Isothermal oxidation of air plasma spray NiCrAlY bond coatings

The isothermal oxidation behaviors of plasma-sprayed NiCrAlY bond coatings (Ni22Cr10Al1Y and Ni31Cr11Al 0.6Y) have been evaluated. Two unique microstructures, oxide stringers and improperly flattened zones, can be observed in the air plasma spray (APS) bond coatings. The structures and chemical compositions of the oxide stringers were examined by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Improperly flattened zones had a high density of open pores. These features affected the oxidation behaviors in both the transient and the steady-state stages. In the transient oxidation stage, NiO, Cr2O3 and α-Al2O3 were observed on the surface simultaneously. The 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, a continuous α-Al2O3 layer reduced the diffusion rate of oxygen and, thus, the rate of isothermal oxidation became low. As the oxidation time was increased, depletions of Al, the formation of NiAl2O4 layers and the formation of Cr2O3 subsequently occurred. Hemispherical protrusions were also detected on the surface after cooling to room temperature.