2600 Journal of the American Ceramic Socien-Kerans et aL. Vol. 85. No. 11 fiber surfaces(a thin carbon layer over a thin SiO, layer) that is weak enough to deflect matrix cracks and protect the fibers from matrix crack stress concentrations. ,s Sliding between fiber and matrix, before and after the fibers fracture, further dissipates energy via friction. These mechanisms give CMCs the tolerance to local overload that makes them useful as structural materials Composites with no carbon layer fail catastrophically with low strength in the manner of poor-quality monolithics cally, had the Nicalon fiber actually been stoichiometric crystalline SiC, carbon layers would not have formed in situ, and attaining mechanically viable ceramic composites would have been more problematic, but perhaps hastened more-detailed understanding of the mechanics governing composite desi Early CMC studies measured strength and load-deflection behavior at room temperature, CMCs with carbon layers on the fibers demonstrated high strength, high strain-to-failure, and non- linear load-deflection behavior. However. when tested at (b temperatures, there was a substantial loss in strength above 900C (Fig. 2).,,+Initially, this was attributed to replacement of the carbon layer by Sio, that strongly bonded fibers to the matrix and allowed matrix cracks to propagate directly through fibers. 7 48 Recent work suggests that oxidative degradation of Nicalon fiber may contribute to composite strength loss to a degree comparable to direct effects of interface property changes. 49-52Nevertheles in either case, carbon interface oxidation allowing oxygen acces to the entire fiber surface area in a Cmc is the first degradation step. Above 1000oC, a self-sealing SiO, layer can prevent acces of oxygen to the interface. 48,53 However, at intermediate temper occurs from uninterrupted oxidation(Fig. 3). 20,48 so ength los atures, typically between 700 and 900C, significant st experiments, analytical modeling s and experiments on Nica- lon/C/SiC composites have contributed to the current under standing of this intermediate-temperature degradation. It has been argued that fibers(and coatings)do not oxidize in a crack-free CMC used at design stresses less then the matrix- cracker Fig. 1. (a)Fracture surface of Nextel 720 fiber/monazite fiber coating/ stress. Such an approach might be acceptable for preservation of the aluminosilicate matrix indicating that crack deflection occurred at or near interface when overloads are infrequent and design stresses are low ber/coating interfaces. Energy dispersive spectroscopy(EDS)analysis enough that cracks are not held open, or if there are mechanisms to indicates that the light phase is monazite and that it is essentially al ways al lightly loaded cracks. A sensible design using this approach left in the trough.( Fiber coating by AFRL/ML; composite by Composi trives to have the regions most likely to crack, the more highly Optics, Inc )(b) Fracture surface of Nextel 610/scheelite fiber coating/ stressed regions, at temperatures that are relatively benign alumina CerablakM matrix indicating that crack deflection occurred at or Although this approach has merit if the cra near fiber/coating interfaces( Coating and composite by Mc Dermott, In and Applied Thin Films, Inc. made sufficiently high and the application environment is well- known. it seems far from an ideal solution. All design stress calculations are approximations based on an idealized situation including mating of perfectly matching surfaces, absence of Ill discusses the design and evaluation of coatings and composites lefects and foreign matter, and predictable environments. These Section IV discusses specific approaches to interface control. For approximations work for metals, because ductile materials blunt completeness, BN coatings and porous-matrix composites also are flaws by local plastic deformation that otherwise cause local stress briefly reviewed in Section IV. Section V discusses coating concentrations. For CMCs, the equivalent local deformation is process technology and fiber degradation. Section VI summarizes local matrix cracking and a few broken fibers, which allows acces and speculates on future options. This review is intended to be a of the atmosphere to the composite interior. Furthermore, there is comprehensive critical review and to provide some thought- evidence that matrix cracking occurs in some CMCs well below provoking speculation on composite design and useful future the proportional limit. The fact that introduction of monolithic ceramics into structural applications has been slow and limited despite very high strength and thorough proof testing, provides circumstantial evidence for this point of view. At least occasional Il. Interface Properties and Mechanics local stress concentrations greater than the matrix-cracking stress almost 's exist in practice. Hence, the ideal composite Initial interest in CFCCs was generated by marketing of equires all constituents to be oxidation resistant, including the Nicalon fiber(Nippon Carbon Co Japan) and the fiber/matrix interface erceived availability of a fiber that had the nsity, creep, and oxidation resistance of sic and the high and fabrication ease of small-diameter filaments in a fiber tow However Nicalon (2) Initiation of Interfacial Cracks and Deflection of is not crystalline SiC, but instead is carbon-and oxygen-rich and atrix cracks Although in most respects Ni Crack deflection is the most important event for achievin excellent fiber, when exposed to high temperatures, it crystallizes tough composites; however, the complexities of the problem and to SiC, rejects carbon and oxygen, and shrinks slightly. ,4During of real materials require simplification for analysis, and confirma matrix processing, this decomposition can form a coating on the tion by experiment is problematic. The details of crack deflectionIII discusses the design and evaluation of coatings and composites. Section IV discusses specific approaches to interface control. For completeness, BN coatings and porous-matrix composites also are briefly reviewed in Section IV. Section V discusses coating process technology and fiber degradation. Section VI summarizes and speculates on future options. This review is intended to be a comprehensive critical review and to provide some thought￾provoking speculation on composite design and useful future work. II. Interface Properties and Mechanics Initial interest in CFCCs was generated by marketing of NicalonTM fiber (Nippon Carbon Co., Tokyo, Japan) and the perceived availability of a fiber that had the low density, creep, and oxidation resistance of SiC and the high strength and fabrication ease of small-diameter filaments in a fiber tow. However, Nicalon is not crystalline SiC, but instead is carbon- and oxygen-rich and nearly amorphous.38–41 Although in most respects Nicalon is an excellent fiber, when exposed to high temperatures, it crystallizes to SiC, rejects carbon and oxygen, and shrinks slightly.40,42 During matrix processing, this decomposition can form a coating on the fiber surfaces (a thin carbon layer over a thin SiO2 layer) that is weak enough to deflect matrix cracks and protect the fibers from matrix crack stress concentrations.3,38 Sliding between fiber and matrix, before and after the fibers fracture, further dissipates energy via friction. These mechanisms give CMCs the tolerance to local overload that makes them useful as structural materials. Composites with no carbon layer fail catastrophically with low strength in the manner of poor-quality monolithics.38,43–45 Ironi￾cally, had the Nicalon fiber actually been stoichiometric crystalline SiC, carbon layers would not have formed in situ, and attaining mechanically viable ceramic composites would have been more problematic, but perhaps hastened more-detailed understanding of the mechanics governing composite design. (1) Oxidation History Early CMC studies measured strength and load–deflection behavior at room temperature.3,4 CMCs with carbon layers on the fibers demonstrated high strength, high strain-to-failure, and non￾linear load–deflection behavior. However, when tested at high temperatures, there was a substantial loss in strength above 900°C (Fig. 2).18,46,47 Initially, this was attributed to replacement of the carbon layer by SiO2 that strongly bonded fibers to the matrix and allowed matrix cracks to propagate directly through fibers.47,48 Recent work suggests that oxidative degradation of Nicalon fiber may contribute to composite strength loss to a degree comparable to direct effects of interface property changes.49–52 Nevertheless, in either case, carbon interface oxidation allowing oxygen access to the entire fiber surface area in a CMC is the first degradation step. Above 1000°C, a self-sealing SiO2 layer can prevent access of oxygen to the interface.48,53 However, at intermediate temper￾atures, typically between 700° and 900°C, significant strength loss occurs from uninterrupted oxidation (Fig. 3).20,48,50,53 Model experiments,54 analytical modeling,55 and experiments on Nica￾lon/C/SiC composites20 have contributed to the current under￾standing of this intermediate-temperature degradation. It has been argued that fibers (and coatings) do not oxidize in a crack-free CMC used at design stresses less then the matrix-cracking stress.56 Such an approach might be acceptable for preservation of the interface when overloads are infrequent and design stresses are low enough that cracks are not held open, or if there are mechanisms to seal lightly loaded cracks.57 A sensible design using this approach strives to have the regions most likely to crack, the more highly stressed regions, at temperatures that are relatively benign. Although this approach has merit if the cracking stress can be made sufficiently high and the application environment is well￾known, it seems far from an ideal solution. All design stress calculations are approximations based on an idealized situation, including mating of perfectly matching surfaces, absence of defects and foreign matter, and predictable environments. These approximations work for metals, because ductile materials blunt flaws by local plastic deformation that otherwise cause local stress concentrations. For CMCs, the equivalent local deformation is local matrix cracking and a few broken fibers, which allows access of the atmosphere to the composite interior. Furthermore, there is evidence that matrix cracking occurs in some CMCs well below the proportional limit.58 The fact that introduction of monolithic ceramics into structural applications has been slow and limited, despite very high strength and thorough proof testing, provides circumstantial evidence for this point of view. At least occasional local stress concentrations greater than the matrix-cracking stress almost always exist in practice. Hence, the ideal composite requires all constituents to be oxidation resistant, including the fiber/matrix interface. (2) Initiation of Interfacial Cracks and Deflection of Matrix Cracks Crack deflection is the most important event for achieving tough composites; however, the complexities of the problem and of real materials require simplification for analysis, and confirma￾tion by experiment is problematic. The details of crack deflection Fig. 1. (a) Fracture surface of Nextel 720 fiber/monazite fiber coating/ aluminosilicate matrix indicating that crack deflection occurred at or near fiber/coating interfaces. Energy dispersive spectroscopy (EDS) analysis indicates that the light phase is monazite and that it is essentially always left in the trough. (Fiber coating by AFRL/ML; composite by Composite Optics, Inc.) (b) Fracture surface of Nextel 610/scheelite fiber coating/ alumina CerablakTM matrix indicating that crack deflection occurred at or near fiber/coating interfaces. (Coating and composite by McDermott, Inc., and Applied Thin Films, Inc.) 2600 Journal of the American Ceramic Society—Kerans et al. Vol. 85, No. 11