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K. Yoshida et al. Composites Science and Technology 61(2001)1323-1329 h in air under a pressure of 20 kPa. In this process, the Bulk density was measured by Archimedes' method. heating rate was 10oC/h from 150 to 300oC in order to Theoretical density of the SiC/SiCr composite was cal- prevent the combustion of Pcs due to rapid oxidation culated as that of a mixture of Sic matrix with sintering [23]. The introduction of oxygen into the PCS structure additives and Hi- Nicalon fiber during the heat-treatment in air, i.e. thermal oxidation- Three-point bending strength was measured at room curing, was promoted. Thermal oxidation curing was temperature in air with a cross-head speed of 0. 1 mm performed in order to prevent the impregnated-PCs min and a lower span of 30 mm. Bending strength into Hi-Nicalon cloths from flowing out due to lowering measurement was performed using a universal testing viscosity of PCs during hot-pressing. machine (Instron 1185, USA). Fracture energy was cal The stacked green body was hot-pressed at 1650, 1700 culated from the area of load-displacement curve in nd 1750oC for I h in Ar atmosphere under a uniaxial bending strength measurement divided by twice the due of the oxidation-cured PCS after pyrolysis at 1300C a scanning electron microscope (SEM re observed by pressure of 40 MPa. It was reported that the weight resi- fracture surface area. Fracture surface w and subsequent heat-treatment at 1500-1700oc in Ar was 43-66%[24. In this study, hot-pressing was performed at 1650-1750oC in Ar atmosphere, and the weight residue 3. Results and discussion of pcs was considered to be similar to these values Two kinds of composites with different volume fr ac- 3.. Microstructure and bulk density ons of fibers were fabricated by use of the green sheets with different thicknesses. Fiber volume fraction of the Fig. 2 shows the difference in microstructure of the composites with thicker or thinner sheets was about 40 SiC/SiCr composites using slurry-impregnated and PCs and 52 vol %o impregnated Hi-Nicalon cloths as the reinforcement. In the case of composite with slurry-impregnated Hi-Nica 2.3. Mechanical properties lon cloth[Fig. 2(a)], the SiC matrix did not form between fibers sufficiently. The fibers contacted directly each Hot-pressed specimens were cut into rectangular bars other, and then they deformed into polyhedral prism as (width: 3.5 mm, thickness: 2.3-3.2 mm, length: 34 mm). seen in Tyrannohex composites [25]. In contrast, in the (a) Slurry-impregnated (b)PCS-impregnated 50m Fig. 2. Microstructure of the SiC/SiCr composites fabricated by hot-pressing at 1750.C(a) SiC slurry-impregnated and(b)PCSimpregnated Hi- Nicalon cloths with BN-coating were used as the reinforcement.h in air under a pressure of 20 kPa. In this process, the heating rate was 10C/h from 150 to 300C in order to prevent the combustion of PCS due to rapid oxidation [23]. The introduction of oxygen into the PCS structure during the heat-treatment in air, i.e. thermal oxidationcuring, was promoted. Thermal oxidation curing was performed in order to prevent the impregnated-PCS into Hi-Nicalon cloths from flowing out due to lowering viscosity of PCS during hot-pressing. The stacked green body was hot-pressed at 1650, 1700 and 1750C for 1 h in Ar atmosphere under a uniaxial pressure of 40 MPa. It was reported that the weight residue of the oxidation-cured PCS after pyrolysis at 1300C and subsequent heat-treatment at 1500–1700C in Ar was 43–66% [24]. In this study, hot-pressing was performed at 1650–1750C in Ar atmosphere, and the weight residue of PCS was considered to be similar to these values. Two kinds of composites with different volume fractions of fibers were fabricated by use of the green sheets with different thicknesses. Fiber volume fraction of the composites with thicker or thinner sheets was about 40 and 52 vol.%, respectively. 2.3. Mechanical properties Hot-pressed specimens were cut into rectangular bars (width: 3.5 mm, thickness: 2.3–3.2 mm, length: 34 mm). Bulk density was measured by Archimedes’ method. Theoretical density of the SiC/SiCf composite was calculated as that of a mixture of SiC matrix with sintering additives and Hi-Nicalon fiber. Three-point bending strength was measured at room temperature in air with a cross-head speed of 0.1 mm/ min and a lower span of 30 mm. Bending strength measurement was performed using a universal testing machine (Instron 1185, USA). Fracture energy was calculated from the area of load-displacement curve in bending strength measurement divided by twice the fracture surface area. Fracture surface were observed by a scanning electron microscope (SEM). 3. Results and discussion 3.1. Microstructure and bulk density Fig. 2 shows the difference in microstructure of the SiC/SiCf composites using slurry-impregnated and PCSimpregnated Hi-Nicalon cloths as the reinforcement. In the case of composite with slurry-impregnated Hi-Nicalon cloth [Fig. 2(a)], the SiC matrix did not form between fibers sufficiently. The fibers contacted directly each other, and then they deformed into polyhedral prism as seen in Tyrannohex composites [25]. In contrast, in the Fig. 2. Microstructure of the SiC/SiCf composites fabricated by hot-pressing at 1750C. (a) SiC slurry-impregnated and (b) PCS-impregnated HiNicalon cloths with BN-coating were used as the reinforcement. K. Yoshida et al. / Composites Science and Technology 61 (2001) 1323–1329 1325