The effect of grain size on the toughening mechanisms and the thermal shock resistance was investigated in Y-TZP ($Y_2O_3$-tetragonal zirconia polycrystal), commercial CZ 202 containing 2.4 m/o $Y_2O_3$ and 6.5 m/o $Al_2O_3$.
The specimen was heat-treaed at 1550℃ for 8 h, and the average grain size increased from 0.54㎛ in the as-received specimen to 0.83㎛ in the heat-treated specimen. X-ray Analysis showed that the monoclinic phase increased from 45% in the as-received specimen to 70% in the heat-treated specimen as grain gets larger.
The thermal shock temperature difference ΔT was found to increase the crack density, and the tendency was stronger in the as-received specimen than in the heat-treated one. The critical temperature difference for crack propagation, ΔTc was 330℃ in the as-received specimen, which is larger than 220℃ in the heat-treated one by 110℃. However, the difference of strength between the two kinds of specimen after the crack propagation was found to be minor.
The above results can be considered to be related to the fact that the increase of grain size caused by heat treatment increases the flaw size, and was resulted in the reduction of transformation toughening.
