Diffusion barrier coating system concept for high temperature applications

The diffusion barrier coating (DBC) system was designed to suppress the interdiffusion between alloy substrate and coatings in addition to ensuring slow oxide growth at high temperatures. In this report, the concept of the DBC system will be introduced, and the results obtained for various heat resistant alloys are presented. In this DBC system, the sigma-phase of the Re-Cr-Ni system was selected as the barrier and was formed by electroplating Re-Ni and Ni films from aqueous solutions, followed by Cr and/or Al pack cementations. A typical DBC system consists of Ni based superalloy/triple sigma-Re based alloy layer/gamma'-Ni3Al and/or beta-NiAl (with or without Pt addition)/alpha-Al2O3. It was found that the DBC system maintained its structure for long time exposures because the sigma-phase has tielines with the gamma and gamma' alloy substrate and the gamma' and/or beta coating. The sigma-Re based alloy can suppress both the inward Al diffusion and the outward diffusion of alloying elements. The DBC system was successfully applied to various heat resistant alloys such as Ni-Mo and Fe-Cr-Ni based alloys, and these alloys with the DBC system were creep deformed slowly under external applied stresses. A simultaneous diffusion coating process for Al and Hf was developed using Hf+NH4Cl+Al(2)O3 pack cementation. To obtain reliable and long life coatings, the combination of alloy substrate, coating and external scale should be considered as a total system, to optimise oxidation resistance, structural stability and mechanical properties.