MICROSTRUCTURAL DESIGN OF TOUGHENED CERAMICS

The fracture toughness of ceramics can be improved by the incorporation of a variety of discontinuous, elastic reinforcing phases that generate a crack-bridging zone. Recent models of toughening by crack-bridging processes are discussed and used to describe the behavior observed in whisker-reinforced ceramics. The toughening response in ceramics reinforced with other types of discontinuous reinforcements is then considered (e.g., matrix and second-phase platelike grains, elongated matrix grains, and grain-size effects in non-cubic matrices). It is shown that crack-bridging toughening processes can be combined with other bridging mechanisms and with other toughening mechanisms (e.g., transformation toughening) to achieve synergistic effects. From these discussions, it is shown that the design of the toughened materials relies heavily on the control of the material properties and microstructural components influencing the toughening behavior to optimize the contributions of both the reinforcing phase and the matrix.