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