November 2002 Interface Design for Oxidarion-Resistant Ceramic Composites 2603 Alumina Alumina (c) Cohesive Fig. 5. (a)Blunting, or deflection, of a matrix crack can occur through the formation of echelon"cracks, as shown in the optical micrograph of the on taking place within a monazite interlayer separating two Al, O, regions. (b)and(c)are schematics of the mechanisms filament microcomposites can be rent from that in full debond length because of insufficient axial strength On the other composites because of the different hand, the local stress state changes and this short coating crack may not provide sufficient stress concentration to greatly influence 3) Interfacial Crack Propagation fiber fracture. In the latter case, the last "debond crack continues If debonding is along a fiber/coating or coating/matrix interface, to grow while the last coating layer develops multiple Mode I then debond propagation is determined by the interfacial energy cracks that are benign in the short term but presumably cause some and the friction generated by shear traction 72.73 If matrix cracks decrease in apparent fiber strength. In the former case, this are deflected in the coating debonding criteria and crack propa- lengths may be short because of a nondeflecting coating/fiber oating is attractive, because a layer of coating remains on the interface, (ii) long debond lengths may require coatings with high fiber, slowing environmental degradation of the fiber. However,it axial strain to, failure, as does fiber oxidation protection by a seems that the remaining coating is unlikely to remain intact coating, (in) failure characterization may find coating/fiber inter- beyond some critical level of strain. This limits the protective face cracks even though crack deflection occurs in the coating, and function and may limit debond length. A"thought experiment" can throughout their entire strain range. Although this discussion is be illustrative(Fig. 6). We imagine that a matrix crack impinges on largely speculative, it is consistent with the behavior observed in a coated fiber, is deflected in the coating (a debond), and advances the lat ter section on easy-cleaving oxides, and it comprises a the matrix crack bypasses the fiber, and the debond advances in the ypothesis for comparison of fracture evidence. coating; therefore, the matrix crack is bridged by a fiber with hinner coating. (This remaining thickness continues to function to slow oxidation and other environmental degradation. )As the composite is loaded further, the coated fiber is strained until the coating fails in Mode I via a surface-initiated crack. However. a coating that deflects cracks can be expected to again deflect a Mode I crack to Mode Il, leaving the fiber with a yet thinner intact coating. The strain-to-failure of thin coatings often increases with refore. the now ing segment can tolerate higher strain before the deflection process repeat Even if the strain-to-failure does not Increase as the lavers become thinner. successive mode i cracks can be expected to initiate in a noncoplanar fashion, either because of coatin random flaw distribution or biased strain fields at the tips of the debonding cracks. In either case, eventually, this Mode I coating crack impinges the fiber, where deflection is governed by a different criterio where i refers to the coating/fib Fig. 6. Schematic of a matrix crack impinging on a coated fiber in a increasing tension along the axis of the fiber (vertical):(a) interface and f to the fiber, r and z to the normals of crack planes initial crack deflection within a coating: (b) subsequent Mode I failure of in cylindrical coordinates with z along the fiber axis. Hence, a the coating, followed by a second deflection: and (c) additional Mode I failures and deflections, until the fiber/matrix interface is reached