Combined Electrochemical and Electron Spectroscopic Investigations of the Surface Oxidation of TiAlN HPPMS Hard Coatings

The surface oxidation of TiAlN High Power Pulsed Magnetron Sputtering (HPPMS) hard coatings was investigated using a combined electrochemical and electron spectroscopic approach. The reversible and irreversible surface redox reactions as well as the resulting surface electronic properties were investigated by means of Cyclic Voltammetry (CV) and Electrochemical Impedance Analysis (EIS). X-ray Photoelectron Spectroscopy (XPS) of the TiAlN near-surface region as oxidised in air after deposition revealed a duplex surface film consisting of oxynitridic TiAl(O,N) covered by a shallow Al rich oxide layer. Complementary EIS revealed metallic conductivity of this TiAl(O,N) phase. A detailed microscopic model of the electrochemical oxidation process highlighting the importance of O interstitials for the oxidation process is presented. The electrochemical oxidation induced by CV resulted into a transition from metallic to n-type semiconducting properties of the surface oxide film. As disclosed by XPS and XPS sputter profiling, a complex multi-layered film was formed comprising segregated a-(TiO2)x(Al(OH)3)y located on top of a more ordered electrochemically inactive oxide film. Further, an intermediary nitrogen doped but segregated a-(TiO2)x(Al(OH)3)y:N phase was also observed. The results are of high importance for understanding the electrochemical oxidation and corrosion resistance of these hard coatings.