J Am Cenanmt. SoH. 85 |1112599-63212002 Interface Design for Oxidation-Resistant Ceramic Composites Ronald J. Kerans, *T Randall S. Hay, Triplicane A. Parthasarathy, *and Michael K. Cinibulk"*t Materials and Manufacturing Directorate, AFRL/MLLN. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 Fiber-reinforced ceramic composites achieve high toughness CMC fibers and matrices also suffer environmental degradation hrough distributed damage mechanisms. These mechanisms However, changes in the mechanical properties of carbon-or are dependent on matrix cracks deflecting into fiber/matrix BN-controlled interfaces after oxidation or enhanced oxidation of interfacial debonding cracks. Oxidation resistance of the fiber fibers or matrices after interface oxidation usually dominate coatings often used to enable crack deflection is an important behavior(see, for example, Refs. 5 and 18-20). his hits CMC limitation for long-term use in many applications. Research on vated research on more oxidation-resistant fiber coatings, viscous alternative, mostly oxide, coatings for oxide and non-oxide sealant phases, and porous-matrix systems that do not require composites is reviewed. Processing issues, such as fiber coat- specific interface control constituents (for concise reviews,see Refs. 21 and 22). From a mechanistic standpoint, the substitution work related to design of crack deflecting coatings is also of BN for carbon has been relatively straightforward: they have reviewed, and implications on the design of coatings and of composite systems using alternative coatings are discussed ery similar structures and elastic and fracture properties. BN and Potential topics for further research are identified carbon are used as solid lubricants and can be expected to provid low sliding friction, Substitution of oxides is a very different matter,and, unfortunately, lack of well-defined interface property L. Introduction requirements complicates the design and evaluation of alternativ interfaces. Identifying viable approaches for use in composites HE discovery that brittle ceramics can be made highl having non-oxide constituents can be further complicated by the tolerant by combining them in fiber/matrix composit need for stability and compatibility in strongly reducing processing CMC. continuous environments. In fact, most oxide-coating work to date has been on composite or CFCC, and ceramic-fiber matrix composite or oxide fibers to be used in oxide matrices. Research on fiber- CFMC)has spawned research spanning appf matrix cracks to the coating processes is also required. For example, coated fibers often display severely degraded tensile strength. which has moti- fiber/matrix interface. leaving intact fibers behind the matrix crack ated research on mechanisms of degradation CMCs has been elec, gh behavior. -6 Crack deflection in most Although development of oxidation-resistant interface control is ted by a relatively weak and compliant complex, there has been progress carbon coating applied to the fibers before matrix processing or (I) There are many interface design parameters, and formed in sim by fiber decomposition during matrix processing etter understood. 25.26 However, long-term use of CMCs has been limited by several 2) Several more oxidation-resistant alternatives to carbon and forms of environmental degradation, the most pervasive of which has been oxidation of the fiber coatings promise for the correct fiber pullout behavior. 27-30 some show (3)There has been progress toward viable fiber-coating processes. systems using BN, and the best BN coatings demonstrate very (4) Definitive evidence of oxide coatings effecting character- good properties. Nevertheless, although BN is a much better istic composite fracture and properties in true yarn-reinforced coating than carbon, it has much poorer oxidation resistance than composites has been observed for two different oxide coatings In this review, progress is summarized in a manner intended to assist in developing guidelines for the design and evaluation of D, B, Marshall-contributing editor tiber coatings and to highlight the most interesting areas for further research. Strategies for oxidation-resistant coatings and relevant interface mechanics are critically reviewed Progress and problems in coating of fibers are summarized. Section II provides back 122. Received November 29, 2000): approved June 13. 20X02 ground in the form of a brief review of historical aspects of interface oxidation. a discussion of the mechanics of crack h UES. Inc. Dayton. OH. under U.S. Air Force deflection and sliding, the effects of coating properties on com- F33615.96( posite behavior, and target values for interface parameters. Section Feature