Retained tensile properties and performance of an oxide-matrix continuous-fiber ceramic composite after elevated-temperature exposure in ambient air

An oxide-matrix continuous-fiber ceramic composite (CFCC) reinforced with Si-C-O (Nicalon(TM)) 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 degrees 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 similar to 135 GPa after exposure at 1000 degrees C. Proportional limit stress decreased from 42 MPa in the as-received condition to 22 MPa after exposure to 1000 degrees C. Ultimate tensile strength decreased from 244 MPa in the as-received condition to 107 MPa after exposure to 1000 degrees C, Finally, modulus of toughness decreased from 1062 kJ/m(3) in the as-received condition to 113 kJ/m(3) after exposure to 1000 degrees C. The Nicalon(TM) 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.