Diamond-coated carbon fibres, the new generation of composites (CFRP)?

Abstract

Carbon-based composites used in aircraft structural applications (wings, fuselage, etc.) consist of sheets of carbon fibre tapes bonded together by a polymer matrix, usually an epoxy resin. This composite material is called Carbon Fibre Reinforced Polymer (CFRP). In recent decades, the proportion of CFRP composites used in aircraft fuselage and wing components has increased to more than 50%. The superior properties of CFRP compared to traditional materials such as titanium or aluminium alloys provide a technological advantage that motivates its use. However, one of the main problems with this material when used in aircraft structures is its poor properties in the perpendicular direction to the carbon fibres (CF), i.e. CFRP is an anisotropic material. Their electrical and thermal conductivities as well as their mechanical properties perpendicular to the CFs are several orders of magnitude lower than in the longitudinal direction. This poses a safety problem for their use in aircraft structures. The main problem is when CFRP components are struck by lightning in flight, as their anisotropy means that they are unable to dissipate the lightning energy, causing damage to the components. The aim of this doctoral thesis work is to provide a solution to improve the electrical and thermal conductivities in the perpendicular direction to the CFs within the CFRP components. To achieve this, highly doped diamond conductor grains are incorporated between the CF rows. This induces transverse conductive percolation between the CFs. The highly doped diamond is deposited as a polycrystalline coating on the surface of the carbon fibres prior to CFRP manufacture. Two different diamond deposition techniques have been used for this purpose: MPCVD and MW-LA-PECVD. The different steps in the formation process in diamond growth on the surface of CFs by these deposition techniques have been elucidated using electron microscopy techniques (TEM and SEM) and Raman spectroscopy. Moreover, the electrical and thermal characterisation of the highly doped diamond coating and the composite resulting from its combination with the CF was carried out by AFM techniques. This analysis shows a halving of the electrical resistance and an increase in heat transfer. This work is a first step towards the future production of a hybrid fibre reinforcement combining the advantages of carbon fibres (excellent mechanical, electrical and thermal properties in the longitudinal direction of the fibre) with those of highly doped diamond (high electrical conductivity, thermal and mechanical properties around the fibre) for the production of CFRP composites without anisotropic conductive properties.