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Dislocation assisted crack healing in h-BN nanosheets

Classical molecular dynamics with a Tersoff potential for hexagonal boron nitride (h-BN) explores how 5|7 dislocations interact with mode-I cracks, reporting substantial toughening (on the order of tens of percent) relative to pristine benchmarks for selected geometries.

Summary

Polycrystalline h-BN sheets contain grain boundaries whose low-angle cores are often described as dislocations—here pentagon–heptagon (5|7) pairs. Kumar and Parashar embed central cracks in large h-BN sheets and vary dislocation–crack arrangements. Quasi-static tensile loading drives mode-I failure; virial stress analysis yields failure stresses and mode-I stress-intensity–related metrics. Relative to reference cases without the beneficial dislocation–crack coupling, the study reports fracture toughness improvements from about 11% to 74%, depending on geometry, and attributes the effect to interaction of dislocation and crack stress fields (shielding, bonding across flanks, crack arrest in favorable configurations).

Methods

1 — MD application (classical fracture MD in LAMMPS). All simulations use Tersoff parameters for h-BN from Albe et al. as cited, in LAMMPS; OVITO is used for post-processing. Monolayer h-BN sheets are ~302 × 302 Ų with centrally embedded mode-I cracks (~25 Å along armchair loading, ~27 Å along zigzag loading after convergence tests on 15–30 Å crack lengths). System size & composition: single-layer h-BN supercells of the above lateral size (~10000 atoms order-of-magnitude for a monolayer sheet of the stated area—exact counts follow from the supercell repeat vectors in the PDF). 5|7 dislocations are embedded in pair configurations consistent with the paper’s Fig. 1 geometries. Relaxation proceeds in two 50 ps segments under NPT (Nose–Hoover thermostat and barostat): first with dislocations present, then after introducing the crack. Quasi-static in-plane uniaxial tension follows, with strain rate ~10⁻³ ps⁻¹ (as stated in the article; the PDF text uses scientific notation). The authors report in-plane boundary conditions to limit size/edge artifacts. Simulation temperature is 50 K to minimize thermal noise in extracted \(K_\text{IC}\) trends; they note insensitivity of \(K_\text{IC}\) to modest thermal distribution choices at this low temperature. Virial stress (their eqn (2)) feeds critical stress intensity estimates (eqn (3)). N/A — integration timestep (fs): not recovered reliably from the extracted PDF text on the pages used for this pass—confirm in the article if a precise Δt is required for reproduction.

2 — Force-field training. N/A — new fit: fixed literature Tersoff parameters (Albe et al.) are used.

3 — Static QM / DFT. N/A — production DFT: the fracture study is classical MD only.

4 — Review / non-simulation framing. N/A: primary PCCP application paper.

Findings

Outcomes and mechanisms. 5|7 dislocations paired with cracks can raise estimated mode-I toughness by ~11%–74% relative to their pristine baselines in the scenarios reported, depending on dislocation–crack geometry. The authors attribute improvements to interaction of dislocation and crack stress fields (compressive shielding near crack edges, lattice trapping of bond-breaking paths, and cross-linking / chain formation across crack flanks in some configurations).

Comparisons. Pristine armchair / zigzag benchmark \(K_\text{IC}\) values from their model are stated to agree with prior published results for h-BN sheets. They also contrast dislocations with literature on Stone–Thrower–Wales (STW) defects being more detrimental to mechanical response in h-BN.

Sensitivity and design levers. Separation parameters between dislocations and crack (\(R_y\), \(R_{xy}\), in-line offsets, etc.—see Fig. 1 families aBN–eBN in the paper) strongly modulate whether toughness enhancement appears, saturates, or disappears; armchair versus zigzag crack alignment changes which bridging motifs dominate.

Limitations and outlook (as authored). The study highlights limited prior work specifically on 5|7 dislocations versus fracture in h-BN and points to further research on other defect types and experimental validation.

Corpus / PDF honesty. Numbers and geometry labels above follow the checked-in PCCP PDF text; if local extracts disagree, prefer the PDF bytes referenced by pdf_path.

Limitations

Classical Tersoff MD omits charge transfer and quantum effects; sheet sizes and loading rates are nanoscale / quasi-static and may not map one-to-one to macroscopic fracture tests. Geometry choices (single dislocation types, periodic supercells) bound generality.

Relevance to group

Illustrates non-ReaxFF classical MD on 2D BN mechanics and fracture, complementary to reactive MD studies on BN and carbon systems in the corpus.

Citations and evidence anchors

Reader notes (navigation)

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