The two-layer structure of Zndtp tribofilms: Part I: AES, XPS and XANES analyses

The nature and properties of polyphosphate glasses formed in the antiwear action of zinc dithiophosphate (Zndtp) is investigated. Special attention is paid to the advantage of coupling three surface analytical techniques on the same Zndtp tribofilm: Auger (AES), XPS and XANES spectroscopies. The data show the two-layer structure of the Zndtp film and permit a clear identification of the chemical composition of each layer: a polymer-like zinc long chain polyphosphate overlying a mixed transition metal short chain phosphate. A Chemical Hardness model is found to predict the formation of such a layered tribofilm. Moreover, a tribochemical reaction between the zinc polyphosphate and the iron oxides species is proposed on the basis of the Hard and Soft Acids and Bases (HSAB) principle. This reaction explains the anti-abrasive mechanism of Zndtp and also predicts a depolymerisation of the long chain zinc polyphosphate glass, in very good agreement with AES/XPS/XANES analytical data. The role of residual sulphur in the lubricant is also explained and the model is in agreement with the formation of metal sulphides embedded in the short chain phosphate matrix. This first paper serves as a basis for a detailed study of the mechanical properties of each film.