High-temperature cyclic fatigue-crack growth behavior in an in situ toughened silicon carbide

The growth of fatigue cracks at elevated temperatures (25-1300 degrees C) is examined under cyclic loading in an in situ toughened, monolithic silicon carbide with AI-B-C additions (termed ABC-SiC), with specific emphasis on the roles of temperature, load ratio, cyclic frequency, and loading mode (static vs cyclic). Extensive crack-growth data are presented, based on measurements from an electrical potential-drop crack-monitoring technique, adapted for use on ceramics at high temperatures. It was found that at equivalent stress-intensity levels, crack velocities under cyclic loads were significantly faster than those under static loads. Fatigue thresholds were found to decrease with increasing temperature up to 1200 degrees C; behavior at 1300 degrees C, however, was similar to that at 1200 degrees C. Moreover, no effect of frequency was detected (between 3 and 1000 Hz), nor evidence of creep cavitation or crack bridging by viscous ligaments or grain-boundary glassy phases in the crack wake. Indeed, fractography and crack-path sectioning revealed a fracture mode at 1200-1300 degrees C that was essentially identical to that at room temperature, i.e. predominantly intergranular cracking with evidence of grain bridging in the crack wake. Such excellent crack-growth resistance is attributed to a process of grain-boundary microstructural evolution at elevated temperatures, specifically involving crystallization of the amorphous grain-boundary films/phases. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.