Isothermal and Cyclic Oxidation Behavior of HVOF-Sprayed NiCoCrAlY Coatings: Comparative Investigations on the Conventional and Nanostructured Coatings

Abstract

Conventional and nanostructured NiCoCrAlY coatings were deposited using the high-velocity oxy-fuel thermal spraying technique. The nanostructured NiCoCrAlY powder feedstock for the coatings was produced by the ball-milling method. The microstructures of the as-received and nanostructured powders as well as their developed coatings were investigated by x-ray diffraction, a field-emission scanning electron microscope equipped with energy-dispersive x-ray spectroscopy, and a transmission electron microscope. Williamson–Hall measurements were also carried out to estimate the crystalline size of the powders and coatings. For the evaluation of the oxidation kinetics, the free-standing coating specimens were subjected to short- and long-term isothermal and cyclic oxidation at 1000 and 1100 °C, respectively, under a laboratory air atmosphere. The results indicated that the as-received NiCoCrAlY coating had parabolic oxidation behavior in short- and long-term exposure tests. For the nanostructured NiCoCrAlY coating, in contrast, the long-term oxidation kinetics deviated from parabolic behavior and showed instead sub-parabolic rate behavior. The obtained results also revealed that the nanostructured NiCoCrAlY coating had greater oxidation resistance to both isothermal and cyclic conditions on account of the formation of a dense and slow-growing Al2O3 layer on the coating surface.