High-entropy carbide (NbTaTiV)C₄ (HEC4), (MoNbTaTiV)C₅ (HEC5), and (MoNbTaTiV)C₅-SiC (HEC5S) multiphase ceramics were prepared by spark plasma sintering (SPS) at 1900 to 2100 °C, using metal carbide and silicon carbide (SiC) as raw materials. Their microstructure, and mechanical and tribological properties were investigated. The results showed that the (MoNbTaTiV)C₅ synthesized at 1900–2100 °C had a face-centered cubic structure and density higher than 95.6%. The increase in sintering temperature was conducive to the promotion of densification, growth of grains, and diffusion of metal elements. The introduction of SiC helped to promote densification but weakened the strength of the grain boundaries. The average specific wear rates for HEC4 were within an order of magnitude of 10⁻⁵ mm³/N·m, and for HEC5 and HEC5S were within a range of 10⁻⁷ to 10⁻⁶ mm³/N·m. The wear mechanism of HEC4 was abrasion, while that of HEC5 and HEC5S was mainly oxidation wear.

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.