Carbon vacancy induced metallicity and lattice distortion for superior mechanical properties in niobium carbide

Niobium carbide (NbC) has been considered as one of the most significant carbides due to the high hardness and thermal stability, although it usually exhibits low fracture toughness. Thus, to improve the toughness without hardness deterioration is one of the most challenging issues in material science. In present work, non-stoichiometric NbC_x (0.6 ≤ x ≤ 1) with controlled carbon vacancies was synthesized under high pressure (5 GPa) and high temperature (2200 °C) conditions trying to enhance the mechanical properties of NbC_x by refined microstructures. The effects of carbon vacancies on the mechanical properties and superconductivity of NbC_x were systematically investigated. The mechanical properties, including Vickers hardness and fracture toughness obtained from Anstis equation, improved with increasing carbon vacancy concentration, with NbC_0.85 exhibiting optimal hardness values of 23.3 ± 0.3 GPa and fracture toughness of 4.3 ± 0.4 MPa m^1/2, representing ∼10 % and ∼23 % improvements, respectively, compared to stoichiometric NbC. This enhancement was attributed to the reduced covalent bonding and increased metallic bonding, which promoted crack deflection. Furthermore, low-temperature resistance measurements revealed decreased superconducting transition temperature from 11.1 K for NbC to 7.1 K for NbC_0.85 due to the effect of carbon vacancies on the electronic structure. These findings highlight the potential of carbon vacancy engineering to enhance the mechanical performance of NbC_x ceramics, providing a new strategy for designing advanced high-performance materials.