Improved oxidation resistance of TiAlN coatings by doping with Si or B

The improved oxidation resistance of TiAlN-based hard coatings is based on the formation of protective oxide scales. Here we report on further improvement of the oxidation resistance of TiAlN coatings by doping with Si and B. An industrial-scale cathodic arc evaporation facility was used to deposit Ti–Al–Si–N and Ti–Al–B–N coatings, with doping contents in the target of up to 2 at.% for Si and 0.5 at.% for B. X-ray diffraction displayed an increased formation of a hexagonal nitride phase for Si and B doped coatings. Oxidation experiments in ambient air showed a decreased thickness of the oxide scale with increasing content of the doping element, where doping with Si was found to be more beneficial than with B. Energy-dispersive X-ray spectroscopy line scans indicated an oxide scale with an Al-rich top-layer and a Ti-rich sub-layer, while Si was homogeneously distributed in both layers. For both doping elements, a modified oxidation sequence of Ti was found, compared to the undoped TiAlN reference coating. In particular, Raman spectroscopy revealed a significantly higher content of the metastable anatase type TiO2, at the expense of the stable rutile type TiO2, in the formed oxide scale. This is explained by hindered grain growth of the anatase phase followed by a retarded anatase–rutile transformation. Consequently, the time for coarsening of rutile and the accompanied generation of compressive stresses in the Ti-rich sub-layer is reduced. This is responsible for the superior oxidation properties of Si and B doped TiAlN.