Elucidating the thermal shock performance of (Nb,X)Si₂ silicide coatings on Nb-Si based alloy: Oxidation kinetics and SiO₂ scale crystallization behavior

Thermal shock resistance is a critical performance indicator for high-temperature service coatings, directly determining their operational reliability. Nb-Si based alloys, as next-generation ultra-high-temperature alloys, still lack clarity regarding the oxidation kinetics, scale formation, and microstructural evolution of their dedicated silicide coatings under thermal shock. To address this, this study systematically investigated the thermal shock behavior of two types of (Nb,X)Si₂ coatings on the alloy at 1250 °C. The results indicate that both coatings exhibited distinct oxidation behavior under thermal shock compared to isothermal oxidation. The repeated rapid heating and cooling cycles induced the crystallization of the amorphous SiO₂ formed on both coatings, whereas the SiO₂ scales were predominantly amorphous under isothermal condition. For the Si-Al-Y coating, the scale after 100 cycles consisted of a surface TiO₂ layer, a SiO₂ matrix, and discontinuous Cr₂O₃ at the bottom, with a thickness comparable to that after isothermal oxidation. The crystallization transformation led to delamination within the SiO₂ matrix, forming an inner cristabilate-SiO₂ layer and an outer amorphous SiO₂ layer. In contrast, for the Si-Cr-Ti coating, SiO₂ underwent more extensive crystallization under thermal shock, which led to a superior barrier capability against the inward diffusion of oxygen. Consequently, the scale grew significantly slower than under isothermal condition, consisting mainly of a thin SiO₂ matrix embedded with dispersed TiO₂ particles. The difference in the degree of SiO₂ crystallization between the two coatings is mainly attributed to their different Al contents. Specifically, the higher Al content in the Si-Al-Y coating suppressed the crystallization of SiO₂.