Strain Shielding from Mechanically Activated Covalent Bond Formation during Nanoindentation of Graphene Delays the Onset of Failure

摘要: Mechanical failure of an ideal crystal is dictated either by elastic instability or a soft-mode instability. Previous interpretations nanoindentation experiments on suspended graphene sheets,1,2 however, indicate anomaly: the inferred strain in sheet directly beneath diamond indenter at measured load anomalously large compared to fracture strains predicted both and acoustic analyses. Through multiscale modeling combining results continuum, atomistic, quantum calculations, analysis experiments, we identify strain-shielding effect initiated mechanochemical interactions graphene-indenter interface as operative mechanism responsible for this anomaly. Transmission electron micrographs molecular model indenter's tip suggest that surface contains facets comprising crystallographic {111} {100} planes. Ab initio dynamics (MD) simulations confirm covalent bond (weld) formation between can be induced compressive contact stresses order achieved tests. Finite element (FEA) MD reveal shear stiction provided bonding restricts relative slip its with indenter, thus initiating local effect. As result, subsequent stress-induced interface, spatial variation continuing incremental substantially redistributed, locally shielding region limiting buildup while imparting deformation surrounding regions. The extent governed strength stiction, which depends upon level hydrogen saturation surface. We show intermediate levels enable support experimentally determined loads displacements without prematurely reaching states stress deformation.