JU Jinchi, HE Pengfei, HU Shujun, SUN Chuan, MENG Hong, DUAN Ximing, HU Zhenfeng, LIANG Xiubing
Ultrahigh-temperature boride-silicon carbide-based ceramics are highly promising thermal-protection materials for the thermal components of high-speed aircraft owing to their high melting point, excellent thermal conductivity, and superior ablation resistance. This review systematically summarizes recent studies pertaining to the oxidation and ablation resistance of ultrahigh-temperature boride-silicon carbide coating materials. It focuses on the enhancement mechanisms achieved by incorporating various modification phases, including refractory metal silicides, rare-earth oxides, and other additives, such as borides, carbides, oxides, and metallic / non-metallic elements. The incorporation of SiC as a second phase significantly enhances the oxidation and ablation resistance of ZrB2 and HfB2 ceramics. The formation of a SiO2-rich glassy phase effectively prevents oxygen diffusion. However, in high-temperature environments exceeding 1 800 ℃, SiC undergoes active oxidation. This results in the formation of a porous SiC depletion layer, which severely degrades the oxidation and ablation resistance of the ultrahigh-temperature boride-silicon carbide coating material. To suppress this phenomenon, researchers have introduced various modification phases to improve the oxidation and ablation resistance by regulating the structure of the oxide scale, increasing the viscosity of the glass phase, and inhibiting oxygen diffusion. Refractory metal silicides (e.g., MoSi2, WSi2, TaSi2, ZrSi2, and HfSi2) enhance the oxidation and ablation resistance by providing an additional Si source, thereby resulting in complex glass phases and promoting densification. MoSi2 and WSi2 form volatile MoO3 and WO3 during oxidation, thus reducing the surface temperature; however, their excessive addition increases porosity. The modifying effect of TaSi2 exhibits significant temperature dependence. At an intermediate temperature (approximately 1 600 ℃), TaSi2 effectively inhibits the formation of a SiC depletion layer. The oxidation product Ta2O5 dissolves in the glass phase or reacts with ZrO2 / HfO2, thus resulting in a denser oxide scale. At ultrahigh temperatures (>1 900 ℃), Ta-containing oxidation products have relatively low melting points and tend to form low-melting eutectic phases, thus reducing the stability of the oxide scale. ZrSi2 and HfSi2 enhance the stability of the oxide scale by forming (Zr,Hf)O2 solid solutions, which inhibit oxygen diffusion. Rare-earth compounds (e.g., La2O3, Y2O3, Sm2O3, and Yb2O3) improve the oxidation and ablation resistance of coatings by refining grains, stabilizing ZrO2 / HfO2 structures, increasing the viscosity of the glass phase, and enhancing material emissivity. Rare-earth cations dissolve in the ZrO2 / HfO2 lattice, thus suppressing volume changes during phase transition. Additionally, La2O3 and LaB6 facilitate the formation of a stable La-Si-O glass layer. Y2O3 can react with SiO2 to form silicate nanocrystals, such as Y2Si2O7 or Y2SiO5, which disperse in the glass phase and increase the viscosity. Sm2O3 significantly enhances the emissivity of the coatings, strengthens thermal radiation, and reduces the surface temperature. Yb2O3 effectively increases the viscosity of the glass phase owing to its small ionic radius and high field strength. However, excessive addition of rare-earth elements may weaken the glass-phase structure and increase the oxygen-vacancy concentration, thus weakening the oxidation and ablation resistance of the coatings. Other modification phases, such as borides (TaB and WB), carbides (ZrC, WC, and TaC), oxides (MgO and Al2O3), and metallic / non-metallic elements (W, Si, and C), can enhance oxidation and ablation resistance through different mechanisms. ZrC can occupy the pores by forming ZrO2, which inhibits the formation of the SiC depletion layer. The oxidation of WC forms WO3, which promotes the liquid-phase sintering of ZrO2. Graphene modification significantly improves the thermal conductivity and reduces the surface temperature of the coatings. The introduction of elemental Si directly increases the Si content and suppresses the active oxidation of SiC. Currently, the thermal-protection performance of ultrahigh-temperature boride-silicon carbide coating materials is limited to 2 300 ℃. Future studies should focus on the following aspects: first, the dynamic evolution mechanisms and quantitative evaluation methods for oxide scales should be investigated comprehensively. Second, data-driven approaches can be used to screen for modification phases and predict material performance. Finally, new active-passive synergistic thermal-protection mechanisms must be identified.