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