Time-dependent, environmentally assisted crack growth in NICALON-fiber-reinforced SiC composites at elevated temperatures

Subcritical crack growth measurements were conducted on ceramic matrix composites of beta-SiC matrix reinforced with NICALON fibers (SiC/SiCf); fiber-matrix interphases were of carbon and boron nitride. Velocities of effective elastic cracks were determined as a function of effective applied stress intensity in pure Ar and in Ar plus 2000, 5000, and 20,000 ppm O-2 atmospheres at 1100 degrees C. Over a wide range of applied stress intensities, the V - K-eff diagrams revealed a stage II pattern in which the crack velocity depends only weakly on the applied stress intensity, followed by a stage III, or power-law, pattern at higher stress intensity. Oxygen increased the crack velocity in stage II and shifted the stage II to III transition to the left. A two-dimensional (2-D) micromechanics approach, developed to model the time dependence of observed crack-bridging events, rationalized the measured effective crack velocities, their time dependence, the stage II to III transition, and the effect of oxygen in terms of the load relaxation of crack-bridging fibers.