Nanoscopic observations for evaluating the failure process of aligned multi-walled carbon nanotube/epoxy composites

This study examined the nanoscopic damage progression of aligned multi-walled carbon nanotubes (CNT)/epoxy composites under tensile loading using transmission electron microscopy (TEM). Aligned CNT/epoxy composite films (30 μm thickness) were processed using a forest-drawn aligned CNT sheet and hot-melt prepreg method. Four film specimens, respectively subjected to tensile stress of 0 MPa, 45 MPa, 95 MPa and 110 MPa, were prepared. After tensile loading, each specimen was machined until the thickness became about 100 nm using a focused ion beam milling machine (FIB) for TEM observations. Damage of three kinds, i.e. CNT break derived from the disordered CNT structures around metallic catalyst, sword-in-sheath type CNT break, and several patterns of interfacial debonding, was observed clearly. The broken CNTs and interfacial debonding per unit area were counted from TEM photographs. Results show that broken CNTs and interface debonding increased considerably at 95–110 MPa, which suggests multiple fracture of CNT under tensile loading. The CNT length at the failure stress (110 MPa) was approximately 45 μm. Estimated values from the strength of CNTs resemble those from macroscopic stress–strain behavior.