Gas-discharging reactions and their effect on the microstructures of greenbodies in gelcasting of non-oxide materials

In situ consolidation colloidal-forming processes, such as gelcasting, are promising techniques for complex shaping of materials with high density, hi gh uniformity and, thus, high reliability of performance. Non-oxide powders , e.g. Si, SiC and Si3N4, are suitable for being well dispersed in alkaline solutions. However, silicon powder and free silicon contained in silicon c arbide and silicon nitride powders have a tendency to hydrolize, especially in a basic aqueous solution for gelcasting, causing a large amount of macr opores in green bodies as a consequence of hydrogen gas evolution. Moreover , silicon nitride itself is chemically unstable in aqueous solutions and re acts with other reagents when processed by aqueous gelcasting, which also a ffects the microstructure of green bodies. After identifying the causes of macropore formation in the gelcast green bodies, some solutions are propose d to overcome these problems. (C) 2000 Elsevier Science B.V. All rights res erved.