Al-O-N based duplex coating system for improved oxidation resistance of superalloys and NiCrAlY coatings

Performance of an Al-O-N based duplex coating system (DCS) was examined in order to improve the high-temperature oxidation resistance of superalloys and the low-pressure plasma sprayed NiCrAlY coatings. The DCS (∼5 μm thick) was deposited using pulsed DC reactive magnetron sputtering as an external coating on Rene'N5 superalloy with and without NiCrAlY coating. DCS coatings were characterized using photo-stimulated luminescence spectroscopy, optical microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy before and after high-temperature oxidation. As-deposited DCS contained varying amounts of α-, γ-Al₂O₃ and amorphous phases depending on the chemistry and surface characteristics of the substrate. Heat treatment at 1121 °C (2050 °F) for 2 h in air facilitated complete phase transformation to equilibrium α-Al₂O₃ in the DCS. Improvement in oxidation resistance was observed for DCS specimens during 250-h oxidation at 1121 °C (2050 °F). In particular, DCS was found to be very effective as an external coating on the rough (as-sprayed) NiCrAlY surface. The superior oxidation resistance of the specimens with DCS may be attributed to the formation of slow-growing and adherent alumina scale and the prevention of Ni/Cr-rich oxide formation.