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/SiC f); fiber-matrix interphases were of carbon and boron nitride. Velocit ies of effective elastic cracks were determined as a function of effec tive applied stress intensity in pure Ar and in Ar plus 2000, 5000, an d 20,000 ppm O-2 atmospheres at 1100 degrees C. Over a wide range of a pplied 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 h igher stress intensity. Oxygen increased the crack velocity in stage I I and shifted the stage II to III transition to the left. A two-dimens ional (2-D) micromechanics approach, developed to model the time depen dence of observed crack-bridging events, rationalized the measured eff ective crack velocities, their time dependence, the stage II to III tr ansition, and the effect of oxygen in terms of the load relaxation of crack-bridging fibers.