The incorporation of carbon nanotubes (CNTs) into the matrix of conventional fibre reinforced composites as fillers offers the potential for mechanical, electrical and thermal multifunctional enhancement without disrupting the pristine in-plane properties. In this research, the CNTs were distributed within a thermosetting matrix system via an extrusion method which alleviated the processing difficulties associated with conventional liquid processing methods. Hierarchical composites (HCs) based on carbon fibres and the manufactured CNT-filled matrix were then produced using a wet powder impregnation technique followed by hot melt consolidation.
The use of pure or mixed matrix powder provided either a homogeneous distribution of CNTs or an engineered heterogeneity at the length scale of the powder particle size (at ~11 µm). Mode I fracture toughness and interlaminar shear strength was measured with double cantilever beam (DCB) and short beam shear (SBS) tests respectively. Through-thickness electrical and thermal conductivities were also characterised to ensure the CNTs had imbued multifunctionality to the HCs.
Heterogeneous HCs had significantly improved initiation fracture toughness (36%) in comparison to that of the baseline carbon fibre epoxy composite. The fracture toughness was also 29% higher than that of HC manufactured with homogenously distributed CNTs, at similar nanoreinforcement content. The interlaminar shear strength increased with average CNT loading up to 5 wt%E; weight percent to the weight of the epoxy; (1.38 vol%C; volume percent to the volume of the hierarchical composite) but was unaffected by the heterogeneity. The mechanisms for these improvements were investigated through extensive fractography. The through-thickness electrical conductivity exhibited a substantial 357% improvement with the inclusion of 10 wt%E (2.67 vol%C) CNTs. Through-thickness thermal conductivity exhibited 22% improvement regardless of CNTs content. These physical property improvements implied that multifunctionality in mechanical, electrical and thermal characteristics were achieved.