Fabrication and Oxidation Resistance of Metallic Ta-Reinforced High-Entropy (Ti,Zr,Hf,Nb,Ta)B2 Ceramics.

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 (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)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.