Carbon/carbon composites with heat-treated pitches II. Development of porosity in composites

C/C composites made with carbon fibres and coal tar pitch as matrix precursor were characterised in relationship with the properties of the pitch. The influence of pitch pretreatment in nitrogen and in air on the development of porosity in the C/C composites during a first impregnation-carbonisation step is studied. The amount of voids present in the final composite as well as the pore size distribution have been determined using quantitative image analysis. Results indicate that the pretreatment in air induces a significant change in the porosity of the C/C composite since the pore size distribution of the intertow and intratow porosity becomes narrower and the fraction of closed voids increases. Thermal annealing at 2000 degreesC of the composite also affects significantly its porosity in a way depending on the pitch properties due to the propagation of microfissures. Thermal programmed desorption of C/C composite and carbon precursors provides informations on the carbonisation behaviour of the composite and therefore confirms the results obtained by image analysis. (C) 2001 Elsevier Science Ltd. All rights reserved.