Improvement of Strength and Toughness of Silicon Nitride Ceramics

Abstract

Mechanisms of and possible benefits obtained by the improvement of the toughness and strength are described. It has been shown that the improvement of fracture toughness arises mainly from crack bridging by elongated silicon nitride grains resulting in rising R-curve behavior. In addition, the recent study suggested that the frontal zone shielding also contributes to the toughening. Coarser microstructures are beneficial to the toughness improvement, and materials with the fracture toughness in excess of 10 MPa√m have been developed following this approach. On the other hand, the attempts to improve strength have been centered on achieving the fine microstructures with minimum sizes of defects while maintaining moderate fracture toughness. Due to these different approaches, it has become recognized that there is a trade-off in improving these properties. The toughness improvement is shown to be beneficial to improving resistance to the machining damage. It is also suggested that the toughening by microcracking is little susceptible to the cyclic fatigue in contrast to the toughening by crack bridging.