Nanoanalysis of Ceramic Composite Interfaces

Instrumentation requirements, particularly for small probes, for high resolution analytical electron microscopy are described. Analysis of structure and chemistry of interfaces in conventionally processed Si₃N₄/SiC(w) ceramic matrix composites (CMCs) and metal/(0001) 6H-SiC single crystal substrates are presented. All of these interfaces are characterized by a chemical and a structural width. In the CMCs, which were processed at high temperatures, the chemical widths formed by diffusion of sintering aids into the bulk microstructure and were always much larger than the corresponding structural widths. The metals on SiC were deposited near room temperature, where diffusion is slow. Significant chemical widths did not form, and microstructure was determined by crystallographic factors. Upon annealing, reaction zones with appreciable chemical widths did form, with interesting morphologies that separated naturally into two groups. For metals that formed stable silicides and carbides, e.g. Ti, all interfaces in the reaction zones remained nearly atomically flat over large distances. For metals that formed only stable silicides interfaces in the reaction zones were often curved, and sometimes formed closed surfaces, for relatively low annealing temperatures, suggesting that interface melting had occurred even at temperatures less than expected for Si-metal eutectics.