ADVANCES IN THE DESIGN OF TOUGHENED CERAMICS

The mechanical performance of brittle ceramics can be improved by increasing their fracture toughness/resistance. This is reflected by increases in resistance to strength degradation from in-service thermal stresses, slow crack growth, and contact or impact damage when the fracture toughness is enhanced. Furthermore, it appears that the distribution of the fracture strengths is narrowed (e.g., increased Weibull modulus) in toughened ceramics. Two approaches which can result in improved fracture toughness are transformation toughening and reinforcement processes. The reinforcement process can be achieved by introducing various microstructural features (e.g., strong whiskers and platelets, large noncubic matrix grains, and elongated matrix grains). However, the design of toughened ceramics employing either transformation toughening or reinforcement mechanisms must consider the material properties and characteristics which control each mechanism. Both the analysis of the toughening contributions and experimental observations can be used to define the influence of microstructure and material parameters on the toughening effects. These observations, then, provide a basis for the design of toughened ceramics.