Characterization of the fibre-matrix interfacial structure in carbon fibre-reinforced polycarbosilane-derived SiC...

This paper presents a characterization study of the microstructural evolution of various carbon fibre-reinforced polycarbosilane (PCS)-derived SiC matrix composites during high temperature heat treatment. Both surface-treated and untreated carbon fibre reinforcements were investigated. The STEM/EELS technique was found to be a particularly useful characterization tool. The results of quantitative EELS linescans have been interpreted in terms of the migration of gaseous SiO and CO, produced by the reaction between the small amount of SiO2 and excess carbon within the PCS-derived SiC matrix, from the central matrix region towards the fibre-matrix interfaces. Generally, the migration of gaseous SiO and CO results in an enrichment of SiO2 at the region adjacent to the fibre-matrix interface. However, differing final composite microstructures are formed depending on the strength of the fibre-matrix bonding. In the case of strong fibre-matrix interfacial bonding where few escape channels are present, a distinct Si-C-O layer was identified within the matrix adjacent to the fibre-matrix interface; both crystalline beta-SiC and the segregated Si-O-C phase coexist in this microstructure up to at least 1450 degrees C. In the case of weak fibre-matrix bonding this oxygen segregated interfacial layer is eventually removed at high enough temperatures. The final interfacial microstructure has important consequences for the mechanical properties of the composite material.