w. Yang et al. Materials Science and Engineering 4345(2003)28-35 among the interlayered composites. Composite T-C50 yielded much lower PLS and UFS, 257 and 410 MPa, respectively, which are about 170 and 200 MPa less than those of T-C100. The density of this composite is 2.41 Mg m. The low density resulted from insufficient CVI-matrix densification and causes a weak inter-fabl layers bonding. Therefore, the inter-laminar debonding occurred(Fig. 4) due to the inter-fabric layers shear stress upon bending load, and caused the shear failure behavior of the composite with low ultimate strengt Similar situation occurred with composite T-Sic/C150 1014· Tyranno-SA/siC▲Hi- Nicalon/siC of which the density is 2.37 Mg m. The flexural modulus also demonstrated density-dependence as shown in Fig. 6. High flexural modulus was obtained Py C layer thickness/nm by the composites with dense matrix (T-NL and T Fig. 7. PyC layer thickness dependence of the UFS 9. As mentioned before, SiC/SiC composites are very or thicker(T-C200) PyC layer. A 150 nm SiC layer was attractive candidates for applications under severe environments. The current key issue is the further deposited on the fibers prior to the deposition of the 150 improvement of the mechanical properties. Develop nm PyC layer in this composite. Another bi-layered ment of appropriate interlayer(s)with desired materials, tendency in Fig. 7. However, the effects of the Sic layer interlayer structures and layer thickness is a main on the composite strength remains unclear because that solution. For the present Tyranno-SA/SIC composites, although Fig. 7 relates the UFS to the PyC layer it was found [17] that the interfacial properties were thickness. it is, in fact, a blend effect of the interlayer, significantly affected by the thickness of the PyC layers density, and fracture mechanism, etc. The low density Fig. 7 relates the UFS of the Tyranno-SA/SiC compo- and inter-laminar shear failure of T-SiC/C150 are sites to the thickness of the PyC layers. The results of 2D considered to be the main reasons causing the low plain-woven CVI-Hi-Nicalon/SiC composites [ll] are ultimate strength. Further studies are necessary to get a also presented for comparison. The UFS of the Tyr- clear understanding of the effects the SiC layer. Any anno-SA/SiC composites increases from 280 to 600 how, Fig. 7 shows a clear trend of the effects of the PyC MPa with increasing the Pyc layer to 100 nm, then layer thickness on the strength of the composites keeps at a similar level up to 200 nm considering the li-Nicalon/SiC composites, the Tyranno-SA/SiC com- 4.3. Effects of lss posites exhibit a slightly less sensitivity to the Pyc layer thickness For the Hi-Nicalon/SiC composites, the high The PLs of Sic/SiC composites is significantly est strength is achieved by the composite with 150 nm affected by the ISS. Inghels and Lamon [21] developed gle PyC layer while the Tyranno-SA/SiC composite a theoretical mode to predict the strength of unidirec- with same thickness of PyC (T-Sic/C150) layer yielded a tional Sic/Sic composites upon flexural loading. The lower strength than those with either thinner(T-C100) PLS was given by 2,mElfa Ec+ Er eVE 4E BEC erve where ,m is the surface energy of CVD-SiC, which was 25 Jm--[21]. T; is the interfacial shear Em, Ef, Vm, and Ve are the Youngs modulus and volume fractions of the matrix and the fiber. Ec is the composite modulus determined from the law of mixture [21] re is the diameter of the fiber The mode was developed based on a ID model cell Composite density/Mg/m3 while the reinforcements in the present composites are 2D plain-woven fabrics. Here for simplicity [22, 23], the Fig.6. Density dependence of the flexural modulus of the composites. 0 fiber bundles are regarded as the reinforcement whileamong the interlayered composites. Composite T-C50 yielded much lower PLS and UFS, 257 and 410 MPa, respectively, which are about 170 and 200 MPa less than those of T-C100. The density of this composite is 2.41 Mg m3 . The low density resulted from insufficient CVI-matrix densification and causes a weak inter-fabric layers bonding. Therefore, the inter-laminar debonding occurred (Fig. 4) due to the inter-fabric layers shear stress upon bending load, and caused the shear failure behavior of the composite with low ultimate strength. Similar situation occurred with composite T-SiC/C150 of which the density is 2.37 Mg m3 . The flexural modulus also demonstrated density-dependence as shown in Fig. 6. High flexural modulus was obtained by the composites with dense matrix (T-NL and T￾C100). As mentioned before, SiC/SiC composites are very attractive candidates for applications under severe environments. The current key issue is the further improvement of the mechanical properties. Develop￾ment of appropriate interlayer(s) with desired materials, interlayer structures and layer thickness is a main solution. For the present Tyranno-SA/SiC composites, it was found [17] that the interfacial properties were significantly affected by the thickness of the PyC layers. Fig. 7 relates the UFS of the Tyranno-SA/SiC compo￾sites to the thickness of the PyC layers. The results of 2D plain-woven CVI-Hi-Nicalon/SiC composites [11] are also presented for comparison. The UFS of the Tyr￾anno-SA/SiC composites increases from
/280 to
/600 MPa with increasing the PyC layer to 100 nm, then keeps at a similar level up to 200 nm considering the relatively large error bars. Compared with that of the Hi-Nicalon/SiC composites, the Tyranno-SA/SiC com￾posites exhibit a slightly less sensitivity to the PyC layer thickness. For the Hi-Nicalon/SiC composites, the high￾est strength is achieved by the composite with 150 nm single PyC layer while the Tyranno-SA/SiC composite with same thickness of PyC (T-SiC/C150) layer yielded a lower strength than those with either thinner (T-C100) or thicker (T-C200) PyC layer. A 150 nm SiC layer was deposited on the fibers prior to the deposition of the 150 nm PyC layer in this composite. Another bi-layered composite, T-SiC/C80, also yielded a strength below the tendency in Fig. 7. However, the effects of the SiC layer on the composite strength remains unclear because that although Fig. 7 relates the UFS to the PyC layer thickness, it is, in fact, a blend effect of the interlayer, density, and fracture mechanism, etc. The low density and inter-laminar shear failure of T-SiC/C150 are considered to be the main reasons causing the low ultimate strength. Further studies are necessary to get a clear understanding of the effects the SiC layer. Any￾how, Fig. 7 shows a clear trend of the effects of the PyC layer thickness on the strength of the composites. 4.3. Effects of ISS The PLS of SiC/SiC composites is significantly affected by the ISS. Inghels and Lamon [21] developed a theoretical mode to predict the strength of unidirec￾tional SiC/SiC composites upon flexural loading. The PLS was given by, sPLSEc
12gmEfV2 f ti EcE2 mVmrf 1=3
Ec EfVf 2EfVf

4EfVf 3Ec EfVf 1=3 ; (1) where gm is the surface energy of CVD-SiC, which was given as 25 J m2 [21]. ti is the interfacial shear strength. Em, Ef, Vm, and Vf are the Young’s modulus and volume fractions of the matrix and the fiber. Ec is the composite modulus determined from the law of mixture [21]. rf is the diameter of the fiber. The mode was developed based on a 1D model cell while the reinforcements in the present composites are 2D plain-woven fabrics. Here for simplicity [22,23], the Fig. 6. Density dependence of the flexural modulus of the composites. 08 fiber bundles are regarded as the reinforcement while Fig. 7. PyC layer thickness dependence of the UFS. W. Yang et al. / Materials Science and Engineering A345 (2003) 28 /35 33