Oxidation behavior of sputter-deposited NiCrAlY coating

The oxidation behavior of the sputtered NiCrAlY coating at 900, 1000 and 1100 °C was investigated. The oxidation kinetics of the coating showed phase transformation from θ-Al2O3 to α-Al2O3. The higher the oxidation temperature, the faster was the transformation from θ-Al2O3 to α-Al2O3. The rapid formation of α-Al2O3 which led to the mass gain of specimen at 1100 °C was the lowest at the initial oxidation stage. Under steady-state condition, the oxidation reaction appeared to be controlled by diffusional transport in α-Al2O3 scale with activation energy of approximately 390 kJ/mol. The scale formed on the coating was alumina exclusively, though there exist different modifications of alumina. This favorable oxidation behavior could be mainly attributed to the high aluminum content, the third element Cr and the nanocrystallized microstructure of the coating. During oxidation, β-NiAl transformed to γ′-Ni3Al and γ phase because of the diffusion and the formation of Al2O3.