Preparation and Properties of Ta Fiber Reinforced High-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-SiC Composite Ceramics

High-entropy boride ceramics are expected to be widely used in aerospace, automotive turbines, and armor protection due to their advantages of high melting point, high hardness, adjustable performance, high-temperature stability, and good oxidation resistance. However, it is urgent to solve the problem of low fracture toughness before application. Therefore, in this paper, a single-phase high-purity (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ powder was prepared by boron/carbon thermal reduction method using a vacuum furnace. The effects of synthesis temperature and C content on the powder were studied. Secondly, HEB ((Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂) powder, SiC powder, and chopped Ta fiber were mixed uniformly, and Ta fiber toughened HEB-SiC composite ceramics were prepared by spark plasma sintering (SPS). The effects of Ta fiber content on the phase composition, microstructure, mechanical properties, and oxidation resistance of the composite ceramics were investigated. The results show that with the increase in synthesis temperature, the HEB powder gradually dissolves, and the solid solution is completely formed at 1700°C. As the C content increases, the oxygen content and particle size of the powder gradually decrease. Single-phase high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ powders with high purity were prepared at 1700°C for 1 h with 6 wt% C content. The addition of C will promote the boron/carbon thermal reduction method, reduce the oxygen content, and inhibit grain growth. With the increase of Ta fiber content, the density of HEB-SiC-Ta_f composite ceramics increased first and then decreased. The hardness gradually decreased, and the fracture toughness gradually increased. When the Ta fiber content was 7 vol%, the fracture toughness was the highest, reaching 5.12 ± 0.39 MPa·m^1/2, which was nearly 45% higher than that of the composite ceramics without Ta fiber. This is because of the synergistic toughening mechanism of metal toughening and fiber toughening, such as crack deflection, crack bridging, fiber debonding, and fiber pullout, which improves the fracture toughness of the composite ceramics. With the increase in oxidation temperature, B₂O₃, SiO₂, Ta₂O₅, and various metal oxides appear on the surface of HEB-SiC-Ta_f composite ceramics. The oxidation depth and weight gain per unit area gradually increase. When the Ta fiber content is 5 vol%, the composite ceramics exhibit the best high temperature stability and oxidation resistance. This is due to the Ta₂O₅ formed by the oxidation of Ta fibers, which dissolves into the B₂O₃ glass phase, increasing viscosity and improving high temperature stability while reducing the oxygen diffusion rate.