Anisotropic mechanical properties and strengthening mechanism in superaligned carbon nanotubes-reinforced aluminum

High-strength carbon nanotubes (CNTs) enhance the mechanical properties in metal matrix composites; however, their extremely high aspect ratio leads to the anisotropy of mechanical properties. This underlying issue has not yet been clarified owing to the complicated multiple strengthening mechanisms. Herein, we report the anisotropic mechanical properties of a CNT-reinforced aluminum composite and strengthening mechanisms. The uniaxial alignment of CNTs and control of alignment angles were achieved via a mechanical pulling method using a vertically grown CNT forest. As a result, the modulus and strengths decreased in proportion to the misorientation angle. Owing to the superaligned CNTs, the experimental tensile strength in the iso-strain state of the Al-0.15 vol% CNT composite (improved by 20.1%) was near the theoretical value (21.8%), and the strengthening efficiency of the composite was ∼1000. On the other hand, there was a significant deviation between the experimental result and theoretical value in the iso-stress state of the composites. This unusual anisotropic tendency was demonstrated by the strengthening effect of the CNT bridges, which tied the aligned CNTs together, in line with the interconnecting model. The anisotropic mechanical properties corroborate well with our predicted model from calculation by the failure criterion theory with the interconnecting model.