Effects of Diffusion on Aluminum Depletion and Degradation of NiAl Coatings

Abstract

Experiments were performed to critically demonstrate the effects of diffusion on the aluminum depletion and degradation of NiAl coatings on superalloys. Pack aluminized IN 100 and Mar‐M200 were diffusion annealed in 5 × 10 − 3 Torr vacuum at 1100°C for 300 hr. Aluminum losses due to oxidation and vaporization were minimal. Metallographic and electron microprobe analyses showed considerable interdiffusion of the coating with the substrate, which caused a large decrease in the original aluminum level of the coating. Subsequent cyclic furnace oxidation tests were performed at 1100°C using 1 hr cycles on prediffused and as‐coated specimens. The prediffusion treatment decreased the oxidation protection for both alloys, but more dramatically for IN 100. Identical oxidation tests of bulk NiAl , where such diffusion effects are precluded, showed no signs of degradation at twice the time needed to degrade the coated superalloys. These results, plus limited tests showing the reduced oxidation resistance of aluminum‐poor NiAl , suggest a degradation model whereby the coating is first depleted of aluminum by diffusion, as opposed to Al 2 O 3 spalling, then rapidly degraded by the formation of spallprone oxides.