Abstract
The mass loss reaction during sintering of Si3N4 ceramics with 5mass% Y2O3 and 3mass% Al2O3, which was reported in the former paper, was discussed in greater detail, including the observation of cross-sectional morphology and the analysis of the distributions of crystal phases and elements. Differences in the conditions of the powder bed and N2 gas pressure (0.1 and 1MPa) during sintering caused significant differences in cross-sectional structures. A porous layer was formed in the surface of samples sintered under 0.1MPa N2 at more than 4h of soaking, while a pore-free densified layer was formed at the surface of samples sintered under 1MPa N2 at more than 4h of soaking. It was found that the packing powder not only suppressed mass loss reaction, but also drew Y-Si-Al-O-N glass from the grain boundary phase via capillary force. With increasing mass loss with longer soaking times, the oxygen content in specimens decreased and α-SiAlON, melilite and other crystals including Y, Si, O and N were formed. As a result, we concluded that mass loss reaction during sintering of Si3N4 ceramics occurred mainly through the following reaction: Si3N4(s)+3SiO2(l)→6SiO(g)+2N2(g).
