High-temperature corrosion of Nicalon reg sign /SiC composites

Abstract

The elevated-temperature stability of Nicalon{reg sign} (Nippon Carbon Company, Tokyo, Japan)/SiC composites with a graphitic carbon interface layer in oxidizing and simulated fossil fuel environments was investigated. Composite specimens with and without an external SiC surface coating were oxidizing in air and exposed to a variety of combustion environments at a temperature of 1273 K. A burner rig furnace was constructed for simulating corrosive fossil fuel environments containing water vapor, sulfur, and sodium. The mechanical properties of unprotected Nicalon{reg sign}/SiC specimens were degraded after short periods of exposure, due to the oxidation of the carbon interface coating. Longer exposures resulted in the oxidation of the fibers and matrix to form silica, which with time bonded the components together and produced brittle behavior. Combustion environments hastened the embrittlement of composites without an external SiC coating. Conversely, the specimens protected by a chemical vapor deposition (CVD) SiC surface coating exhibited only small decreases in strength after oxidation or corrosion in combustion environments. The SiC layer sealed off the surface of the composites, protecting the exposed fibers' ends, and thus prevented oxidation at the fiber-matrix interface. 26 refs., 11 figs., 1 tab.