2600 Journal of the American Ceramic Societ)Kerans ef al Vol. 85. No. II tiber 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. .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. 8.4.3-4Ironi- ly, had the Nicalon fiber actually been stoichiometric crystalline SiC. carbon layers would not have formed in sit, and attaining mechanically viable ceramic composites would have been more problematic, but perhaps hastened more-detailed understanding of the mechanics governing composite design, ()Oxidation History 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 higl temperatures, there was a substantial loss in strength above 900"C (Fig. 2).8.46.47Initially. this was attributed to replacement of the carbon layer by Sio, that strongly bonded fibers to the matrix an allowed matrix cracks to propagate directly through fibers. 7.48 Recent work suggests that oxidative degradation of Nicalon fiber to direct effects of interface property chain a deg? Nevertheless. may contribute to composite strength loss comparable 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 SiO, layer can prevent access of oxygen to the interface. .However, at intermediate temper atures, typically between 700 and 900C, significant strength loss occurs from uninterrupted oxidation(Fig. 3).20.4N. 50).3Model experiments, analytical modeling. 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- cracking Fig.I. (a) Fracture surface of Nextel 720 fiber/monazite fiber coating/ ress, 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 coalI enough that cracks are not held open, or if there are mechanisms to dicates that the light phase is monazite and that it is essentially always seal lightly loaded cracks, A sensible design using this approach left in the trough. Fiber coating by AFRL/ML; composite by Composite strives 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 umina CerablakM matrix indicating that crack deflection occurred at or Although this approach has merit if the cracking stress can be ear fiber/coating interfaces( Coating and composite by McDermott. Inc. made sufficiently high and the application environment is well- and Applied Thin Films Inc. s far from an ideal solution. All design stress alculations are approximations based on an idealized situation. including mating of perfectly matching surfaces, absence of Ill discusses the design and evaluation of coatings and composite defects 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 on Section VI summarizes local matrix cracking and a few broken fibers, which allows access and speculates on future options. This review is intended to be a of the atmosphere to the composite interior. Furthermore, there is rooking 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 almost always exist in practice. Hence, the ideal composite Initial interest in CFCCs was generated by marketing of requires all constituents to be oxidation resistant, including the Nicalon"M fiber (Nippon Carbon Co., Tokyo, Japan) and the fiber/matrix intertace 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 (2) Initiation of Interfacial Cracks and Deflection of is not crystalline SiC, but instead is carbon- and oxygen-rich and Matrix Cracks nearly amorphous. Although in most respects Nicalon is an rack deflection is the most important event for achieving excellent fiber, when exposed to high temperatures, it crystallizes ugh composites: however, the complexities of the problem and to SiC, rejects carbon and oxygen, and shrinks slightly. 0.d During of real materials require simplification for analysis, and confirma atrix processing. this decomposition can form a coating on the tion by experiment is problematic. The details of crack deflection