The mechanics of stretch-graphitization of glassy carbon fibres |
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Authors: | H M Hawthorne |
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Affiliation: | (1) Centre for Materials Research, The University of British Columbia, Vancouver, B.C., Canada;(2) Present address: MacTainAir Systems Ltd, 420 Cowley Crescent, Vancouver Airport, Richmond, B.C., Canada |
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Abstract: | 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 Å3, 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. |
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