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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 45 SiC has been codeposited and characterized. Chemically vapor deposited carbon coatings containing 7 to 17 weight percent SiC have been used commercially for heart valves for several years. TABLE 4 Dispersed-Phase Ceramic Composites Prepared by CVD (Lackey et al., 1987) Matrix Dispersoid Carbon SiC, TiC, B4C, BeO Si3N4 C, TiN, BN, AlN, or SiC SiC TiSi2 Ti-Si-C Ti-Si-C Ti-Ge-C Ti-Ge-C Al2O3 ZrO2 Si3N4 Matrix Composites Dispersoid-type composites having a Si3N4 matrix have been prepared by Hirai (1982). The dispersed particles were TiN, C, BN, or SiC. Dispersoid concentrations were and have been as high as 32, 10, and 83 weight percent for TiN, C, and BN respectively. The thermal conductivity of -Si3N4 containing less than 10 volume percent of 10-nm TiN particles was a factor of 10 less than for pure Si3N4. It appears that the TiN particles resist phonon transport. The morphology of the dispersed phase and crystallinity of the matrix can be varied by control of deposition temperature and pressure. Deposits containing TiN whiskers in Si3N4 matrix have been codeposited. SiC + TiSi2 Stinton and Lackey (1985) have deposited SiC coatings containing TiSi2 as a dispersed phase. Coatings were deposited on graphite substrates, some of which were suspended in a particulate fluidized bed. For nonfluidized bed coatings, the TiSi2 particles were columnar, whereas fluidized bed coatings produced smaller, nearly equiaxed particles and the coatings were more uniform. The composite coatings, compared to single-phase SiC, were more adherent and possessed significantly greater fracture toughness.
