Journal of the American Ceramic Society'Kerans et al. Vol 81. No. 7 matrix coan三 Fig. 5. 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 failures and deflections, until the fiber/matrix interface is reached fiber interface, and(ii) any oxidation protection provided by in drawing conclusions regarding initial crack deflection, I the coating may be limited by the tensile strain-to-failure of the on inspection of regions of the composite away from clusions based on the arguments of the preceding section be tive of the fibers in the neighborhoodof ss that can be coating. The first of these consequences requires that the con tiating matrix crack. Furthermore, coating matrix cracks should based on sound evidence regarding the location of the initial not be expected to provide effective protection at high stresses, crack deflection, because all debonding cracks will have a ten unless they are axially very strain tolerant(i.e, the coatings dency to eventually traverse into or near the interface. This have strain-to-failure values that are comparable to that of the enario is also consistent with the analysis that is based on two fiber) successive modes of cracking behavior used in the preceding Consideration of the implications of debond-crack roughness section, wherein the initial short-period crack regime would has led to valuable insights regarding the possible roles of fiber correspond to initial crack deflection in the coating. ar inter- in turn, has led to the identification of several parameters that facial crack should be considered in the design and evaluation of alternative Although the possibility of crack deflection and tough be- fiber coatings. If they are not considered, it is entirely possible havior, despite higher-strength interfaces, is a positive result, that viable coating schemes will be discarded for invalid rea- to vary the course of damage evolution in the overall compos- appreciation of the complexity of the problem, they should not te. Such things as notch sensitivity and fatigue behavior can be considered to be prohibitive. There are simple approaches to managing eac tems will require re-evaluation of the failure behavior of the re yet to be ide composite Acknowledgments: Author RJK would like to express his appreciation collaboration that included this Rough-crack formalism seems to be useful in providing an work. Mr Kelly Brown is also acknowledged for his assistance with manuscript interpretation of the push-out behavior of high-strength, preparation treated-fiber NicalonTM/C/SiC composites. It also offers a rea- sonable resolution of the wide variety of interface properties of References such composites inferred from different test techniques and H. C. Cao E. of Interfaces on the Properties of Fiber-Reinforced Soc,7361691-99(1990 amon,""Fracture Toughness of 2D Woven SiC/SiC tests indicate that the allowable toughness of coatings for de Composites with Multilayered Interphases, 'J. Am. Ceram. Soc., 79 [4]849-58 flecting cracks may be higher than previously believed 3C. Droillard, J. Lamon, and X. Bourrat, The interfacial fracture behavior of the two types of com- dition for Efficient multilay ng Interfaces in CMCs Con- later. Res. Soc. Symp. Proc. posites that have been considered provides strong evidence that 365,371-76(1995) the debonding process in the weaker-interface material initiates R. Naslain, Fiber-Matrix Interphases and Interfaces in Cerami s a mode I crack ahead of the matrix crack Composites Processed by CVI, "" Compos. Interfaces, I 3]253-86 However, consideration of the failure pro aces and Interfacial Mechanics: Influence on the deflection in a coating indicates that car cal Behavior of Ceramic Matrix Composites, J. Phys. 11,311]fiber interface, and (ii) any oxidation protection provided by the coating may be limited by the tensile strain-to-failure of the coating. The first of these consequences requires that the con￾clusions based on the arguments of the preceding section be based on sound evidence regarding the location of the initial crack deflection, because all debonding cracks will have a ten￾dency to eventually traverse into or near the interface. This scenario is also consistent with the analysis that is based on two successive modes of cracking behavior used in the preceding section, wherein the initial short-period crack regime would correspond to initial crack deflection in the coating and the long-period crack would correspond to the coating/fiber inter￾facial crack. Although the possibility of crack deflection and tough be￾havior, despite higher-strength interfaces, is a positive result, significant variation in the interface properties can be expected to vary the course of damage evolution in the overall compos￾ite. Such things as notch sensitivity and fatigue behavior can change significantly;28,29 hence, changes in the coating sys￾tems will require re-evaluation of the failure behavior of the composite. IV. Summary Rough-crack formalism seems to be useful in providing an interpretation of the push-out behavior of high-strength, treated-fiber Nicalon™/C/SiC composites. It also offers a rea￾sonable resolution of the wide variety of interface properties of such composites inferred from different test techniques and resolves macroscopic measurements of pyrocarbon properties with composite interface measurements. Finally, the analyzed tests indicate that the allowable toughness of coatings for de￾flecting cracks may be higher than previously believed. The interfacial fracture behavior of the two types of com￾posites that have been considered provides strong evidence that the debonding process in the weaker-interface material initiates as a mode I crack ahead of the approaching matrix crack. However, consideration of the failure process after initial crack deflection in a coating indicates that care should be exercised in drawing conclusions regarding initial crack deflection, based on inspection of regions of the composite away from the ini￾tiating matrix crack. Furthermore, coatings that can be protec￾tive of the fibers in the neighborhood of matrix cracks should not be expected to provide effective protection at high stresses, unless they are axially very strain tolerant (i.e., the coatings have strain-to-failure values that are comparable to that of the fiber). Consideration of the implications of debond-crack roughness has led to valuable insights regarding the possible roles of fiber coatings in influencing composite behavior. This observation, in turn, has led to the identification of several parameters that should be considered in the design and evaluation of alternative fiber coatings. If they are not considered, it is entirely possible that viable coating schemes will be discarded for invalid rea￾sons. Although these additional design parameters add to our appreciation of the complexity of the problem, they should not be considered to be prohibitive. There are simple approaches to managing each of these parameters and, no doubt, more-subtle ones that are yet to be identified. Acknowledgments: Author RJK would like to express his appreciation to Prof. Roger Naslain (Director, Laboratoire des Composites Thermostruc￾turaux), the U.S. Air Force Office of Scientific Research, and Wright Labora￾tory Materials Directorate for facilitating the collaboration that included this work. Mr. Kelly Brown is also acknowledged for his assistance with manuscript preparation. References 1 H. C. Cao, E. Bischoff, O. Sbaizero, M. Ru¨hle, A. G. Evans, D. B. Marshall, and J. J. Brennan, ‘‘Effect of Interfaces on the Properties of Fiber-Reinforced Ceramics,’’ J. Am. Ceram. Soc., 73 [6] 1691–99 (1990). 2 C. Droillard and J. Lamon, ‘‘Fracture Toughness of 2D Woven SiC/SiC Composites with Multilayered Interphases,’’ J. Am. Ceram. Soc., 79 [4] 849–58 (1996). 3 C. Droillard, J. Lamon, and X. Bourrat, ‘‘Strong Interfaces in CMCs, Con￾dition for Efficient Multilayered Interphases,’’ Mater. Res. Soc. Symp. Proc., 365, 371–76 (1995). 4 R. Naslain, ‘‘Fiber–Matrix Interphases and Interfaces in Ceramic Matrix Composites Processed by CVI,’’ Compos. Interfaces, 1 [3] 253–86 (1993). 5 J. Lamon, ‘‘Interfaces and Interfacial Mechanics: Influence on the Mechani￾cal Behavior of Ceramic Matrix Composites,’’ J. Phys. IV, 3 [11] 1607–16 (1993). Fig. 5. 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). 1886 Journal of the American Ceramic Society—Kerans et al. Vol. 81, No. 7