The microstructures of silicon nitride ceramics during hot-pressing transformations

Abstract

The microstructure of silicon nitride hot-pressed with a magnesium oxide additive has been studied by transmission electron microscopy. This includes material at various stages in a hot-pressing process: the initial (∼ 90%α) silicon nitride powder; specimens partially densified and partially transformed from α-silicon “nitride” (Si_11.5N₁₅O_0.5) to β-silicon nitride (Si₃N₄); and almost fully dense and fully transformed β-Si₃N₄. The observations substantiate a solid/liquid/solid transformation mechanism, whereby Si and N are transported from α grains through a silicate liquid phase to nucleation sites for β at α/liquid interfaces or to β grains nucleated homogeneously in the liquid phase.