w. Yang et al. Materials Science and Engineering 4345(2003)28-35 Inter-fabric layers delaminating the same CVI process with similar fiber volume fraction, T-C50 composite density and interlayers are considered to be the main reasons causing the differences of the e lexar properties. The highest PLS and UFS, 426 and 606 MPa, respectively, were obtained comi C100, which was coated with 100 nm PyC interlayer This composite possesses the highest density of the interlayered composit I mm Tensile surface 4. Discussion Fig. 4. SEM image of cross section of osite T-C50, showing inter-fabric layer debonding. 4. 1. Effects of the fiber The Tyranno-SA/SiC composites exhibited high PLS but relatively small displacement at load maximums Fig. 5 and Table 3). High PLS was a direct benefit from the advanced Tyranno-SA fiber. The tensile modulus the Tyranno-SA fiber is 410 Gpa (Table 1),versus ----T-NL lower value of 200 and x 280 GPa for the nicalon and Hi-Nicalon fibers, respectively [18]. Stronger fiber causes less load share by the matrix in a SiC/SiC composite and therefore, results in a larger PLS [13] Table 3 shows quite large ISS, from 195 to 414 MPa, for the interlayered composites. Large IsS generally makes 0.000050.100.150.20 250300.35 it more difficult for the interfacial debonding and fiber D isplacement/mm liding to occur. This will reduce the abilities for the interfaces and the fibers to deviate and bridge the Fig. 5. Representative load-displacement curves of the Tyranno-SA/ already initiated matrix cracks upon bending, and Sic composites. hence, cause quicker failure of the composites at relatively small displacement. Large ISS is a result of 3.3. Flexural properties he pure SiC surface chemistry and rough surface of the Tyranno-SA fiber [17]. Therefore, weaker interfacial The flexural properties that were extracted from the bonding(thicker PyC interlayer or more suitable alter- load-displacement curves according to ASTMC 1341- nating compliant interlayers)may be necessary for the 97[19] are summarized in Table 3. The proportional Tyranno-SA/SiC composites to achieve further im- limit stress(PLS) was the stress corresponding to 0.019 proved fracture tolerance offset strain. The ultimate flexural strength(UFS)was calculated by the simple elastic beam theory [20]. The 4.2. Effects of density and interlayer interfacial shear strengths(ISS)were obtained from Ref Table 3 exhibits high PLS but relatively small structures dependence of the flexuralper and interlayer displacements at load maximums for the Tyranno-SA/ highest PLS and UFS were obtained with composite T Sic composites. Since the composites were fabricated by C100, which had the highest densities of 2.63 Mg m ecimen I.D. Flexural modulus(GPa) PLS (MPa) UFS(MPa) Displacement at load maximum(mm) ISS (MPa) T.NL 0.07(0.02) >633 T-C50 135(13) 0.13(0.05) 1(140) TC100 57(1 426(2) 0.18(00 42(97) TC200 33918) 549(58) 0.15(0.01) 195(51 T-SiC/C80 148(2) 339(45) 4l4(117) T-SiC/C150 0.14(0.02)3.3. Flexural properties The flexural properties that were extracted from the load/displacement curves according to ASTM C 1341- 97 [19] are summarized in Table 3. The proportional limit stress (PLS) was the stress corresponding to 0.01% offset strain. The ultimate flexural strength (UFS) was calculated by the simple elastic beam theory [20]. The interfacial shear strengths (ISS) were obtained from Ref. [17]. Table 3 exhibits high PLS but relatively small displacements at load maximums for the Tyranno-SA/ SiC composites. Since the composites were fabricated by the same CVI process with similar fiber volume fraction, composite density and interlayers are considered to be the main reasons causing the differences of the flexural properties. The highest PLS and UFS, 426 and 606 MPa, respectively, were obtained with composite T￾C100, which was coated with 100 nm PyC interlayer. This composite possesses the highest density of the interlayered composites. 4. Discussion 4.1. Effects of the fiber The Tyranno-SA/SiC composites exhibited high PLS but relatively small displacement at load maximums (Fig. 5 and Table 3). High PLS was a direct benefit from the advanced Tyranno-SA fiber. The tensile modulus of the Tyranno-SA fiber is
/410 Gpa (Table 1), versus lower value of
/200 and
/280 GPa for the Nicalon and Hi-Nicalon fibers, respectively [18]. Stronger fiber causes less load share by the matrix in a SiC/SiC composite and therefore, results in a larger PLS [13]. Table 3 shows quite large ISS, from 195 to 414 MPa, for the interlayered composites. Large ISS generally makes it more difficult for the interfacial debonding and fiber sliding to occur. This will reduce the abilities for the interfaces and the fibers to deviate and bridge the already initiated matrix cracks upon bending, and hence, cause quicker failure of the composites at relatively small displacement. Large ISS is a result of the pure SiC surface chemistry and rough surface of the Tyranno-SA fiber [17]. Therefore, weaker interfacial bonding (thicker PyC interlayer or more suitable alter￾nating compliant interlayers) may be necessary for the Tyranno-SA/SiC composites to achieve further im￾proved fracture tolerance. 4.2. Effects of density and interlayer Table 3 shows a composite densities and interlayer structures dependence of the flexural performance. The highest PLS and UFS were obtained with composite T￾C100, which had the highest densities of 2.63 Mg m3 Fig. 4. SEM image of cross section of composite T-C50, showing inter-fabric layer debonding. Fig. 5. Representative load-displacement curves of the Tyranno-SA/ SiC composites. Table 3 Flexural properties and ISS of the Tyranno-SA/SiC composites Specimen I.D. Flexural modulus (GPa) PLS (MPa) UFS (MPa) Displacement at load maximum (mm) ISS (MPa) T-NL 156(13) 275(48) 281(60) 0.07(0.02) 633 T-C50 135(13) 257(5) 410(92) 0.13(0.05) 331(140) T-C100 157(11) 426(32) 606(28) 0.18(0.02) 342(97) T-C200 140(10) 339(18) 549(58) 0.15(0.01) 195(51) T-SiC/C80 148(2) 339(45) 440(1) 0.12(0.03) 414(117) T-SiC/C150 141(2) 370(43) 495(85) 0.14(0.02) 284(96) 32 W. Yang et al. / Materials Science and Engineering A345 (2003) 28 /35