The effect of sintering aids and kinds of SiC whisker on the densification phenomena and mechanical properties of SiC whisker reinforced $Si_3N_4$ composites has been studied. Also, the effect of content of SiC whisker on mechanical properties of composites has been studied. SiC whisker-$Si_3N_4$ composites doped with 5wt% MgO and 10wt% $Y_2O_3$-5wt% $Al_2O_3$ respectively, and the content of SiC whisker varied up to 30 vol%. Composites were fabricated by pressureless sintering at 1750°C for 1h and hot-pressed at 1750°C, 35MPa for 1h.
Composites were fabricated with SiC whisker and $Si_3N_4$ powder treated by sedimentation have better microstructure and mechanical properties than composites with untreated SiC whisker and $Si_3N_4$. Density of composites fabricated by pressureless sintering decreased with increase of whisker content. But almost full density and more than 95% of theoretical density were obtained by addition of 5wt% MgO and 10wt% $Y_2O_3$-5wt% $Al_2O_3$ respectively, as sintering additives.
Homogeneous microstructure was obtained by using SiC whisker and $Si_3N_4$ powder treated by sedimentation method. Composites having homogeneous microstructure have better mechanical properties than other composites having nonhomogeneous microstructure.
In $Al_2O_3$-SiC whisker composites, it was reported that mechanical properties were improved by removal of oxide layer on the surface of SiC whisker. But SiC whisker-$Si_3N_4$ composites mixed with SiC whisker treated by HF acid and as-received $Si_3N_4$ revealed low flexural strength and toughness. Thus, HF treatment of SiC whisker was not effective to improve the mechanical properties in this systems, because eutectic liquid between oxide layer on the surface of $Si_3N_4$ and sintering aids. Also, poor mixing obtained by using acetone for prohibiting reformation of oxide layer on the HF treated surface resulted in the decrease of mechanical properties.
Flexural strength and toughness of composites containing Tokai whisker increased with whisker content. Flexural strength and toughness of $Y_2O_3-Al_2O_3$ doped composites are 970MPa and 9.3MP$am^{\frac{1}{2}}$, respectively, at the content of 30 vol% whisker. From the microstructural analysis, the mechanical properties are enhanced by crack deflection mechanism.
