Rare-earth nitrate additives for the sintering of silicon carbide

Effect of Milling on the Densification of SiC-BasedComposites from Polysilazane

“…In order to investigate the effect of ZrO 2 impurity in the composites, we added intentionally 5 wt.% and 10wt.% of ZrO 2 (mean particle size: 200 nm) to the amorphous Si-C-N-O and hot pressed at 1700 °C in the same condition. The densities were 2.04 g/cm 3 and 2.21 g/cm 3 for 5 wt.% and 10 wt.% addition of ZrO 2 , respectively. It seemed that the ZrO 2 introduced by the high energy milling created almost no effect to enhance the densification of HEM1600 and HEM1700.…”

“…Regardless of the milling process, the composite density increased with the increase of hot pressing temperature as shown in Figure 3. The densities of HEM1600 and HEM1700 were 2.80 and 3.04 g/cm 3 , respectively, and 1.53 and 2.47 g/cm 3 for 1600 and 1700, respectively. There were two possibilities of liquid phase sintering mechanism in this study: the presence of ZrO 2 impurity that acted as sintering additives or the formation of Si 2 N 2 O that involved liquid.…”

“…In addition, the formation of Si 2 N 2 O was desirable because this phase conferred the excellent high temperature strength and oxidation resistance [28][29]. Furthermore, the typical hot pressing temperature of >1800 °C for Si 3 N 4 /SiC composites can be used to achieve the density of >3.00 g/cm 3 with the addition of 8-15 wt. % sintering additives [13,[30][31][32], which was higher compared to this study.…”

“…Since no other phase such Si 2 N 2 O and ZrO 2 was found, the microstructure of 1600 (Figure 4 (c)) was very fine, and the grain size cannot be seen clearly with SEM. However, at 1700 (Figure 4 (d)), the grain grew tremendously without densification mechanism, resulting in density of only 2.47 g/cm 3 corresponding to 76.9% relative density (theoretical density of SiC = 3.21 g/cm 3 ). On the other hand, SiC-based composite with dense microstructure was obtained at 1700 °C as seen in Figure 4 (b).…”

“…In liquid phase sintering, sintering additives react with thin layer SiO 2 that exists on the surface of SiC to form a liquid phase and enhance the densification of SiC. Although the sintering additives can increase the density of SiC, the grain growth can be enhanced with the addition of sintering additives [3], which might decrease the mechanical properties of SiC.…”