Abstract
The static fatigue test have been carried out at 1200°C in air by useing plain and surface cracked Si3N4 bend specimen. This Si3N4 was developed for heat resisting spring. The dependence of fracture stress on the crack length is examined at room temperature and elevated temperature of 1100°C and 1200°C, and the applicability of the process zone size failure criterion for silicon nitride ceramics is studied. The results showed that the process zone size failure criterion is useful for evaluating crack length problems both at room and at elevated temperature.
In this experiment, high fracture toughness has been obtained by crack deflection and bridging of rod-like grains. And high fracture strength was also obtained because of well controlled microstructure of the Si3N4.
The cracked specimen with surface crack length of 650μm has showed static fatigue limit of 120MPa, and the time is 107 sec which is longer than the time reported. As a result of these static and static fatigue test, this Si3N4 is assumed to be useful material for heat resisting spring.
