EFFECTS OF FIBER SURFACE-TREATMENT ON FRACTURE AND ACOUSTIC-EMISSION FROM SINGLE AND HYBRID CFRC

Two classes of unidirectional carbon fibre reinforced carbon composites (CFRC) with resin carbon matrix and heat-treated to 1100 and 2500-degrees-C were manufactured by using PAN-based carbon fibres with contrasting surface activities; a third class of hybrid, unidirectional CFRC, was made from alternating laminae containing the fibres with different surface activities. The CFRC were subject to flexural loading with acoustic emission (AE) monitoring. For the 1100-degrees-C CFRC-containing surface-treated fibres, there was brittle failure, whereas failure of CFRC with untreated fibres was dominated by fibre bundle sliding. The flexural strength of the hybrid CFRC was greater than that of the other two composites. For the CFRC with surface-treated fibres, there was a substantial increase in flexural strength on heat treatment to 2500-degrees-C and a tougher failure mode; for the CFRC with untreated fibres, there was a reduction in flexural strength on heat treatment to 2500-degrees-C but no change in the failure mode; these changes were related to stress graphitisation at the fibre/matrix interface. The flexural strength of the hybrid CFRC lay between those for the other two composites. Analysis of AE event counts and AE amplitude distributions supported these interpretations of the deformation and flexural failure of the three types of composite. The development of hybrid composites containing different carbon fibre types seems to be a promising route for controlling properties of CFRC.