STUDIES ON THE INFLUENCE OF SECONDARY HEAT-TREATMENT ON STRUCTURE ANDPROPERTIES OF UNIDIRECTIONAL CARBON CARBON AND CARBON/FURFURAL-SILOXANE COMPOSITES/

Unidirectional carbon/carbon composites have been prepared using high strength and high modulus carbon fibres as reinforcements and phenolic , polyfurfuryl alcohol and furfuryl-siloxane resins as matrix precurso rs. The composites were pyrolyzed to 1000 degrees C and subsequently h eat treated to 1450 and 2800 degrees C in the presence of argon. Chang es in volume and density were measured and were found to be dependent on reinforcement and matrix precursors. Porosity and pore size distrib ution measured by mercury porosimetry suggests that during secondary h eat treatment pore and crack closing takes up if the fibre/matrix bond ing is strong and the opposite phenomenon takes place otherwise. This has been substantiated by optical and SEM microscopy. The porosity and pore size distribution influences the flexural strength of the compos ites while the dynamic modulus has been found to be dependent on the g raphitic contents of the composite. The secondary heat treatment enhan ces the oxiresistivity of the composites though porosity is still a de termining factor. Presence of silicon compounds in the matrix enhances the oxidation resistance and in the composites with this matrix syste m is found to be more effective at the fibre/matrix interface. Copyrig ht (C) 1996 Elsevier Science Ltd.