Structural and corrosion behaviour of stoichiometric and substoichiometric TiN thin films

This paper reports the structural and electrochemical behaviour of TiNx thin films prepared by d.c. reactive magnetron sputtering. X-Ray diffraction showed the development of the hexagonal α-Ti phase, with strong [002] orientation, for low nitrogen contents. For nitrogen contents of 20 and 30 at.%, the ε-Ti2 N phase appears with [200] orientation. With further increasing the nitrogen content, the δ-TiN phase becomes dominant. Composition and the resulting changes in microstructure (crystalline phases and the lattice distortion induced by the growth conditions) are the two main parameters that seem to rule coating properties. Results of potentiodynamic polarisation tests showed that all films have a high corrosion resistance reflected by corrosion current densities below 0.7 μA/cm2. Also, electrochemical impedance spectroscopy tests corroborated the results obtained in the polarisation tests, showing that films containing low percentages of nitrogen (less than 8%) reveal the best corrosion resistance. Further increases in nitrogen content lead to a decrease in the corrosion resistance. An exception to this behaviour was found for the film with 30 at.% N. This sample presents an excellent corrosion resistance, which in fact, increases with the immersion time. Higher nitrogen contents (52 and 55 at.%) promote a relative increase in the corrosion resistance when compared with 50 at.% films. This behaviour might be explained by the particular microstructural characteristics of the films.