Radial textured SiAlON ceramics were prepared from sinter-forging ring-shaped SiAlON ceramic green bodies at 1750 ℃. The effects of Ni/Co (0–2.25 wt.%) additives on phase composition, texture degree, microstructure and mechanical properties of theh textured SiAlON ceramics were studied. On the section parallel to the grain orientation (RS, Radial Section), the orientation factor increased from 0.38 to 0.74 in the SiAlON ceramics due to the introduction of 2.25 wt.% Ni/Co. The radial texture microstructure was formed after sinter-forging. On the section perpendicular to the grain orientation direction (TS, Tangential Section), the grains exhibited equiaxed microstructure, while on the RS plane, the grains with rod-like microstructure are aligned in the same direction. The hardness of RS plane for the textured SiAlON ceramics is higher than that of TS plane, while the hardness decreases with increasing content of additives. The fracture toughness increases from (6.93±0.31) MPa·m^1/2 to (7.52±0.28) MPa·m^1/2 as the Ni/Co additives content increases from 0 to 2.25 wt.% in the direction perpendicular to the grain orientation of the RS plane.

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.

Low-temperature hot pressed Si₃N₄ ceramics show a greater Vickers hardness and a lower fracture toughness, while high-temperature hot pressed Si₃N₄ ceramics show a lower Vickers hardness and a higher fracture toughness. To prepare Si₃N₄ ceramics with a great Vickers hardness and a high fracture toughness, a low-temperature hot pressed Si₃N₄ ceramic was prepared via sintering at 1500 ℃ with 20% SiC whisker (SiC_w, in volume fraction) and 2.5% ZrB₂. The phase composition, relative density, microstructure and mechanical properties of low-temperature hot pressed Si₃N₄ ceramic were investigated, compared with high-temperature hot pressed Si₃N₄ ceramics at 1800 ℃ . The results show that the addition of SiC_w inhibits the densification of Si₃N₄ ceramics sintered at 1500 ℃ , and the relative density decreases from 97.9% to 92.9%, the Vickers hardness decreases from 20.5 GPa to 16.4 GPa, and the fracture toughness increases from 2.9 MPa·m^1/2 to 3.4 MPa·m^1/2. However, the addition of SiC_w and ZrB₂ promotes the phase transition from α-Si₃N₄ to β-Si₃N₄. The relative density of Si₃N₄–SiC_w–ZrB₂ reaches 97.5%, and the Vickers hardness and fracture toughness are 20.7 GPa and 5.1 MPa·m^1/2, respectively. Compared with low-temperature hot pressed Si₃N₄ ceramic, the fracture toughness of Si₃N₄–SiC_w–ZrB₂ ceramic is improved without reducing the Vickers hardness. Compared with high-temperature hot pressed Si₃N₄ ceramic, the hardness is greatly improved. The Si₃N₄–based ceramic with high hardness and toughness prepared via a low-temperature hot pressing with SiC_w and ZrB₂ can have a promising application in ceramic structural parts.