Effect of microstructure on early oxidation of MCrAlY coatings

Two MCrAlY coatings with the same nominal composition, but different microstructures are deposited onto Hastelloy® X superalloy substrates by low-pressure plasma spraying (LPPS) and high velocity oxygen fuel (HVOF) spraying, respectively. The early oxidation behaviour of the coatings at 1150 °C is studied. For the LPPS coating with relative coarse grains, the γ and β-phase show different oxidation behaviours: the oxide on the γ-phase shows a multi-layer structure, consisting of outer (Ni,Co) (Cr,Al)2O4, intermediate (Cr,Al)2O3 and inner α-Al2O3; the oxidation of the β-phase, however, is characterised by selective oxidation of aluminium and the oxide is composed of a mixture of θ and α-Al2O3. For the nanocrystalline HVOF coating, it does not show site-specific oxidation and the oxidation is characterised by formation of an exclusive α-Al2O3 scale over the surface. The microstructure-dependent oxidation is quantitatively described from the perspective of oxidation kinetics, with contribution from both lattice and grain boundary diffusion taken into consideration. A critical grain size of the γ-phase, below which formation of an exclusive Al2O3 scale can be achieved, is predicted and verified. The study gives new insights into oxidation of MCrAlY coatings.