J Mater sci(2007)42:763-771 microstructural and mechanical properties of the brought about by heat treatment. Changes in the composites can be affected by the environment with microstructure of the specimen were monitored using time. Some studies carried out on glass ceramic matrix standard microstructural characterisation techniques composites [13-16 show that the interface between and physical properties such as thermal diffusivity of fibre and matrix in the composites can be affected by the specimen were measured before and after heat the temperature of the environment. Oxidation occurs treatment at the interface during the heat treatment with temper ature and this results in a degradation of the mechanical properties of the composites. This was confirmed using Experimental mechanical tests such as tensile, three- or four-point bending, fibre pullout, creep, etc. These studies suggest Materials that this behaviour can also be studied using thermal property tests for composites exposed to a heat treat- A 0/90 laminated SiC/BMAS composite was supplied ment at varous temperatures n UK. The by the National Physical Laborator S $ g Limited amount of work has been carried out on the composite was manufactured by Harwell Technology thermal properties of such composites. Hasselmann in England. The preparation route was for the Tyranno and co-workers have carried out the most comprehen- fibre tow to be desized in a furnace, taken through a sive series of thermal diffusivity measurement on a slurry of glass frit, removed and wound on a wheel unit, wide range of composite systems 3, 17-24. Thermal allowed to dry for 20 min, cut and laid up manually in diffusivity or conductivity can be affected by the layers for hot pressing in a graphite die at-1, 200C for relative volume fraction of the constituents(fibre/ 10 min. It was then crystallised via a proprietary heat matrix and porosity), the orientation of the fibres, the treatment, which involved in heating to a temperature particular processing route chosen, and the structure of not exceeding 1, 300C [AEA Harwell Technology Heating to high temperatures can affect the thermal private communication the fibre/matrix interface Thermal diffusivity and thermal expansion measure diffusivity of SiC fibre-reinforced composites [19, ments were carried on the as-received material and primarily because of the change in fibre/matrix inter- after heat treatments in air at temperatures of 700C, face in thermal exposure. a number of studies have800°C,900°C,1,000°C,1,100°C,1,200° C for times of indicated that thermal conductivity and diffusivity of ranging from 1 to 30 h composites can be affected by a thermal barrier resistance of the interface [18-21, 24]. The direction Microstructural examination of heat flow also plays an important role in determining le effective diffusivity of composites in which there is X-ray diffraction analysis fibre/matrix interface resistance. The greatest effe will be observed when heat flow is perpendicular to the X-ray diffraction studies were carried out to identi fibre/matrix interface the phases present in the composites. These were Oxidation resulting in the removal of carbon at performed using a PHILIPS E'XPERT diffractometer interface behaved as a thermal barrier. When the Pw 3710 by using nickel-filtered copper K radiation carbon layer is oxidised, the thermal conductivity at with a graphite secondary monochromator Scans at a the fibre/matrix interface occurs by gaseous conduction step width of 0.005 for 20 values from 20 to 700 were and resulted in lower thermal diffusivity in composites used on samples that were solid bulk plates 10 mm [18. However, there have been relatively few reported square by 2 mm thick. The diffraction traces obtained observations of the effect of thermal exposure on the were compared against standard Xrd patterns for a thermal properties. Certainly there is not any system- range of materials atic investigation undertaken on this. It is possible that the measurement of the thermal diffusivity can be used Optical and scanning electron microscopy as a qualitative non-destructive tool to determine the integrity of the fibre/matrix interfaces and to monitor Sample preparations were taken in three stages; the microstructural changes occurring in the fibres or specimens were ground on a Buehler DATAMET microprocessor grinding/polishing system. METLAP 4 In this work, a detailed microstructural character- wheel with 9 um METaDI diamond slurry, with the ization of the BMAS/SiC system has been presented wheel contra-rotating at 25 r.P. m, then on a Beuhler along with subsequent changes in microstructure Metlap 2 wheel with 6 um diamond slurry, the wheel 2 Springermicrostructural and mechanical properties of the composites can be affected by the environment with time. Some studies carried out on glass ceramic matrix composites [13–16] show that the interface between fibre and matrix in the composites can be affected by the temperature of the environment. Oxidation occurs at the interface during the heat treatment with temper￾ature and this results in a degradation of the mechanical properties of the composites. This was confirmed using mechanical tests such as tensile, three- or four-point bending, fibre pullout, creep, etc. These studies suggest that this behaviour can also be studied using thermal property tests for composites exposed to a heat treat￾ment at various temperatures. Limited amount of work has been carried out on the thermal properties of such composites. Hasselmann and co-workers have carried out the most comprehen￾sive series of thermal diffusivity measurement on a wide range of composite systems [3, 17–24]. Thermal diffusivity or conductivity can be affected by the relative volume fraction of the constituents (fibre/ matrix and porosity), the orientation of the fibres, the particular processing route chosen, and the structure of the fibre/matrix interface. Heating to high temperatures can affect the thermal diffusivity of SiC fibre-reinforced composites [19], primarily because of the change in fibre/matrix inter￾face in thermal exposure. A number of studies have indicated that thermal conductivity and diffusivity of composites can be affected by a thermal barrier resistance of the interface [18–21, 24]. The direction of heat flow also plays an important role in determining the effective diffusivity of composites in which there is fibre/matrix interface resistance. The greatest effect will be observed when heat flow is perpendicular to the fibre/matrix interface. Oxidation resulting in the removal of carbon at interface behaved as a thermal barrier. When the carbon layer is oxidised, the thermal conductivity at the fibre/matrix interface occurs by gaseous conduction and resulted in lower thermal diffusivity in composites [18]. However, there have been relatively few reported observations of the effect of thermal exposure on the thermal properties. Certainly there is not any system￾atic investigation undertaken on this. It is possible that the measurement of the thermal diffusivity can be used as a qualitative non-destructive tool to determine the integrity of the fibre/matrix interfaces and to monitor microstructural changes occurring in the fibres or matrix. In this work, a detailed microstructural character￾ization of the BMAS/SiC system has been presented along with subsequent changes in microstructure brought about by heat treatment. Changes in the microstructure of the specimen were monitored using standard microstructural characterisation techniques and physical properties such as thermal diffusivity of the specimen were measured before and after heat treatment. Experimental Materials A 0/90 laminated SiC/BMAS composite was supplied by the National Physical Laboratory in UK. The composite was manufactured by Harwell Technology in England. The preparation route was for the Tyranno fibre tow to be desized in a furnace, taken through a slurry of glass frit, removed and wound on a wheel unit, allowed to dry for 20 min, cut and laid up manually in layers for hot pressing in a graphite die at ~1,200 C for 10 min. It was then crystallised via a proprietary heat treatment, which involved in heating to a temperature not exceeding 1,300 C [AEA Harwell Technology, private communication]. Thermal diffusivity and thermal expansion measure￾ments were carried on the as-received material and after heat treatments in air at temperatures of 700 C, 800 C, 900 C, 1,000 C, 1,100C, 1,200C for times of ranging from 1 to 30 h. Microstructural examination X-ray diffraction analysis X-ray diffraction studies were carried out to identify the phases present in the composites. These were performed using a PHILIPS E’XPERT diffractometer PW 3710 by using nickel-filtered copper K radiation with a graphite secondary monochromator. Scans at a step width of 0.005 for 2h values from 20 to 70 were used on samples that were solid bulk plates 10 mm square by 2 mm thick. The diffraction traces obtained were compared against standard XRD patterns for a range of materials. Optical and scanning electron microscopy Sample preparations were taken in three stages; the specimens were ground on a Buehler DATAMET￾microprocessor grinding/polishing system. METLAP 4 wheel with 9 lm METADI diamond slurry, with the wheel contra-rotating at 25 r.p.m, then on a Beuhler Metlap 2 wheel with 6 lm diamond slurry, the wheel 123 764 J Mater Sci (2007) 42:763–771