Strong Hierarchical Reinforcement Effect of Fe3O4-Terminated Nanostructures of Finely Rebuilt Carbon Nanotubes at Short Carbon Fiber Surface on Epoxy Composites

In this study, a hierarchical reinforcement with multiple structures is innovated to improve the mechanical properties of short carbon fiber-filled epoxy resin composites (SCFs-EP) through the carbon fiber surface modification by Fe3O4 nanocluster-anchored unzipped carbon nanotubes (CNTs). The results demonstrate that the Fe3O4–unzipped CNTs (Fe3O4–UCNTs)-assisted SCFs-EP exhibit enhanced mechanical properties, tensile strength, and flexural strength up to 200 and 267 MPa, respectively, which are 300 and 231% higher than that of the pure epoxy matrix. The dominant fracture mechanism of the optimized SCFs-EP changes from adhesive failure to cohesive failure after the incorporation of Fe3O4–UCNTs composites. The enhancement effect is mainly attributed to the Fe3O4-terminated nanostructures of finely rebuilt CNTs, which offer sufficient chemical bonding and admirable pinning effect at the interface. Additionally, a large amount of Fe3O4–UCNTs hybrids endow the interface with a favorable modulus transition, decreasing the stress concentration. This research provides a promising strategy to realize substantial reinforcement of short carbon fiber-based composites by optimizing interfacial interactions.