1876 Communications of the American Ceramic Society Vol. 80, No. 7 2A. G. Evans. f. W. Zok Davis,“ The Role of I P/B es, "Compos. Sci K、Ce Mater, Res, Soc. Symp. Proc., 20, 697-712(1984) 4A. Kelly, Design of a Possible Microstructure for High Temperature Ser MP B vice, 'Ceram. Trans., 57, 117-29(1995 SR Lundberg and L. Eckerbom, Design and Processing of All-Oxide Com- posites, " Ceram. Trans., 58, 95-104(1995) Oxide Composites, "Mater Sci. Eng, A162, 15(1993) J. Whalen. D Narasimhan, C. G. Gasdaska, w. ODell, and R C. morris, G. Razzell, and 3, Gent, ""Devel f Interfaces in Oxide and Silicate-Matrix Composites, Ceram, Trans. 58,41-52(1995 loA. G. Evans and D. B. Marsha Mechanical Behaviour of Ceramic hexaaluminate Matrix Composites, Acta Metall. 37 [10] 2567-83(1989) Fig. 5. Schematic showing preferential growth of outer hexaalumi nate laye 12D. C. Hitchcock and L C De Jonghe, ""Fracture Toughness Anisotropy of Sodium-B-Alumina, J, Am. Ceram Soc., 66 19]1053-67(1983) N. lyi, S. Takekawa, and S. Kimura, "The Crystal Chemistry of Hexaal tes: B-Alumina and Magnetoplumbite Structures, "J. Solid State Chem to remain in solid solution with the zirconia, directly stabilize 83, 8-19(1989) the hexaaluminate through solid solution reaction with the alu- P.E. D. Morgan and J. A Miles, ""M: mina. The gradient in the oxygen partial pressure is responsible Further Discussion J, A Ceram Soc., 69 the zirconia coating. In reducing conditions, the defect center in tween Dissimilar Materials, "Int J. Solids Struct, 25, 1053-67(1989/e be. SM. Y. He and J w. Hutchinson ' Crack Deflection at the Interf Ce-stabilized zirconia is an oxygen ion vacancy which com M. Miura, H. Hongoh, T. Yogo, S. Hiran T Fuji, Formation of es for the reduced valent Ce3+ and can be La-B-Aluminate Crystal in Ce-TZP, J, Mater. Sci., 29, 262-c 2Ce+++0o 2Ce3++v6+102T with V representing an (1994) oxygen vacancy and Oo representing an oxygen on an oxygen 17P.-L Chen and I.-w. Chen, "" In-situ Alumina/Aluminate Platelet Compos- ites, 'J Am. Ceram Soc., 75 [9]2610-12(1992) site. Clearly, in atmospheres of excess oxygen the reaction is buffered and proceeds in the opposite direction thereby stabi- Plasticity and Toughening in CeO-Partially-Stabilized Zirconia-Alumina(Ce and others, even slightly reducing conditions can result in the 38(1992) loss of oxygen from the system leading to the stabilization of r9R. A. Cutler, R J. Mayhew, K M. Prettyman, and A. v. virkar stable only in oxidizing atmospheres. The stability of the RE \Ww1/S Ce-TZP the 3+ valent state. It thus appears that Ce-stabilized zirconia is B/MP-aluminas in oxide site systems has been demon- 2H- D. Kim, L-S. Lee, S.-w. Kang, and J.w. Ko, The Formation of strated in high-temperature oxidizing heat treatment studies up ties of Alumina, "J. Mater. Sci, 29, 4119-24(1994) Mechanical Proper- to 1500C, against alumina and unstabilized zirconia inter 2T. B. Troczynki and P. S. Nicholson, Resistance to Fracture of a Partia faces.8. 31 Stabilized Zirconia/B-Alumina Composite, J. Am. Ceram Soc., 68[10]123- 35(1985) M. K. Cinibulk and R S Hay, Textured I. Conclusions Ite Fiber-Matrix The reduction of Ce++to Ce+(and possibly Zr+to Zr+)has 1233-46(1996 been shown to result in the development of in situ reacted ture and Stability of Synthetic Interphases in CMCs, "Key Eng. Mater, 127 hexaaluminate interphases. The reaction occurs via one of two l3I[Part137-50(1997 routes:(1)A Ce solute diffusion from the tetragonal solid solution and phase partitioning into an unstabilized zirconia has not been observed in these in situ reacted studies, it is from 1350 c to ig and T. Sata, "Phase Studies in the Sy Zr02 -Ce2 O3 C, Bull. Tokyo Inst Tech 8,25-32(1972) expected that such a phase would react with alumina, forming 2H. -Y, Zhu, T. Hirata, and Y Muramatsu, Phase Separation in 12 mol% zirconia and hexaaluminate. (2)The destabilization of the Ce- Soc, 75 (10 2843-48(1992) Induced by Heat Treatment in H, and Ar, J. Am Ceran luminate via directed diffusion along grain boundaries and/or onal Zirconia polycrystals by reduc the columnar structure of the PVd coatings through a REDOX Am. Ceram Soc. 72(6)1044 46(1902nduced Phase Trans type reaction. Preferred orientational growth of the hexaalumi ZRA. L Leo A. B. Andreeva, and E.K. Keler. Influence of the Gas nate parallel to the coating interface was interpreted from its Atmos of Zirconium dioxide with Oxides of Cer morphological character which is in the correct habit for en- [zv. Akad. Nauk SSSR, Neorg. Mater. 2 [1] 137-44(1966) hanced micro/macromechanical properties. nd E. K. Keler. *I hemical Reactions and Polymorphic Transfor tem Zirconium Dioxide-Cerium Oxides, Ogneupory, 31 [3] 42-48 301. Manassidis and M. J. Gillan, ""Structure and Energetics of nd P c. hall faces Calculated from First Principles, " J. Am. Ceram. Soc., 77[2]335-38 with Ap m.Soc,765]1265-73(1993) 3IM. G. Cain; unpublished work, 19951876 Communications of the American Ceramic Society Vol. 80, No. 7 Fig. 5. Schematic showing preferential growth of outer hexaalumi￾nate layer. to remain in solid solution with the zirconia, directly stabilize the hexaaluminate through solid solution reaction with the alu￾mina. The gradient in the oxygen partial pressure is responsible for setting up the gradient in the Ce3+/Ce4+ ratio which spans the zirconia coating. In reducing conditions, the defect center in Ce-stabilized zirconia is an oxygen ion vacancy which com￾pensates for the reduced valent Ce3+ and can be written as 2ce4+ + o0 4 2ce3+ + G- + 1/20,T with 6- representing an oxygen vacancy and Oo representing an oxygen on an oxygen site. Clearly, in atmospheres of excess oxygen the reaction is buffered and proceeds in the opposite direction thereby stabi￾lizing the 4+ valent state. However, from these experiments and others, even slightly reducing conditions can result in the loss of oxygen from the system leading to the stabilization of the 3+ valent state. It thus appears that Ce-stabilized zirconia is stable only in oxidizing atmospheres. The stability of the RE P/MF’-aluminas in oxide composite systems has been demon￾strated in high-temperature oxidizing heat treatment studies up to 150O0C, against alumina and unstabilized zirconia inter￾face~.~”’ IV. Conclusions The reduction of Ce4+ to Ce3+ (and possibly ZP+ to 213’) has been shown to result in the development of in siru reacted hexaaluminate interphases. The reaction occurs via one of two routes: (1) A Ce solute diffusion from the tetragonal solid solution and phase partitioning into an unstabilized zirconia and a pyrochlore, P, phase Zr,Ce,O,. 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