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The degradation of boron filament reinforced aluminum composites at temperatures over 400 C severely limits this material to relatively low-temperature applications. In order to extend the elevated temperature usefulness of such composites boron filament was coated with a diffusion and oxidation inhibiting coating of β-silicon carbide. The coating was applied by thermally decomposing a halogenated silane compound on the surface of previously prepared boron filaments. Electron microprobe analysis showed that the boron had diffused part way through the silicon carbide layer during the deposition process at approximately 1200 C. The ultimate tensile strengths of the coated fibers, however, were found to be approximately the same as the uncoated fibers, namely, 450,000 to 550,000 psi.
The stability of the silicon carbide coated filaments in air was studied to determine the extent of oxidation inhibition of the silicon carbide layer. Test specimens of uncoated and silicon carbide coated boron filament were taken from previously evaluated control fiber and heated at several temperatures up to 600 C, for various times up to 5000 h in air. The room-temperature tensile properties of these heat treated specimens indicate that silicon carbide coated boron fibers retain most of their strength after heating to 600 C in air for 5000 h, whereas uncoated boron fibers degrade after only 1 h at 500 C or at lower temperatures for longer periods of time. It appears that the degradation in tensile properties is controlled by the rate of boron diffusion through the silicon carbide layer and, therefore, is dependent on the thickness of the coating.
To study the interfacial stability of the coated filaments in metals, test specimens of coated and uncoated fibers were packed into aluminum alloy (2024) and titanium powders, heated at several temperatures up to 600 C for various times up to 1000 h, cooled, extracted with acid, and their ultimate tensile strength measured. The aluminum powder specimens were heated in air and the titanium specimens heated in argon to prevent oxidation of the titanium. The results of the tension tests indicate that uncoated boron fibers in contact with aluminum or titanium degrade at temperatures as low as 200 C with long time exposure and degradation is very rapid at temperatures over 400 C. Silicon carbide coated fibers on the other hand had approximately the same room-temperature strength after 1000 h at 400 C and retained a room-temperature tensile strength of over 300,000 psi after 1000 h at 600 C. The results of the metal matrix studies also indicate that the boron diffusion rate through the silicon carbide layer is the controlling step in the fiber degradation process.
boron, silicon carbide, composites, filaments, interfaces, evaluation, tests
Senior materials scientist, United Aircraft Research Laboratories, East Hartford, Conn.