Unveiling the role of Ta in Cr-Ta-Ti modified silicide coatings for Nb alloy with superior oxidation resistance up to 1500 ℃
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To address the insufficient high temperature oxidation resistance of Nb521 alloy in engineering applications, Cr-Ta-Ti modified silicide coatings with different Ta contents were fabricated on its surface via slurry sintering. The oxidation behavior and elemental diffusion characteristics of the coatings were systematically investigated at 1400–1500 ℃, and the mechanism by which Ta affects the microstructural evolution and oxidation resistance of the coatings was revealed. The results show that the Cr-Ta-Ti modified silicide coating exhibits a distinct multilayer structure, which from top to bottom consists of a (Nb,Cr,Ti,Ta)Si 2 /Cr 4 Nb 2 Si 5 composite layer, a dense NbSi 2 layer, a NbSi 2 /Nb 5 Si 3 layer, and a Nb 5 Si 3 transition layer. The introduction of Ta influences the diffusion of Si, Cr, and other elements, resulting in a reduced coating thickness. An appropriate Ta content (5Ta) can markedly decrease the growth rate of the oxide scale, providing effective protection for the substrate for at least 150 h at 1400 ℃ and 100 h at 1500 ℃, respectively. The dissolution of Ta into SiO 2 and TiO 2 enhances the high temperature viscosity and thermal stability of the oxide scale, restrains the inward diffusion of oxygen and the outward diffusion of coating elements, thereby significantly improving the high temperature oxidation resistance of the coating.
