Effect of Nickel Aluminide Microcrystalline Coating on High-Temperature Oxidation Behavior of NiAl-33.5%Cr-0.5%Zr Alloy

The effect of nickel aluminide (NiAl) microcrystalline coating prepared by magnetron-sputtering on the high-temperature oxidation behavior of the NiAl-33.5Cr-0.5Zr (at%) alloy was investigated in static air from 1,000°C to 1,150°C. The additions of Cr and Zr changed the single β phase structure into the multiphase structure (β-NiAl, α-Cr, and Heusler [Ni₂AlZr] phase). The NiAl-33.5Cr-0.5Zr alloy formed a nonprotective Al₂O₃ + Cr₂O₃ + ZrO₂ (aluminum oxide + chromium oxide + zirconium dioxide) mixed scale and exhibited relatively large weight gains. The large weight gains were attributed to extensive internal oxidation. The sputtered NiAl microcrystalline coating remarkably improved the oxidation resistance compared to the NiAl-33.5Cr-0.5Zr alloy due to the formation of a continuous Al₂O₃ scale from 1,000°C to 1,150°C. The metastable θ-Al₂O₃ → α-Al₂O₃ transformation was identified by x-ray diffraction (XRD) results and a change in the morphology of scales also occurred with the transformation.