w, Yang et al. /Ceramics International 31(2005)47-5 51 is because of the moderate ISS, which is owing to the spon- alternative source material (in stead of MTS) for the fab- taneous formed graphite interlayer rication of CVI-SiC/SiC composites with advantage of Inghels and Lamon [16] have developed a theoretical ap- spontaneously formed graphite interlayer (the additional proach to predict the strength of unidirectional SiC/SiC com- interlayer deposition process might become unnecessary in posites upon flexural loading from the properties of the fiber, this case). However, further studied on the detailed mecha matrix and the interface. Yang et al. [8] applied the theory nism of the spontaneously formation of the graphite inter- to calculate the Pls of plain-woven Hi-Nicalon fabric cloth layer is necessary for the controlling the thickness of the reinforced CVI-SiC/SiC composites with a simple assump- tion that the 90 bundles could be regarded as'matrix', and an empirical coefficient, K(=0.66), defined from a compar- ison between the model prediction and experimental results Acknowledgements for their composite syster PLS= KOpLS This work is supported by the Crest, Japan Science and Technology Corporation and conducted at the National where apis is the theoretical prediction giving by Institute for Materials Science. A part of this study was financially supported by the Budget for Nuclear Research ps =Ec 12ym Er vISS of the Ministry of Education, Culture, Sports, Science and EE品(1-v-V)r Technology, based on the screening and counseling by the commissie Ec+ Efv 4ErV\1/3 2Et v 3Ec+ Ef v where ym is the surface energy of CVD-SiC, which was References given as 25 J/m-[16]. Em and Er are the Young's modulus Plea [GN)m加5dCm如m2{m and vn are the volume fractions of the o bundle fibers and porosity, which are 20 and 17% for the present composite 2] T. Noda, H. Araki, F. Abe, M. Okada, Microstructure and mechanical properties of FCVI carbon fiber/SiC composites, J. Nucl. Mater. respectively. Ec is the composite modulus determined from the law of mixture 3 D. Brewer, HSR/EPM Ec= Erve+ Em(1-Vp-ve) ater.sci.Eng.A261(1999284-291 4K. M. Prewo, J.J. Brennan, Silicon carbide fiber Using Eqs. (3H5), the PLS of present composite was cal- culated to be 389 MPa. The calculated value is slightly lower ness,J. Mater.Sci.17(1982)2371-2383. 5]TM. Besmann, D P Stinton, E.R. Kupp, S Shanmugham, than the experimental observation(Table 1). The model, as In ceramIc composites, well as the empirical coefficient K=0.66, was derived symp.Poc.48(197)147-159 from a family of SiC/SiC composites with plain-woven re- [6]RA. Lowden, Fiber coatings and the mechanical properties of a inforcement. further modification of the model is neces- ber-reinforced ceramic composite, Ceram. Trans. 19(1991)619- sary for an improved estimation of the strength of Sic/Sic [7 w. Yang, H. Araki, T. Noda, J.Y. Park, Y. Katoh, T. Hinoki, J composite from ETS with eight harness satin-woven cloth Yu,A.Kohyama, Hi-NicalonTM fiber-reinforced CVI-SiC matrix reinforcement opposites: I effects of PyC and PyC-SiC multilayers on the fracture behaviors and flexural properties, Mater. Trans. 43(10)(2002)2568- 4. Conclusions [8]W. ma, Y. Katoh, Q. Hu, H. Suzuki, T. Noda, Hi-NicalonM fiber-reinforced CVI-SiC matrix composites:Il interfacial shear strength and its effects on the flexural properties A new source gas, ETS, was used for fabricating a SiC/SiC Mater. Trans.43(10)(2002)2574-2577 composite with eight harness satin-woven Hi-Nicalon cloth [9]R. Naslain, The concept of layered interlayers in SiC/SiC, Ceram. as the reinforcement. A spontaneous graphite interlayer Trans.58(1995)23-29 formation was observed in the composite during the cvi [10J F. Rebillat, J. Lamon, R. Naslain, E. Lare-Curzio, M K. Ferber. T.M. Besmann, Interfacial bond strength in SiC matrix densification. which was attributed to the extra materials. as studied ngle-fiber pushout tests, J. Am. Ceram. carbon from the thermal decomposition of the Ets. This graphite interlayer adjusted the interfacial shear strength to [11]C. Droillard, J. Lamon, X. Bourrat, Strong interface in CMCs a a reasonable range, and therefore, yielded the composite ondition for efficient multilayered interlayers, Mater. Res. Soc with ductile fracture behavior and high flexural strength (12)W. Yang, Development of CVI process and property evaluation of of450±65 MPa and567±75 MPa for Pls and UFS, CVI-SiC/SiC composites. Ph. D thesis, Institute of Advanced Energy respectively. This study indicates that ETS might be an Kyoto University, 2002W. Yang et al. / Ceramics International 31 (2005) 47–52 51 is because of the moderate ISS, which is owing to the spon￾taneous formed graphite interlayer. Inghels and Lamon [16] have developed a theoretical ap￾proach to predict the strength of unidirectional SiC/SiC com￾posites upon flexural loading from the properties of the fiber, matrix and the interface. Yang et al. [8] applied the theory to calculate the PLS of plain-woven Hi-Nicalon fabric cloth reinforced CVI-SiC/SiC composites with a simple assump￾tion that the 90◦ bundles could be regarded as ‘matrix’, and an empirical coefficient, K(=0.66), defined from a compar￾ison between the model prediction and experimental results for their composite system: PLS = KσTh PLS (3) where σTh PLS is the theoretical prediction giving by: σTh PLS = Ec
12γmEfV2 f ISS EcE2 m(1 − Vp − V f)rf 1/3 × Ec + EfV f 2EfV f  4EfV f 3Ec + EfV f 1/3 (4) where γm is the surface energy of CVD-SiC, which was given as 25 J/m2 [16]. Em and Ef are the Young’s modulus of the matrix and the fiber, 400 and 270 GPa for typical CVI-SiC matrix and the Hi-Nicalon fiber, respectively. V f and Vp are the volume fractions of the 0◦ bundle fibers and porosity, which are 20 and 17% for the present composite, respectively. Ec is the composite modulus determined from the law of mixture: Ec = EfV f + Em(1 − Vp − V f) (5) Using Eqs. (3)–(5), the PLS of present composite was cal￾culated to be 389 MPa. The calculated value is slightly lower than the experimental observation (Table 1). The model, as well as the empirical coefficient K = 0.66, was derived from a family of SiC/SiC composites with plain-woven re￾inforcements. Further modification of the model is neces￾sary for an improved estimation of the strength of SiC/SiC composite from ETS with eight harness satin-woven cloth reinforcement. 4. Conclusions A new source gas, ETS, was used for fabricating a SiC/SiC composite with eight harness satin-woven Hi-Nicalon cloth as the reinforcement. A spontaneous graphite interlayer formation was observed in the composite during the CVI matrix densification, which was attributed to the extra carbon from the thermal decomposition of the ETS. This graphite interlayer adjusted the interfacial shear strength to a reasonable range, and therefore, yielded the composite with ductile fracture behavior and high flexural strength of 450 ± 65 MPa and 567 ± 75 MPa for PLS and UFS, respectively. This study indicates that ETS might be an alternative source material (in stead of MTS) for the fab￾rication of CVI-SiC/SiC composites with advantage of spontaneously formed graphite interlayer (the additional interlayer deposition process might become unnecessary in this case). However, further studied on the detailed mecha￾nism of the spontaneously formation of the graphite inter￾layer is necessary for the controlling the thickness of the layer. Acknowledgements This work is supported by the CREST, Japan Science and Technology Corporation and conducted at the National Institute for Materials Science. A part of this study was financially supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology, based on the screening and counseling by the Atomic Energy Commission. References [1] G.N. Morscher, J.D. Cawley, Intermediate temperature strength degradation in SiC/SiC composites, J. Eur. Ceram. Soc. 22 (14/15) (2002) 2777–2787. [2] T. Noda, H. Araki, F. Abe, M. Okada, Microstructure and mechanical properties of FCVI carbon fiber/SiC composites, J. Nucl. Mater. 191–194 (1992) 539–543. [3] D. Brewer, HSR/EPM combustor materials development program, Mater. Sci. Eng. A261 (1999) 284–291. [4] K.M. Prewo, J.J. Brennan, Silicon carbide fiber reinforced glass-ceramic matrix composites exhibiting high strength and tough￾ness, J. Mater. Sci. 17 (1982) 2371–2383. [5] T.M. Besmann, D.P. Stinton, E.R. Kupp, S. Shanmugham, P.K. Liaw, Fiber–matrix interfaces in ceramic composites, J. Mater. Res. Soc. Symp. Proc. 458 (1997) 147–159. [6] R.A. Lowden, Fiber coatings and the mechanical properties of a fiber-reinforced ceramic composite, Ceram. Trans. 19 (1991) 619– 663. [7] W. Yang, H. Araki, T. Noda, J.Y. Park, Y. Katoh, T. Hinoki, J. Yu, A. Kohyama, Hi-NicalonTM fiber-reinforced CVI-SiC matrix composites: I effects of PyC and PyC–SiC multilayers on the fracture behaviors and flexural properties, Mater. Trans. 43 (10) (2002) 2568– 2573. [8] W. Yang, H. Araki, A. Kohyama, Y. Katoh, Q. Hu, H. Suzuki, T. Noda, Hi-NicalonTM fiber-reinforced CVI-SiC matrix composites: II interfacial shear strength and its effects on the flexural properties, Mater. Trans. 43 (10) (2002) 2574–2577. [9] R. Naslain, The concept of layered interlayers in SiC/SiC, Ceram. Trans. 58 (1995) 23–29. [10] F. Rebillat, J. Lamon, R. Naslain, E. Lare-Curzio, M.K. Ferber, T.M. Besmann, Interfacial bond strength in SiC/C/SiC composite materials, as studied by single-fiber pushout tests, J. Am. Ceram. Soc. 81 (4) (1998) 965–978. [11] C. Droillard, J. Lamon, X. Bourrat, Strong interface in CMCs a condition for efficient multilayered interlayers, Mater. Res. Soc. Symp. Proc. 365 (1995) 371–376. [12] W. Yang, Development of CVI process and property evaluation of CVI-SiC/SiC composites. Ph.D. thesis, Institute of Advanced Energy, Kyoto University, 2002.