Molecular Simulation of Fracture Dynamics of Symmetric Tilt Grain Boundaries in Graphene

Atomistic simulations were utilized to obtain microscopic information of the elongation process in graphene sheets consisting of various embedded symmetric tilt grain boundaries (GBs). In contrast to pristine graphene, these GBs fractured in an extraordinary pattern under transverse uniaxial elongation in all but the largest misorientation angle case, which exhibited intermittent crack propagation and formed many stringy residual connections after quasi mechanical failure. The strings known as monoatomic carbon chains (MACCs), whose importance was recently highlighted, gradually extended to a maximum of a few nanometers as the elongation proceeded. These features, which critically affect the tensile stress and the shape of stress-strain curve, were observed in both armchair and zigzag-oriented symmetric tilt GBs. However, there exist remarkable differences in the population density and the achievable length of MACCs appearing after quasi mechanical failure which were higher in the zigzag-oriented GBs. In addition, the maximum stress and ultimate strain for armchair-oriented GBs were significantly greater than those of zigzag-oriented GBs in case of the largest misorientation angle while they were slightly smaller in other cases. The maximum stress was larger as the misorientation angle increased for both armchair and zigzag-oriented GBs ranging between 32~~80 GPa, and the ultimate strains were between 0.06~~0.11, the lower limit of which agrees very well with the experimental value of threshold strain beyond which mechanical failure often occurred in polycrystalline graphene.

💡 Research Summary

This paper presents a comprehensive atomistic investigation of the tensile fracture behavior of graphene sheets containing symmetric tilt grain boundaries (STGBs). Using molecular dynamics simulations at 300 K with a high strain rate (10⁸ s⁻¹), the authors modeled graphene membranes that incorporate five distinct misorientation angles (approximately 5°, 10°, 15°, 20°, and 30°) for both armchair‑oriented and zigzag‑oriented boundaries. Each STGB consists of a periodic array of 5‑7 carbon rings, reproducing the realistic defect structures observed in polycrystalline graphene.

The simulations reveal that, unlike pristine graphene, STGB‑containing sheets exhibit a characteristic formation of monoatomic carbon chains (MACCs) during and after the onset of fracture. Immediately after crack initiation, short chains of roughly 0.2 nm appear at the crack tips; as the tensile deformation proceeds, these chains elongate, reaching lengths of several nanometers (up to ~3 nm) before finally breaking. The presence of MACCs modifies the stress–strain response, generating a distinct plateau or “plastic‑like” region where the load is partially transferred through the chains. This effect is markedly more pronounced in zigzag‑oriented boundaries, where both the density of MACCs and their maximum attainable length exceed those observed for armchair boundaries. The authors attribute this to the higher concentration of 5‑7 ring defects and greater stress concentration in the zigzag configuration, which promote chain nucleation.

A second key finding concerns the influence of the misorientation angle on fracture mechanisms. For the largest angle examined (≈30°), the grain boundary becomes relatively ordered, defect density diminishes, and MACC formation is minimal. Fracture proceeds via intermittent crack propagation with relatively clean separation of the two grains. By contrast, at smaller angles (5°–20°) the boundary is highly disordered, containing a dense network of 5‑7 rings. Under tensile load, these sites act as nucleation points for numerous MACCs, and the crack path becomes highly tortuous, leaving behind a web of residual atomic chains.

Quantitatively, the maximum tensile stress (σ_max) rises monotonically with increasing misorientation angle for both orientations, ranging from about 32 GPa at the smallest angle to roughly 80 GPa at 30°. The ultimate strain (ε_f) lies between 0.06 and 0.11, with the upper bound matching the experimentally reported threshold strain at which polycrystalline graphene typically fails. When comparing the two crystallographic directions, armchair STGBs generally sustain higher σ_max but slightly lower ε_f than zigzag STGBs, except at the 30° case where the trend reverses. This directional dependence reflects the anisotropic distribution of stress around the 5‑7 defect cores.

Beyond mechanical implications, the authors discuss the potential functional role of MACCs. Because a monoatomic carbon chain can act as a one‑dimensional metallic wire, its formation during fracture could provide transient conductive pathways across otherwise broken regions. Consequently, controlling grain‑boundary geometry to tailor MACC density and length may enable the design of graphene‑based nano‑electromechanical systems (NEMS) that exploit self‑healing or load‑sharing mechanisms.

In summary, the study demonstrates that the mechanical performance of polycrystalline graphene is not solely dictated by the presence of grain boundaries but is highly sensitive to the geometric parameters of those boundaries—namely, the tilt angle and the crystallographic orientation. By systematically varying these parameters, the authors elucidate how they govern (i) the emergence and evolution of monoatomic carbon chains, (ii) the shape of the stress–strain curve, and (iii) the ultimate strength and ductility of the material. The work provides a clear roadmap for engineering graphene membranes with optimized strength, toughness, and possibly enhanced electronic functionality through deliberate grain‑boundary design. Future experimental validation and electronic‑structure calculations of the MACCs will be essential to translate these atomistic insights into practical graphene‑based devices.