Strength‐Toughness Relations in Sintered and Isostatically Hot‐Pressed ZrO2‐Toughened Al2O3

Abstract

The fracture toughness of fine‐grained undoped ZrO2‐toughened Al2O3 (ZTA) was essentially unchanged by postsintering hot isostatic pressing and increased monotonically with ZrO2 additions up to 25 wt%. The strength of ZTA with 5 to 15 wt% tetragonal ZrO2, which depended monotonically on the amount of ZrO2 present before hot isostatic pressing, was increased by pressing but became almost constant between 5 and 15 wt% ZrO2 addition. The strength appeared to be controlled by pores before pressing and by surface flaws after pressing; the size of flaws after pressing increased with ZrO2 content. The strength of ZTA containing mostly monoclinic ZrO2 (20 to 25 wt%) remained almost constant despite the noticeable density increase upon hot isostatic pressing because the strength was controlled by preexisting microcracks whose extent did not change on postsintering pressing. These strength‐toughness relations in sintered and isostatically hot‐pressed ZTA are explained on the basis of R‐curve behavior. The importance of the contribution of microcracks to the toughness of ZTA is emphasized.