552 N igawa et al. /Journal of Physics and Chemistry of Solids 66(2005)551-554 20.0 center middle outer bottom center middle outer Fig. I. Typical distribution of the(a) porosity and(b)thickness of carbon interphase of the FCVl-SiC/SiC. The precursors were 99% purity propylene(Matheson, decreasing the porosity; the average porosity was decreased Morrow, GA, USA)for carbon deposition and the technical to 15% with increasing the fiber volume fraction to 44%.In grade methyltrichlorosilane(MrS, Gelest Inc, Tullytown, the typical cross-section of SiC/SiC composite shown in PA,USA)for SiC deposition and infiltration. The SiC fabric Fig 3, two types of pores were observed; one is a large pore with a fabric layer orientation of [-30/0/301 around fiber bundles, and the other is a small pore in a [0/90] were retrained in a graphite fixture. The fiber bundle. The size of this inter-bundle pores was decreased perform was 75 mm in diameter and 12.5 mm thick and the with increasing the fiber fraction, though that of intra average fiber volume fraction was 35.2 vol%. The carbon bundle pores was nearly constant as a function of fiber interphase was deposited on the fiber surface by decompo- fraction. The distance between fiber bundles is shorter when on of propyle 5×10-2dm3/ min and the fiber fraction is higher, so the inter-bundle pore became I dm /min Ar at 5 Pa, 1100C. The Sic interphase was smaller in SiC/Sic with the higher fiber fraction by the end deposited by the decomposition of MTS with the flow rate of of FCVI process. 0.15g/min and 0.25 dm/min H2 at 5 Pa, 1"C. The Although the carbon interphase of the bottom region was average coating rates of carbon and SiC layer were 1. 2 and thicker than that of the top region by the conventional 5 nm/min, respectively. The multilayer SiC/C interphase infiltration, the distribution of that thickness was signifi consists of 6-set of SiC/C layers: the thickness of carbon layers was fixed at 50 nm and those of Sic were 50 nm(1 cantly improved by flipping midway through the interphase layer), 100 nm(2nd-4th layers)and 500 nm(5th-6th layers) infiltration(Fig. 1(b). Fig. 3(c)shows the typical SEM After interphase deposition, the matrix was formed by image of multilayer SiC/C interphase. It was found that high FCVI process. Details of FCVI process were described uniform layers were formed in this method elsewhere [71 Fig. 4 shows the tensile strength of SiC/Sic with the To estimate the distribution of porosity and interpha carbon interphase as a function of the carbon interphase ickness in a specimen, the specimen was cut into nine thickness. In the present study, the tensile stress-strain sections: the porosity was calculated from the dimensions curve in the results of tensile test exhibited pseude-ductile and the mass of the cut specimen. The microstructure and fracture mode, which means that carbon interphase was interphase thickness was measured by scanning electron effective of the fiber pullout and excellent tensile properti microscope The tensile strength was slightly increased with increasing Tensile tests were carried out at room temperature in air and at 1300C in Ar. The schematic illustration dimension of tensile specimen and details of tensile testing were described in Ref. 81 3. Results and discussion 815 The bottom region, which is the upstream side of cursor in the composite. This tendency was improved by decreas SiC/C ing the precursor and carrier gas flow rates at the latter part of the FCVI process and a much better uniform porosity in the composite was obtained(Fig. 1(a)). The average Fiber fraction(vol % porosity as a function of fiber volume was shown in Fig. 2. It was found that the higher fiber fraction is very effective in Fig. 2. Effect of the fiber volume fraction on the reduction of the porosity.The precursors were 99% purity propylene (Matheson, Morrow, GA, USA) for carbon deposition and the technical grade methyltrichlorosilane (MTS, Gelest Inc., Tullytown, PA, USA) for SiC deposition and infiltration. The SiC fabric layers with a fabric layer orientation of [K308/08/30] and [08/908] were retrained in a graphite fixture. The fiber perform was 75 mm in diameter and 12.5 mm thick and the average fiber volume fraction was 35.2 vol.%. The carbon interphase was deposited on the fiber surface by decompo￾sition of propylene with flow rate of 5!10K2 dm3 /min and 1 dm3 /min Ar at 5 Pa, 1100 8C. The SiC interphase was deposited by the decomposition of MTS with the flow rate of 0.15 g/min and 0.25 dm3 /min H2 at 5 Pa, 1100 8C. The average coating rates of carbon and SiC layer were 1.2 and 9.5 nm/min, respectively. The multilayer SiC/C interphase consists of 6-set of SiC/C layers: the thickness of carbon layers was fixed at 50 nm and those of SiC were 50 nm (1st layer), 100 nm (2nd–4th layers) and 500 nm (5th–6th layers). After interphase deposition, the matrix was formed by FCVI process. Details of FCVI process were described elsewhere [7]. To estimate the distribution of porosity and interphase thickness in a specimen, the specimen was cut into nine sections: the porosity was calculated from the dimensions and the mass of the cut specimen. The microstructure and interphase thickness was measured by scanning electron microscope. Tensile tests were carried out at room temperature in air and at 1300 8C in Ar. The schematic illustration, dimension of tensile specimen and details of tensile testing were described in Ref. [8]. 3. Results and discussion The bottom region, which is the upstream side of precursor gas and lower temperature, had higher porosity in the composite. This tendency was improved by decreas￾ing the precursor and carrier gas flow rates at the latter part of the FCVI process and a much better uniform porosity in the composite was obtained (Fig. 1(a)). The average porosity as a function of fiber volume was shown in Fig. 2. It was found that the higher fiber fraction is very effective in decreasing the porosity; the average porosity was decreased to 15% with increasing the fiber volume fraction to 44%. In the typical cross-section of SiC/SiC composite shown in Fig. 3, two types of pores were observed; one is a large pore around fiber bundles, and the other is a small pore in a bundle. The size of this inter-bundle pores was decreased with increasing the fiber fraction, though that of intra￾bundle pores was nearly constant as a function of fiber fraction. The distance between fiber bundles is shorter when the fiber fraction is higher, so the inter-bundle pore became smaller in SiC/SiC with the higher fiber fraction by the end of FCVI process. Although the carbon interphase of the bottom region was thicker than that of the top region by the conventional infiltration, the distribution of that thickness was signifi- cantly improved by flipping midway through the interphase infiltration(Fig. 1(b)). Fig. 3(c) shows the typical SEM image of multilayer SiC/C interphase. It was found that high uniform layers were formed in this method. Fig. 4 shows the tensile strength of SiC/SiC with the carbon interphase as a function of the carbon interphase thickness. In the present study, the tensile stress–strain curve in the results of tensile test exhibited pseude-ductile fracture mode, which means that carbon interphase was effective of the fiber pullout and excellent tensile properties. The tensile strength was slightly increased with increasing Fig. 1. Typical distribution of the (a) porosity and (b) thickness of carbon interphase of the FCVI-SiC/SiC. Fig. 2. Effect of the fiber volume fraction on the reduction of the porosity. 552 N. Igawa et al. / Journal of Physics and Chemistry of Solids 66 (2005) 551–554