NUCLEATION AND GROWTH OF SILICON OXYNITRIDE GRAINS IN A FINE-GRAINED SILICON-NITRIDE MATRIX

Annealing induces significant microstructural changes in beta-Si3N4 ce ramics densified with a silica containing additive, as signified by th e precipitation of elongated Si2N2O grains in the fine-grained beta-Si 3N4 matrix, leading to a reinforced structure. The phases and microstr ucture evolution and the interface structural features in this composi te 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, typical ly the spatial distribution of the stacking faults and the size, morph ology, phase, and the spatial distribution of the particles trapped wi thin the Si2N2O, it has been generally established that Si2N2O grains nucleate heterogeneously on the beta-Si3N4 grain surface and grow rath er quickly. The low nucleation rate and high growth speed of Si2N2O in dicate 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 beta-Si3N4 grains wh ich either serve as a nucleation site for Si2N2O or are simply encapsu lated during Si2N2O grain growth can be observed; instead, the interfa ce between the Si3N4 and the Si2N2O features an amorphous phase of une ven thickness. Sialon polytypoid phase particles similarly trapped in Si2N2O show an epitaxial relationship with Si2N2O grains. From the obs erved interface structure between the Si2N2O and beta-Si3N4, a possibl e partial ordering of a glass film in the form of Si2N2O is also discu ssed.