The mechanics of stretch-graphitization of glassy carbon fibres

The mechanics of stretch-graphitization of glassy carbon fibres made from two pitch precursors was studied by determining the plastic deformation characteristics of monofilaments during their elongation to 50% strain in a continuous apparatus. By monitoring fibre tension under various processing conditions and analysing deformation profiles quenched into extending fibres, the temperature and strain-rate dependence of induced fibre stresses has been obtained. Over the temperature range 2200 to 3000° C and strainrate range 3.0×10^?4 to 2.5×10^?1 sec^?1, tensile stresses in the glassy carbon fibres ranged from 7000 to 50 000 psi. The deformation can be described by the empirical equation \(\dot \varepsilon = A\sigma ^n \) exp (? ΔH/RT), where A is a constant, n=8.6±1 and ΔH ～ 290±50 kcal mole^?1, independent of temperature and strain-rate, for both fibre types. The apparent activation energy is consistent with the controlling operation of a microfibrillar reorientation process, accompanied by atomic diffusion (ordering). The high strain-rate stress dependence, indicating an apparent activation volume ～ 4000 ų, also suggests a molecular-scale rate-limiting process. Results are compared with those of various high temperature processes in carbonaceous solids and deformation in organic polymers and it is suggested that stretch-graphitization can be considered analogous to the affine drawing of amorphous polymer fibres.