Electron probe microanalysis of a sample of Nicalon fibre showed it to consist of 54.9 wt% Si, 32.1 wt% C and 11.6 wt% O. Studies of the fine structure of the X-ray emission bands suggested these elements were combined as 46 vol % silicon carbide, 34 vol % silicon oxycarbide and 20 vol% free carbon, with the oxycarbide in the outermost regions of the fibre being significantly richer in oxygen. The silicon carbide was composed of microcrystallites several micrometres in diameter and the remaining material formed an amorphous network of material surrounding the microcrystallites.

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SUMMARY 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°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 β‐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.

Nicalon is a fibre produced commercially by melt spinning a mixture of silanes and pyrolising the product. This gives a ceramic fibre, based upon silicon carbide, used for reinforcing metallic and composite materials. The composition of Nicalon is not fully understood but in addition to silicon carbide the presence of free carbon, and silica have been reported together with a non-stoichiometric oxycarbide.EPMA was carried out on transverse fibre sections prepared from a plate of metal matrix composite using a JEOL 8600M fitted with wavelength dispersive spectrometers. Quantitative data were obtained at a voltage of 15kV using reference standards of silicon carbide (for silicon and carbon analysis) and silica (for oxygen). The analyzing crystals employed for recording CK, OK and SiKα radiation were lead stearate, a W-Si multlilayer (2d = 7nm) and PET respectively. Peak intensity measurements were used except in the case of CK radiation where area measurements were necessary because of obvious differences in peak shape from Nicalon and SiC.

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