T. Taguchi et al. I Journal of Nuclear Materials 329-333(2004)572-576 The normalized flexural and tensile strengths of the siC/ Sic composite with SiC/C multi-layer were approxi- mately 10%o higher than those of the composite with single C interphase The SEM micrographs of fracture surfaces of Sic/ Sic composite with SiC/C multi-layer after flexural and tensile tests are shown in Fig. 4. The cylindrical steps around the fiber were observed after flexural and tensile tests and several crack deflections occurred within SiC/C multi-layer interphase. Furthermore, pull-out of fiber bundles occurred. These results indicate that the second to fourth Sic layers and fifth to sixth Sic layers oper 00615K035,的 5 um ated efficiently to improve the fracture behavior. Since Fig. 1. Typical cross-sectional SEM microphotograph of Sic both multiple pull-out of fibers and fiber bundles oc SiC composite with SiC/C multi-layer interphase. curred, higher fracture energy was absorbed compared to single C layer. The grain size of inner Sic layer was The tested composites had different porosities and fiber volume fractions. Araki et al. [5]reported that the 700 flexural strength increased with decrease aComposite with single C layer composite. Since the load is mainly maintained by un a composite with SiC/C multiLayer. fractured fibers and friction between fractured fiber and interphase above the proportional limit stress, the flex- ural and tensile strengths depend on the porosity and a400 tensile strengths were, therefore, normalized by the fol lowing equation c200 Normalized strength original strength Vr 100 where v and vr are fiber volume fraction and the 0 average fiber volume fraction of the composites he normalized flexural and tensile strengths of the Fig. 3. Normalized flexural and tensile strengths of the Sic/Sic SiC/SiC composite with SiC/C multi-layer and the composite with SiC/C multi-layer and the composite with single composite with single C interphase are shown in Fig 3 500nm Fig. 2. Typical cross-sectional TEM microphotographs of Sic/Sic composite with SiC/C multi-layer interphase and the electron diffraction patterns for(A)SiC fiber, (B)second Sic layer and (C)Sic matrix.The tested composites had different porosities and fiber volume fractions. Araki et al. [15] reported that the flexural strength increased with decreased porosity of composite. Since the load is mainly maintained by un￾fractured fibers and friction between fractured fiber and interphase above the proportional limit stress, the flex￾ural and tensile strengths depend on the porosity and fiber volume fraction of composite. The flexural and tensile strengths were, therefore, normalized by the fol￾lowingequation: Normalized strength ¼ original strength 1 porosity Vf V f ; where Vf and V f are fiber volume fraction and the average fiber volume fraction of the composites. The normalized flexural and tensile strengths of the SiC/SiC composite with SiC/C multi-layer and the composite with single C interphase are shown in Fig. 3. The normalized flexural and tensile strengths of the SiC/ SiC composite with SiC/C multi-layer were approxi￾mately 10% higher than those of the composite with single C interphase. The SEM micrographs of fracture surfaces of SiC/ SiC composite with SiC/C multi-layer after flexural and tensile tests are shown in Fig. 4. The cylindrical steps around the fiber were observed after flexural and tensile tests and several crack deflections occurred within SiC/C multi-layer interphase. Furthermore, pull-out of fiber bundles occurred. These results indicate that the second to fourth SiC layers and fifth to sixth SiC layers oper￾ated efficiently to improve the fracture behavior. Since both multiple pull-out of fibers and fiber bundles oc￾curred, higher fracture energy was absorbed compared to single C layer. The grain size of inner SiC layer was Fig. 1. Typical cross-sectional SEM microphotograph of SiC/ SiC composite with SiC/C multi-layer interphase. Fig. 2. Typical cross-sectional TEM microphotographs of SiC/SiC composite with SiC/C multi-layer interphase and the electron diffraction patterns for (A) SiC fiber, (B) second SiC layer and (C) SiC matrix. Fig. 3. Normalized flexural and tensile strengths of the SiC/SiC composite with SiC/C multi-layer and the composite with single C interphase. 574 T. Taguchi et al. / Journal of Nuclear Materials 329–333 (2004) 572–576