Studies on the influence of secondary heat treatment on structure and properties of unidirectional carbon/carbon and carbon/furfural-siloxane composites Original Research Article

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 precursors. The composites were pyrolyzed to 1000 °C and subsequently heat treated to 1450 and 2800 °C in the presence of argon. Changes in volume and density were measured and were found to be dependent on reinforcement and matrix precursors. Porosity and pore size distribution measured by mercury porosimetry suggests that during secondary heat treatment pore and crack closing takes up if the fibre/matrix bonding 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 composites while the dynamic modulus has been found to be dependent on the graphitic contents of the composite. The secondary heat treatment enhances the oxiresistivity of the composites though porosity is still a determining factor. Presence of silicon compounds in the matrix enhances the oxidation resistance and in the composites with this matrix system is found to be more effective at the fibre/matrix interface.