wwceramics. org/ACT Properties of Sic Matrix Composite 315 between the matrix and fiber, as a result, the fexural sample C, because of the larger opening width of crack strength of sample B did not increase apparently with(Fig. Ib)than that in sample C(Fig. Ic). In summary, the decrease of open porosity. the hybrid fibers can significantly improve the oxidation The brittle fracture mode of carbon fibers limits the resistance of Sic omposite when compared with strength and toughness of the hybrid composite. It is carbon-fiber-reinforced SiC matrix composites in air at supposed that the thickness of PyC interphase(150 nm) 700C in Cr/Py C/SiC composite was too thin to make the car- bon fiber be pulled out. To increase the thickness of PyC int only decrease the fract Conclusions nergy of PyC interphase but also reduce the roughness asperity interactions between the fber and matrix, Microstructure and mechanical properties of SiC which can make the Prc interphase debond during and carbon hybrid fibers reinforced SiC matrix com- posite were investigated, which were compared with thicknesses also can serve to accommodate residual ther- those of C/Py C/SiC and C/PyC/SiC mal mismatch stresses In a word, increasing the thick With the decrease of the open porosity, the fexural ness of PyC interphase may further improve the strength of the composites fitted the exponential func mechanical properties of the hybrid composite. tion. It was revealed that the ke the fracture behavior is the ratio of fracture energy of The Oxidation Behaviors of the Composite PyC interphase to the adjacent fiber. He and Hutchin- son's model that support these results are presented an The weight changes of the three kinds of compos- discussed in the present work. The oxidation properties ites as a function of oxidation time in air at 700C fe of the three kinds of composites were studied in air at 10h are shown in Fig. 8. The strength retention ratios 700 C for 10h. The main conclusions were as follows of the three kinds of composites were 99%, 67%, and (1)Both(SiC-C)/Py C/SiC hybrid composite and 78%,respectively, after oxidation for 10 h, as shown in C/PyCHT/SiC composite had higher flexural strength Table Il, Almost no weight change was found for sam- than C/PyC/SiC composite. With the decrease of th ple a after 10 h oxidation. However, there is an appar open porosity, the fexural strength of( SiC-C)/PyC/ ent weight loss for samples B and C, which were SiC and C/PyC /SiC composites exhibited the sig consistent with that of Cheng and colleagues. The nificantly incr nificantly increasing tendency but that of C/PyC/S microcracks in samples B and C acted as channels for was nearly independent on the open porosity xygen to diffuse into the interior of the composites. (2)(SiC-C)/PyC/SiC hybrid composite had th The weight loss of sample b was larger than that of less TRS-induced matrix microcracks compared with those in C/PyC/SiC composite and C/PyC /SiC composite. Compared with the C/PyC/SiC composite, the mechanical properties of hybrid composite were improved significantly by the pullout of SiC fibers due to a working PyC interphase. However, compared with the C/Py iC composite, the properties of hybrid -Sample C -Sample B composite at room temperature were not different ignificantl (3)The crystalline degree of the PyC interphase was modified by the heat treatment, which made the pullout of the carbon fibers possible for C/PyC/SiC (4)The fracture behaviors of the composites were supported by the He and Hutchinsons model. Whether Oxidation time(Hrs) the interphase can be debonded or not was mainly de rmined by the ratio of fracture of PyC inter phase to the adjacent fiber. Both higher fracture energybetween the matrix and fiber, as a result, the flexural strength of sample B did not increase apparently with the decrease of open porosity. The brittle fracture mode of carbon fibers limits the strength and toughness of the hybrid composite. It is supposed that the thickness of PyC interphase (150 nm) in Cf/PyC/SiC composite was too thin to make the car￾bon fiber be pulled out. To increase the thickness of PyC interphase may not only decrease the fracture energy of PyC interphase but also reduce the roughness asperity interactions between the fiber and matrix, which can make the PyC interphase debond during loading.6 Meanwhile, PyC interphase with increased thicknesses also can serve to accommodate residual ther￾mal mismatch stresses. In a word, increasing the thick￾ness of PyC interphase may further improve the mechanical properties of the hybrid composite. The Oxidation Behaviors of the Composites The weight changes of the three kinds of compos￾ites as a function of oxidation time in air at 7001C for 10 h are shown in Fig. 8. The strength retention ratios of the three kinds of composites were 99%, 67%, and 78%, respectively, after oxidation for 10 h, as shown in Table II, Almost no weight change was found for sam￾ple A after 10 h oxidation. However, there is an appar￾ent weight loss for samples B and C, which were consistent with that of Cheng and colleagues.3–5 The microcracks in samples B and C acted as channels for oxygen to diffuse into the interior of the composites. The weight loss of sample B was larger than that of sample C, because of the larger opening width of cracks (Fig. 1b) than that in sample C (Fig. 1c). In summary, the hybrid fibers can significantly improve the oxidation resistance of SiC matrix composite when compared with carbon-fiber-reinforced SiC matrix composites in air at 7001C. Conclusions Microstructure and mechanical properties of SiC and carbon hybrid fibers reinforced SiC matrix com￾posite were investigated, which were compared with those of C/PyC/SiC and C/PyCHT/SiC composites. With the decrease of the open porosity, the flexural strength of the composites fitted the exponential func￾tion. It was revealed that the key parameter to control the fracture behavior is the ratio of fracture energy of PyC interphase to the adjacent fiber. He and Hutchin￾son’s model that support these results are presented and discussed in the present work. The oxidation properties of the three kinds of composites were studied in air at 7001C for 10 h. The main conclusions were as follows: (1) Both (SiC–C)/PyC/SiC hybrid composite and C/PyCHT/SiC composite had higher flexural strength than C/PyC/SiC composite. With the decrease of the open porosity, the flexural strength of (SiC–C)/PyC/ SiC and C/PyCHT/SiC composites exhibited the sig￾nificantly increasing tendency but that of C/PyC/SiC was nearly independent on the open porosity. (2) (SiC–C)/PyC/SiC hybrid composite had the less TRS-induced matrix microcracks compared with those in C/PyC/SiC composite and C/PyCHT/SiC composite. Compared with the C/PyC/SiC composite, the mechanical properties of hybrid composite were improved significantly by the pullout of SiC fibers due to a working PyC interphase. However, compared with the C/PyCHT/SiC composite, the properties of hybrid composite at room temperature were not different significantly. (3) The crystalline degree of the PyC interphase was modified by the heat treatment, which made the pullout of the carbon fibers possible for C/PyC/SiC composite. (4) The fracture behaviors of the composites were supported by the He and Hutchinson’s model. Whether the interphase can be debonded or not was mainly de￾termined by the ratio of fracture energy of PyC inter￾phase to the adjacent fiber. Both higher fracture energy Fig. 8. The weight changes of the three kinds of composites in air at 7001C versus oxidation time. www.ceramics.org/ACT Properties of SiC Matrix Composite 315