CHEMICAL CHARACTERIZATION OF TRIBOCHEMICAL AND THERMAL FILMS GENERATED FROM NEUTRAL AND BASIC ZDDPS USING X-RAY-ABSORPTION SPECTROSCOPY

X-ray absorption near edge structure (XANES) spectroscopy at the phosp horus L-edge and sulphur L-edge has been used to characterize the chem ical nature of tribochemical and thermally generated films from severa l ZDDP antiwear agents in the neutral and basic forms. Using the P and S L-edge XANES spectra of model compounds with known structure as fin gerprints, the chemical structures of P and S species in the films hav e been identified. P appears in all the films as polyphosphates in dif ferent proportions of short and long chain polyphosphates. In some fil ms, polyphosphates are accompanied by unchanged ZDDP. Generally films generated from neutral and basic ZDDPs show similar P and S chemistry (polyphosphates and sulphides) but contain different proportions of un changed ZDDP. However, the aryl ZDDP films have different polyphosphat e structure compared to the alkyl ZDDP films. The sulphur proportion i n the tribochemical films is decreased a great deal, but remains in th e reduced form. However, S in the thermo-oxidatively generated films, appears both in the reduced and oxidized form, depending on the ZDDP a nd the temperature. (C) 1997 Elsevier Science Ltd.