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 β-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 2,000, 5,000, and 20,000 ppm O2 atmospheres at 1,100 C. Over a wide range of applied stress intensities, the V - Keff diagrams revealed a stage 2 pattern in which the crack velocity depends only weakly on the applied stress intensity, followed by a stage 3, or power-law, pattern at higher stress intensity. Oxygen increased the crack velocity in stage 2 and shifted the stage 2 to 3 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 2 to 3 transition, and the effect of oxygen in terms of the load relaxation of crack-bridging fibers