Improved damage tolerance and oxidation resistance of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂–SiC by introducing chopped carbon fibers

High-entropy diborides (HEBs) are considered as promising high-temperature structure materials owing to their high melting point and excellent thermal stability. However, the intrinsic brittleness is the main obstacle that seriously limits their practical applications. To overcome with this obstacle, carbon fibers (C_f) with outstanding mechanical properties are used in the present work as a first attempt to improve the damage tolerance of HEBs. The as-prepared C_f/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂–SiC composite (C_f/HEB–SiC) shows high relative density (97.9%) and good mechanical properties with flexural strength of 411±3 MPa and fracture toughness of 6.15±0.11 MPa·m^1/2. More importantly, the damage tolerance parameter (D_t) has increased from 0.10 m^1/2 for HEB–SiC to 0.29 m^1/2 for C_f/HEB–SiC. Through microstructural analysis and Vickers indentation of the composite, the toughening mechanisms are disclosed. The carbon fibers coated with carbon coatings demonstrate unique capacity for prolonging the crack propagation path, which promotes the reliability of the composite effectively. Moreover, the C_f/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂–SiC composite also exhibits good static oxidation resistance in the temperature range of 1100–1500 ℃ in air due to the formation of the protective oxide layer constituting of multicomponent oxides (Zr)HfTiO₄ and (Zr)Hf₆Ta₂O₁₇ embedded in a continuous SiO₂ glass. These results are promising, and this primary work can be used as a reference to the synthesis of C_f/HEBs for thermal protection materials under high-temperature serving conditions.