Enhancing the mechanical properties and oxidation resistance of high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C0.95 ceramics through nitrogen doping

Abstract

High-entropy carbide ceramics (HECCs) exhibit superior properties compared to their constituent binary compounds. However, high synthesis and sintering temperature are main obstacles that limit their widespread applications. To address this issue, compositional and particle size controllable high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_x powders were successfully prepared by a sugar hydrogel combined with the carbothermal reduction method. Owing to the introduction of carbon vacancy, the temperature for the formation of single-phase solid solution of the high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_x powders was decreased, and the addition of nitrogen decreased the densification temperature of the high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_0.95 ceramic by 200°C. In addition, the flexural strength and fracture toughness of the high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_0.95 ceramic were improved by 29% and 30%, respectively, compared with those without nitrogen doping. Atomic-resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive spectroscopy (EDS) mapping reveal that the segregation of N and small cation Ti as well large lattice strains are responsible for the enhanced mechanical properties. Furthermore, with the introduction of nitrogen, the onset oxidation temperature (OOT) was increased, while the parabolic oxidation rate constant was decreased, revealing the beneficial effect of nitrogen doping on oxidation resistance. These results demonstrate that nitrogen doping can not only improve the mechanical properties of HECCs but also enhance the oxidation resistance, which paves the way for the wide application of HECCs.