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.
