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 H₂ to BCl₃ 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.

BBr₃ can be reduced by hydrogen at slightly lower temperatures than can BCl₃, but this advantage is probably outweighed by the lower boron content, lower volatility and higher cost of BBr₃.

When stoichiometric amounts of BCl₃ and CH₄ were reacted with an excess of H₂ at 1,300°C, B₄C deposits were obtained. An excess of CH₄ in the reaction mixture resulted in the codeposition of B₄C and free carbon. B₄C deposits contained 74.7 to 76.0% boron, compared with 78.3% for pure B₄C. The specific gravity of the B₄C was 2.32, which is 90% or more of the theoretical value. The Knoop microhardness of one B₄C deposit on a mullite substrate was 3,472.

A method of etching B₄C was developed and used. Etching revealed that the B₄C deposits had a columnar microstructure with the columns perpendicular to the substrate.

When BCl₃ was reduced with H₂ at 1,500°C with a graphite substrate, the depositing boron reacted with the graphite to produce uniform, strongly adherent coatings of B₄C on graphite. The Knoop microhardness of two of these B₄C coatings was 3,065 and 3,195, which corresponds to a Mohs hardness of approximately 9. Graphite coated with B₄C has many physical and chemical properties that would be desirable in rocket nozzle applications.