J Mater Sci(2007)42:763-771 DOI10.1007/s10853-006-1443-3 Effect of heat treatment in air on the thermal properties of siC fibre-reinforced composite. Part 1: a barium osumilite (BMas) matrix glass ceramic composite R. Yilmaz·R. Taylor October 2003/ Accepted: 13 April 2006/Published online: 12 January 2007 Springer Science+Business Media, LLC 2007 Abstract The thermal properties have been studied Introduction on a glass ceramic composite comprised of a barium umilite(BMAS)matrix reinforced with SiC (Tyran- SiC fibres have been developed and used in a variety of no) fibres which has been subjected to a heat treatment reinforced glass ceramic composites over the last two in air in the range of 700-1, 200C. Microstructural decades. They are intended to provide reinforcement studies were carried out especially on of the interface and to improve the performance of structural ceramics between fibre and matrix. The presence of a carbon for high temperature applications [1] thin layer in the interface is a typical observation in Sic Ceramics are brittle, have low fracture toughness fibre-reinforced glass ceramic matrix composite sys- and they fail in a catastrophic manner. The brittleness tems. The microstructural evaluation and thermal of the material results from sudden propagation of a properties showed a degradation of interfacial layer crack under the applied stress. Thus, for the future ccurred at low heat treatment temperatures, (700- application of ceramics at high temperatures it is 800C)this was attributed to the fact that, at those necessary to develop ceramic fibres as reinforcement in heat treatment temperatures the carbon rich layer suitable matrices. It is also necessary to understand the formed during processing was oxidised away leaving fracture behaviour of these composites and behaviour voids between fibre and matrix, which were linked by of the interface for design matters in order to optimise isolated silicon-rich bridges. After heat treatment at their mechanical properties higher temperatures of 1,000-1, 200C, the thermal One of the most promising glass ceramic matrix properties were retained or even enhanced by leaving a systems is barium osumilite(BMAS), first developed thick interfacial layer. by Brennan et al. [2]. Thermal properties of BMAS glass ceramic matrix composites have been studied by Johnson et al. 3]. Although the use of such materials ill be governed by the development of the suitable mechanical properties, the accurate thermo-physical properties are also needed such as thermal expansion, thermal diffusivity and conductivity During manufacturing stage, due to fibr R. Yilmaz(凶) reaction, a carbon layer is formed [4, 5] in SiC fibre- Technical Education Faculty. Metal Education Division Sakarya University, Esentepe Campus, 54187 Sakarya reinforced glass ceramic composites. This carbon rich Turkey layer has been observed in MAS, calcium aluminium e-mail: ryilmaz@sakarya. edu.tr silicate (CAS), barium aluminium silicate (BAS) barium magnesium aluminium silicate(BMAs)glass R. Pavlo Manchester Materials Science Centre, University of ceramic matrix composites 6-12 Manchester Institute of Science and Technology. Grosvenor The in-service performance of these composites Street, Manchester M1 7HS. England depends on the environmental conditions. The thermal
Effect of heat treatment in air on the thermal properties of SiC fibre-reinforced composite. Part 1: a barium osumilite (BMAS) matrix glass ceramic composite R. Yilmaz Æ R. Taylor Received: 13 October 2003 / Accepted: 13 April 2006 / Published online: 12 January 2007 Springer Science+Business Media, LLC 2007 Abstract The thermal properties have been studied on a glass ceramic composite comprised of a barium osumilite (BMAS) matrix reinforced with SiC (Tyranno) fibres which has been subjected to a heat treatment in air in the range of 700–1,200 C. Microstructural studies were carried out especially on of the interface between fibre and matrix. The presence of a carbon thin layer in the interface is a typical observation in SiC fibre-reinforced glass ceramic matrix composite systems. The microstructural evaluation and thermal properties showed a degradation of interfacial layer occurred at low heat treatment temperatures, (700– 800 C) this was attributed to the fact that, at those heat treatment temperatures the carbon rich layer formed during processing was oxidised away leaving voids between fibre and matrix, which were linked by isolated silicon-rich bridges. After heat treatment at higher temperatures of 1,000–1,200 C, the thermal properties were retained or even enhanced by leaving a thick interfacial layer. Introduction SiC fibres have been developed and used in a variety of reinforced glass ceramic composites over the last two decades. They are intended to provide reinforcement and to improve the performance of structural ceramics for high temperature applications [1]. Ceramics are brittle, have low fracture toughness and they fail in a catastrophic manner. The brittleness of the material results from sudden propagation of a crack under the applied stress. Thus, for the future application of ceramics at high temperatures it is necessary to develop ceramic fibres as reinforcement in suitable matrices. It is also necessary to understand the fracture behaviour of these composites and behaviour of the interface for design matters in order to optimise their mechanical properties. One of the most promising glass ceramic matrix systems is barium osumilite (BMAS), first developed by Brennan et al. [2]. Thermal properties of BMAS glass ceramic matrix composites have been studied by Johnson et al. [3]. Although the use of such materials will be governed by the development of the suitable mechanical properties, the accurate thermo-physical properties are also needed such as thermal expansion, thermal diffusivity and conductivity. During manufacturing stage, due to fibre and matrix reaction, a carbon layer is formed [4, 5] in SiC fibrereinforced glass ceramic composites. This carbon rich layer has been observed in MAS, calcium aluminium silicate (CAS), barium aluminium silicate (BAS), barium magnesium aluminium silicate (BMAS) glass ceramic matrix composites [6–12]. The in-service performance of these composites depends on the environmental conditions. The thermal, R. Yilmaz (&) Technical Education Faculty, Metal Education Division, Sakarya University, Esentepe Campus, 54187 Sakarya, Turkey e-mail: ryilmaz@sakarya.edu.tr R. Taylor Manchester Materials Science Centre, University of Manchester Institute of Science and Technology, Grosvenor Street, Manchester M1 7HS, England J Mater Sci (2007) 42:763–771 DOI 10.1007/s10853-006-1443-3 123
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 Springer
microstructural 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 temperature 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 treatment 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 comprehensive 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 interface 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 systematic 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 characterization 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 measurements 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 DATAMETmicroprocessor 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
J Mater Sci(2007)42:763-771 contra-rotating at 120 r.P. m. After that polishing was were considered but the most suitable condition was done using a TEXMET platen with a wheel using 1 um found to be as follows: 5 kV ion beam energy 0.4A diamond slurry. Final polishing was carried out with current at 30, 25 and 16 impingement angle for 30 h. colloidal silica. Each section takes 10 min and the After milling the foils were then taken from the io polishing pressure was set to 1n per sample and beam thinner and placed directly into the electron maintained at that level during preparation. The microscope for examination. The analytical electron samples were then finished by washing with water for microscopy was carried out using a Philips EM 400 and 1 min and dried. After preparation of the specimens, CM 20 operated at 120 kV and 200 kv, respectively they were mounted on to an aluminium stub and Both were equipped with an Energy dispersive spec- initially coated with carbon or gold in order to prevent troscopy(EDS)system, and the investigation was charging in the microscope. An Edwards coating conducted using bright field, lattice imaging diffraction system E 306A was used for coating. A conducting and micro diffraction techniques silver paste was used with the carbon-coated samples painted on the edge of the sample connecting it with Thermal diffusivity measurement the stub to improve electrical contact. The surface of the heat-treated samples were examined using Philips Thermal diffusivity measurements were carried out 525 scanning electron microscopes(SEM) operating at using the laser flash method originally described by 20 kV with scanning facility operating with a computer Parker et al. [25]. The thermal diffusivity equipment programme-connected microscope used at UMIST has been previously described by Taylor [ 26] Transmission electron microscopy The specimens used in the measurements were in the form of 10 mm- plates with a thickness of appro Discs of the same diameter as the electron microscope mately 2 mm. The specimens were cut from the specimen holder (3 mm in diameter and 2 mm thick) composite plate by using a slow speed diamond saw. were cut from the heat-treated specimens. The spec- In order to ensure optimum absorption of the laser flash imens were prepared for transmission electron micros- at the sample front surface and maximum emissivity for copy(tEm) by a combination of mechanical polishing monitoring the transient temperature at the opposite and ion beam thinning techniques. The specimens were face of the sample, the both faces of the sample were initially ground on 1200 grit wet Sic abrasive until the coated with a colloidal graphite film. Measurements thickness of the specimen was reduced to -150 m. were performed from 100C up to 1,000C with the Further grinding was carried out until the specimen 100C interval. At least three measurements were thickness was 70 um. The foils were then transferred to taken at each measured temperature and averaged a Gatan ion beam thinner. Several milling conditions value was taken for plotting of the graphs Fig. 1 X-ray diffraction spectrum of as-received terial of bMaS/SiC CN Co: Cordierite B: Barium osumilite S: SiC fibre 8 400 300 8 1001B o"T"Jo""4o""5o""6("6
contra-rotating at 120 r.p.m. After that polishing was done using a TEXMET platen with a wheel using 1 lm diamond slurry. Final polishing was carried out with colloidal silica. Each section takes 10 min and the polishing pressure was set to 1 N per sample and maintained at that level during preparation. The samples were then finished by washing with water for 1 min and dried. After preparation of the specimens, they were mounted on to an aluminium stub and initially coated with carbon or gold in order to prevent charging in the microscope. An Edwards coating system E 306A was used for coating. A conducting silver paste was used with the carbon-coated samples painted on the edge of the sample connecting it with the stub to improve electrical contact. The surface of the heat-treated samples were examined using Philips 525 scanning electron microscopes (SEM) operating at 20 kV with scanning facility operating with a computer programme-connected microscope. Transmission electron microscopy Discs of the same diameter as the electron microscope specimen holder (3 mm in diameter and 2 mm thick) were cut from the heat-treated specimens. The specimens were prepared for transmission electron microscopy (TEM) by a combination of mechanical polishing and ion beam thinning techniques. The specimens were initially ground on 1200 grit wet SiC abrasive until the thickness of the specimen was reduced to ~150 m. Further grinding was carried out until the specimen thickness was 70 lm. The foils were then transferred to a Gatan ion beam thinner. Several milling conditions were considered but the most suitable condition was found to be as follows; 5 kV ion beam energy 0.4 A current at 30, 25 and 16 impingement angle for 30 h. After milling, the foils were then taken from the ion beam thinner and placed directly into the electron microscope for examination. The analytical electron microscopy was carried out using a Philips EM 400 and CM 20 operated at 120 kV and 200 kV, respectively. Both were equipped with an Energy dispersive spectroscopy (EDS) system, and the investigation was conducted using bright field, lattice imaging diffraction and micro diffraction techniques. Thermal diffusivity measurement Thermal diffusivity measurements were carried out using the laser flash method originally described by Parker et al. [25]. The thermal diffusivity equipment used at UMIST has been previously described by Taylor [26]. The specimens used in the measurements were in the form of 10 mm2 plates with a thickness of approximately 2 mm. The specimens were cut from the composite plate by using a slow speed diamond saw. In order to ensure optimum absorption of the laser flash at the sample front surface and maximum emissivity for monitoring the transient temperature at the opposite face of the sample, the both faces of the sample were coated with a colloidal graphite film. Measurements were performed from 100 C up to 1,000 C with the 100 C interval. At least three measurements were taken at each measured temperature and averaged value was taken for plotting of the graphs. Fig. 1 X-ray diffraction spectrum of as-received material of BMAS/SiC CMC 123 J Mater Sci (2007) 42:763–771 765
J Mater sci(2007)42:763-771 Results between heating and cooling measurement runs. Te ssess material variability take Characterisation of as-received material on four samples cut from different regions of the as-received plate. The results are plotted in Fig. 2 and To identify the phases present in the as-received show a scatter of *3%. The median value obtained material, the X-ray diffraction studies were carried from this curve will be included, for comparison out on the samples. Figure 1 shows a typical scan over purposes in all future diffusivity/temperature plots the range 10< 20 <70 from which three crystalline Changes in thermal diffusivity were noted for the phases have been identified: Barium osumilite(BaMg2 samples heat treated for times as short as 1 h at Al3(SigAl3O30)), hexacelsian(BaAl3Si2Os)and cor- temperatures of 700 and 900C. However for higher dierite(Mg2AlgSis O1s) temperature anneals at 1,000-1, 200C, a negligible change was noted. The thermal diffusivity data after 1 h Thermal properties heat treatment in the range 700-1, 200C are presented in Fig 3. It can be seen that the greatest degradation Thermal diffusivity measurements thermal diffusivity is noted for the sample heat treated at700° C with the value of44×10-3cm2s-lwas Thermal diffusivity was measured over the tempera- measured at 100C and fair ture range 100-1,000C. There was no change noted 3.44 x 10 cm-s of being recorded above 350C. Fig. 2 Measurement of thermal diffusivity of BMAS/ a AR2 SiC as-received materials AR: Samples cut from different regions of the as- 0.007 0.006 0.005 010020030040050060070080090010001100 erc] Fig 3 Measurement of 0.008 thermal diffusivity of BMAS/ Sic after heat treatment for 900°c I h in air 0.007 0.005 0.003 010020030040050060070080090010001100 Temperature rcl
Results Characterisation of as-received material To identify the phases present in the as-received material, the X-ray diffraction studies were carried out on the samples. Figure 1 shows a typical scan over the range 10 < 2h < 70 from which three crystalline phases have been identified: Barium osumilite (BaMg2 Al3 (Si9Al3O30)), hexacelsian (BaAl3Si2O8) and cordierite (Mg2Al4Si5O18). Thermal properties Thermal diffusivity measurements Thermal diffusivity was measured over the temperature range 100–1,000 C. There was no change noted between heating and cooling measurement runs. To assess material variability, measurements were taken on four samples cut from different regions of the as-received plate. The results are plotted in Fig. 2 and show a scatter of ±3%. The median value obtained from this curve will be included, for comparison purposes in all future diffusivity/temperature plots. Changes in thermal diffusivity were noted for the samples heat treated for times as short as 1 h at temperatures of 700 and 900 C. However for higher temperature anneals at 1,000–1,200 C, a negligible change was noted. The thermal diffusivity data after 1 h heat treatment in the range 700–1,200 C are presented in Fig. 3. It can be seen that the greatest degradation in thermal diffusivity is noted for the sample heat treated at 700 C with the value of 4.4 · 10–3 cm2 s –1 was measured at 100 C and fairly constant value 3.44 · 10–3 cm2 s –1 of being recorded above 350 C. Fig. 2 Measurement of thermal diffusivity of BMAS/ SiC as-received materials. AR: Samples cut from different regions of the asreceived composite Fig. 3 Measurement of thermal diffusivity of BMAS/ SiC after heat treatment for 1 h in air 123 766 J Mater Sci (2007) 42:763–771
J Mater Sci(2007)42:763-771 This value is 39% lower than the value measured for the thermal diffusivity values after 700-900C heat treat as-received material ments all show lower values than the as-received A more detailed set of thermal diffusivity results materials with the ranking order of greatest change after 10 h heat treatments are shown in Fig. 4. The shown after annealing at 700C, 800C and 900oC 1, 200C treatment shows a slight enhancement of thermal diffusivity, as does the 1,100C, whereas heat Microstructural studies treatment at 1000oC shows similar values to the values of the as-received material. all the other heat SEm studies treatments at temperatures lower than 1,000C show lower thermal diffusivities than the as-received mate- Limited SEM studies were carried out on samples heat rial. However, the 700C heat-treated sample again treated at temperatures between 700C and 1, 200C. shows the lowest thermal diffusivity values The most noteworthy observation was that a gap These trends are also maintained for the 30-h heat between fibre and matrix was noted at low tempera treatments for which thermal diffusivity results are ture. This is illustrated in Fig. 6 for a sample heat shown in Fig. 5. Again the heat treatment at 1, 200 treated at 700oC for 30 h. The shows higher thermal diffusivity values than those for residual stresses exist in the samples heated at lowe the as-received material, whereas data for samples temperatures. Residual glassy phases would tend to heated at 1,000C and 1, 100C are very close to that fow through any gaps in the matrix but the extent to of the as-received composite. On the other hand, the which this occurs depends on temperature of the heat Fig. 4 Measurement of 000 thermal diffusivity of BMAS/ Sic CMC after heat treatment in air for 10 h 0008 900°C 0D07 0006 日As recelved △△ 0004 0003 01002003040050060070080090010001100 Temperature rCl Fig 5 Measurement of 0.009 thermal diffusivity of BMAS/ 700°C 末 Sic CMC after heat treatment in air for 30 h 0.008 1100°c 0,006 x1200℃C 0.005 0.003 10020030040050060070080090010001100 Temperature rc
This value is 39% lower than the value measured for the as-received material. A more detailed set of thermal diffusivity results after 10 h heat treatments are shown in Fig. 4. The 1,200 C treatment shows a slight enhancement of thermal diffusivity, as does the 1,100 C, whereas heat treatment at 1,000 C shows similar values to the values of the as-received material. All the other heat treatments at temperatures lower than 1,000 C show lower thermal diffusivities than the as-received material. However, the 700 C heat-treated sample again shows the lowest thermal diffusivity values. These trends are also maintained for the 30-h heat treatments for which thermal diffusivity results are shown in Fig. 5. Again the heat treatment at 1,200 C shows higher thermal diffusivity values than those for the as-received material, whereas data for samples heated at 1,000 C and 1,100 C are very close to that of the as-received composite. On the other hand, the thermal diffusivity values after 700–900 C heat treatments all show lower values than the as-received materials with the ranking order of greatest change shown after annealing at 700 C, 800 C and 900 C. Microstructural studies SEM studies Limited SEM studies were carried out on samples heat treated at temperatures between 700 C and 1,200 C. The most noteworthy observation was that a gap between fibre and matrix was noted at low temperature. This is illustrated in Fig. 6 for a sample heat treated at 700 C for 30 h. These may indicate that residual stresses exist in the samples heated at lower temperatures. Residual glassy phases would tend to flow through any gaps in the matrix but the extent to which this occurs depends on temperature of the heat Fig. 4 Measurement of thermal diffusivity of BMAS/ SiC CMC after heat treatment in air for 10 h Fig. 5 Measurement of thermal diffusivity of BMAS/ SiC CMC after heat treatment in air for 30 h 123 J Mater Sci (2007) 42:763–771 767