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.
The present article focuses on changes in the mechanical properties of an all-oxide fiber-reinforced composite following long-term exposure (1000 h) at temperatures of 1000 -1200°C in air. The composite of interest derives its damage tolerance from a highly porous matrix, precluding the need for an interphase at the fiber-matrix boundary. The key issue involves the stability of the porosity against densification and the associated implications for long-term durability of the composite at elevated temperatures. For this purpose, comparisons are made in the tensile properties and fracture characteristics of a 2D woven fiber composite both along the fiber direction and at 45°to the fiber axes before and after the aging treatments. Additionally, changes in the state of the matrix are probed through measurements of matrix hardness by Vickers indentation and through the determination of the matrix Young's modulus, using the measured composite moduli coupled with classical laminate theory. The study reveals that, despite evidence of some strengthening of the matrix and the fiber-matrix interfaces during aging, the key tensile properties in the 0°/90°orientation, including strength and failure strain, are unchanged. This strengthening is manifested to a more significant extent in the composite properties in the ؎45°o rientation, wherein the modulus and the tensile strength each exhibit a twofold increase after the 1200°C aging treatment. It also results in a change in the failure mechanism, from one involving predominantly matrix damage and interply delamination to one which is dominated by fiber fracture. Additionally, salient changes in the mechanical response beyond the maximum load suggest the existence of an optimum matrix strength at which the fracture energy in the ؎45°orientation attains a maximum. The implications for long-term durability of this class of composite are discussed.
This study focuses broadly on synthesis and characterization of porous mullite/alumina mixtures for use as matrices in oxide fiber composites. Specifically, an assessment is made of the utility of a precursor‐derived alumina (PDA) in controlling both the modulus and the toughness of mullite‐rich particle mixtures. Property changes are probed through models of the mechanical behavior of bonded particle aggregates. Consideration of the conditions needed to cause crack deflection at a fiber–matrix interface yields an upper allowable limit on the concentration of PDA to ensure damage tolerance in a fiber composite. The predicted critical concentration lies in the range of about 7–9%, depending on the mullite/alumina ratio in the particle slurry and the subsequent aging treatment. Values slightly below this limit should yield composites that exhibit a desirable balance between fiber‐ and matrix‐dominated properties.
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