Comparative study on phase formation, microstructure and high-temperature oxidation behavior of Nb-silicide coatings: Role of different substrate alloys

The insufficient oxidation resistance of Nb based alloys is the primary challenge they face in actual service conditions. Due to differences in composition, phase constituents, and microstructure among various types of Nb alloys, the structure and oxidation behavior of their coatings can vary significantly. Therefore, investigating the formation and oxidation resistance of silicide coatings on different substrate is of great significance for developing coatings suitable for various Nb alloys. Thus, this study employed the slurry sintering to prepare Nb-silicide coatings on both Nb521 and Nb-Si based alloys. The microstructure formation of the coatings, and their microstructural evolution and oxidation behavior at high temperatures were investigated. The coating on Nb521 alloy is primarily composed of (Nb,X´)Si₂, Cr₄Nb₂Si₅ and pure NbSi₂, whereas that on Nb-Si based alloy is predominantly (Nb,X)Si₂. At 1250 °C, a tri-layered scale formed on the coating of the Nb-Si based alloy, while a thinner, crystalline SiO₂ scale with dispersed TiO₂ particles was developed on the coating of the Nb521 alloy. The difference in the oxidation behavior of the two coatings primarily stems from the structural differences of the SiO₂. On Nb521, although crystalline SiO₂ hinders oxygen diffusion, its low flowability leads to the depletion of Cr₂O₃ and failure at the edges, whereas on the Nb-Si based alloy, the amorphous SiO₂ exhibits good fluidity, which can not only suppress further oxidation of Cr₂O₃ but also promote crack healing. Due to better healing ability for cracks, the coating on Nb-Si based alloy exhibited superior oxidation resistance at 1250 °C. At 1350 °C, the scale formation process of both coatings was accelerated, but their microstructures did not exhibit significant changes.