Microstructural characterisation of monolithic ceramic matrix composites from polysiloxane and SiC powder

The microstructure of ceramic matrix composites (CMCs) prepared by moulding α-SiC powder in the presence of preceramic polysiloxane as a binder was investigated. During pyrolysis, the powder/polymer system was converted into a ceramic body, in which the shrinkage and the porosity contents could be tailored. Different amounts of polymeric binder and also different firing temperatures were found to have strong influence on the microstructural parameters, such as average pore size, pore size distribution and overall porosity, as determined by both mercury porosimetry and nitrogen adsorption techniques. These parameters were related to shrinkage rate. The morphological characterisation of the composites obtained at 1000 °C, performed by field emission scanning electron microscopy, showed a uniform distribution of the α-SiC, in the SiCxO4−x phase, as well as SiC whisker particles in the pores of the CMCs. Additional characterisation of the ceramic phase, obtained from preceramic polysiloxane, was performed by 29Si and 13C nuclear magnetic resonance and infrared spectroscopies and also by X-ray diffraction. The results suggested a random distribution of silicon sites, SiCxO4−x with 0≤x≤4, in the ceramic bulk, at 1000 °C. This phase evolved to β-SiC at higher temperatures.