While the use of low-melting-point metals as sintering aids for high-entropy carbide (HEC) ceramics has been well-established, their existence can compromise hardness due to residual metallic inclusions. This study demonstrates an innovative strategy to meet this challenge, where (Ti, Zr, Nb, Ta, Mo)C high-entropy carbide ceramics with ultrafine grain and enhanced hardness are obtained through Cr-metal-assisted spark plasma sintering at a temperature as low as 1600 ℃. Result shows that the addition of 5 vol.% Cr metal promotes the formation of highly densified single HEC phase ceramics with a high relative density (98.4 %) and an ultra-fine grained microstructure (0.17 μm). This low-temperature densification mechanism can be attributed to Cr's solid-solution effect within the matrix and the increased carbon vacancies generated during sintering. The grain size of the (Ti, Zr, Nb, Ta, Mo)C ceramics with 5 vol.% Cr metal addition is significantly smaller than that of Cr-free (Ti, Zr, Nb, Ta, Mo)C ceramics sintered at 2000 ℃ (3.03 μm) or via traditional low-temperature liquid-phase sintering (1.3-1.5 μm). Importantly, the addition of 5 vol.% Cr metal substantially increases the hardness of ceramics, with a remarkable increase from 23.57 GPa to 28.16 GPa as compared to pure (Ti, Zr, Nb, Ta, Mo)C ceramics, owing to fine-grain strengthening and solid-solution strengthening mechanism. This work highlights the uniqueness of Cr metal as a sintering aid in achieving densification and hardness improvements in (Ti, Zr, Nb, Ta, Mo)C ceramics, offering a promising strategy of property improvement for further development of HEC materials in the near future.

Aiming to achieve silicon nitride (Si₃N₄) ceramics with high hardness and high toughness, the relationships among phase composition, microstructure, and mechanical properties of Si₃N₄ ceramics prepared by spark plasma sintering (SPS) at temperatures ranging from 1500 to 1800 ℃ were investigated in this study. Two stages with different phase and microstructure features were observed and summarized. The α–β phase transformation occurs first, and the development and growth of grains lag behind. During the first stage, the average grain size remains basically unchanged, and the hardness maintains at a value of ~20.18±0.26 GPa, despite the β-Si₃N₄ phase fraction increases from 7.67 to 57.34 wt%. Subsequently, the equiaxed grains transform into rod-like grains with a high aspect ratio via the reprecipitation process, resulting in a significant increase in the fracture toughness from 3.36±0.62 to 7.11±0.15 MPa·m^1/2. In the second stage of sintering process, the fraction of β-Si₃N₄ phase increases to 100.00 wt%, and the grain growth also rapidly occurs. Thus, the fracture toughness increases slightly to 7.61±0.42 MPa·m^1/2, but the hardness reduces to 16.80± 0.20 GPa. The current results demonstrate that the phase contents of β-Si₃N₄ and the microstructure shall be carefully tailored to achieve high-performance Si₃N₄ ceramics. Si₃N₄ ceramics with a fine-grained bimodal microstructure, consisting of the main α- and β-phases, can exhibit the optimized combination of hardness and toughness.

In order to prepare high toughness (Ti,Zr,Nb,Ta,Mo)C ceramics at low temperatures while maintaining high hardness, a liquid-phase sintering process combined with Co-based liquid-phase extrusion strategy was adopted in this study. The densification temperature can be lowered to 1350 ℃, which is much lower than the solid-state sintering temperature (~2000 ℃) generally employed for high-entropy carbide ceramics. When sintered at 1550 ℃ and 30 MPa applied pressure, part of the Co-based liquid-phase was squeezed out of the graphite mold, such that only ~3.21 vol% of Co remained in the high-entropy ceramic. Compared to the Co-free solid-state sintered (Ti,Zr,Nb,Ta,Mo)C ceramics, prepared at 2000 ℃ and 35 MPa, the hardness was slightly decreased from 25.06±0.32 to 24.11±0.75 GPa, but the toughness was increased from 2.25±0.22 to 4.07±0.13 MPa·m^1/2. This work provides a new strategy for low-temperature densification of high-entropy carbides with both high hardness and high toughness.

High-entropy boride-silicon carbide (HEB-SiC) ceramics were fabricated using boride-based powders prepared from borothermal and boro/carbothermal reduction methods. The effects of processing routes (borothermal reduction and boro/carbothermal reduction) on the HEB powders were examined. HEB-SiC ceramics with > 98% theoretical density were prepared by spark plasma sintering at 2000 ℃. It was demonstrated that the addition of SiC led to slight coarsening of the microstructure. The HEB-SiC ceramics prepared from boro/carbothermal reduction powders showed a fine-grained microstructure and higher Vickers’ hardness but lower fracture toughness value as compared with the same composition prepared from borothermal reduction powders. These results indicated that the selection of the powder processing method and the addition of SiC phase could contribute to the optimal preparation of high-entropy boride-based ceramics.