Journal Am. Crra SeN、8721261-67(2004 Mullite/Alumina Mixtures for Use as Porous Matrices in Oxide Fiber Composites Hiroki Fujita, George Jefferson, Robert M. McMeekings and Frank W. Zok*t Materials Department, Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, California 93106 Department of Aeronautics and Astronautics. Air Force Institute of Technology, Wright Patterson Air Force Base, Ohio 45433 Weakly bonded particle mixtures of mullite and alumina are tolerance under fiber-dominated loadings. These offsetting effects assessed as candidate matrixes for use in porous matrix suggest the existence of an optimum in matrix properties at which ceramic composites. Conditions for the deflection of a matrix a prescribed balance of properties is attained. However. the crack at a fiber- matrix interface are used to identify the relationships between matrix structure and composite performance combinations of modulus and toughness of the fibers and the are understood presently at only a rudimentary level. Conse- natrix for which damage-tolerant behavior is expected to quently, the pathway to optimization remains ill defined occur in the composite. Accordingly, the present study focuses on the modulus and toughness of the particle mixtures, as well connections between composition, microstructure, and mechanical as the changes in these properties following aging at elevated properties of a candidate family of porous matrices, with emphasis temperature comparable to the targeted upper- use tempera aggregates are presented, assessed, and calibrated. The exper- addressed elsewhere. 2 The work stems from concurrent activities imental and modeling results are combined to predict the on the development of all-oxide CFCCs for use in future gas critical aging times at which damage tolerance is lost because turbine systems. Because of the interest in mullite/alumina of sintering at the particle junctions and the associated changes fibers for use in these applications. the present article focuses in mechanical properties. For an aging temperature of 1200C, specifically on mullite/alumina matrixes. These matrices are both the critical time exceeds 10 000 h for the mullite-rich mixtures. chemically compatible with the fibers and exhibit mechanical characteristics that make them attractive for use in ccCS. A L. Introduction perspective based on the mechanics of crack deflection is used to guide the experimental measurements and provide a framework for T interpreting the property values. Consideration is given to the USE of porous matrixes to enable damage tolerance in stability of the properties following long-term exposure at elevated ontinuous-fiber ceramic composites( CFCCs) has emerged as temperature, comparable to the targeted upper use temperature for paradigm in high-performance materials. The concept o* we present a review of the mechanics of crack deflection at a tide CFCS obviates the need for fiber coatings to affect crack deflection thereby providing opportunities for lower-cost manufacturing relative to that of conventional coated-fiber systems Furthermore fiber-matrix interface. The results are used to identify the critical on selection of all-oxide constituents, the long-term durability combination of matrix modulus and matrix toughness needed for requirements in the targeted high-temperature applications can be deflection and to motivate the subsequent mechanical measure- ments. The nature of the candidate matrix system, the processing Although the porous matrix concept offers new opportunities route, and the measurement procedures are described. Results for modulus and toughness are presented, along with predictions of for the development of damage-tolerant CFCCs, it also presents these properties from models of bonded-particle aggregates. Fi. hallenges in the design and synthesis of microstructures that meet nally, we address the implications on crack deflection in CFCCs. in the absence of fiber coatings, the matrix must be sufficiently aging time at which the crack deflection condition is no longer yet retain adequate strength to ensure acceptable off-axis proper atisfied and thus the damage tolerance is lost. ties. In principle, the combination of properties can be tailored through changes in the state of the matrix. For instance, improve- ments in the interlaminar strength and off-axis in-plane strength l. Mechanics of Crack Deflection can be obtained by reducing the matrix porosity: however, these mprovements come at the expense of a reduction in the damage As a minimum requirement for damage tolerance in CFCCs cracks in the matrix must either arrest at or deflect into the fiber/matrix interface rather than penetrate into the fibers. The conditions that satisfy this requirement are obtained from the He T A. Panthasarathy-contnbuting editor and Hutchinson diagram shown in Fig. 1. Crack arrest or deflection is predicted to occur when the ratio of interface toughness, Ii, to fiber toughness, T falls below the corresponding ratio of energy release rates. G /G, associated with deflection into Manuscript No 10088 Received March 28, 2003: approved September I the boundary and penetration into the fiber. The critical ratio is under Contract No, F49620-02-1-0128, monitored by Dr B. L Lee, as well controlled by the elastic mismatch parameter from NGK Insulator Ican Ceramic Society Department of Mechanical and Environmental Engineering -(E+