Cryomilling was applied to conventional gas-atomized CoNiCrAlY powder to produce powder with nanocrystalline grains. The cryomilled powder and gas-atomized powder were HVOF sprayed onto mild steel sheets to prepare two coatings with fine-grained (~15 nm) and coarse-grained (~one micron) microstructure, respectively. Isothermal oxidation tests in air at 1000°C were conducted for the two coatings for up to 330 h. The morphology and composition of the thermally grown oxides (TGOs) formed on the two coatings were characterized with XRD, SEM, and EDS and compared with each other. After oxidation for up to 24 h, a pure alumina scale formed on the cryomilled coating, while a mixed oxide layer formed on the conventional coating. However, after oxidation for 330 h, non-alumina oxides formed also on the cryomilled coating. These results indicate that, while a fine-grained microstructure can promote the formation of a pure alumina scale after short-term oxidation by increasing the Al diffusion rate toward the surface, it can also accelerate the Al depletion by increasing the Al diffusion rate toward the substrate, which results in the formation of non-alumina oxides after long-term oxidation. The mechanisms governing the oxide formation are discussed in terms of atomic diffusion and thermodynamic stability.

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