Pressureless sintering of dense Si3N4 and Si3N4/SiC composites with nitrate additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si3N4 and a Si3N4/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si3N4 particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si3N4 particles milled with oxide additives. Monolithic Si3N4 could be sintered to 94% of theoretical density (TD) at 1500 degrees C with nitrate additives. The sintering temperature was about 100 degrees C lower than the case with oxide additives. After pressureless sintering at 1750 degrees C for 2 h in N-2, the bulk density of a Si3N4/20 wt% SIC composite reached 95% TD with nitrate additives.