CVD Processing
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CVD of FGMs
Various methods are used in functionally gradient materials technology to control the composition and structure of a composite.1 The chemical vapor deposition (CVD) method yields a deposit with source gases by applying various forms of energy (heat, light, plasma, etc.) to the gases after they are introduced into a CVD reactor. Hydride, bromide, and chloride are generally used for source materials. By continually changing the mixture ratio of source gases or by controlling the CVD conditions such as deposition temperatures, gas pressure, and gas-flow rate, CVD permits relatively easy syntheses of various FGMs (to a maximum thickness on the order of a centimeter).
Figure 1 shows the experimental setup used for preparing SiC/C FGMs by CVD.2 For CVD-SiC/C FGMs, an SiCl 4 -CH 4 -H 2 combination was used as the source gas. SiCl4 vapor was transported into the CVD furnace by bubbling hydrogen carrier gas. The gas-flow rate of SiCl4 and CH 4 was controlled by a program controller. The deposition temperature was 1673-1773 K. The total gas-pressure range was from 1.3 to 6.5 kPa. The CVD-SiC/C FGMs (0.2-0.8 mm in thickness) can be obtained on a graphite substrate through an appropriate change of the SiC/C ratio in the source gas by controlling SiCl4 and CH 4 gas-flow rates. Uemura et al. prepared a rocket nose cone with a hemisphere of 50 mm diameter and a model combustor with a cylinder diame-
RIBBON HEATER
PRESSURE REGULATOR
H2GAS WATER-COOLED REACTION CHAMBER SiCL, RESERVOIR
OPTICAL PYROMETER /
GRAPHITE SUBSTRATE
U/
V
PUMP PRESSURE REGULATOR
Figure 1. Schematic diagram of the CVD setup for preparing SiCIC FGMs.
Table I: Examples of FGMs Prepared by CVD. ZrC/C,5 SiC/C, 278 TiC/C, 9 SiC/TiC,10'11 SiC/TiC(SiC),12 C/B 4 C/SiC 6 SiO2/SiO2(GeO2) BN/Si3N46
MRS BULLETIN/JANUARY 1995
CVD is one of the most effective methods for preparing an FGM as a thermalbarrier coating. In order to reduce thermal stress in a thermal-barrier coating, Hirai et al. have successfully prepared, by CVD, an SiC/C FGM having the optimum compositional distribution profile predicted by a design model. 7 For application to a combustion nozzle, an infinite cylinder with a thickness of 10 mm and inner radius of 95 mm is fixed at one end. This model was used to study the stress distribution, assuming zero axial stress. The temperature of the cylinder's inner surface was set at 1600 K and the outer surface at 300 K, creating a temperature difference of 1300 K. In an effort to reduce the thermal stress, calculations were made on the infinite cylinder to determine the distribution profiles of temperature, stress, Young's modulus, and strength/stress ratio by changing the composition ratio of SiC/(SiC+C) in the thickness direction. Figure 2 shows calculated results and the optimum compositional distribution profile for this case.
Characteristics and Applications of CVD-SiC/C FGMs
MASS FLOWMETER
Carbide Oxide Nitride
Design of SiC/C FGMs
GRAPHITE HEATER
CONSTANT TEMPERATURE BATH
CH4 GAS
ter of 30 mm, made of three-dimensional C-C
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