Radially Grown Graphene Nanoflakes for Tough and Strong Carbon Fiber Epoxy Composites

A long-standing challenge in structural material design is the simultaneous attainment of high strength and toughness, a conflicting requirement rarely met in engineering materials, with important technological applications in aerospace, defense, automobile, and marine industries. Motivated from examples in biological materials, to address this challenge, we demonstrate that strong and damage-tolerant carbon-fiber-reinforced polymers (CFRPs) can be realized via the direct growth of self-assembled radially aligned graphene nanoflakes (GNFs) on carbon fibers (CFs). Here, we report a first-of-its-kind study on the dependence of strength and toughness on the surface morphology of GNFs in CFRPs. The results indicated that fracture toughness was dependent on the density and waviness of the GNFs, whereas the tensile strength was also affected by the periodicity of the coated carbon fiber layers into the laminated structures. Notably, GNFs with reduced waviness and increased number of layers exhibited enhancement in interlaminar fracture toughness for modes I and II by 93.8% and 43.3%, respectively, whereas GNFs with increased waviness led to a marginal increase or preserved tensile strength. The highly interconnected and wavy nature of GNFs facilitated effective load transfer in both in-plane and out-of-plane directions. Moreover, the out-of-plane through-volume conductivity was remarkably enhanced by 527%. The results of this work demonstrated for the first time the unique potential of GNFs, as an excellent nanoreinforcement and electrically conducting interface, for achieving simultaneously strong, tough, and conducting multifunctional CFRP composites.