Summary 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.

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Oxidation heat treatments between 375°C and 600°C for 100 hours in air, have been performed on the calcium aluminosilicate glass-ceramic matrix / Sic fibre reinforced composite CASMicalon (manufactured by Corning, USA). Using a commercial nano-indentation system to perform fibre push-down tests, the fibre-matrix interfacial debond fracture surface energy (Gi) and fictional shear stress (T) have been determined. Modification of interface properties, compared to the asfabricated material, was observed at heat treatment temperatures as low as 37SoC, where a significant drop in G, and an increase in r were recorded. With 45OoC, 525°C and 600°C heat treatments, an increase in G, but a dramatic increase in T were recorded. Under four-point flexure testing, the as fabricated and the 375°C heat treated materials displayed tough, composite behaviour with extensive fibre pull out, but at 245O"C, brittle failure with minimal fibre pull out, was observed. This transition from tough mechanical response to one of brittleness is due to the large increase in T reducing fibre pull out to a mmimum and therefore reducing the total required work of fracture. The large increases in r and Gi have been attributed to the oxidative removal of the lubricating, carbon interface and the compressive residual stresses across the interface.

SUMMARY Electron microscopy and associated analytical spectroscopic techniques have been used to characterize interfaces in SiC‐fibre/silicate‐matrix composites. Interface structure, formed via reaction during hot‐press fabrication, is a function of time, temperature, matrix composition and fibre type. Interfaces with Nicalon or Tyranno fibres vary from amorphous carbon in fine precipitated form to continuous graphitic layers. Interface behaviour in a stressed composite, and hence the matrix microcracking stress, is a sensitive function of microstructure. Interface debond and shear properties have been assessed using indent‐based ‘push‐down’ and ‘push‐through’ tests using a specially developed instrument within a scanning electron microscope. This uses piezoload measurement and translation, and is capable of dynamic image recording of the indentation sequence. Interface micromechanical (indent) measurements have been correlated with structure and macromechanical response in bend testing for a range of fibre/matrix types, processing and post‐processing thermal treatments. An example is also given of interfaces prepared by fibre precoating.