In $Si_3N_4-ZrO_2$ composite, the effects of zirconia and $Y_2O_3$ dissolved in zyttrite on the densification and the $\alpha/\beta$ phase transformation of $Si_3N_4$ were studied by using hot-pressing of $Si_3N_4$ with the addition of pure 3, 6 and 8 mol% $Y_2O_3$-doped zirconia. And the reaction couples between $Si_3N_4$ and $ZrO_2$ or zyttrite were made to observe the reaction phenomena.
The addition of pure zirconia was not effective to obtain the full density of $Si_3N_4-ZrO_2$ composite. However, $Y_2O_3$ diffused from the added zyttrite promoted densification; density of $Si_3N_4$ with 5 vol% of pure $ZrO_2$ composite was 71 % of theoretical, and nearly full density (>97%) could be obtained in $Si_3N_4$ with 5 vol% of 6, 8 mol% $Y_2O_3$-doped $ZrO_2$ composite.
On the basis of observation with $Si_3N_4$-pure $ZrO_2$ reaction couple, the reaction between $Si_3N_4$ and $ZrO_2$ resulted in the formation of $Si_2N_2O$ phase, and the $\alpha/\beta$ phase transformation of $Si_3N_4$ occurred via this $Si_2N_2O$ phase. From the XRD analysis of reaction layer between $Si_3N_4$ and zyttrite, the reaction products, $Y_2Si_2O_7$, $Y_2Si_3N_4O_3$ and $Si_2N_2O$ phase were detected.
From the EDS analysis in STEM, it is suggested that the formation of Y - Zr - Si oxynitride glass in grain boundary occur local equilibrium and restrict the diffusion of $Y_2O_3$ in zyttrite. As a result, the same amount of $Y_2O_3$ remained in zyttrite grain was detected in both $Si_3N_4$ with 6 and 8 mol% $Y_2O_3$-doped $ZrO_2$ composite.
Fracture toughness of $Si_3N_4$-zyttrite composite increased with the addition of $ZrO_2$. It seems that the toughening mechanism, that is, stress induced phase transformation or microcracking related to $ZrO_2$ phase was not acted in this composite. The increase of toughness with the addition of $ZrO_2$ was mainly related to the crack deflection due to the difference of thermal expansion coefficient between $Si_3N_4$($\sim3\times10^{-6}$/$^\circ$C) and $ZrO_2$ ($\sim8-10\times10^{-6}$/$^\circ$C).
Also, fracture toughness was increased by heat treatment of $Si_3N_4$-zyttrite composite at 700$^\circ$C in air. From the phase analysis of $Si_3N_4$-$ZrO_2$ and $Si_3N_4$-zyttrite composite heat treated in vac. and air, it is suggested that the oxidation of ZrN which is the reaction product between $Si_3N_4$ and zirconia induce the compressive stress on the surface of composite resulting in the increase of toughness.
