Oxidation behaviour of nanoscale TiAlN/VN multilayer coatings

Nanoscale multilayer TiAlN/VN coatings were grown using combined cathodic arc/unbalanced magnetron deposition (arc bond sputtering, ABS) in an industrial-sized physical vapour deposition machine. The thermal behaviour of TiAlN/VN coatings in air was studied using thermo-gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy (TEM). Coatings were heat treated in air for 30 min at 550, 600, 638 and 672 °C. Surface oxidation occurred at ⩾550 °C at the coating surface. V2O5 was found to form at ⩾600 °C by XRD. V2O5, TiO2, AlVO4 were identified at 600 °C, with possible Al2TiO5 and VO2 phases. TEM revealed a non-uniform oxide/coating interface, with preferential oxidation along columnar grain boundaries, presumably because these provide a fast track for diffusion. At 672 °C, the entire coating volume was oxidised, and the coating spalled.