The high-toughness mechanism of heterogeneous solid solution HfC-TaC-HfO2 composite ceramics

Homogenisation and low-temperature sintering of multicomponent ultra-high temperature ceramics (UHTCs) are crucial technologies for their applications. However, the potential of utilizing the heterogeneous solid solution between UHTCs as a means of toughening has been neglected. The current work proposes a novel inhomogeneous solid solution phase composed of isomorphic HfC and TaC, which is designed to induce additional fracture energy dissipation. This phase is achieved using an ingenious powder screening method combined with the introduction of HfO₂ sintering additive. The hardness and fracture toughness of the composite ceramics reached 14.9 ± 1.3 GPa and 6.5 ± 0.4 MPa m^1/2, respectively. The toughening mechanism was studied using real two-dimensional structure stress simulation and density functional theory (DFT) calculations. Uneven valence electron concentration results in the ductile to brittle transition of Hf_1-xTa_xC. Crack deflection and bridging toughening mechanisms originate from the second phase stress of HfO₂ particles and the heterogeneous matrix. This discovery will provide a noteworthy research direction for the design of high toughness multicomponent UHTCs.