Improved fiber coatings for nicalon/SiC composites

Boron-doped carbon and boron nitride were examined as improved interface coatings for Nicalon SiC composites. The interlayers were deposited on fiber performs prior to densification by chemical vapor infiltration (CVI). Room-temperature flexure strengths of as-fabricated composites and specimens exposed for 24 h in air at 1273 K were measured to assess the effectiveness of the fiber coatings. Compositions and microstructures of the interlayers were characterized using electron microscopy. Beyond a minimum concentration, boron doping of the carbon interface enhanced the oxidation resistance of the composites. Higher levels of boron, however, produced decreased strength and toughness. Composites with BN fiber coatings exhibited improved post-exposure strength retention, but were embrittled by the treatments. The loss of toughness appeared to be a results of chemical interaction between the fiber and oxygen in the BN interlayer.