Boron and boron carbide by vapor deposition

Abstract

Boron trichloride can be reduced by hydrogen at 1,300°C with a recovery of approximately 85% as elemental boron. The optimum H 2 to BCl 3 molar ratio was 30 under the test conditions used. No reaction between tungsten substrates and the boron deposits could be detected. Other substrate materials tested, in order of increasing reactivity with the depositing boron, were fused silica, mullite, graphite, titanium and nickel. Alumina reacted extensively with the chloride gases to form aluminum chloride. BBr 3 can be reduced by hydrogen at slightly lower temperatures than can BCl 3, but this advantage is probably outweighed by the lower boron content, lower volatility and higher cost of BBr 3. When stoichiometric amounts of BCl 3 and CH 4 were reacted with an excess of H 2 at 1,300°C, B 4C deposits were obtained. An excess of CH 4 in the reaction mixture resulted in the codeposition of B 4C and free carbon. B 4C deposits contained 74.7 to 76.0% boron, compared with 78.3% for pure B 4C. The specific gravity of the B 4C was 2.32, which is 90% or more of the theoretical value. The Knoop microhardness of one B 4C deposit on a mullite substrate was 3,472. A method of etching B 4C was developed and used. Etching revealed that the B 4C deposits had a columnar microstructure with the columns perpendicular to the substrate. When BCl 3 was reduced with H 2 at 1,500°C with a graphite substrate, the depositing boron reacted with the graphite to produce uniform, strongly adherent coatings of B 4C on graphite. The Knoop microhardness of two of these B 4C coatings was 3,065 and 3,195, which corresponds to a Mohs hardness of approximately 9. Graphite coated with B 4C has many physical and chemical properties that would be desirable in rocket nozzle applications.