S.M. Dong et al. Ceramics International 28(2002)899-905 age by the pure polymer precursor during the pyrolysis process Microstructure of intra-bundle matrix is shown in Fig. 3. In composite TSA/SiC, as the PIMP process progresses, the matrix became dense. Meanwhile, the initially formed matrix around fibers could also be identified in some areas. The following cycles of PIMP left a clear profile in intra-bundle matrix. Even in this case, the matrix relatively maintained the uniformit around fibers. However, some debonding between fibers and matrix could be observed. In TSA/C/SiC compo site, matrix seems not to be dense even though large amounts of intra-bundle matrix and inter -bundle matrix effectively formed in the first cycle impregnation Fig 1. Typical SEM micrograph on the polished cross-section of the and the particles were not strongly bonded together at composite TSA/SiC the present PIMP temperature. This"porous"matrix might be ascribed to the difficulty for achieving effective polymer impregnation after the matrix was formed in the first cycle impregnation. Generally, during PIMP phenomenon in conventional PIP prepared samples. decrease gradually in size when PIMP cycles proceeded Further observation of the intra-bundle and inter-bun- and then hindered further polymer impregnation. When dle matrix formation was conducted by optical micro- the pores were small enough, the viscous polymer pre- scopy on the polished cross-section, as shown in Fig. 2. cursor could not be effectively impregnated into the Dense intra-bundle matrix can be identified. Meanwhile, consolidated body. At this time, the process should be microcracks still remained but more extensively in the stopped. With particulate loading, the inter-bundle and TSA/C/SiC composite. In this composite, relatively intra-bundle spaces were relatively easier to be filled. large amount of matrix formed in both intra-bundle and After few cycles of PIMP, the remaining porosity, espe inter-bundle areas(Fig. 2b), indicating that the infiltra- cially the open porosity on the outer surface of the tion efficiency was high when particulates were loaded composites was greatly lowered, making the impregna in the first-cycle of impregnation. In composite TSA/ tion more difficult. Since both of the composite densifi SiC (using non-coated Tyranno SA fiber preforms as cation and matrix strengthening are highly dependent reinforcement and without particulate loading during on the polymer impregnation and pyrolysis, insufficient impregnation), intra-bundle fibers and fiber layers were impregnation of polymer precursor implies that the tightly bonded together. Only thin inter-layer matrix matrix might not be strongly bonded In the TSa/C/SiC could be evidenced, as shown by arrow in Fig 2a. These composite, polymer impregnation was stopped after six results might be ascribed to low mass inclusion( without cycles of PIMP, while for TSA/SiC, eight cycles could the addition of Sic particles) and large volume shrink- Fig. 2. Optical photographs of the polished cross-section of the composites showing the intra-bundle and inter-bundle matrix formation and the cracks propagation: (a) TSA/SiC, (b) TSA/C/SiC.areas and inter-layers. This is also the typically observed phenomenon in conventional PIP prepared samples. Further observation of the intra-bundle and inter-bun￾dle matrix formation was conducted by optical micro￾scopy on the polished cross-section,as shown in Fig. 2. Dense intra-bundle matrix can be identified. Meanwhile, microcracks still remained but more extensively in the TSA/C/SiC composite. In this composite,relatively large amount of matrix formed in both intra-bundle and inter-bundle areas (Fig. 2b),indicating that the infiltra￾tion efficiency was high when particulates were loaded in the first-cycle of impregnation. In composite TSA/ SiC (using non-coated Tyranno SA fiber preforms as reinforcement and without particulate loading during impregnation),intra-bundle fibers and fiber layers were tightly bonded together. Only thin inter-layer matrix could be evidenced,as shown by arrow in Fig. 2a. These results might be ascribed to low mass inclusion (without the addition of SiC particles) and large volume shrink￾age by the pure polymer precursor during the pyrolysis process. Microstructure of intra-bundle matrix is shown in Fig. 3. In composite TSA/SiC,as the PIMP process progresses,the matrix became dense. Meanwhile,the initially formed matrix around fibers could also be identified in some areas. The following cycles of PIMP left a clear profile in intra-bundle matrix. Even in this case,the matrix relatively maintained the uniformity around fibers. However,some debonding between fibers and matrix could be observed. In TSA/C/SiC compo￾site,matrix seems not to be dense even though large amounts of intra-bundle matrix and inter-bundle matrix were effectively formed in the first cycle impregnation, and the particles were not strongly bonded together at the present PIMP temperature. This ‘‘porous’’ matrix might be ascribed to the difficulty for achieving effective polymer impregnation after the matrix was formed in the first cycle impregnation. Generally,during PIMP process,the micropores left in the matrix would decrease gradually in size when PIMP cycles proceeded and then hindered further polymer impregnation. When the pores were small enough,the viscous polymer pre￾cursor could not be effectively impregnated into the consolidated body. At this time,the process should be stopped. With particulate loading,the inter-bundle and intra-bundle spaces were relatively easier to be filled. After few cycles of PIMP,the remaining porosity,espe￾cially the open porosity on the outer surface of the composites was greatly lowered,making the impregna￾tion more difficult. Since both of the composite densifi- cation and matrix strengthening are highly dependent on the polymer impregnation and pyrolysis,insufficient impregnation of polymer precursor implies that the matrix might not be strongly bonded. In the TSA/C/SiC composite,polymer impregnation was stopped after six cycles of PIMP,while for TSA/SiC,eight cycles could be performed. Fig. 1. Typical SEM micrograph on the polished cross-section of the composite TSA/SiC. Fig. 2. Optical photographs of the polished cross-section of the composites showing the intra-bundle and inter-bundle matrix formation and the cracks propagation: (a) TSA/SiC,(b) TSA/C/SiC. S.M. Dong et al. / Ceramics International 28 (2002) 899–905 901