In $Al_2O_3-ZrO_2$ composite, first, the effect of $MoO_2$ on the tetragonal → monoclinic martensitic phase transformation of $ZrO_2$ was confirmed. Second, using above result, the magnitude of toughening accomodated by stress-induced phase transformation toughening mechanism and that by microcrack toughening mechanism was compared. And finally, a new method of surface strengthening in $Al_2O_3-ZrO_2$ was provided and its results was presented.
In chapter three, three kind of $Al_2O_3-15$ vol % $ZrO_2$ composites, containing 3 mol. % $Y_2O_3$-doped $ZrO_2$(STD-15), $ZrO_2$(AZT-15), and 200 ppm $MoO_2$-doped $ZrO_2$(AZM-15) respectively, were hot pressed such that the grain size distribution of $ZrO_2$ and $Al_2O_3$ of each specimen would be same. After hot pressing, the volume percents of $ZrO_2$ transformed to monoclinic in these composites were 0 for STD-15,46 for AZT-15 and 67 for AZM-15. When $Y_2O_3$ is added into $ZrO_2$, as it was known to be, the structural stability of t-$ZrO_2$ increase because the ionic radius of $Y^{3+}$ ion is larger than that of $Zr^{4+}$ ion. It was the reason for the retention of $t-ZrO_2$ in STD-15. The reason for the enhanced transformation of $ZrO_2$ in AZM-15 is that, $MoO_2$, in contrast with the case of $Y_2O_3$, has destabilized $t-ZrO_2$ structure. The structural stability of $t-ZrO_2$ seemed to reduce because smaller $Mo^{4+}$ ion substitute for larger $Zr^{4+}$ ion. The fact that $MoO_2$ destabilized $t-ZrO_2$ was also confirmed from the result that the critical grain size of $ZrO_2$ for t→m transformation in AZM-15 was smaller than that in AZT-15. From these results, it could be concluded that $MoO_2$ increased the driving force for t→m transformation of $ZrO_2$, i.e., it has destabilized $ZrO_2$ like $HfO_2$ which is known to be a unique unstabilizer thus far.
From theoretical analyses, the toughness increases accomodated by stress-induced phase transformation toughening mechanism is proportional to the volume fraction of $ZrO_2$ transformed from tetragonal to monoclinic during fracture. On the other hands, on the basis of theoretical results and in-situ TEM observation of others, the fracture toughness increases due to microcrack toughening mechanism is supposed to be proportional to volume fractions of $ZrO_2$ transformed to monoclinic before fracture. In chapter four, the toughening effects of two toughening mechanism were compared by comparing the proportional coefficients of $ZrO_2$ volume fraction. The volume percents of $ZrO_2$ transformed to monoclinic during fracture were 0 for STD-15 and AZM-15, and 15 for AZT-15. The fracture toughness of each $Al_2O_3-15$ vol % $ZrO_2$ composite was 4.38 for STD-15, 4.92 for AZT-15, 4.50 for AZM-15. From these experimental results of $ZrO_2$ transformation before and after fracture and measured fracture toughness of each specimens, it was concluded that the magnitude of toughness increases accomodated by transformation toughening mechanism was larger than those by microcrack toughening mechanism.
In chapter five, the variation of fracture toughness and flexural strength of each specimens above mentioned were investigad with $ZrO_2$ content. In case of $Y_2O_3$-doped system (STD series), though neither transformation toughening mechanism nor microcrack mechanism operate, the fracture toughness and flexural strength increased with $ZrO_2$ content. It was believed to be the second phase effect of $ZrO_2$ particles that increase the mechanical properties of STD specimens. $Al_2O_3$-pure $ZrO_2$ composites (AZT series) show the highest value of fracture toughness and strength among three class of specimens, which is supposed to be the effect by transformation toughening mechanism. When large part of $ZrO_2$ was made to transform to monoclinic with the addition of $MoO_2$ (AZM series), fracture toughness has similar level of value to STD series, but flexural strength shows least among three class of specimens because of largest flaw size which is introduced by t→m transformation of $ZrO_2$ and porosity.
In chapter six, by using the roll of $MoO_2$ as a destabilizer, a new method of surface strengthening was provided in $Al_2O_3-ZrO_2$ composite. By heattreating $Al_2O_3$-pure $ZrO_2$ composite in $ZrO_2$-(0.3~10 wt.%)$MoO_2$ mixing powder, and thereby let $MoO_2$ diffuse into $ZrO_2$ existing at surface region of composite, the volume fraction of monoclinic $ZrO_2$ was made increase from 0.3 to 0.5~0.6 depending on the composite of atmosphere powder. The enhanced transformation results in an increase in flexural strength up to 30%, because of compressive stress built in at the surface, the increase in flexural strength is proportional to the difference in volume fraction of monoclinic $ZrO_2$ between surface layer and bulk matrix, in agreement with two layer composite model.
