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 micr ostructural stability (specifically interface stability) after environ mental exposure at elevated temperature. Characterization of the as-re ceived material demonstrated the presence of a thin 'carbon-rich' inte rfacial layer between fibre and matrix, as typically observed in glass -ceramic/silicon carbide fibre composite systems. Samples have been su bjected to heat-treatments in an oxidizing atmosphere at temperatures between 723 and 1473 K, for up to 500 h. Intermediate-temperature agei ng, between 873 and 1073 K, results in strong fibre/matrix bonding, wi th consequent degradation of strength and composite 'ductility'. This is due to oxidative removal of the carbon interfacial layer and subseq uent oxidation of the fibre surface, forming a silica bridge. Carbon i s retained at higher ageing temperatures due to the formation of a pro tective surface oxide scale at exposed fibre ends, Attempts to pretrea t the BMAS composite at high temperature (1273-1473 K), designed to in hibit 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.