| The isothermal oxidation behaviors of plasma sprayed NiCrAlY bond coatings (Ni22Cr10Al1Y and Ni31Cr11Al 0.6Y) were evaluated. Two unique microstructures such as oxide stringers and improperly flattened zones can be observed in the atmospheric plasma sprayed bond coatings. Structure and chemical composition of oxide stringer 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. At transient`oxidation stage, NiO, Cr_2O_3 and α-Al_2O_3 were observed on the surface simultaneously. Oxide stringers and improperly flattened zones had an especially high density of NiO and Cr_2O_3, which implied that these regions had inherently lower contents of Al. During steady-state oxidation, continuous α-Al_2O_3 reduced the diffusion rate of oxygen and thus rate of isothermal oxidation became slow. As the oxidation times increased, depletion of Al, the formation of NiAl_2O_4 layer and the formation of Cr_2O_3 occurred subsequently. And the hemispherical protrusions were also detected on the surface at room temperature. These protrusions were examined carefully. |
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