Multi-scale intrinsic deformation mechanisms of 3D graphene foam

Abstract

The mechanical properties and multi-scale deformation mechanisms of a freestanding 3D graphene foam are evaluated for the first time. Nanoindentation is used to evaluate the nanoscale properties in compression whereas in situ tensile testing inside scanning electron microscope (SEM) is used to evaluate the tensile properties of the bulk foam. Nano-compression results show that the hardness (19.9–26.1kPa) and elastic modulus (1.2–1.5MPa) of the foam are relatively low. The deformation mechanisms in compression are graphene branch bending and branch wall elastic depression, which do not utilize the exceptionally high in-plane mechanical properties of graphene. The elastic modulus (69.9GPa) during tensile loading is found to be four orders of magnitude higher owing to graphene branch alignment which enables branches to bear load along the high strength in-plane direction of graphene. In situ SEM tensile testing of free standing 3D graphene foam supports the proposed mechanisms and reveals that the ductile graphene branches gradually become aligned by rotating at rates of ∼0.59°/s, while the brittle node junctions become aligned abruptly at rates of ∼3.08°/s. It is observed that due to defects such as cracked branches and discontinuous graphene sheets, only a fraction of graphene branches bear significant loads.