| This dissertation reports the results of a project aimed at the improvement of the oxidation resistance of carbon fiber reinforced carbon composites by specifically enhancing the ability of the carbon fiber/matrix interface to resist attack by oxidation. The fiber/matrix interface plays a key role in the mechanical properties of composites, and oxidative attack at the interface leads to composite mechanical failure. This dissertation focuses upon chemically modifying the interface to provide better oxidation protection.; The interfaces of carbon fiber reinforced carbon composites and carbon fiber reinforced polymeric composites were modified with the aim of establishing fiber/matrix polymer chemical interactions. The surface of carbon fibers was treated by electrochemical oxidation in three electrolytes, namely, phosphoric acid, nitric acid, and ammonium carbonate solution, to introduce various functional groups on fiber surface. A series of interfaces with chemical bonding have been designed with specific considerations to the exact surface chemistry of the carbon fibers and the chemistry of the phenolic resin.; The fiber/polymer interface was modelled as a thin layer of resin on fiber. X-ray photoelectron spectroscopy (XPS) was utilized as the principal technique to monitor the interface chemistry. The spectra were analyzed by ab initio and multiple scattered wave X{dollar}alpha{dollar} molecular orbital calculations, which, by simulating valence band spectra based upon particular chemical interaction models, identified interfacial chemical interactions involving several fiber/resin crosslinks. Valence band XPS, interpreted by model calculations, proved valuable in understanding the nature of these chemical interactions. The effect of the interface enhancement on air oxidation of the resin coated fiber was examined by thermogravimetric analysis (TGA).; An acetal crosslinking structure was identified between phosphoric acid treated fibers and phenolic resin with the use of coupling agent propionaldehyde in an acidic environment. When carbon fiber was electrochemically oxidized in ammonium carbonate solution, it was found that an ether linkage was formed directly between the oxidized fiber surface and the resin, and that the use of glutaraldehyde as a coupling agent led to a probable hemi-acetal crosslinking between the oxidized fiber and the resin. The air oxidation on resin coated fibers showed that improved oxidation resistance could be achieved in cases where interfacial chemical interaction occurred between the resin and fiber.; When titanate coupling agents were used, it was found that for phosphoric acid treated fibers interfacial crosslinking proceeded as a simultaneous process involving oxidized fiber surface, resin, and titanate coupling agent, and that for ammonium carbonate solution treated fibers interfacial crosslinking proceeded as a step process. XPS and TGA results show that tetrakis (2-ethyl-hexyl) titanate is the most effective in establishing fiber/resin chemical interaction, which leads to improved oxidation resistance. This work reports the first case of chemical modification and interfacial chemical analysis to enhance oxidation resistance in composite systems.; In the course of these studies it was discovered that extensive sodium ion adsorption could be induced on oxidized surfaces by treating the surface with sodium ions in a concentrated aqueous acetone solution. |
