Controlling microstructural evolution to in situ toughen and strengthen silicon carbide
Nonoxide ceramics are desirable for high temperature structural applications, however, they have typically exhibited inferior room temperature fracture toughness. Similar to processing developments to toughen Si3N4, SiC has recently been processed via control of a phase transformation to produce in situ toughened microstructures. An elongated grain shape, coupled with a tortuous fracture path around grains, can provide bridging behind an advancing crack tip, which increases the crack resistance (rising R curve) and halts crack propagation. Most in situ toughened nonoxide ceramics incorporate upwards of 10-20% secondary phase(s), which simplifies crack propagation through this weaker phase to improve toughness, but typically at the expense of substantially reducing strength at high temperatures. The ABC-SiC in this study can be processed with <3% secondary phases and consequently exhibits record toughness and higher strength than commercial (Hexoloy SA) SiC.