Multiscale Fracture in Peeling of Highly Oriented Pyrolytic Graphite

Abstract

Micromechanical cleavage is one of the methods used for isolation of single-and few-layer graphene sheets from bulk graphite. On the surface of peeled graphite flakes, nanosteps of precisely multiple-layer thickness are often observed. The nanosteps are believed to be termination edge of graphene sheets and formed by tearing graphene sheets sandwiched in the mouth of a main cleavage crack during the peeling process. In the present work, we introduce a continuum model to examine the peeling process that involves multiple fractures: the main cleavage fracture at the microscale, delamination of a graphene sheet from bulk graphite at the nanoscale, and tearing fracture of graphene at the atomistic scale. We apply von Karman's plate theory to model the graphene layer, the elastic fracture mechanics for the microscale cleavage crack, and a cohesive zone model for the nanoscale interlayer delamination and for the lattice-scale tearing fracture as well. With a reliable empirical interlayer potential, we could reveal the characteristic length scales involved in the multiscale fracture process. We show that the graphene layer is locally stretched to fracture in mode-I when von Karman's finite deflection effect in a plate is invoked, although the loading by the sandwiching cleavage crack faces is nominally tearing in mode-III.