High-entropy boride (HEB) ceramics combine ultra-high melting points, superior hardness, and compositional tunability, enabling service in extreme environments; however, difficult densification and limited fracture toughness still constrain their aerospace applications. In this study, metallic Ta was introduced into high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ as both a sintering aid and a toughening phase. Bulk HEB-Ta composites were fabricated by spark plasma sintering to investigate the effect of Ta content on densification behavior, microstructure, mechanical properties, and high-temperature oxidation resistance. The results show that an appropriate amount of Ta markedly promotes densification; at 10 vol% Ta, the open porosity reaches a minimum of 0.15%. Hardness and fracture toughness exhibit an increase-then-decrease trend with Ta content, attaining maxima at 15 vol% Ta (20.79 ± 0.17 GPa and 4.31 ± 0.12 MPa·, respectively). During oxidation at 800–1400 °C, the extent of oxidation increases with temperature, yet the composite with 10 vol% Ta shows the best oxidation resistance. This improvement arises from the formation of a viscous, protective Ta₂O₅-B₂O₃ glassy layer that effectively suppresses oxygen diffusion and enhances high-temperature stability. Overall, incorporating metallic Ta is an effective route to improve the manufacturability and service durability of HEB ceramics, providing a composition guideline and a mechanistic basis for simultaneously enhancing densification, toughness, and oxidation resistance.

ZrB₂-SiC-Zr₂Al₄C₅ multi-phase ceramics with uniform structure and high density were successfully prepared through the introduction of in situ synthesized Zr₂Al₄C₅ into ZrB₂-SiC ceramic via SPS at 1800 °C. A systematic analysis and discussion of the experimental results and proposed mechanisms were carried out to demonstrate the composition-dependent sintering properties, mechanical properties and oxidation behavior. The results showed that the in situ synthesized Zr₂Al₄C₅ could be evenly distributed in the ZrB₂-SiC ceramic matrix and inhibited the growth of ZrB₂ grains, which played a positive role in the sintering densification of the composite ceramics. With increasing Zr₂Al₄C₅ content, the Vickers hardness and Young’s modulus of composite ceramics gradually decreased. The fracture toughness showed a trend that first increased and then decreased, and was increased by about 30% compared with ZrB₂-SiC ceramics. The major phases resulting from the oxidation of samples were ZrO₂, ZrSiO₄, aluminosilicate and SiO₂ glass. With increasing Zr₂Al₄C₅ content, the oxidative weight showed a trend that first increased then decreased; the composite ceramic with 30 vol.% Zr₂Al₄C₅ showed the smallest oxidative weight gain. We believe that the presence of Zr₂Al₄C₅ results in the formation of Al₂O₃ during the oxidation process, subsequently resulting in a lowering of the viscosity of the glassy silica scale, which in turn intensifies the oxidation of the composite ceramics. This would also increase oxygen permeation through the scale, adversely affecting the oxidation resistance of the composites with high Zr₂Al₄C₅ content.