Microstructural design of stable porous matrices for all-oxide ceramic composites

A nonconventional paradigm for enabling damage tolerance in all-oxide ceram ic composites is examined. The strategy is based on the use of a porous mat rix for crack deflection and frictional dissipation, obviating the need for the debonding interphases used in the mon conventional materials. The prin ciples guiding the microstructural design are reviewed, and the different d esign concepts are compared and contrasted. The discussion focuses on a mic rostructural design concept that affords stability of the porous structure, and hence preserves the damage tolerance properties, upon prolonged exposu re to high temperature. The key feature of the design is the use of two par ticulate oxide constituents, in different size scales and with distinctly d ifferent sintering kinetics, to form the porous matrix. The implementation of the concept is described, with emphasis on the factors that influence th e scale and uniformity of distribution of the porosity and their relationsh ip with the process. The resulting material is shown to achieve the design goals in terms of its damage tolerance characteristics, as well as its long -term stability at temperatures up to 1200 degrees C. It is anticipated tha t the concept can be extended to higher temperatures once fibers with impro ved capabilities become available.