Carbon nanotube grafted fibres: a route to advanced hierarchical composites

Abstract

The feasibility of reinforcing conventional fibre/polymer composites by grafting carbon nanotubes (CNTs) onto the fibre surfaces has been investigated. Different methods were developed for directly growing CNTs on silica and carbon (C320 and IM7) fibres. Pure and N-doped CNTs with controllable length were grown on silica fibres using an injection chemical vapour deposition (ICVD) method. The diameter and crystallinity of both types of CNTs increased during growth, which proceeded via the base-growth mechanism. However, the nature surface of carbon fibres is not favourable for the ICVD method. As an alternative, CNT-grafted carbon fibres were produced using the incipient wetness technique or electrochemical deposition to pre-load catalyst for subsequent CNT growth. The effects of growth parameters on the morphology, density, and alignment of CNTs were explored. The CNT-grafting process maintained or improved the fibre tensile modulus, but resulted in strength degradations, to different extents, depending on the fibre type and growth parameters. The impact of CNT-grafting on the interfacial shear strength (IFSS) was studied using different micromechanical interface tests, based on different fibre/polymer systems. The IFSS was unchanged in push-out tests, likely due to an unusual internal failure of the fibres. In all other cases, the IFSS was significantly increased. Single fibre pull-out tests on C320 carbon fibre/epoxy composites showed a 60% increase, whilst fragmentation tests on poly(methyl methacrylate) composites yielded improvements of 26% and 80-150% for IM7 carbon fibres and silica fibres, respectively. The improvements can be attributed to the increased surface area, excellent fibre wettability by the matrix and mechanical interlocking of CNTs with the matrix. In addition, a new combined in situ AFM/Raman technique proved to be a useful tool to study CNT distribution and orientation within hierarchical composites.