BRENNAN: INTERFACIAL CHARACTERIZATION Hi-Nicalon 5HS SyIramieTM 5HS 650C, 160 MPa, 272 cycles, 545 hrs to runout 650C, 160 MPa, 273 cycles, 46 hrs to runout Residual 650C UTS=172 MPa, Ef=0.08%) (Residual 650C UTS=318 MPa, ef=0.23%) UTRC Fig. 6. Fracture surface comparison between Hi-Nicalon and Sylramic SiC fiber MI SiC/SiC composites after 650° tensile fatigue Carbon-Rich (near fiber Si Ire of Hi-Nicalon fiber/BN/MI SiC/SIC composite interfacial region(as-processed) of 140 MPa. This sample failed after 154 h(76 path, and not down the fiber/BN/matrix interface. At cycles), with the primary fracture surface s below the matrix microcracking stress, no some effects of oxidation, but the secondary are formed in the composite during testing, and such as that shown in Fig. 10 showing limit nly oxidation that occurs is confined to the sur- dation of the BN fiber coating only along the crack face of the composite. A Sylramic fiber MI composite4624 BRENNAN: INTERFACIAL CHARACTERIZATION Fig. 6. Fracture surface comparison between Hi-Nicalon and Sylramic SiC fiber MI SiC/SiC composites after 650°C tensile fatigue. Fig. 7. TEM microstructure of Hi-Nicalon fiber/BN/MI SiC/SiC composite interfacial region (as-processed). of |140 MPa. This sample failed after 154 h (76 cycles), with the primary fracture surface showing some effects of oxidation, but the secondary cracks such as that shown in Fig. 10 showing limited oxi￾dation of the BN fiber coating only along the crack path, and not down the fiber/BN/matrix interface. At stresses below the matrix microcracking stress, no cracks are formed in the composite during testing, and the only oxidation that occurs is confined to the sur￾face of the composite. A Sylramic fiber MI composite