September 2004 CVI Furnace Process and Mechanical Properties of in Situ SiC Nanowire-Reinforced CVI Sic/SiC Composite mber of the geometry. The stack was located in the vertical hot chan Hot Chamber CVI furnace, as shown in Fig. 1. Before the nanowire-growing process was begun. the furnace(preform) was heated to 1223 K under vacuum(<0. I Pa) using a rotary pump. The rotary pump Composite Preform then was replaced by a diaphragm pump, and CH, was introduced Carbon Mesh Gas Distributor nto the specimen chamber at a constant flow rate of 200 sccm standard cubic centimeters per minute) to deposit a carbon coating on the fibers in the preform as the fiber/matrix interlayer. The Valve exhausted gas was evacuated using the diaphragm pump. During H the CVD carbon-coating process, the pressure in the chamber was H2 carrier maintained at 14.7 kPa (by adjusting the evacuation rate of the Cold Diaphragm gm pump). After the specimen was processed for 70 min Trap Pump the CHa flow was stopped, and the system was maintained at 1223 K for 10 min to make certain of complete reaction of the residual CHa in the chamber. The specimen then was heated to 1373 K and Temperature b od MTS Bath Rotary MTS was introduced into the chamber to grow SiC nanowires in the preform. The MTS was carried by hydrogen-gas using a typical bubble system(Fig. I). A bypass hydrogen flow also was used to Fig. 1. Schematic diagram of the CvI system dilute the MrS concentration in the chamber. During this process. the MTS bath temperature was maintained at 293 K. The flow rates of carrier and bypass hydrogen-gas were 20 and 1000 sccm. tens of micrometers have been produced in our recent work 23 respectively. Therefore, the flow rate of the MTS was 1.8X 10 mol/min. The nanowire process was conducted for 2 h at a Chemical vapor infiltration (CVD) essentially has been developed decreased specimen chamber pressure of 1. 47 kPa. The MTS and preform held at an elevated temperature. Matrix materials are hydrogen-gas flows then were stopped. After the system wa maintained at this level for- 10 min, it was cooled to deposited on the substrate structure via a standard CVD reaction temperature, and the weight of the preform was measured to Because of such advantages, CVI is widely applied for the estimate the amount of SiC nanowires in the preform. One fabric fabrications of fiber/whisker reinforced/ceramic matrix compos- sheet from the center of the preform was taken out for microscopic ites, including SiC/SiC composites examination before the synthesized SiC nanowires in the preform The SiC nanowires in our previous work were a plain-woven Tyranno-SA fabric sheet using a typical C\ were the same as described before but for a shorter processing system. Here we report, for the first time, the fabrication of a Sic time, i.e., 20 min. This carbon coating was supposed to decrease growth of Sic nanowires in the composite preform before the this carbon-coating process, another fabric sheet was used to the nanowires to modify nanowire/matrix interfacial bonding. The After the completion of the second carbon-coating process, the main interests were the flexural strength and fracture toughness of preform was compressed with a set of graphite fixtures to 2.2 mm the composite and the effects of the SiC nanowire in thickness, which resulted in a volume fraction of fibers of -43%6. Finally, the preform was densified with SiC matrix IL. Experimental Procedures material for 18 h using a CVI process at 1273 K under 14. 7 kPa The total hydrogen flow was 1000 sccm, and the MTS: hydrogen I) Processes for Carbon-Coated SiC Nanowires and volume ratio was 1: 10. The composite was named NF-C SiC/SiC Composites (nanowire- and fiber-reinforced composite) The nanowires were synthesized directly in a compo A conventional Tyranno-SA/SiC composite, named F-C. also form using a CVI system, as schematically shown in Fig was fabricated for comparison. A similar CVI process as described composite preform was prepared by stacking 14 sheets (40 above was used, except the nanowire portion of the process was diameter) of plain-woven Tyranno-SA fiber cloths in a deleted a) 20m 5u m Fig. 2. SiC nanowires on the Tyranno-SA fabric sheet.September 2004 Process and Mi'cbunical Properties oj in Sini SiC Nanowire-Reinforced CVl SiC/SiC Camposile 1721 C\'l Furnace Hot Chamber Tlienitocouple Composite Preform Carbon Mesh Gas Distributor Klowmeler, Valve . CH4 H2 by pass H2 carrier Temperature Controller fiii? t t t Diaphragm Pump MTS Bath Rotary Pump Fig. 1. Schematic diagram of the CVI system. tens of micrometers have been produced in our recent Chemical vapor infiltration (CVI) essentially has been developed from CVD.""^'^ In CVI. gaseous reactants infiltrate a porous preform held at an elevated temperature. Matrix materials are deposited on the substrate structure via a standard CVD reaction. Because of such advantages. CVI is vi/idely applied for the fabrications of fiber/whisker reinforced/ceramic matrix compos￾ites, including SiC/SiC composites.*""^ The SiC nanov^-ires in our previous SNV-TY were directly grown on a plain-woven Tyranno-SA fabric sheet""^ using a typical CVI system. Here we report, for the first time, the fabrication of a SiC nanowire/[iher reinforced/SiC matrix composite through the in silu growth of SiC nanowires in the composite preform before the CVI-matrix intiltration. A thin carbon coating was deposited on the nanowires to modify nanuwire/matrix interfacial bonding. The main interests were the flexurat strength and fracture toughness of the composite and the effects of the SiC nanowires. II. Experimental Procedures (I) Processes for Carbon-Coaled SiC Nanowires and SiC/SiC Composites The nanowires were synthesized directly in a composite pre￾form using a CVI system, as schematically shown in Fig. 1. The composite preform was prepared by stacking 14 sheets (40 mm in diameter) of plain-woven Tyranno SA fiber cloths in a 0790° \ geometry. The stack was located in the vertical hot chamber of the CVI furnace, as shown in Fig. 1. Before the nanowire-growing process was begun, the furnace (preform) was heated to 1223 K under vacuum (<0.1 Pa) using a rotary pump. The rotary pump then was replaced by a diaphragm pump, and CH4 was introduced into the specimen chamber at a constant flow rate t)f 200 seem (standard cubic centimeters per minute) to deposit a carbon coating on the fibers in the preform as the fiber/matri\ intcrlayer. The exhausted gas was evacuated using the diaphragm pump. During the CVD carbon-coat ing process, the pressure in the chamber was maintained at 14.7 kPa (by adjusting the evacuation rate of the diaphragm pump). After the specimen was processed for 70 min. the CH4 flow was stopped, and the system was maintained at 1223 K for 10 min to make certain of complete reaction of the residual CH4 in the chamber. The specimen then was heated to 1373 K and MTS was introduced into the chamber to grow SiC nanowires in the preform. The MTS was carried by hydrogen-gas using a typical bubble system (Fig. 1). A bypass hydrogen flow also was used to dilute the MTS concentration in the chamber. During this process, the MTS bath temperature was maintained at 293 K. The flow rates of carrier and bypass hydrogen-gas were 20 and 1000 seem, respectively. Therefore, the flow rate of the MTS was 1.8 X lO""" mol/min. The nanowire process was conducted for 2 h at a decreased specimen chamber pressure of 1.47 kPa. The MTS and hydrogen-gas flows then were stopped. After the system was maintained at this level for—10 min, it was cooled to room temperature, and the weight of the preform ft-as measured to estimate the amount of SiC nanowires in the preform. One fabric sheet from the cenier of the preform was taken out for microscopic examination before the synthesized SiC nanowires in the preform were CVD carbon coated again. The carbon coating conditions were the same as described before but for a shorter processing time, i.e., 20 min. This carbon coating was supposed to decrease the bonding strength between the nanowires and the matrix. After this carbon-coat ing process, another fabric sheet was used to examine the carbon coating on the nanowires. After the completion of the second carbon-coating process, the preform was compressed with a set of graphite fixtures to 2.2 mm in thickness, which resulted in a volume fraction of fibers of -^43%. Finally, the preform was densified with SiC matrix material for 18 h using a CVI process^' at 1273 K under 14.7 kPa. The total hydrogen flow was 1000 seem, and the MTS:hydrogen volume ratio was 1:10. The composite was named NF-C (nanowire- and fiber-reinforced composite). A conventional Tyranno-SA/SiC composite, named F-C. also was fabricated for comparison. A similar CVI process as described above was used, except the nanowire portion of the process was deleted. Fig. 2. SiC nanowires on the Tyranno-SA fabric sheet