INTERFACE COMPATIBILITY IN NICALON-FIBER REINFORCED METAL AND CERAMIC COMPOSITES

Microstructural characteristics of the fibre-matrix interface of two composite systems which utilize Nicalon fibre reinforcement are analysed and discussed. An Al-based composite produced by liquid-metal infiltration was found to contain crystals of aluminium carbide and alumina at the fibre-matrix interface, which produced a strong interfacial bond, restricted fibre pullout, and resulted in an essentially brittle composite. A ceramic-matrix composite based upon calcium aluminosilicate and produced by hot pressing exhibited substantial fibre pullout during testing; microstructural analysis of the interface showed the presence of a C-rich layer. Treatment of the composite in air over a range of temperatures (600-1200-degrees-C) progressively oxidized the carbon and formed silica 'bridges' between fibre and matrix, which resulted in increased brittleness. Electron-probe microanalysis combined with electron microscopy of the Nicalon fibre showed that approximately half the material consisted of microcrystalline beta-SiC and the remainder was free carbon and silicon oxycarbide. Thus the carbon constituent was largely responsible for carbide formation in the Al-based material, which restricted fibre pullout, whilst free carbon, plus the additional free carbon formed by chemical reaction between silicon carbide in the fibre and the calcium aluminosilicate matrix, provided the interfacial carbon layer which gave enhanced fibre pullout in the ceramic-based composite; the decreased fibre pullout and increased brittleness of the latter after heat treatment in air could thus be explained by the removal of the carbon layer and the development of silica bridges between fibre and matrix.