w. Yang et al./Ceramics International 31(2005)525-531 Multilayer Data from Ref [14] Popped fiber SA-MI Fiber SA-SL ISS=340MP surface 300400500600 Interfacial shear strength/ MPa Matrix Fig. 7. Effects of ISS on PLS of plain-woven Tyranno-SA/SiC composites. Onm Fig. 6. The pushed and popped fiber in composite TSA-ML after pushou effects of Iss on PLS (Fig. 7)and UFS, this study indicates test(the multilayers remained in the matrix) that the incorporation of Sic sub-layer in multilayered Tyranno-SA/SiC composites, such as fiber/PyC Sic+ Py C/matrix, may not cause a decrease of the mechanical layer next to the fibers in TSA-ML affected the ISs of the properties, but depend on the value of ISs, which is largely material far larger than that of the PyC sub-layer next to the determined by the thickness of the first PyC sub-layer. matrix. Similar experimental observation, the interfacial Similarly, Bertrand et al. [24] has found that the mechanical bonding strength seemed to be related to the thickness of the behaviors of unidirectional reinforced Hi-Nicalon/Pyc/Sic first carbon layer(nearest to the fibers) of Nicalon fiber and Hi-Nicalon/(PyC-SiC),SiC composites upon tensile reinforced CvI-SiC/Sic with(C-SiC) inter loading did not differ significantly except when the thickness layers, was reported by Rebillat et al. [7] of the Py C-layer became very low. 3.4. Effect of SiC sub-layer on flexural strength 4. Conclusions Both theoretical and experimental studies [14, 22, 23 showed that ISS is a critical factor on determining the Two plain-woven Tyranno-SA/SiC composites with flexural strength of SiC/SiC composites. SiC/SiC compo- designed interlayers of 100 nm PyC and 50 nm Pyc+ sites with different ISSs generally show different flexural 150 nm SiC 50 nm PyC, respectively, were fabricated. strengths. However, in this study, composite TSA-ML High-magnification S.E. M. examinations of the interlayer showed much higher Iss but yielded a similar level of structures and the thickness of each layer confirmed a flexural strength(PLS and UFS) to TSA-SL. This result successful deposition of the thin single and multilayers in the seems contrary to already established knowledge. It was composites with quite well thickness and uniformity control found [14] that for CVI-Tyranno-sa (plain-woven)/Sic toward the designed value by the CVI process composites, there exists an optimum Iss regarding the Composite with PyC SiC PyC interlayers showed flexural strength(PLS and UFS). The strength increases with much larger ISS compared with that of composite with the increasing of the ISS up to the optimum value, beyond single PyC interlayer. The ISS of the multilayered Tyranno- which the strength decreases gradually. A graphic illustra- SA/SiC composite is predominantly affected by the tion of the flexural strength of present composites against thickness of the first Pyc sub-layer on the fiber, rather than their ISSs might be able to get an easier understanding of the total and/or the second PyC sub-layer. The interfacial present results. Fig. 7 relates the PLSs of several composites debonding and fiber pushouts occurred at the fiber/first PyC (the present two composites and those from ref. [14))to their layer interface upon single fiber pushout loading ISSs. All the PLSs in Fig. 7 were normalized to composite The incorporation of stiff SiC sub-layer in multilayers did density to minimize the effect of composite density. The ot cause noticeable change of the flexural strength of the ISSs of the present two composites fall at either side of the present composite. However, this may not be always true optimum ISs (340 MPa) and happened to obtain almost This study indicates that SiC sub-layer(s) in multilayered the same value of PLS, owing to the near symmetrical shape SiC/SiC composites may not cause a decrease of the of the curve around 340 MPa of Iss. Similar situation mechanical strength, but depend on the value of Iss, which occurred when relating the UFSS to ISS. Combining the is largely determined by the thickness of the first PyC layer effects of Pyc and Sic layers on the ISS(Fig. 5), and thelayer next to the fibers in TSA-ML affected the ISS of the material far larger than that of the PyC sub-layer next to the matrix. Similar experimental observation, the interfacial bonding strength seemed to be related to the thickness of the first carbon layer (nearest to the fibers) of Nicalon fiber reinforced CVI-SiC/SiC composites with (C-SiC)n inter￾layers, was reported by Rebillat et al. [7]. 3.4. Effect of SiC sub-layer on flexural strength Both theoretical and experimental studies [14,22,23] showed that ISS is a critical factor on determining the flexural strength of SiC/SiC composites. SiC/SiC compo￾sites with different ISSs generally show different flexural strengths. However, in this study, composite TSA-ML showed much higher ISS but yielded a similar level of flexural strength (PLS and UFS) to TSA-SL. This result seems contrary to already established knowledge. It was found [14] that for CVI-Tyranno-SA (plain-woven)/SiC composites, there exists an optimum ISS regarding the flexural strength (PLS and UFS). The strength increases with the increasing of the ISS up to the optimum value, beyond which the strength decreases gradually. A graphic illustra￾tion of the flexural strength of present composites against their ISSs might be able to get an easier understanding of present results. Fig. 7 relates the PLSs of several composites (the present two composites and those from ref. [14]) to their ISSs. All the PLSs in Fig. 7 were normalized to composite density to minimize the effect of composite density. The ISSs of the present two composites fall at either side of the optimum ISS (
340 MPa) and happened to obtain almost the same value of PLS, owing to the near symmetrical shape of the curve around 340 MPa of ISS. Similar situation occurred when relating the UFSs to ISS. Combining the effects of PyC and SiC layers on the ISS (Fig. 5), and the effects of ISS on PLS (Fig. 7) and UFS, this study indicates that the incorporation of SiC sub-layer in multilayered Tyranno-SA/SiC composites, such as fiber/PyC + SiC + PyC/matrix, may not cause a decrease of the mechanical properties, but depend on the value of ISS, which is largely determined by the thickness of the first PyC sub-layer. Similarly, Bertrand et al. [24] has found that the mechanical behaviors of unidirectional reinforced Hi-Nicalon/PyC/SiC and Hi-Nicalon/(PyC-SiC)n/SiC composites upon tensile loading did not differ significantly except when the thickness of the PyC-layer became very low. 4. Conclusions Two plain-woven Tyranno-SA/SiC composites with designed interlayers of 100 nm PyC and 50 nm PyC + 150 nm SiC + 50 nm PyC, respectively, were fabricated. High-magnification S.E.M. examinations of the interlayer structures and the thickness of each layer confirmed a successful deposition of the thin single and multilayers in the composites with quite well thickness and uniformity control toward the designed value by the CVI process. Composite with PyC + SiC + PyC interlayers showed much larger ISS compared with that of composite with single PyC interlayer. The ISS of the multilayered Tyranno￾SA/SiC composite is predominantly affected by the thickness of the first PyC sub-layer on the fiber, rather than the total and/or the second PyC sub-layer. The interfacial debonding and fiber pushouts occurred at the fiber/first PyC layer interface upon single fiber pushout loading. The incorporation of stiff SiC sub-layer in multilayers did not cause noticeable change of the flexural strength of the present composite. However, this may not be always true. This study indicates that SiC sub-layer(s) in multilayered SiC/SiC composites may not cause a decrease of the mechanical strength, but depend on the value of ISS, which is largely determined by the thickness of the first PyC layer on the fibers. 530 W. Yang et al. / Ceramics International 31 (2005) 525–531 Fig. 6. The pushed and popped fiber in composite TSA-ML after pushout test (the multilayers remained in the matrix). Fig. 7. Effects of ISS on PLS of plain-woven Tyranno-SA/SiC composites