Nanomechanical Characterization of Boron Nitride and Carbon Nanotubes-Metal Interfaces

The light, strong and durable characteristics of nanofiber-reinforced metal-matrix nanocomposites (MMNC) hold promise for tackling some of the most demanding applications, such as the body of aerospace vehicles. The reinforcing mechanism in nanofiber-reinforced MMNC critically relies on adequate load transfer on the nanofiber-metal interface. Boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) are two of the most promising reinforcing fillers for disruptive MMNC technology due to their ultra-strong, resilient, and low-density properties. However, the understanding of the interfacial load transfer on these nanotube-metal interfaces remains elusive, which has been a major scientific obstacle in the development of the nanotube-reinforced MMNC technology. Here we investigate the mechanical strengths of the interfaces formed by individual BNNTs/CNTs with aluminum or titanium matrices by using <i>in situ</i> electron microscopy nanomechanical single-nanotube pull-out techniques. By pulling out individual nanotubes from metal matrices using atomic force microscopic force sensors inside a high-resolution scanning electron microscope, both the pull-out force and the embedded tube length were measured with resolutions of a few nano-Newtons and nanometers, respectively. The load transfer on the nanotube-metal interface is found to follow a shear lag behavior and the interfacial strength is significantly influenced by thermal-induced reaction products. Density functional theory calculations provide insights into the binding interaction and sliding behavior on the nanotube-metal interfaces that are highly dependent on the electronic structure of the underlying chemical bonds. The research findings help to better understand the load transfer on the nanotube-metal interface and the reinforcing mechanism of nanotube-reinforced metal nanocomposites.