Effect of Changes in Grain Boundary Toughness on the Strength of Alumina

Abstract

ABSTRACTIndentation/strength data on a series of Ca-doped aluminas are presented, revealing considerable microstructural influence and increasing fracture resistance during crack propagation. The source of the increasing resistance is considered in terms of a restraining zone of grain-localized ligamentary bridges behind the crack tip. A simple fracture mechanics model is developed to characterize the effect of the restraining ligaments, which also allows the steady state toughness, applicable to large cracks, and the intrinsic strength, applicable to small cracks, to be predicted. The segregation of Ca to the grain boundaries is shown to correlate with decreases in grain boundary toughness and increases in toughness in the large crack limit. These latter two combined effects lead to decreases in intrinsic strength with increasing steady state toughness.