During the past decade, $Si_3N_4$ has received considerable attention, primarily directed towards heat engine applications. Its intrinsic properties, i.e. low thermal expansion and high strength, minimize stress developed during severe thermal transient better than other structural ceramics. Despite its exceptionally high strength at room temperature, $Si_3N_4$ exhibits significant strength degradation at higher temperatures.
In the present study, a composition containing $α-Si_3N_4$ with 5 W/O $Y_2O_3$ and 4W/O $Al_2O_3$ was hot-pressed at 1,650℃ and 350kg/㎠ for 1.5hr, and specimens of the same composition were pressureless-sintered at 1,750℃ for 1.5 and 5 hrs, in order to know the variations of the high temperature strength with the crystallization of grain-boundary phase. By X-ray diffraction, hot-pressed specimens consisted of α-and $β-Si_3N_4s$, and sintered specimens consisted of $β-Si_3N_4$ and $Si_3N_4ㆍY_2O_3$ which was crystallized out from grain-boundary phase. The 5-hr sintered specimens had higher degree of crystallization than the 1.5-hr sintered specimens. And all the hot-pressed and sintered specimens had microstructures interlocked by elongated β-grains.
Among these three different specimens, the 5-hr sintered specimens showed the highest strength after MOR test at 1,000℃. This result assures that the crystallization of grain-boundary phase has a significant effect on the high temperature strength of $Si_3N_4$. The SPT diagram for the 5-hr sintered $Si_3N_4$ was generated by measurements of the stressing rate dependence of fracture strength. This diagram enables predictions of a safe working stress to be made for specific component lifetimes and survival probabilities.
