I.. Davies et al. Composites Science and Technology 59(1999)801-811 (b) 3 um 3 um c)2(d) 4 um Fig. 7. Scanning electron micrographs illustrating fracture surfaces of Tyranno" Si-TiC-O fibres tested in situ at 1350 and 1380C: (a) general view of fibre exhibiting"debris"on the fracture surface(1350 C), (b) general view of a fibre that failed at 1380C, (c)detailed view of a fibre that failed at 380C illustrating the grain structure, and (d) general view of a fibre which shows evidence of a fracture mirror centred on a surface flaw on the right hand side of the fibre(1380C) 3.1.3. 1350 and 1380.C in vacuum accurately measured. For this reason, fracture mirror Typical fracture surfaces for fibres within composite data was only obtained for fibres within specimens tes specimens tested at 1350 and 1380C in vacuum have ted up to 1300.C in vacuum. been presented in Fig. 7. Although broadly similar to fibres tested at 1300 C in vacuum, those at 1350 C pos- 3.1.4. 1100 and 1200C in air ssed a coarser grain structure together with additional Fracture surfaces for fibres in composite specimens features on the fracture surface that at first sight tested at 1000 and 1 C in air were essentially simi- appeared to be debris [Fig. 7(a)]. However, further lar-the main feature being that <20% of fibres investigation showed similar"debris"to be present on showed evidence of fracture mirrors. Instead, the vast the fracture surface of nearly all fibres tested at 1350.c majority of fibres possessed smooth fracture surfaces in vacuum, suggesting these features to be an integral accompanied by negligible fibre pull-out(Fig 8). Simi- part of the fibre structure. One point of note is that suc lar characteristics have been observed in previous features were not generally observed in fibres tested at CMCs following testing in air at elevated temperature the test temperatures on either side of 1350.C, i.e. 1300 and attributed to oxidation and removal of the carbon and 1380C, implying that temperature was not a sig- layer present at the fibre /matrix interface [22] and its nificant factor in their appearance. Further work replacement with a silica layer resulting from oxidation required to determine the exact nature of these features. of the fibre surface[8, 23]. Such a phenomenon would be The grain structure of fibres tested at 1380.C in expected to dramatically increase t due to the replace- vacuum [Fig. 7(b d)] was similar to that observed at ment of carbon, whose friction coefficient, u, is esti- 1200C apart from a noticeable increase in average mated to be 0.01 [24], with SiO2 that has u A03-0.8 rain size and attributed to the increased test tempera ture. Although some evidence of fracture mirrors was Previous work has shown matrix cracks perpend observed in the 1350 and 1380 C fibres [Fig. 7(d)]. the cular to the fibre to only be deflected at the fibre matrix/ features were generally not sufficiently distinct to be interface when the following inequality is satisfied [26]3.1.3. 1350 and 1380C in vacuum Typical fracture surfaces for ®bres within composite specimens tested at 1350 and 1380C in vacuum have been presented in Fig. 7. Although broadly similar to ®bres tested at 1300C in vacuum, those at 1350C pos￾sessed a coarser grain structure together with additional features on the fracture surface that at ®rst sight appeared to be debris [Fig. 7(a)]. However, further investigation showed similar ``debris'' to be present on the fracture surface of nearly all ®bres tested at 1350C in vacuum, suggesting these features to be an integral part of the ®bre structure. One point of note is that such features were not generally observed in ®bres tested at the test temperatures on either side of 1350C, i.e. 1300 and 1380C, implying that temperature was not a sig￾ni®cant factor in their appearance. Further work is required to determine the exact nature of these features. The grain structure of ®bres tested at 1380C in vacuum [Fig. 7(b)±(d)] was similar to that observed at 1200C apart from a noticeable increase in average grain size and attributed to the increased test tempera￾ture. Although some evidence of fracture mirrors was observed in the 1350 and 1380C ®bres [Fig. 7(d)], the features were generally not suciently distinct to be accurately measured. For this reason, fracture mirror data was only obtained for ®bres within specimens tes￾ted up to 1300C in vacuum. 3.1.4. 1100 and 1200C in air Fracture surfaces for ®bres in composite specimens tested at 1000 and 1100C in air were essentially simi￾larÐthe main feature being that <20% of ®bres showed evidence of fracture mirrors. Instead, the vast majority of ®bres possessed smooth fracture surfaces accompanied by negligible ®bre pull-out (Fig. 8). Simi￾lar characteristics have been observed in previous CMCs following testing in air at elevated temperature and attributed to oxidation and removal of the carbon layer present at the ®bre/matrix interface [22] and its replacement with a silica layer resulting from oxidation of the ®bre surface [8,23]. Such a phenomenon would be expected to dramatically increase  due to the replace￾ment of carbon, whose friction coecient, , is esti￾mated to be 0.01 [24], with SiO2 that has   0:3±0.8 [25]. Previous work has shown matrix cracks perpendi￾cular to the ®bre to only be de¯ected at the ®bre matrix/ interface when the following inequality is satis®ed [26]: Fig. 7. Scanning electron micrographs illustrating fracture surfaces of Tyranno1 Si±Ti±C±O ®bres tested in situ at 1350 and 1380C: (a) general view of ®bre exhibiting ``debris'' on the fracture surface (1350C), (b) general view of a ®bre that failed at 1380C, (c) detailed view of a ®bre that failed at 1380C illustrating the grain structure, and (d) general view of a ®bre which shows evidence of a fracture mirror centred on a surface ¯aw on the right hand side of the ®bre (1380C). 806 I.J. Davies et al. / Composites Science and Technology 59 (1999) 801±811