J. Am Ceram Soc..83[92287-96(2000 urna Thermochemical Reactions and equilibria between Fluoromicas and silicate matrices A Petromimetic Perspective on Structural Ceramic Composites Todd T. King,* f Walter Grayeski, and Reid F. Cooper*, s Department of Materials Science and Engineering, University of wisconsin-Madison, Madison. Wisconsin 53706-1595 A petromimetie (geologicaF-analog) approach is applied to the interface. Both of these criteria emphasize the importance of design of alumina-fiber-reinforced glass-ceramic -matrix com engineering thermodynamically and mechanically functional inte posites that use a refractory, trioctahedral fluoromica fiber- faces or interphases between the fiber and the matrix phase. matrix interphase and feldspar matrixes. Studies of the solid- In considering these interfacial requirements, one realizes that state reaction couples between these silicate phases are the achievement of effective ceramic composite materials will be pursued to address the chemical tailorability of the interphase/ predicated on polyphase ceramic equilibria possessing high matrix interface from an engineering perspective. The mini- mechanical-behavior contrast among phases. Therefore, one ap- nization of alumina and silica activities within polyphase proach toward the fulfillment of these general design criteria is to feldspar-based matrixes via Mgo buffering is shown to be an onsider the wealth of data and understanding in the geologic ffective route toward a stable fluoromica interphase. An record: present right below our very feet is evidence of polyphase anorthite-2-vol%-alumina(CaAl Si2Os a-Al2O3)substrate, ceramic equilibria of phases-specifically silicates-with a wide hemically buffered with MgO, is shown to exhibit thermody variation of mechanical behavior. The combined disciplines of amie stability against fluorokinoshitalite(BaMg3lAlSiJO1oF2, igneous and metamorphic petrology and geochemistry indicate the up to temperatures potentially as high as 1460oC. The key to plethora of multicomponent silicate and oxide assemblages that he approach is the reduction of alumina activity via the exhibit high-temperature stability over geologic time scales. Thus, formation of MgAI O spinel. Similarly, the formation of one can consider the design of environmentally robust, elevated- forsterite(Mg, SiO4) stabilizes the mica in contact with matrix temperature ceramic composites from a petromimetic perspective compositions otherwise containing excess silica. The cationic Silicates possess a vast structural diversity, emanating from the interdiffusion between solid-solution feldspars and fluoromi- degree of Si-O--Si polymerization, that can provide the neces- as also is characterized. Coupled interdiffusion of k and sary mechanical contrast (i.e, intrinsic fracture toughness)to Si+ in exchange for Ba2+ and Al+ was observed between achieve debonding in fiber-reinforced ceramic composites as K-Ba solid-solution celsian and the barium-rich solid-solution articulated, for example, by the bimaterial interfacial crac end-member fuorokinoshitalite at 1300oC. A similar cationic deflection criterion developed by He and Hutchinson. Silicate xchange also is observed against the potassium-rich end- mechanical behavior ranges from extremely strong but brittle member fluorophlogopite(KMg3lAISi3JOJoF,), although in a framework silicates(fully polymerized, almost covalently bonded reverse direction, at temperatures of <1280C. The interfacial structures) to flexible sheet silicates(phyllosilicates )such as micas compositions identified via electron microprobe analysis spec- and clays(less-polymerized structures that incorporate notably fy one set of local equilibrium conditions from which global weak ionic or van der Waals bonding on specific crystallographic ceramic composite equilibrium can be achieved planes ). Thus, a refractory ceramic composite that uses micas as a thin, thermodynamically stable interphase to protect alumina fibers ically from cracks in an oxide/silicate matrix becomes a L. Introduction IBER-REINFORCED ceramic composites must fulfil two general as have a tetrahedral-cation-to-oxygen-anion(T: o)ratio of criteria to be effective structural materials for high-temperature 1: 2.5 and can be described by the structural formula aerobic environments. First, long- term thermochemical stability A0,.s-BY-[Alo-2Si4-2O1o(OH)2 (1) must be established between each of the ceramic component phases (i.e, fiber, matrix, and interphase) and the oxidizing where the Roman-numeral superscripts refer to the cation coordi environment. Second, useful ceramic composite systems must nation with O and/or OH anions, A represents a 12 exhibit significant toughness at elevated temperatures and rapid oordinated (i.e, large-diameter) alkali or alkaline-earth cation oading rates. Achievement of this second requirement is largely(known as the interlayer cation), and B represents a divalent or dependent on the debonding characteristics of the fiber/matrix trivalent octahedral (6-coordinated) cation. The T: O ratio require ment means that the relative amounts of al and si on the tetrahedral sites are affected by the specific occupancy of the octahedral and interlayer cation sites, for the above-described R. A. Condrate--contributing editor formula, with 100- anions, the sum of the tetrahedral Al and si ions must total 4. Cation-site occupancy defines the adjectives used to describe the micas. For example, the mineral muscovite which has the formula KAL,[Al SiJO,o(OH),, is described as a 189258 Received June 30. 1999 dioctahedral (i. e, only two of the three available octahedral sites ystems Program in Mechanics and Materials, through Grant No are occupied), trisilicic (i.e, three of the four tetrahedral sites contain Si), true(or flexible, i.e., the inter layer cation is an alkali) Member. American Ceramic Socie urrent ly with intel Corp, Santa car ight Center, Greenbelt,MD ca,the mineral kinoshitalite(BaMg [Al,Si,J0,o(OH),)is a trioctahedral, disilisic, brittle (i.e, the interlayer cation is an AUthor to whom correspondence should be addressed alkaline-earth element) mica. 287Thermochemical Reactions and Equilibria between Fluoromicas and Silicate Matrices: A Petromimetic Perspective on Structural Ceramic Composites Todd T. King,* ,† Walter Grayeski,‡ and Reid F. Cooper* ,§ Department of Materials Science and Engineering, University of Wisconsin–Madison, Madison, Wisconsin 53706–1595 A petromimetic (geological–analog) approach is applied to the design of alumina-fiber-reinforced glass-ceramic-matrix com￾posites that use a refractory, trioctahedral fluoromica fiber– matrix interphase and feldspar matrixes. Studies of the solid￾state reaction couples between these silicate phases are pursued to address the chemical tailorability of the interphase/ matrix interface from an engineering perspective. The mini￾mization of alumina and silica activities within polyphase, feldspar-based matrixes via MgO buffering is shown to be an effective route toward a stable fluoromica interphase. An anorthite–2-vol%-alumina (CaAl2Si2O8 1 a-Al2O3) substrate, chemically buffered with MgO, is shown to exhibit thermody￾namic stability against fluorokinoshitalite (BaMg3[Al2Si2]O10F2), up to temperatures potentially as high as 1460°C. The key to the approach is the reduction of alumina activity via the formation of MgAl2O4 spinel. Similarly, the formation of forsterite (Mg2SiO4) stabilizes the mica in contact with matrix compositions otherwise containing excess silica. The cationic interdiffusion between solid-solution feldspars and fluoromi￾cas also is characterized. Coupled interdiffusion of K1 and Si41 in exchange for Ba21 and Al31 was observed between K-Ba solid-solution celsian and the barium-rich solid-solution end-member fluorokinoshitalite at 1300°C. A similar cationic exchange also is observed against the potassium-rich end￾member fluorophlogopite (KMg3[AlSi3]O10F2), although in a reverse direction, at temperatures of <1280°C. The interfacial compositions identified via electron microprobe analysis spec￾ify one set of local equilibrium conditions from which global ceramic composite equilibrium can be achieved. I. Introduction FIBER-REINFORCED ceramic composites must fulfil two general criteria to be effective structural materials for high-temperature aerobic environments. First, long-term thermochemical stability must be established between each of the ceramic component phases (i.e., fiber, matrix, and interphase) and the oxidizing environment. Second, useful ceramic composite systems must exhibit significant toughness at elevated temperatures and rapid loading rates. Achievement of this second requirement is largely dependent on the debonding characteristics of the fiber/matrix interface. Both of these criteria emphasize the importance of engineering thermodynamically and mechanically functional inter￾faces or interphases between the fiber and the matrix phase. In considering these interfacial requirements, one realizes that the achievement of effective ceramic composite materials will be predicated on polyphase ceramic equilibria possessing high mechanical-behavior contrast among phases. Therefore, one ap￾proach toward the fulfillment of these general design criteria is to consider the wealth of data and understanding in the geologic record: present right below our very feet is evidence of polyphase ceramic equilibria of phases—specifically silicates—with a wide variation of mechanical behavior. The combined disciplines of igneous and metamorphic petrology and geochemistry indicate the plethora of multicomponent silicate and oxide assemblages that exhibit high-temperature stability over geologic time scales. Thus, one can consider the design of environmentally robust, elevated￾temperature ceramic composites from a petromimetic perspective. Silicates possess a vast structural diversity, emanating from the degree of SiOOOSi polymerization, that can provide the neces￾sary mechanical contrast (i.e., intrinsic fracture toughness) to achieve debonding in fiber-reinforced ceramic composites as articulated, for example, by the bimaterial interfacial crack￾deflection criterion developed by He and Hutchinson.1 Silicate mechanical behavior ranges from extremely strong but brittle framework silicates (fully polymerized, almost covalently bonded structures) to flexible sheet silicates (phyllosilicates) such as micas and clays (less-polymerized structures that incorporate notably weak ionic or van der Waals bonding on specific crystallographic planes). Thus, a refractory ceramic composite that uses micas as a thin, thermodynamically stable interphase to protect alumina fibers mechanically from cracks in an oxide/silicate matrix becomes a very real possibility. Micas have a tetrahedral-cation-to-oxygen-anion (T:O) ratio of 1:2.5 and can be described by the structural formula A0.5–1 XII B2–3 VI @Al0 –2Si4 –2# IVO10~OH!2 (1) where the Roman-numeral superscripts refer to the cation coordi￾nation with O22 and/or OH2 anions, A represents a 12- coordinated (i.e., large-diameter) alkali or alkaline-earth cation (known as the interlayer cation), and B represents a divalent or trivalent octahedral (6-coordinated) cation. The T:O ratio require￾ment means that the relative amounts of Al and Si on the tetrahedral sites are affected by the specific occupancy of the octahedral and interlayer cation sites; for the above-described formula, with 10 O22 anions, the sum of the tetrahedral Al and Si ions must total 4. Cation-site occupancy defines the adjectives used to describe the micas. For example, the mineral muscovite, which has the formula KAl2[Al1Si3]O10(OH)2, is described as a dioctahedral (i.e., only two of the three available octahedral sites are occupied), trisilicic (i.e., three of the four tetrahedral sites contain Si), true (or flexible; i.e., the interlayer cation is an alkali) mica; the mineral kinoshitalite (BaMg3[Al2Si2]O10(OH)2) is a trioctahedral, disilisic, brittle (i.e., the interlayer cation is an alkaline-earth element) mica. R. A. Condrate—contributing editor Manuscript No. 189258. Received June 30, 1999; approved March 20, 2000. Research supported, in part, by the National Science Foundation, Division of Civil and Mechanical Systems Program in Mechanics and Materials, through Grant No. CMS-9414756. *Member, American Ceramic Society. † Currently with NASA/Goddard Space Flight Center, Greenbelt, MD. ‡ Currently with Intel Corp., Santa Clara, CA. § Author to whom correspondence should be addressed. J. Am. Ceram. Soc., 83 [9] 2287–96 (2000) 2287 journal