Fracture of Hot-Pressed Silicon Nitride at Elevated Temperatures

306 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1983.MgO-doped and Al(,2)O(,3)-Y(,2)O(,3)-doped hot-pressed silicon nitrides were fractured between 1100(DEGREES)C and 1350(DEGREES)C in four point bend at three displacement rates. Fracture stress and critical stress intensity factor were calculated and plotted versus temperature for each displacement rate. The scanning electron microscope, transmission electron microscope and scanning Auger electron spectrometer were used to analyze as-received and fractured material. Fracture stress and critical stress intensity factor showed a strain rate dependence above 1200(DEGREES)C. The MgO-doped Si(,3)N(,4) exhibited a peak in fracture stress before the values decreased at high temperatures. This peak corresponds to the onset of subcritical crack growth prior to catastrophic failure. Subcritical cracking is believed to occur by the nucleation, growth and coalescence of cavities in the secondary, amorphous grain boundary phase. The coalescence of cavities along with grain boundary separation results in the formation of microcracks in the vicinity of the main crack. The main crack then advances slowly by joining up with these nearby microcracks. Cavities and regions of grain boundary deformation were observed with SEM and TEM. A map was drawn of displacement rate versus temperature showing regions where subcritical cracking was and was not observed as well as a transition region. Both silicon nitrides exhibited a peak in effective critical stress intensity factor prior to the decrease. A decrease in K(,1C) results as the material weakens at high temperatures. The Y(,2)O(,3)-Al(,2)O(,3)-doped silicon nitride sustained its strength and K(,1C) to higher temperatures due to its more refractory Y(,2)O(,3)-Al(,2)O(,3)-SiO(,2) crystalline compound grain boundary phase than the amorphous magnesium-silicate phase present in the MgO-doped silicon nitride.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD