Nucleation and Growth of Silicon Oxynitride Grains in a Fine‐Grained Silicon Nitride Matrix

Abstract

Annealing induces significant microstructural changes in β‐Si3N4 ceramics densified with a silica containing additive, as signified by the precipitation of elongated Si2N2O grains in the fine‐grained β‐Si3N4 matrix, leading to a reinforced structure. The phases and microstructure evolution and the interface structural features in this composite system have been systematically studied by using X‐ray diffraction, and conventional and high‐resolution transmission electron microscopy. Traced from the indispensable features of the Si2N2O grains, typically the spatial distribution of the stacking faults and the size, morphology, phase, and the spatial distribution of the particles trapped within the Si2N2O, it has been generally established that Si2N2O grains nucleate heterogeneously on the β‐Si3N4 grain surface and grow rather quickly. The low nucleation rate and high growth speed of Si2N2O indicate a high nucleation barrier of Si2N2O and fast diffusion of (N,O) in the liquid. Experimentally, no epitaxial orientation relationship between Si2N2O grains and intragranularly trapped β‐Si3N4 grains which either serve as a nucleation site for Si2N2O or are simply encapsulated during Si2N2O grain growth can be observed; instead, the interface between the Si3N4 and the Si2N2O features an amorphous phase of uneven thickness. Sialon polytypoid phase particles similarly trapped in Si2N2O show an epitaxial relationship with Si2N2O grains. From the observed interface structure between the Si2N2O and β‐Si3N4, a possible partial ordering of a glass film in the form of Si2N2O is also discussed.