Microstructure evolution of vacuum plasma sprayed CoNiCrAlY coatings after heat treatment and isothermal oxidation

The microstructure of a CoNiCrAlY coating vacuum plasma sprayed onto an aluminized MARM002 Ni-base superalloy was investigated in the as-deposited condition, after heat treatment to promote adhesion, and after oxidation in air for 100 h at 1100 °C. The as-sprayed coating comprised three main phases: the solid solution γ-(Ni, Co), the intermetallic compound β-(Ni, Co)Al and the intermetallic compound γ′-(Ni, Co)3Al that grew as small precipitates inside γ grains. γ′ phase was present even in the as-sprayed condition because of Al diffusion into the coating from the aluminized substrate during manufacture at temperatures of 800–1000 °C. The microstructure was fine-scale and complex with β phase within γ grains, at γ grain boundaries and large β grains surrounded by γ matrix. After heat treatment, the microstructure comprised β grains typically of 3-μm diameter randomly distributed throughout the γ matrix. There was little or no Al depletion of the coating because of Al diffusion from the substrate aluminized layer. Diffusion of C from the substrate induced the formation of Cr23C6 and M5Y (where M was predominantly Ni and Co) in the as-sprayed coating; with M5Y transforming to Y2O3 during heat treatment due to internal oxidation. After oxidation, there was a loss of Al despite the aluminized layer, because of the formation of a thermally grown oxide (TGO) layer on the upper surface of the coating, mainly comprising Al2O3 with some Cr2O3. There was a corresponding reduction in β phase fraction and a substantial reduction in the substrate aluminized layer thickness. Some β grains persisted close to the TGO at grain boundaries, together with Y2O3 and Cr23C6 particles.