The Chemical Vapor Deposition of Dispersed Phase Composites in the B-Si-C-H-Cl-Ar System
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T.S. MOSS, W.J. LACKEY, and G.B. FREEMAN Georgia Tech Research Institute, EOEML, Atlanta, GA 30332-0826
ABSTRACT The CVD of the coatings in the B-Si-C-H-C1-Ar system was accomplished using a statistically designed experiment. The experimental design used five factor half-fraction factorial with a central composite design that was both rotatable and orthogonal. Deposits were thick and dense and were composed Of BD3C2 and fl-SiC with compositions ranging from 0 to 100%. Response surfaces were generated using multivariate regression for unit cell volumes
of
B,3C2
and fl-SiC,
%BI3C2/%SiC in
the coating, and the Si to B ratio in the deposit. These
equations could then be used to examine the significant variables in the reaction, as well as for tailoring and optimizing the deposition process.
INTRODUCTION The importance of the B-Si-C-H-C1-Ar system lies in its application to oxidation prone applications. An example of such a use is that of carbon-carbon composites that undergo significant attack by oxygen at temperatures as low as 500TC, which can lead to catastrophic failure of the component. The additions of B- and Si-containing compounds to the composite are thought to protect it from the attack of oxygen through the formation of a glassy phase, either that of silicate or borate. ' The diffusion of oxygen through the glass is reduced, and the material below is protected from attack. Chemical vapor deposition (CVD) is an attractive method by which compounds from this system may be applied to the outer surface of a carbon-carbon composite or to the interior carbon matrix through a chemical vapor infiltration (CVI) technique. CVD offers a versatile method by which high purity, high density coatings may be deposited.
PROCEDURE In the case of the B-Si-C-H-Cl-Ar system, it was found that there were three crystalline compounds that were observed during experimentation: boron carbide (B, 3 C2), beta-silicon carbide (/3-SiC), and graphitic carbon (C). This paper concentrates on the deposition of these compounds from mixtures of boron trichloride (BCI 3 ), methyltrichlorosilane or MTS ((CH 3)SiC13), methane (CH4 ), hydrogen (112), and argon (Ar) within a hot wall deposition reactor in the temperature range of 1000 to 1400°C. A flow diagram of the system is shown in Figure I. Experiments were done using ATJ graphite substrates at a system pressure of 76 torr. A modified impinging jet geometry was employed to simplify the diffusional analysis for use in physical modeling. 2 Samples coated with the modified impinging jet geometry had a unique circular deposition pattern which develops on the substrate. The center stagnation region was used for characterization. 239 Mat. lies. Soc. Syrup. Proc. Vol. 363 ©1995 Materials Research Soceley
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Figure 1. The schematic for the deposition of B-Si-C compounds from mixtures of BCI 3 +MTS+H 2+CH 4 +Ar is shown. However, as is the case of many CVD systems, the number of control variables is very high. As a result, a programmed sta
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