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.