J Mater sci(2007)42:763-771 in significant diffusivity changes at the lower heat treatment temperatures (700-800C). There wa marked change in the microstructures of samples heat treated in this temperature range where voids and gap can be seen at the interfaces. In Fig. 8, there is a clearly identifiable gap of -8 nm at the interface between fibre and matrix but also linkages between fibre and matrix were also seen as noted by Plucknett et al. [14] After heat treatment to higher temperature no evidence of voids was detected. Examples of these are the micrographs in Fig 8(a) and(b), which show interfaces in samples heated at 1, 100C and 1, 200C. It is noticeable that the interface has now thickened to Fig6 Back-scattered electron SEM images showing by arrows -20-25 nm. For an even higher heat treatment tem- glassy phases around the fibre after heat treatment at 700C for perature the interface is even thicker(40-50 nm), as evident with sample heat treated at 1, 200C(Fig. 9b) The change on the interface structure as a result of reatment. However, it was not easy for all the glass to thermal treatment can be summarised as a transition flow. Therefore, local concentrations of the glassy from a carbon-rich interphase in the as-fabricated phase occurred. After a higher temperature anneal at composite to a carbon-free interphase after heat treat higher than 900C( Fig. 7)and reduced glassy phase ments in the intermediate temperature range content was seen in the matrix. This reduction sugge (700-800C)and finally to retention of a carbon-rich that some recrystallisation of the residual glassy pha interface at higher ageing temperatures(900-1, 200C) had occurred The interphase formed at the higher heat treatment temperature is also much thicker (45 nm)than that TEM studies formed in the as-received composite(20 nm)[12] Figure 10 collates information from the EDs traces TEM studies were carried out on the selected samples. of the various regions of the interface and compare Whereas, SEM studies were intended primarily to the information with that obtained for as-received identify the changes in phase structure and glassy material after heat treatments at 700C and 1, 200C phase content in the TEM studies. A special emphasis Although EDS analysis does not give reliable results was paid to studying the fibre/matrix interface where due to its limitations for low atomic weight elements microstructural changes had been previously reported such as o and C and the results were affected by [13-15]. Because of difficulties in preparing of suitable the back ground noise, this can provide a useful TEM samples, it was decided that the heat-treated samples to be studied were those heat treated for 30 h nce heat treatment for this period had resulted fbre Fig8 TEM bright field images of the sample heated at 700C Fig. 7 SEM micrograph after heat treatment at 1,000C for 30 h showing gap and voids at the interfac 2 Springertreatment. However, it was not easy for all the glass to flow. Therefore, local concentrations of the glassy phase occurred. After a higher temperature anneal at higher than 900 C (Fig. 7) and reduced glassy phase content was seen in the matrix. This reduction suggests that some recrystallisation of the residual glassy phases had occurred. TEM studies TEM studies were carried out on the selected samples. Whereas, SEM studies were intended primarily to identify the changes in phase structure and glassy phase content in the TEM studies. A special emphasis was paid to studying the fibre/matrix interface where microstructural changes had been previously reported [13–15]. Because of difficulties in preparing of suitable TEM samples, it was decided that the heat-treated samples to be studied were those heat treated for 30 h in air since heat treatment for this period had resulted in significant diffusivity changes at the lower heat treatment temperatures (700–800 C). There was a marked change in the microstructures of samples heat treated in this temperature range where voids and gaps can be seen at the interfaces. In Fig. 8, there is a clearly identifiable gap of ~8 nm at the interface between fibre and matrix but also linkages between fibre and matrix were also seen as noted by Plucknett et al. [14]. After heat treatment to higher temperature no evidence of voids was detected. Examples of these are the micrographs in Fig. 8(a) and (b), which show interfaces in samples heated at 1,100 C and 1,200 C. It is noticeable that the interface has now thickened to ~20–25 nm. For an even higher heat treatment tem￾perature the interface is even thicker (~40–50 nm), as evident with sample heat treated at 1,200 C (Fig. 9b). The change on the interface structure as a result of thermal treatment can be summarised as a transition from a carbon-rich interphase in the as-fabricated composite to a carbon-free interphase after heat treat￾ments in the intermediate temperature range (700–800 C) and finally to retention of a carbon-rich interface at higher ageing temperatures (900–1,200 C). The interphase formed at the higher heat treatment temperature is also much thicker (45 nm) than that formed in the as-received composite (20 nm) [12]. Figure 10 collates information from the EDS traces of the various regions of the interface and compares the information with that obtained for as-received material after heat treatments at 700 C and 1,200 C. Although EDS analysis does not give reliable results due to its limitations for low atomic weight elements such as O and C and the results were affected by the back ground noise, this can provide a useful Fig. 7 SEM micrograph after heat treatment at 1,000 C Fig. 8 TEM bright field images of the sample heated at 700 C for 30 h showing gap and voids at the interface Fig. 6 Back-scattered electron SEM images showing by arrows glassy phases around the fibre after heat treatment at 700 C for 30 h 123 768 J Mater Sci (2007) 42:763–771