MECHANICAL-PROPERTIES AND MICROSTRUCTURES OF SOL-GEL DERIVED CERAMIC-MATRIX COMPOSITES

A sol-gel process involving rapid freezing (''freeze gelation'') permits the fabrication of ceramic-matrix composite components at low sintering temperatures, to near-net shape and with low shrinkage. The effects of matrix composition and sintering temperature on the microstructures, mechanical properties and damage modes of sol-gel-silica/unidirectional carbon-fibre composites obtained by filament winding were explored. Matrix properties were modified by the incorporation of amorphous silica and glass-ceramic particles and amorphous silica and quartz particles into the colloidal silica sol. Flexural testing and scanning electron microscopy of fracture surfaces were used to determine mechanical properties and fracture mechanisms, whilst transmission and scanning electron microscopy, optical microscopy and X-ray diffraction were used to characterize the microstructures. A transition from a tough mode of fracture, involving appreciable fibre pull-out, to a brittle mode was observed when the sintering temperature was increased beyond 900 degrees C. The brittleness was attributed to the formation of alpha-cristobalite whose high thermal expansion coefficient caused matrix cracking and fibre clamping. In one matrix system, a-cristobalite was formed in the amorphous silica filler particles, and in the other in the sol-gel matrix itself. A complex pattern of directional porosity, an artefact of the freeze gelation process, was found to influence the crack-growth behaviour during mechanical testing.