Transient oxidation behavior of nanocrystalline CoCrAlY coating at 1 050 °C

Nanocrystalline CoCrAlY overlay coating was prepared on M38G superalloy by magnetron sputtering deposition. To investigate the oxidation behavior and phase transformation of alumina during oxidation, the oxidation experiments were conducted at 1 050 °C for various time in the range of 5–180 min. The phase compositions of the oxide scales were investigated by using glancing angle X-ray diffraction(XRD). The microstructure analysis of oxide scales was carried out by means of scanning electron microscopy(SEM). The growth process of metastable alumina at the grain boundaries and transformation to stable alumina were discussed. The results show that at the initial oxidation stage the mixture of δ-Al2O3, γ-Al2O3 and α-Al2O3 is formed on the sample surface rapidly. Especially, δ-Al2O3 and γ-Al2O3 prefer growing at the grain boundaries of CoCrAlY coating. With increasing oxidation time,δ-Al2O3 and γ-Al2O3 transform to θAl2O3, afterwards θ-Al2O3 transforms to α-Al2O3 gradually. After 180 min oxidation, θ-Al2O3 transforms into α-Al2O3 completely.