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K. Yoshida et al. / Composites Science and Technology 61(2001)1323-1329 internal compressive stress from the matrix due to the difference of thermal expansion coefficient between matrix and fiber, and then interfacial strength between fiber and matrix may increase. In the case of the com- posite obtained in this study, the thermal expansion coefficient of the matrix was considered to be higher than that of the fiber because the sic matrix contained components with higher thermal expansion coeficient, such as Al2O3( 8.8x10K- )and yttrium aluminum garnet(YAG: Y3Al5O12, 5.1x10-6K-)compared with Hi-Nicalon fiber (3. 5x10-6K-). Then, the tensile resi- dual stress on the matrix and the compressive residual stress on the fibers would have taken place and the tight interface between fiber and matrix was formed. as a result, the composite would show a redu strength for matrix cracking and have difficulty in fiber (b) pull-out at RT The present authors [30] investigated the mic structure of Y2O3-Cao additives using slurry-impregnated Hi Nicalon cloth by transmission electron microscopy. It was shown that some crystals grew from the matrix into a bn layer and the thickness of the layer was not homogeneous. Furthermore, growth of Sic crystals in the fiber, which induced degradation of fiber strength, was accelerated by the diffusion of sintering additives from the matrix. Therefore it was confirmed that some reactions between matrix/fiber coating/fiber occurred in the case of slurry-impregnated cloths, but these interac- tions between matrix/fiber coating/fiber were effectively suppressed in the case of PCS-impregnated cloths In the case of the hot-pressing temperature of 1650oC the strengthening of the matrix is not enough due to higher viscosity of the glassy phase at sintering tempera ture. Higher viscosity causes insufficient matrix impreg nation between fibers, therefore, fiber/matrix interfacial strength would be weak, resulting in decreasing maximum strength and large fiber pull-out. In contrast, in the case of the hot-pressing temperature of 1750oC, the trengthening of the matrix progressed due to lot of the glassy phase sintering temperature. Then the interfacial strength would be relatively high, but the degradation of fiber due to the exposure to h progressed, resulting in low maximum strength, short fiber pull-out and then low fracture energy. In this study, 0 d that the temperature of 1700C would be a better condition for hot-pressing to obtain the SiC/SiCt Fig. 8. SEM micrographs of the fracture surface of the Sic/SiCt composites hot- pressed at(a)1650°C,(b)1700°Cand(c)1750° after composite with good mechanical properties. The differ a three-point bending test at room ter ure. Fiber volume fraction ence in maximum strength, and then fracture energy was about 52 vol% depended greatly on the fiber volume fraction. From these results it is concluded that the use of considered to be one of the most important factors PCS-impregnated Hi-Nicalon cloths as the reinforce- influencing the characteristics of interfacial strength ment and an increase in fiber volume fraction are effec [20]. As sintering temperature is increased, the reactions tive ways to improve the mechanical properties of SiC/ between fiber/fiber coating/matrix will progress, and SiCr compositesconsidered to be one of the most important factors influencing the characteristics of interfacial strength [20]. As sintering temperature is increased, the reactions between fiber/fiber coating/matrix will progress, and internal compressive stress from the matrix due to the difference of thermal expansion coefficient between matrix and fiber, and then interfacial strength between fiber and matrix may increase. In the case of the composite obtained in this study, the thermal expansion coefficient of the matrix was considered to be higher than that of the fiber, because the SiC matrix contained components with higher thermal expansion coefficient, such as Al2O3 (8.8106 K1 ) and yttrium aluminum garnet (YAG: Y3Al5O12, 5.1106 K1 ) compared with Hi-Nicalon fiber (3.5106 K1 ). Then, the tensile residual stress on the matrix and the compressive residual stress on the fibers would have taken place and the tight interface between fiber and matrix was formed. As a result, the composite would show a reduction in strength for matrix cracking and have difficulty in fiber pull-out at R.T. The present authors [30] investigated the microstructure of the composites hot-pressed with Al2O3– Y2O3–CaO additives using slurry-impregnated HiNicalon cloth by transmission electron microscopy. It was shown that some crystals grew from the matrix into a BN layer and the thickness of the layer was not homogeneous. Furthermore, growth of SiC crystals in the fiber, which induced degradation of fiber strength, was accelerated by the diffusion of sintering additives from the matrix. Therefore, it was confirmed that some reactions between matrix/fiber coating/fiber occurred in the case of slurry-impregnated cloths, but these interactions between matrix/fiber coating/fiber were effectively suppressed in the case of PCS-impregnated cloths. In the case of the hot-pressing temperature of 1650C, the strengthening of the matrix is not enough due to higher viscosity of the glassy phase at sintering temperature. Higher viscosity causes insufficient matrix impregnation between fibers, therefore, fiber/matrix interfacial strength would be weak, resulting in decreasing maximum strength and large fiber pull-out. In contrast, in the case of the hot-pressing temperature of 1750C, the strengthening of the matrix progressed due to lower viscosity and higher diffusion rate of the glassy phase at sintering temperature. Then the interfacial strength would be relatively high, but the degradation of fiber due to the exposure to high-temperature simultaneously progressed, resulting in low maximum strength, short fiber pull-out and then low fracture energy. In this study, it was concluded that the temperature of 1700C would be a better condition for hot-pressing to obtain the SiC/SiCf composite with good mechanical properties. The difference in maximum strength, and then fracture energy depended greatly on the fiber volume fraction. From these results, it is concluded that the use of PCS-impregnated Hi-Nicalon cloths as the reinforcement and an increase in fiber volume fraction are effective ways to improve the mechanical properties of SiC/ SiCf composites. Fig. 8. SEM micrographs of the fracture surface of the SiC/SiCf composites hot-pressed at (a) 1650C, (b) 1700C and (c) 1750C after a three-point bending test at room temperature. Fiber volume fraction was about 52 vol.%. 1328 K. Yoshida et al. / Composites Science and Technology 61 (2001) 1323–1329