Mechanical properties of vapour-grown carbon fibres

Abstract

Vapour-grown carbon fibres are formed by depositing layers of carbon from hydrocarbon pyrolysis on precursors of filamentous carbon. They are partially graphitic with the basal planes preferentially oriented as nested coaxial cylinders. As the fibres are axially strained, their stiffness increases by about 28%, probably due to strain-induced increase in the preferred orientation. Vapour-grown fibres 7.5 mu m in diameter have a tensile strength of 2.92 GPa (0.42*106 PSI) and a modulus of 237 GPa (34*106 PSI). Both the tensile strength and the modulus shows an apparent dependence on fibre diameter. Larger-diameter fibres have a lower modulus and tensile strength than smaller-diameter fibres. This apparent dependence of fibre mechanical properties on diameter is shown to be due to nonuniform carbon deposition rates along the length of the fibre growth reactor. In a given growth experiment, a range of fibre diameters are produced in a fixed period with the larger-diameter fibres deposited at a higher rate. X-ray studies of fibres having a range of diameters and hence deposition rates show that the graphitic ordering of small-diameter fibres (low deposition rates) is more complete than that of large-diameter fibres, thus accounting for the larger modulus of small-diameter fibres.