1012 Joumal of the American Ceramic Sociery-Kovar et al. Vol 8I. No 4 fracture resistance causes more energy to be dissipated through BN the creation of interfacial crack area. however if the interfacial fracture resistance is too high, crack kinking will reduce the delamination crack area. This observation suggests that, for a Sin 4 resistance that maximizes the energy-absorption capability, and his optimum value is determined by the transition from de lamination cracking to crack kinking References w.J. Clegg, K. Kendall, N. MeN. Alford, T, w. Button, and J. D. Birchall, 455-57(1990 and T. w. Clyne, "Fracture Behavior of Ce- mic Laminates in Bend perimental Data, Acta Metal. Mater, 41 [3]819-27(1993) A. Folsom, F. w. Zok, and F. F. Lange, "Flexural Properties of Britt Multilayer Materials: IL, Experiments, J. Amm. Ceram Soc., 77[82081-8 H. Liu and S H. Hsu, ""Fracture Behavior of Multilayer Silicon Nitride/ Boron Nitride Ceramics, J. A. Ceram. Soc., 79 19]2452-57(1996) E. Hilmas, G A. Brady, and J W. Halloran, "SiC and Si Si3 NI Monoliths: Non-brittle Fracture from Powder Processed Ceramics Coextrusion": Pp. 609-14 in Ceramic Transactions, Vol, 51, Ceran ing Science and Technology. Edited by H. Hausner, G. L. Messin 数 Hirano. American Ceramic Society, Westerville, OH, 19 Si, NA 'D. Kovar, B. H. King, R. W. Trice, and J. W. Halloran, "" Fibrous Monolithic Ceramics, J. Amm. Ceram. Soc., 80 [10J2471-87(199 ontrol of Crack Propa- gation in All-Brittle Syst Dissimilar Elastic Materials, Int. J. Solids Struct, 25[9]1053-67(1989 Fig. 15. Schematic depiction of the possible reasons for crack kink Eng.Maer,1l6-117,193-208(1996) dealized situation considered by He et al.is shown in Fi D. Kovar, G. A. Brady, M. D. Thouless, and J. W. Halloran, ""Interfacial where a crack is growing on the interface between Si3N4 and Fracture between Boron Nitride and Silicon Nitride and Its Applications to the bn before being drawn out of the interface by a flaw in the Si,N Failure Behavior of Fibrous Monolithic Ceramics he Si3N,/BN system, delamination cracking occur Scaling, and Ductile/Brittle Behavior. Edited by R L. Blumberg Selinger, J.J within the BN layer until the crack is drawn out of the interphase by Mecholsky, A E. Carlsson, and E.R. Fuller Jr. Materials Research Society, a flaw in the Si3N4(Fig. 15(b))or it is driven out of the interphase by 2D, Kovar and M. D. Thouless, "Simple Method for Measuring Frictional Sliding Resistance and Energy Dissipation in Layered Ceramics, "J.Am. Ce ram.Soc,803673-79(1997) ISW. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceram- V. Conclusions lP. G. Charalambides, J. Lund, A G. Evans, and R. M. McMee Silicon nitride(Si3N4)layered ceramics separated by weal est Specin Si aphases that contain a mixture of boron nitride(BN)andface 4pp.Mech,563]77-82(1989 MPa and work-of-fracture (WOF) values tha als", pp. 64-191 in Advances in Applied Mechanics, Vol, 29. Edited by J xcess o exceed 5000 J/m were achieved. The strength was insensitive Hutchinson and T Y. Wu. Academic Press, San Diego, CA, 1992. to the composition of the interphase, however, the woF de- Reinforced Brittle Matrix Composites,"JMater. Sct, 29, 3857-9 creased dramatically as the Si3N4 content in the interphase was increased. Observations of the crack path revealed that the Pushout behavior of a model Composite, " J. Am. Ceram. Soc., 77[12]3232-36(1994) energy-absorption capacity of these materials was related di ST J. Mackin, P D. Warren, and A G. Evans, "Effects of Fiber rectly to the length of delamination cracks. In materials that on Interface Sliding in Composites, "Acta MetalL, Mater.,40,1251-5 exhibited low energy absorption, the delamination crack lengths were limited by crack kinking The crack-kinking behavior that was observed as the SiN oS. Hashemi, A J. Kinloch, and G. williams, ""Mixed content in the interphase was increased was attributed to inter- 四 facial flaws that act to draw the delamination crack out of the ited by T K. O'Brien. American Society for Testing and Ma nterphase. A relationship was derived that related the interfa 20. Sbaizero, P. G Charalambides, and A. G. Ev ial fracture resistance and interfacial flaw size to the tende Laminated Ceramic-Matrix Composite, J, Am. Ceram Soc., 73 7) favored at high relative values of the interfacial fracture resis- tance. agreement between this model and the observed crack of Interfacial Grain-Bridging Sliding paths was good Friction in the Crack-Resistance and Strength Properties of Nontransforming These results indicate that promotion of crack deflection is not a sufficient condition to achieve high energy absorption in layered ceramics. Rather, high energy absorption requires that Crack out of an Interface: RA. G. Evans, and J. W.Hutchinson, Kinking of a of In-Plane Stress, J. Am. Ceram. Soc., 74 [4] delamination cracks propagate a substantial distance. Long de- 2J. H. Edgar, ""Crystal Structure, Mechanical Properties and Thermal lamination distances are favored when the interfacial fracture erties of BN Pp. 7-21 in Properties of Group III Nitrides. Edited by resistance is low, the flaw size in the layers is small, and the on,UK,1994. fracture resistance of the layers is high. At very low values of entation in S, N/BN Fibrous Monoliths "to be submitted to J. am the interfacial fracture resistance, increasing the interfacial Soc.V. Conclusions Silicon nitride (Si3N4) layered ceramics separated by weak interphases that contain a mixture of boron nitride (BN) and Si3N4 were manufactured and tested in flexure. Strengths in excess of 500 MPa and work-of-fracture (WOF) values that exceed 5000 J/m2 were achieved. The strength was insensitive to the composition of the interphase; however, the WOF de￾creased dramatically as the Si3N4 content in the interphase was increased. Observations of the crack path revealed that the energy-absorption capacity of these materials was related di￾rectly to the length of delamination cracks. In materials that exhibited low energy absorption, the delamination crack lengths were limited by crack kinking. The crack-kinking behavior that was observed as the Si3N4 content in the interphase was increased was attributed to inter￾facial flaws that act to draw the delamination crack out of the interphase. A relationship was derived that related the interfa￾cial fracture resistance and interfacial flaw size to the tendency for crack kinking to occur. Crack kinking was predicted to be favored at high relative values of the interfacial fracture resis￾tance. Agreement between this model and the observed crack paths was good. These results indicate that promotion of crack deflection is not a sufficient condition to achieve high energy absorption in layered ceramics. Rather, high energy absorption requires that delamination cracks propagate a substantial distance. Long de￾lamination distances are favored when the interfacial fracture resistance is low, the flaw size in the layers is small, and the fracture resistance of the layers is high. At very low values of the interfacial fracture resistance, increasing the interfacial fracture resistance causes more energy to be dissipated through the creation of interfacial crack area. However, if the interfacial fracture resistance is too high, crack kinking will reduce the delamination crack area. This observation suggests that, for a given material system, there is an optimum interfacial fracture resistance that maximizes the energy-absorption capability, and this optimum value is determined by the transition from de￾lamination cracking to crack kinking. References 1 W. J. Clegg, K. Kendall, N. McN. Alford, T. W. Button, and J. D. Birchall, ‘‘A Simple Way to Make Tough Ceramics,’’ Nature (London), 357 [Oct. 4] 455–57 (1990). 2 A. J. Phillipps, W. J. Clegg, and T. W. Clyne, ‘‘Fracture Behavior of Ce￾ramic Laminates in Bending—II. Comparison of Model Predictions with Ex￾perimental Data,’’ Acta Metall. Mater., 41 [3] 819–27 (1993). 3 C. A. Folsom, F. W. Zok, and F. F. Lange, ‘‘Flexural Properties of Brittle Multilayer Materials: II, Experiments,’’ J. Am. Ceram. Soc., 77 [8] 2081–87 (1994). 4 H. Liu and S. H. Hsu, ‘‘Fracture Behavior of Multilayer Silicon Nitride/ Boron Nitride Ceramics,’’ J. Am. Ceram. Soc., 79 [9] 2452–57 (1996). 5 G. A. Danko, D. Popovic, K. Stuffle, B. H. King, J. W. Halloran, J. W. Holmes, and D. F. Hasson, ‘‘Commercial Development of Fibrous Monolithic Ceramics,’’ Ceram. Eng. Sci. Proc., 16 [5] 673–80 (1995). 6 G. E. Hilmas, G. A. Brady, and J. W. Halloran, ‘‘SiC and Si3N4 Fibrous Monoliths: Non-brittle Fracture from Powder Processed Ceramics Produced by Coextrusion’’; pp. 609–14 in Ceramic Transactions, Vol. 51, Ceramic Process￾ing Science and Technology. Edited by H. Hausner, G. L. Messing, and S.-I. Hirano. American Ceramic Society, Westerville, OH, 1994. 7 D. Kovar, B. H. King, R. W. Trice, and J. W. Halloran, ‘‘Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 80 [10] 2471–87 (1997). 8 J. Cook and J. E. Gordon, ‘‘A Mechanism for the Control of Crack Propa￾gation in All-Brittle Systems,’’ Proc. R. Soc. London, 282, 508–20 (1964). 9 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). 10W. Lee and W. J. Clegg, ‘‘The Deflection of Cracks at Interfaces,’’ Key Eng. Mater., 116–117, 193–208 (1996). 11D. Kovar, G. A. Brady, M. D. Thouless, and J. W. Halloran, ‘‘Interfacial Fracture between Boron Nitride and Silicon Nitride and Its Applications to the Failure Behavior of Fibrous Monolithic Ceramics’’; pp. 243–48 in Materials Research Society Symposium Proceedings, Vol. 409, Instability Dynamics, Scaling, and Ductile/Brittle Behavior. Edited by R. L. Blumberg Selinger, J. J. Mecholsky, A. E. Carlsson, and E. R. Fuller Jr. Materials Research Society, Pittsburgh, PA, and Boston, MA, 1996. 12D. Kovar and M. D. Thouless, ‘‘Simple Method for Measuring Frictional Sliding Resistance and Energy Dissipation in Layered Ceramics,’’ J. Am. Ce￾ram. Soc., 80 [3] 673–79 (1997). 13W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceram￾ics, 2nd Ed.; p. 774. Wiley, New York, 1976. 14P. G. Charalambides, J. Lund, A. G. Evans, and R. M. McMeeking, ‘‘A Test Specimen for Determining the Fracture Resistance of Bimaterial Inter￾faces,’’ J. Appl. Mech., 56 [3] 77–82 (1989). 15J. W. Hutchinson and Z. Suo, ‘‘Mixed Mode Cracking in Layered Materi￾als’’; pp. 64–191 in Advances in Applied Mechanics, Vol. 29. Edited by J. W. Hutchinson and T. Y. Wu. Academic Press, San Diego, CA, 1992. 16A. G. Evans and F. W. Zok, ‘‘Review—The Physics and Mechanics of Fibre￾Reinforced Brittle Matrix Composites,’’ J. Mater. Sci., 29, 3857–96 (1994). 17T. A. Parthasarathy, D. R. Barlage, P. D. Jero, and R. J. Kerans, ‘‘Effect of Interfacial Roughness Parameters on the Fiber Pushout Behavior of a Model Composite,’’ J. Am. Ceram. Soc., 77 [12] 3232–36 (1994). 18T. J. Mackin, P. D. Warren, and A. G. Evans, ‘‘Effects of Fiber Roughness on Interface Sliding in Composites,’’ Acta Metall. Mater., 40, 1251–57 (1992). 19M. D. Thouless, H. C. Cao, and P. A. Mataga, ‘‘Delamination from Surface Cracks in Composites,’’ J. Mater. Sci., 24, 1406–12 (1989). 20S. Hashemi, A. J. Kinloch, and G. Williams, ‘‘Mixed-Mode Fracture in Fiber–Polymer Composite Laminates’’; pp. 143–68 in Composite Materials: Fatigue and Fracture, Vol. 3, ASTM Special Technical Publication 1110. Ed￾ited by T. K. O’Brien. American Society for Testing and Materials, Philadel￾phia, PA, 1991. 21O. Sbaizero, P. G. Charalambides, and A. G. Evans, ‘‘Delamination Crack￾ing in a Laminated Ceramic-Matrix Composite,’’ J. Am. Ceram. Soc., 73 [7] 1936–40 (1990). 22P. E. D. Morgan and D. B. Marshall, ‘‘Ceramic Composites of Monazite and Alumina,’’ J. Am. Ceram. Soc., 78 [6] 1553–63 (1995). 23S. J. Bennison and B. R. Lawn, ‘‘Role of Interfacial Grain-Bridging Sliding Friction in the Crack-Resistance and Strength Properties of Nontransforming Ceramics,’’ Acta. Metall., 37 [10] 2659–71 (1989). 24M.-Y. He, A. Bartlett, A. G. Evans, and J. W. Hutchinson, ‘‘Kinking of a Crack out of an Interface: Role of In-Plane Stress,’’ J. Am. Ceram. Soc., 74 [4] 767–71 (1991). 25J. H. Edgar, ‘‘Crystal Structure, Mechanical Properties and Thermal Prop￾erties of BN’’; pp. 7–21 in Properties of Group III Nitrides. Edited by J. H. Edgar. INSPEC, London, U.K., 1994. 26R. J. Moon, K. J. Bowman, D. Kovar, and J. W. Halloran, ‘‘Preferred Ori￾entation in Si3N4/BN Fibrous Monoliths,’’ to be submitted to J. Am. Ceram. Soc. h Fig. 15. Schematic depiction of the possible reasons for crack kink￾ing. The idealized situation considered by He et al.24 is shown in Fig. 15(a), where a crack is growing on the interface between Si3N4 and BN before being drawn out of the interface by a flaw in the Si3N4 layer. However, in the Si3N4/BN system, delamination cracking occurs within the BN layer until the crack is drawn out of the interphase by a flaw in the Si3N4 (Fig. 15(b)) or it is driven out of the interphase by a local region of high interfacial fracture resistance (Fig. 15(c)). 1012 Journal of the American Ceramic Society—Kovar et al. Vol. 81, No. 4