Mechanisms and criteria for failure in polycrystalline graphene

Abstract

Graphene is a two-dimensional material that consists of a single layer of carbon atoms covalently bonded in a hexagonal lattice that has incredible mechanical, electrical, and thermal properties. Nanoindentation experiments on freely-suspended circular membranes of mechanically exfoliated single crystal graphene have demonstrated it as the strongest material ever characterized. Chemical Vapor Deposition (CVD) techniques have offered an industrially scalable method to synthesize large area continuous polycrystalline graphene films. Subsequent nanoindentation experiments reveal the presence of grain boundaries only slightly diminishes its strength. Herein, we investigate the probability of failure of grain boundaries in graphene through the Finite Element Method (FEM) within the context of the nanoindentation experiment of a two-grain graphene domain with a single straight grain boundary defined at varying distances from the indentation point. We introduce a novel formulation for a Cohesive Zone Model (CZM) within membrane elements to admit fracture within the grain boundary that accounts for the nonplanar kinematics of membranes. We examine the transition in failure mechanisms from one of rupture within the grain boundary to one of structural instability within the grain. Our analysis reveals three distinct failure regions that provide insight into the factors that influence the probability of failure, such as the indenter tip curvature and the grain boundary properties.