| Two chemically different phases, boron carbide (B{dollar}sb{lcub}13{rcub}{lcub}rm C{rcub}sb2){dollar} and silicon carbide ({dollar}beta{dollar}-SiC), were simultaneously deposited by chemical vapor deposition (CVD) for potential use as a carbon-carbon composite oxidation resistant coating. The films were grown in a hot wall reactor on graphite substrates using a modified impinging jet geometry from mixtures of BCl{dollar}sb{lcub}3{rcub}{dollar}-MTS-CH{dollar}sb4{dollar}-H{dollar}sb2{dollar}-Ar. In addition, single phase coatings were prepared to allow for comparison with the dispersed phase deposits. The effects of processing parameters such as temperature, pressure, and reagent concentration on the deposition rate and microstructure of the B{dollar}sb{lcub}13{rcub}{lcub}rm C{rcub}sb2,{dollar} SiC, and B{dollar}sb{lcub}13{rcub}{lcub}rm C{rcub}sb2+{lcub}rm SiC{rcub}{dollar} coatings were systematically investigated. The coatings were characterized using x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy. In general, dispersed phase coatings were composed of very fine B{dollar}sb{lcub}13{rcub}{lcub}rm C{rcub}sb2{dollar} and {dollar}beta{dollar}-SiC grains as two distinctly different crystalline phases.; A mass transfer model was developed to identify the deposition mechanisms for the individual single phase systems. At typical operating conditions, both the B{dollar}sb{lcub}13{rcub}{lcub}rm C{rcub}sb2{dollar} and SiC deposition reactions were found to be kinetically limited. However, the dispersed phase reaction could not be adequately described as the independent sum of the two reactions. Based on statistical analysis of the deviation away from independence, critical factors were found which led to the acceleration or retardation of the deposition. This information allowed for a qualitative explanation of the dispersed phase mechanism.; The dispersed phase samples were also statistically analyzed to develop response surfaces for several dependent variables. The use of these surfaces, when coupled with the deposition mechanism, could allow for the tailoring of the deposition to fit specific material and processing criteria. |
