Dependence of fracture stress on applied stress rate in a Yb2O3-SiO2-dopedhot-pressed silicon nitride ceramic

High-temperature fracture behavior of a Yb2O3-SiO2-doped hot-pressed silico n nitride (Si3N4) ceramic was investigated in four-point flexure between 10 00 and 1500 degreesC at five crosshead speeds, using the specimens precrack ed with three indentation loads. Above 1000 degreesC, a temperature and str essing rate dependence of fracture stress was seen. At 1200 degreesC the fr acture stress of the precracked specimens increased with decreasing stressi ng rates due to a toughening effect, the absence of slow crack growth (SCG) . However, at 1400 and 1500 degreesC the fracture stress decreased with dec reasing stressing rate. In particular, this dependence was stronger at 1500 degreesC than at 1400 degreesC. The SCG was observed only in the specimens precracked with indentation loads of 98 and 196 N. This crack extended wit h increasing test temperature and/or decreasing stressing rate. The depende nce of fracture stress on stressing rate was attributed to a SCG behavior a t higher temperatures.