September 2000 quilibria between Fluoromicas and Silicate Matrices stabilized h celsian fluorophlogopite Distance (um ig. 5. EDS linescan across the stabilized-celsian/fluorophlogopite inter- face reacted at 1300.C for 10 h. The annotation of the plot notes the schematic representation of the interdiffusion process between the solid- solution feldspar and the solid-solution trioctahedral mica. case of te,isl460°or 1388°C. ely. These same chemical-buffering technique should be equally applicable to alumina-rich stabilized-celsian matrices. Preliminary studies also indicate that Mgo additions to barium-stuffedcordierite-matrix compositions can sufficiently reduce the alumina or silica activities to provide thermochemical abilized fluorophlogopite feldspar mineral, the interlocking doublering structure of cordi. rite is quite similar to that of framework silicates, which suggests elian an ideal matrix-phase mechanical response. This aspect is one The second approach toward stable interfaces uses the chemical flexibility offered between phases exhibiting a solid-solution equilibrium. The interdiffusional attributes of K-Ba solid-solution celsian against both end-members of the fluorokinoshitalite- fluorophlogopite K-Ba solid-solution series, involving the cationic exchange of K and Si for Ba- and Al, provides a wide compositional and thermal stability range. The compositions characterizing the interfacial response between stabilized celsian and fluoromica establish the exact thermochemical conditions necessary for maintaining global equilibrium between the inter- phase and matrix, up to temperatures that are limited only by the fluoromica melting temperature Both of these approaches offer substantial latitude in designing functional interfaces between fluoromica interphases and alumi 10 MICRONS nosilicate feldspar matrices. This design flexibility can be directly attributed to the chemical and structural complexity of the fluoromi- cas:the manipulation of multivariable, polyphase systems, although tedious to characterize, may offer the best approach toward property- Fig. 4. Interfacial morphology of a stabilized-celsian/fluorophlogopite tailorable interfaces in engineering ceramic composites. reaction couple processed at 1300.C. Figure 4(a) is a BEl micrograph illustrating the interfacial morphology. Within the fluorophlogopite, one easily can see the incorporation of heavier-Z elements near th spectroscopy reveals the incorporation of Ba- into the mica(see Fig. 5). The arrow in the figure indicates the incomplete interdiffusion in one mica ed and grain whose basal plane is approximately parallel to the reaction interface Figure 4(b) is an SEl micrograph identify ing the interfacial reaction Drs. Kenneth Chyung and Steven Dawes( Corming, Inc, Corning, NY). The authors products. As confirmed via WDS, points"B" and"C in the figure are truly grateful for the help of Doug Waelchli(University of wisconsin-Madison) ouple specimens. The authors also thank Dr. Y. Austin orrespond to spinel and leucite, respectively, these phases indicate the manv A and"E in the figure correspond to unreacted stabilized celsian and fluorophlogopite, respectively, whereas point"D"in the figure indicates a appreciative of Mark Paquette for his patience and perseverance in the frustrating task cul rain into which ion exchange(K+ and Si++for Ba2+and Al+)has of preparing laminate specimens for optical and electron microscopy. Namaste References need to form more product phases is minimized, effectively protectin IM. Y. He and J. W. Hutchinson,"Crack Deflection at an Interface Between the fluoromica interphase from a catastrophic loss of Mgo 2H. R Shell and K. H. Ivey, "Fluorine Micas, "Bull. U.S. Bur. Mines, 647(1969) Following this methodology, thermodynamic equilibrium can R. F. Giese Jr."The Effect of F/OH Substitution on Some Laver-Silicate and a fluoromica interphase up to very high temperatures that are TaG. H. Beall, k. Chyung, S B. Dawes, K P. Giadkare, and S N. Hoda, be established between a Mgo-buffered anorthite-matrix phase limited only by the fluoromica melting temperature, which, in the 4935387, June 19, 1990.(b)G. H Beall, K Chyung, S. B Dawes, K. P. Gadkaree,need to form more product phases is minimized, effectively protecting the fluoromica interphase from a catastrophic loss of MgO. Following this methodology, thermodynamic equilibrium can be established between a MgO-buffered anorthite-matrix phase and a fluoromica interphase up to very high temperatures that are limited only by the fluoromica melting temperature, which, in the case of fluorokinoshitalite or fluorophlogopite, is 1460° or 1388°C, respectively. These same chemical-buffering techniques should be equally applicable to alumina-rich stabilized-celsian matrices. Preliminary studies also indicate that MgO additions to “barium-stuffed” cordierite-matrix compositions can sufficiently reduce the alumina or silica activities to provide thermochemical stability against fluoromica interphases. Although not truly a feldspar mineral, the interlocking double-ring structure of cordi￾erite is quite similar to that of framework silicates, which suggests an ideal matrix-phase mechanical response. This aspect is one focus of our continuing research. The second approach toward stable interfaces uses the chemical flexibility offered between phases exhibiting a solid-solution equilibrium. The interdiffusional attributes of K-Ba solid-solution celsian against both end-members of the fluorokinoshitalite– fluorophlogopite K-Ba solid-solution series, involving the cationic exchange of K1 and Si41 for Ba21 and Al31, provides a wide compositional and thermal stability range. The compositions characterizing the interfacial response between stabilized celsian and fluoromica establish the exact thermochemical conditions necessary for maintaining global equilibrium between the inter￾phase and matrix, up to temperatures that are limited only by the fluoromica melting temperature. Both of these approaches offer substantial latitude in designing functional interfaces between fluoromica interphases and alumi￾nosilicate feldspar matrices. This design flexibility can be directly attributed to the chemical and structural complexity of the fluoromi￾cas; the manipulation of multivariable, polyphase systems, although tedious to characterize, may offer the best approach toward property￾tailorable interfaces in engineering ceramic composites. Acknowledgments All fluoromica and feldspar powder precursors were prepared and provided by Drs. Kenneth Chyung and Steven Dawes (Corning, Inc., Corning, NY). The authors are truly grateful for the help of Doug Waelchli (University of Wisconsin–Madison) in preparing the reaction-couple specimens. The authors also thank Dr. Y. Austin Chang (University of Wisconsin–Madison) and Dr. Doug Swenson (Michigan Technology University, Houghton, MI) for many fruitful discussions concerning the modeling of multicomponent interdiffusion systems. Lastly, the authors also are appreciative of Mark Paquette for his patience and perseverance in the frustrating task of preparing laminate specimens for optical and electron microscopy. Namaste. References 1 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). 2 H. R. Shell and K. H. Ivey, “Fluorine Micas,” Bull. U.S. Bur. Mines, 647 (1969). 3 R. F. Giese Jr., “The Effect of F/OH Substitution on Some Layer-Silicate Minerals,” Z. Kristallogr., 141, 138–44 (1975). 4 (a)G. H. Beall, K. Chyung, S. B. Dawes, K. P. Gadkaree, and S. N. Hoda, “Fiber-Reinforced Composite Comprising Mica Matrix or Interlayer,” U.S. Pat. No. 4 935 387, June 19, 1990. (b)G. H. Beall, K. Chyung, S. B. Dawes, K. P. Gadkaree, Fig. 4. Interfacial morphology of a stabilized-celsian/fluorophlogopite reaction couple processed at 1300°C. Figure 4(a) is a BEI micrograph illustrating the interfacial morphology. Within the fluorophlogopite, one easily can see the incorporation of heavier-Z elements near the interface; spectroscopy reveals the incorporation of Ba21 into the mica (see Fig. 5). The arrow in the figure indicates the incomplete interdiffusion in one mica grain whose basal plane is approximately parallel to the reaction interface. Figure 4(b) is an SEI micrograph identifying the interfacial reaction products. As confirmed via WDS, points “B” and “C” in the figure correspond to spinel and leucite, respectively; these phases indicate the presence of excess alumina within the feldspar substrate originally. Points “A” and “E” in the figure correspond to unreacted stabilized celsian and fluorophlogopite, respectively, whereas point “D” in the figure indicates a mica grain into which ion exchange (K1 and Si41 for Ba21 and Al31) has occurred. Fig. 5. EDS linescan across the stabilized-celsian/fluorophlogopite inter￾face reacted at 1300°C for 10 h. The annotation of the plot notes the schematic representation of the interdiffusion process between the solid￾solution feldspar and the solid-solution trioctahedral mica. September 2000 Thermochemical Reactions and Equilibria between Fluoromicas and Silicate Matrices 2295