Solid-Gas Reaction during Sintering of Si_3N_4 Ceramics (Part 4) -Mass Loss Reaction and Morphology of Si_3N_4 Ceramics-

摘要

The mass loss reaction during sintering ofSi_3N_4 ceramics with 5 masspercent Y_2O_3 and 3masspercent Al_2O_3 which was reported in the formerpaper, was discussed in greater detail, including theobservation of cross-sectiona1 morphology and theanalysis of the distributions of crystal phases andelements. Differences in the conditions of the powderbed and N2 gas pressure (0.1 and 1 MPa) during sinteringcaused significant differences in cross-sectionalstructures. A porous layer was formed in the surfaceof samples sintered under 0.1 MPa N_2 at more than 4 hof soaking, while a pore-free densified layer wasformed at the surface of samples sintered under 1 MPaN_2 at more than 4 h of soaking. It was found that thepacking powder not only suppressed mass loss reaction,but also drew Y-Si-Al-O-N glass from the grain boundaryphase via capillary force. With increasing mass losswith longer soaking times, the oxygen content inspecimens decreased and #alpha#-SiAlON, melilite andother crystals including Y, Si, O and N were formed.As a result, we concluded that mass loss reaction duringsintering of Si_3N_4 ceramics occurred mainly throughthe following reaction: Si_3N_4(s) +3SiO_2(l)->6SiO(g) +2N_2(g).