Multichannel hollow carbon fiber reinforcement in an epoxy resin matrix for direct ink writing of high-performance composites

Abstract

Carbon-fiber reinforced polymers are widely used in additive manufacturing for high-performance composites. However, the aerospace and automotive sectors seek lighter materials compatible with practical processing methods. This study introduces hollow carbon fibers (HCFs) with a honeycomb cross-section as lightweight reinforcements in composites, fabricated via direct ink writing. The printability, mechanical performance, and microstructural features of HCF-based composites were systematically evaluated. Rheological testing showed that HCF-based inks exhibit similar pre-printing properties to conventional, densified carbon fiber (DCF) inks. However, mechanical tests revealed superior strength in traditional DCF composites due to differences in fiber morphology, density, and diameter. Microstructural analysis using small-angle X-ray scattering (SAXS) and optical microscopy indicated comparable fiber alignment, while scanning electron microscopy (SEM) showed complete epoxy infiltration in HCF channels, evidenced by the pullout of cured epoxy strands. While fiber–matrix interlocking was expected to enhance strength, weak bonding within HCF interiors contributed to reduced mechanical strength. Despite lower strength, HCFs offer advantages for applications prioritizing weight reduction, thermal insulation, or fluid permeability, such as lightweight aerospace and automotive components, thermal management systems, and filtration media. The hollow structure also enables integration with functional materials for smart materials and energy storage.