H Mei et al. Materials Science and Engineering A 460-461(2007)306-313 Thermal cycle number, N After 50 thermal cycles in the oxidizing atmosphere, resid- 10 al strengths of the C/SiC specimens were measured with a loading rate of 0.001 mm/s at room temperature. The statisti 9808 cal mean strengths of the thermally cycled 2D and braided 3D composite specimens retained 83.2 and.6% of the initial prop- erties. Obviously, related to the 3D braided architecture, the 2D C/SiC composites suffer greater loss in the strength. The differ ent reduction in the retained strengths could be better interpreted by the following observed microstructures, which caused differ s ent damage resistance of the composites against oxidation and thermal shock SiC fitting 33. Microstructural observations SEMmicrographs of the outer coatings and fractured sections 0 1200 2400 3600 4800 6000 of the thermally cycled composites are presented in Figs. 5- Time(s) It can be seen from Figs. 5 and 6 that the irregular netty cracks and the highly oriented wavy cracks could be found on the top Fig. 4. Strain vs time curves of the 2D and braided 3D C/SiC composites surfaces(as defined in Fig. la and b)of the 2D and braided 3D ubjected to thermal cycling and mechanical fatigue, and their fitting curves specimens, whilst the coating cracks on the side surfaces for the two architectures exhibit the same transverse direction(verti resisted and hindered by the transverse fibers. As a result, it is cal to tensile axis). It is strongly believed that thermal cycles more and more difficult for the 2D architecture to be elongated made the coating cracks strictly arranged in the direction per- with increasing transverse compression resistance. By contrast, pendicular to the fibers in composites since the SiC matrix has the 3D braided architecture can easily extend in the longitudi- a greater CtE than the longitudinal fiber. As a result, the netty nal direction since all the fibers are laid at a small angle(22%) cracks and wavy cracks were formed on the top surface coatings along the tensile axis and the porous CVI-Sic matrix has a poor of the 2D architecture and 3D braided architecture composites, transverse compression resistance. As mentioned previously in respectively. Simultaneously, both side surface coatings always Section 3. 1, when the 3D braided architecture preform is loaded generated the transverse cracks. It is more important that the with a longitudinal stress, all the neighboring two fiber bundles arrangement orientation of the cracks have a significant effect will move closer and rotate around their contacts, and the braid- on the oxidation resistance of the composites since the coating ing angle decreases gradually. Consequently, the longitudinal cracks will serve as avenues for the ingress of the environment strain of the braided 3D composites seems not to be limited into the composite. The highly ordered orientations are con- until the fibers are stretched straightly up to final rupture. Fur- sidered to be of advantage to the crack closure for the braided thermore, under the effect of the reloading and unloading in the 3D composites upon heating. Comparatively, the irregular netty tensile axis, the coating regions between the two neighboring crack distributions are of impediment for the crack closure for fiber bundles can be compressed and corrugated, leading to the the 2D composites. As a consequence, because oxygen has an wave-shaped coating cracking at relatively regular spacing(as easier path into the 2D composite, the fibers in the 2D composite are more exposed to oxidation than the fibers in the braided 3D 5.0Kv 101mm x45 SE Fig. 5. Typical micrographs showing the outer coating cracks on the 2D C/SiC composites after 50 thermal cycles: (a)top surface and (b)side surfaceH. Mei et al. / Materials Science and Engineering A 460–461 (2007) 306–313 309 Fig. 4. Strain vs. time curves of the 2D and braided 3D C/SiC composites subjected to thermal cycling and mechanical fatigue, and their fitting curves. resisted and hindered by the transverse fibers. As a result, it is more and more difficult for the 2D architecture to be elongated with increasing transverse compression resistance. By contrast, the 3D braided architecture can easily extend in the longitudi￾nal direction since all the fibers are laid at a small angle (∼22◦) along the tensile axis and the porous CVI-SiC matrix has a poor transverse compression resistance. As mentioned previously in Section 3.1, when the 3D braided architecture preform is loaded with a longitudinal stress, all the neighboring two fiber bundles will move closer and rotate around their contacts, and the braid￾ing angle decreases gradually. Consequently, the longitudinal strain of the braided 3D composites seems not to be limited until the fibers are stretched straightly up to final rupture. Fur￾thermore, under the effect of the reloading and unloading in the tensile axis, the coating regions between the two neighboring fiber bundles can be compressed and corrugated, leading to the wave-shaped coating cracking at relatively regular spacing (as shown later in Fig. 6). After 50 thermal cycles in the oxidizing atmosphere, resid￾ual strengths of the C/SiC specimens were measured with a loading rate of 0.001 mm/s at room temperature. The statisti￾cal mean strengths of the thermally cycled 2D and braided 3D composite specimens retained 83.2 and 91.6% of the initial prop￾erties. Obviously, related to the 3D braided architecture, the 2D C/SiC composites suffer greater loss in the strength. The differ￾ent reduction in the retained strengths could be better interpreted by the following observed microstructures, which caused differ￾ent damage resistance of the composites against oxidation and thermal shock. 3.3. Microstructural observations SEM micrographs of the outer coatings and fractured sections of the thermally cycled composites are presented in Figs. 5–8. It can be seen from Figs. 5 and 6 that the irregular netty cracks and the highly oriented wavy cracks could be found on the top surfaces (as defined in Fig. 1a and b) of the 2D and braided 3D specimens, whilst the coating cracks on the side surfaces for the two architectures exhibit the same transverse direction (verti￾cal to tensile axis). It is strongly believed that thermal cycles made the coating cracks strictly arranged in the direction per￾pendicular to the fibers in composites since the SiC matrix has a greater CTE than the longitudinal fiber. As a result, the netty cracks and wavy cracks were formed on the top surface coatings of the 2D architecture and 3D braided architecture composites, respectively. Simultaneously, both side surface coatings always generated the transverse cracks. It is more important that the arrangement orientation of the cracks have a significant effect on the oxidation resistance of the composites since the coating cracks will serve as avenues for the ingress of the environment into the composite. The highly ordered orientations are con￾sidered to be of advantage to the crack closure for the braided 3D composites upon heating. Comparatively, the irregular netty crack distributions are of impediment for the crack closure for the 2D composites. As a consequence, because oxygen has an easier path into the 2D composite, the fibers in the 2D composite are more exposed to oxidation than the fibers in the braided 3D Fig. 5. Typical micrographs showing the outer coating cracks on the 2D C/SiC composites after 50 thermal cycles: (a) top surface and (b) side surface