EFFECT OF HEAT-TREATMENT IN AIR ON THE STRUCTURE AND PROPERTIES OF BARIUM OSUMILITE REINFORCED WITH NICALON FIBERS

Microstructural studies have been carried out on glass-ceramic matrix composites, consisting of barium osumilite reinforced with Nicalon fibres, which have been subjected to heat treatment in air in the range 600-1100-degrees-C. Parallel studies have involved the measurement of the friction stress between fibre and matrix and the flexural strength of the composite. The matrix was shown to consist of barium osumilite, hexacelsian, mullite and a silica-rich glass, the thermal mismatch of these different phases leading to the development of appreciable strains. Whilst high-temperature treatments caused the formation of voids due to flow of the glassy phase, the major factor controlling the mechanical properties of the composite was the fibre/matrix interface. A change in microstructure, from a weak carbon-rich interface to one where the fibre and matrix were strongly bonded together by a silica layer, was thus reflected in an increase in the interfacial friction stress and a change in fracture behaviour from one showing fibre pull-out and delamination to one with brittle characteristics.