INFLUENCE OF ZRO2 GRAIN-SIZE AND CONTENT ON THE TRANSFORMATION RESPONSE IN THE AL2O3-ZRO2 (12 MOL-PERCENT CEO2) SYSTEM

Based on experimental and modeling studies, the rate of increase in the martensite start temperature M(s) for the tetragonal-to-monoclinic transformation with increase in zirconia grain size is found to rise with decrease in ZrO2 content in the zirconia-toughened alumina ZTA system. The observed grain size dependence of M(s) can be related to the thermal expansion mismatch tensile (internal) stresses which increase with decrease in zirconia content. The result is that finer zirconia grain sizes are required to retain the tetragonal phase as less zirconia is incorporated into the alumina, in agreement with the experimental observations. At the same time, both the predicted and observed applied stress required to induce the transformation are reduced with increase in the ZrO2 grain size. In addition, the transformation-toughening contribution at temperature T increases with increase in the M(s) temperature brought about by the increase in the ZrO2 grain size, when T > M(s). In alumina containing 20 vol% ZrO2 (12 mol% CeO2), a toughness of approximately 10 MPa. square-root m can be achieved for a ZrO2 grain size of approximately 2 mum (M(s) approximately 225 K). However, at a grain size of approximately 2 mum, the alumina-40 vol% ZrO2 (12 mol% CeO2) has a toughness of only 8.5 MPa. square-root m (M(s) approximately 150 K) but reaches 12.3 MPa. square-root m (M(s) approximately 260 K) at a grain size of approximately 3 mum. These findings show that composition (and matrix properties) play critical roles in determining the ZrO2 grain size to optimize the transformation toughening in ZrO2-toughened ceramics.