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.