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.