BRENNAN: INTERFACIAL CHARACTERIZATION Hi-Nicalon Fiber BN Layer Fig. 8. TEM microstructure of Hi-Nicalon fiber/BN/MI SiC/SiC composite interfacial region(after 650C that deal with the Sylramic fiber/BN interfacial region. These two issues related to the roughness of the surface of the Sylramic fiber and its effect on composite properties, and the high-temperature moist ure susceptibility of the BN interface On occasion, composites tested under this prog exhibited rather weak and brittle tensile strengths, ompared with the usual composite properties. From BN Layer scanning electron microscopy( SEM)examination of the fracture surfaces of these composites it appeared that the Sylramic SiC fibers in the weak and brittle composites were not as dense as those in the strong and tough composites. On further examination of composite cross-sections, as shown in Fig. Il, the cvisc porosity in the fibers was found to actually consist of raphite particles that were concentrated in the center of the fibers This is an indication that the cessing of these particular fibers from their carbon Fig 9.TEM microstructure of Sylramic fiber/BN/MI SiC/SiC rich precursor did not proceed to completion. How ever. since tensile tests on these fibers indicated that they were almost as strong as fibers without the graphite inclusions, another reason for the poor com was fatigued at 1200C, 117 MPa stress, for a total posite properties must be present. From atomic force of 9870 h before failure occurred. No oxidation of microscopy(AFM) of the fiber surfaces, it was found the fibers or BN interface was found except in a that the graphite-containing Sylramic SiC fibers had region within -50 um of the tensile sample surface. much rougher surface topography [root-mean square 3.3. Issues of concern with Sylramic Sic fiber/BN roughness(RMS)-34 nm] than the dense fibers nterface MI SiC/SiC composites (RMS-10 nm), as shown in Fig. 12. It was thus con- luded that the rougher surface of these fibers made From the results discussed above. and others relat- it much more difficult for these fibers to debond and ing to thermal conductivity of the composites, the then pull out of the matrix compared with the system of Sylramic SiC fiber/BN interface/MI smoother fibers; i.e., the interfacial sliding stress was SiC/SiC composite was selected for further develop- too high [13]. The manufacturer of the Sylramic SiC ment under the HSCT/EPM program. During this fibers, Dow Corning Corp, took steps during sub- development phase, two issues of concern were raised sequent fiber processing to ensure that the sinteringBRENNAN: INTERFACIAL CHARACTERIZATION 4625 Fig. 8. TEM microstructure of Hi-Nicalon fiber/BN/MI SiC/SiC composite interfacial region (after 650°C tensile fatigue). Fig. 9. TEM microstructure of Sylramic fiber/BN/MI SiC/SiC composite interfacial region (after 650°C, 159 MPa tensile fatigue). was fatigued at 1200°C, 117 MPa stress, for a total of 9870 h before failure occurred. No oxidation of the fibers or BN interface was found except in a region within |50 µm of the tensile sample surface. 3.3. Issues of concern with Sylramic SiC fiber/BN interface MI SiC/SiC composites From the results discussed above, and others relat￾ing to thermal conductivity of the composites, the system of Sylramic SiC fiber/BN interface/MI SiC/SiC composite was selected for further develop￾ment under the HSCT/EPM program. During this development phase, two issues of concern were raised that deal with the Sylramic fiber/BN interfacial region. These two issues related to the roughness of the surface of the Sylramic fiber and its effect on composite properties, and the high-temperature moist￾ure susceptibility of the BN interface. On occasion, composites tested under this program exhibited rather weak and brittle tensile strengths, compared with the usual composite properties. From scanning electron microscopy (SEM) examination of the fracture surfaces of these composites it appeared that the Sylramic SiC fibers in the weak and brittle composites were not as dense as those in the strong and tough composites. On further examination of composite cross-sections, as shown in Fig. 11, the porosity in the fibers was found to actually consist of graphite particles that were concentrated in the center region of the fibers. This is an indication that the pro￾cessing of these particular fibers from their carbon￾rich precursor did not proceed to completion. How￾ever, since tensile tests on these fibers indicated that they were almost as strong as fibers without the graphite inclusions, another reason for the poor com￾posite properties must be present. From atomic force microscopy (AFM) of the fiber surfaces, it was found that the graphite-containing Sylramic SiC fibers had much rougher surface topography [root-mean square roughness (RMS)|34 nm] than the dense fibers (RMS|10 nm), as shown in Fig. 12. It was thus con￾cluded that the rougher surface of these fibers made it much more difficult for these fibers to debond and then pull out of the matrix compared with the smoother fibers; i.e., the interfacial sliding stress was too high [13]. The manufacturer of the Sylramic SiC fibers, Dow Corning Corp., took steps during sub￾sequent fiber processing to ensure that the sintering