Unravelling the impact of CoNiCrAlY coating microstructure and composition on the formation of duplex oxide scales

The formation process of Al2O3-based duplex oxide scales growing on high velocity oxygen fuel (HVOF) CoNiCrAlY alloy was investigated at 1135 oC. The dual-phase nature of the alloy originated a site-specific oxidation behaviour that facilitated the formation of an inhomogeneous duplex oxide scale with widespread embedded mixed oxides. The early-stage formation of these oxides was explained by a modified Gaussian diffusion model where grain boundary proximity is the main variable. The subsequent nucleation/growth of mixed oxides with oxidation time was attributed to continuous cationic diffusion of non-aluminium elements across the scale enabled by the presence of cationic impurities in the scale.