Slurry aluminizing mechanisms of Ni-based superalloys incorporating an electrosynthesized ceria diffusion barrier

This study investigates the effect of an electrosynthesized ceria interlayer on the growth mechanisms of a full TBC system sintered from a slurry containing spherical Al micro-particles. A potential Selective Diffusion Barrier (SDB) ability of the ceria interlayer appears. It is demonstrated that ceria allows the Al enrichment of the substrate while limiting the Cr upward diffusion and segregation of AlxCry. It is found that the ceria interlayer partially hinders the flow of solid and molten Al into the substrate at 700 °C from the spheres. Simultaneously, the flow of Al among the spheres results in the establishment of bridges and in the loss of sphericity all by maintaining a metallic core. In contrast, emptying of the particles and quick formation of δ-Ni2Al3 rapidly occur in the absence of ceria interlayer. Increasing the annealing temperature to 1100 °C for 2 h brings about the formation of a β-NiAl coating thicker than the one obtained without an interlayer and promotes thicker crusts and a greater sintering of the Al2O3 shell of the particles. This is proposed to result both from delayed Al inward diffusion and enhanced Ni outward diffusion promoted by the heat release accompanying the exothermic formation of the NixAly phases.