Reaction-Layer Interfaces in SiC-Fiber-Reinforced Glass-Ceramics: A High-Resolution Scanning Transmission Electron Microscopy Analysis

The reaction layers between silicon carbide continuous fibers (Nicalon) and a calcium aluminosilicate glass‐ceramic (anorthite composition) matrix in hot‐pressed composites have been characterized both structurally and chemically using high‐resolution, field‐emission scanning transmission electron microscopy. Chemical compositions at 10‐nm spacings, with a resolution of ∼5 nm, were collected across the fiber‐matrix interface zone. The reaction sequence in the material is silicon carbide (fiber)—carbon (in this case, graphite)—silica‐rich glass—anorthite. The composition of the carbon layer is constant across its width; the interfaces between the four phases are planar. This morphology and the chemical gradients observed are consistent with the simple, ‘carbon‐condensed’ oxidation displacemetn reaction, SiC + O2→ SiO2+ C, being responsible for interface phase formation in the composites. The planar interfaces indicate that the rate‐limiting process in the interface formation reaction is the diffusion of oxygen through the matrix and silica glass layer; a corollary of this conclusion is that the diffusion of silicon through the carbon layer is a relatively faster process.