Advanced Coatings on High Temperature Applications

To suppress interdiffusion between the coating and alloy substrate in addition to ensuring slow oxide growth at very high temperatures advanced coatings were developed, and they were classified into four groups, (1) the diffusion barrier coating with a duplex layer structure, an inner σ−(Re-Cr-Ni) phase as a diffusion barrier and outer Ni aluminides as an aluminum reservoir formed on a Ni based superalloy, Hastelloy X, and Nb-based alloy. (2) the up-hill diffusion coating with a duplex layer structure, an inner TiAl2 + L12 and an outer β-NiAl formed on TiAl intermetallic and Ti-based heat resistant alloys by the Ni-plating followed by high Al-activity pack cementation. (3) the chemical barrier coating with a duplex layer structure, an inner* γ + β + Laves three phases mixture as a chemical diffusion barrier and an outer Al-rich γ-TiAl as an Al reservoir formed by the two step Cr / Al pack process. (4) the self-formed coating with the duplex structure, an inner α-Cr layer as a diffusion barrier and an outer β-NiAl as an Al-reservoir on Ni-(2050)at% Cr alloy changed from the δ-Ni2Al3 coating during oxidation at high temperature. The oxidation properties of the coated alloys were investigated at temperatures between 1173 and 1573K in air for up to 1,000 hrs (10,000 hrs for the up-hill diffusion coating). In the diffusion barrier coating the Re-Cr-Ni alloy layer was stable, existing between the Ni-based superalloy (or Hastelloy X) and Ni aluminides containing 1250at%Al when oxidized at 1423K for up to 1800ks. It was found that the Re-Cr-Ni alloy layer acts as a diffusion barrier for both the inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate. In the chemical barrier coating both the TiAl2 outermost and Al-rich γ-TiAl outer layers maintained high Al contents, forming a protective Al2O3 scale, and it seems that the inner, γ, β, Laves three phase mixture layer suppresses mutual diffusion between the alloy substrate and the outer/outermost layers.