October 2001 Communications of the American Ceramic Sociery 453 testing geometry (i.e, the second SIC/Al,O, interface that the that the threshold energy release rate ratio for cont deflection crack encounters). A micrograph of the fractured surface of the s-0.6.This threshold criterion can be applical pper Sic bar is shown in Fig. 3, where there is a substantial interphase, where a crack is likely to meet an acial flaw amount of Al,O, adhering to the relatively smooth SiC surface. However, the interfacial flaws in our system may not be large This suggests significant microstructural inhomogeneity in either enough to be treated as interfacial cracks. The effect of porosity on the porous interphase or at the SiC/Al,O3 interface. The tortuous ontinued crack deflection also has been studied in a similar fracture path indicates that the deflected crack is not moving manner, by treating the porosity as flaws smoothly along the upper interface. This effect should increase the fracture resistance of a deflected crack, which is potentially consistent with the experimentally measured fracture resistance IV. Conclusions values. Unfortunately, the effects of such microstructure distribu- tions on the deflection criterion are not clearly understood. A Model laminates were fabricated using thin, porous-AL2O3 already described, the Al2O/Al2O3 system that was investigated interphases between SiC substrates. The interfacial fracture resis previously has a porous microstructure that is similar to the tance in these systems was lower than that observed in interphase investigated here. However, the type of fracture surface AL,O,porous-AL2O3 system processed under identical conditions in Fig 3 was not observed. Thus, flaw population effects leading Experiments also showed that crack deflection occurred in speci mens where the interfacial fracture resistance significantly ex to the higher than expected T, values are more likely to be related ceeded the deflection limit defined by the He-Hutchinson crite to the SiC/porous- A2 O, interface, rather than to the bulk porous. rion. Porosity and flaw distributions along the interphase may have ignificant effects on the deflection criterion and a more detailed somewhat different because of the higher hot-pressing temperature examination of those issues may have important implications for and the possible presence of Sio,(native oxide on SiC) Work by Mammoli et al. suggests that the presence of flaws the design of brittle-matrix composites has a considerable effect on the crack deflection criterion. Based on their analysis, flaws increase the allowable energy release ratio, Acknowledgmen such that higher interfacial resistances allow crack deflection. This ay partly explain how interfacial fracture resistances above the We are grateful to Dr, Jit Goela of Rohm and Haas Advanced Materials for He-Hutchinson limit may lead to crack deflection. However, the providing the SiC substrate material, and to Professors Janet Rankin and David Green for their comments on the manuscript. effects of flaws are significant only when the crack tip gets close to the flaw, and energy release rate ratios well in excess of the He-Hutchinson threshold can lead to crack deflection only when References the crack meets a flaw at the interface. Residual stress effects ior of Ceramic Matrix such crack-flaw interactions and their implications with regard to Composites Metall. mater.,37012567-83(1989) the competition between crack deflection and penetration are Dissimilar Elastic Materials, "Int J. Solids Struct, 25 19)1053-67(1989) problems that have not been addressed For the specific case of the 3M. J. O'Brien, F M. Capaldi, crack meeting a flaw at the interface, the flaw can be considered as an extension of the crack onto the interface With a homogeneous K.S. Blanks, A Kristoffersson, E Carlstrom, and w.J. Clegg, "Crack Deflectio interface, where a defect lies along the interface, it has been shown in Ceramic Laminates Using Porous Interlayers,J. Ear. Ceram. Soc., I8 L.A. Simpson, "Effect of Microstructure on Measurements of Fracture Energy of Al2O3,J Am Ceram Soc., 56 117-11(19 gh-T. O'Brien, "Fabrication of a Tailored Oxidation-Resistant Interface and W. Hutchinson, "Crack Deflection at an Interface between Dissimilar Elastic Materials: Role of Residual Stresses, Int J Solids Struct. 31口243443-55(199 SM. J. O'Brien and B. W. Sheldon, "Porous Alumina Co Fracture Resistance for Alumina Composites, ".Am. Ceram. Soc., 82[12]3567- PP. G Charalambides, J. Lund, A. G. Evans, and R. M. McMeeking,"A Test P G. Charalambides, H. C Cao, J. Lund, and A G. Evans, " Development of a Test Method for Measuring the Mixed Mode Fracture Resistance,J. App/ Mech, 8 269-83(1989 IM. A. Pickering, R. L. Taylor, J. T. Keeley, and G.A. Graves,"Chemically Vapor Deposited Silicon Carbide (SiC)for Optical Applications, Nucl. Instrum. Methods,A291,95-110(1990) 协 er.Si.Eg,A07,135-43(1989) Y F. Liu, Y. Tanaka, and C. Masuda, "Debonding Mechanisms in the Presence 078“15001Nmc0招 s," dcta metall. Mater,465]5237-47(1998 mmoli, A. L. Graham, I. E. Reimanis, and D. L. Tullock, "The Effect of Flaws on the Propagation of Cracks at Bimaterial Interfaces, Acta Metall. Mater Fig 3. Micrograph of the fracture surface on the upper SiC interface M. Y. He and J. w. Hutchinson."Kinking of a Crack Out of an Interface contact with the porous interphase J. AppL. Mech.,56,270-78(1989)testing geometry (i.e., the second SiC/Al2O3 interface that the crack encounters). A micrograph of the fractured surface of the upper SiC bar is shown in Fig. 3, where there is a substantial amount of Al2O3 adhering to the relatively smooth SiC surface. This suggests significant microstructural inhomogeneity in either the porous interphase or at the SiC/Al2O3 interface. The tortuous fracture path indicates that the deflected crack is not moving smoothly along the upper interface. This effect should increase the fracture resistance of a deflected crack, which is potentially consistent with the experimentally measured fracture resistance values. Unfortunately, the effects of such microstructure distribu￾tions on the deflection criterion are not clearly understood. As already described, the Al2O3/Al2O3 system that was investigated previously has a porous microstructure that is similar to the interphase investigated here. However, the type of fracture surface in Fig. 3 was not observed.8 Thus, flaw population effects leading to the higher than expected
i values are more likely to be related to the SiC/porous-Al2O3 interface, rather than to the bulk porous￾Al2O3 structure. It is also possible that the bulk porous structure is somewhat different because of the higher hot-pressing temperature and the possible presence of SiO2 (native oxide on SiC). Work by Mammoli et al.14 suggests that the presence of flaws has a considerable effect on the crack deflection criterion. Based on their analysis, flaws increase the allowable energy release ratio, such that higher interfacial resistances allow crack deflection. This may partly explain how interfacial fracture resistances above the He–Hutchinson limit may lead to crack deflection. However, the effects of flaws are significant only when the crack tip gets close to the flaw, and energy release rate ratios well in excess of the He–Hutchinson threshold can lead to crack deflection only when the crack meets a flaw at the interface. Residual stress effects on such crack–flaw interactions and their implications with regard to the competition between crack deflection and penetration are problems that have not been addressed. For the specific case of the crack meeting a flaw at the interface, the flaw can be considered as an extension of the crack onto the interface. With a homogeneous interface, where a defect lies along the interface, it has been shown that the threshold energy release rate ratio for continued deflection is 0.6.15 This threshold criterion can be applicable with a porous interphase, where a crack is likely to meet an interfacial flaw. However, the interfacial flaws in our system may not be large enough to be treated as interfacial cracks. The effect of porosity on continued crack deflection also has been studied in a similar manner, by treating the porosity as flaws.4 IV. Conclusions Model laminates were fabricated using thin, porous-Al2O3 interphases between SiC substrates. The interfacial fracture resis￾tance in these systems was lower than that observed in an Al2O3/porous-Al2O3 system processed under identical conditions. Experiments also showed that crack deflection occurred in speci￾mens where the interfacial fracture resistance significantly ex￾ceeded the deflection limit defined by the He–Hutchinson crite￾rion. Porosity and flaw distributions along the interphase may have significant effects on the deflection criterion, and a more detailed examination of those issues may have important implications for the design of brittle-matrix composites. Acknowledgment We are grateful to Dr. Jit Goela of Rohm and Haas Advanced Materials for providing the SiC substrate material, and to Professors Janet Rankin and David Green for their comments on the manuscript. References 1 A. G. Evans and D. B. Marshall, “The Mechanical Behavior of Ceramic Matrix Composites,” Acta Metall. Mater., 37 [10] 2567–83 (1989). 2 M. Y. He and J. W. Hutchinson, “Crack Deflection at an Interface between Dissimilar Elastic Materials,” Int. J. Solids Struct., 25 [9] 1053–67 (1989). 3 M. J. O’Brien, F. M. Capaldi, and B. W. Sheldon, “A Layered Alumina Composite Tested at High Temperature in Air,” J. Am. Ceram. Soc., 83 [12] 3033–40 (2000). 4 K. S. Blanks, A. Kristoffersson, E. Carlstrom, and W. J. Cleggs, “Crack Deflection in Ceramic Laminates Using Porous Interlayers,” J. Eur. Ceram. Soc., 18 [13] 1945–51 (1998). 5 L. A. Simpson, “Effect of Microstructure on Measurements of Fracture Energy of Al2O3,” J. Am. Ceram. Soc., 56 [1] 7–11 (1973). 6 M. J. O’Brien, “Fabrication of a Tailored Oxidation-Resistant Interface and High-Temperature Testing of a Laminated Composite”; Ph..D. Thesis, Brown University, Providence, RI, 1998. 7 M. Y. He, A. G. Evans, and J. W. Hutchinson, “Crack Deflection at an Interface between Dissimilar Elastic Materials: Role of Residual Stresses,” Int. J. Solids Struct., 31 [24] 3443–55 (1994). 8 M. J. O’Brien and B. W. Sheldon, “Porous Alumina Coating with Tailored Fracture Resistance for Alumina Composites,” J. Am. Ceram. Soc., 82 [12] 3567–74 (1999). 9 P. G. Charalambides, J. Lund, A. G. Evans, and R. M. McMeeking, “A Test Specimen for Determining the Fracture Resistance of Bimaterial Interfaces,” J. Appl. Mech., 56, 77–82 (1989). 10P. G. Charalambides, H. C. Cao, J. Lund, and A. G. Evans, “Development of a Test Method for Measuring the Mixed Mode Fracture Resistance,” J. Appl. Mech., 8, 269–83 (1989). 11M. A. Pickering, R. L. Taylor, J. T. Keeley, and G. A. Graves, “Chemically Vapor Deposited Silicon Carbide (SiC) for Optical Applications,” Nucl. Instrum. Methods, A291, 95–110 (1990). 12Z. Suo and J. W. Hutchinson, “Sandwich Test Specimens for Measuring Interface Toughness,” Mater. Sci. Eng., A107, 135–43 (1989). 13Y. F. Liu, Y. Tanaka, and C. Masuda, “Debonding Mechanisms in the Presence of an Interphase in Composites,” Acta Metall. Mater., 46 [15] 5237–47 (1998). 14A. A. Mammoli, A. L. Graham, I. E. Reimanis, and D. L. Tullock, “The Effect of Flaws on the Propagation of Cracks at Bimaterial Interfaces,” Acta Metall. Mater., 43 [3] 1149–56 (1995). 15M. Y. He and J. W. Hutchinson, “Kinking of a Crack Out of an Interface,” J. Appl. Mech., 56, 270–78 (1989). Fig. 3. Micrograph of the fracture surface on the upper SiC interface that was in contact with the porous interphase. October 2001 Communications of the American Ceramic Society 2453