Direct measurement of grafting strength between an individual carbon nanotube and a carbon fiber

Hierarchical structures consisting of carbon nanotubes (CNTs) grafted onto a carbon fiber (CF) have the potential to improve the performance of fiber/polymer composites. The strength between a CNT and a CF is a key factor that influences the load-transfer behavior and inter-laminar properties. Here, we directly measured the grafting strength of a chemically bonded CNT–CF hierarchical structure by detaching individual CNT from the CF substrate and simultaneously recording the force–displacement characteristics in a scanning electron microscopy equipped with a nano-manipulator. We observed a relatively wide distribution of the maximum forces at complete detachment for different grafted CNTs, which ranges from below the van der Waals (vdW) force existing at the CNT–CF interface up to 7 times higher than that. For a typical configuration where a CNT is partially anchored on a CF, we obtained grafting strengths in the range of 5–90 MPa, which are dominated by the vdW force as well as other factors such as chemical bonding. Our results, based on the measurements at individual nanostructure level, might be useful for designing and fabrication of high performance hierarchical composites.