High-temperature chemical phenomena affecting silicon nitride joints

Knudsen cell mass spectrometry was used to study the chemical processes responsible for joint degradation in joined silicon nitride ceramics. Vapor species present above two commercial hot-pressed silicon nitrides and above three joining glasses were identified, and partial pressures were estimated at 1480 K. Oxide vaporization products related to reducing conditions were observed. The implications of these results on proposed silicon nitride joining processes are discussed. It appears that oxygen potential gradients within both glazed and unglazed hot-pressed Si3N4 samples are responsible for the enhanced vaporization rates of the sample and the observed instability of glazed joints at high temperatures. Observed vaporization behavior of oxide additives correlates well with that predicted for the chemically reducing environment of Si3N4.