P. Bansal, J.A. Setlock/Composites: Part A 32(2001)1021-1029 in Table 1. The fiber volume fraction in the composites was calculated to be -42%. From their densities, the composites ppear to be almost fully dense. The XRD pattern taken from the polished surface of the hot pressed composite was similar to that shown in Fig. 3. Monoclinic celsian was the only crystalline phase present and the undesired hexacelsian phase was not detected from XRD. This implies that the desired, thermodynamically stable, monoclinic celsian phase is formed in situ, from the mixed oxides recursor, during hot pressing of the composite. Doping with Sro facilitates 9, 10 the formation of monoclinic celsian in the matrix. In a recent study [17], a minor amount Fig 9. EDS spectra from(a)the coarsened BN region and(b) the corre of hexacelsian phase has been detected in the matrix using ponding microdiffraction pattern Raman micro-spectroscopy SEM micrographs taken from the polished cross-section (a-quartz) and BaAlO were the major phases preser of the unidirectional hot pressed composite are shown Small amounts of Ba2SiO4, a-Al2O3, and Ba2Sr2Al2O, Fig. 7. Uniform fiber distribution and good matrix infiltra- were also identified. Fig 4 shows the XRD pattern taken tion within the fiber tows is evident. Some of the filaments from the surface of a monolithic sample made by hot press- are of irregular shape rather than having circular cross- ing the precalcined powder at 1300.C for 2 h under section. The manufacturer reports an average fiber diameter 27.5 MPa(4 ksi). All the XRD peaks correspond to the of -14 um, but a large variation in the diameter of the monoclinic celsia filaments within a fiber tow can be seen. The BN/SiC undesirable hexacelsian phase. surface coating has been detached from some of the fibers SEM micrographs showing the surface and the cross- during metallography or composite processing Debonding tions of the BN/SiC coated Hi-Nicalon fibers are given or loss of the fiber coating may lead to adverse reactions in Fig. 5. The coatings on some of the fibers appear to be between the fibers and the oxide matrix at high temperature smooth and of uniform thickness whereas on other fibers the resulting in strong fiber -matrix bonding coating is very thick and granular. SEM micrographs taken TEM micrographs showing the fiber/matrix interface from the polished cross-sections of the fibers are shown in region of the Hi-Nicalon/BN/SIC/BSAS composite ig. 6. The double coating on the fiber surface is clearly presented in Fig 8. The BN visible. The dark layer is BN and the bright layer is Sic. layers as seen in Fig. 8(a). From EDS analysis, the fiber/BN Some of the composite physical characteristics are given interface was found to be rich in C along with the presence 800 08 10 Fig. 10. Stress-strain curves recorded in three-point flexure for unidirectional Hi-Nicalon/BN/SiC/BSAS composites hot pressed under(a) less and(b)more aggressive conditions> K 0?  .,1 !  F        .,1  .,/  ...,* !   
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