Effect of morphology on the strain recovery of vertically aligned carbon nanotube arrays: An in situ study

We report on the distinctly different mechanical responses of two vertically aligned carbon nanotube (VACNT) films, subjected to large displacement (up to 70 μm) flat punch indentations. The VACNT films were synthesized using the same chemical vapor deposition (CVD) technique but for varying reaction times, which resulted in their different thicknesses (480 and 160 μm, respectively) and morphologies. In situ tests reveal that the shorter, more aligned VACNT film deforms via an instantaneous vertical shearing of the material directly underneath the indenter tip, which is manifested as a rapid displacement burst in the load–displacement response when tested at rates of 100 nm/s and above. The resultant buckles were of a more permanent nature leading to their low recoverability (22–40%). In contrast, we find the thicker, more tortuous VACNT film to show a higher (∼80%) recovery and a more compliant response. These differences in the mechanical response of the VACNTs are discussed in the framework of foam-like deformation with a particular emphasis on their different morphological features, namely density and tortuosity.