Effect of microstructure on early oxidation of MCrAlY coatings

Two MCrAlY coatings with the same nominal composition, but different microstructures are deposited onto Hastelloy (R) 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 degrees C is studied. For the LPPS coating with relative coarse grains, the gamma and beta-phase show different oxidation behaviours: the oxide on the gamma-phase shows a multi-layer structure, consisting of outer (Ni,Co) (Cr,Al)(2)O-4, intermediate (Cr,Al)(2)O-3 and inner alpha-Al2O3; the oxidation of the beta-phase, however, is characterised by selective oxidation of aluminium and the oxide is composed of a mixture of theta and alpha-Al2O3. a For the nanocrystalline HVOF coating, it does not show site-specific oxidation and the oxidation is characterised by formation of an exclusive alpha-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 gamma-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. (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.