Effect of High-Entropy Boride Content on Microstructure and Mechanical Properties of Silicon Nitride Ceramics

Silicon nitride (Si₃N₄) ceramics have a wide potential application in industrial processes, but high fracture toughness and hardness are always difficult to be handled, thus affecting the application of Si₃N₄ ceramics. In this paper, silicon nitride ceramics were fabricated via spark plasma sintering at 1600 ℃ with high-entropy boride phase (Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ as sintering aids. The effect of (Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ on the phase assemblage, densification, microstructure and mechanical properties was investigated. Compared with (Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂-free Si₃N₄ ceramic, the addition of only 1.0% (in volume fraction) (Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ in the ceramic can increase the mass fraction of β-Si₃N₄ from 38% to 53% at 1600 ℃ , showing a bimodal microstructure. As a consequence, the fracture toughness of Si₃N₄-based ceramic increases from (5.4±0.3) MPa·m^1/2 to (6.9±0.2) MPa·m^1/2, and the hardness maintains (20.2±0.2) GPa. The relative density of Si₃N₄-based ceramics decreases and the phase transformation enhances with increasing the high-entropy boride fraction to 2.5% and 5.0%, thus leading to a decrease in the hardness.