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An oxide-matrix continuous-fiber ceramic composite (CFCC) reinforced with Si-C-O (Nicalon™) fibers coated with an oxidation-resistant SiC/BN interphase was investigated for its resistance to elevated-temperature degradation. Tensile specimens at zero load were exposed to ambient air environments at 800 and 1000°C for 1, 24, and 100 h. Room-temperature tension tests of the exposed specimens were conducted per ASTM Test Method for Monotonic Tensile Strength Testing of Continuous Fiber-Reinforced Advanced Ceramics with Solid Rectangular Cross Section Specimens at Ambient Temperatures (C 1275) at 0.003 mm/s to ascertain retained mechanical properties. While elastic constants and strengths decreased on the order of 10 to 55%, the modulus of toughness, a direct measure of the inherent damage tolerance of CFCCs, decreased 90%. For example, elastic modulus decreased with exposure temperature and time from 150 GPa in the as-received condition to ∼135 GPa after exposure at 1000°C. Proportional limit stress decreased from 42 MPa in the as-received condition to 22 MPa after exposure to 1000°C. Ultimate tensile strength decreased from 244 MPa in the as-received condition to 107 MPa after exposure to 1000°C. Finally, modulus of toughness decreased from 1062 kJ/m3 in the as-received condition to 113 kJ/m3 after exposure to 1000°C. The Nicalon™ fibers showed some degradation effects of elevated-temperature exposure. Fractography revealed damage to both the fibers and matrix. Brittle fracture (accompanied by minimal fiber pullout) was the dominant failure mode at after exposure to all elevated temperatures.
continuous fiber ceramic composite, oxide matrix, tensile test, elevated-temperature exposure, bending, proportional limit stress, ultimate tensile strength
Associate engineer, EG&G Structural Kinematics Inc, Troy, MI
Assistant professor, University of Washington, Seattle, WA