ournal Am Cerum,soc,879120-17252004 Process and Mechanical Properties of in Situ Silicon Carbide-Nanowire-Reinforced Chemical Vapor Infiltrated Silicon Carbide/Silicon Carbide Composite Wen Yang, Hiroshi Araki, Akira Kohyama, *. Somsri Thaveethavorn, Hiroshi Suzuki, and Tetsuji Noda National Institute for Materials Science, Tsukuba 305-0047, Japan Institute of Advanced Energy. Kyoto University, Uji, Kyoto 611-0011, Japan A SiC nanowire/Tyranno-SA fiber-reinforced SiC/SiC com- transverse matrix cracks propagate easily through those fiber posite was fabricated via simple in situ growth of SiC nano- bundles perpendicular to the direction of the main stress in the wires directly in the fibrous preform before CVI matrix materials on either tensile or flexural external force, especially densification; the purpose of the SiC nanowires was to mark when these bundles are insufficiently deposited within the matrix edly improve strength and toughness. The nanowires consisted ak points tes and mak of single-crystal B-phase Sic with a uniform -5 nm carbon little contribution to strength and toughness. Challenges for further shell; the nanowires had diameters of several tens to one improvements of strength and toughness of SiC/SiC composites hundred nanometers. The volume fraction of the nanowires in should follow on strengthening and toughening these weak points the fabricated composite was -5%. However, the composite as well as the brittle matrix did not show significant increase in strength and toughness One-dimensional nanostructures, such as SiC nanowires and likely because of strong bonding between the nanowires and carbon nanotubes, have great potential for use in composite the matrix caused by the very thin carbon coating on the nano- materials as reinforcements because of their significantly greater wires. Little debonding and pullout of SiC nanowires from the strength than their bulk counterparts. 0- Several efforts have matrix were observed at the fracture surfaces of the composite. been made to fabricate carbon-nanotube-reinforced compos ites. 3. However, a significant reinforcement effect of the strong and expensive nanotubes on the mechanical properties has not been observed, likely because of several critical difficulties, as HERE has been strong interest in ceramic-matrix composites for roposed by Zhan et al. First, the properties of the individual nany decades for a variety of high-temperature, high-stress carbon nanotube must be optimized. Second, the carbon nanotubes applications in aerospace, hot engine, and energy conversion must be sufficiently bonded to the matrix so that they actuall devices, The low fracture toughness of the ceramics can be carry the load. Third, the load must be distributed throughout the readily improved by the incorporation of reinforcement materials, nanotubes to ensure that the outermost layer does not shear off such as continuous -SiC-fiber-reinforced SiC-matrix (SiC/SiC) SiC nanowires also have been suggested as good reinforcement opposites . 6 In a SiC/SiC composite, a transverse matrix crack materials for ceramic-matrix composites. o The reported elastic can be deflected with energy dissipation that occurs via debonding modulus and ultimate bending strengths of SiC nanorods are at the fiber/matrix interface, crack deflection, crack bridging by the 610-660 GPa and 53.4 GPa, respectively. These values are much fibers, fiber sliding, and eventual fiber fracture. These energy. larger than those of SiC-based fibers, including the advanced dissipating mechanisms provide for improved apparent fracture Tyranno-SA fibers, which are newly developed Sic fibers and toughness and result in a noncatastrophic mode of failure. Many widely used for advanced Sic/SiC composites. Although several Sic/SiC composites for the purpose of improving strength and wires. no effort has been reported in the current literature on fracture tolerance, and a good understanding of the results has been Sic-nanowire-reinforced composites. This is possibly because of established the substantial challenges on homogeneity that disperse the Although the fracture tolerance of bulk Sic can be readily nanowires in the matrix with sound nanowire/matrix interfacial gains in fracture tolerance are basically from the fiber/matrix matrix, such as SiC, is likely to produce strong interfacial bonding, interfacial debonding and the bridging/deflection of transverse which results in unnecessarily improved strength over the pure matrix cracks by the fibers. The SiC matrix displays a brittle matrix. Hence, a compliant coating on the nanowires is necessary behavior similar to its bulk counterparts. In addition, when the for modified bonding between the nanowires and the matrix. composite is reinforced with two- or three-dimensional fabrics, Chemical vapor deposition( CVD) is a suitable and widely used method to produce SiC of various shapes of thin films, powders whiskers and nanorods 8-20 Kirchner and Knoll have obtained microscale CVD-SiC whiskers through thermal decomposition of T M Besmann--contributing editor CH, SiCI, (MTS), which is carried by hydrogen. Lespiaux et al. have performed a detailed study on the correlation between ga phase and supersaturation, nucleation process, and physicochem cal characteristics of CVD-SiC deposited from the MTS-H2 system; they have found that the supersaturation, Y and the type of Culture, Sports, Science, and Technology, based on screening and counseling by the kinetic process are determining factors for the control of the Atomic Energy Commission. morphology and microstructure of the deposited SiC. Based on this rican Ceramic Society National Institute for Materials Science knowledge, a simple CVD process has been developed, and B-SiC "Institute of Advanced Energy nanowires that have a thickness of -100 nm and length of severaljournal / Im Ceram. Soc.. 87 [91 1720-17:5 12004) Process and Mechanical Properties of in Situ Silicon Carbide-Nanowire-Reinforced Chemical Vapor Infiltrated Silicon Carbide/Silicon Carbide Composite Wen Yang/ Hiroshi Araki,' Akira Kohyama.** Somsri Thaveethavom.^ Hiroshi Suzuki/ and Tetsuji. Noda' National Institute for Materials Science, Tsukuba 305-0047, Japan Institute of Advanced Energy. Kyoto University, Uji, Kyoto 611-0011, Japan A SiC nanowire/Tyranno-SA fiber-reinforced SiC/SiC com￾posite was fabricated via simple in situ growth of SiC nano￾wires directly in the fibrous preform hefore CVI matrix densification; the purpose of the SiC nanowires was to mark￾edly improve strength and toughness. The nanowires consisted of single-crystal p-phase SiC with a uniform —5 nm carbon shell; the nannwires had diameters of several tens to one hundred nanometers. The volume fraction of the nanowires in the fabricated composite was —5%. However, the composite did not show significant increase in strength and toughness, likely because of strong bonding between the nanowires and the matrix caused by the very thin carbon coating on the nano￾wires. Little debonding and pullout of SiC nanowires from the matrix were observed at the fracture surfaces of the composite. I. Introduction T HF.RE has been strong interest in ceramic-matrix composites for many decades for a variety of high-temperature, high-stress applications in aerospace, hot engine, and energy conversion devices.'""" The low fracture toughness of the ceramics can be readily improved by the incorporation of reinforcement materials, such as continuous-SiC-t1ber-reinforced SiC-matrix (SiC/SiC) composites.-^'' In a SiC/SiC composite, a transverse matrix crack can be deflected with energy dissipation that occurs via debonding at the fiber/matrix interface, crack deflection, crack bridging by the fibers, fiber sliding, and eventual flber fracture,'"^ These energy￾dissipating mechanisms provide for itnproved appareni fracture toughness and result in a noncatastrophic mode of failure. Many efforts have been made to upgrade the interfacia! properties of SiC/SiC composites for the purpose of improving strength and fracture tolerance, and a good understanding of the results has been established.''"'' Although the fracture tolerance of bulk SiC can be readily improved by the incorporation of SiC reinforcement flbers, such gains in fracture tolerance are basically from the flber/matrix interfacial debonding and the bridging/deflection of transverse matrix cracks by the fibers. The SiC matrix displays a brittle behavior similar to its bulk counterparts. In addition, when the composite is reinforced with two- or three-dimensional fabrics. T. M. Besniann—contributing editor ManusL-ripi No. 10681. Received November 26. 2003; approved May 25. 2CK)4. Supported by the Budget lor Nuclear Research of the Ministry of Education, Culture, Sports, Science, and Technology, based on screenittg and counseling by the Atotiiic Energy Commission. 'Member. American Ceramic Society. ^National Institute for Materials Science. *lnstilute of Advanced Energy. transverse matrix cracks propagate easily through those flber bundles perpendicular to the direction of the main stress in the materials on either tensile or flexural external force, especially when these bundles are insufficiently deposited within the matrix. These fiber bundles are weak points in the composites and make little contribution to strength and toughness. Challenges for further improvements of strength and toughness of SiC/SiC composites should follow on strengthening and toughening these weak points as well as the brittle matrix. One dimensional nanostructures, such as SiC nanowires and carbon nanotubes, have great potential for use in composite materials as reinforcements because of their significantly greater strength than their bulk counterparts.'""'" Several efforts have been made to fabricate carbon-nanotubc-reinforced compos￾ites.'"^"'* However, a significant reinforcement effect of the strong and expensive nanotubes on the mechanical properties has not been observed, likely because of several critical difflculties, as proposed by Zhan et ai^^ First, the properties of the individual carbon nanoiube must be optimized. Second, the carbon nanotubes must be sufficiently bonded to the matrix so that they actually carry the load. Third, the load must be distributed throughout the nanotubes to ensure that the outermost layer does not shear off. SiC nanowires also have been suggested as good reinforcement materials for ceramic-matrix composites.'" The reported elastic modulus and ultimate bending strengths of SiC nanorods are 610-660 GPa and 53.4 GPa, respectively. These values are much larger than those of SiC-based fibers, including the advanced Tyranno-SA flbers. which are newly developed SiC fibers and widely used for advanced SiC/SiC composites.'^ Although several methods have been developed for fabrication of SiC nano￾wires,'^'"* no effort has been reported in the current literature on SiC-nanowire-reinforced composites. This is possibly because of the substantial challenges on homogeneity that disperse the nanowires in the matrix with sound nanowire/matrix interfacial bonding strength. Direct burying of the SiC nanowires in a ceramic matrix, such as SiC, is likely to produce strong interfacial bonding, which results in unnecessarily improved strength over the pure matrix. Hence, a compliant coating on the nanowires is necessary for modified bonding between the nanowires and the matrix. Chemical vapor deposition (CVD) is a suitable and widely used method to produce SiC of various shapes of Ihin films, powders, whiskers, and nanorods.'" ^" Kirchner and Knoll"' have obtained microscale CVD-SiC whiskers through thermal decomposition of CHiSiCli (MTS), which is carried by hydrogen. Lespiaux et a(}~ have performed a detailed study on the correlation between gas phase and supersaturation, nucleation process, and physicochem￾ical characteristics of CVD-SiC deposited from the MTS-H^ system; they have found that the supersaturation, •y. and the type of kinetic process arc determining factors for the control of the morphology and microstructure of the deposited SiC. Based on this knowledge, a simple CVD process has been developed, and p-SiC nanowires that have a thickness of —100 nm and length of several 1720