Structural imperfection of β-Si3N4 crystals associated with nucleation

Some kinds of dense strain contrast in [bgr]-Si3N4 have been considered as dislocation tangles formed during mechanical milling. However, our present electron microscopic analyses indicate that the strain contrast with radial contours is not in the least attributed to the dislocations but rather to the subgrain boundaries associated with the initial growth of the crystal, i.e. the nucleation. A number of growth variants, independently nucleated on the surface of a single Si-N cluster, coarsen and form closure mistakes when they approach, giving rise to the formation of subgrain boundaries extended from the core. The subgrain boundaries are found to locate only on {100}, {110} and {101} planes of the [bgr]-Si3N4 crystal structure.