Microstructural Design of Silicon Nitride with Improved Fracture Toughness: I, Effects of Grain Shape and Size

The use of self‐reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady‐state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady‐state values of up to 10 MPam 1/2 and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady‐state) toughness and the R ‐curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R ‐curve response with a steady‐state toughness of only 3.5 MPam 1/2 coupled with a reduced fracture strength.