1028 N P Bansal, J.A. Setlock/Composites: Part A 32(2001)1021-1029 Table 2 Room temperature mechanical properties of Hi-Nicalon fiber-reinforced Baa7sSro2s Al Si2Os celsian composites(Unidirectional; 12 Plies)(Measured in three- point flexure Yield Ultimate Ultimate content. modulus E(GPa) (MPa) (MPa) HI-NIC-BSAS-1-29. 9 BN/SiC 0.283 0.258 HI-NIC-BSAS-1-31-96 BN/SiC 0.42 0.283 0.771 Fig. 11. SEM micrographs from the fracture surfaces of unidirectional Nicalon/BNSIC/BSAS composites showing extensive fiber pullout; composites hot pressed under(a) less and(b)more aggressive condit of Si and O. The Sic overcoating was often found to be and elastic modulus of 160-170 GPa were observed. The missing from the BN/matrix interface, but was intact in this measured elastic modulus is in good agreement with a particular case. A crack running along the fiber/BN interface value of 169 GPa, calculated from the rule-of-mixtures may also be seen. The bn coating, when viewed via the (Ec= VmEm V Er where V is the volume fraction and TEM, did reveal an internal reaction. Some areas show no the subscripts c, m, and f refer to the composite, matrix, evidence of coarsening(Fig. &a) while others show varying and fiber, respectively) using Em=96 GPa[13]and Er amounts of reaction. Fig 8(b)shows coarsening of three Bn 270 GPa[14, 15]. Values of yield strain and the ultimate layers, from outside in towards the fiber, during hot pressing strain are -0.26-0.28% and -0.6-0.8%, respectively of the composite. However, the fiber/BN interface and the SEM micrographs of fracture surfaces of the two inner BN layer remain unchanged. EDS spectra obtained composites, after the three-point flexure tests, are shown from the coarsened region(Fig. 9a) show no obvious in Fig. 11. Extensive long lengths of fiber pullout are chemical changes in the various BN layers. Corresponding microdiffraction patterns(Fig. 9b) show coarsening to the point where individual diffraction spots are seen. It may be pointed out that reaction through all four BN layers or visi- ble degradation of the fiber was never observed. The sic layer remains apparently unaffected. Typical stress-strain curves recorded in three-point flex ure for the unidirectional BSAS matrix composites rein- forced with BN/SiC-coated Hi-Nicalon fibers hot pressed at two different temperatures are shown in Fig. 10. The fiber volume in the composites was%. Both the compo- sites show graceful failure. The values of yield stress, oy, yield strain, Ey, elastic modulus, E, ultimate strength, ou, and ultimate strain, Eu, of the composites hot pressed at two ifferent temperatures are given in Table 2. Values of yield Fig. 12. SEM micrograph from the polished cross-section of a unidirec- stress of 400-470 MPa, ultimate strength of 850-960 MPa tional Hi-Nicalon/BN/SiC/BSAS composite after the flexure test   , ( '  & !        &   %     !            &  &  %    #     (   & ! !   (;4          !       & >
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