C/N Ratio Effects on Mechanical, Thermodynamic and Electronic Properties of (TiTaZrHf)C1-N High-Entropy Carbonitride Ceramics: A Comprehensive First-Principles Study

High-entropy carbonitride ceramics (HECNCs) have emerged as highly promising ultra-high-temperature materials for extreme environments owing to their exceptional combination of mechanical, thermal, and chemical properties. This work employs first-principles calculations and quasi-harmonic approximation method to systematically investigate the effects of C/N ratio on the structural, mechanical, thermodynamic, and electronic properties of (TiTaZrHf)C 1- x N x ( x =0, 0.25, 0.50, 0.75, 1) HECNCs. Results show that progressive nitrogen substitution reduces the lattice parameter, enhances lattice distortion and structural stability, lowers formation enthalpy, suppresses thermal expansion, increases high-temperature bulk modulus, and markedly improves thermodynamic stability by decreasing Gibbs free energy. Mechanically, higher N content decreases shear/Young’s moduli, hardness, and yield strength while increasing elastic anisotropy and ductility, driving a brittle-to-ductile transition near x =0.75–1. Electronically, nitrogen incorporation shifts the total density of states to lower energies and depletes deep valence states, reflecting stronger ionic–covalent TM–N bonding. These findings demonstrate that tailoring C/N ratio offers an effective strategy to simultaneously optimize mechanical strength, ductility, thermodynamic stability, and ultra-high-temperature performance, thereby providing clear theoretical guidance for the rational design of next-generation advanced high-entropy ceramics.