S Tang et al. Materials Science and Engineering A 465 (2007)290-294 chemical vapor infiltration(HCVI) technique in a very short ried out by three-point bending, using 80 mm x 10 mm x 6mm time; and the materials were characterized in terms of density, size longitudinal specimens with a span equal to 70 mm and microstructure,composition,mechanical properties and thermal loaded at I mm/min. Compressive tests were performed on 10 mm x 10 mm x 10 mm specimens with a loading speed of I mm/min Fracture toughness was measured on longitudinal 2. Experimental samples of 450mm x 100mm x 4.8 mm at 0.05 mm/min to reduce the deformation rate for stable crack propagation through In the present work, a three-stage and easily controlled pro- a single-edge notch beam(SENB)test using a 40 mm span, a ess was used to fabricate the C/SiC composites. At first, the 5 mm notch depth and a 0. 2 mm notch width[15]. Five, five and 2D preform with a size of 280 mm x 70 mm x 8 mm was fabri- three samples were measured for the flexural, compressive and cated by altematively stacked weftless plies and short-cut-fiber SENB tests, respectively. In-plane CTE from 200 to 1000C was webs using a needle-punching technique and two successive measured on bars with dimensions of 65 mm x8 mm x mm plies were oriented at an angle of 90. Carbon fiber types of them Through-thickness specific heat and TC were obtained through were both PAN-based carbon fiber T700, 12 K tow(from Toray, 12.0 mm x 2.5 mm samples on a FlashlineTM-5000 thermal Japan). The fiber contents of the weftless plies, the webs and the properties analyzer needle fibers were 24.0, 4.5 and 1.5 vol %, respectively. The pre- form was clamped in a graphite clamp with a purpose of wiping 3. Results and discussion off the fiber sizing and maintaining the preform shape. The pro- cessing temperature was increased to 1200C and kept there for Fig. I shows a typical SEM micrograph of the polished cross- 2 h in a vacuum furnace. Secondly, in order to produce a pyro- sections of as-processed C/SiC composites. The cross-section carbon(PyC)interphase and increase the TC, a small amount of area includes O fiber plies, short-cut-fiber webs, 90 fiber plies, Pyc was added to the treated preform by pyrolysing natural gas needle fibers, dense SiC matrix and pores, which is mainly dom- for 10 h at 1000oC in an ICVI apparatus. In the third stage, the inated by well-consolidated parts(Fig. 1(a)). A large amount of treated preforms, clamped between two graphite electrodes for matrix is formed and a few isolated macropores about from 50 to directly heating by passing an electric current in a 50kw cold- 300 um are retained in both the inter-bundles and the inter-ply wall and normal-pressure furnace, were rapidly densified by regions. In a traditional ICVI prepared sample, this is a more forming a SiC matrix by the HCVItechnique. The technique par- typically observed phenomenon and the sizes of these pores are ially overcame the limits of mass transfer and chemical kinetics larger. With a further observation of the intra-bundle, the dense in the traditional ICVI process since a strong temperature gra- inter-fiber matrix can be identified and some residual microp- dient was formed in the fiber preform and an electromagnetic ores are found(Fig. 1(b). From the micrograph, it is possible ield was established around the fiber [14], while one piece of to measure the Sic coating thickness in the inter-fiber pores sample can be prepared one time. Methyltrichlorosilane(MTS, The linear deposition rate in the pores, which is a more practical CH3 SiCl3) was used as a precursor for depositing the SiC matrix, method to evaluate the efficiency of the process, is obtained by and was carried by bubbling hydrogen gas into the chamber. dividing the coating thickness by the infiltration time. The lin- Argon was used as a dilute gas to slow the reaction rate. The ear deposition rates of the SiC matrix within the large inter-fiber leposition conditions were as follows: the hydrogen-to-MTs pores are 0.48-1.01 um/h, partially depending on the spaces temperature below 1000.C and the deposition time 25h. mon mole ratio 1-2, the total flow rate 0.17m/h, the deposition between these pores, which indicates the high deposition effi- ciency. The overall deposition rate can be calculated by dividin Bulk density and open porosity of the C/SiC composites the weight gain per unit time by the volume, and is 0. 10 g/hcm were evaluated by a water immersion technique. Microstruc- As expected, the composites obtain a density of 2.32 g/cm after ture and composition were analyzed using scanning electron a 25-h deposition and its open porosity is only 9.8% microscopy(SEM), energy dispersive spectroscopy (EDS)and The quantitatively chemical compositions of C and Si in the electron probe microanalysis(EPMA). Flexural tests were car- SiC matrix at the center and the edge of the sample according fiber ply Needle~fibe Fig. 1. SEM micrographs of: (a) the macropores between the fiber plies and between the fiber bundles and (b)the micropores between the fibersS. Tang et al. / Materials Science and Engineering A 465 (2007) 290–294 291 chemical vapor infiltration (HCVI) technique in a very short time; and the materials were characterized in terms of density, microstructure, composition, mechanical properties and thermal properties. 2. Experimental In the present work, a three-stage and easily controlled pro￾cess was used to fabricate the C/SiC composites. At first, the 2D preform with a size of 280 mm × 70 mm × 8 mm was fabri￾cated by alternatively stacked weftless plies and short-cut-fiber webs using a needle-punching technique and two successive plies were oriented at an angle of 90◦. Carbon fiber types of them were both PAN-based carbon fiber T700, 12 K tow (from Toray, Japan). The fiber contents of the weftless plies, the webs and the needle fibers were 24.0, 4.5 and 1.5 vol.%, respectively. The pre￾form was clamped in a graphite clamp with a purpose of wiping off the fiber sizing and maintaining the preform shape. The pro￾cessing temperature was increased to 1200 ◦C and kept there for 2 h in a vacuum furnace. Secondly, in order to produce a pyro￾carbon (PyC) interphase and increase the TC, a small amount of PyC was added to the treated preform by pyrolysing natural gas for 10 h at 1000 ◦C in an ICVI apparatus. In the third stage, the treated preforms, clamped between two graphite electrodes for directly heating by passing an electric current in a 50 kW cold￾wall and normal-pressure furnace, were rapidly densified by forming a SiC matrix by the HCVI technique. The technique par￾tially overcame the limits of mass transfer and chemical kinetics in the traditional ICVI process since a strong temperature gra￾dient was formed in the fiber preform and an electromagnetic field was established around the fiber [14], while one piece of sample can be prepared one time. Methyltrichlorosilane (MTS, CH3SiCl3) was used as a precursor for depositing the SiC matrix, and was carried by bubbling hydrogen gas into the chamber. Argon was used as a dilute gas to slow the reaction rate. The deposition conditions were as follows: the hydrogen-to-MTS mole ratio 1–2, the total flow rate 0.17 m3/h, the deposition temperature below 1000 ◦C and the deposition time 25 h. Bulk density and open porosity of the C/SiC composites were evaluated by a water immersion technique. Microstruc￾ture and composition were analyzed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and electron probe microanalysis (EPMA). Flexural tests were car￾ried out by three-point bending, using 80 mm × 10 mm × 6 mm size longitudinal specimens with a span equal to 70 mm and loaded at 1 mm/min. Compressive tests were performed on 10 mm × 10 mm × 10 mm specimens with a loading speed of 1 mm/min. Fracture toughness was measured on longitudinal samples of 45.0 mm × 10.0 mm × 4.8 mm at 0.05 mm/min to reduce the deformation rate for stable crack propagation through a single-edge notch beam (SENB) test using a 40 mm span, a 5 mm notch depth and a 0.2 mm notch width [15]. Five, five and three samples were measured for the flexural, compressive and SENB tests, respectively. In-plane CTE from 200 to 1000 ◦C was measured on bars with dimensions of 65 mm × 8 mm × 5 mm. Through-thickness specific heat and TC were obtained through Ø 12.0 mm × 2.5 mm samples on a FlashlineTM-5000 thermal properties analyzer. 3. Results and discussion Fig. 1 shows a typical SEM micrograph of the polished cross￾sections of as-processed C/SiC composites. The cross-section area includes 0◦ fiber plies, short-cut-fiber webs, 90◦ fiber plies, needle fibers, dense SiC matrix and pores, which is mainly dom￾inated by well-consolidated parts (Fig. 1(a)). A large amount of matrix is formed and a few isolated macropores about from 50 to 300m are retained in both the inter-bundles and the inter-ply regions. In a traditional ICVI prepared sample, this is a more typically observed phenomenon and the sizes of these pores are larger. With a further observation of the intra-bundle, the dense inter-fiber matrix can be identified and some residual microp￾ores are found (Fig. 1(b)). From the micrograph, it is possible to measure the SiC coating thickness in the inter-fiber pores. The linear deposition rate in the pores, which is a more practical method to evaluate the efficiency of the process, is obtained by dividing the coating thickness by the infiltration time. The lin￾ear deposition rates of the SiC matrix within the large inter-fiber pores are 0.48–1.01m/h, partially depending on the spaces between these pores, which indicates the high deposition effi- ciency. The overall deposition rate can be calculated by dividing the weight gain per unit time by the volume, and is 0.10 g/h cm3. As expected, the composites obtain a density of 2.32 g/cm3 after a 25-h deposition and its open porosity is only 9.8%. The quantitatively chemical compositions of C and Si in the SiC matrix at the center and the edge of the sample according Fig. 1. SEM micrographs of: (a) the macropores between the fiber plies and between the fiber bundles and (b) the micropores between the fibers.