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