ENVIRONMENTAL AGING EFFECTS IN A SILICON-CARBIDE FIBER-REINFORCED GLASS-CERAMIC MATRIX COMPOSITE

A silicon carbide fibre-reinforced glass-ceramic composite, based upon a BaO-MgO-Al2O3-SiO2 (BMAS) matrix, has been used for a study of microstructural stability (specifically interface stability) after environmental exposure at elevated temperature. Characterization of the as-received material demonstrated the presence of a thin 'carbon-rich' interfacial layer between fibre and matrix, as typically observed in glass-ceramic/silicon carbide fibre composite systems. Samples have been subjected to heat-treatments in an oxidizing atmosphere at temperatures between 723 and 1473 K, for up to 500 h. Intermediate-temperature ageing, between 873 and 1073 K, results in strong fibre/matrix bonding, with consequent degradation of strength and composite 'ductility'. This is due to oxidative removal of the carbon interfacial layer and subsequent oxidation of the fibre surface, forming a silica bridge. Carbon is retained at higher ageing temperatures due to the formation of a protective surface oxide scale at exposed fibre ends, Attempts to pretreat the BMAS composite at high temperature (1273-1473 K), designed to inhibit intermediate-temperature degradation via the formation of silica plugs at exposed fibre ends, has given mixed results due to the high residual porosity content in these materials, allowing paths of 'easy' oxygen ingress to be retained.