HIGH-TEMPERATURE CRACK-GROWTH IN MONOLITHIC AND SICW-REINFORCED SILICON-NITRIDE UNDER STATIC AND CYCLIC LOADS

Experimental results are presented on subcritical crack growth under s ustained and cyclic loads in a HIPed Si3N4 at 1450 degrees C and a hot -pressed Si3N4-10 vol% SiCw composite in the temperature range 1300 de grees-1400 degrees C. Static and cyclic crack growth rates are obtaine d from the threshold for the onset of stable fracture with different c yclic frequencies and load ratios. Fatigue crack growth rates for both the monolithic and SiCw-reinforced Si3N4 are generally higher than th e crack growth velocities predicted using static crack growth data. Ho wever, the threshold stress intensity factor ranges for the onset of c rack growth are always higher under cyclic loads than for sustained lo ad fracture. Electron microscopy of crack make contact and crack-tip d amage illustrate the mechanisms of subcritical crack growth under stat ic and cyclic loading. Critical experiments have been conducted system atically to measure the fracture initiation toughness at room temperat ure, after advancing the crack subcritically by a controlled amount un der static or cyclic loads at elevated temperatures. Results of these experiments quantify the extent of degradation in crack-wake bridging due to cyclically varying loads. The effects of preexisting glass phas e on elevated temperature fatigue and fracture are examined, and the c reep crack growth behavior of Si3N4-based ceramics is compared with th at of oxide-based ceramics.