1722 Journal of the American Ceramic Sociery-Yang et al Vol, 87. No 9 (2) Flexure and Toughness Tests <. The flexural strength and fracture behaviors were investigated (a) ing three-point bending tests. Rectangular specimens were cut from the composites parallel to one of the fiber bundle directions and were carefully polished to dimensions of 30 mm x 4.0 mm x 1.5 mm. The measurement of fracture toughness was conducted using single-edge notched-beam (SENB) specimens of 25 mm X 3.0 mm X 2.0 mm. The precracks were cut using a diamond blade 0.3 mm in width and 1. 4 mm in depth. The bridging distance was 16 mm, and the loading velocity was 0.5 mm/min for flexure and SENB tests. The flexural strength was derived from the load. displacement curves of three bending specimens, according to ASTM C 1341-97. 6 The fracture toughness (K,e) was calculated from load/displacement curves of four SENB tests according ASTM C1421-99.27 (3) Microstructure Characterizatio The nanowires were examined using scanning electron micros- opy (SEM; Model JSM-6700F, JEOL, Tokyo, Japan), high resolution transmission electron microscopy (HRTEM: Model JEM-3000F, JEOL), and selected-area electron diffractometry (SAED: Model JEM-3000F, JEOL). The chemistry of the nano- wires was examined using energy-dispersive spectroscopy(EDS) attached to the HRTEM. The interlayers, microstructures of the composite, and fracture surfaces after tests were examined using SEM 10nm UL. Resul (I) SiC Nanowires on the Fabric Sheer Figure 2(a) shows the SEM image of a Tyranno-SA fabric sheet the nanowire growth process. The sheet is covered with a (b) like layer, which is an aggregation of nanowires, as con- by the higher-magnification SEM image in Fig. 2(b). The nanowires are generally several tens to more then one hundred micrometers in length and randomly oriented with straight or curved morphologies. The diameter of most of the nanowires is c100 nm. However nanowires with diameters <50 nm or even 10 nm often were observed using HRTEM. Figure 2 shows that the amount of the nanowires on the fabric sheet is not large. The volume ratio of the nanowires in the preform was estimated to be 5%0 from the weight gains before and after the nanowire process HRTEM images of an individual nanowire after the CVD carbon-coating process are shown in Figs. 3(a)and(b)(where(b) A 3nm is a higher magnification of (a)), The corresponding SAED is inserted in Fig. 3(a). The HRTEM images and corresponding 1200 SAED show that the nanowire consists of single-crystal B-phase SiC core structure and a thin amorphous shell. The diameter of the c) core crystal nanowire is -20 nm, and it possesses high-density 1000 stacking faults and microtwins in the crystal plane normal to the axis of the nanowire. The distance between these planes is 0. 25 nm, which is the same as the distance between (111) planes in a -C-coated SiC nanowire SiC crystal. Therefore, the nanowire grows along the <lll> 800 direction, which is confirmed using the defocus SAED technique The faint streaks along the nanowire axis in the SAEd are due to the dense stacking faults along the <111> direction. Figure 3(b) shows that the B-SiC nanowire is covered with a continuous + Cu peaks, from TEM copper mesh amorphous coating with a thickness of -5 nm. This amorphous coating did not appear at the surfaces of the Sic nanowires before the CVD carbon-coating process. As mentioned before, during the CVD carbon-coating process, only CH, was used as the source s. Therefore, this am us layer is concluded to be carbon 200 coating from the thermal decomposition of CHa. This has been … SiC nanowire confirmed using comparative EDS studies, as shown in Fig. 3(c) The relative intensity of carbon in the Sic nanowires increased after they were processed with the carbon-coating deposition process (the carbon: silicon atomic ratio increased from near toichiometric to-60/40). The copper peaks in the spectrum were from the copper mesh on which the nanowires were loaded. When Energy /kev the same process was used, the carbon coating was frequently Fig. 3. (a) and(b) HRTEM images with insert in (a) of SAED and deposited on continuous SiC fibers as the fiber/matrix interlayer in EDS spectrum of an individual carbon-coated SiC nanowire.1722 Journal of the American Ceramic Society—Yang et al. Vol. 87. No. 9 (2) Flexure and Toughness Tests The flexural strength and fracture behaviors were investigated using three-point bending tests. Rectangular specimens were cut from the composites parallel to one of the fiber bundle directions and were carefully polished to dimensions of 30 mm X 4.0 mm X 1,5 mm. The measurement of fracture toughness was conducted using single-edge notched-beam (SENB) specimens of 25 mm x 3.0 mm X 2,0 mm. The precracks were cut using a diamond blade 0.3 mm in width and 1.4 mm in depth. The bridging distance was 16 mm. and the loading velocity was 0.5 mm/min for flexure and SENB tests. The flexural strength was derived from the load/ displacement curves of three bending specimens, according to ASTM C 1341-97.^^ The fracture toughness (KiJ was calculated from load/displacement curves of four SENB tests according to ASTMC 1421-99." (3) Microstructure Characterization The nanowires were examined using scanning electron micros￾copy (SEM; Model JSM-6700F. JEOL. Tokyo, Japan), high￾resolution transmission electron microscopy (HRTEM; Model JEM-3000F. JEOL), and selected-area electron diffractometry (SAED; Model JEM-3000F, JEOL). The chemistry of the nano￾wires was examined using energy-dispersive spectroscopy (EDS) attached to the HRTEM. The interlayers, microstructures of the composite, and fracture surfaces after tests were examined using SEM. i n. Results and Discussion (1) SiC Nanowires on the Fabric Sheet Figure 2(a) shows the SEM image of a Tyranno-SA fabric sheet after the nanowire growth process. The sheet is covered with a spongelike layer, which is an aggregation of nanowires, as con￾finned by the higher-magnification SEM image in Fig. 2(b), The nanowires are generally several tens to more then one hundred micrometers in length and randomly oriented with straight or curved morphologies. The diameter of most of the nanowires is — 100 nm. However, nanowires with diameters <50 nm or even —10 nm often were observed using HRTEM. Figure 2 shows that the amount of the nanowires on the fabric sheet is not large. The volume ratio of the nanowires in the preform was estimated to be ~ 5 % from the weight gains before and after the nanowire process. HRTEM images of an individual nanowire after the CVD carbon-coating process are shown in Figs. 3 (a) and (b) {where (b) is a higher magnification of (a)). The corresponding SAED is inserted in Fig. 3(a), The HRTEM images and corresponding SAED show that the nanowire consists of single-crystal [3-phase SiC core structure and a thin amorphous shell. The diameter of the core crystal nanowire is —20 nm, and it possesses high-density stacking faults and microtwins in the crystal plane normal to the axis of the nanowire. The distance between these planes is 0.25 nm. which is the same as the distance between {111} planes in a p-SiC crystal. Therefore, the nanowire grows along the <111> direction, which is confirmed using the defocus SAED technique. The faint streaks along the nanowire axis in the SAED are due to the dense stacking faults along the < 111 > direction. Figure 3(b) shows that the P-SiC nanowire is covered with a continuous amorphous coating with a thickness of ~5 nm. This amorphous coating did not appear at the surfaces of the SiC nanowires before the CVD carbon-coating process. As mentioned before, during the CVD carbon-coating process, only CH4 was used as the source gas. Therefore, this amorphous layer is concluded to be carbon coating from the thermal decomposition of CH4. This has been confirmed using comparative EDS studies, as shown in Fig, 3(c). The relative intensity of carbon in the SiC nanowires increased after they were processed with the carbon-coating deposition process (the carbon:silicon atomic ratio increased from near stoichiometric to —60/40). The copper peaks in the spectrum were from the copper mesh on which the nanowires were loaded. When the same process was used, the carbon coating was frequently deposited on continuous SiC fibers as the fiber/matrix interlayer in (c) -coate(J SiC nanowire XJ:^ Cu peaks, from JWl copper mesh • —-SiC nanowire J 'L. 2 4 6 Energy / keV 10 Fig. 3. (a) ana (b) HRTEM images with insert In (a) (if SAED mid (c) EDS spectrum 0!' an individual carbon-coated SiC nanowire