Microwave reaction bonding of silicon nitride using an inverse temperature gradient and ZrO2 and Al2O3 sintering additives

Si preforms containing Si3N4, Al2O3 and ZrO2 sintering additives were produced via an aqueous gelcasting route with 45 vol.% dry matter solids loading. Samples up to 10 mm thick could be produced without cracking. Microwave nitridation of the preforms was carried out using a fiberboard insulation box without packing powders in order to effect an inverse temperature gradient. Nitridation began at ∼800 °C and up to 40% nitridation was achieved by nitriding at 930–950 °C. Nitriding at higher temperatures caused a decrease in the amount of nitridation. All the samples displayed an inverse temperature gradient. In the centre of the samples, overheating occurred, which led to sintering and melting. Sample melting could be reduced by reducing the nitridation gas pressure from 0.2 to 0.1 MPa.