Abstract

The isothermal oxidation behaviors of plasma sprayed NiCrAlY bond coatings were evaluated. Two unique microstructures, characterized by oxide stringers and improperly flattened zones can be observed in the APS bond coatings. Structure and chemical composition of the oxide stringers were examined by TEM and EDS. Improperly flattened zones had a high density of open porosities. These features affected the oxidation behaviors at both transient and steady state stages. Under transient oxidation, NiO, Cr2O3, and α-Al2O3 were present together on the surface. Oxide stringers and improperly flattened zones had an especially high density of NiO and Cr2O3, which implied that these regions had inherently lower contents of Al. During steady state oxidation, continuous α-Al2O3 reduced the diffusion rate of oxygen and, in turn, the rate of isothermal oxidation. Over longer oxidation times, the depletion of Al led to the formation of NiAl2O4 and Cr2O3 layers along with hemispherical protrusions.

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