Kinetic aspects of superfast consolidation of silicon nitride based ceramics by spark plasma sintering

A newly developed novel rapid densification process named Spark Plasma Sintering (SPS) has been successfuly applied to compact silicon nitride based ceramics. In this process the precursor powders are pressed uniaxially in a carbon die, and an on-off pulsed DC voltage is applied simultaneously. The current passes through the carbon pressure die as well as the sample, so that the sample is heated both from the outside and the inside, which allows very fast heating rates to be applied (up to 600 degreesC min(-1)). In addition to heat and pressure this process utilises the self-heating action caused by spark discharges between the particles, which occurs in the initial stage of the current-voltage pulse. These processes promote material transfer and allow the preparation of fully dense compacts of silicon nitride based ceramics within a few minutes. In this presentation, the phase transformation sequences during the initial stages of the sintering process, as well as the consolidation mechanisms are discussed from a kinetic point of view for Si3N4, beta -sialon, and alpha-beta -sialon composite materials.