Microstructures of NiCrAlY Coating Applied on CMSX-4 by Cathodic Arc Deposition

NiCrAlY overlay coatings were applied on Ni-base single-crystal superalloy CMSX-4 substrates by cathodic arc deposition. The coated specimens were subjected to one- and two-step heat treatments performed under 870°C/20h/AC (AC stands for air cooling) and 1140°C/6h/AC+870°C/20h/AC conditions, respectively. The microstructure of the original NiCrAlY cathodes fabricated by spark plasma sintering and the as-deposited as well as heat-treated coatings were characterized using the scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction techniques. It was observed that the cathodes contained multiple phases uniformly distributed across the cathode volume. The as-deposited coatings exhibited a duplex microstructure consisting of splats and inter-splat regions with the latter showing internal porosity and a significantly lower Al concentration compared to that of the cathodes. The post-deposition heat treatments eliminated the porosity but further reduced the Al content, resulting in a coating microstructure dominated by the solid solution γ-Ni phase. The total thickness of the coating including the interdiffusion zone increased significantly during the two-step heat treatment. The oxide scale formed on the coating surface during heat treatment consisted primarily of α-Al2O3, with Y2O3 pegs underpinning the scale to the coating. The oxide scale was continuous on the smooth sections of the coating surface, but became fragmented at the protruding macroparticles present on the coating surface.Copyright © 2014 by Her Majesty the Queen in right of Canada