w. Yang et aL/Ceramics International 31 (2005)525-531 Pyc tri-layer interphase are 120 and 58/140/50 nn because of the failure of a significant fraction of the fibers respectively, slightly deviated from the designed values Certain downhill load is remained till large displacement (100 and 50/150/50 nm, respectively ) The total amount of depending mainly on the fraction of the remained intact fiber PyC in the two composites is almost the same, considering bundles in the specimen. These features are in accordance the estimated resolution (10 nm)of the measurement with the matrix cracking and fracture surface observations. and the standard deviations of the thickness of each layer For both composites, transverse matrix cracks initiated at the (Table 1). Table I and Fig. 2 show a successful deposition of tensile surfaces of the specimens and propagated towards the either single PyC layer or thin PyC/SiC/PyC multilayers in compression surfaces with multiple deflections/deviations the composites by the Cvi process with quite fine thickness by the fiber bundles, as shown typically in the insert d space homogeneity control. The interfaces between the micrograph in Fig 3. The fracture surfaces of both com interphase layers(Fig. 2)in composite TSA-ML are fairly posites showed interfacial debonding and sound fiber rough, apparently because of the grain growth in pullouts fracture behaviors (Fig. 4(a)), owing to the de- layer as well as the rough surface of the Tyranno-SA fiber It posited PyC or PyC/SiC/PyC interlayers. Fracture surface is shown [20] that the morphology/surface roughness of examinations revealed that the matrix cracks generally went CVD-SiC is closely dependent on the CVD conditions hrough the interlayers and were deflected at the very fiber Smoother SiC layer in present composite might be produced surfaces, independent on whether they were single PyC or by optimization of the CVI conditions PyC/SiC/PyC multilayers, as shown in high magnification SEM micrograph in Fig. 4(b), with bare fiber pullouts 3. 2. Mechanical properties and fracture behaviors Clearly, weaker bonding exists between the fiber surface and the first PyC-layer deposited from methane Representative load-displacement curves of the two The average PLSs and UFSs of the two composites are composites are shown in Fig 3. Both composites exhibited summarized in Table 2. The average PLS and UFS of typical fracture behaviors for SiC/SiC upon bending composite TSA-ML are 350+ 53 and 520+ 20 MPa loading:(1)an initial linear region, reflecting the elastic respectively, which are slightly lower than those of TSA-SL response of the composites, followed by(2) a non-linear (370+ 30 and 570 26 MPa, respectively). As shown in domain of deformation until the load maximum, due mainly Table 1, composite TSA-ML possesses a slightly lowe to the matrix cracking, interfacial debonding, and fiber density than TSA-SL, which might have negative effects on sliding and pullouts, and individual fiber failures, (3)quick the strength. A statistic study (21]on the flexural strength of drop of the load after it reached its maximum, perhaps a CVI-Tyranno-SA/SiC composite showed that the flexural Load 160 Imm Transverse ISA-MI TSA-SL 0.000.050.100.150.200.250.300.35040 Fig. 3. Typical load-displacement curves of the composites.PyC tri-layer interphase are 120 and 58/140/50 nm, respectively, slightly deviated from the designed values (100 and 50/150/50 nm, respectively). The total amount of PyC in the two composites is almost the same, considering the estimated resolution (
10 nm) of the measurement and the standard deviations of the thickness of each layer (Table 1). Table 1 and Fig. 2 show a successful deposition of either single PyC layer or thin PyC/SiC/PyC multilayers in the composites by the CVI process with quite fine thickness and space homogeneity control. The interfaces between the interphase layers (Fig. 2) in composite TSA-ML are fairly rough, apparently because of the grain growth in the SiC layer as well as the rough surface of the Tyranno-SA fiber. It is shown [20] that the morphology/surface roughness of CVD-SiC is closely dependent on the CVD conditions. Smoother SiC layer in present composite might be produced by optimization of the CVI conditions. 3.2. Mechanical properties and fracture behaviors Representative load–displacement curves of the two composites are shown in Fig. 3. Both composites exhibited typical fracture behaviors for SiC/SiC upon bending loading: (1) an initial linear region, reflecting the elastic response of the composites, followed by (2) a non-linear domain of deformation until the load maximum, due mainly to the matrix cracking, interfacial debonding, and fiber sliding and pullouts, and individual fiber failures, (3) quick drop of the load after it reached its maximum, perhaps because of the failure of a significant fraction of the fibers. Certain downhill load is remained till large displacement depending mainly on the fraction of the remained intact fiber bundles in the specimen. These features are in accordance with the matrix cracking and fracture surface observations. For both composites, transverse matrix cracks initiated at the tensile surfaces of the specimens and propagated towards the compression surfaces with multiple deflections/deviations by the fiber bundles, as shown typically in the insert micrograph in Fig. 3. The fracture surfaces of both com￾posites showed interfacial debonding and sound fiber pullouts fracture behaviors (Fig. 4(a)), owing to the de￾posited PyC or PyC/SiC/PyC interlayers. Fracture surface examinations revealed that the matrix cracks generally went through the interlayers and were deflected at the very fiber surfaces, independent on whether they were single PyC or PyC/SiC/PyC multilayers, as shown in high magnification SEM micrograph in Fig. 4(b), with bare fiber pullouts. Clearly, weaker bonding exists between the fiber surface and the first PyC-layer deposited from methane. The average PLSs and UFSs of the two composites are summarized in Table 2. The average PLS and UFS of composite TSA-ML are 350  53 and 520  20 MPa, respectively, which are slightly lower than those of TSA-SL (370  30 and 570  26 MPa, respectively). As shown in Table 1, composite TSA-ML possesses a slightly lower density than TSA-SL, which might have negative effects on the strength. A statistic study [21] on the flexural strength of a CVI-Tyranno-SA/SiC composite showed that the flexural 528 W. Yang et al. / Ceramics International 31 (2005) 525–531 Fig. 3. Typical load–displacement curves of the composites