Sq. Guo et al., Dependence of fracture stress on applied stress rate in a Yb2O3-SiO2-dopedhot-pressed silicon nitride ceramic, J MATER RES, 16(11), 2001, pp. 3254-3261
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.