Y Liu et al. Materials Science and Engineering A 475(2008)217-223 (b) CVI SiC matrix CVI SiC matrix SiC particle Residual pore CvI SiC matrix (d) CVI SiC whisker CVI SiC al microstructure of SiC(P/SiC composites(a) polished cross-section of SiCp/SiC composites; (b)SEM image of intra-agglomeration; (c)TEM image between two SiC reinforcing particles; (d) SEM image of SiC whisker formed intra-agglor hundreds nanometer, and the lengths of the whiskers were about sen diffusion. There is Sic globularity at the whiskers terminal, which shows the growth mechanism of the CVI SiC whiskers is According to the whole CVI process, microstructure differ- vapor-liquid-solid process ence of Sic deposits could be explained based on the deposition The flexural strength of specimen as a function of the first eaction and above gas diffusion process. The whole deposition step pressure and second-step pressure is shown in Fig 3. The reaction of SiC is shown in reaction(2). strengths of Sic(P/SiC composites with 15 MPa first-step pres- sure are all higher than those with 20 MPa. Under the 15 MPa CH3 SiCl3 -> SiC+ 3HCI (2) first-step pressure, the strength of the composites decreases when The reaction gases transfer to the agglomeration surfaces nrough transition diffusion. Some of the reaction gases diffuse into the inner of the agglomerations through Knudsen diffusion then the deposition reaction occurs, and the Sic is deposited in 22s0 the inter- and intra-agglomeration pores as shown in Fig. 2(a) and(b). The CH3SiCl3 will also traverse the residual porous carbon layer through Knudsen diffusion, which leads to the t formation of CVI SiC on the surface of Sic reinforcing par- icle as shown in Fig. 2(c). As deposition time increased, more nd more Sic deposits were deposited on the agglomeration surface and intra-agglomeration. The intra-agglomeration pores would first be blocked out due to the smaller pore size. There- after, the inter-agglomeration pores would also be blocked out There existed inter-and intra-agglomerations pores because of Second-step pressure(MPa) the release of by-product gases. In some intra-agglomeration Fig 3. Flexural strength of composites as a function of the first-step pressure pores,the CvI SiC whiskers grow at the low super-saturation, and second-step pressure. N ) First-step pressure 15MPa; (aa) Second-step because the small amount reaction gas transfers through Knud- pressure 20MPa220 Y. Liu et al. / Materials Science and Engineering A 475 (2008) 217–223 Fig. 2. Typical microstructure of SiC(P)/SiC composites (a) polished cross-section of SiCp/SiC composites; (b) SEM image of intra-agglomeration; (c) TEM image of interfaces between two SiC reinforcing particles; (d) SEM image of SiC whisker formed intra-agglomeration pores. hundreds nanometer, and the lengths of the whiskers were about 20m. According to the whole CVI process, microstructure differ￾ence of SiC deposits could be explained based on the deposition reaction and above gas diffusion process. The whole deposition reaction of SiC is shown in reaction (2). CH3SiCl3 1000 ◦C −→H2 SiC + 3HCl (2) The reaction gases transfer to the agglomeration surfaces through transition diffusion. Some of the reaction gases diffuse into the inner of the agglomerations through Knudsen diffusion, then the deposition reaction occurs, and the SiC is deposited in the inter- and intra-agglomeration pores as shown in Fig. 2(a) and (b). The CH3SiCl3 will also traverse the residual porous carbon layer through Knudsen diffusion, which leads to the formation of CVI SiC on the surface of SiC reinforcing par￾ticle as shown in Fig. 2(c). As deposition time increased, more and more SiC deposits were deposited on the agglomeration surface and intra-agglomeration. The intra-agglomeration pores would first be blocked out due to the smaller pore size. There￾after, the inter-agglomeration pores would also be blocked out. There existed inter- and intra-agglomerations pores because of the release of by-product gases. In some intra-agglomeration pores, the CVI SiC whiskers grow at the low super-saturation, because the small amount reaction gas transfers through Knud￾sen diffusion. There is SiC globularity at the whiskers terminal, which shows the growth mechanism of the CVI SiC whiskers is vapor–liquid–solid process. The flexural strength of specimen as a function of the first￾step pressure and second-step pressure is shown in Fig. 3. The strengths of SiC(P)/SiC composites with 15 MPa first-step pres￾sure are all higher than those with 20 MPa. Under the 15 MPa first-step pressure, the strength of the composites decreases when Fig. 3. Flexural strength of composites as a function of the first-step pressure and second-step pressure. ( ) First-step pressure 15 MPa; ( ) Second-step pressure 20 MPa.