Improved fiber coatings for controlling interfacial forces in SiC matrix composites. Part 3: Boron nitride
Conference
·
OSTI ID:140109
- Oak Ridge National Lab., TN (United States)
Boron nitride was examined as an improved interface coating for Nicalon/SiC composites. The interlayers were deposited on fiber preforms from gas mixture containing boron trichloride, ammonia, and hydrogen employing a chemical vapor infiltration (CVI) technique. The preforms were subsequently densified with a silicon carbide matrix, and test bars prepared. Specimens were heated in air for 24, 100, and 1,000 h at 1,273 K, and for 100 h at 1,473 and 1,673 K. Room-temperature flexure strengths of as fabricated and heat-treated composites were measured to assess the oxidation resistance of the fiber coating. Compositions and microstructures of the interlayers were characterized using electron microscopy. The composites with BN fiber coatings exhibited improved short-term (24 h) oxidation resistance as compared to those with carbon interlayers, but were embrittled by the heat treatment. Subsequently, strength was decreased and toughness was further diminished by extended exposure time and higher temperatures. Transmission electron microscopic analysis of the fiber-interlayer boundary revealed the presence of silica and carbon layers, products formed by the decomposition of the fiber. Thus the loss of strength and toughness appeared to be a result of chemical interaction between the fiber and oxygen in the BN interlayer, which was progressive with exposure time and temperature.
- Research Organization:
- Oak Ridge National Lab., TN (United States)
- DOE Contract Number:
- AC05-84OR21400
- OSTI ID:
- 140109
- Report Number(s):
- ORNL/FMP--93/1; CONF-9305135--; ON: DE94001091
- Country of Publication:
- United States
- Language:
- English
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