Transition metal carbides are promising candidates for thermal protection materials due to their high melting points and excellent mechanical properties. However, the relatively high thermal conductivity is still a major obstacle to its application in an ultra-high-temperature insulation system. In this work, the low thermal conductivity of dense (TiZrHfVNbTa)C_x (x = 0.6–1) high-entropy carbides has been realized by adjusting the carbon stoichiometry. The thermal conductivity gradually decreases from 10.6 W·m⁻¹·K⁻¹ at room temperature to 6.4 W·m⁻¹·K⁻¹ with carbon vacancies increasing. Due to enhanced scattering of phonons and electrons by the carbon vacancies, nearly full-dense (97.9%) (TiZrHfVNbTa)C_0.6 possesses low thermal conductivity of 6.4 W·m⁻¹·K⁻¹, thermal diffusivity of 2.3 mm²·s⁻¹, as well as electrical resistivity of 165.5 μΩ·cm. The thermal conductivity of (TiZrHfVNbTa)C_0.6 is lower than that of other quaternary and quinary high-entropy carbide ceramics, even if taking the difference of porosity into account in some cases, which is mainly attributed to compositional complexity and carbon vacancies. This provides a promising route to reduce the thermal conductivity of high-entropy carbides by increasing the number of metallic elements and carbon vacancies.

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot- pressing using different contents of 3Y₂O₃-5Al₂O₃ (molar ratio of 3:5) as the sintering additive. During hot-pressing, the liquid Y₃Al₅O₁₂ (YAG) phase showing good wettability to h-BN grains was in situ formed through the reaction between Y₂O₃ and Al₂O₃, and a coherent relationship between h-BN and YAG was observed with [010]_h-BN// $[\overline{1}11]$_YAG and (002)_h-BN//(321)_YAG. In the YAG liquid phase environment formed during hot-pressing, plate-like h-BN grains were rotated under the uniaxial sintering pressure and preferentially oriented with their basal surfaces perpendicular to the sintering pressure direction, forming textured microstructures with the c-axis of h-BN grains oriented parallel to the sintering pressure direction, which give these composite ceramics anisotropy in their mechanical and thermal properties. The highest texture degree was found in the specimen with 30 wt% YAG, which also possesses the highest anisotropy degree in thermal conductivity. The aggregation of YAG phase was observed in the specimen with 40 wt% YAG, which resulted in the buckling of h-BN plates and significantly reduced the texture degree.

Phase relation and microstructure evolution in the pressure-less sintered TiB₂‒TiC ceramics preceded with mechanical alloying were systematically studied by a combination of SEM analysis. WC debris from milling balls promotes sintering by dissolving into the TiC phase to achieve dense microstructures at 1600 ℃. Variation of W solution in TiC grains exposes two types of core‒rim structures, with no or more W in dark and white cores respectively but with common medium W in both rims. Diminishing white-cores reveal an exchange reaction between WC and TiC via mechanical alloying to form the Ti_1-zW_zC phase prior to sintering. The dark-cores inherit from the as-milled TiC power to further enable the reprecipitation of rims from a mixed liquid-phase, which facilitated also the anisotropic growth of TiB₂ grains. The dark-cores grow persistently in the second-step at 2000 ℃ enabled by this liquid-phase, which coarsens the TiB₂ grains too. With more alloyed phase, sintering was insufficient at 1500 ℃ with only the surface fluidity from the primary powders, and the second-step sintering increased the fluidity in the liquid-phase to fully densify the binary microstructure. Re-distribution of the alloyed W by two-step sintering rationalizes the evolution process of the binary microstructures and leads to better understanding of the mechanical behaviors.