| Vapor phase processing of ceramic materials is attractive because lower process temperatures (;Microwave heating was used to create temperature gradients inside bulk composites to produce inside-to-outside densification through chemical vapor infiltration (CVI) of alumina matrix onto alumina fibers. Modeling suggested that substantial temperature gradients could be achieved with hybrid microwave heating, however experiments showed that control of microwave heating could only be achieved while having small temperature gradients. Experiments also showed that low temperature, high pressure, low dilution, high reactant gas flow rates, and an optimum gas composition led to deeper infiltration and more uniform densification inside the specimen. High densities of 82% were achieved with optimum conditions.;The evolution of the microstructure during CVI was simulated, and was used to calculate the thermal conductivity, gas permeability, and surface area as a function of porosity and temperature. These physical properties were implemented in a model of the CVI process to determine the affects of processing parameters on the resulting deposition profiles in a specimen.;A knowledge of the dielectric properties of a changing material were found to be important for achieving controlled microwave heating. Materials that changed from a conductor to an insulator lead to localized heating and a loss of control. On the other hand, conversion from a lossy dielectric to an insulator represents a class of materials that provides controlled microwave heating.;A composite coating was adhered to monolithic tubes using CVI processing. The coating consisted of a porous interlayer of bonded fibers overcoated by a dense CVD coating. The coating provided damage tolerance for the tubes from externally applied surface flaws, and represents an affordable alternative to bulk composites. |
