Some microstructural aspects of vapour-grown carbon fibres to disclose their failure mechanisms

The microstructure of vapour-grown carbon fibres has been studied by an SEM examination of the transverse section. In this way the duplex structure, of catalytic and pyrolytic carbon, can be differentiated by the fracture of each phase; pyrolytic carbon shows concentric circles, termed free trunk structure, while a glassy appearance characterizes the pyrolytic phase. It was observed that fracture was strongly influenced by the breaking mode of the fibre, because in tensile failure of a thick fibre, fracture similar to the tree trunk appearance can be formed in the outer layer of the pyrolytic phase. Thus it is necessary to study the transverse microstructure of vapour-grown carbon fibres without any failure process. Using a preparation of fibre samples, in slides as thin as necessary for TEM study, the internal structure was disclosed. The pyrolytic phase was constituted of randomly oriented small crystals, while the tree trunk structure was really formed by very elongated crystals with preferential orientation. Electron diffraction of both phases shows a different degree of texture according to the structures. In addition to crystals, TEM examination showed the existence of hollow cavities, that have a clear influence on the failure mechanism. Owing to these faults, the failure process forms parallel grooves, that constitute the tree trunk appearance.