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

Abstract

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-Al 2O 3 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-Al 2O 3 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.