Mullite/Alumina Mixtures for Use as Porous Matrices in Oxide Fiber Composites Hiroki Fujita, † George Jefferson, ‡ Robert M. McMeeking †,§ and Frank W. Zok* ,† 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 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 tempera- ture for oxide composites. Models based on bonded particle aggregates are presented, assessed, and calibrated. The exper- imental 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. I. Introduction T HE USE of porous matrixes to enable damage tolerance in continuous-fiber ceramic composites (CFCCs) has emerged as a new paradigm in high-performance materials. 1 The concept 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, on selection of all-oxide constituents, the long-term durability requirements in the targeted high-temperature applications can be met. Although the porous matrix concept offers new opportunities for the development of damage-tolerant CFCCs, it also presents challenges in the design and synthesis of microstructures that meet the opposing property requirements placed on the matrix. That is, in the absence of fiber coatings, the matrix must be sufficiently weak to enable damage tolerance under fiber-dominated loadings, yet retain adequate strength to ensure acceptable off-axis proper- 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 can be obtained by reducing the matrix porosity; however, these improvements come at the expense of a reduction in the damage tolerance under fiber-dominated loadings. These offsetting effects suggest the existence of an optimum in matrix properties at which a prescribed balance of properties is attained. However, the relationships between matrix structure and composite performance are understood presently at only a rudimentary level. Conse- quently, the pathway to optimization remains ill defined. The objective of the present article is to elucidate some of the connections between composition, microstructure, and mechanical properties of a candidate family of porous matrices, with emphasis on the conditions needed to enable damage tolerance in fiber- dominated loadings. Effects of the matrix on off-axis properties are addressed elsewhere. 2 The work stems from concurrent activities on the development of all-oxide CFCCs for use in future gas- turbine systems. 3–6 Because of the interest in mullite/alumina fibers for use in these applications, the present article focuses specifically on mullite/alumina matrixes. These matrices are both chemically compatible with the fibers and exhibit mechanical characteristics that make them attractive for use in CFCCs. A perspective based on the mechanics of crack deflection is used to guide the experimental measurements and provide a framework for interpreting the property values. Consideration is given to the stability of the properties following long-term exposure at elevated temperature, comparable to the targeted upper use temperature for oxide CFCCs. We present a review of the mechanics of crack deflection at a fiber-matrix interface. The results are used to identify the critical combination of matrix modulus and matrix toughness needed for deflection and to motivate the subsequent mechanical measure- ments. The nature of the candidate matrix system, the processing route, and the measurement procedures are described. Results for modulus and toughness are presented, along with predictions of these properties from models of bonded-particle aggregates. Fi- nally, we address the implications on crack deflection in CFCCs. For this purpose, calibrated models are used to establish the critical aging time at which the crack deflection condition is no longer satisfied and thus the damage tolerance is lost. II. Mechanics of Crack Deflection 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 and Hutchinson 7 diagram shown in Fig. 1. Crack arrest or deflection is predicted to occur when the ratio of interface toughness,  i , to fiber toughness,  f , falls below the corresponding ratio of energy release rates, G d /G p , associated with deflection into the boundary and penetration into the fiber. The critical ratio is controlled by the elastic mismatch parameter:    E  f - E  m  E  f + E  m  (1) T. A. Parthasarathy—contributing editor Manuscript No. 10088. Received March 28, 2003; approved September 12, 2003. Funding for this work was provided by the Air Force Office of Scientific Research under Contract No. F49620-02-1-0128, monitored by Dr. B. L. Lee, as well as a gift from NGK Insulators. *Member, American Ceramic Society. † Materials Department. ‡ Department of Aeronautics and Astronautics. § Department of Mechanical and Environmental Engineering. J. Am. Ceram. Soc., 87 [2] 261– 67 (2004) 261 journal