Processing of all-oxide ceramic composites for damage tolerance without fiber coatings

Ceramic composite systems based on polycrystalline alumina fibers and mullite-alumina matrices are of interest for high temperature applications owing to their inherent oxidative stability but must also be designed to exhibit damage tolerance. One microstructural design concept relies on crack deflection through the matrix rather than at fiber/matrix interfaces, which is the more conventional approach to toughening in CFCC's. The enabling element of this concept is a matrix with an optimum combination of toughness and strength which may be achieved by incorporating a minimum amount of fine, well distributed porosity, while simultaneously reducing the population of large flaws. Implementation of this concept by vacuum infiltration of aqueous mullite-alumina slurries into 2-D woven fiber preforms has been investigated. Initial evaluation of these materials shows promising properties under tension and potential for notch insensitive performance. Microstructural and processing issues relevant to the attainment of this behavior are discussed.