Design of thermal and environmental barrier coatings for Nb-based alloys for high-temperature operation

The design of thermal and environmental barrier coatings (T/EBCs) applied to Nb-based alloys for high-temperature operation (>1200 °C) was studied. Different multilayer T/EBCs consisting of rare earth aluminates, silicates, and zirconates (with Y₃Al₅O₁₂ (YAG), Y₂SiO₅ (YMS), and Gd₂Zr₂O₇ (GZO) as exemplars) are proposed. Mechanics models were used to determine the magnitude of thermal stresses developed and the tendency for delamination or penetration cracking. The calculated energy release rates (ERRs) for crack formation show that the T/EBC composed of a porous YAG layer (porosity > 0.15) and a dense YAG + YMS layer is effective in preventing the delamination and can potentially arrest cracks penetrating from the surface before reaching the substrate. Phase equilibria calculations indicate that all combinations are stable to above their expected use temperatures. The greatest likelihood for melting is the scenario where a YAG + YMS solid mixture contains excess SiO₂ due to process variability, but even in this case the melting temperature, 1430 °C, exceeds the corresponding temperature in the prescribed temperature profile.