The article examines degradation of a SiC‐based fiber composite containing Tyranno ZMI fibers in water vapor at elevated temperatures (800°C and 1100°C). Degradation is characterized through mechanical tests under cyclic and quasi‐static tensile loading in the near‐threshold regime, at stresses at or slightly above the matrix cracking limit. These tests are augmented by examinations of fracture surfaces and polished cross‐sections, measurements of fracture mirror radii, and measurements of interfacial debond toughness and sliding resistance. Degradation involves highly localized consumption of fibers through reactions of water vapor with the fibers and the BN coatings in regions adjacent to the few matrix cracks present at low stresses; the global hysteresis response and the average interfacial properties are minimally affected. Boria formed by oxidation of BN appears to play a fluxing role; it combines with silica on the fibers to form a non‐protective molten glass. Inhomogeneous fiber consumption leads to stress concentrations in the fibers and hence reduced fiber strength. Spatial variations in the degradation process occur at two length scales: at the macroscopic scale, because of cracking of the external CVI SiC overcoat and subsequent water ingress through the cracks, and at the tow‐scale, because of cracking of the CVI SiCmore »
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