Defect-Driven Brittleness in Hexagonal Boron Nitride (h-BN) Atomic Layers

Monolayer hexagonal boron nitride (h-BN), a two-dimensional wide-bandgap material for electronic devices, can exhibit reduced structural reliability due to defect-induced mechanical degradation. The mechanical properties and failure behavior of defect-driven monolayer single-crystal h-BN are systematically investigated. In specimens without defects large or sharp enough to trigger immediate instability, fracture proceeds through stable crack growth associated with a rising R -curve, allowing stepwise crack extension and about 30% higher energy dissipation than predicted by classical brittle fracture. By contrast, nanopore-containing specimens fail by abrupt catastrophic brittle fracture without experimentally resolvable stable crack propagation, and their fracture strength decreases to 11.2–32.4 GPa. The transition between the two modes is interpreted using stability analysis that compares the crack driving force with the rising resistance curve. A modified Griffith criterion indicates that the characteristic defect size associated with crack-initiation-controlled catastrophic fracture is several nanometers, with a predicted range of 2.6–29.8 nm, consistent with nanopores observed by electron microscopy. Statistical analysis further shows a “narrower is stronger” width dependence of fracture strength. These results establish quantitative correlations among defect scale, fracture mode, and mechanical performance, and provide guidance for defect engineering in h-BN-based devices.