Mechanical properties of pristine and nanocrystalline graphene up to ultra-high temperatures

Abstract

Despite being the basic building block of most carbon materials used in high and ultra-high temperature applications, the mechanical properties of graphene at such temperatures remain totally unknown. Here we compute these properties using tensile molecular dynamics simulations at temperatures ranging from room temperature to 5000 K (i. e. about the melting temperature) for pristine and nanocrystalline graphene. Mechanical simulations at such temperatures are made possible via the use of the screened environment dependent reactive empirical bond order potential [Perriot et al. Phys. Rev. B 88, 064101 (2013)]. We show that even at the highest temperature, both graphene and nanocrystalline graphene retain high stiffness, strength and fracture strain. Analyses of tensile curves and trajectories (including an analysis of in-plane non-affine atomic displacements) show that both pristine and nanocrystalline graphene show brittle rupture, whatever the temperature, even though some early sign of ductility can be detected for the nanocrystalline graphene at 5000 K. This rules out the possibility of a ductile fracture in graphene at any temperature below the melting point. Finally, the simulations also highlight the importance of bond rotation (Stone-Wales) based defects in the fracture initiation mechanism for pristine graphene. Especially at intermediate temperature (3000 K) crack nucleation in pristine graphene is shown to take place exactly at the moment and place of the first Stone-Wales defect formation.