosphorus content of the tribofilm was reduced substantially when the overbased detergent was combined with MTP additive. XANES spectroscopy of the P L-edge provided direct evidence for the formation of calcium phosphate in tribofilms from the ashless additives in combination with the detergent. S K-edge revealed that sulfate is the main sulfur species formed in the bulk for all three tribofilms in the presence of the neutral detergent while a mixture of sulfite and sulfide species are formed when the overbased was used. S L-edge XANES showed that calcium sulfonate has undergone some oxidation at the surface. A more antagonistic effect was observed for MTP, with the formation of a very thin phosphate film. Tribological performance was also evaluated. Surprisingly, combination of the neutral detergent with any of the AW additives did not result in any significant change in wear to the substrate. For MTP plus neutral detergent, the thinner phosphate film produced, combined with very little change in wear protection confirms that, not only is the tribochemistry dominated by calcium sulfonate, but also confirms the anti-scuffing and AW properties that are associated with it as well. Even more surprising, was the significant decrease in wear when the overbased detergent is used. This illustrates not only that the tribochemistry was dominated by the detergent, but also the exceptional AW properties of calcium carbonate.