Effects of interfacial depletion on the degradation of SiAlN coating

Thermally stable 0.4 µm, 0.8 µm, and 1.6 µm thick SiAlN coatings with Mo interlayers have been deposited on Ti substrates to serve as a protective barrier layer for aeroengine applications. After 50 h (10 cycles) of exposure at 800 °C in air, the 0.4 µm, 0.8 µm thick SiAlN coatings were depleted to tens of nanometers in thickness and then formed oxide scales, whereas 1.6 µm thick SiAlN coating retained a 0.5 µm thick remnant SiAlN layer without any observable oxidation. The depletion of the SiAlN coating is induced by purely interfacial diffusion/reactions with the underlying substrate. Once depleted to a few tens of nanometers thick, i.e., close to being fully depleted, the SiAlN coating starts to oxidise, along with an elemental composition change in the remnant SiAlN. The degradation mechanism of the SiAlN coating is determined by its depletion, as opposed to the interfacial reaction induced microstructural change of the remnant coating.