Deposition of NiAl coating for improvement of oxidation resistance of cold-rolled Ni3Al foils

Abstract

beta-NiAl, an important intermetallic compound in Ni-Al systems, together with gamma'-Ni3Al, has been used as a coating material for Ni-base superalloys, because of its excellent oxidation resistance. In this study, NiAl-based coating through RF magnetron sputtering was selected for deposition on Ni3Al thin foils which were cold-rolled up to 96% without any intermediate annealing steps. The thickness of the coating on Ni3Al foils is restricted below 10 mum, which is much thinner than general coatings on bulk alloys. Insufficient coating thickness can induce rapid Al depletion in the NiAl coating layer. However, the addition of 3 at.% Ti was very effective in retarding atomic diffusion and Al depletion in the coating layer. The binary Ni-50Al coating did not form a stable alpha-Al2O3 oxide during cyclic oxidation tests at 1000 degreesC. Moreover, rapid Al diffusion between the coating layer and substrate was observed. Ni-47Al-3Ti-0.1Y was then selected as a modified coating composition. The Ni-47Al-3Ti-0.1Y coating showed superior oxidation resistance due to stable alpha-Al2O3 formation and effective retardation of Al diffusion from the coating layer. In addition, Ni-oxide and alpha-Al2O3 coexisted in the bare Ni3Al foil without any coating, and the bare Ni3Al foil exhibited more stable oxidation behavior than the bulk Ni3Al alloy. (C) 2004 Elsevier Ltd. All rights reserved.