The Influence of Microstructure on the Mechanical Behavior of Silicon Nitride Ceramics

The introduction of elongated silicon nitride grains during densification in the presence of a liquid phase can impart considerable improvement to the fracture toughness. This toughening is not universally attained but depends on the activation of intergranular rather than transgranular fracture. This is reminiscent of the requirement of interfacial debonding in whiskerreinforced ceramics. In fact, additional observations such as bridging in the crack wake by elongated grains and pullout of some of these grains further suggest that the crack wake mechanisms that contribute to the toughening of whisker-reinforced ceramics can also operate in silicon nitrides containing elongated grains. Various investigators have found that, consistent with crack wake mechanisms, the fracture toughness of silicon nitrides increases with increase in the diameter of the larger elongated grains. However, little is known about the effects of the grain boundary phase(s) and their properties on the interfacial debonding/intergranular fracture in such silicon nitrides. This is critical as observations show that crack propagation in some systems exhibiting larger elongated grains occurs transgranularly and no toughening occurs.