April 2001 Toughened Oxide Composites Based on Porous Alumina-Platelet Interphases overall strength, however, the curve was characteristic of graceful References failure and significant crack diversion along the interphase. This expectation was consistent with the sEM micrographs that are IA shown in Fig 4: the platelet interphase with 5 vol% of powder Soc,73[2】187 2D. B. Marshall, B. N. Cox, and A G. Evans, " The Mechanics of Matrix Cracking exhibited brittle failure behavior(Fig. 4(a), whereas the platelet o 5g. Wogavidge, "Fiber-Reinforced Ceramics, "Composites(Guildford, UK), 18(2) terphase with additions of 3 vol% of 3Al203 2SiO2 produced noticeably more crack deflection along the interphase(Fig. 4(b). MY. He and J nson,“ Crack Det at an Interface Between Again, Fig 9 demonstrates more-extensive crack deflection along Dissimilar elas t.J. Solids struct,25{91053-67(1989) the platelet interphase that contains only 3 vol% of 3Al 2O3 2SiO .A. G. Evans, J. w. Hutchinson, "Interface Debonding and Fiber racking in brittle mposites,J.Am. Ceram Soc., 72[121 2300-303 Following the concept of a bimodal microstructure, it is W. Hutchinson and H. M. Jensen, "Models of Fiber Debonding and Pullout in postulated that regions with one layer of high matrix: interphase Brittle Composites with Friction," Mech. Mater, 9, 139-63(1990) ratio(high strength)should be alternated with regions of low R.J. Kerans, R. S Hay, N.J. Pagano, and T. A. Parthasarathy, "The Role of the matrix:interphase ratio(high toughness), which consist of several 429-42(1989). iber-Matrix Interface in Ceramic Composites, Am. Ceram. Soc. Bull, 68 [21 such thin layers---not just a couple of layers, as was fabricated A. G. Evans, "The Mechanical Performance of Fiber-Reinforced Ceramic Matrix ” Mater.Sci.Eng,A,A107,227-39(1989 has been hypothesized to have high kinetic energy. The role of the F be rein forced Band e m i (ck, isetesew of the. hy sizes. a8s gc h. of several layers with a low matrix: interphase ratio is to slow the low.J. Clegg, K. Kendall, N M. Alford, D. Birchall, and T W.Button, "A Simple crack, by causing it to deflect along a tortuous Way to Make Tough Ceramics, Natire (london), 347, 455-57(1990) where some of its energy is dissipated, hence imparting toughnes on and Failure of Laminar Ceramic Composites to the composite. Further work investigating such a mechanism of AM1,4013085-93(92 Y. Shigegaki, M. E. Brito, K. Hirao, M. Toriyama, and S. Kanzak intimate mixing of strength and toughness, on a microstructural of a Novel Multilayered Silicon Nitride, J. Am. Ceram. Soc., 79 [8] 2197-200 preliminary data-such as, for example, that illustrated in Figs. 8 Pat. n: 477 524, sept 20. 1988. tnc ceramic and menod for oucton, and 9(for alternating sequence of just one layer with a 12: 1 ratio, followed by layers with a 5: 1 ratio(Table IShow the Ceramics: 1, Fabrication, Microstructure, and Indentation Behavior, "J.Am. Ceram potential for such a system to exhibit sustained toughness and flaw Soc. 7619)2209-16(1993) Baskaran, S. D. Nunn, D. Popovic, and J. W. Halloran, "Fibrous Monolithic tolerance. as well as a significant If this mechanism is Ceramic cs: Il, Flexural Strength and Fracture Behavior of the Silicon Carbide/Graphite oupled with an intrinsically strong matrix (e.g 3Y-1ZP), the Systsm Bas aman. Cer m oH a6 9)22ib-234(Mo3) Additional work to investigate these speculations and hypothes and Oxidation Behavior of the Silicon Carbide/Boron Nitride System, "J. Am. Ceram. Soc,77[51249-55(1994 is currently underway. In addition, the high-temperature stability D Kovar, B H. King, R. W. Trice, and J. w. Halloran,"Fibrous Monolithic of the pores and the high-temperature mechanical properties will Ceramics, J. Am Cera Soc., 80[10] 2471-87(1997) be investigated and described in a future publication. P. E. D. Morgan and D. B. Marshall."Functional Interfaces for Oxide/Oxide Composites," Mater Sci Eng, A, A162, 15-25(Is P. E. D. Morgan, D. B. Marshall, and R, M. Housley, "High Temp Stability of Monazite-Alumina Composites, " Mater. Sci. Eng A, A195, 215-22 V. Conclusions P: E.D. Morgan and t. B, hssa erams composites of Monazite and A novel mechanism of interphase debonding in an all-oxide Soc. 78(9)2574(199 Kriven, "A Strong and Damage-Tolerant Oxide Laminate, composite system has been introduced. This concept is based on J. Am. Ceram Soc, 80(9)2421-24(1997). the engineering of a suitably weak interphase, through the use of D. H Kuo and W. M. Kriven, "Fracture of Multilayer Oxide Composites, "Mater. relatively unsinterable alumina platelets 10-15 um(or 5-10 um) Sci Eng- A, A241, 241-50(1998). in diameter and I um thick. The room-temperature strength of the interphase can be adjusted using minor additions of matrix powders(on the order of 1-3 vol%). Laminated composites of both t, C. B. Cart PA,1997 mullite and alumina have been fabricated. Modified fibrous 2w. M. Kriven and D H. Kuo, "High-Strength, Flaw-Tolerant, Oxide Cerami monoliths of alumina that consist of a triple layer "core/interphase/ Composite,'VS. Pat No. 5948516, Sept, 7, I of the processing and microstructural-design parameters has not 305-16(1998) Cristobalite Transformation Weakened Interphases, " Ceram. Eng. Sci. Proc., 19 [3 fully conducted, preliminary mechanical data and scanning elec- w. M. Kriven and S J. Lee, " Toughening of Mullite/Cordierite Laminates by on microscopic observation of the crack profiles nevertheless ansformation Weakening of B-Cristobalite Interphases, to be submitted to.Am. ram.Soc. demonstrate this procedure to be a viable high-temperature, 27W. M. Kriven, C M. Huang, D. Zhu, and Y. Xu, "Toughening of Titania by oxidation-resistant, toughening mechanism in chemically compat Transformation Weakening of Enstatite(MgSiO)Interphases, submitted to Acta ble oxide composites. In terms of chemistry, the platelets provid and effective mechanism for debonding in air, indepen- Fiber Coating Concepts for Brittle-Matrix Composites, J. Am. Ceram Soc., 76 [51 dent of temperature, up to the melting point of the alumina or the 12492. May, K Keller, T. A Parthasarathy, and J. Guth, "Fugitive Interface Coating matrix A mechanism that intimately mixes strength and toughness, on 922-30(1993) in Oxide-Oxide Composites: A Viability Study, " Ceram. Eng. Sci. Proc, 14 [9-101 a microstructural scale, through an optimally tailored bimodal C G. Levi, J. Y. Yang, B. J Dalgleish, F. W. Zok, and A G. Evans, "Processing microstructure has been introduced. It has been postulated that and Performance of an Al-Oxide Ceramic Composite,J.Am. Ceram Soc., 81 181 strength)could be alternated with regions of low matrix interphase of Composite Powders, "J. Mater Res, 2[1]59-65(1987). atio(high toughness), and these regions may consist of several uch thin layers. After the crack passes through a thick matrix Bsmm,时 yer, it is believed to have high kinetic energy. The role of the several layers with a low matrix interphase ratio is to slow the 34R.K. and G. W. Scherer, On Constrained Sintering-IIl: Rigid crack, by causing it to deflect along a tortuous interphase path, Inclusions, "Acta Metall. Mater, 36[912411-16(1988). 350. Sudre and F. F. Lange."Effect of Inclusions on Densification: 1. Microstruc- where some of its energy is dissipated and imparts toughness to the tural Development in an Al20, Matrix Containing a High Volume Fraction of ZrO2 composite Am. Ceram.Soe.,753l519-24(1992)overall strength; however, the curve was characteristic of graceful failure and significant crack diversion along the interphase. This expectation was consistent with the SEM micrographs that are shown in Fig. 4: the platelet interphase with 5 vol% of powder exhibited brittle failure behavior (Fig. 4(a)), whereas the platelet interphase with additions of 3 vol% of 3Al2O3z2SiO2 produced noticeably more crack deflection along the interphase (Fig. 4(b)). Again, Fig. 9 demonstrates more-extensive crack deflection along the platelet interphase that contains only 3 vol% of 3Al2O3z2SiO2 powder. Following the concept of a bimodal microstructure, it is postulated that regions with one layer of high matrix:interphase ratio (high strength) should be alternated with regions of low matrix:interphase ratio (high toughness), which consist of several such thin layers—not just a couple of layers, as was fabricated here. The crack, after it has passed through a thick matrix layer, has been hypothesized to have high kinetic energy. The role of the several layers with a low matrix:interphase ratio is to slow the crack, by causing it to deflect along a tortuous interphase path, where some of its energy is dissipated, hence imparting toughness to the composite. Further work investigating such a mechanism of intimate mixing of strength and toughness, on a microstructural scale, through a tailored microstructure, is underway. However, preliminary data—such as, for example, that illustrated in Figs. 8 and 9 (for an alternating sequence of just one layer with a 12:1 ratio, followed by layers with a 5:1 ratio (Table III))—show the potential for such a system to exhibit sustained toughness and flaw tolerance, as well as a significant WOF. If this mechanism is coupled with an intrinsically strong matrix (e.g., 3Y-TZP), the absolute strength of the composite can be enhanced further. Additional work to investigate these speculations and hypotheses is currently underway. In addition, the high-temperature stability of the pores and the high-temperature mechanical properties will be investigated and described in a future publication. IV. Conclusions A novel mechanism of interphase debonding in an all-oxide composite system has been introduced. This concept is based on the engineering of a suitably weak interphase, through the use of relatively unsinterable alumina platelets 10–15 mm (or 5–10 mm) in diameter and 1 mm thick. The room-temperature strength of the interphase can be adjusted using minor additions of matrix powders (on the order of 1–3 vol%). Laminated composites of both mullite and alumina have been fabricated. Modified fibrous monoliths of alumina that consist of a triple layer “core/interphase/ matrix” arrangement have been designed. Although optimization of the processing and microstructural-design parameters has not fully conducted, preliminary mechanical data and scanning elec￾tron microscopic observation of the crack profiles nevertheless demonstrate this procedure to be a viable high-temperature, oxidation-resistant, toughening mechanism in chemically compat￾ible oxide composites. In terms of chemistry, the platelets provide a simple and effective mechanism for debonding in air, indepen￾dent of temperature, up to the melting point of the alumina or the matrix. A mechanism that intimately mixes strength and toughness, on a microstructural scale, through an optimally tailored bimodal microstructure has been introduced. It has been postulated that regions with one layer of high matrix:interphase ratio (high strength) could be alternated with regions of low matrix:interphase ratio (high toughness), and these regions may consist of several such thin layers. After the crack passes through a thick matrix layer, it is believed to have high kinetic energy. The role of the several layers with a low matrix:interphase ratio is to slow the crack, by causing it to deflect along a tortuous interphase path, where some of its energy is dissipated and imparts toughness to the composite. References 1 A. G. Evans, “Perspective on the Development of High-Toughness Ceramics,” J. Am. Ceram. Soc., 73 [2] 187–206 (1990). 2 D. B. Marshall, B. N. Cox, and A. G. Evans, “The Mechanics of Matrix Cracking in Brittle Fiber Composites,” Acta Metall., 33, 2013–21 (1985). 3 R. W. Davidge, “Fiber-Reinforced Ceramics,” Composites (Guildford, UK), 18 [2] 92–98 (1987). 4 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). 5 A. G. Evans, M. Y. He, and J. W. Hutchinson, “Interface Debonding and Fiber Cracking in Brittle Matrix Composites,” J. Am. Ceram. Soc., 72 [12] 2300–303 (1989). 6 J. W. Hutchinson and H. M. Jensen, “Models of Fiber Debonding and Pullout in Brittle Composites with Friction,” Mech. Mater., 9, 139–63 (1990). 7 R. J. Kerans, R. S. Hay, N. J. Pagano, and T. A. Parthasarathy, “The Role of the Fiber–Matrix Interface in Ceramic Composites,” Am. Ceram. Soc. Bull., 68 [2] 429–42 (1989). 8 A. G. Evans, “The Mechanical Performance of Fiber-Reinforced Ceramic Matrix Composites,” Mater. Sci. Eng., A, A107, 227–39 (1989). 9 A. G. Evans and F. W. Zok, “Review of the Physics and Mechanics of Fiber-Reinforced Brittle Matrix Composites,” J. Mater. Sci., 29, 3857–96 (1994). 10W. J. Clegg, K. Kendall, N. M. Alford, D. Birchall, and T. W. Button, “A Simple Way to Make Tough Ceramics,” Nature (London), 347, 455–57 (1990). 11W. J. Clegg, “The Fabrication and Failure of Laminar Ceramic Composites,” Acta Metall., 40 [11] 3085–93 (1992). 12Y. Shigegaki, M. E. Brito, K. Hirao, M. Toriyama, and S. Kanzaki, “Processing of a Novel Multilayered Silicon Nitride,” J. Am. Ceram. Soc., 79 [8] 2197–200 (1996). 13W. S. Coblenz, “Fibrous Monolithic Ceramic and Method for Production,” U.S. Pat. No. 4 772 524, Sept. 20, 1988. 14S. Baskaran, S. D. Nunn, D. Popovic, and J. W. Halloran, “Fibrous Monolithic Ceramics: I, Fabrication, Microstructure, and Indentation Behavior,” J. Am. Ceram. 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Bordia and G. W. Scherer, “On Constrained Sintering—I: Constitutive Model for a Sintering Body,” Acta Metall. Mater., 36 [9] 2393–97 (1988). 33R. K. Bordia and G. W. Scherer, “On Constrained Sintering—II: Comparison of Constitutive Models,” Acta Metall. Mater., 36 [9] 2399–407 (1988). 34R. K. Bordia and G. W. Scherer, “On Constrained Sintering—III: Rigid Inclusions,” Acta Metall. Mater., 36 [9] 2411–16 (1988). 35O. Sudre and F. F. Lange, “Effect of Inclusions on Densification: I, Microstruc￾tural Development in an Al2O3 Matrix Containing a High Volume Fraction of ZrO2 Inclusions,” J. Am. Ceram. Soc., 75 [3] 519–24 (1992). April 2001 Toughened Oxide Composites Based on Porous Alumina-Platelet Interphases 773