July 2006 Communications of the American Ceramic Society volume content was the highest(14.5 vol%)when vapor silicon filtrated at 1973 K Figure 3 shows scanning electron micrographs on the cross section of a Co/SiC composite processed at 1973 K. The dense matrix and regions of some microporosity can be seen 16 Fig. 3(a)). At higher magnification(Fig. 3(b)), fiber coatings (C and Sic) can be identified. However, it was difficult to iden- 12 tify the fiber coatings in many areas, although there was no evidence of reaction between the matrix and the fibers The composite prepared at 1873 K had a low flexural strength, about 160 MPa(as shown in Fig. 2), which resulted 1850 1950 2000 from its low bulk density. Because the amount of Sic matrix T/K formed by vapor silicon and carbon was small, the fiexural tress-displacement curves appeared more fiat. For composites Fig. 1. Density and porosity of the composites. filtrated at 1923 K, the flexural strength was around 288 MPa This composite also demonstrated the higher bending displace- ment. When the infiltration temperature increased to 1973 K displacement curves shown in Fig. 2. The densely formed matrix 1923K contributed to the improvement of the mechanical properties 250 Figure 4 shows scanning electron micrographs of the fracture urfaces of these composites, which revealed the occurrence of fiber pull-out, although the pull-out length was larger for the 150 lower infiltration temperature. The composite with higher den- sity demonstrated a relatively short fiber pull-out, as shown Fi 50 IV. Conclusion Displacement/ Dense Cr sic composites were fabricated at 1973k by vapor silicon infiltration. The density and porosity were 2.25 g/cmand Fig. 2. Stress-displacement curves for carbon 6%, respectively. The flexural strength reached nearly 300 carbide matrix composites via vapor silicon inf ion at different MPa and the material exhibited non-brittle fracture behavior It was found that the density and flexural strength of the composites decreased with decreasing infiltration temperature. During vapor silicon infiltration process, the amount of vapor layer Fig 3. Scanning electron microscopy observation on the polished cross section of carbon fiber reinforced silicon carbide matrix composites via vapor Si infiltration at 1973 K Fig 4. Fracture surfaces of carbon fiber reinforced silicon carbide matrix composites via vapor Si infiltration at different temperatures: (a)1873 K; (b)1923K;and(c)1973Kvolume content was the highest (14.5 vol%) when vapor silicon infiltrated at 1973 K. Figure 3 shows scanning electron micrographs on the cross section of a Cf/SiC composite processed at 1973 K. The dense matrix and regions of some microporosity can be seen (Fig. 3(a)). At higher magnification (Fig. 3(b)), fiber coatings (C and SiC) can be identified. However, it was difficult to iden￾tify the fiber coatings in many areas, although there was no evidence of reaction between the matrix and the fibers. The composite prepared at 1873 K had a low flexural strength, about 160 MPa (as shown in Fig. 2), which resulted from its low bulk density. Because the amount of SiC matrix formed by vapor silicon and carbon was small, the flexural stress–displacement curves appeared more flat. For composites infiltrated at 1923 K, the flexural strength was around 288 MPa. This composite also demonstrated the higher bending displace￾ment. When the infiltration temperature increased to 1973 K, the flexural strength reached nearly 300 MPa. Its stiffness was also higher, as demonstrated by the initial slope of the stress– displacement curves shown in Fig. 2. The densely formed matrix contributed to the improvement of the mechanical properties. Figure 4 shows scanning electron micrographs of the fracture surfaces of these composites, which revealed the occurrence of fiber pull-out, although the pull-out length was larger for the lower infiltration temperature. The composite with higher den￾sity demonstrated a relatively short fiber pull-out, as shown in Fig. 4(c). IV. Conclusion Dense Cf/SiC composites were fabricated at 1973 K by vapor silicon infiltration. The density and porosity were 2.25 g/cm3 and B6%, respectively. The flexural strength reached nearly 300 MPa and the material exhibited non-brittle fracture behavior. It was found that the density and flexural strength of the composites decreased with decreasing infiltration temperature. During vapor silicon infiltration process, the amount of vapor 1850 1900 1950 2000 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 T/K Denstiy/g/cm3 4 8 12 16 20 24 28 Open porosity /% Fig. 1. Density and porosity of the composites. 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 250 300 350 Flexure Stress/MPa Displacement/mm 1873K 1923K 1973K Fig. 2. Stress–displacement curves for carbon fiber reinforced silicon carbide matrix composites via vapor silicon infiltration at different temperatures. Fig. 3. Scanning electron microscopy observation on the polished cross section of carbon fiber reinforced silicon carbide matrix composites via vapor Si infiltration at 1973 K. Fig. 4. Fracture surfaces of carbon fiber reinforced silicon carbide matrix composites via vapor Si infiltration at different temperatures: (a) 1873 K; (b) 1923 K; and (c) 1973 K. July 2006 Communications of the American Ceramic Society 2339