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

## TL;DR

Adding metallic tantalum improves the strength and oxidation resistance of high-entropy boride ceramics, making them better for extreme environments.

## Contribution

Metallic Ta is introduced as a sintering aid and toughening phase to enhance densification and oxidation resistance in HEB ceramics.

## Key findings

- 10 vol% Ta minimizes open porosity to 0.15% and improves oxidation resistance.
- 15 vol% Ta maximizes hardness (20.79 GPa) and fracture toughness (4.31 MPa·m^(1/2)).
- Ta2O5-B2O3 glassy layer suppresses oxygen diffusion during high-temperature oxidation.

## Abstract

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.