ELEVATED-TEMPERATURE EFFECTS OF OXYGEN ON SIC SIC COMPOSITES/

High-temperature exposures of SiC/SiC composites to oxidizing environm ents can lead to substantial changes in mechanical behavior. In the wo rk reported here, results from flexure and crack growth experiments ar e used to demonstrate such effects. Flexure tests of graphite-coated N icalon-reinforced SiC previously oxidized in air at 950-degrees-C reve aled that degradation of fracture resistance began after very short ex posure times (less than 1 h) and could be described in terms of distin ct oxidation effects on strength and fiber pullout. Crack velocities w ere determined as a function of applied stress intensity and time for varying O2 levels. It was observed that crack velocities increased at 1,100-degrees-C in the presence of oxygen, which also shifted the onse t of stage III (power law) growth to lower values of applied stress in tensity. The crack growth observations were described using a two-dime nsional micro-mechanical model developed to simulate cracks bridged by continuous fibers. Fiber creep relaxation predicted the correct crack velocity and time-dependence in argon, but other mechanisms, such as interface removal, are required to explain the data in Ar + O2.