Development of an oxidation resistant coating on Nb-based ultra-high temperature alloy

A coating with a duplex layer structure, outer beta-NiAl and inner sigma-Re-Cr-Ni layers, was formed on the third generation Nb-5Mo-15W-16Si-5Hf-5C alloy by using successively Re-pack cementation, electroplating of Re-Ni film, and Cr/Al pack cementation. The duplex layer coating changed during high temperature oxidation to form a coating with a four layer structure: an outermost Ni(2)Al(3), an outer Ni, an inner sigma-Re-Cr-Ni, and an innermost chi-Re-Nb. The Re-pack cementation was carried in an alumina crucible where the specimen was buried in Re metal powder, in vacuum at 1573K to form a Re film, and then a Re-Ni film was electroplated onto the Re-pack treated alloy. The coated alloy formed a protective alpha-Al(2)O(3) scale when oxidized at 1573K in air for 14.4ks. It was concluded that the sigma-Re-Cr-Ni phase can act as a diffusion-barrier against both inward Al difflusion and outward diffusion of alloying elements from the alloy substrate to the beta-NiAl.