Overcoming the long-term thermal protection bottleneck above 3100 K through a novel hybrid active-passive thermal protection strategy

Ultra-high temperature thermal protection materials play key roles for protecting hypersonic flight, re-entry vehicles, and propulsion systems from thermal attack. However, even the top-level passive protection materials such as C_f/HfB₂-SiC composites are unable to withstand the long-term harsh-environment above 3100 K as they suffer severe ablations. To overcome this bottleneck, herein a synergistic active-passive strategy is proposed. Embed aligned cooling channels were fabricated via microelectrical discharge machining (Micro-EDM) in C_f/HfB₂-SiC composite and liquid water was used as the cooling medium, forming an integrated system combining the composite's passive ablation resistance and the channels' active temperature regulation capability. Benefiting from this novel strategy, the surface temperature was significantly reduced by 46 % (from over 3100 K to below 1700 K). Furthermore, a qualitative leap in ablation resistance was achieved, e.g., the ablation rate was dropped from 5.12 × 10⁻³ mm/s to a non-ablation state after 400 s at such a high temperature, with almost no surface oxidation. These achievements were believed to stem from the synergistic coupling of the “passive barrier” (high melting point, good ablation resistance) and the liquid water's “active reinforcement” (enhanced surface heat transfer). Notably, for 0.4 mm-diameter channels, heat and mass transfer were optimized, blockages and pressure fluctuations were mitigated, ensuring the cooling stability. This work demonstrates that the 3100 K ablation resistance limit of thermal protection materials can be broken, providing a promising route for developing materials suitable for service at temperatures above 3100 K, and lays a solid foundation for upgrading thermal protection in next-generation propulsion systems.