Mullite/Alumina Mixtures for Use as Porous Matrices in Oxide Fiber Composites

Weakly bonded particle mixtures of mullite and alumina are assessed as candidate matrixes for use in porous matrix ceramic composites. Conditions for the deflection of a matrix crack at a fiber-matrix interface are used to identify the combinations of modulus and toughness of the fibers and the matrix for which damage-tolerant behavior is expected to occur in the composite. Accordingly, the present study focuses on the modulus and toughness of the particle mixtures, as well as the changes in these properties following aging at elevated temperature comparable to the targeted upper-use temperature for oxide composites. Models based on bonded particle aggregates are presented, assessed, and calibrated. The experimental and modeling results are combined to predict the critical aging times at which damage tolerance is lost because of sintering at the particle junctions and the associated changes in mechanical properties. For an aging temperature of 1200°C, the critical time exceeds 10 000 h for the mullite-rich mixtures.