Journal of the American Ceramic Socien-Kerans et aL. Vol. 85. No. 1I fracture energy to deflect cracks itself. This discussion also failure criterion leaves the matter open to speculation. This suggests that coatings that are intended to deflect cracks by failure sequence has been observed in laminates and in model fracture within the coating can be evaluated independently of the composites Nicalon/C/SiC composites made with fibers treated to promote rack direction from perpendicular to parallel to a fiber surface dence of interfacial failure preceding matrix crack impinge- Fig. 4(a), but there are other possibilities. Mode I interface cracks can form in the tensile stress field normal to the fiber surface ahead connect to debonded coating/fiber interfaces, with no deflection in f a matrix crack z(Fig 4(b). Modeling of an annular matrix the coating itself, i.e., the interface, not the coating, fails. In crack has predicted that, for most reasonable choices of properties, otherwise identical composites made with treated fibers, matrix cracks connect to diffuse cracks in the coating without debonding other failure event(e. g, fiber fracture)intervenes before the coating/fiber interface; ie, the coating itself fails.66, 68The the matrix crack can be driven to the interface. unless the inter interfacial toughness, friction, and composite strength are higher face is debonded ahead of it. 63 Interface stresses can be high for the treated-fiber CMC If the matrix crack runs through the enough to make interfacial debond ahead of the matrix crack a plausible mechanism, but lack of a completely understood local coatings on untreated fibers before the interface debonds, it deflects in the coating, as it does in the identical coating on the treated fiber; hence, the coating/fiber interface must fail before the matrix crack enters the coating. When the crack does pass through the coating, the elastic constraint of the fiber is mostly removed by the preexisting debond, and the crack runs directly to the debonded interface. If there are truly no material difference besides interface strength, a definitive sequence of events consistent with the model is implied. These composites have other interesting properties that are discussed in later sections Another deflection mechanism preceding matrix crack impinge ment has been suggested for a composite comprising SiC mono- filaments with successive coatings of carbon and TiB, in a glass matrix.9 The coating was calculated to be in triaxial compressio d model ested that matrix cracks would run to th coating only after coating or interfacial failure. Experiment re- LATRIX vealed that debond cracks ran very near the fiber surface except for ular-section C rings around the fiber with their peaks at the matrix crack planes. Shear stresses on planes in the approximate orientation of the sides of the C rings(about 45. from the matrix crack plane) were calculated to be the highest coating stresses that could lead to fracture. The suggested failure sequence was(i)the matrix crack approached the coating, (ii) the coating failed in shear on planes +45 from the crack plane, ahead of the crack, and formed C rings, (iii) the coating cracks turned parallel to the fiber urface at or near the fiber surface, and(iv) the matrix crack advanced until it joined the coating shear cracks at their intersec- tion(see Fig. 4(c)) A further possibility is growth of periodic echelon cracks In an analysis of thin laminates with a Mode I crack normal to the nterface. the maximum tensile stresses in the coatings were 45o allel to a "half-turn of the nto eries of parallel"periodic echelon"microcracks on these hi stress planes at the center of the coating. As the microcrack approached the coating/plate interfaces, they turned parallel to the interface and joined to form a debond. Evidence for a similar- appearing but different sequence of events has been observed in monazite interlayers in Al, O /Al,O3 laminates(Fig. 5).27In that case, the echelon cracks appeared after initial deflection of the main crack into the coating/ laminate interface. [二A Detailed fracture observations are difficult: therefore. the se- quence of events for fiber/matrix debonding in CMCs speculative Debonding mechanisms may vary with the particular composite and global stress state. The coating most ortant to crack deflection depends on subtle differences in onstituent elastic properties and residual stresses. In the ideal case, coatings are engineered material "components'" of a compo ite system selected for phase, microstructure, and geometry to promote a specific failure mechanism. Enhanced understanding of crack deflection is necessary to allow such a priori design Simple Fig. 4. Three possible sequences leading to crack deflection:(a)matrix crack grows into the coating and then bifurcates and turn models are often useful, but they can sometimes be misleading running parallel to the fiber surface in each direction(as well hence, detailed analysis and comparison of microstructure, crack on in the matrix),(b) coating or interface fails in the tensile deflection behavior, and analytical models may be necessary. The he matrix crack before arrival of the matrix crack at the interface region same considerations apply to the interpretation of micromechani- and (c)in the matrix crack at the coating, the crack bifurcates and turns as cal tests. For example, fiber pushout/pullout tests may not direct the coating fails in shear at an intermediate angle, then turns parallel to the measure the parameters that actually determine debonding during fiber surface at or near the fiber surface composite failure. It is even possible that debonding in singlefracture energy to deflect cracks itself. This discussion also suggests that coatings that are intended to deflect cracks by fracture within the coating can be evaluated independently of the fiber. Crack deflection is usually assumed to be a local change in crack direction from perpendicular to parallel to a fiber surface (Fig. 4(a)), but there are other possibilities. Mode I interface cracks can form in the tensile stress field normal to the fiber surface ahead of a matrix crack62 (Fig. 4(b)). Modeling of an annular matrix crack has predicted that, for most reasonable choices of properties, some other failure event (e.g., fiber fracture) intervenes before the matrix crack can be driven to the interface, unless the inter￾face is debonded ahead of it.63 Interface stresses can be high enough to make interfacial debond ahead of the matrix crack a plausible mechanism, but lack of a completely understood local failure criterion leaves the matter open to speculation. This failure sequence has been observed in laminates64 and in model composites.65 Nicalon/C/SiC composites made with fibers treated to promote higher coating/fiber interface strengths also provide indirect evi￾dence of interfacial failure preceding matrix crack impinge￾ment.66,67 In composites made with untreated fibers, matrix cracks connect to debonded coating/fiber interfaces, with no deflection in the coating itself; i.e., the interface, not the coating, fails. In otherwise identical composites made with treated fibers, matrix cracks connect to diffuse cracks in the coating without debonding the coating/fiber interface; i.e., the coating itself fails.66,68 The interfacial toughness, friction, and composite strength are higher for the treated-fiber CMC. If the matrix crack runs through the coatings on untreated fibers before the interface debonds, it deflects in the coating, as it does in the identical coating on the treated fiber; hence, the coating/fiber interface must fail before the matrix crack enters the coating. When the crack does pass through the coating, the elastic constraint of the fiber is mostly removed by the preexisting debond, and the crack runs directly to the debonded interface. If there are truly no material differences besides interface strength, a definitive sequence of events consistent with the model is implied.63 These composites have other interesting properties that are discussed in later sections. Another deflection mechanism preceding matrix crack impinge￾ment has been suggested for a composite comprising SiC mono￾filaments with successive coatings of carbon and TiB2 in a glass matrix.69 The coating was calculated to be in triaxial compression, and modeling suggested that matrix cracks would run to the coating only after coating or interfacial failure. Experiment re￾vealed that debond cracks ran very near the fiber surface except for triangular-section C rings around the fiber with their peaks at the matrix crack planes. Shear stresses on planes in the approximate orientation of the sides of the C rings (about 45° from the matrix crack plane) were calculated to be the highest coating stresses that could lead to fracture. The suggested failure sequence was (i) the matrix crack approached the coating, (ii) the coating failed in shear on planes 45° from the crack plane, ahead of the crack, and formed C rings, (iii) the coating cracks turned parallel to the fiber surface at or near the fiber surface, and (iv) the matrix crack advanced until it joined the coating shear cracks at their intersec￾tion (see Fig. 4(c)). A further possibility is growth of periodic echelon cracks. In an analysis of thin laminates with a Mode I crack normal to the interface, the maximum tensile stresses in the coatings were 45° to the interface plane, that is, parallel to a “half-turn” of the impinging crack into the interface plane.70 Failure initiated as a series of parallel “periodic echelon” microcracks on these high￾stress planes at the center of the coating. As the microcracks approached the coating/plate interfaces, they turned parallel to the interface and joined to form a debond. Evidence for a similar￾appearing but different sequence of events has been observed in monazite interlayers in Al2O3/Al2O3 laminates (Fig. 5).27 In that case, the echelon cracks appeared after initial deflection of the main crack into the coating/laminate interface. Detailed fracture observations are difficult; therefore, the se￾quence of events for fiber/matrix debonding in CMCs remains speculative. Debonding mechanisms may vary with the particular composite and global stress state. The coating property most important to crack deflection depends on subtle differences in constituent elastic properties and residual stresses. In the ideal case, coatings are engineered material “components” of a compos￾ite system selected for phase, microstructure, and geometry to promote a specific failure mechanism. Enhanced understanding of crack deflection is necessary to allow such a priori design. Simple models are often useful, but they can sometimes be misleading; hence, detailed analysis and comparison of microstructure, crack deflection behavior, and analytical models may be necessary. The same considerations apply to the interpretation of micromechani￾cal tests. For example, fiber pushout/pullout tests may not directly measure the parameters that actually determine debonding during composite failure.71 It is even possible that debonding in single￾Fig. 4. Three possible sequences leading to crack deflection: (a) matrix crack grows into the coating and then bifurcates and turns with fronts running parallel to the fiber surface in each direction (as well as continuing on in the matrix); (b) coating or interface fails in the tensile field ahead of the matrix crack before arrival of the matrix crack at the interface region; and (c) in the matrix crack at the coating, the crack bifurcates and turns as the coating fails in shear at an intermediate angle, then turns parallel to the fiber surface at or near the fiber surface. 2602 Journal of the American Ceramic Society—Kerans et al. Vol. 85, No. 11