K Shimoda et al. Composites Science and Technology 68(2008)98-105 Fibers Mat c) Matri> Fiber Pyc m Fig. 6. Backscattered electron images of polished cross-section at intra-fibers of SiC/SiC composites with various PyC interface thickness: (a)uncoated, (b) CVD-025,(c)CVD-050 and(d) CVD.1.00 mental stability, compared to CVI-SiC/SiC. The differences The increase of PyC thickness decreased the bulk density in properties between NITE and CVI composites come No big pores could be identified at inter-fiber-tows, as from the difference in matrix porosity, which is less than shown in Fig. 5b-d. Pores are mainly distributed in the a few percent in the former. As shown in Fig. 4, This intra-fiber-tows regions. The composites with PyC thick may be supposed that macro-pores(hundreds microns ness of 1.00 um maintained about 7% porosity The forma in size) at inter-fiber-tows and closed micro-pores (oa tion of porous intra-fiber-tows matrix might be due to the few microns)at intra-fiber-tows are mainly observed in poor SiC nano-powder infiltration. As shown in Fig. 6d CVI based composites, on the other hand distributed increasing PyC coating thickness caused the difficulty of micro-pores(below 10 um)at inter-fiber-tows are only SiC nano-powder infiltration due to the promotion of nar- observed in NITE composites. Figs. 5 and 6 show SEM row aisle and/or the attachment between PyC layers For images of cross section and backscattered electron images all that, its density is comparatively higher than that of at intra-fiber-tows of cross section of SiC/SiC composites conventional CVI-, PIP-SiC/SiC composites. Only when with various PyC interface thickness after polishing. In this the thickness of Pyc is about 0. 25 um, PyC layer was dis- study, nearly fully densified(almost 99% of theoretical den- appeared and degraded severely, as shown in Fig. 6b ity) SiC/SiC composites without PyC coating fiber tows Moreover, the thickness of PyC was decreased in the cases could be obtained with the sufficient infiltration of Sic of CVD-050 and CVD-1.00. These can explain through nano-powder into the narrow region of the intra-fiber- the reactions between PyC layer and additional oxide addi- wS,as shown in Fig. 6a. However, uncoated fibers tives. From thermodynamics, the following reaction is con- showed attachment to matrix and irregular fiber edges. sidered as main [21, 22: Table 4 Mechanical properties of hot-pressed SiC/SiC composites with various interface conditions Pyc thickness(nm) Flexural strength(MPa Uncoated 0 l140±50 CVD-0. 25 780±50 247±13 CvD-0.50 l00-250 600±26 185±12 CVD.L.00 409±60 164±8mental stability, compared to CVI-SiC/SiC. The differences in properties between NITE and CVI composites come from the difference in matrix porosity, which is less than a few percent in the former. As shown in Fig. 4, This may be supposed that macro-pores (hundreds microns in size) at inter-fiber-tows and closed micro-pores (a few microns) at intra-fiber-tows are mainly observed in CVI based composites, on the other hand distributed micro-pores (below 10 lm) at inter-fiber-tows are only observed in NITE composites. Figs. 5 and 6 show SEM images of cross section and backscattered electron images at intra-fiber-tows of cross section of SiC/SiC composites with various PyC interface thickness after polishing. In this study, nearly fully densified (almost 99% of theoretical den￾sity) SiC/SiC composites without PyC coating fiber tows could be obtained with the sufficient infiltration of SiC nano-powder into the narrow region of the intra-fiber￾tows, as shown in Fig. 6a. However, uncoated fibers showed attachment to matrix and irregular fiber edges. The increase of PyC thickness decreased the bulk density. No big pores could be identified at inter-fiber-tows, as shown in Fig. 5b–d. Pores are mainly distributed in the intra-fiber-tows regions. The composites with PyC thick￾ness of 1.00 lm maintained about 7% porosity. The forma￾tion of porous intra-fiber-tows matrix might be due to the poor SiC nano-powder infiltration. As shown in Fig. 6d, increasing PyC coating thickness caused the difficulty of SiC nano-powder infiltration due to the promotion of nar￾row aisle and/or the attachment between PyC layers. For all that, its density is comparatively higher than that of conventional CVI-, PIP-SiC/SiC composites. Only when the thickness of PyC is about 0.25 lm, PyC layer was dis￾appeared and degraded severely, as shown in Fig. 6b. Moreover, the thickness of PyC was decreased in the cases of CVD-0.50 and CVD-1.00. These can explain through the reactions between PyC layer and additional oxide addi￾tives. From thermodynamics, the following reaction is con￾sidered as main [21,22]: Fig. 6. Backscattered electron images of polished cross-section at intra-fibers of SiC/SiC composites with various PyC interface thickness: (a) uncoated, (b) CVD-0.25, (c) CVD-0.50 and (d) CVD-1.00. Table 4 Mechanical properties of hot-pressed SiC/SiC composites with various interface conditions PyC thickness (nm) Flexural strength (MPa) Elastic modulus (GPa) Uncoated 0 1140 ± 50 250 ± 7 CVD-0.25 0 780 ± 50 247 ± 13 CVD-0.50 100–250 600 ± 26 185 ± 12 CVD-1.00 450–600 409 ± 60 164 ± 8 102 K. Shimoda et al. / Composites Science and Technology 68 (2008) 98–105