Toughening in 2D Materials

Two dimensional (2D) nanomaterials have been intensively investigated as emerging materials for future devices, including electronics, photonics, and electrochemical energy storage devices. The mechanical stability of each 2D component is critical to the reliability of the fabricated devices. Currently, research on experimental mechanics of 2D materials has been focused on quantifying mechanical properties and understanding fracture behaviours using different techniques. Confined to 2D geometry, cracks in 2D materials generally favour a brittle behaviour with minimum plasticity at room temperature, which continues the dilemma of mutually exclusive fracture toughness and mechanical strength in bulk materials.<br/>Considerable research has been devoted to improving fracture toughness of 2D materials. For example, carbon nanotubes (CNTs) were integrated into graphene as an extrinsic toughening strategy. The fabricated rebar graphene displays a zigzag fracture surface, guided and redirected by the embedded CNTs. Such toughening mechanism is similar to improving fracture resistance extrinsically by introducing fiber/lamella bridging, oxide wedging, transformation toughening, etc. In addition to rebar graphene, <i>h</i>-BN has been carefully investigated as it has the same structure of graphene but is composed of two elements. <i>h</i>-BN retains the high strength (~100 GPa) and elastic modulus (~0.8 TPa). The fracture behaviour of monolayer single crystalline <i>h</i>-BN has long been taken as an ideal brittle material subject to Griffith’s law. By combining computational analysis and <i>in situ</i> tensile test, the monolayer <i>h</i>-BN has an exceptionally high fracture toughness. The effective energy release rate is up to one order of magnitude higher than its Griffith energy release rate and that of graphene, yet their strength is very close. Stable crack propagation in monolayer <i>h</i>-BN is observed, and the corresponding crack resistance curve is obtained. The crack deflection and branching occur repeatedly due to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which toughens <i>h</i>-BN tremendously and enables stable crack propagation not seen in graphene. The novel lattice asymmetry induced intrinsic toughening mechanism discovered at the 2D limit shed important light into toughening of heterogeneous 2D materials.