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 MPaؒm 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 MPaؒm 1/2 coupled with a reduced fracture strength.

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2-dimensional silica-silica Continuous Fiber-reinforced Ceramic-matrix Composites (CFCCs) were fabricated by a sol-gel infilitration method that has a changing processing condition, such as the repetitions of infilitration. In order to investigate the relationship between the processing condition and the mechanical properties of composites, the mechanical properties of specimens were measured by means of a 4-point flexural strength test while the evidence of strength degradation were microstructurally characterized. There seemed to be a minimum density value that existed at which the delamination between the fabrics would not occur. In the case that the density of silica CFCCs exceeded 1.55 g/cm 3 , the flexural strength also exceeded approximately 18 MPa at least. By applying the Minimum Solid Area (MSA) analysis of the porous structure, the correlation between the relative density and the mechanical properties of composites will be discussed.