ournal Am. Ceram.Soc.85m2599-632(2002) Interface Design for Oxidation-Resistant Ceramic Composites Ronald J. Kerans, *f Randall S Hay, f Triplicane A. Parthasarathy, and Michael K. Cinibulk*f Materials and Manufacturing Directorate, AFRL/MLLN, Air Force Research Laboratory Wright-Patterson Air Force Base, Ohio 45433 fibers and matrices also suffer environmental through distributed damage mechanisms. These mechanisms r, changes in the mechanical properties of carbon- or lent on matrix cracks deflecting into fiber/matrix rolled interfaces after oxidation or enhanced oxidation of interfacial debonding cracks. Oxidation resistance of the fiber fibers or matrices after interface oxidation usually dominates CMC coatings often used to enable crack deflection is an importal behavior(see, for example, Refs. 5 and 18-20). This has mot 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 alant phases, and porous-matrix systems that do not require composites is reviewed. Processing issues, such as fiber coat ific interface control constituents (for concise reviews, see ngs and fiber strength degradation, are discussed. Mechanics 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 very similar structures and elastic and fracture properties. BN and composite systems using alternative coatings are discussed. carbon are used as solid lubricants and can be expected to provide Potential topics for further research are identified low sliding friction. Substitution of oxides is a very different matter, and, unfortunately, lack of well-defined interface property L. Introduction equirements complicates the design and evaluation of alternative viable approaches for use in composites IE discovery that brittle ceramics can be made highly having non-oxide constituents can be further complicated by the tolerant by combining them in fiber/matrix composi sIte form need for stability and compatibility in strongly reducing processin (ceramic-matrix composite or CMC, continuous-fiber environments. In fact, most oxide-coating work to date has been on e or CFCC, and ceramic-fiber matrix composite or oxide fibers to be used in oxide matrices. Research on fiber CFMC) has spawned research spanning approximately three de- coating processes is also required. For example, coated fibers often ades. Early work revealed that deflection of matrix cracks to the dis splay severely degraded tensile strength, 3, 24 which has moti- fiber/matrix interface, leaving intact fibers behind the matrix crack vated research on mechanisms of degradation. tip, was essential for tough behavior. Crack deflection in mos Although development of oxidation-resistant interface control is CMCs has been effected by a relatively weak and compliant complex, there has been progress carbon coating applied to the fibers before matrix processing o (1) There are many interface design parameters, and they are formed in situ by fiber decomposition during matrix processing better understood 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 bn have the correct crack deflection behavior. and some show as been oxidation of the fiber coatings promise for the correct fiber pullout behavior. 2-30 To improve oxidation resistance, BN has been substituted for (3) There has been progress toward viable fiber-coating carbon(see, for example, Refs. 7-17). Progress has been made on process ystems 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 yarm-reinforced coating than carbon, it has much poorer oxidation resistance than composites has been observed for two different oxide coatings most candidate fiber and matrix constituents (Fig. 1). In this review, progress is summarized in a manner intended to ssist in developing guidelines for the design and evaluation of B. Marshall-contributing editor fiber coatings and to highlight the most interesting areas for further esearch. Strategies for oxidation-resistant coatings and relevant interface mechanics are critically reviewed. Progress and problems in coating of fibers are summarized. Section Il provides back- Manuscript No 188122 Received November 29, 2000, approved June 13, 2002. ground in the form of a brief review of historical aspects of interface oxidation. a discussion of the mechanics of crack Also affiliated with UES, Inc, Dayton, OH, under U.S. Air Force Contract No. deflection and sliding, the effects of coating properties F33615-96-C5258 posite behavior, and target values for interface parameters. Section FeatureInterface Design for Oxidation-Resistant Ceramic Composites Ronald J. Kerans,* ,† Randall S. Hay,* ,† Triplicane A. Parthasarathy,* ,‡ and Michael K. Cinibulk* ,† Materials and Manufacturing Directorate, AFRL/MLLN, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 Fiber-reinforced ceramic composites achieve high toughness through distributed damage mechanisms. These mechanisms are dependent on matrix cracks deflecting into fiber/matrix interfacial debonding cracks. Oxidation resistance of the fiber coatings often used to enable crack deflection is an important limitation for long-term use in many applications. Research on alternative, mostly oxide, coatings for oxide and non-oxide composites is reviewed. Processing issues, such as fiber coat￾ings and fiber strength degradation, are discussed. Mechanics work related to design of crack deflecting coatings is also reviewed, and implications on the design of coatings and of composite systems using alternative coatings are discussed. Potential topics for further research are identified. I. Introduction THE discovery that brittle ceramics can be made highly damage tolerant by combining them in fiber/matrix composite form (ceramic-matrix composite or CMC, continuous-fiber ceramic composite or CFCC, and ceramic-fiber matrix composite or CFMC) has spawned research spanning approximately three de￾cades. Early work revealed that deflection of matrix cracks to the fiber/matrix interface, leaving intact fibers behind the matrix crack tip, was essential for tough behavior.1–6 Crack deflection in most CMCs has been effected by a relatively weak and compliant carbon coating applied to the fibers before matrix processing or formed in situ by fiber decomposition during matrix processing. However, long-term use of CMCs has been limited by several forms of environmental degradation, the most pervasive of which has been oxidation of the fiber coatings. To improve oxidation resistance, BN has been substituted for carbon (see, for example, Refs. 7–17). Progress has been made on systems using BN, and the best BN coatings demonstrate very good properties. Nevertheless, although BN is a much better coating than carbon, it has much poorer oxidation resistance than most candidate fiber and matrix constituents. CMC fibers and matrices also suffer environmental degradation. However, changes in the mechanical properties of carbon- or BN-controlled interfaces after oxidation or enhanced oxidation of fibers or matrices after interface oxidation usually dominates CMC behavior (see, for example, Refs. 5 and 18–20). This has moti￾vated research on more oxidation-resistant fiber coatings, viscous sealant phases, and porous-matrix systems that do not require specific interface control constituents (for concise reviews, see Refs. 21 and 22). From a mechanistic standpoint, the substitution of BN for carbon has been relatively straightforward; they have very similar structures and elastic and fracture properties. BN and carbon are used as solid lubricants and can be expected to provide low sliding friction. Substitution of oxides is a very different matter, and, unfortunately, lack of well-defined interface property requirements complicates the design and evaluation of alternative interfaces. Identifying viable approaches for use in composites having non-oxide constituents can be further complicated by the need for stability and compatibility in strongly reducing processing environments. In fact, most oxide-coating work to date has been on oxide fibers to be used in oxide matrices. Research on fiber￾coating processes is also required. For example, coated fibers often display severely degraded tensile strength,23,24 which has moti￾vated research on mechanisms of degradation. Although development of oxidation-resistant interface control is complex, there has been progress. (1) There are many interface design parameters, and they are better understood.25,26 (2) Several more oxidation-resistant alternatives to carbon and BN have the correct crack deflection behavior, and some show promise for the correct fiber pullout behavior.27–30 (3) There has been progress toward viable fiber-coating processes.23,31–37 (4) Definitive evidence of oxide coatings effecting character￾istic composite fracture and properties in true yarn-reinforced composites has been observed for two different oxide coatings (Fig. 1). In this review, progress is summarized in a manner intended to assist in developing guidelines for the design and evaluation of fiber 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￾ground in the form of a brief review of historical aspects of interface oxidation, a discussion of the mechanics of crack deflection and sliding, the effects of coating properties on com￾posite behavior, and target values for interface parameters. Section D. B. Marshall—contributing editor Manuscript No. 188122. Received November 29, 2000; approved June 13, 2002. *Member, American Ceramic Society. † Air Force Research Laboratory. ‡ Also affiliated with UES, Inc., Dayton, OH, under U.S. Air Force Contract No. F33615-96-C-5258. journal J. Am. Ceram. Soc., 85 [11] 2599–632 (2002) Feature