Study of the structural changes induced by air oxidation in Ti–Si–N hard coatings

3-μm thick Ti–Si–N coatings were deposited on polished X38CrMoV5 substrates by sputtering a composite Ti–Si target in Ar–N2 reactive mixture. Oxidation tests were performed in air at 700 °C during 2 h. Whatever the silicon content in the range 0–4 at.%, no silicon containing compound was detected by XRD before air oxidation and only the TiN phase was evidenced. The mean grain size estimated from the full width at half maximum of the TiN (111) diffraction peak was close to 10 nm. As commonly reported for Ti–Si–N films, the hardness showed a maximum at 51 GPa versus the Si content. After oxidation of the TiN film, XRD and micro-Raman analyses revealed the occurrence of the TiO2 rutile phase in the whole films thickness, indicating the total oxidation of the TiN film. On the other hand, the addition of silicon into the TiN-based coatings induced a strong improvement of the film oxidation resistance. Indeed, the oxide thickness was reduced to nearly 0.4 μm for films containing 1.2 at.% Si. Moreover, the silicon addition gave rise to a change in the structure of the oxide layer. In fact, weak diffraction peaks of the TiO2 anatase phase were detected by XRD. The presence of the anatase phase was clearly shown by micro-Raman spectroscopy, which is a very sensitive method to detect this TiO2 phase. The intensity of the anatase micro-Raman bands increased with the silicon concentration, whereas that of rutile decreased.