In this work, high-entropy composite bulk ceramic of (Hf_0.2Zr_0.2Ti_0.2Nb_0.2Ta_0.2)N/silicon carbide (HEN–SiC) was fabricated via spark plasma sintering (SPS) at 2100 °C with submicron-sized single-phase (Hf_0.2Zr_0.2Ti_0.2Nb_0.2Ta_0.2)N (HEN) powder as a raw material and SiC particles as a sintering aid. The crystal structure, phase composition, and grain size/morphology of the composite were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The relative density and mechanical properties (i.e., bending strength, Vickers hardness, and fracture toughness) of the composite bulk ceramics were analyzed. The results show that the relative density of the composite bulk ceramics with increasing SiC particle content from 0 to 10.0 wt% increases from 93.08%±0.13% to 99.12%±0.12%. The high-entropy composite bulk ceramic with SiC particle content of 10.0 wt% exhibits the optimum mechanical properties (i.e., Vickers hardness of 23.34±0.67 GPa, fracture toughness of 4.35±0.13 MPa·m^1/2, and bending strength of 409±11 MPa) compared with the ceramic without SiC particles (i.e., Vickers hardness of 19.16±0.56 GPa, fracture toughness of 3.78±0.09 MPa·m^1/2, and bending strength of 335±11 MPa). These results indicate that SiC particles can be distributed and fill the HEN grain boundaries in the ceramic matrix, promoting composite densification via pinning grain boundaries and inhibiting grain growth. The enhanced fracture toughness can be due mainly to fine-grain toughening in addition to lamellar/chain structure toughening. The use of SiC as a sintering aid can promote the densification and fracture toughness of high-entropy nitride bulk ceramics, providing a promising approach for the preparation of high-density, high-performance high-entropy nitride bulk ceramics.

High-entropy nitride powders are one of prerequisite materials for the preparation of high-performance high-entropy nitride ceramics. In this paper, high-entropy (HfZrTiNbTa)N powders were synthesized via nitride (i.e., silicon nitride (Si₃N₄)) thermal reduction with soft mechano-chemical assistance. The results show that metal oxides like hafnium dioxide (HfO₂), zirconium dioxide (ZrO₂), titanium dioxide (TiO₂), niobium pentoxide (Nb₂O₅), and tantalum pentoxide (Ta₂O₅) can all be transformed into the corresponding metal nitrides in the presence of Si₃N₄ at 1700 ℃, and solid solution of the metal nitrides can be formed as the temperature increases to 2100 ℃. The high-entropy (HfZrTiNbTa)N powders with submicron-sized particles, a narrower size distribution, and a single face-centered cubic (fcc) structure are obtained from raw material mixtures ground for 10 h and subsequently sintered at 1800 ℃. In addition, the high-entropy bulk nitride ceramics with relative density (R_w) of 94.31%±0.76%, Vickers hardness of 21.00±0.94 GPa, and fracture toughness (K_IC) of 3.18±0.16 MPa·m^1/2 are obtained with submicron-sized powders, which are superior to those obtained with micron-sized powders.

Zirconium nitride (ZrN) ceramics were prepared via hot pressed sintering (HP) at 1750 ℃ in N₂ atmosphere with ZrO₂–Y₂O₃ as sintering additive. X-ray diffraction was applied to analyze the phase composition of the as-prepared ceramics to study the high temperature phase relation in ZrN–ZrO₂–Y₂O₃ ternary system and establish ZrN–ZrO₂–Y₂O₃ ternary phase diagrams. The results show that ZrN and tetragonal ZrO₂ (t-ZrO₂) solid solution, face-centered cubic ZrO₂ (c-ZrO₂) solid solution, body-centered cubic Y₂O₃ (c-Y₂O₃) solid solution coexist in the system of ZrN–ZrO₂–Y₂O₃.