A Comprehensive Physical Insight into WC Polymorphs via First-Principles

Abstract

Tungsten carbide (WC) has attracted immense attention for extreme-environment applications owing to its outstanding hardness, high melting point, thermal stability, and chemical inertness. First-principles calculations based on density functional theory (DFT) are performed to investigate the key physical properties of the cubic (α-WC) and hexagonal (β-WC) phases. The optimized lattice parameters show close agreement with reported values. The calculated negative formation energies indicate thermodynamic stability, and the phonon dispersion further reveals dynamical instability in α-WC at 0 K, in contrast to the absence of any imaginary modes in β-WC. They are both metallic polymorphs with no bandgaps (E_g = 0 eV). Both the phases exhibit mechanical stability and elastic anisotropy, with Vickers hardness of 30.96 GPa for the ductile α-WC and 62.86 GPa for the brittle β-WC. The Debye temperature of β-WC (670 K) is higher than that of α-WC (551 K), and the direct free energy of β-WC is more stable. Both the polymorphs exhibit ultrahigh melting temperatures. The β-WC possesses a much higher lattice thermal conductivity of 273 Wm⁻¹K⁻¹ at 300 K compared to 35 Wm⁻¹K⁻¹ for α-WC. These findings demonstrate that WC polymorphs are ideal candidates for ultrahigh-temperature applications, including nuclear, aerospace, and energy technologies.