Pressure-driven grain fusion and mechanical properties improvement of high-entropy (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C ceramics

• Highly dense (Ti 0.2 Zr 0.2 Nb 0.2 Hf 0.2 Ta 0.2 )C ceramics were successfully sintered under high pressure to show the advantages of ultra-high pressure. • The high relative density, 98.8 %, of (Ti 0.2 Zr 0.2 Nb 0.2 Hf 0.2 Ta 0.2 )C ceramics result in exceptional high hardness and unprecedented fracture toughness of 27.9 GPa and 8.9 MPa·m 1/2 , respectively. • Highly dense microstructure give (Ti 0.2 Zr 0.2 Nb 0.2 Hf 0.2 Ta 0.2 )C ceramics high oxidation resistance ability. Dense and fine-grained high entropy transition metal carbides are considered as one of the most promising materials with superior hardness, fracture toughness and electrical conductivity. However, the difficulty in preparing high-quality fine-grained samples limits their wide applications. In this work, fully dense and fine-grained (Ti 0.2 Zr 0.2 Nb 0.2 Hf 0.2 Ta 0.2 )C ceramics were prepared by high temperature and high pressure technique. Ultra-high pressure significantly accelerates the densification process and significantly lowers the sintering temperature due to the pressure-induced grain fusion and grain growth suppression effect. The monolith sintered at 1200 ℃ and 15 GPa exhibits a Vickers hardness of 27.9 GPa (9.8 N), and a high fracture toughness of 8.9 MPa·m 1/2 , both of which are the highest values for the reported high-entropy carbide ceramics. Advanced characterization demonstrates that high hardness and toughness are closely related to the high dislocation density, fine grain size, and the high relative density. Additionally, the sintering temperature is significantly reduced by applying pressure, which provides a general route for preparing advanced polycrystalline high-entropy carbide ceramics for more superior mechanical properties.