X-ray absorption study of the effect of calcium sulfonate on antiwear film formation generated from neutral and basic ZDDPs: Part 2 - Sulfur species

In Part 2, the XANES spectra, the identification of spectral features and the interpretation of the results for the sulfur species are presented. The experimental work required to obtain the spectra is described in Part I (preceding paper). The appearance of S-IV and S-VI in the tribofilms is interpreted as evidence for disproportionation of sulfonate during sliding. Iron sulfate at the tribofilm surface may be due to sulfonate oxidation. Basic ZDDPs form films with higher S concentrations than do neutral ZDDPs. Aryl ZDDPs form films containing more S than do the corresponding alkyl ZDDPs. Most of the sulfur in the tribofilms is in the form of sulfide. The increased intensities of the Ca++ peak is evident in the S spectra and consistent with the deposition of CaCO3 in the tribofilm. Calcite tribofilms have been shown to be anti-wear. Perhaps this compensates for the loss in antiwear efficacy reported in Part 1.