NONISOTHERMAL MICROWAVE PROCESSING OF REACTION-BONDED SILICON-NITRIDE

The size and density of reaction-bonded silicon nitride (RBSN) specime ns are limited by the reduction in pore size and pore volume associate d with the nitridation reaction. In particular, under conventional hea ting, pores at the surface of dense compacts close before the center h as reacted fully. Microwave heating offers a unique advantage over con ventional heating for the processing of RBSN. A temperature gradient c an be maintained within the compact, which causes the reaction to occu r preferentially in the interior. This increases the amount of silicon converted to Si3N4 because the center of compacts with a high green d ensity finishes reacting before the porosity near the surface closes. This study follows the reaction process and shows that partially nitri ded silicon compacts have composition gradients in the radial directio n. Microwave processing also facilitates control of the reaction rate.