Oxide formation on two-phase Ni–3Cr–20Al and Ni–10Cr–11Al–8Ti microcrystalline coatings

Due to the metastable state in the as-sputtered Ni–3Cr–20Al and Ni–10Cr–11Al–8Ti nanocrystalline coatings, γ′ phase precipitation will occur during oxidation and then produce a γ and γ′ two-phase microstructure. In this paper, as-sputtered Ni–3Cr–20Al and Ni–10Cr–11Al–8Ti nanocrystalline coatings were initially oxidized at 1000 °C in air for 200 h and 90 min, respectively in order to produce a two-phase microstructure in these two coatings. And then the formed oxide scales were removed. The oxide formation on the Ni–3Cr–20Al and Ni–10Cr–11Al–8Ti coating surface with a γ and γ′ two-phase microstructure has been examined after 200 h oxidation at 1000 °C in air. Compared with the as-sputtered coating, the oxide formation on the Ni–10Cr–11Al–8Ti two-phase coating surface was not altered, both TiO2 dispersions on Al2O3 scales were formed. Whereas the oxide dispersions on Al2O3 scale formed on the Ni–3Cr–20Al coating has changed from NiAl2O4 on the as-sputtered single phase coating to NiO on two-phase coating surface. The formation mechanism is discussed.