Comparison of oxidation resistance of NiCoCrAlTaY-coated and -uncoated Mar-M247 superalloys in the air at 1150 °c

Oxidation resistance of NiCoCrAlTaY coated and uncoated Mar-M247 superalloys were compared experimentally in the air at 1150 °C. It was found that the oxidation behavior of the NiCoCrAlTaY coated and uncoated Mar-M247 superalloys generally followed the parabolic kinetics with the rate of 1.5 and 9.8 × 10 ^-3 mg ²/cm ⁴ h for each one. Microstructural observation and elemental analysis indicated that after 200 h the uncoated alloy covered with an oxide scale of 6-10 times thicker than the coated one, beneath which a fairly continuous and relatively thin α-Al ₂O ₃ could be found along the interface close to the substrate. Comparatively, the NiCoCrAlTaY coated alloy uniformly produced the continuous, dense and thick α-Al ₂O ₃ layer adhesive to the coating to prevent the metal elements against excessive oxidation, and finally formed the steady oxide scales: outer (Ni, Co)(Al, Cr) ₂O ₄ spinels, NiO, (Al, Cr) ₂Ni ₃; inner protective α-Al ₂O ₃ with a little Cr ₂O ₃. On the surface of the uncoated alloy, however, it mainly formed the Ni-rich oxides in which a large quantity of cracks propagated to result in the more oxidation.