Machine-Learning Potentials Predict Orientation- and Mode-Dependent Fracture in Refractory Diborides

Fracture toughness ($K_{\mathrm{I}\mathrm{c}}$) and fracture strength (${\sigma }_{\mathrm{f}}$) are key criteria in the selection and design of reliable ceramics. However, their experimental characterization remains challenging – especially for ceramic thin films, where size and interfacial effects hinder accurate and reproducible measurements. Here, machine-learning interatomic potentials (MLIPs) trained on $\mathit{\text{ab}}\mathit{\text{initio}}$ datasets of single crystal models deformed up to fracture are used to characterize transgranular cleavage in pre-cracked ceramic diboride TMB${}_2$ (TM = Ti, Zr, Hf) lattices through stress intensity factor ($K$)-controlled loading. Mode-I simulations performed across distinct crack geometries show that fracture is primarily driven by straight crack extension along the original plane. The corresponding macroscale fracture-initiation properties ($K_{\mathrm{I}\mathrm{c}}\approx 1.7$-2.9 MPa$\cdot \sqrt{\text{m}}$, ${\sigma }_{\mathrm{f}}\approx 1.6$-2.4 GPa) are extrapolated using established scaling laws. Considering TiB${}_2$ as a representative system, additional simulations explore loading conditions ranging from pure Mode-I (opening) to Mode-II (sliding). TiB${}_2$ models containing prismatic cracks exhibit their lowest fracture resistance under mixed-mode conditions, where the crack deflects onto pyramidal planes–as confirmed by nanoindentation tests on TiB${}_2$(0001) thin films. This study establishes $K$-controlled, MLIP-based simulations as predictive tools for orientation- and mode-dependent fracture in ceramics. The approach is readily extendable to finite temperatures for evaluating fracture behavior under conditions relevant to refractory applications.