September 2004 Process and Mechanical Properties of in Situ SiC Nanowire Reinforced CVI SiC/SiC Composite load-transferring bridges between them. The SEM images in Fig. provides more reasonable nanowire/matrix interfacial bonding 6 show that some of the microrods are broken while others remain and/or increased load of the nanowires unbroken. More fracture energy is dissipated when the matrix cracks propagate through these bundles to break the microrods. On References the other hand. the unbroken microstructures enable the cracked matrix to suspend a certain level of strength. It is believed that D Brewer, "HSR/EPM Combustor Materials Development Program. "Mater. Sci. these SiC-nanowire- based microscale structures, as well as these Eng,A261.28491(19 and O.J. Schwarz,"Advanced SiC Composites for Fusion Applica SiC nanowires buried in densely deposited matrix, serve as tion. " J. Nucl. Mater. 219 3-14(1995) H. Matsui, S. Jitsuknwn, and S Matsuda, "Interactions Between Fusion Materials R&D and Other Technologies. However, when the very high strength of the single-crystal SiC J Nucl. Mater. 283-287, 20-27(2000) nanowires is considered, o the reinforcement effects of the sic C: Droillard and J, Lamon, "Fracture Toughness of 2-D Woven SiC/SiC nanowires in the current composite are not significant. One of the Cv- Composites with Multi-Interlayered Interlayers"/.Am. Ceram. Soc.79 141 important issues for fiber or nanowire reinforcement of materials is tw. Yang. A Kohyama, Y, Katoh, H. Araki. J. Yu, and T Noda, "Effect of C and the control of the interfacial bonding between the reinforcement SiC/C Interlayers on Mechanical Behavior of Tyranno-SA Fiber-Reinforced Sic and the matrix, which must be neither too strong nor too weak ome degree of fiber/nanowire pullout allows energy to be ab- R. Naslain, "Fiber-Matrix Interphases and Interfaces in Ceramic-Matrix Compos- ites Processed by CVI, "Compos. Interfaces, 1 341 253-86(1993) orbed in breaking reinforcement/matrix bonding. The fracture I M. Besmann, D. P, Stinton, E R. Kupp S Shanmugham, and P. KLiaw surface examinations reveal that the bonding between the Tyranno-SA fibers and the matrix in composite NF-C is reason- 45%.- 9 g/rfaces n Ceramic Compesite Mater. Res Se, swnp. Pre. omposite, No debonding and pullout of SiC nanowires from(s able, because sound fiber pullouts occur during the failure of the w. Yang, T Noda, H. Araki. J. Yu, and A Kohyama."Mechanical Properties of Several Advanced Tyranno- SA Fiber-Reinforced CvI-SiC- Matrix Composites, Ala- ter.Sdi.Eng,A345.28-35(2003 matrix is observed at the fracture surfaces, which indicates a too IE W. Wong, P. E Sheehan, and C M. Lieber. "Nanobeam Mechanics: Elastici strong bonding between the nanowires and matrix. This is likely 277 126)1971-75(1997) due to the very thin(-5 nm) compliant carbon coating on the W. R. L Lambrecht, B. Segall, M Methfessel, and M. van Schilfgaard nanowires, Such a thin carbon coating is insufficient to produce "Calculated Elastic Constants and Defonnation Potential of Cuhic SiC, Phvs. Rev ound bonding strength between the nanowires and the matrix to Condens Matter, 44, 3685-91 11991) allow interfacial debonding during the failure of the composite IP. Calvert, "A Receipe for Strength. "Nature (London), 399. 210-11(1999) IR. W. Sirgel, S K Chang,, B J. Ash. J. Stone. P. M. Ajayan R W.Doremus, and Bridging of matrix cracks by the Sic nanowires rarely takes place L S Schadler, "Mechanical Behavior of Polymer and Ceramic-Matrix Nanocompos- in this case. Therefore, the efficiency of the reinforcement of SiC tes,"Scr.Maer,44.2061-64(2001 E. Flahaut. A. Peigney, Ch. Laurent, Ch. Marliere, F, Chastel, and A. Rousset. nanowires might be largely decreased. Nevertheless, this smdy"Carbon Nanotuhe-Metal Oxide Nanocomposites: Microstructure,Electrical Conduc- demonstrates the possibility to fabricate in situ SiC-nanowire- einforced ceramic-matrix composites with hopefully markedI D. Zhan, J Huntz, J. Wan, and A. Mukherjee,"Single. Wall Carbon Nano- improved strength and toughness, which provides thicker carbon ubes as Attractive Toughening Agents in Alumina- Based Nanocomposites, "Nar or alternative compliant coatings on the surfaces of the nanowires nd/or increases the load of the nanowires "T, Ishikawa, Y, Kohtoku, K Kumagawa, T. Yamamura, and T, Nagasawa gh-Strength Alkali-Resistance Sintered SiC Fiber Stable to 2200C,Nature london)39166691773-751998 H. Dai. E. W. Wong. Y, Z Lu S. Fan, and C. M. Lieber. ""Synthesis and Clrae Nature .3751291769-72(1995 IV. Conclusion I. T. Zhou N, Wang. F. C. K. Au, H L Lai, H. Y, Peng, L Bello, C.S. Lee, and S. T. Lee, "Growth and Emussion Properties of B-SiC Nanorods. " Mater. Sci. Eng A SiC-nanowire/Tyranno-SA-fiber-reinforced SiC/SiC com- A286.119-24(2000 posite was fabricated via the simple in situ growth of SiC C. C. Chiu. S. B. Desu, and C. Y Tsai."Low-Pressure Chemical Vapor nanowires directly in the fibrous preform before CVI matrix Deposition (LPCVD) of B-SiC on Si( 100) Using MTS in a Hot Wall Reactor. J. Mater.Res.,8o2617-25(1993 densification. The volume fraction of the SiC nanowires in the composite was estimated to be -5% The nanowires were characterized as pure and single-crystal H. P, Kirchner and P. Knoll, "Silicon Carbide Whiskers, "J. Am. Ceran Soc., 46 299-300(1963 B-phase SiC with diameters of several tens to one hundred nanometers and lengths of several tens of micrometers, A uniform e2D. Lespiaux. F. Langlais,R. Naslain. A. Schamm, and J.Sevely "Correlation carbon coating of-5 nm was successfully deposited on the Characteristics of Silicon Carbide Deposited from Si-C-H-CI System on Silica nanowires in situ as the nanowire/matrix interlayer Substrate. "J. Mater. Sci, 301. 1500-10(199 ow. Yang. H. Araki, A. Kohyama, Q-LHu, H. Suzuki, andT Noda, "Grow The composites with Sit nanowire showed higher flexural SiC Nanowires SiC Nanowires on Tyranno-SA SiC Fibers, J. An. Cera. Soc. $7141 733- strength and fracture toughness compared with those of the companion conventional composite, despite its thinner carbon T. M. Besmann, B. w. Sheldon, R. A Lowden, and D P. Stinton. "Vapor-Phase fiber/matrix interlayer and lower density. The increased strengt Fabrication and Properties of Continuous-Filament Ceramic Composites, Science Washington, DC.253.104-10901991) and toughness were attributed to the SiC nanowires in the R. Naslain and F. Langlais, "CVD- Processing of Ceramic-Ceramic Composite mposite. However, the increased strength and toughness were As in Tailoring Multiphase and Composite Ceramics. Edited by R.E. not remarkable, and little debonding and pullout of SiC nanowires Tressler, G. L Messing. G. G. Pantano, and R. E Newnham. Plenum, New York, observed. It seemed that the carbon coating on the current sic Advanced Ceramic Composites. ASTM Designation C 1341-97 2000 ASTM Annual nanowires was not thick enough to produce a sound interfacial Book of Standards, Part 15. ASTM International, West Conshohocken. PA bounding to allow nanowire/matrix debonding during the failure of H Standard Test Method for Determination of Fracture Toughness of Advanced the composite. The efficiency of the reinforcement by the SiC Annual Book of Standands Part 15. ASTM International, West Conshohocken.PA nanowires was, therefore, believed to be decreased The present process demonstrated the possibility to fabricate in situ SiC-nanowire-reinforced ceramic-matrix composite with Transactions, Vol. 144. Advanced SiC/SiC Ceramie Composites-Developmernts and Applications to Energy Systems. Edited by A Koby ama, M. Singh, H. T. Lin, and Y hopefully markedly improved strength and toughness, which Katob. American Ceramie Society. Westerville. OH, 2002.September 2004 Process and Mechanical Properties of in Situ SiC Nanowire-Reinforced CVl SiC/SiC Composite 1725 load-transferring bridges between them. The SEM images in Fig. 6 show thai some of the microrods are broken while others remain unbroken. More fracture energy is dissipated when the matrix eracks propagate through these bundles to break the microrods. On the other hand, the unbroken microstructures enable the cracked matrix to suspend a certain level of strength. It is believed thai these SiC-nanowire-based microscale structures, as well as these SiC nanowires buried in densely deposited matrix, serve as additional reinforcement elements besides the Tyranno-SA fibers, which results in a contribution to the increase of flexural strength and fracture toughness. However, when the very high strength of the single-crystal SiC nanowires is considered,"^ the reinforcement effects of the SiC nanowires in the current composite are not significant. One of the important issues for fiber or nanowire reinforcement of materials is the control of the interfacial bonding between the reinforcement and the matrix, which must be neither too strong nor too weak; some degree of fiber/nanowire pullout allows energy to be ab￾sorbed in breaking reinforcement/matrix bonding. The fracture surface examinations reveal that the bonding between the Tyranno-SA fibers and the matrix in composite NF-C is reason￾able, because sound fiber pullouts t>ccur during the failure of the composite. No debonding and pullout of SiC nanowires from the matrix is observed at the fracture surfaces, which indicates a too strong bonding between the nanowires and matrix. This is likely due to the very thin (~5 nm) compliant carbon coating on the nanowires. Such a thin carbon coating is insufficient to produce sound bonding strength between the nanowires and the matrix to allow interfacial debonding during the failure of the composite. Bridging of matrix cracks by the SiC nanowires rarely takes place in this case. Therefore, the efficiency of the reinforcement of SiC nanowires might be largely decreased. Nevertheless, this study demonstrates the possibility to fabricate in situ SiC-nanowire￾reinforced ceramic-matrix composites with hopefully markedly improved strength and toughness, which provides thicker carbon or alternative compliant coatings on the surfaces of ihe nanowires and/or increases the load of the nanowires. IV. Conclusion A SiC-nanowire/Tyranno-SA-fiber-reinforced SiC/SiC com￾posite was fabricated via the simple in .situ growth of SiC nanowires directly in the fibrous preform before CVI matrix densification. The volume fraction of the SiC nanowires in the composite was estimated to be —5%. The nanowires were characterized as pure and single-crystal P-phase SiC with diameters of several tens to one hundred nanometers and lengths of several tens of micrometers. A uniform carbon coating of ~5 nm was successfully deposited on the nanowires in situ as the nanowire/matrix interlayer. The composites with SiC nanowires showed higher flexural strength and fracture toughness compared with those of the companion conventional composite, despite its thinner carbon llber/matrix interlayer and lower density. The increased strength and toughness were attributed to the SiC nanowires in the composite. However, the increased strength and toughness were not remarkable, and little debonding and pullout of SiC nanowires from the matrix at the fracture surface of the composite were observed. It seemed that the carbon coating on the current SiC nanowires was not thick enough to produce a sound interfacial bounding to allow nanowire/matrix debonding during the failure of the compt>site. The efficiency of the reinforcement by the SiC nanowires was, therefore, believed to be decreased. 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