Nanoindentation Assessment of the Interphase in Carbon Nanotube-Based Hierarchical Composites

Optimizing the interphase properties in polymer fiber laminates represents a major issue in the development of materials for high structural applications. Carbon nanotube-based hierarchical composites offer a challenging route to produce polymer fiber laminates with enhanced interlaminar properties. The influence of nanotube reinforcement on the interphase properties between the polymer matrix and the microfiber is of major concern and drives the present study. Nanoindentation is used to explore the local mechanical properties of single-walled carbon nanotube (SWCNT)-based multiscale (or hierarchical) composites comprising glass fiber mats alternated with SWCNT-reinforced poly(ether ether ketone). The continuous stiffness measurement (CSM) technique is used to determine elastic modulus and hardness at the fiber, at the matrix, and across the interface, as a function of indentation depth from small penetrations of around 50 nm. It is shown that the polymer fiber interface can be readily detected using CSM. Most importantly, the interphase has been identified after consideration of fiber-bias effects. It is found that the SWCNTs not only reinforce the polymer matrix at the nanoscale but also enhance the interphase properties with the microscale fibers. A mapping of the mechanical properties reveals that nanoindentation outstandingly reproduces the surface topography and precisely locates the interphase properties at the boundary between the fibers and the matrix: Multiscale reinforcement is found to be a convenient route to provide polymer fiber laminates with improved interphases.