W. Yang et al. /Ceramics International 31(2005)525-531 (b) Debond at fiber/PyC layer interface (a) fracture surfac Fiber surface Multilayers 1Our 500nm Fig 4. SEM images of fracture surface of composite TSA-ML strength increased with higher specimen density. Here, by composites, as shown in Fig. 5( the composites in Fig. 5 were imply normalizing the strengths of present composites fabricated using the same Cvi process as in this study). Fig densities, composites TSA-ML and sl showed similar level 5 shows that the Iss of TSA-sl falls well into the trend of of both pls and UFS. as shown in Table 2. he PyC thickness dependence of the Iss, while the ISs of The obtained ISSs of the two composites are also given in TSA-ML is obviously higher than those with single but Table 2. Composite TSA-ML exhibited an average ISs of similar total amount of PyC layer. When relating the Iss to 430+ 165 MPa, much larger than that of TSA -SL, which the first PyC sub-layer(5& nm), the Iss showed much better 300±72MPa fit to the Pyc-lss trend, as indicated by the dashed circle in Fig. 5. SEM examinations after the single fiber pushout tests 3.3. Effects of SiC sub-layer on /SS revealed that the interfacial debondings and fiber pushouts predominantly occurred at the very fiber surfaces for both Generally, for given reinforcement fibers and process composites, as shown in Fig. 6 for TSA-ML, which shows conditions for interlayer(s)and matrix, the ISSs of Sic/Sic that the fiber was pushed and popped while the PyC + SiC+ composites are mainly determined by the amount of PyC tri-layers remained within the matrix. Such interfacial compliant(PyC)interlayer(s)[4-6, 9]. The total PyC layer debonding and fiber pushout behaviors indicate that the thickness in TSA-ML is 110 nm (Table 1), which is interfaces between fiber and PyC layer(for TSA-SL)and slightly less than that in TSA-SL (120 nm). Considering the first PyC sub-layer(for TSA-ML) are the weakest link in the resolution(10 nm)of the thickness measurements and the composites under single fiber pushout loading. From these standard deviations of the interlayers, the difference in total observations, it is reasonable to assume that the PyC sub- PyC layer thickness between the two composites might be negligible. Both composites and interlayers were fabricated using the same CVI processes. Therefore, it is obvious that the increased Iss in TSA-ML is because of the incorporation of the sandwiched SiC sub-layer by the two PyC sub-layers TSA-MI This becomes more evident when the Iss is graphically related to the PyC layer thickness, together with previous sults [19] on investigating the effects of Pyc layer TSA hickness on the IsSs of several Tyranno-SA/PyC/Sic Table 2 o Data from Ref [191 The mechanical properties of the composites Composite ISS(MPa) PLS (MPa) UFS(MPa) PLS TSA-SL300±72370±30570±26140±11210±9 PyC interlayer thickness/nm TSAML430±165350±53520±20140±20200±7 Fig. 5. PyC layer thickness dependence of the ISS of plain-woven Tyrann normalized by composite densities (MPa/mg/m) SA/SIC composites.strength increased with higher specimen density. Here, by simply normalizing the strengths of present composites to densities, composites TSA-ML and SL showed similar level of both PLS and UFS, as shown in Table 2. The obtained ISSs of the two composites are also given in Table 2. Composite TSA-ML exhibited an average ISS of 430  165 MPa, much larger than that of TSA-SL, which is 300  72 MPa. 3.3. Effects of SiC sub-layer on ISS Generally, for given reinforcement fibers and process conditions for interlayer(s) and matrix, the ISSs of SiC/SiC composites are mainly determined by the amount of compliant (PyC) interlayer(s) [4–6,9]. The total PyC layer thickness in TSA-ML is
110 nm (Table 1), which is slightly less than that in TSA-SL (120 nm). Considering the resolution (
10 nm) of the thickness measurements and the standard deviations of the interlayers, the difference in total PyC layer thickness between the two composites might be negligible. Both composites and interlayers were fabricated using the same CVI processes. Therefore, it is obvious that the increased ISS in TSA-ML is because of the incorporation of the sandwiched SiC sub-layer by the two PyC sub-layers. This becomes more evident when the ISS is graphically related to the PyC layer thickness, together with previous results [19] on investigating the effects of PyC layer thickness on the ISSs of several Tyranno-SA/PyC/SiC composites, as shown in Fig. 5 (the composites in Fig. 5 were fabricated using the same CVI process as in this study). Fig. 5 shows that the ISS of TSA-SL falls well into the trend of the PyC thickness dependence of the ISS, while the ISS of TSA-ML is obviously higher than those with single but similar total amount of PyC layer. When relating the ISS to the first PyC sub-layer (58 nm), the ISS showed much better fit to the PyC-ISS trend, as indicated by the dashed circle in Fig. 5. SEM examinations after the single fiber pushout tests revealed that the interfacial debondings and fiber pushouts predominantly occurred at the very fiber surfaces for both composites, as shown in Fig. 6 for TSA-ML, which shows that the fiber was pushed and popped while the PyC + SiC + PyC tri-layers remained within the matrix. Such interfacial debonding and fiber pushout behaviors indicate that the interfaces between fiber and PyC layer (for TSA-SL) and first PyC sub-layer (for TSA-ML) are the weakest link in the composites under single fiber pushout loading. From these observations, it is reasonable to assume that the PyC sub￾W. Yang et al. / Ceramics International 31 (2005) 525–531 529 Fig. 4. SEM images of fracture surface of composite TSA-ML. Table 2 The mechanical properties of the composites Composite ISS (MPa) PLS (MPa) UFS (MPa) PLS UFSa TSA-SL 300  72 370  30 570  26 140  11 210  9 TSA-ML 430  165 350  53 520  20 140  20 200  7 a Normalized by composite densities (MPa/mg/m3 ). Fig. 5. PyC layer thickness dependence of the ISS of plain-woven Tyranno￾SA/SiC composites