3rd stage retard the densification. On the other hand, densification of composites with high-V was not sufficient at lower fabrication 2nd stage could effectively enhance the densification. At 1900C, almost 99% of the theoretical density could be obtained. Densification of the composite with high-V had the difficulty to explain by the clas 3. 2. Microstructural evolutions The matrix in NITE composites consists of well-crystallized Sic grains with small amount of oxide remnants including alum Monolithic Sic um and yttrium(mostly YAG: yttrium aluminum garnet)from b-Composites(High-V XRD analysis[17 The presence of YAG should improve the densi Composites(Low-vy fication of matrix in composites. Fig. 3 shows low-magnification and intra-fiber-bundle microstructures of polished cross-section of Sic/SiC composites. Typically, no inter-fiber-bundle pores were 1700 observed, even in the composites with high-V fabricated at 1800C. Many small pores were observed in the intra-fiber-bun- Fabrication temperature (oc) dles for the composites with high-V fabricated at 1800C. The Fig. 2. Effects of the fabrication temperature on densification of monolithic SiC and composites maintained about 6. 2% open porosity, so that the for- wo SiC/SiC composites. mation of porous intra-fiber-bundle matrix could be identified. With increasing temperature, the pore size and the number of pores decreased, and simultaneously densification in the intra-fi omposites, densification was well reflected to the classic liquid ber-bundles was promoted better. At 1900C, intra-fiber-bundle phase sintering, and thus could be divided into three stages, as pores were strongly eliminated, as shown in Fig. 3e. Even for the shown in Fig. 2. Densification of the composites with low-V was well-densified intra-fiber-bundle matrix, some differences co dramatically promoted at the temperature of 1750-1800C prob- be identified with different fabrication temperature. In the com- ably due to the liquid phase formation of sintering additives lead- posites with low-V fabricated at 1800C, some consolidated parts ing to the rearrangement of SiC nano-powder, and then saturated were loosely formed On the other hand, in the composites with above 1800C In NITE process, Al2O3-Y2O3-SiOz system is used low- and high-V/ fabricated at 1900C, consolidated parts were tightly formed, and matrix and Py c interface also tightly ture for liquid phase between Al203 and Y203 can be lowered to bonded together. In CFCCS, the distance among fibers of intra-ii 1700-1800C by additional SiO2 [23]. The composites with low- ber-bundles is shortened when fiber volume fraction increases V demonstrated similar characteristics of monolithic Sic corre- [24]. Even for Sic nano-powder, infiltration and densification of sponding to the matrix although the introduction of fibers might matrix in the intra-fiber-bundles strongly depended on the fiber 10u 50um 50um um s 50um Ou 50um 50um Fig3. FE-SEM images of polished cross-section of Sic/siC composites with low-V/(a)at 1800C and (b)at 1900.C and with high-V:(c)at 1800C(d)at 1850.C and(e)atcomposites, densification was well reflected to the classic liquid phase sintering, and thus could be divided into three stages, as shown in Fig. 2. Densification of the composites with low-Vf was dramatically promoted at the temperature of 1750–1800 C prob￾ably due to the liquid phase formation of sintering additives lead￾ing to the rearrangement of SiC nano-powder, and then saturated above 1800 C. In NITE process, Al2O3–Y2O3–SiO2 system is used for SiC/SiC composites as sintering additives because the tempera￾ture for liquid phase between Al2O3 and Y2O3 can be lowered to 1700–1800 C by additional SiO2 [23]. The composites with low￾Vf demonstrated similar characteristics of monolithic SiC corre￾sponding to the matrix although the introduction of fibers might retard the densification. On the other hand, densification of the composites with high-Vf was not sufficient at lower fabrication temperature (1800 C). The increasing fabrication temperature could effectively enhance the densification. At 1900 C, almost 99% of the theoretical density could be obtained. Densification of the composite with high-Vf had the difficulty to explain by the clas￾sic liquid phase. 3.2. Microstructural evolutions The matrix in NITE composites consists of well-crystallized b￾SiC grains with small amount of oxide remnants including alumi￾num and yttrium (mostly YAG: yttrium aluminum garnet) from XRD analysis [17]. The presence of YAG should improve the densi- fication of matrix in composites. Fig. 3 shows low-magnification and intra-fiber-bundle microstructures of polished cross-section of SiC/SiC composites. Typically, no inter-fiber-bundle pores were observed, even in the composites with high-Vf fabricated at 1800 C. Many small pores were observed in the intra-fiber-bun￾dles for the composites with high-Vf fabricated at 1800 C. The composites maintained about 6.2% open porosity, so that the for￾mation of porous intra-fiber-bundle matrix could be identified. With increasing temperature, the pore size and the number of pores decreased, and simultaneously densification in the intra-fi- ber-bundles was promoted better. At 1900 C, intra-fiber-bundle pores were strongly eliminated, as shown in Fig. 3e. Even for the well-densified intra-fiber-bundle matrix, some differences could be identified with different fabrication temperature. In the com￾posites with low-Vf fabricated at 1800 C, some consolidated parts were loosely formed. On the other hand, in the composites with low- and high-Vf fabricated at 1900 C, consolidated parts were tightly formed, and matrix and PyC interface were also tightly bonded together. In CFCCs, the distance among fibers of intra-fi- ber-bundles is shortened when fiber volume fraction increases [24]. Even for SiC nano-powder, infiltration and densification of matrix in the intra-fiber-bundles strongly depended on the fiber Fig. 2. Effects of the fabrication temperature on densification of monolithic SiC and two SiC/SiC composites. Fig. 3. FE-SEM images of polished cross-section of SiC/SiC composites with low-Vf: (a) at 1800 C and (b) at 1900 C and with high-Vf; (c) at 1800 C, (d) at 1850 C and (e) at 1900 C. K. Shimoda et al. / Composites Science and Technology 69 (2009) 1623–1628 1625