Carbon Stoichiometry Effects on the Structure, Mechanical Properties, and Ablation Resistance of (HfZrTiTaNb)Cx , (HfZrTiTaV)Cx , and (HfZrTiMoV)Cx High-Entropy Carbides

This study investigates the influence of carbon stoichiometry on the structure and properties of high-entropy carbides (HECs) (HfZrTiTaNb)C _x , (HfZrTiTaV)C _x , and (HfZrTiMoV)C _x . Bulk samples with carbon content ranging from 30 to 65 at.% were synthesized via mechanical alloying and spark plasma sintering (SPS). X-ray diffraction analysis revealed a monotonic increase in the FCC lattice parameter with increasing carbon content until a plateau was reached, corresponding to the formation of a limiting stoichiometric phase. Experimentally, carbon concentrations of 40 at.% were found to provide maximum microhardness (28–32 GPa) and compressive strength (2500–2800 MPa), attributed to the minimization of vacancies in the carbon sublattice. Gas-dynamic testing in a high-enthalpy oxidative flow showed that optimal ablation resistance is achieved at 45–50 at.% C. This is associated with a compromise between the chemical stability of the carbide phase and enhanced radiative properties due to dispersed free carbon. Based on the optimized (HfZrTiTaNb)C _x composition with 40 at.% C, a technology for depositing a single-phase carbide barrier coating (620 µm) on a carbon-carbon composite was developed. The coating demonstrated high microhardness (20.3 GPa) and thermochemical stability; however, its service life is limited by detachment due to the mismatch of coefficients of thermal expansion (CTE) with the substrate. The obtained results establish a quantitative relationship between carbon stoichiometry, lattice parameter, mechanical, and thermal properties of high-entropy carbides, which is critically important for their application as ultra-high-temperature ceramics and protective coatings.