Development of an Oxidation Resistant Coating on Nb-Based Ultra-High Temperature Alloy
A coating with a duplex layer structure, outer β-NiAl and inner σ-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 Ni2Al3, an outer Ni, an inner σ-Re-Cr-Ni, and an innermost χ-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 α-Al2O3 scale when oxidized at 1573K in air for 14.4ks. It was concluded that the σ-Re-Cr-Ni phase can act as a diffusion-barrier against both inward Al diffusion and outward diffusion of alloying elements from the alloy substrate to the β-NiAl.
Shigeji Taniguchi, Toshio Maruyama, Masayuki Yoshiba, Nobuo Otsuka and Yuuzou Kawahara
K. Saito et al., "Development of an Oxidation Resistant Coating on Nb-Based Ultra-High Temperature Alloy ", Materials Science Forum, Vols. 522-523, pp. 309-316, 2006