Mullite/zirconia composite was obtained by $in$ $situ$-reactive sintering of zircon and alumina mixture, and its mechanical properties were studied in conjunction with interpretation of microstructure of this composite.
Dissociation of zircon into zirconia and silica and the subsequent reaction of silica with $Al_2O_3$ to form mullite initiated at 1475±10℃, simultaneously. And above 1565℃, no zircon phase was remained in the mixture.
Martensitic transformation of zirconia phase thus being dispersed in the composite occurred at 600℃ as a 4-mol% $Y_2O_3$ is added to the content of zirconia.
Nucleation and extension of microcrack in this composite were interpreted for the toughening mechanism from the observation of microstructure. On cooling tetragonal (t) to monoclinic (m) phase transformation induced microcrack at the grain-boundary of mullite matrix, which in turn absorbs the fracture energy created in stress field.
M.O.R. (4-point) and fracture toughness ($K_{1c}$) of this composite were 250 $MN\cdot{m}^{-2}$ (max. 330) and 3.3 $MN\cdot{m}^{-3/2}$, respectively.
The weight fraction of t-$ZrO_2$ being retained at room temperature was 0.33. This t-$ZrO_2$ phase did not transform to m-$ZrO_2$ phase under stress field such as polishing. Therefore, the transformation toughening mechanism was not applicable in this composite. Increasing of M.O.R. and $K_{1c}$ value was insignificant even the weight fraction of t-$ZrO_2$ phase increase 0.12 to 0.33.
It was concluded that the toughening mechanism of mullite/zirconia composite was attributed to the grain-boundary strengthening, resulted from a "meta-stable solid solution" in which substitute $Al^{3+}$ and $Si^{4+}$ for $Zr^{4+}$, and $\it{vice versa}$, between mullite matrix and zirconia grains by EPMA.
