Design of Materials and Processing Methods for High Temperature Composites

In this dissertation, various studies are described which seek to improve the properties of high temperature ceramic-matrix and carbon-matrix composites. Processes were developed and optimized to control the compositions, microstructures, and morphologies of ceramic materials. Focus was on the chemical vapor infiltration (CVI) of coatings on continuous fiber substrates. Other processes were involved as well, such as polymer impregnation & pyrolysis (PIP) and silicon melt infiltration (SMI). Both carbon and SiC fibers were used as substrates. Materials deposited using CVI include carbon, SiC, TiC, BN, and B4C. Characterization methods include scanning electron microscopy, mechanical testing, X-ray diffraction, and others. A brief introduction of high temperature composite materials is outlined in the first chapter. Chapter 2 explains how the CVI parameters for depositing TiC onto carbon fibers was optimized. The TiC layer serves to prevent chemical degradation of the fibers. In Chapter 3, carbon fibers are given different CVI coatings to explore ways of protecting them from molten silicon during the SMI process. BN was determined to be beneficial for providing protection and also improved mechanical properties. An effort was made in Chapter 4 to perform CVI of a mixed B4C-Carbon matrix material to optimize thermal properties of carbon/carbon composites. Coatings with different compositions and structures were produced which may be useful for different applications of CMCs. A CMC with a ferromagnetic matrix was developed in Chapter 5 using the PIP method. This was done by dissolving cobalt salts in the preceramic polymer. The final chapter was devoted to suggesting how this work may be continued by future researchers to further improve these materials. Partial studies which show promise for CMC research are also presented.