The two-layer structure of zndtp tribofilms Part 1: AES, XPS and XANES analyses

The nature and properties of polyphosphate glasses formed in the antiwear a ction of zinc dithiophosphate (Zndtp) is investigated. Special attention is paid to the advantage of coupling three surface analytical techniques on t he same Zndtp tribofilm: Auger (AES), XPS and XANES spectroscopies. The dat a show the two-layer structure of the Zndtp film and permit a clear identif ication of the chemical composition of each layer: a polymer-like zinc long chain polyphosphate overlying a mixed transition metal short chain phospha te. A Chemical Hardness model is found to predict the formation of such a l ayered tribofilm. Moreover, a tribochemical reaction between the zinc polyp hosphate and the iron oxides species is proposed on the basis of the Hard a nd Soft Acids and Bases (HSAB) principle. This reaction explains the anti-a brasive 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 expl ained and the model is in agreement with the formation of metal sulphides e mbedded in the short chain phosphate matrix. This first paper serves as a b asis for a detailed study of the mechanical properties of each film. (C) 20 01 Elsevier Science Ltd. All rights reserved.