Chemical Vapor Deposition of Multiphase Boron-Carbon-Silicon Ceramics
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CHEMICAL VAPOR DEPOSITION OF MULTIPHASE BORON-CARBON-SILICON CERAMICS E.
Michael Golda and B.
Gallois
Department of Materials Science and Engineering Stevens Institute of Technology, Hoboken, NJ, 07030. ABSTRACT Specific compositions of boron-carbon-silicon ceramics exhibit improved abrasive wear and good thermal shock resistance, but require bulk sintering at temperatures in excess of 2100K. The formation of such phases by chemical vapor deposition was investigated in the temperature range of 1073K-1573K. Methyltrichlorosilane (CH3SiCl 3), boron trichloride, and methane were chosen as reactant gases, with hydrogen as a carrier gas and diluent. The coatings were deposited in a computercontrolled, hot-wall reactor at a pressure of 33 MPa (200 Torr). Below 1473K the coatings were amorphous. At higher temperatures non-equilibrium reactions controlled the deposition process. The most common coating consisted of a silicon carbide matrix and a silicon boride, SiB6, dispersed phase. Multiphase coatings of B+B4C+SiB6 and SiC+SiB6+SiB14 were also deposited by controlling the partial pressure of methane and boron trichloride. Non-equilibrium thermodynamic analysis qualitatively predicted the experimentally deposited multiphase coatings. INTRODUCTION Boron-carbon-silicon ceramics have traditionally been fabricated either by sintering or by electric arc melting. Both processes require temperatures in excess of 2100K. Dispersion strengthened ceramics can be formed in this system which exhibit superior properties compared to the single phase material. Proper selection of the matrix and the dispersed phase can be used to control specific materials properties. A boron carbide matrix with a dispersed silicon carbide phase exhibits a significant improvement in abrasive resistance compared to pure boron carbide.(l) A silicon carbide matrix with a boron carbide dispersed phase exhibits superior thermal cycling stability compared not only to pure boron carbide but to other refractory oxides as well. Boron-carbon-silicon ceramics also exhibit good oxidation resistance up to 1473K.[2] Surface modification, in which the ceramic is applied as a coating, is an alternative processing approach. Chemical vapor deposition (CVD) is one of the most versatile techniques for applying ceramic coatings. Dispersion strengthening can be accomplished by the simultaneous deposition, or codeposition, of two or more ceramics. In spite of the potential for improved coating performance, there is a dearth of published information on CVD of boron-carbon-silicon ceramics. Niemyski et al. (3] used boron trichloride, silicon tetrachloride and carbon tetrachloride as precursors. The deposition pressure was 100 MPa (760 Torr) at temperatures greater than 1873K. X-ray diffraction identified the deposit as a bulk substitution compound, B12 (CSi,B) 3. In the present work we report on the formation of boron-carbon-silicon ceramics using different precursors at Mat. Res. Soc. Symp. Proc: Vol. 250. @1992 Materials Research Society
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lower deposition temperatures
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