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