PRESSURELESS SINTERING OF DENSE SI3N4 AND SI3N4 SIC COMPOSITES WITH NITRATE ADDITIVES/

The effect of aluminum and yttrium nitrate additives on the densificat ion of monolithic Si3N4 and a Si3N4/SiC composite by pressureless sint ering 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. Mon olithic Si3N4 could be sintered to 94% of theoretical density (TD) at 1500 degrees C with nitrate additives. The sintering temperature was a bout 100 degrees C lower than the case with oxide additives. After pre ssureless 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 .