M. Kotani et al. Composites Science and Technology 62(2002)2179-2188 Apparent difference was not identified between these 3. 2.2. Heating rate conditions. There seemed to be enough matrix as to Fig. 10 shows relative densities of the consolidated bind fibers tightly in both samples, though many pores bodies at various heating rates. There could be con- of various sizes still remained all over the specimen. sidered two main influences by heating rate, i.e. volu- These pores would be formed due to volume shrinkage metric yield of a polymer-pyrolyzed product as of matrix precursor. While, stratiform structure where demonstrated in Fig. 4 and gas evolution rate. Volu- both fiber rich and matrix rich areas were formed was metric yield of a precursor directly affected porosity recognized all over the specimen in micrograph(c). It while gas evolution may cause crack initiation if it is too due to poor fluidity of the precursor. This structure tinuously improved by slowing the heating rate down o would take over intermediate layer between cured sheet much. In case of 583 K. relative densities were co could provide a negative influence on mechanical beha 60 K/h. However, it declined at low heating rate from viors such as delamination, in spite of superior relative 30 to 10 K/h. While, relative densities of the composites density [27]. Micrograph(d) is a typical example of the at 643 K were continuously improved as heating rate microstructure cured at beyond 623 K. Many cracks as became slower all over the range, although those differ well as lamination layers were observed all over the ences were fairly smaller. There could not be seen the specimen. It would be initiated by pressure applied for declination of the density that was shown in case of 583 excessively hardened sheets. K. According to the previous report that showed that With above careful inspection for the consolidated the evolution of Sic structure didn, t depend on only bodies, it was demonstrated that the density and micro- temperature but time exposed at high temperature [2 structure of a polymer-derived composite depend on thethe crystallization of the polymer-pyrolyzed product consolidation conditions, which closely related with the could be employed as one of reason for the declination. characteristics of a precursor. Fig. 9 shows schematic But, considering that the declination was shown only ummary of the effect of consolidation conditions in the for the green bodies cured up to 583 K, it was possibly aspects of density and microstructure. Microstructures related with heating condition under pressure before the seemed to be classified into following four condition polymer was solidified. As fiber distribution in a green areas: (1)effective consolidation was achieved with well- body cured up to 643 K could not be changed during balanced pressure for cured sheets, where the homo- consolidation so much, the improvement of the density geneity of matrix was dependent on curing temperature, was considered to be owed only to the increase of cera- (2)pores and cracks were formed due to insufficient mic yield. Greater increase and subsequent decline of supression,(3) fibrous body with little matrix was pro- relative density which were shown in case of 583 K must duced due to excess pressure, (4)laminate structure and include any other factors than the yield. crack were formed due to poor fluidity of a precursor Fig. 1l exhibits cross sectional micrographs of the From the standpoint of density and microstructure, the consolidated bodies at the heating rates of (a)600,(b) condition(603, 5)seemed to be most promising process 30 and(c)10 K/h, respectively. Those composites were candidate for high performance composite production derived from the green bodies cured up to 583 K. There 583K 2 1000 10000 Curing Temperature Heating Rate/Kh Fig 9. Schematic summary of consolidation conditions in the aspects Fig. 10. Relative densities of as-consolidated bodies at various heating of density and microstructureApparent difference was not identified between these conditions. There seemed to be enough matrix as to bind fibers tightly in both samples, though many pores of various sizes still remained all over the specimen. These pores would be formed due to volume shrinkage of matrix precursor. While, stratiform structure where both fiber rich and matrix rich areas were formed was recognized all over the specimen in micrograph (c). It would take over intermediate layer between cured sheets due to poor fluidity of the precursor. This structure could provide a negative influence on mechanical beha￾viors such as delamination, in spite of superior relative density [27]. Micrograph (d) is a typical example of the microstructure cured at beyond 623 K. Many cracks as well as lamination layers were observed all over the specimen. It would be initiated by pressure applied for excessively hardened sheets. With above careful inspection for the consolidated bodies, it was demonstrated that the density and micro￾structure of a polymer-derived composite depend on the consolidation conditions, which closely related with the characteristics of a precursor. Fig. 9 shows schematic summary of the effect of consolidation conditions in the aspects of density and microstructure. Microstructures seemed to be classified into following four condition areas: (1) effective consolidation was achieved with well￾balanced pressure for cured sheets, where the homo￾geneity of matrix was dependent on curing temperature, (2) pores and cracks were formed due to insufficient supression, (3) fibrous body with little matrix was pro￾duced due to excess pressure, (4) laminate structure and crack were formed due to poor fluidity of a precursor. From the standpoint of density and microstructure, the condition (603, 5) seemed to be most promising process candidate for high performance composite production. 3.2.2. Heating rate Fig. 10 shows relative densities of the consolidated bodies at various heating rates. There could be con￾sidered two main influences by heating rate, i.e. volu￾metric yield of a polymer-pyrolyzed product as demonstrated in Fig. 4 and gas evolution rate. Volu￾metric yield of a precursor directly affected porosity, while gas evolution may cause crack initiation if it is too much. In case of 583 K, relative densities were con￾tinuously improved by slowing the heating rate down on 30 K/h. However, it declined at low heating rate from 30 to 10 K/h. While, relative densities of the composites at 643 K were continuously improved as heating rate became slower all over the range, although those differ￾ences were fairly smaller. There could not be seen the declination of the density that was shown in case of 583 K. According to the previous report that showed that the evolution of SiC structure didn’t depend on only temperature but time exposed at high temperature [28], the crystallization of the polymer-pyrolyzed product could be employed as one of reason for the declination. But, considering that the declination was shown only for the green bodies cured up to 583 K, it was possibly related with heating condition under pressure before the polymer was solidified. As fiber distribution in a green body cured up to 643 K could not be changed during consolidation so much, the improvement of the density was considered to be owed only to the increase of cera￾mic yield. Greater increase and subsequent decline of relative density which were shown in case of 583 K must include any other factors than the yield. Fig. 11 exhibits cross sectional micrographs of the consolidated bodies at the heating rates of (a) 600, (b) 30 and (c) 10 K/h, respectively. Those composites were derived from the green bodies cured up to 583 K. There Fig. 9. Schematic summary of consolidation conditions in the aspects of density and microstructure. Fig. 10. Relative densities of as-consolidated bodies at various heating rates. M. 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