Comparative study on the initial oxidation behavior of conventional and nanocrystalline MCrAlY coatings - effect of microstructure evolution and dynamic mechanisms

Two MCrAlY coatings with the same nominal composition but different microstructures are prepared by magnetron sputtering (MS) and arc ion plating (AIP). Their oxidation at 1000°C led to very different results. The relatively coarse grained AIP coating formed a duplex-layer scale, consisting of external θ-Al2O3 and internal α-Al2O3, with small amounts of NiCr2O4. Severe surface undulation was accompanied by the development of numerous cracks and voids. Aluminum depletion transformed the AIP coating from β to γ, causing volume shrinkage, promoting scale spallation and leading to premature failure. In contrast, the nanocrystalline MS coating was less degraded due to its columnar crystal structure and high-density of grain boundaries which provided rapid Al diffusion. Oxidation of this coating was characterized by a faster θ-Al2O3 to α-Al2O3 transformation and formation of an exclusive α-Al2O3 scale, protecting the surface.