Microstructural Evolution in Some Silicate Glass–Ceramics: A Review

Linda R. Pinckneyw and George H. Beall

Corning Incorporated, Corning, New York 14831

Just as the microstructures in glass–ceramics encompass therange from nanocrystalline transparent materials to microcrys-talline tough materials, so the paths of microstructural evolutionin glass–ceramics vary widely. Evolution can proceed in numer-ous ways, their genesis being a perturbation of some type, in-cluding the surface nucleation used in glass frit processing,crystallization of the primary phase or phases upon distinctcrystalline nuclei, and nucleation promoted by nano- or micro-scale amorphous phase separation in the parent glass. Examplesof the crystallization history of several glass–ceramic materialsare described, with emphasis on how their microstructural evo-lution influences their ultimate physical and optical properties.

I. Introduction

GLASS–CERAMICS are micro- or nanocrystalline materials pro-duced by the controlled nucleation and crystallizationof a glass precursor. The properties of glass–ceramics dependon composition, phase assemblage, and microstructure. Thecomposition and heat treatment determine the potential phaseassemblage, which in turn governs many physical and chemicalcharacteristics such as hardness, density, and thermal expansion.Equally important is the microstructure that develops in theglass–ceramic, as it also plays a key role in controlling the prop-erties of glass–ceramic materials.

A wide variety of microstructural configurations can resultfrom tailoring both composition and thermal treatment. Eithersurface nucleation/crystallization or internal nucleation or acombination of both can be used to design a glass–ceramicwith the desired properties. For glass–ceramics based on internalnucleation and growth, a general evolutionary pattern is ob-served in the crystallization cycle: amorphous phase separationand/or precipitation of primary crystalline nuclei, nucleationand growth of metastable crystalline phases, and approach to astable crystalline assemblage. Unique microstructures, most ofwhich cannot be duplicated by other ceramic processes, can beproduced by interrupting the cycle at a desired point.1

Amorphous phase separation is generally the first stagein glass–ceramic formation. Phase separation results frommicro- or nanoscale immiscibility in the glass, whereby a dis-persed liquid phase structurally incompatible with the host glassforms either on cooling or on reheating the glass. This phase isnormally highly unstable as a glass and will precipitate primarycrystalline nuclei on heating at temperatures near the annealingpoint of the host glass. These nuclei may form at the interface ofthe phase-separated droplets or may be homogeneously pro-duced within the unstable droplet. These primary nuclei crys-tallites are composed of species enriched in the dispersed glassyphase during phase separation; examples include oxides (such astitanates and zirconates), phosphates, fluorides, and metals. The

next nucleation stage normally involves the heterogeneous nu-cleation and growth of metastable crystalline phases on the pri-mary crystalline nuclei, resulting in a fine-grained metastablesolid solution assemblage. Finally, with increasing temperature,this metastable phase assemblage breaks down into stable crys-talline phases by means of crystal phase transformations, reac-tions between metastable phases, exsolution, or a combinationof several of these mechanisms.

In some multicomponent glass–ceramic systems, multiphaseseparation can occur, with two types of nucleation occurringalmost simultaneously. An example of this is seen in certainglass–ceramics based on fluormica and fluorapatite.2 Moreover,by utilizing the phase separation phenomenon in the base glass,internal crystallization can be achieved at an earlier stage of theheat-treatment process or may be delayed because of variationsin the composition of the matrix phase.3

II. Nucleation and Growth

(1) Surface Crystallization

Without the internal nucleation process as a precursor to crys-tallization, nucleation is initiated at lower energy surface sites.Normally, this leads to undesirable, uncontrolled devitrification,but the phenomenon can be capitalized on in certain glass–ceramic technologies such as those based upon devitrifying frits.In these materials, the relict surfaces of glass grains (frit) serve asnucleating sites for the crystal phases, and the glass compositionand processing conditions are chosen such that the glass softensbefore sintering and undergoes viscous sintering to full densityjust before the crystallization process is completed. Given theseconditions, the final crystalline microstructure may be essentiallythe same as that produced from the bulk process. Devitrifyingfrits are commonly used for high-temperature-compatible coat-ings and joining materials, such as seals for solid oxide fuelcells,4,5 or as low thermal expansion materials for joining silicaglass components in the manufacture of lightweight telescopemirrors.

The devitrifying frit technology has also been used for fabri-cating architectural panels for buildings. The panels consist of afrit-generated, surface-crystallized wollastonite glass–ceramic.6,7

The manufacturing process begins with glass frit particles1–7 mm in size, which are sintered to a dense monolith byheat treatment of around 8501C. Curved shapes can be fabri-cated at this step. At temperatures above 9501C, controlled sur-face crystallization of b-wollastonite (CaSiO3) begins to grow atthe boundaries of the former glass grains. At 10001C, wollasto-nite grows in a needle-like form from the surface of the glasstoward the interior of the glass grain (Fig. 1). Controlled crys-tallization ends after a heat treatment of 11001C for 2 h, with thecrystallized boundaries joining together to form large needleswith lengths of 1–3 mm. The boundaries of the initial glassgrains are virtually indiscernible in the final product. Crystalscomprise about 40 wt% of the final product. The microstructureof needle-shaped crystals, coupled with the different refractiveindices of the b-wollastonite and glassy matrix, gives the glass–ceramic the translucence of marble but with superior chemicaldurability. This type of glass–ceramic has been manufacturedand sold by Nippon Electric Glass Co. Ltd., Otsu, Japan, underthe name Neoparies

s

.

M. Hall—contributing editor

Presented at the joint meeting of the ACerS Glass and Optical Materials Division andthe University Conference on Glass held in Rochester, New York, May 20–23, 2007.

wAuthor to whom correspondence should be addressed. e-mail: pinckneylr@corning.com

Manuscript No. 23369. Received June 21, 2007; approved September 7, 2007.

Journal

J. Am. Ceram. Soc., ]] []]] 1–7 (2008)

DOI: 10.1111/j.1551-2916.2007.02129.x

r 2008 Corning Incorporated (NY)

1

Designed surface-crystallized bulk glasses also have been de-veloped for use in certain optical applications. Examples of sucha material are surface-crystallized glasses in the system TiO2–BaO–B2O3, in which spontaneously preferred orientation ofb-BaB2O4 (b-BBO) yields second-harmonic generation as highas 50% of the value obtained with single-crystal b-BBO.8

(2) Distinct Nucleating Crystal Phase

Numerous mechanisms exist for inducing controlled crystalliza-tion in bulk glasses. One common mechanism is the use of dis-tinct, heterogeneous nucleating agents. The structures of theprimary crystalline nuclei that typically precipitate following aninitial phase separation are of particular importance, in that thestructure of the early protocrystalline phases is signifi-cantly different from that of the host glass. For example, inaluminosilicates, the structure of the oxide nucleants involvesedge-shared or corner-shared octahedra and edge-shared cubesas the basic structural units. This is distinct from the generalcorner-shared tetrahedral random network of the base glass.The fact that many of these primary crystalline nuclei are meta-stable phases that do not persist when the glass–ceramic is fullycrystallized suggests that they may reflect, in terms of polyhedralcoordination and linkage, the original structure of the dispersedamorphous phase.9

The earliest glass–ceramics developed by Stookey10 werebased on heterogeneously nucleated metal particles. In thesephotosensitive materials, the metals are incorporated into theglass in ionic form (e.g., Ag1 and Au1) along with cerium ionsin the form of Ce31. When exposed to ultraviolet (UV) light,Ce31 is oxidized to Ce41, and the metals, then reduced to ametallic form, serve as nucleating particles for the crystallizationof lithium metasilicate. These crystals grow dendritically in askeletal fashion, with crystal growth proceeding in specific latticedirections. Because lithium metasilicate crystals preferentiallyetch in dilute hydrofluoric acid solutions, high-precision struc-tures can be fabricated by suitable masking of the substrate,exposure to UV light, and selected etching of the exposed areas.

Glass–ceramics based on crystals of stuffed b-quartz(b-quartz solid solution) in the lithium aluminosilicate systemcan be heterogeneously nucleated with certain oxide crystals.For example, additions of ZrO2 and/or TiO2 in the 2–4 mol%range serve as efficient nucleating agents for these glasses and

produce primary nuclei—either ZrTiO4, Al2Ti2O7, or ZrO2, de-pending on the composition—when heated to about 8001C.These nuclei then act as sites for efficient heterogeneous nucle-ation of metastable stuffed b-quartz crystals upon heating tohigher temperatures of about 9001C. The resulting glass–ceram-ics are highly transparent for several reasons. First, the efficientnucleation of b-quartz produces very small crystallites (typicallyo50 nm). Moreover, the birefringence of the b-quartz solid so-lution is very low in the typical commercial compositions withclose to 70 mol% silica.11 Finally, the persistence of a thin vis-cous residual glass layer around each primary b-quartz crystal,also known as a cellular membrane microstructure, restricts thesize of the crystallites and delays secondary crystal growth.12

Transmission electron microscopy (TEM) has been used tostudy the development of a stuffed b-quartz phase upon nucleiof ZrTiO4.

13 A specific heat treatment was designed such thatonly a few nucleating crystals form at a low temperature andb-quartz crystals then grow on these nucleating crystals at ahigher temperature. This special microstructure comprised a rel-atively few large b-quartz crystals, most of which can be seen tohave a nucleating crystal in their center, as shown in Fig. 2. TheZrTiO4 nucleating phase appears as a small dot a few nanome-ters in size. This microstructure suggests that an epitaxial rela-tionship exists between the ZrTiO4 and the stuffed b-quartzphase. Extra nucleating crystals can be seen at the b-quartz grainboundaries, as the residual glass becomes supersaturated withTiO2 and ZrO2 during the b-quartz growth phase.

Another highly crystalline glass–ceramic with a very differentmorphology is that based on canasite.14 In these materials, thebulk glass has a composition very close to that of stoichiometricfluorcanasite Na4K2Ca5Si12O30F4, with an excess amount ofCaF2. Internal nucleation is obtained through the precipitationof CaF2 crystallites and subsequent spherulitic growth of cana-site upon these nuclei (Fig. 3(a)). The final glass–ceramic is denseand highly crystalline, with a microstructure of interpenetratingblades (Fig. 3(b)) that confers the material with abraded flexuralstrengths of 300 MPa and toughness values of up to5 MPa �m1/2.

(3) Amorphous Phase Separation

(A) Droplet Phase Separation: Metastable immiscibilityis observed in many glass systems and is a primary origin forcontrolled crystallization in glass–ceramics; a distinct nucleation

Fig. 1. Scanning electron microscopic image showing the microstruc-ture of sintered b-wollastonite glass–ceramic, showing needle-like wol-lastonite crystals growing from the surfaces of the glass grains. Scalebar51 mm. (Courtesy M. Wada, in Höland and Beall17).

Fig. 2. Transmission electron micrograph of ZrTiO4-nucleatedb-quartz glass–ceramic. Scale bar5 200 nm.

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phase is not always required. One such glass–ceramic is derivedfrom low-alumina glasses in the binary SiO2–Al2O3 system(Fig. 4). For example, in a glass of 15% Al2O3, 10 nm dropletsof a high-alumina dispersed glass phase form within a silica-richglass.15 Nucleation and subsequent crystallization of mullitewithin this dispersed phase takes place upon reheating. The mi-crostructure of such a glass heated at 9501C is shown in Fig. 5.Because the original nanoscale pattern of phase separation hasbeen preserved, the glass–ceramic is transparent and remains soup to temperatures of 12001C.

Another system with metastable droplet phase separation arespinel glass–ceramics in the SiO2–Al2O3–ZnO–MgO–TiO2–ZrO2 system,

16 in which alumina- and titania-rich amorphousdroplets on the order of 10 nm in size become dispersed within a

silica-rich glass. As crystallization of the spinel proceeds, thefluxes in the glass—MgO and ZnO—selectively partition intothese alumina-rich droplets, forming a complex spinel solidsolution within the approximate phase field ZnAl2O4–MgAl2O4–Mg2TiO4. Crystallization occurs at high viscosity asthe spinel phase incorporates these fluxes during the process.The final microstructure consists of 10–20 nm spinel crystalsdispersed uniformly throughout a continuous highly siliceousglass matrix (Fig. 6). This continuous siliceous glass confers theglass–ceramic with high use temperature (strain points up to9501C) and excellent chemical durability, while the crystal sizeconfers transparency.

The microstructural evolution of machinable fluormica glass–ceramics can be quite complex, as described in Höland andBeall,17 after Chyung et al.18 The crystallization sequence of aMACOR

s

-like glass–ceramic (Corning Inc., Corning, NY), forexample, is illustrated in Fig. 7. The process begins with amor-phous phase separation into B0.5 mm droplets composed of afluorine-rich, potassium aluminoborosilicate glass dispersedthroughout a relatively Mg-rich matrix glass. Primary crystalli-zation begins at approximately 6501C as dendritic chondroditecrystals begin to grow in the Mg-rich matrix glass, nucleatedalong the phase separation boundaries. As the temperature in-creases, the chondrodite reacts completely with the phase-sepa-rated droplets to form norbergite, which in turn is consumed attemperatures above 8501C to form the phlogopite mica. The

Fig. 4. Phase diagram for Al2O3–SiO2 binary system (after phase dia-grams for ceramists).

Fig. 5. Relict microstructure in low-alumina Al2O3–SiO2 glass–ceramicshowing raised droplets crystallized to mullite. Scale bar5 1 mm.

Fig. 3. (a) Transmission electron micrograph showing spherulitic growth of canasite upon CaF2 nuclei at approximately 8001C. Diameter B0.5 mm.(b) Final highly crystalline canasite glass–ceramic microstructure. Scale bar5 1 mm.

2008 Microstructural Evolution in Some Silicate Glass–Ceramics 3

bulk composition of a mica glass–ceramic can also be tailored toprovide a range of mica morphologies from blocky crystals to aflaky, house-of-cards-like microstructure.9 Blocky crystals areobtained by designing in a viscous siliceous glass that inhibitsgrain growth, while the most machinable glass–ceramics areachieved with a more fluid, B2O3-containing residual glass and amica crystal designed to be K deficient, thereby yielding a lowercleavage energy.

A significantly different nucleation and growth mechanism isresponsible for yielding cabbage-head mica microstructures, as

shown in Fig. 8.19 Phase separation has proved to be very diffi-cult to discern, particularly with conventional replica microsco-py techniques, presumably because there is but a minorcompositional difference between the two phases. More recent-ly, careful TEM analysis indicated that there is indeed phaseseparation on the scale of 5 mm.20 These authors suggest that aradial composition gradient around these phase-separated drop-lets leads to the peculiar cabbage-head morphology, with crys-tallization following isocompositional lines. They, moreover,show that the mica crystals themselves are not bent, but ratherthe ‘‘cabbage leaves’’ are composed of equally spaced segmentsabout 200 nm wide with slightly different orientations.

Fig 6. Transmission electron micrograph of transparent glass–ceramicbased on spinel solid solution crystals. Scale bar5 0.1 mm.

Fig. 7. Microstructural evolution in machinable phlogopite glass–ceramics. Scale bars5 1 mm. (a) Transmission electron microscopic (TEM) replicaimage of phase-separated base glass showing amorphous phase separation of glass droplets of approximately 0.5 mm. (b) TEM image showing dendriticcrystallization of primary Mg-rich chondrodite-like phase in matrix glass, after 7001C—1 h. (c) TEM image showing further crystallization in whichnorbergite develops at 8501C. N, norbergite; P, phlogopite. (d) Final microstructure showing phlogopite mica crystals after 9501C—4 h.

Fig. 8. Scanning electron microscopic cross section through globularmicrostructure of cabbage head mica glass–ceramic. Diameter B100–200 mm.

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(B) Interlocking Phase Separation: Glass-in-glass phaseseparation where both constituents occupy roughly similar vol-umes often displays a wormy and cocontinuous geometry. Thisgeometry is common in spinodal decomposition, which resultsfrom formation of two liquids on cooling of a glass compositionin the central regions of a miscibility gap.21 In such cases wherethere are two cocontinuous glassy phases, one of the phases maycrystallize upon reheating while the other remains amorphous.The situation often results in the growth of anhedral (irregular)-shaped crystals with an internal microstructure that inherits themorphology of the original phase separation.22 This phenome-non can be called ‘‘anhedral’’ crystallization in phase-separatedglasses, and the resulting microstructures are called relictspinodal.

Certain mullite glass–ceramics in the SiO2–Al2O3 system ex-hibit anhedral crystallization of this type. These glasses possess ahigher alumina content than the mullite glass–ceramics thatyield droplet-type phase separation. The difference in the na-ture of the phase separation can be explained by reference to thephase diagram in Fig. 4. For example, a glass with about 30%Al2O3 falls near the center of the spinodal miscibility gap, whilea lower-alumina glass, such as that with 15% Al2O3, falls within

the nonspinodal miscibility gap. (These latter glasses show drop-let-style phase separation.) A glass with a composition of 30%Al2O3–70% SiO2 is unstable and rapidly phase separates intococontinuous glasses: one with a higher-alumina, mullite-likestoichiometry and the other very siliceous. The high-aluminaglass crystallizes, while the siliceous component does not.Figure 9 illustrates the microstructure of an HF-etched sampleof this mullite glass–ceramic. The mullite crystals grow in asemielliptical form, encompassing the siliceous glass. The relicttexture inherited from the original phase separation is evidentwithin these crystals where fine interconnected siliceous glassremains. Zones of totally uncrystallized material are less etchedthan the partially crystalline ellipsoids. TEM analysis shows thatthese mullite ellipsoids have a constant orientation and are sin-gle-crystal despite being riddled with siliceous glass.

In the SiO2–Al2O3–ZnO–ZrO2 system, a metastable miscibil-ity gap exists on the join SiO2–ZnAl2O4, within which glassesspontaneously phase separate upon cooling. A glass of approx-imate molar composition 73 SiO2–27 ZnAl2O4 separates into awormy cocontinuous network of high-silica glass and glass richin ZnO and Al2O3 on a scale of B100 nm. Heat treatment near8501C produces a microstructure in which Zn-stuffed b-quartzcrystals, very close in composition to the lower-silica phase,form along the original continuous network of this phase. WhileZrO2 is added for enhanced nucleation of the b-quartz phase,nucleation is still relatively inefficient and so large b-quartz crys-tals several micrometers in diameter are formed. The more sta-ble siliceous glassy phase, cocontinuous with the crystallizinglower silica glass, remains amorphous but is entrapped withinthe b-quartz domains, as shown in Fig. 10. Thus, while the op-tical microscope shows an apparent highly crystalline, relativelycoarse, and uniform microstructure, electron microscopy revealsthat the b-quartz grains are indeed single crystal and are inter-woven with silica glass following the geometry of the originalphase separation. This microstructure provides a number ofpractical advantages. For example, despite its considerableglassy content of 430%, the glass–ceramic provides a modu-lus of rupture of 140 MPa and low overall expansion, becauseboth the siliceous glass and the b-quartz crystals have lowexpansions.

This type of interlocking, cocontinuous phase separation canalso be observed in relatively complex glass–ceramic systems,such as that based on SiO2–MgO–CaO–Na2O–K2O–F–Li2O–P2O5–Al2O3. While the ultimate primary crystal phase in these

Fig. 9. Replica electron micrograph of HF-etched sample of mulliteglass–ceramic, showing relict microstructure of ellipsoidal growth ofmullite encompassing phase-separated siliceous glass. Scale bar5 1 mm.

Fig. 10. Replica micrographs (two magnifications) showing microstructure of large crystals of Zn-stuffed b-quartz with a substructure of relictamorphous phase separation. Scale bars5 1 mm.

2008 Microstructural Evolution in Some Silicate Glass–Ceramics 5

glass–ceramics is fluor-K-richterite, an amphibole structure(KNaCaMg5Si8O22F2), the microstructural evolution compris-es numerous phase changes and a complex solid-state reactionprocess. Nucleation in these glasses begins with a spinodal-likeamorphous phase separation in which one of the two phases isclose in composition to a lithium-containing, tetrasilicic fluor-mica of approximate composition KMg2LiSi4O10F2. The crys-tallization of this component is rapid near 6001C. The fluormicacrystals incorporate the major fluxes into the system, andso crystallization occurs at high viscosity. Precipitation ofdiopside—CaMgSi2O6—follows at 7001C. Subsequently, themetastable mica and diopside react to form the stable phase,fluor-K-richterite.

Figure 11 shows the microstructural evolution in these glass–ceramics. The nucleation and growth of spherulitic fluormicacrystals begin at around 5501C. These rounded crystals encom-pass a noncrystallizing component of the original phase-sepa-rated glass in a relict microstructure. The single-crystal nature ofthe mica crystals is suggested by a constant cleavage directionwithin each spheroid. At 6501C, the spherulites have grown andmica cleavage is pronounced. The spherulites have partiallyrecrystallized by 7001C to lens-shaped books and the mica hasnow taken on its common morphology, with preferred planargrowth. Nevertheless, the glassy droplets inherited from theoriginal amorphous phase separation still persist within themica lenses. It is likely that the original subspherical morphol-ogy and the enclosed glass have combined to create this lent-icular morphology instead of the usual tabular form of micasuch as that obtained in machinable glass–ceramics. A totalrecrystallization to interlocking diopside, K-richterite, and micais obtained by 9001C, with the diopside phase totally consumedby 9801C, the top crystallization temperature. The final micro-structure consists of tightly interlocked fine-grained acicularamphibole crystals with aspect ratios of 10:1 in a matrix ofmica, cristobalite, and minor residual glass. The interlocked rodmicrostructure yields toughness values of 3.2 MPa �m1/2 andbending strengths of 150 MPa.

III. Summary

Microstructure plays a key role in determining the ultimateproperties of glass–ceramic materials. A wide variety of micro-structural configurations can result from tailoring both compo-sition and thermal treatment. Either surface nucleation/crystallization or internal nucleation or a combination ofboth can be used to design a glass–ceramic with the desiredproperties.

Controlled nucleation and crystallization from surfaces canbe employed for products ranging from high-performance sealsand coatings through architectural panels and products withuseful optical characteristics. For glass–ceramics based on in-ternal nucleation and growth, a general evolutionary pattern isobserved in the crystallization cycle: amorphous phase separa-tion and/or precipitation of primary crystalline nuclei, nucle-ation and growth of metastable crystalline phases, and approachto the stable crystalline assemblage. Amorphous phase separa-tion can produce isolated droplets of one phase within anotheror a spinodal-like cocontinuous intergrowth of two glassy phas-es, both of which can yield intricate microstructures with com-plex phase evolutions. Moreover, multiphase separation canoccur in some glass–ceramic systems, with two types of nucle-ation occurring together. This rich spectrum of microstructuralevolution confers glass–ceramics with their broad array of prop-erties from transparency and high use temperature through highstrength and toughness.

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