Hardness-strength-toughness synergy of (NbMoTaW)C through TiC-TiO2 dual-phase engineering

Abstract

High-entropy carbide ceramics (HECs) face critical limitation in engineering applications due to their inherent brittleness and inadequate flexural strength. Notably, there are pronounced trade-offs between hardness and toughness, and between strength and toughness in HECs. To address these challenges, a dual-phase engineering strategy is proposed in this study to synergistically enhance the mechanical properties of HECs, with (NbMoTaW)C as a representative example. Through the combined effects of TiC solid solution strengthening and in-situ TiO₂ toughening, the (NbMoTaW)C demonstrated superior comprehensive mechanical properties, achieving simultaneously high Vickers hardness (20.23±0.26 GPa), high flexural strength (857±23 MPa), and high fracture toughness (4.97±0.16 MPa m^1/2), thus resolving the trade-offs between these properties. The formation and adjustment mechanisms of the oxide phase were elucidated. The integrated multiscale characterizations and density functional theory (DFT) calculations illustrated that the lattice distortion induced by the TiC solid solution, along with d-orbital hybridization in the electronic structure, resulted in a significant enhancement in both structural stability and mechanical properties of HEC. Furthermore, the TiO₂ particles, strategically formed from oxygen impurities and predominantly located at grain boundaries, significantly enhanced the HEC’s flexural strength. At the same time, the TiO₂ particles remarkably improved the toughness of the HEC through crack deflection, bridging, and branching mechanisms. This work has established a synergistic phase optimization strategy that enables overcoming the traditional trade-offs in mechanical properties of HECs.