SILICON-NITRIDE BASED CERAMIC NANOCOMPOSITES

Nanocomposites (SL(3)N(4)/SiC) were studied by combined high-resolutio n transmission electron microscopy and electron energy-loss spectrosco pic imaging (ESI) techniques. In ESI micrographs three types of crysta lline grains were distinguished: Si3N4 matrix grains (similar to 0.5 m u m), nanosized SiC particles (<100 nm) embedded in the Si3N4, and lar ge 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 bounda ry film thickness varied from 1 to 2.5 nm. Two kinds of embedded SiC p articles 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 th e matrix and the particles of type A depends on the lattice mismatch a cross the interface. The mechanisms of nucleation and growth of beta-S i3N4 grains are discussed on the basis of these experimental results.