CHEMICAL AND STRUCTURAL WIDTHS OF INTERFACES AND GRAIN-BOUNDARIES IN SILICON NITRIDE-SILICON CARBIDE WHISKER COMPOSITES

The extent of chemical distributions into crystals bounding whisker/matrix interfaces and matrix grain boundaries and the lateral continuity of the distributions was investigated by analytical electron microscopy methods and compared to their structural widths determined by high-resolution TEM. The extent of the distributions into the bounding crystals, defined as chemical widths, was 10 to 120 times the structural widths; this ratio was larger for grain boundaries than for the interfaces, Further, the chemical distributions were laterally discontinuous at interfaces but continuous at grain boundaries, The elements from the sintering aids, Y2O3 and Al2O3, were the primary chemical distribution constituents, The distributions were examined by the new position-resolved electron energy loss spectroscopy and Z-contrast scanning TEM methods, Microstructural observations indicated that chemical widths resulted from solid-state diffusion into the bounding crystals and that lateral discontinuities in the distributions resulted from preferential Gibbs-Thompson solution effects of the whisker surfaces at interfaces, These nonequilibrium distributions are process-dependent, and are expected to affect composite properties.