Synthesis and Sintering of Nonstoichiometric (Mo_0.2Nb_0.2Ta_0.2Ti_0.2W_0.2)C_x Nanoscale Powders for Highly-Dense Ceramics

Guided by thermodynamic calculations, this study successfully synthesized nonstoichiometric high-entropy carbide (Mo_0.2Nb_0.2Ta_0.2Ti_0.2W_0.2)C_x (x = 0.875-0.972) nanometer-sized powders using micrometer-sized metal oxides (MoO₃, Nb₂O₅, Ta₂O₅, TiO₂, and WO₃) and carbon black as raw materials through carbothermic reduction at 1400-1550 °C. The powders synthesized above 1500 °C exhibited a single-phase rock-salt structure with an average grain size as low as 270 nm. TEM analysis confirmed the lattice parameters increased from 0.4378 nm to 0.4395 nm with decreasing carbon content and synthesis temperature. After ball milling, the optimal powder was densified into a (Mo_0.2Nb_0.2Ta_0.2Ti_0.2W_0.2)C_0.9 ceramic block through spark plasma sintering (SPS, 1950 °C/10 min/20 MPa), achieving a relative density of 99.1% and an average grain size of 4.3 μm. This ceramic exhibited remarkable mechanical properties (17.3 GPa Vickers hardness, 25.9 GPa nano-hardness, 524 GPa Young's modulus, and 4.43 MPa·m^1/2 fracture toughness) and a relatively low room-temperature thermal conductivity of 8.3 W·m^-1·K^-1. This study provides a theoretical basis and technical approach for the preparation of high-hardness and low-thermal-conductivity nonstoichiometric high-entropy carbide ceramics via low-temperature carbothermic reduction and sintering.