I.. Davies et al. Composites Science and Technology 59(1999)801-811 defect size at the fibre surface caused by the surface tested specimens were similar at room temperature and reatment. The chemical depth profile in Fig 3(b)shows 1200oC(400 MPa), followed by a gradual decrease of the surface-modified LoxM fibre to possess a 10 nm approximately 50% until 1380oC. Tensile strain to fail- SiOx-rich layer at the surface surrounding an inner 40 ure was approximately 1. 2%for specimens tested at nm carbon-rich layer. The justification for such a sur- room temperature and up to 1380.C in vacuum face chemistry is that the outer SiOx- rich layer will bond Although a suitable strain measurement technique for strongly to the matrix with the 40 nm carbon layer specimens tested in air at elevated temperature was not effectively acting as the fibre/matrix interface. In this available, values of typically <0.05% would be expec- case, the fibres would be expected to fail at the carbon ted when considering the reduced tensile strength of layer within the fibre surface [16] rather than at the specimens tested in air at elevated temperature(Fig 3) actual fibre/matrix interface that is usually the case in However, it should be emphasised that specimens inves- CMCS tigated in this report possessed no oxidation protection Prior to matrix densification the fibres were woven system. Tensile tests at elevated temperature in air for nto an orthogonal 3-D structure with fibre volume similar specimens, but surface sealed using a proprietary fractions in the x, y, and z directions being 0.19, 0.19, glass-based technique, indicate tensile strength to be simi- and 0.02, respectively. Weaving technology was utilised lar to that for unsealed specimens tested in vacuum [4] for this composite as it possesses great versatility as Following tensile failure, specimen fracture surfaces regards shape and dimension control [17], which should were investigated using a JEOL JSM-6300F scanning reduce machining costs in final applications electron microscope(SEM). The general nature of the Matrix densification consisted of a polymer similar to composite fracture surface was assessed whilst a detailed polytitanocarbosilane(PTCS) that was impregnated study of fibre pull-out behaviour is reported elsewhere into the fibre preform and pyrolysed to form a matrix [19]. Fibre fracture surfaces were characterised, with the similar in chemistry to that of the fibres. Eight cycles of fracture mode and flaw mirror radius being noted impregnation and pyrolysis were required to maximise Between 100 and 800 fibres were examined for each test the composite density [16] condition whilst in situ fibre properties were derived Tensile testing was undertaken with the specimen with the aid of Eqs.(2H(7) axis parallel to the loading direction at temperatures between room temperature and 1380 C in vacuum and from room temperature to 1200 C in air For specimens 3. Results and discussion tested at elevated temperature, heating rates between 300C and the specified test temperature were approxi- 3.I. Microstructural observation mately 0. 75C s-I whilst failed specimens were furnace cooled at an estimated rate above 1000c of 3. 3C 3. .1. Room temperature and 1200 C in vacuum The total time spent at the test temperature was believed The majority of fibres within specimens tested at to be approximately 600 s-further experimental details room temperature and 1200C in vacuum possessed being given elsewhere [15, 18 fine-grained structures with a well-defined mirror zone The tensile strengths [2] of specimens examined in this and crackle region that originated at the fibre surface report are presented in Fig 4. The strengths of vacuum-(as indicated in Fig. 1). Surface flaws thus controlled fibre strength under these conditions with relatively few fibres failing as a result of internal flaws for the room temperature case. The percentage of fibres failing at internal flay her for the 1200@C case compared to room temperature as indicated in Table 2. However, this appeared to have no effect on composite tensile ￡30 strength shown in Fig. 4, which would be consistent with fibres failing at the surface and in the bulk having similar strength distributions; this will be the topic of ■ vacuum further research. An example of a fibre that failed 100 through an internal flaw during testing at 1200oC in vacuum is presented in Fig. 5 with a mirror zone also being present around the flaw. From Table 2 it can be 10001100120013001400 seen that the failure mode could not be determined for Test temperature(C) 13% of fibres that failed at room temperature Although these fibres had no fracture mirrors visible, it Fig. 4. Tensile strength [2] of SiC/SiC-based specimens tested in was concluded that they probably did fail due to surface vacuum and air up to 1380.C. Note the non linear temperature scale flaws and may have represented the strongest populationdefect size at the ®bre surface caused by the surface treatment. The chemical depth pro®le in Fig. 3(b) shows the surface-modi®ed LoxM ®bre to possess a 10 nm SiOx-rich layer at the surface surrounding an inner 40 nm carbon-rich layer. The justi®cation for such a sur￾face chemistry is that the outer SiOx-rich layer will bond strongly to the matrix with the 40 nm carbon layer e€ectively acting as the ®bre/matrix interface. In this case, the ®bres would be expected to fail at the carbon layer within the ®bre surface [16] rather than at the actual ®bre/matrix interface that is usually the case in CMCs. Prior to matrix densi®cation the ®bres were woven into an orthogonal 3-D structure with ®bre volume fractions in the x, y, and z directions being 0.19, 0.19, and 0.02, respectively. Weaving technology was utilised for this composite as it possesses great versatility as regards shape and dimension control [17], which should reduce machining costs in ®nal applications. Matrix densi®cation consisted of a polymer similar to polytitanocarbosilane (PTCS) that was impregnated into the ®bre preform and pyrolysed to form a matrix similar in chemistry to that of the ®bres. Eight cycles of impregnation and pyrolysis were required to maximise the composite density [16]. Tensile testing was undertaken with the specimen y-axis parallel to the loading direction at temperatures between room temperature and 1380C in vacuum and from room temperature to 1200C in air. For specimens tested at elevated temperature, heating rates between 300C and the speci®ed test temperature were approxi￾mately 0.75C sÿ1 whilst failed specimens were furnace￾cooled at an estimated rate above 1000C of 3.3C sÿ1 . The total time spent at the test temperature was believed to be approximately 600 sÐfurther experimental details being given elsewhere [15,18]. The tensile strengths [2] of specimens examined in this report are presented in Fig. 4. The strengths of vacuum￾tested specimens were similar at room temperature and 1200C (400 MPa), followed by a gradual decrease of approximately 50% until 1380C. Tensile strain to fail￾ure was approximately 1.2% for specimens tested at room temperature and up to 1380C in vacuum. Although a suitable strain measurement technique for specimens tested in air at elevated temperature was not available, values of typically <0.05% would be expec￾ted when considering the reduced tensile strength of specimens tested in air at elevated temperature (Fig. 3). However, it should be emphasised that specimens inves￾tigated in this report possessed no oxidation protection system. Tensile tests at elevated temperature in air for similar specimens, but surface sealed using a proprietary glass-based technique, indicate tensile strength to be simi￾lar to that for unsealed specimens tested in vacuum [4]. Following tensile failure, specimen fracture surfaces were investigated using a JEOL JSM-6300F scanning electron microscope (SEM). The general nature of the composite fracture surface was assessed whilst a detailed study of ®bre pull-out behaviour is reported elsewhere [19]. Fibre fracture surfaces were characterised, with the fracture mode and ¯aw mirror radius being noted. Between 100 and 800 ®bres were examined for each test condition whilst in situ ®bre properties were derived with the aid of Eqs. (2)±(7). 3. Results and discussion 3.1. Microstructural observation 3.1.1. Room temperature and 1200C in vacuum The majority of ®bres within specimens tested at room temperature and 1200C in vacuum possessed ®ne-grained structures with a well-de®ned mirror zone and crackle region that originated at the ®bre surface (as indicated in Fig. 1). Surface ¯aws thus controlled ®bre strength under these conditions with relatively few ®bres failing as a result of internal ¯aws for the room temperature case. The percentage of ®bres failing at internal ¯aws was higher for the 1200C case compared to room temperature as indicated in Table 2. However, this appeared to have no e€ect on composite tensile strength shown in Fig. 4, which would be consistent with ®bres failing at the surface and in the bulk having similar strength distributions; this will be the topic of further research. An example of a ®bre that failed through an internal ¯aw during testing at 1200C in vacuum is presented in Fig. 5 with a mirror zone also being present around the ¯aw. From Table 2 it can be seen that the failure mode could not be determined for 13% of ®bres that failed at room temperature. Although these ®bres had no fracture mirrors visible, it was concluded that they probably did fail due to surface ¯aws and may have represented the strongest population Fig. 4. Tensile strength [2] of SiC/SiC-based specimens tested in vacuum and air up to 1380C. Note the non-linear temperature scale. 804 I.J. Davies et al. / Composites Science and Technology 59 (1999) 801±811