Gelcasting of silicon compositions for SRBSN

Compositions of high purity silicon metal containing various oxides as sintering aids were gelcast to prepare green bodies for subsequent nitridation and sintering to form sintered reaction bonded silicon nitride (SRBSN). An acidic aqueous and an alcohol-based gelcasting system were used. The solids content in the slurry affected the green density of the cast samples, however, there was no apparent correlation between the green density and the percent nitridation. Samples were nitrided under flowing, high-purity nitrogen in either a tungsten element furnace, a graphite element furnace, or in a microwave furnace. The samples nitrided in the graphite and microwave furnaces showed 9--16% higher levels of nitridation than in the tungsten furnace. A N{sub 2}-4% H{sup 2} gas mixture was compared to pure nitrogen in the tungsten furnace. The addition of the hydrogen increased nitridation levels by 14--19%. The nitrided and sintered SRBSN ceramics had densities of 95--98% of theoretical, with no direct relationship being observed between the level of nitridation and the fired density. The average 4-point flexure strength of the samples ranged from 428 to 741 MPa