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2606 Journal of the American Ceramic Socien-Kerans et aL. Vol. 85. No. 1I implying very short debond lengths, they also demonstrate high and coating surface roughnesses. o Therefore, if debonding is strength and toughness. Nevertheless, there is such a thing as within a coating and the crack meanders in the coating, a thinner debond lengths that are too short, even though that value is coating may decrease the fracture surface roughness and, there- considerably less than has been widely assumed before analysis of fore, increase toughness. If debonding initiates and remains at the these composites. 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 I(), the result is apparently benign. That However, if the debonding crack tends to approach th is, either(i)the interface, although stronger than the coating itself, surface via Mode I steps as it propagates and the interfa is weak enough to fail before the fiber, (ii) the changed local stress debond criterion is not satisfied( the situation discussed in state and short crack do not pose substantial stress concentration Il(), then greater coating thickness leads to longer debond on the fiber, or (iii) the resulting failure event is sufficiently late to ngths and higher toughness. yet allow excellent composite properties 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 (I General Interface Considerations roughness, but it may fail to relieve compressive residual stress In Ideally, the choice of composite constituents and geometry such cases, two coating layers might be considered. The weak should lead to the best balance of properties throughout the crack-deflecting layer would be thin, and the compliant layer for omponent service lifetime. In fact, many possibilities must await relief of residual compressive stress would be thick. The added the development of more constituent options, and optimizing complexity and expense is not desirable, but it may not be properties 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 understanding 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 approach that develops a framework into choices of fiber, coating, and matrix that are thermochemical which new tools can be fitted as they become available and that stable individually and in combination in the temperature range 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 elaxed to include materials that react at acceptably slow rates. In before the fiber fails, thereby removing matrix -imposed stress fact, almost no structural materials are at thermodynamic equili concentrations on the fiber. The second function is that the coating rium in their use environments. a common example of acceptable must allow some sliding along the fiber/matrix debond after environmental instability is SiC 20,- Sio,+ co,, whe 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. -s 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 For debonding at the fiber/coating interface, allowable T-/ limited by the processing requirements values based on the He and Hutchinson criterion 9,60 vary with Excessive thermal stress in the coating may cause it to sp fiber/coating elastic modulus mismatch from -0.25 for matrix processing. This is particularly important for CMC mismatch to almost 0.7 when the fiber is 6 times stiffer than the atings, because they are designed to be weak, or weaki coating or matrix, as in SiC-reinforced glass-matrix CMCs.A 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 eliminating steps that bend fibers excessively ahead of the matrix crack. Excessive handling can be avoided by applying fiber coatings to Once debonding starts. it 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 in chemical vapor infiltration(CVD) processing, rather than to fiber of the debond crack(distance from the matrix crack plane to the tows. Preform-coating processes using other than cvi or in situ debond crack tip) depends on the interfacial sliding friction. The ocesses using fiber constituents have not been demonstrated lower the friction, the longer the crack and the greater the distance opposites that perform poorly may require careful evaluation to from the matrix crack plane required to transfer the excess load on determine if an ineffective coating, a damaged coating, or a the fiber back to the matrix. Higher friction along this Mode l damaged fiber is responsible ack causes the fiber stress to decrease faster with distance from Thermal expansion mismatch, roughness, and coating com the matrix crack plane. That is, the highly stressed portion of the ance interplay determine the postslidin ber is shorter, and there is a higher probability that fibers fracture and they should be considered simultaneously. For example, if the at or near the matrix crack plane. Therefore, toughne 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 and the coeffi- cient of friction Residual stress is d CTEs. the coating thickness the fiber volup tion and the use ( Coating Evaluation emperature. In many systems, the coating is the most compliant The properties a coating must possess to provide good compo component; therefore, coating thickness can provide some adjust ite properties are not well-known. Hence, coating evaluation is 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%. This process can be toughness development of new fiber-coating and matrix-processing metha c time consuming and expensive. Each new approach can requi Potential opposite effects of coating thickness on crack pat should be considered. The maximum fracture surface roughness is Replacement of the CMC matrix with a glass matrix that is easier bounded by the sum of the coating thickness as well as the fiber to process also can be considered for coating evaluation, althoughimplying very short debond lengths, they also demonstrate high strength and toughness. Nevertheless, there is such a thing as debond lengths that are too short, even though that value is considerably less than has been widely assumed before analysis of these composites. If the coating cracks ultimately reach the coating/fiber interface, as discussed in Section II(3), the result is apparently benign. That is, either (i) the interface, although stronger than the coating itself, is weak enough to fail before the fiber, (ii) the changed local stress state and short crack do not pose substantial stress concentration on the fiber, or (iii) the resulting failure event is sufficiently late to yet allow excellent composite properties. III. Coating System Design and Evaluation (1) General Interface Considerations Ideally, the choice of composite constituents and geometry should lead to the best balance of properties throughout the component service lifetime. In fact, many possibilities must await the development of more constituent options, and optimizing properties requires more highly sophisticated models. Eventually, 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 understanding is incomplete and often speculative. Nevertheless, it is useful to take a logical approach that develops a framework into which new tools can be fitted as they become available and that can provide insight for the refinement of approaches. The first function of the coating, or interface, is that it must fail before the fiber fails, thereby removing matrix-imposed stress concentrations on the fiber. The second function is that the coating must allow some sliding along the fiber/matrix debond after deflection. As discussed earlier, results from carbon- and BNinterface CMCs and models for their behavior suggest that the debond may be at either the fiber/coating interface or within the coating. Coating design strategies can be based on either possibility. For debonding at the fiber/coating interface, allowable i r/ f z values based on the He and Hutchinson criterion59,60 vary with fiber/coating elastic modulus mismatch from 0.25 for zero mismatch to almost 0.7 when the fiber is 6 times stiffer than the coating or matrix, as in SiC-reinforced glass-matrix CMCs. A similar criterion based on interface strengths also can be used.91 For debonding within the coating, fracture anisotropy of the coating is the most important parameter. Although the He and Hutchinson criterion is a very useful guide, as discussed earlier, it may not always be relevant because of effects such as debonding ahead of the matrix crack. Once debonding starts, it must continue to propagate as a cylindrical Mode II crack between the fiber and matrix. The length of the debond crack (distance from the matrix crack plane to the debond crack tip) depends on the interfacial sliding friction. The lower the friction, the longer the crack and the greater the distance from the matrix crack plane required to transfer the excess load on the fiber back to the matrix. Higher friction along this Mode II crack causes the fiber stress to decrease faster with distance from the matrix crack plane. That is, the highly stressed portion of the fiber is shorter, and there is a higher probability that fibers fracture at or near the matrix crack plane. Therefore, toughness may decrease with increasing friction. Friction is controlled by residual and applied stress, the fracture surface roughness, and the coefficient of friction.81 Residual stress is determined by constituent CTEs, the coating thickness, the fiber volume fraction, and the use temperature.88 In many systems, the coating is the most compliant component; therefore, coating thickness can provide some adjustment of residual stresses. Specifically, where the coating is more compliant and/or has higher thermal expansion than the other constituents, thicker coatings can be expected to provide higher toughness. Potential opposite effects of coating thickness on crack path should be considered. The maximum fracture surface roughness is bounded by the sum of the coating thickness as well as the fiber and coating surface roughnesses.86 Therefore, if debonding is within a coating and the crack meanders in the coating, a thinner coating may decrease the fracture surface roughness and, therefore, increase toughness. If debonding initiates and remains at the coating/fiber interface, fracture surface roughness can be varied only by modifying the fiber surface roughness. However, if the debonding crack tends to approach the fiber surface via Mode I steps as it propagates and the interface/fiber debond criterion is not satisfied (the situation discussed in Section II(3)), then greater coating thickness leads to longer debond 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 effect by allowing higher fracture surface roughness; conversely, a thin coating may contribute to decreasing friction by minimizing roughness, but it may fail to relieve compressive residual stress. In such cases, two coating layers might be considered. The weak, crack-deflecting layer would be thin, and the compliant layer for relief of residual compressive stress would be thick. The added complexity and expense is not desirable, but it may not be prohibitive. (2) CMC Design Steps The first step in a logical CMC design sequence might be the choices of fiber, coating, and matrix that are thermochemically stable individually and in combination in the temperature range and environment of interest. In practice, that condition is often relaxed to include materials that react at acceptably slow rates. In fact, almost no structural materials are at thermodynamic equilibrium in their use environments. A common example of acceptable environmental instability is SiC 2O2 3 SiO2 CO2, where oxidation of SiC is defined by the low diffusion rates of oxygen in the SiO2 scale.93–95 The second step that must be considered in design is processing. Processing should not excessively degrade the fiber or coating; therefore, matrix choice can be, and often is, limited by the processing requirements. Excessive thermal stress in the coating may cause it to spall during matrix processing. This is particularly important for CMC fiber coatings, because they are designed to be weak, or weakly bonded, to the fiber. Many excellent review articles discuss debonding of coatings from thermal stress (see, for example, Ref. 96). If possible, choice of a fiber–coating combination with minimal thermal stress should be considered. Debonding of coatings during handling or weaving of coated fibers might be decreased by eliminating steps that bend fibers excessively. Excessive handling can be avoided by applying fiber coatings to woven cloth or, better yet, the final fiber preform, as is often done in chemical vapor infiltration (CVI) processing, rather than to fiber tows. Preform-coating processes using other than CVI or in situ processes using fiber constituents have not been demonstrated. Composites that perform poorly may require careful evaluation to determine if an ineffective coating, a damaged coating, or a damaged fiber is responsible. Thermal expansion mismatch, roughness, and coating compliance interplay to determine the postsliding stresses and friction, and they should be considered simultaneously. For example, if the fiber is known to have a comparatively rough surface, residual stresses should be low and coating compliance should be high. (3) Coating Evaluation The properties a coating must possess to provide good composite properties are not well-known. Hence, coating evaluation is most convincingly done via behavior of a composite that is analogous to a practically usable material form: for example, in sheet form with fiber volume fraction 25%. This process can be time consuming and expensive. Each new approach can require development of new fiber-coating and matrix-processing methods. Replacement of the CMC matrix with a glass matrix that is easier to process also can be considered for coating evaluation, although 2606 Journal of the American Ceramic Society—Kerans et al. Vol. 85, No. 11