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
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
November 2002 Interface Design for Oxidation-Resistant Ceramic Composites 260l (mm) (a) 0.2 200 800°C 60 Time(10s) CO, CO, 250 CROSS-HEAD DISPLACEMENT (IN (mm) FIBER 0127 0.254 (b 4 PYROCARBON Fig. 3. (a) Measured oxidative weight change of a Nicalon/carbon/SiC composite. Oxidation starts with a weight loss fr xidation Above 900 C, the oxidation of SiC results in plugs I rther carbon 35 indicated in the(b)schematic, "00 C, the sealing and associated statistics of the thin coatings and associated interfaces are not well-known. The second problem is that there are no proven local failure criteria. That is, even if the stresses can be 3 calculated, there is no appropriate failure criterion for a very small volume of material where bulk properties do not apply and the flaw distribution is unknown. Energy-based analyses assume particular virtual crack extensions and may not be appropriate for predicting behavior on this scale. Continued progress can be 2 t'y ii not yet understood what level of detail a model must ted with increasing comparison of analyses to experiments 10 capture to properly predict actual behavior. Treating coating layers as being properly described by properties of an infinitesimally thin interface surely must be misleading in many situations. For example. the He and Hutchinson analysis" considered the cri- terion for deflection of a mode I matrix crack to an interfacial crack in an ideal planar interface perphould be expected only for endicular to the matrix crack plane. They found that deflection sl ratios of interface toughness to"fiber" toughness less than a 0005 certain value dependant on elastic properties but generally less CROSS-HEAD DISPLACEMENT (IN) than about 1/4.9-I This often has been applied without regard to the details of crack deflection, However if cracks deflect inside the coating, propagation of the crack in either sense is determined Fig. 2. Effect of oxidation on the mechanical behavior at 900C by coating fracture properties, i.e., the ratio of coating debond nforced lithium a aluminosilicate (LAS) matrix comp where c indicates coating r and z indicate crack surface normals in and at(b)900C in air. K cylindrical coordinates with z along the fiber axis, and applied tractions are along +z.). Fiber toughnesses are typically a few determine the interface property that engineered, Micro- MPam";therefore, coating toughness can be higher than fiber nechanics analyses have contributed toughness. A coating can fail the test of (debond fracture energy y standing of composite behavior, but are two pervis (fiber fracture energy ) 1/4, but can deflect cracks. because the problems that limit the utility of the analyses in guiding composite ratio of coating toughnesses for the two types of cracks do satisfy design. The first of these problems is that the properties, geometry the criterion: that is, the coating is sufficiently anisotropic in
2602 Journal of the American Ceramic Sociery-Kerans et al Vol. 85. No. I to deflect cracks itself. This discussion also failure criterion leaves the matter open to specula This the gs that are intended to deflect cracks by failure sequence has been observed in laminatesoar coating can be evaluated independently of the composites fiber Crack deflection is usually assumed to be a local change in higher coating/fiber interface strengths also provide indirect evi- ack direction from perpendicular to parallel to a fiber surface dence of interfacial failure preceding matrix crack impinge Fig. 4(a)), but there are other possibilities. Mode I interface cracks ment.b(/In composites made with untreated fibers, matrix cracks can form in the tensile stress field normal to the fiber surface ahead connect to debonded coating/fiber interfaces. with no deflection in the coating itself; i.e., the interface, not the coating, fails. In of a matrix crack"(Fig. 4(b), Modeling of an annular matrix otherwise identical composites made with treated fibers, matrix crack has predicted that, for most reasonable choices of properties some other failure event(.g,, fiber fracture) intervenes befor cracks connect to diffuse cracks in the coating without debonding he coating/fiber interface: i. e, the coating itself fails. o,b8 The the matrix crack can be driven to the interface, unless the inter- interfacial toughness, friction, and composite strength are higher face is debonded ahead of it. Interface stresses can be high for the treated-fiber CMC If the matrix crack runs through the enough to make interfacial debond ahead of the matrix crack a plausible mechanism, but lack of a completely understood local coatings on untreated fibers before the interface debonds, it deflects in the coating, as it does in the identical coating on the treated fiber: hence, the coating/fiber interface must fail before the matrix crack enters the coating. When the crack does pass through the coating, the elastic constraint of the fiber is mostly removed by he preexisting debond, and the crack runs directly to the debonded nterface. If there are truly no material differences besides interface strength, a definitive sequence of events consistent with the model c implied 63 These composites have other interesting properties are discussed in later sections ment has been suggested for a composite comprising SiC mono- filaments with successive coatings of carbon and TiB, in a glas matrix,The coating was calculated to be in triaxial compression and modeling suggested that matrix cracks would run to the coating only after coating or interfacial failure. Experiment re ATRIX vealed that debond cracks ran very near the fiber surface except for triangular-section C rings around the fiber with their peaks at the (a) matrix crack planes. Shear stresses on planes in the approximate mentation of the sides of the C rings(about 45 from the matrix crack plane)were calculated to be the highest coating stresses that could lead to fracture. The suggested failure sequence was(i) the matrix crack approached the coating, (ii) the coating failed in shear n planes =45 from the crack plane, ahead of the crack, and formed C rings, (iii)the coating cracks turned parallel to the fiber surface at or near the fiber surface, and (iv) the matrix crack advanced until it joined the coating shear cracks at their intersec tion(see Fig. 4( A further possibility is growth of periodic echelon cracks In an analysis of thin laminates with a Mode I crack normal to the MATRIX nterface, the maximum tensile stresses in the coatings were --45% to the interface plane, that is, parallel to a"half-turn"of the impinging crack into the interface plane. Failure initiated as a series of parallel"periodic echelon"microcracks on these high- stress planes at the center of the coating. As the microcracks approached the coating/plate interfaces, they turned parallel to the interface and joined to form a debond. Evidence for a simila ppearing but different sequence of events has been observed in monazite interlayers in Al,O/AL,O, laminates(Fig. 5). In that ase, the echelon cracks appeared after initial deflection of the main crack into the coating/laminate interface Detailed fracture observations are difficult: therefore. the se. quence of events for fiber/matrix debonding in CMCs remains omposite and global stress state. The coating property most oportant to crack deflection depends on subtle differences in constituent elastic properties and residual stresses. In the ideal case, coatings are engineered material "components"of a compos e system selected for phase, microstructure, and geometry to promote a specific failure mechanism. Enhanced understanding of Fig 4. Three possible sequences leading to crack deflection:(a models are often useful, but they can sometimes be misleading crack grows into the coating and then bifurcates and turns w running parallel to the fiber surface in each direction(as well as co nce, detailed analysis and comparison of microstructure, crack in the matrix);(b)coating or interface fails in the tensile field eflection behavior, and analytical models may be necessary. The he matrix crack before arrival of the matrix crack at the interface region same considerations apply to the interpretation of micromechani and (e)in the matrix crack at the coating, the crack bifurcates and turns as cal tests. For example, fiber pushout/pullout tests may not directly he coating fails in shear at an intermediate angle, then turns parallel to the measure the parameters that actually determine debonding during fiber surface at or near the fiber surface composite failure. It is even possible that debonding in single-
November 2002 Interface Design for Oxidarion-Resistant Ceramic Composites 2603 Alumina Alumina (c) Cohesive Fig. 5. (a)Blunting, or deflection, of a matrix crack can occur through the formation of echelon"cracks, as shown in the optical micrograph of the on taking place within a monazite interlayer separating two Al, O, regions. (b)and(c)are schematics of the mechanisms filament microcomposites can be rent from that in full debond length because of insufficient axial strength On the other composites because of the different hand, the local stress state changes and this short coating crack may not provide sufficient stress concentration to greatly influence 3) Interfacial Crack Propagation fiber fracture. In the latter case, the last "debond crack continues If debonding is along a fiber/coating or coating/matrix interface, to grow while the last coating layer develops multiple Mode I then debond propagation is determined by the interfacial energy cracks that are benign in the short term but presumably cause some and the friction generated by shear traction 72.73 If matrix cracks decrease in apparent fiber strength. In the former case, this are deflected in the coating debonding criteria and crack propa- lengths may be short because of a nondeflecting coating/fiber oating is attractive, because a layer of coating remains on the interface, (ii) long debond lengths may require coatings with high fiber, slowing environmental degradation of the fiber. However,it axial strain to, failure, as does fiber oxidation protection by a seems that the remaining coating is unlikely to remain intact coating, (in) failure characterization may find coating/fiber inter- beyond some critical level of strain. This limits the protective face cracks even though crack deflection occurs in the coating, and function and may limit debond length. A"thought experiment" can throughout their entire strain range. Although this discussion is be illustrative(Fig. 6). We imagine that a matrix crack impinges on largely speculative, it is consistent with the behavior observed in a coated fiber, is deflected in the coating (a debond), and advances the lat ter section on easy-cleaving oxides, and it comprises a the matrix crack bypasses the fiber, and the debond advances in the ypothesis for comparison of fracture evidence. coating; therefore, the matrix crack is bridged by a fiber with hinner coating. (This remaining thickness continues to function to slow oxidation and other environmental degradation. )As the composite is loaded further, the coated fiber is strained until the coating fails in Mode I via a surface-initiated crack. However. a coating that deflects cracks can be expected to again deflect a Mode I crack to Mode Il, leaving the fiber with a yet thinner intact coating. The strain-to-failure of thin coatings often increases with refore. the now ing segment can tolerate higher strain before the deflection process repeat Even if the strain-to-failure does not Increase as the lavers become thinner. successive mode i cracks can be expected to initiate in a noncoplanar fashion, either because of coatin random flaw distribution or biased strain fields at the tips of the debonding cracks. In either case, eventually, this Mode I coating crack impinges the fiber, where deflection is governed by a different criterio where i refers to the coating/fib Fig. 6. Schematic of a matrix crack impinging on a coated fiber in a increasing tension along the axis of the fiber (vertical):(a) interface and f to the fiber, r and z to the normals of crack planes initial crack deflection within a coating: (b) subsequent Mode I failure of in cylindrical coordinates with z along the fiber axis. Hence, a the coating, followed by a second deflection: and (c) additional Mode I failures and deflections, until the fiber/matrix interface is reached
Journal of the American Ceramic Sociery-Kerans et al Vol. 85. No. II Consideration of protection of fibers by residual coating layers on interfacial stresses and sliding friction. Rea hat rough raises the issue of the degree of protection that might be expected. ness misfit effects can be substantial in oxide Luthra" has discussed the issue of Sic-fiber protection from reexamination of conventional composites for oxidation in some detail. It is evident that very thin coatings can compliance effects compliance effects. Modeling has shown that roughness in ow oxidation only to a limited degree. Small-diameter fibers- creases the compre creases the compressive radial stress in a hypothetical uncoated SiC filaments are typically 8-12 um in diameter--are desirable Nicalon fiber/SiC composite from -150 MPa before sliding to 450 for easy handling, weaving, and shape-making, but the surface MPa after sliding. These stresses are decreased by 1/3 by including volume ratio is very high. Consequently, oxidation depths that are a 0.5 um thick carbon coating: therefore, changes in coatin significant thickness can be expected to affect debond length and composite roperties. In general, oxides are less compliant than carbon and 4) Interfacial Friction modate misfit stresses, Assuming a Nicalon/SiC composite l BN: therefore, thicker coatings are required to similarly accom- CMC behavior also depends strongly on the fiber/matrix slidi practical lower limit of 70 GPa for the elastic modulus of a porous friction. The ultimate strength, strain-to-failure, matrix crack oxide. the compliance provided by 500 nm of carbon requires-2 spacing, and toughness are affected. .Coulomb friction is um of oxide, If coatings of such thickness are not practical proportional to the radial clamping stress on the fiber, which can suitable friction levels may need to be engineered in other ways. be caused by residual stress from differential thermal expansion or e.g. by controlling roughness, matrix compliance, and residual stress state, or by other deformation mechanisms. Coatings of such els and experiments focus on residual stresses. 73, 78-80 but, re- thickness are also a large volume fraction of the composite and can cently, more attention has been given to roughness-induced stress es..- A large roughness effect on sliding friction has beer affect other composite properties, such as modulus, thermal conductivity, and thermal expansion. Astute design allows for the shown by fiber push back or"seating drop"measurements. 2. effects on composite properties Initial modeling of the roughness effect'is based on an approx imation that debond roughness of amplitude h causes a mismatch strain of h/R, where R is the fiber radius, that adds to the thermal (6)EJects of Coating Properties on Composite Analysis Many calculations of radialer only the thermoelastic properties aspects of the behavior for many interfacial crack roughness debonding and sliding consic geometries and, for most systems, during sliding of long fiber of the fiber and matrix. The discussion above implies that serious lengths. However, modeling has shown that the effect of rough errors may result. A rigorous treatment of the coating elastic ness in the early stages of debond crack propagation(Fig. 7)can effects exists, but the results are not easily incorporated into be much more pronounced and can have a significant influence on existing models of behavior. An approach that utilizes an approx composite properties. This effect is due to the initial unseating of imation of this work in a method that represents the coated fiber by he matching rough surfaces just behind the crack tip. In th an"effective"(transversely isotropic) fiber in simple fiber/matrix region, the work required to further compress the fiber and matrix composites allows simple inclusion of coating elasticity in existing to accommodate the misfit is done. Furthermore, the sliding analyses s This work also indicates that many conventional surfaces are not parallel to the fiber axis; therefore, there is nalyses that have neglected carbon and BN coatings in a Nicalon/ component of applied force that increases the friction. Perhaps the SiC system are significantly in error, Plots of normalized elastic I example of a system where this effect is important is the modulus and coefficient of thermal expansion(CTE) for isotropic treated-fiber SiC composite system discussed earlier: a rough effective" fibers are given in Fig. 8. There are limits to the interface model is necessary to decrease pushout data, and rough- geometries for which this approach yields good results. The plots ness appears to be the primary source of the high friction that work well for compliant (carbon, BN) coating thickness up to 69 dictates the very good fracture properties. .m Models of such thickness up to 10%e. The thickness constraints relax somewhat processes are now available and can be used to study debonding with increasing coating stiffness. Other limitations are discussed roughness contributions to composite behavior. 71.8 Effects pre- dicted for oxide fiber coatings are discussed later elsewhere This approach is applicable to many hat assume transversely isotropic fibers. For example. (5) Interfacial Layer Compliance ties can be directly used in the shear-lag Although the coating is not often explicitly considered in and pushout, as well as the Budiansky-Hutchinson-Evans analysis, the compliance of the coating can have significant effects (BHE)model for matrix-cracking stress (7) Necessary Values of Interfacial Toughness and Friction Many CMCs fit in one of two categories: those with negligible Matrix c (bond Crack-tip D interfacial strength, moderate to low interfacial friction, and tough behavior: and those with high interfacial strength and elastic behavior. From these categories, it often has been inferred that negligible interfacial strength and low friction are necessary for toughness. 5, 0.9 When combined with the ease of using one parameter to describe the interface, this practice has led to the Bridging Fiber Nicalon/C/SiC composites made with fibers treated to enhance Matri coating/fiber bond strength". evidence interface properties that defy common assumptions regarding what is required for good or.compositesmadewithtreatedfibershay 30% higher tensile strength(from 250 to 350 MPa)at the same strain-to-failure, much finer matrix crack spacing, and signifi Fig. 7. Illustration of the effect of interfacial roughness during cantly different stress-strain behavior (Fig. 9). The change i ive debonding progressing away from a matrix crack in a composi attributed to interfacial friction (T) that increases from5 to -150 tension. Three different regions, labeled 1, Il, and Ill, can be env MPa. Strong and tough composites with hi Roughness amplitude, h, period, 2d. and fiber radius, R, are th (0.5%) are observed even when T=370 MPa. The lure mportant parameters that influence interfacial frictio composite strength has been attributed to the decrease in effect
Interface Design for Oxidation-Resistant Ceramic Composite 1.3 1,2,57.5,1010°C 0.9 A(GPa) 1.2t/R<6% 0.8 口20(BN 0.7 1.1 V150 0.6 000 521 0.5 t<0.5_m:EE 005 04 00.2040.60.8 1.2 -0.1-0.0500.050.10.1502025 R{(Ect1)1+0y}tR}{aa)-1}1+0.EE (b) Fig. 8. Universal plots can be used to obtain the models that use transversely isotropic moduli and C of (a) effective modulus and (b) effective cte in the i Plots are a good approximation for up to 0.5 um thi ting on an 8 um fiber radius (R). Symbols E and c refer ubscripts t, c, and f stand for the transverse, coatit in the original reference, where the matrix on the side pectively. Asterisk ()denotes effective properties. (Plot(b)was in Eq (8)should be inverted gauge length of bridging fibers resulting from short debond isotropic coating--it can be as high as 0.7 for an elastic anisotropy that are, in turn, a consequence of high T. As discussed of 6)from the He and Hutchinson. o analysis is not satisfied. matrix cracks in high-strength material deflect into even in the coating. A similar discrepancy has been noted for interfacial cracks, rather than a single debond. Therefore, crack deflection criteria using a laminate geometry. Although this deflection for this CMC is decided primarily by fracture anisotropy result is not well understood, it is encouraging with regard to the within the coating, rather than at the coating/fiber or coating development of alternative coatings in that the fracture energies matrix interface. Unusual fiber pushout load-deflection curves and the sliding friction may not be ired to be as low as suggest substantial effects of rough interfaces, and subsequent reviously thought. In any event, many of the coating approaches analysis implies that the critical strain energy to propagate cracks discussed later are likely to exhibit sufficiently high fracture in this interfacial region may be as high as 25 J/m". This is more energy and friction to greatly restrict debond lengths. It is helpful than half the fracture energy across the strongest graphite planes. to know that, although the composites discussed above exhibit The criterion of fracture energy anisotropy of -1/4 or less(for an matrix crack spacings of from one to three fiber diameters 0 0.4 LONGITUDINAL TENSILE STRAIN (%) rain behaviors in tension measured on the two-dimensional Si abricated from (I untreated or () treated nical amic grade) fibers. Complex crack deflection within the coating on treated fibers(schematic upper left) leads to higher friction than smooth interfacial
606 Journal of the American Ceramic Sociely-Kerans et al. Vol. 85. No. II implying very short debond lengths, they also demonstrate high and coating surface roughnesses. Therefore, if debonding is strength and toughness. Nevertheless, there is such a thing a debond lengths that are too short, even though that value is within a coating and the crack meanders in the coating, a thinner coating may decrease the fracture surface roughness and. there onsiderably less than has been widely assumed before analysis of fore, increase toughness. If debonding initiates and remains at the these composite coating/fiber interface, fracture surface roughness can be varied If the coating cracks ultimately reach the coating/fiber interface only by modifying the fiber surface roughness. as discussed in Section Il(3), the result is apparently benign. That However, if the debonding crack tends to approach the fiber is, either(i) the interface, although stronger than the coating itself. surface via Mode I steps as it propagates and the interface/fiber is weak enough to fail before the fiber, (ii) the changed local stress debond criterion is not satisfied(the situation discussed in Section state and short crack do not pose substantial stress concentration nl(3), then greater coating thickness leads to longer debond on the fiber, or(ii) the resulting failure event is sufficiently late to lengths and higher toughness There are conflicts between some coating design parameters For example, a thicker coating can provide a route to lower friction by decreasing the compressive residual stresses, but it counters that Ill. Coating System Design and Evaluation effect by allowing higher fracture surface roughness; conversely, a thin coating may contribute to decreasing friction by minimizing (1) General Interface Considerations should lead to the best balance of properties throughout the crack-deflecting layer would be thin, and the compliant laNe ed gomponent service lifetime. In fact, many possibilities must await development of more constituent options, and optimizing complexity and expense is not desirable, but it may not be roperties requires more highly sophisticated models. Eventually prohibitive. there may be more fibers, coatings, and matrices to choose from but, presently, composite design is constrained by constituents for which there are no viable alternatives. Likewise, mechanistic (2) CMC Design Steps nderstanding is incomplete and often speculative. Nevertheless, it The first step in a logical CMC design sequence might be the is useful to take a logical ch that develops a framework into choices of fiber, coating, and matrix that are thermochemically which new tools can be fitted as they become available and that an provide insight for the refinement of approaches and environment of interest. In practice, that condition is often The first function of the coating, or interface, is that it must fail relaxed to include materials that react at acceptably slow rates. In concentrations on the fiber. The second function is that the coating rium in their use environments. A common example of acceptable environmental instability is SiC 20.- Sio,+ CO., where deflection. As discussed earlier, results from carbon- and BN- oxidation of SiC is defined by the low diffusion rates of oxygen in interface CMCs and models for their behavior suggest that the the SiO, scale. The second step that must be considered in debond may be at either the fiber/coating interface or within the design is processing. Processing should not excessively degrade coating. Coating design strategies can be based on either possibil- the fiber or coating; therefore, matrix choice can be, and often is. ly For debonding at the fiber/coating interface, allowable T, /I limited by the processing values based on the He and Hutchinson criterion,o vary wit Excessive thermal stress in the coating may cause it to spall fiber/coating elastic modulus mismatch from -0.25 for zero during matrix processing. This is particularly important for CMC mismatch to almost 0. 7 when the fiber is 6 times stiffer than the iber coatings, because they are designed to be weak, or weakly coating or matrix, as in SiC-reinforced glass-matrix CMCs. A bonded, to the fiber, Many excellent review articles discuss similar criterion based on interface strengths also can be used. 9 debonding of coatings from thermal stress(see, for example, Ref For debonding within the coating, fracture anisotropy of the 96). If possible, choice of a fiber-coating combination with coating is the most important parameter. Although the He and minimal thermal stress should be considered. Debonding of Hutchinson criterion is a very useful guide, as discussed earlier. it coatings during handling or weaving of coated fibers might be may not always be relevant because of effects such as debonding decreased by eliminatin hat bend fibers e ahead of the matrix crack Excessive handling can be avoided by applying fiber coatings to Once debonding starts, it must continue to propagate as a woven cloth or. better yet, the final fiber preform, as is often done cylindrical Mode Il crack between the fiber and matrix. The length of the debond crack (distance from the matrix crack plane to the in chemical vapor infiltration(CVI) processing, rather than to fiber debond crack tip)depends on the interfacial sliding friction. The processes using fiber constituents have not been demonstrated lower the friction, the longer the crack and the greater the distance Composites that perform poorly may require careful evaluation from the matrix crack plane required to transfer the excess load on determine if an ineffective coating, a damaged coating,or the fiber back to the matrix. Higher friction along this mode ll damaged fiber is responsible crack causes the fiber stress to decrease faster with distance from Thermal expansion mismatch, roughness, and coating compli the matrix crack plane. That is, the highly stressed portion of the ance interplay to determine the postsliding stresses and friction at or near the matrix crack plane. Therefore, toughness may fiber is known to have a comparatively rough surface, residual decrease with increasing friction. Friction is controlled by residual stresses should be low and coating compliance should be high and applied stress, the fracture surface roughness, and the coeffi cient of friction. Residual stress is determined by constituent CTEs, the coating thickness, the fiber volume fraction, and the use ( Coating eraluation temperature. In many systems, the coating is the most compliant The properties a coating must possess to provide good compos- component; therefore, coating thickness can provide some adjust- ite properties are not well-known. Hence, coating evaluation ment of residual stresses. Specifically, where the coating is more most convincingly done via behavior of a composite that is compliant and/or has higher thermal expansion than the other analogous to a practically usable material form: for example, in constituents, thicker coatings can be expected to provide higher sheet form with fiber volume fraction >25%0. This process can be toughness. time consuming and expensive. Each new approach can require Potential opposite effects of coating thickness on crack path development of new fiber-coating and matrix-processing methods should be considered. The maximum fracture surface roughness is Replacement of the CMC matrix with a glass matrix that is easier unded by the sum of the coating thickness as well as the fiber to process also can be considered for coating evaluation, although
November 2002 Interface Design for Oxidation-Resistant Ceramic Composites the change in chemistry, and probably elastic properties. may environmental resistance have been studied, Periodic matrix introduce some ambiguity in interpretation of results cracks, nonlinear load displacement, and hysteresis during unload- Porous-matrix CMCs without fiber coatings can have attractive load cycles have been observed, from which debond properties via distributed damage mechanisms, because cracks and the average friction( T)have been estimated. However. full deflect around fibers without need for a coating (see Section confidence in validity of the results for property prediction in a full V().Matrix pore volume fractions at which significant tough CMC has not been established. ening is observed range from >30% to 15%..Hence, porous Oxide/oxide microcomposites have been fabricated and tested matrices complicate evaluation of fiber coatings, because the to evaluate the effectiveness of monazite(LaPO)and hibonite hening Therefore, better understanding of damage mechanisms in porous- composites. Using sapphire monofilaments in an Al,O, matrix matrix composites may be necessary for complete understanding as the control composites, the fractography and fracture strengths of damage mechanisms in coated-fiber composites with imper were compared. For interlayer thicknesses of 0.3-0.5 um, both fectly densified matricesusually the case. interlayers showed evidence of crack deflection; however debond lengths in hibonite-coated specimens were limited to just a small (4) Micro- and Mini-CMCs fraction of the fiber diameter. Monazite-coated specimens showed Use of micro or mini-CMCs for more-rapid evaluation has nultiple matrix cracks and extensive debonding at the coating/ received increasing attention. 00. Io A micro-CMC is defined as a is defined as a matrix interface. In both cases, the load-displacement curves were cylindrical matrix reinforced with one fiber, while a mini-CMC almost linear to failure: therefore there was no unload-reload uses one or multiple fiber tows(200-3000 fibers/tow and up to hysteresis from which to measure interfacial friction. Failure four tows). The mechanical behavior of a mini-CMC is more strength was the only measurable mechanical parameter. The difficult to interpret, but it includes the statistical nature of fiber extent of nonlinearity in tension of specimens of any typ fracture and is more representative of a real composite. These high fiber modulus, straight fibers, and low matrix volume fraction micro- and mini-CMCs are easier to fabricate than full CMCs, and relaxed sintering constraints on matrix densification can allow hypothesis that, even if the coating and matrix volume fraction is denser matrices to be more easily made. 2103 Most such tests very low, there is severe degradation in apparent fiber strength if have been limited to carbon and BN fiber/matrix interfaces and there is no mechanism to deflect cracks. The matrix and coating mostly CVI-SiC matrices. Effects of fiber surface treatments or crack at relatively low strain, and, unless the crack deflects, it acts coating procedures on interface properties and evaluation of as a large flaw in the fiber. In this experiment, composite strengths Sapphire Matrix Hibonite Debonded Surface (Matrix dislodged Alumina CMC-Control CMC-Control 1.18 GPa 0F 1.18GPa m=52 Fiber-l45Q°chh 225GP F Tiber/Mon A FiberHibonite CMC- Hibonite 1450c2h -4 1.18 GPa 1. 84GPa 日 1.28 GPa m=10.3 0.500 11.5 -1-0.50 0.5 1.5 Ln Stress, GPa I Ln Stress, GPa Fig. 10. Single-filament forced/Al, O, matrix microcomposites tested in tension: (a) cracks deflect within the hibonite intertace but a fiber diameter, because of the roughness;(b) present at the monazite/matrix interface are revealed by matrix regions that fell and (d)microcomposites with coatings have no coatings, but the Weibu he same mean strengths as the control composites with about the same as the coated fibers, Results he matrix is not sufficiently dense for evaluation of
2608 Journal of the American Ceramic Society-Kerans et al. Vol. 85. No. I1 were relat gh for both coatings, considering the fiber the nature of the process. Although the desired Al,O, phase strength degradation during processing: the strengths were greater remains difficult to process, work has been reported where almost e matrix-cracking stresses (Fig. 10). However, the mean 85%-dense ZrO, has been deposited around woven preforms. re not significantly different from that of the fiber/ However, the stability of the interface coating during CVD control specimens, although coated- fiber composites had higher Weibull moduli. The lack of difference in strength is processing is unknown and likely to be a major issue because of the use of gaseous hydrogen and CO in the attributed to the porosity in the matrix; porous-matrix composites are known to perform well without interface treatments(see next This has led to the use of glass matrices to test coating concepts. section). The results imply that the matrix density needs to be Preliminary work using Blackglas"(Allied Signal, Inc.(now Honeywell), Morristown. NJ) polymer-derived glass as matrix The processing of even minicomposites having a dense oxide shows some promise, although the matrices remain far from ideal matrix can be challenging. Use of chemical vapor deposition CVD) to deposit oxides remains in the developmental stage array of shrinkage microcracks. Oxide-fiber-reinforced minicom CVD-deposited Al, O, matrices are amorphous and do not bond posites having a dense but microcracked glassy matrix of Black readily to coated or uncoated fiber tows, which causes debonding as have been used in two studies to test oxidation-resistant even in control specimens. o There is no known work on oatings. In one study, the technique was used to evaluate Nextel polycrystalline-oxide-matrix composites with high enough matrix 610(3M Corp. St. Paul, MN)/Blackglas composites with and densities to definitively suppress the mechanism of debonding via without porous lanthanum hexaluminate fiber coatings. 06The matrix cracking(say 90%). CVI of dense stable polycrystalline minicomposites with the fiber coatings had significantly higher oxides is made difficult by the formation of amorphous or ultimate strengths than the uncoated control specimens. In a metastable oxides( which later crystallize or transform, introducing another study, porous oxide(ZrO - SiO, mixture)and monazite significant stresses and cracking) and by the ity to reach were evaluated in Nextel 720-reinforced Blackglas. 7BN- porosity levels below the permeation threshold (-15)because of oated and uncoated fibers were used as controls for comparison. (a Control(uncoated) (b BN 2 CMC- Control CMC. Control (uncoated) 265MPa;m=8.8 265MPa;m=8.8 00 90 2 TOw: Contro uncoated) cMc·BN 742MPa;m=6.5 383MPa;m=64 5.5 6.5 7 5 5.5 6 Ln[ Stress, MPa] Ln[ Stress, MPa] Porous Zro2-sio (d Monazite CMC- Control CMC. Control (uncoated) (uncoated) 265MPa;m=8.8 265MPa;m=8.8 与 cMc· porous Zro-sie CMC. Monazite 356MPa;m=6.0 353MPa;m=8.8 5.56 6.5 6.5 Ln Stress, MPa] Ln Stress, MPa Fig. 11 plots of the strengths of minicomposites using dense Blackglas as the matrix show that porous (c) ZrO - SiO, and (d) monazite coatings ers are as effective as the(b) BN-coated fibers. (a) Control is significantly weaker than the fiber, showing that Blackglas might be a good model ma alate interface coatings