K Shimoda et al. Composites Science and Technology 68(2008)98-105 cation matrix with the severe fiber deformation. In this layer. PyC coating used on fibers is very reactive with study, the thickness of PyC coating is indicated to be one sintering additives, and about 0.30-0.40 um is sacrifi- of the essential issues for matrix densification cially consumed during hot-pressing. (3) Higher strength with a pseudo-ductile fracture behav- 3.3. Mechanical properties and fracture behavior or could be obtained using 0.50 um of PyC coating hickness. This is supposed that suficient Sic nano- Mechanical properties of hot-pressed SiC/SiC compos- owder infiltration at intra-fiber -tows enhanced den ites are listed in Table 4, and typical flexural stress-dis fication process with the existence of PyC coating placement curves and fracture surfaces after bending test layer to protect fibers and PyC interface after are shown in Figs. 7 and 8, respectively. The average flex induced-processing ural strength of the SiC/SiC composites without PyC coat fiber tows was very high, above I GPa. Fracture surface Acknowledgement of the composites was smooth and fiber-tows were not so lear, therefore flexural stress-displacement curve showed This work was supported by Fundamental r&D on Ad- a brittle fracture behavior (without fiber pull-out). The vanced Material system for(High Efficiency Environment SiC/SiC composites with the PyC thickness of 0.25 um also conscious) Very High Temperature Gas-cooled Fast Reac- showed a brittle fracture behavior with smooth fracture tor Core Structures, the program funded by Ministry of surface(without fiber pull-out) due to the absence of Pyc Education, Culture, Sports and Technology of Japan interface to deflect cracks. It is assumed that most PyC interface to deflect cracks was consumed by the reaction References (4)during hot-pressing in the case of the SiC/SiC compos- ites with the PyC thickness of 0. 25 um. The difference of [1] Tressler RE. Recent developments in fibers and interphases for high maximum strength might be due to porosity and fiber vol- ume fraction at intra-fiber-tows. Higher strength with 199930:429-37. pseudo-ductile fracture behavior could be obtained using 2] Naslain R. Design, preparation and properties of non-oxide CMCs 0.50 um of PyC thickness, where a lot of deflects and for application in engines and nuclear reactors: an overview. Compos ci Technol 2004: 64- 155-70 branches of the propagating cracks and fiber pull-out were [3]Kohyama A, Seki M, Abe K, Muroga T, Matsui H, Jitsukawa S, observed (see Fig. 8c). The extra PyC thickness (to et al. Interactions between fusion materials r&d and other technol- 00 um), which enhanced the prevention of Sic nano- ogies. J Nucl Mater 2000: 283-287: 20- owder infiltration at intra-fiber -tows caused the severe 4] Snead LL, Jones R, Kohyama A, Fenici P Status of silicon carbide composites for fusion. J Nucl Mater 1996: 233-237: 26-36. degradation of Pyc interface as well as fibers by induced 5]Araki H, Yang w, Suzuki H, Hu Q, Busabok C, Noda T Fabrication pressure (see Fig. 6d), resulting in poor mechanical and flexural properties of Tyranno-SA/SIC composites with carbon properties. From these results, it is concluded that the infil- interlayer by Cvl. J Nucl Mater 2004: 329: 56 tration efficiency of SiC nano-powder into the narrow [6]Kohyama A, Kotani M,Katoh Y,Nakayasu T,Sato M,Yamamura region of intra-fiber-tows improves the densification T, et al. High-performance SiC/SiC composites by improved PIP processing with new precursor polymers. J Nucl Mater 2000: 283- process and simultaneously protects fibers and PyC 87:565-9 interface during induced-processing [7 Sayano A, Sutoh C, Suyama S, Itoh Y, Nakagawa S Development of a reaction-sintered silicon carbide matrix composite. J Nucl Mate 999;271-272:467-71 4. Conclusions [8] Kotani M, Kohyama A, Katoh Y. Development of SiC/SiC composites by PIP in combination with Rs. J Nucl Mater 2001:289:37-41 Unidirectional SiC/SiC composites were fabricated by [9] Nozawa T, Hinoki T, Katoh Y, Kohyama A. Effects of fibers and NitE process using SiC nano-powder infiltration tech fabrication processes on mechanical properties of neutron irradiated nique with sintering additives for matrix formation. CVD SiC/SiC composites. J Nucl Mater 2002: 307-311: 1173-7 process can coat appropriate PyC thickness with unifor- [) Hinoki T. Spead LL, katoh Y,, Hasegawa A, Nozawa T, Kohyama nity Induced PyC interface conditions strongly affect the fibers and their silicon carbide composites. J Nucl Mater 2002: 283- y, microstructural evolution, and therefore dominate nical properties and fracture behaviors. The results [11] Snead LL, Katoh Y, Kohyama A, Bailey JL, Vaughn NL, Lowden follow RA. Evaluation of neutron irradiated near-stoichiometric silicon carbide fiber composites. J Nucl Mater 2000: 283-287: 551-5 (1) Nearly full-dense SiC/SiC composites with [12] Kohyama A, Dong SM,Katoh Y.Development composites by nano-infiltration and transient eutectic (NITE)pro- uncoated fibers caused strong interaction between cess Ceram Eng Sci Proc 2002: 23: 311-8. fibers and matrix, resulting in a brittle fracture [13]Katoh Y, Dong SM, Kohyama A. A novel processing technique of behavior silicon carbide-based ceramic composites for high temperature (2)Increasing PyC thickness caused the difficulty of sic applications. Ceram Trans 2002: 144: 77-86. nano-powder infiltration with the promotion of nar. l4]Katoh Y Kohyama A, Dong SM, HinokiT, Kai JJ.Microstructure and properties of liquid phase sintered SiC/SiC composites. Ceram row aisle and/or the attachment between PyC coating Eng Sci Proc 2002: 23(3 ): 363.cation matrix with the severe fiber deformation. In this study, the thickness of PyC coating is indicated to be one of the essential issues for matrix densification. 3.3. Mechanical properties and fracture behavior Mechanical properties of hot-pressed SiC/SiC compos￾ites are listed in Table 4, and typical flexural stress–dis￾placement curves and fracture surfaces after bending test are shown in Figs. 7 and 8, respectively. The average flex￾ural strength of the SiC/SiC composites without PyC coat￾ing fiber tows was very high, above 1 GPa. Fracture surface of the composites was smooth and fiber-tows were not so clear, therefore flexural stress–displacement curve showed a brittle fracture behavior (without fiber pull-out). The SiC/SiC composites with the PyC thickness of 0.25 lm also showed a brittle fracture behavior with smooth fracture surface (without fiber pull-out) due to the absence of PyC interface to deflect cracks. It is assumed that most PyC interface to deflect cracks was consumed by the reaction (4) during hot-pressing in the case of the SiC/SiC compos￾ites with the PyC thickness of 0.25 lm. The difference of maximum strength might be due to porosity and fiber vol￾ume fraction at intra-fiber-tows. Higher strength with a pseudo-ductile fracture behavior could be obtained using 0.50 lm of PyC thickness, where a lot of deflects and branches of the propagating cracks and fiber pull-out were observed (see Fig. 8c). The extra PyC thickness (to 1.00 lm), which enhanced the prevention of SiC nano￾powder infiltration at intra-fiber-tows caused the severe degradation of PyC interface as well as fibers by induced￾pressure (see Fig. 6d), resulting in poor mechanical properties. From these results, it is concluded that the infil￾tration efficiency of SiC nano-powder into the narrow region of intra-fiber-tows improves the densification process and simultaneously protects fibers and PyC interface during induced-processing. 4. Conclusions Unidirectional SiC/SiC composites were fabricated by NITE process using SiC nano-powder infiltration tech￾nique with sintering additives for matrix formation. CVD process can coat appropriate PyC thickness with unifor￾mity. Induced PyC interface conditions strongly affect the density, microstructural evolution, and therefore dominate mechanical properties and fracture behaviors. The results are as follows: (1) Nearly full-dense SiC/SiC composites with uncoated fibers caused strong interaction between fibers and matrix, resulting in a brittle fracture behavior. (2) Increasing PyC thickness caused the difficulty of SiC nano-powder infiltration with the promotion of nar￾row aisle and/or the attachment between PyC coating layer. PyC coating used on fibers is very reactive with sintering additives, and about 0.30–0.40 lm is sacrifi- cially consumed during hot-pressing. (3) Higher strength with a pseudo-ductile fracture behav￾ior could be obtained using 0.50 lm of PyC coating thickness. This is supposed that sufficient SiC nano￾powder infiltration at intra-fiber-tows enhanced den￾sification process with the existence of PyC coating layer to protect fibers and PyC interface after induced-processing. Acknowledgements This work was supported by Fundamental R&D on Ad￾vanced Material system for (High Efficiency Environment￾conscious) Very High Temperature Gas-cooled Fast Reac￾tor Core Structures, the program funded by Ministry of Education, Culture, Sports and Technology of Japan. References [1] Tressler RE. Recent developments in fibers and interphases for high temperature ceramic matrix composites. Composites A 1999;30:429–37. [2] Naslain R. Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Compos Sci Technol 2004;64:155–70. [3] Kohyama A, Seki M, Abe K, Muroga T, Matsui H, Jitsukawa S, et al. Interactions between fusion materials R&D and other technol￾ogies. J Nucl Mater 2000;283–287:20–7. [4] Snead LL, Jones R, Kohyama A, Fenici P. Status of silicon carbide composites for fusion. J Nucl Mater 1996;233–237:26–36. [5] Araki H, Yang W, Suzuki H, Hu Q, Busabok C, Noda T. Fabrication and flexural properties of Tyranno-SA/SIC composites with carbon interlayer by CVI. J Nucl Mater 2004;329:567–71. [6] Kohyama A, Kotani M, Katoh Y, Nakayasu T, Sato M, Yamamura T, et al. High-performance SiC/SiC composites by improved PIP processing with new precursor polymers. J Nucl Mater 2000;283– 287:565–9. [7] Sayano A, Sutoh C, Suyama S, Itoh Y, Nakagawa S. Development of a reaction-sintered silicon carbide matrix composite. J Nucl Mater 1999;271–272:467–71. [8] Kotani M, Kohyama A, Katoh Y. Development of SiC/SiC composites by PIP in combination with RS. J Nucl Mater 2001;289:37–41. [9] Nozawa T, Hinoki T, Katoh Y, Kohyama A. Effects of fibers and fabrication processes on mechanical properties of neutron irradiated SiC/SiC composites. J Nucl Mater 2002;307–311:1173–7. [10] Hinoki T, Snead LL, Katoh Y, Hasegawa A, Nozawa T, Kohyama A. The effect of high dose/high temperature irradiation on high purity fibers and their silicon carbide composites. J Nucl Mater 2002;283– 287:1157–62. [11] Snead LL, Katoh Y, Kohyama A, Bailey JL, Vaughn NL, Lowden RA. Evaluation of neutron irradiated near-stoichiometric silicon carbide fiber composites. J Nucl Mater 2000;283–287:551–5. [12] Kohyama A, Dong SM, Katoh Y. Development of SiC/SiC composites by nano-infiltration and transient eutectic (NITE) pro￾cess. Ceram Eng Sci Proc 2002;23:311–8. [13] Katoh Y, Dong SM, Kohyama A. A novel processing technique of silicon carbide-based ceramic composites for high temperature applications. Ceram Trans 2002;144:77–86. [14] Katoh Y, Kohyama A, Dong SM, Hinoki T, Kai JJ. Microstructure and properties of liquid phase sintered SiC/SiC composites. Ceram Eng Sci Proc 2002;23(3):363–70. 104 K. Shimoda et al. / Composites Science and Technology 68 (2008) 98–105