Interface Design for Oxidation-Resistant Ceramic Composites 2603 Alumina Alumina ng, or deflection, of a matrix crack can occur through the formation of"echelon"cracks, as shown in the optical micrograph of the phenomenon taking place within a monazite interlayer separating two Al,O, regions.(b)and(c)are schematics of the mechanisms filament microcomposites can be different from that in full debond length because of insufficient axial strength On the other composites because of the different constraints hand, the local stress state changes and this short coating crack nay not provide sufficient stress concentration to greatly influence ( 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 energ cracks that are benign in the short term but presumably cause some d the friction generated by shear traction 72,73 If matrix crack decrease in apparent fiber strength. In the former case, this are deflected in the coating, debonding criteria and crack propa- scenario implies that(i)even if the coating deflects cracks, debond gation can be expected to be more complex. Deflection within the lengths may be short because of a nondeflecting coating/fit coating is attractive, because a layer of coating remains on the nterface, (ii) long debond lengths may require coatings with high fiber, slowing environmental degradation of the fiber. However, it xial strain to failure, as does fiber oxidation protection by a seems that the remaining coating is unlikely to remain intact coating,(iii) 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 I Athought experiment "can (iv)residual coating layers should not be expected to"seal" fibers be illustrative(Fig. 6). We imagine that a matrix crack impinges or throughout their entire strain range. Although this discussion is through the coating away from the matrix crack plane. Eventually, the later sectore, it is consistent with the behavior observed in larg on easy-cleaving oxides, and it comprises the matrix crack bypasses ypothesis for comparison of fracture evidence coating; therefore, the matrix crack is bridged by a fiber with a thinner coating. (This remaining thickness continues to function to slow oxidation and other environmental degradation. )As the mposite 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 decreasing thickness: 4 therefore, the now thinner coating segment can tolerate higher strain before the deflection process repeats chaps many times. Even if the strain-to-failure does not increase as the layers become thinner. successive mode i cracks can be expected to initiate in a noncoplanar fashion, either because of 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 criterion. T_/T where i refers to the coating/fiber Fig. 6. Schematic of a matrix crack impinging on a coated fiber in a interface and f to the fiber, r and z to the normals of crack planes mposite under increasing tension along the axis of the fiber(vertical):(a) 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 coating can successfully deflect cracks but not provide sufficient failures and deflections. until the fiber/matrix interface is reached.filament microcomposites can be different from that in full composites because of the different constraints.72 (3) Interfacial Crack Propagation If debonding is along a fiber/coating or coating/matrix interface, then debond propagation is determined by the interfacial energy and the friction generated by shear traction.72,73 If matrix cracks are deflected in the coating, debonding criteria and crack propa￾gation can be expected to be more complex. Deflection within the coating is attractive, because a layer of coating remains on the fiber, slowing environmental degradation of the fiber. However, it seems that the remaining coating is unlikely to remain intact beyond some critical level of strain. This limits the protective function and may limit debond length. A “thought experiment” can be illustrative (Fig. 6). We imagine that a matrix crack impinges on a coated fiber, is deflected in the coating (a debond), and advances through the coating away from the matrix crack plane. Eventually, the matrix crack bypasses the fiber, and the debond advances in the coating; therefore, the matrix crack is bridged by a fiber with a thinner 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 II, leaving the fiber with a yet thinner intact coating. The strain-to-failure of thin coatings often increases with decreasing thickness;74 therefore, the now thinner coating segment can tolerate higher strain before the deflection process repeats, perhaps many times. Even if the strain-to-failure does not increase as the layers become thinner, successive Mode I cracks can be expected to initiate in a noncoplanar fashion, either because of 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 criterion, i
r/ f
z, where i refers to the coating/fiber interface and f to the fiber, r and z to the normals of crack planes in cylindrical coordinates with z along the fiber axis. Hence, a coating can successfully deflect cracks but not provide sufficient debond length because of insufficient axial strength. On the other hand, the local stress state changes and this short coating crack may not provide sufficient stress concentration to greatly influence fiber fracture. In the latter case, the “last” debond crack continues to grow while the last coating layer develops multiple Mode I cracks that are benign in the short term but presumably cause some decrease in apparent fiber strength. In the former case, this scenario implies that (i) even if the coating deflects cracks, debond lengths may be short because of a nondeflecting coating/fiber interface, (ii) long debond lengths may require coatings with high axial strain to failure, as does fiber oxidation protection by a coating, (iii) failure characterization may find coating/fiber inter￾face cracks even though crack deflection occurs in the coating, and (iv) residual coating layers should not be expected to “seal” fibers throughout their entire strain range. Although this discussion is largely speculative, it is consistent with the behavior observed in the later section on easy-cleaving oxides, and it comprises a hypothesis for comparison of fracture evidence. 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 phenomenon taking place within a monazite interlayer separating two Al2O3 regions. (b) and (c) are schematics of the mechanisms.27 Fig. 6. Schematic of a matrix crack impinging on a coated fiber in a composite under increasing tension along the axis of the fiber (vertical): (a) initial crack deflection within a coating; (b) subsequent Mode I failure of the coating, followed by a second deflection; and (c) additional Mode I failures and deflections, until the fiber/matrix interface is reached. November 2002 Interface Design for Oxidation-Resistant Ceramic Composites 2603