The Treatment Conditions of Some Diffusion Coating and the Structure of Coating Layers on Ni-Base Superalloys

Abstract

For the purpose of improving the corrosion resistance of superalloys for gas turbine, the single coating of Al and Cr, and the composite coating of Cr-Al, Cr-Si and Al-Si by diffusion coating method were studied on Ni-base superalloy. In this paper, the coating conditions and the structure of coating layers are reported. The formation rate of Al coating layers was measured as a function of temperature, time and powder composition to find out the optimum coating condition. For the Cr coating, the uniform coating layers were obtained by using the Fe-free Cr powder, rather than Fe-containg powder. To achive the more improved corrosion resistance, the composite coating of each two elements of Cr, Al and Si were tried by means of diffusion coating and the combination of diffusion coating and ion plating. The most uniform and adherent coating layer was obtained by the combination of ion plating of Si and the subsequent diffusion coating of Cr in this study. From the results of EPMA and X-ray diffraction studies, it was found that the Cr rich, coating layer cosists of α-Cr phase and the Al rich coating layer is of NiAl intermetallic phase, each layer containing the additional elements from the alloy substrate and coating materials, respectively.