In the zirconia toughened alumuna (ZTA), thoughness enhancement is known to be related to the phase transformation of $ZrO_2$ particles from tetragonal to monoclinic. Two different toughening mechanisms have been proposed; transformation toughening and microcracking. So far, the effect of tetragonal $ZrO_2$ particles retained at room temperature on toughness has been a main research content in order to find the major mechanism for the toughening.
In this investigation, specimens with transformed $ZrO_2$ (monoclinic) and untransformed $ZrO_2$ (tetragonal) particles have been prepared to study the toughening mechanism. Monoclinic $ZrO_2$ particles at room temperature (over 93%) can be made by doping monoclinic $ZrO_2$ to $ZrO_2$. Room temperature and high temperature toughness of the specimens with monoclinic $ZrO_2$ and tetragonal $ZrO_2$ have been measured.
Room temperature toughness is much reduced in the specimens with monoclinic $ZrO_2$. Arround the $ZrO_2$ particles severe microcracks in alumina matrix are observed. This result implies that too much microcracks have been formed due to the phase transformation from tetragonal to monoclinic and deteriorate mechanical strength of the material. In contrast, high temperature toughness is not reduced in the specimen with monoclinic $ZrO_2$ compared to that with tetragonal $ZrO_2$ because of microcrack healing and softening of alumina matrix. Transformation toughening is believed to be a major mechanism in the studied range.
