Abstract
The cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation induced flaw at the center of the reduced gage section, were tested, where the cyclic load was applied by four-point bending up to frequencies of 10Hz using an electrohydraulic testing machine.
Sintered silicon nitride showed a susceptibility to cyclic fatigue, and its lifetime decreased with increasing maximum applied stress. An apparent fatigue limit was about 70% of the flexural strength. A couple of crack arrest position marks around the indentation were detected in the fracture surfaces of both flexure and fatigue test specimens, and the size of each mark well corresponded with the analytical result based on a contact residual stress. Frequency did not affect the lowest limit of the fatigue lifetime, although a scatter of the data and the highest limit of the life-time increased as the frequency was lowered from 10Hz to 0.01Hz. High magnification fractography revealed an intergranular dominant fatigue failure with partial transgranular failure at perpendicularly elongated crystals. It suggests that the intergranular fatigue crack can be arrested at grain boundary triplets, and also can be reactivated by subsequent cyclic loadings.
The crack growth rate calculated from the fatigue lifetime showed a three region characteristics having a plateau at 70 to 90% of the fracture toughness, which suggests the possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.
