Fatigue, Creep and Fracture Behavior of Silicon Nitride Ceramics

Significant progress has been made in the recent years in developing silicon nitride structural ceramics for heat engine applications due to their excellent high temperature mechanical properties. Pure uniaxial tensile tests were conducted at room and at elevated temperatures on GTE-PY6 a hot isostatically pressed (HIPed) and GTE SNW-1000 a sintered silicon nitride ceramics. It was observed that the tensile strength of these materials dropped significantly after 1000°C and the drop was more in the case of sintered silicon nitride ceramic. Tensile creep tests were performed at 1200° and 1300°C showed that the steady state creep rate was influenced by both the temperature and the applied stress. The higher stress exponent in HIPed as compared to a sintered silicon nitride shows higher creep resistance in the case of HIPed materials. The linking of small pores were also observed during creep process. Energy dispersive spectroscopy (EDS) shows that there is a difference in the silicon and yttrium contents across the pores in both creep tested (CT) and not creep tested (NCT). High resolution electron microscopy (HREM) analysis also showed that isolated ellipsoidal-shaped cavities were generated within the narrow interfaces between Si3N4 grains during creep test. Studies showed that precycling improved significantly the high temperature tensile strength in sintered silicon nitride (GTE SNW-1000). HREM was used to study specimens subjected to a combination of fatigue and tensile loads. The high temperature mechanical behavior was found to be controlled by the presence of amorphous phases along grain boundaries and multi-grain junctions. Precycling at elevated temperature led to nearly complete crystallization of the amorphous phases at the multi-grain junctions which is thought to be a reason for the observed improvement in tensile strength. It was also seen that the grain-boundary film thickness was affected due to precycling at elevated temperature.