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

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.