Indentation size effects in graphene oxide under suspended nanoindentation

In this study, measurements of the elastic properties and the pre-stress of free-standing multilayers graphene oxide (GO) sheets were performed by suspended nanoindentation tests, using two indenter radius with values of approximately 50 nm and 636 nm, in an atomic force microscope and a triboindenter system respectively. The GO sheets were obtained by a modified Hummers' method and deposited by drop-casting method over a specialized transmission electronic microscopy (TEM) grid. The GO sheets had about 3 layers, an average area of 3 mu m2, and a roughness average, ra, of approximately 3 nm. The results showed that the GO sheets had differences in the magnitude of the mechanical properties, according to the number of layers. However, once the measurements were normalized based on their thickness, the bulk elastic modulus from both types of nanoindentation systems had a different value of approximately 130 GPa. Thus, it can be argued that also the mechanical behavior was influenced by the roughness as well as by magnitude of contact depth according to the tip radius size used. Besides, finite element (FE) simulations of both nanoindentations performed on a representative GO sheet, were carried out to verify the elastic modulus obtained experimentally, and then to approximate the pre-stresses involved in the regarding suspended nanoindentations. It was demonstrated that GO sheet roughness, as the tip radius, were also fundamental factors that played a significant role in the different mechanical properties obtained by the considered experimental and computational procedures.