Effect of Defects and Grain Size on Strength of Mullite Ceramics

Abstract

Many studies have been conducted on the strength and fracture toughness K_IC of ceramics. However, more extensive studies are still necessary to explain the relationship between defects and grain size¹, and its effects on the strength and fracture toughness. Recent experimental observations have revealed the following interesting phenomenon on the defect/grain size effect². If the defect size was smaller than 30 times of the grain size, the fracture toughness value [K_IC(defect)] calculated from the inherent defect size became smaller than that [K_IC(SEPB)] measured using a fracture toughness specimen with an artificially-introduced initial crack. These observations were reported on several engineering ceramics including silicon nitride, silicon carbide and alumina. Based on these observations, alumina with fine grain was developed to improve the strength. In the study of alumina³, the fracture toughness increased with increasing grain size(d), and showed the maximum value when d=100μm. It seems to be impossible to increase both strength and fracture toughness values simultaneously by controlling the grain size.