ELECTROCHEMICAL AND THERMAL-OXIDATION OF TIN COATINGS STUDIED BY XPS

X-ray photoelectron spectroscopy (XPS) has been used to investigate the electrochemical and thermal oxidation of titanium nitride (TiN) coatings prepared by physical vapour deposition (PVD) at 200 degrees C. Electrochemical oxidation of TiN was carried out at various potentials in phthalate buffer solution (pH 5.0). Evaluation of the XPS Ti 2p and N 1s spectra showed the presence of nitride, oxynitride and oxide species in the layer formed by anodic oxidation. The electochemical oxidation of TiN to TiO2 proceeds through the formation of a mixed oxynitride/oxide layer, which transforms into oxide (TiO2) at sufficiently positive potentials (E > 1.1 V vs. SHE). The oxidation of TiN to TiO2 is accompanied by the formation of molecular nitrogen (N-2). The thickness of the oxide layer reaches similar to 7 nm after oxidation at the highest potential (1.9 V), A complete coverage of the TiN surface by TiO2 leads to an anodic peak in the polarization curve. On the basis of angle-resolved XPS measurements, two types of oxynitride species are identified, which are distributed differently throughout the oxidized layer, X-ray photoelectron spectroscopy depth profiles of TiN oxidized at 450 degrees C and 600 degrees C in an oxygen flow reveal that at the lower temperature an oxynitride layer is formed, whereas a thick TiO2 layer appears on top of TiN at the higher temperature. The interface between the nitride and oxide phases is relatively sharp. It is suggested that the mechanism of TiN oxidation proceeds by a progressive replacement of nitrogen by oxygen. The TiN coatings can be used up to 600 degrees C as a protective coating in an oxygen atmosphere. Valance band spectra of TiN, as well as of electrochemically and thermally oxidized TiN, are presented and discussed.