October 1998 Stress Concentration Due to Fiber-Matrix Fusion in Ceramic-Matrix Composites 100um Silica Silica 10 Fig 9. SEM micrographs of a microcomposite specimen treated at ig. 8. SEN 1100.C for 40 h in air. Following tensile fracture. the sheath was displaced from its initial position on the fiber to reveal the region fused matrix section sheath, the radius of curvature assumed by the Sio, scale is-2 um sistent with published data for oxidation in air, o which indi- problems involving a SiOx-fused system have been o similar resented cates these conditions would produce -0.5 um of SiO2, enoug previously and indicate that the properties of a SiO -SiC in to cause partial fusion in eccentric fibers, but not enough to terface are not conducive to debonding. 11, 12 Thus, crack initia- ensure fusion tion in the SiO2 seal also is expected to lead to microcomposite The fact that the observed strength in oxidized and HF failure etched monofilaments returns to that observed in as-received The results presented here indicate that fusion of the fiber fibers suggests that it is the presence of the SiO, layer that and matrix local to a matrix crack results in the loss of"com- governs failure in the oxidized bare fiber. The physical fail posite behavior" because of the inability of the fiber and ma- ure origins in the sealed microcomposites have not been de trix to act independently. As well as the corresponding de- termined unambiguously, but two possibilities seem most crease in toughness, the load-bearing capacity of a fused likely. The first involves failure of the SiO2 layer in the re- composite system is expected to decrease sharply because of elon where the fiber is coupled to the matrix, and a crack in stress concentration. Thus, when interfacial sealing is the he SiO2 extends into the fiber. The second possible failure means of interphase preservation, composite designs must also initiation in the fiber occurs in the region where the stress is address the issue of stress concentration, if composite systems concentrated because of sealing. Although FEM analysis of that use this approach are to be effective in the SiO2 layer that are 60% that in the SiC fiber, the stress d he microcomposite specimens in this study have been oxi- the microcomposite system predicts maximum tensile stresses in the SiO, layer at the point of maximum concentration con- room temperature. If the oxidation-induced sealing is to occur tinues to be expected to exceed 1 GPa at the nominal fiber at a constant load, a stress concentration is not predicted. It is, stress at which microcomposite failure occurs. Therefore therefore, conceivable that a composite that remains at a con- although it is not unambiguous that failure is initiated in stant load during high-temperature service would not be af- the SiO, layer in the microcomposite specimens, it is likely. If fected by stress concentration following fiber-matrix bonding the case, then microcomposite failure is a function However, if the loading conditions on the fused system are to of crack deflection(debonding)versus penetration(failure)at change(e.g, by further matrix cracking or failure in a neighsistent with published data for oxidation in air,10 which indi￾cates these conditions would produce ∼0.5 mm of SiO2, enough to cause partial fusion in eccentric fibers, but not enough to ensure fusion. The fact that the observed strength in oxidized and HF￾etched monofilaments returns to that observed in as-received fibers suggests that it is the presence of the SiO2 layer that governs failure in the oxidized bare fiber. The physical fail￾ure origins in the sealed microcomposites have not been de￾termined unambiguously, but two possibilities seem most likely. The first involves failure of the SiO2 layer in the re￾gion where the fiber is coupled to the matrix, and a crack in the SiO2 extends into the fiber. The second possible failure initiation in the fiber occurs in the region where the stress is concentrated because of sealing. Although FEM analysis of the microcomposite system predicts maximum tensile stresses in the SiO2 layer that are 60% that in the SiC fiber, the stress in the SiO2 layer at the point of maximum concentration con￾tinues to be expected to exceed 1 GPa at the nominal fiber stress at which microcomposite failure occurs. Therefore, although it is not unambiguous that failure is initiated in the SiO2 layer in the microcomposite specimens, it is likely. If this is the case, then microcomposite failure is a function of crack deflection (debonding) versus penetration (failure) at the SiO2–SiC interface. Energy release rate solutions to similar problems involving a SiO2-fused system have been presented previously and indicate that the properties of a SiO2–SiC in￾terface are not conducive to debonding.11,12 Thus, crack initia￾tion in the SiO2 seal also is expected to lead to microcomposite failure. The results presented here indicate that fusion of the fiber and matrix local to a matrix crack results in the loss of ‘‘com￾posite behavior’’ because of the inability of the fiber and ma￾trix to act independently. As well as the corresponding de￾crease in toughness, the load-bearing capacity of a fused composite system is expected to decrease sharply because of stress concentration. Thus, when interfacial sealing is the means of interphase preservation, composite designs must also address the issue of stress concentration, if composite systems that use this approach are to be effective. The microcomposite specimens in this study have been oxi￾dized in an unstressed condition and fractured in tension at room temperature. If the oxidation-induced sealing is to occur at a constant load, a stress concentration is not predicted. It is, therefore, conceivable that a composite that remains at a con￾stant load during high-temperature service would not be af￾fected by stress concentration following fiber–matrix bonding. However, if the loading conditions on the fused system are to change (e.g., by further matrix cracking or failure in a neigh￾Fig. 8. SEM micrographs of the mating fiber to the fracture surface depicted in Fig. 7. Rim of SiO2 scale pulled from the surface of the SiC matrix section is observed around the fiber. Where the fiber enters the sheath, the radius of curvature assumed by the SiO2 scale is ∼2 mm. Fig. 9. SEM micrographs of a microcomposite specimen treated at 1100°C for 40 h in air. Following tensile fracture, the sheath was displaced from its initial position on the fiber to reveal the region fused by the SiO2 reaction product. October 1998 Stress Concentration Due to Fiber–Matrix Fusion in Ceramic-Matrix Composites 2601