Bamboo-inspired fibrous monolithic high-entropy carbide ceramics with enhanced toughness and thermal insulation performance

High-entropy carbides (HECCs) are promising ultra-high temperature ceramics with exceptional properties, but their brittleness limits their practical applications. Inspired by bamboo’s structure, fibrous monolithic high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C-based ceramics (FMCs) with continuous weak cell boundaries were designed and fabricated through a combination of phase inversion and hot-pressing techniques. By optimizing the composition of the cell boundary, FM721 achieves a high fracture toughness of 8.3 ± 1.5 MPa∙m^1/2, a 51.9% improvement over HECC, and a work of fracture of 784.0 ± 190.8 J/m², a 1132.7% increase. The toughening mechanisms include crack deflection, crack branching, and load redistribution at the cell boundary, which increase the crack propagation path, consuming more energy. Meanwhile, the introduction of cell boundaries reduces defect sensitivity and enhances damage tolerance as well, FM721, for instance, maintains 77.8% of its initial flexural strength even after a 294 N indentation. Moreover, the relatively low density of FMCs and the thermal barrier effect at the cell boundaries significantly enhance thermal insulation performance. As the temperature increases from room temperature (25°C) to 1000°C, the thermal conductivity of FM721 decreases by 22.9% and 34.5%, respectively, compared to conventional HECC. This work presents a novel strategy for optimization of both mechanical strength and thermal insulation performance of HECC, providing insights for the design of thermal protection materials in extreme environments.