A magnetron sputtered microcrystalline β-NiAl coating for SC superalloys. Part I. Characterization and comparison of isothermal oxidation behavior at 1100 °C with a NiCrAlY coating

A microcrystalline β-NiAl coating was prepared on a single-crystal (SC) superalloy substrate via magnetron sputtering and subsequent vacuum annealing. The grain sizes of the coating ranged from about 300 nm to 1 μm. A reference NiCrAlY coating, which was mainly comprised of γ′-Ni3Al and α-Cr, was prepared by means of vacuum arc evaporation (VAE). Isothermal oxidation tests were carried out at 1100 °C in air for 50 h. Both coatings formed thin and adherent α-Al2O3 scales during tests, while the oxide scales on the bare superalloy primarily consisted of spinel (Ni,Co)Al2O4 with underlying α-Al2O3 scale. The parabolic rate constant of the NiAl-coated specimens was about one order of magnitude lower than that of the NiCrAlY coated specimens. After oxidation tests, only a small amount of γ′ phase was detected at some columnar boundaries of the β-NiAl coating, and about 2/3 parts of the NiCrAlY coating transformed into γ phase which resolved the α-Cr precipitations, while an Al-depleted zone in thickness of about 10 μm formed beneath the TGO of the bare superalloy. Inter-diffusion zones and secondary reaction zones were observed on the specimens coated by either β-NiAl or NiCrAlY. The oxidation mechanism and microstructure evolvement of the specimens during high temperature exposures were discussed.