Silicon Nitride Based Ceramic Nanocomposites

Nanocomposites (Si₃N₄/SiC) were studied by combined high-resolution transmission electron microscopy and electron energy-loss spectroscopic imaging (ESI) techniques. In ESI micrographs three types of crystalline grains were distinguished: Si₃N₄ matrix grains (0.5 μΩ), nanosized SiC particles (<100 nm) embedded in the Si₃N₄, and large SiC particles (100–200 nm) at grain boundary regions (intergranular particles). Amorphous films were found both at Si₃N₄ grain boundaries and at phase boundaries between Si₃N₄ and SiC. The Si₃N₄ grain boundary film thickness varied from 1 to 2. 5 nm. Two kinds of embedded SiC particles were observed: type A has a special orientation with respect to the matrix, and type B possesses a random orientation with respect to the matrix. The surfaces of type B particles are completely covered by an amorphous phase. The existence of the amorphous film between the matrix and the particles of type A depends on the lattice mismatch across the interface. The mechanisms of nucleation and growth of Ω-Si₃N₄ grains are discussed on the basis of these experimental results.