15-porcelain-raw materials, processing, phase evolution, and mechanical behavior.pdf

Porcelain-Raw Materials, Processing, Phase Evolution, and Mechanical Behavior

William M. Cam* and Udayan Senapati* New York State Center for Advanced Ceramic Technology-Whiteware Research Center,

14802 New York State College of Ceramics at Alfred University, Alfred, New York

Porcelain represents the foundation of the ceramics disci- pline and one of the most complex ceramic materials. Com- posed primarily of clay, feldspar, and quartz, porcelains are heat-treated to form a mixture of glass and crystalline phases. This review focuses on raw materials, processing, heat treatment, and mechanical behavior. Because of the complexities of the porcelain system and despite the substantial amount of research already conducted within the field, there remain significant opportunities for research and study, particularly in the areas of raw material understanding, processing science, and phase and microstructure evolution.

I. Introduction

ONTEMPORARY whitewares produced worldwide represent C the foundation of much of the ceramic industry, as well as one of the cornerstones upon which the American Ceramic Society was founded in 1898. Porcelains comprise some of the most complicated of all ceramics in almost all aspects of the manufacturing process: from raw materials, processing, and forming, to the kinetic limitations and complexities of the microstructure and phase development. Even temperature measurement of porcelain firing has required the development of its own unique system-pyrometric cones-because thermocouples simply do not provide sufficient information to indicate the heat work associated with the porcelain firing process.

From an economic perspective, the commercial whiteware industry accounts for -7% of the entire ceramic market worldwide, at sales levels of $6.1 billion in 1994 and $8.5 billion in 1996., In 1994, industrial production was divided into floor and wall tile (31%), artware and pottery (14%), dinnerware and

G. L. Messing-contributing edit01

Manuscript No. 190529. Received October 1, 1997; approved December 1, 1997. Member, American Ceramic Society.

fine china (13%), sanitaryware (12%), foodserviceware (1 l%), and other (19%). Between 1984 and 1993, the whitewares industry experienced a steady net sales increase from $4.55 billion to $8.21 b i l l i ~ n . ~ (These numbers can be used only as indicators, because the statistical accuracy is dependent on the response to a survey by the industry.)

A (ceramic) whiteware is defined as a fired ware consisting of a glazed or unglazed ceramic body that is commonly white and of fine texture, designating such product classifications as tile, china, porcelain, semivitreous ware, and earthenware.4 A whiteware is formed from natural raw materials of which the major portion is clay. Porcelain, stoneware, china, and earthenware bodies historically have been distinguished by their firing temperature and compositions (Table I).5 Porcelains also are distinguished by the lack of open porosity in the fired b ~ d y . ~ , ~ Compositions for the various types of industrial porcelains are presented graphically as a portion of the K,O-AI,O,-SiO, phase diagram in Fig. l.x

The name porcelain is believed to have originated from the Portuguese word Porcellana and presumably first denoted products manufactured from the shell mother-~f-pearl.~ In the 20th century, porcelain products have received wide application in a variety of fields ranging from electrical insulators to dinnerware; hence, the 20th-century connotation of porcelain suggests a product more appropriately defined as a fine-crystalline, strong, impervious ceramic product and re- late(d) to the structure and type of product rather than to any particular composition. lo

This review addresses porcelains via four topical areas: raw materials; processing, including colloidal aspects; firing, including chemical reactions, phase development, and microstructural evolution; and mechanical properties. Within each context, the discussion of porcelain refers primarily to triaxial blends of clay, feldspar, and a filler material (usually quartz or alumina) and, consequently, ignores stoneware and earthenware bodies, as well as glazing and decorating issues.

11. Historical Perspective The high level of intrinsic plasticity in clays and clay-based

systems and the resulting ease of forming objects precipitated

3

Vol. 81, No. 1 4 Journal of the American Ceramic Society-Carty and Senapati

Table I. Traditional Definitions of Whitewares Name ASTM definition4 Maturing temnerature'

Porcelain A glazed or unglazed vitreous ceramic whiteware made by the procelain process, and used for technical purposes, designating such products as electrical, chemical, mechanical, structural, and thermal wares when they are vitreous

Nepheline syenite: 2-6 Sanitaryware: 8-1 2 Electrical: 8-1 2 Hard: 10-18'

China A glazed or unglazed vitreous ceramic whiteware made by the china process, and used for Hotel: 10-13' nontechnical purposes, designating such products as dinnerware, sanitaryware, and artware when they are vitreous

fire clay Stoneware &lo5

Earthenware A glazed or unglazed nonvitreous ceramic whiteware 06-0S6

A vitreous or semivitreous ceramic ware of fine texture, made primarily from nonrefractory

+Provided in pyrometric cone values.

Silica 17 13' sioz n

TRIDYM ITE \ I r

semivitreous whiteware

vitreous sanitary wall

Electrical insulators

Fig. 1. Leucite-mullite-cristobalite portion of the K,O-Al,O,-SiO, phase diagram. Metakaolin is identified on the diagram, representing the location of the clay portion of a porcelain batch following dehydroxylation. Ranges for typical commercial porcelain compositions are identified. Potash feldspar is incongruently melting and the eutectic liquid formation temperature is 990"C8

the development of pottery skills more than 14 centuries ago. Whitewares were formed entirely by hand prior to the development of modem automated forming techniques early in the 20th century. The first hand jiggers were introduced around 1925: and the development of automated jiggers did not be- come commonplace in the dinnerware industry until the late 1940s. Roller-tool jiggers are now the most commonly used method for forming flatware. The tile industry is by far the most automated within whiteware manufacturing, led by equip- ment developments in Italy' ' that provide for almost entirely automated production (other than maintenance workers); i.e., once the process is initiated, the only human input is in the final inspection stages. Currently, automation continues apace within the dinnerware industry, with movement toward dry pressing and pressure casting. l2 It is likely that dinnerware, sanitaryware, and electrical insulator production will remain second to tile production in terms of automation for some time because of the complexity of the shapes being produced.

Corresponding to the increase in automation is the improvement in process control. Prior to automation, dinnerware production loss rates of up to 60% were not uncommon. Today, loss rates within a modem dinnerware production facility are 15%-20%. This higher level of procJuctivity is accomplished partly through improved understanding of the underlying forces governing the rheology and plasticity of porcelain bodies. In the preautomation era, it was necessary to have relatively soft bodies with plasticity suitable for hand forming. With the introduction of automated forming techniques, it be- came necessary to create stiffer bodies with more controlled plasticity through the use of dispersants and increased particle packing. Prior to the introduction of dispersants, casting slips were "scooped into the mold" and the resulting casts were uneven and difficult to handle. l 3 Dispersants-initially K2C0, in 1844, then Na,CO,, sodium bicarbonate, and NazSi03- were introduced as a means of decreasing the slip viscosity and thus improving the drain-casting process. l3

January 1998 Porcelain-Raw Materials, Processing, Phase Evolution, and Mechanical Behavior 5

Table 11. Temperature Equivalents for Standard-Sized Orton Pyrometric Cones

Temperature Temperature Temperature Cone ("C) Cone ("C) Cone ("C)

022 02 1 020 019 018 017 016 015 014 013 012 01 1 010 09 08 07 06 05 Y2 05

589 61 1 634 685 725 752 784 807 83 1 859 864 884 894 923 955 984 999

1023 1046

04 03 02 01

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15

1060 1101 1120 1137 1154 1162 1168 1186 1196 1222 1240 1263 1280 1305 1315 1326 1346 1366 143 1

16 17 18 19 20 21 23 26 27 28 29 30 31 32 32% 33 34 35 36

1473 1485 1506 1528 1549 1569 1590 1605 1627 1633 1645 1654 1679 1717 1730 1741 1759 1784 1796

Assuming a heating rate of 150'Ch over the last few hundred degrees. If altema- tive heating rates are used or if a dwell is used in the firing process, the cone falls at a temperature below that listed. Similarly, if accelerated heating rates are used, the cone falls at a higher temperature. Cone designations continue to he commonly used to denote the firing range of whiteware bodies.

From the firing perspective, commercial porcelain production initially relied on periodic "beehive" kilns, using coal as the primary fuel. In small potteries, the kilns were fired infre- quently, dependent on the production of sufficient ware to fill the kiln volume, and firing cycles on the order of several days were used.I4 As the industry progressed, so did the firing ca- pabilities, and circular tunnel kilns designed to fit into the footprint of the beehive kiln were de~eloped.~." Eventually, linear tunnel kilns were developed that allowed significant increases in production volumes. The most advanced firing ca- pabilities are the current roller-hearth kilns, which can produce large quantities of high-quality porcelain dinnerware or vitri- fied tile with firing cycles as short as one hour.15

Most of the reactions occurring in porcelain bodies are ki- netically governed processes that do not reach thermodynamic equilibrium. Hence, measurement of temperature alone does not provide a true indication of the heat work done on the body

and its degree of maturity. Instead, heat work recorders, such as pyrometric cones and other similar devices (e.g., rings and "check" keys), have been developed to follow the firing prog- ress as it responds to time and temperature variations, demon- strating the appropriate heating rate for maturation. Pyrometric cones have been the most commonly used method for thermal work determinations, and, although less commonly used today in industry, bodies continue to be designated by their "cone value." Pyrometric cones are three-sided pyramids made from mixtures of kaolin, feldspar, quartz, and other raw materials resembling the body composition that function as true indicators of chemical reaction kinetics in porcelain bodies. The first standard and regularly used pyrometric cones were introduced by Seger;I6 in the United States, Orton pyrometric cones17,18 are the norm. Table I1 lists the Orton pyrometric cone designations and their temperature equivalents. Modem commercial firing processes are monitored with thermocouples and often are checked with pyrometric cones or keys for verification of the heat work accomplished.

111. Raw Materials

Because of their three-part composition of clay, feldspar, and quartz, porcelains are referred to as triaxial whitewares. Alu- mina can be substituted for quartz to increase the mechanical strength of the fired ware, and nepheline syenite can be substituted for feldspar. These primary raw materials are listed in Table 111; less commonly used raw materials are listed as secondary raw materials in Table IV. (Excellent introductions to these and other common raw materials are published annu-

Another important raw material, although one less commonly monitored or controlled within manufacturing, is the process water. It is common to use tap or potable water, which usually contains a wide range of cations, including Ca2', Mg2+, Na+, and K+. Table V provides typical tap water cation levels from several locations. Even with well-controlled urban water supplies, the level of these ions can vary, leading to substantial fluctuations in suspension rheology and plasticity. Also, because the ionic concentration can vary dramatically with a geo- graphic location, water chemistry generalizations are difficult. Although it is important to realize that water quality can impact the manufacturing process and substantially affect the repro- ducibility of rheological measurements, the contribution of local water sources is not addressed here.

ally. 1 9 q

Table 111. Primary Raw Materials Used in the Manufacturing of Commerical Whitewares Raw material Nominal composition Common impurities

Ball (plastic) clay A1,0, .2Si0,.2H20 quart^,^ TiO,, Fe,O, Kaolin (china) clay A1,0,~2Si0,~2H20 Montmorillonite, quartz Soda feldspar Na20~A1,0,~6Si0, K,O, CaO, MgO, quartz Potash feldspar K,O~Al2O,~6SiO, Na,O, CaO, MgO, quartz Nepheline syenite K2O~3Na,0~4A1,0,~9Si0, CaO, MgO, quartz A 1 u m i n a '412% Na,O Quartz SiO, TiO,, Fe,O,

+Quartz impurity in ball clay may he as high as 35 wt%.

Table IV. Secondary Raw Materials Used in the Manufacturing of Commerical Whitewares

Raw material Nominal composition Common impurities

Bentonite/montmorillonitet (M2+)(M3'),(Si,A1),O,,(OH),~nH,O Not applicable Glass frits Company specific Not applicable Petalite (lithium feldspar) Li,O~Al,0,~6SiO2 Na,O, K,O Bone ash$ Ca,(PO,), Unknown Talc 3Mg0.2Si0,.2H20 Chrysotile, CaO

Zircon ZrO,.SiO, Not applicable Whiting CaCO, MgCO,

'This unconventional form for presenting the composition was chosen for the highly variable divalent (Mz') and trivalent (M3') cation levels present in montmorillonites. *Bone ash is commonly used as a flux for hone china.

6 Journal of the American Ceramic Society-Carty and Senapati Vol. 81, No. 1

Table V. Typical Chemical Analysis of Several Tap Water Samdes Showing a Broad Range of Variabilitvt

Cation concentration (ppm) Tap water site Ca Mg Na K Si

Distilled water BDL BDL BDL ADL BDL Alfred, NY 52.3 20.2 28 4 5 Buffalo, NY 28.7 9.1 12 6 ADL Syracuse, NY 40.2 9.4 14 ADL BDL Victor, NY 41.5 12 21 BDL 1.5 Columbus, OH 7.5 1.6 ADL 4 ADL East Liverpool, OH 28.2 7.3 21 2 1.9 York, PA 21.3 6.2 9 7 2.5

Dectection limit 0.1 0.1 1. 1. 0.5 +Induction coupled plasma spectroscopy was used to obtain the results. BDL in-

dicates that the cation level was below the detection limit (as specified); ADL indicates species concentration at the detection limit.

(1) CkYS In whiteware production, the term clay refers to raw ma-

terials that provide plasticity and green strength during the forming stages of porcelain production and that contribute substantially to the color of the fired ware.

Kaolinite, the most common clay mineral, is a 1:l sheet silicate composed of a [Si,O,I2- layer and an [Al2(0H),l2 layer, as illustrated schematically in Fig. 2.8 The theoretical formula for kaolinite is Si,Al,O,(OH), (frequently expressed as A1,0,*2Si0,.2H20). Clay minerals are formed by the decomposition of feldspathic rock via geological processes; a typical reaction sequence using potassium feldspar (microcline or orthoclase) can be written:,l

f I

1 Kaolinite

2KA1Si,08 + 3H,O + Al,Si,O,(OH), + 4Si02 + 2KOH If the potassium is not properly removed following the weath- ering process, illitic clays (2:l sheet silicates) are formed instead of kaolinite.

Of primary importance for the domestic whitewares industry are English and U.S. clays; these clays are the most widely studied in the literature. In England, kaolins (or china clays) and ball clays are distinguished as being primary and secondary clays, r e ~ p e c t i v e l y . ~ . ~ ~ In the United States, however, both kaolins and ball clays usually are secondary clays, having been deposited at a considerable distance from their place of origin.23 There are primary clay deposits within the United States but none that are currently commercially exploited. Geographi- cally, the major ball clay deposits are located east of the Ap- palachian Mountains and in western Kentucky and Tennessee; the major kaolin deposits are located in central Georgia and South Carolina.

Mineralogical and chemical differences between ball clays and kaolins are minor-both are kaolinitic in nature, containing quartz as the major impurity and iron (in the form of Fe,O,) and titania (in the form of anatase) as minor i m p ~ r i t i e s . ~ ~ - ~ ~ , ~ ~ The quartz is removed via wet processing of the kaolins but is not commonly removed from the ball clays. English clays are generally lower in titania content than U S . clays but are similar in iron levels, leading to white bodies from English clays and buff-colored bodies from U.S. clays.21 Table VI lists typical chemical analyses of a Georgia kaolin, an English kaolin, and a Kentucky ball clay. Powder X-ray diffraction (XRD) patterns are presented in Fig. 3.

Ball clays often are referred to as plastic clays, because they possess a finer particle size, which produces greater plasticity

dd-0 0

Mica (Muscovite)

Water layers

Water layers I I

About

Montmorilbnite (Hydrated)

Fig. 2. Angstroms). Kaolinite crystal is a 1:l sheet silicate, whereas mica and montmorillonite are 2:l sheet silicates.8

Schematic illustration of the layer structures of kaolinite, mica, and montmorillonite, showing the relative spacings between the layers (in

January 1998 Porcelain-Raw Materials, Processing, Phase Evolution. and Mechanical Behavior 7

Table VI. Typical Chemical Analysis of a Georgia Kaolin, a Kentuckv Ball Clav. and an English Kaolin

Comuosition (%I

Component Georgia kaolin2' Kentucky ball clayz6 English kaolin"

SiO, 45.30 44.70 46.77

Fez03 0.30 0.60 0.36

0.25 0.10 0.24 0.05 0.10 0.13 CaO

Na,O 0.27 0.10 0.05 0.04 0.10 1.49

13.97 13.60 12.97 LO1 (950C)

A1203 38.38 38.30 37.79

TiO, 1.44 2.40 0.02 MgO

K,O

in a whiteware body. The name "ball clay" originated from the method of mining the clays-in large cubes or balls of clay that were cut from the deposits.26 Processed ball clays and kaolins have similar specific surface areas, but the quartz impurity level can be significantly higher in ball clays (up to 35% on a dry-weight basis), requiring a much finer particle size for the kaolinite fraction. Because ball clays frequently contain up to several weight percent of decayed biological or organic mat- ter, they are commonly dark in color-and sometimes black- in the raw form. The organic impurities can be removed entirely during heat treatment to produce a white body.

Kaolins commonly contain montmorillonites and smectites, 2:1 sheet silicates of variable composition (Fig. 2). Montmo- rillonites often incorporate water, as well as other impurity ions, between the sheets (to compensate for ionic substitution within the tetrahedral and octahedral layers), producing variable interlayer spacings. Smectites are particularly problematic from a characterization perspective, because they often are present as individual layers of 2:l sheet silicates and, consequently, are undetectable via XRD. The presence of montmorillonites and smectites can have a significant impact on the rheology and plasticity of a porcelain body, although the specific extent of their contribution is unknown. Commercially, it

2 8

Fig. 3. Powder XRD patterns for a domestic ball clay and kaolin, and an English kaolin. Diffraction peaks at -6" and -8.8" 28 correspond to the interlayer spacings for mica and montmorillonite, respectively, as illustrated in Fig. 1. Large reflection at -12.3" 28 represents the interlayer spacing for kaolinite. All three of the samples contain impurity quartz.

is rare to have a complete mineralogical understanding of a clay, which becomes problematic only when the impurity content varies significantly between batches.

The sheet silicate structure differentiates clays from other common ceramic powders. The platey morphology of a clay particle is responsible primarily for the high specific surface areas of clays (18-30 m2/g), compared to other powders with similar particle size. The mean diameter of ball clays and kaolins is -0.3-0.5 p,m. The high specific surface area, combined with the platelike morphology, is believed to be responsible for the intrinsic plasticity of clay-water systems.

From the clay mineralogy perspective, kaolinites are grouped by their degree of Crystallinity. Standard clay samples are available from the Clay and Clay Minerals Society, including examples of well-crystallized (KGalb) and poorly crystallized (KGa2) kaolinites. Both of these clays are Georgia kaolins; the powder XRD patterns and electron photomicrographs are illustrated in Figs. 4 and 5. The degree of crystallinity is determined by the "Hinkley Index," which compares the intensity of the peaks within the 0,211 domain region.27 The greater the ratio, the greater the degree of crystallinity. It is argued that poorer crystallinity offers greater potential plasticity and that the degree of crystallinity corresponds to a greater degree of ionic substitution within the kaolinite lattice. How- ever, the degree of crystallinity rarely is used as a characteristic measurement of a clay by the ceramics industry.

(2) Fluxes Historically, potash feldspars (microcline and orthoclase)

have been the most commonly used fluxes in porcelain^.^^^^^^^ Potash feldspars rarely are pure, usually containing the minerals albite (sodium feldspar) and anorthite (calcium feldspar). Albite sometimes is used as the flux component in commercial porcelains, but anorthite is not, because it does not occur in commercially viable deposits within the United States. Figure 6 illustrates a -400 mesh (45 km) potash feldspar.

Nepheline syenite has replaced feldspars in many commercial body formulations, reducing the firing temperature and increasing the alkali level in the glass pha~e .~O.~ l Nepheline syenite is composed of the minerals nepheline, albite, and microcline and possesses a higher alka1i:silica ratio (4:9) than the

1 " " I " " I " " I " " I " " l " " I " " l " " l " "

Fig. 4. Powder XRD patterns for a well-crystallized and poorly crystallized kaolinite standards (Clay and Clay Minerals Society). Hinck- ley index evaluates the ratio of peaks in the 0,211 domain. Well- crystallized sample contains a small amount of free quartz.

8 Journal of the American Ceramic Sociev-Gary and Senapati Vol. 81, No. 1

deformation. Quartz and flint are the most commonly used fillers2* in porcelain bodies and are essential to the microstructural evolution by dissolution of silica in the feldspathic glass. The quartz grain size is important to the forming of the body; in most modem commercial porcelain operations, a -325 mesh (63 pm) quartz is used. However, undissolved quartz often is responsible for a deterioration in mechanical properties as a result of the displacive p- to a-quartz inversion at 573C during the cooling process. Figure 7 shows a photomicrograph of a -325 mesh quartz sample.

Calcined alumina also is used as a filler in place of quartz to circumvent the quartz inversion and thereby improve the mechanical properties of the body. The dissolution rate of alumina is extremely slow compared to quartz because of the limited solubility of alumina in the feldspathic glass. The significantly greater cost of alumina compared to quartz is the major draw- back to its use.

Fluxes and the fillers are termed nonplastics, because, by themselves, they possess no intrinsic plasticity. The nonplastics have a significantly larger particle size than the clays and, consequently, provide networklike support for the body; they also significantly reduce the viscosity of the slip.33 The nonplastics also allow greater plasticity to be achieved at lower water contents, probably because of the reduction in specific surface area within the Also, the larger particle size allows for a substantial increase in packing density in the green body, resulting in increased strength and reduced shrinkage.

In industrial practice, it is common to characterize raw materials to obtain the specific surface area, particle size distribution, chemistry, mineralogy, and other data using a variety of test methods. These results then are compiled and tracked with the material in the process, but the correlations with manufacturing loss rates, process performance, and ware quality are generally weak. Consequently, additional tools and more rig- orous observation are needed to predict more accurately process behaviors within industry.

Fig. 5. SEM photomicrographs of (a) well-crystallized and (b) poorly crystallized kaolinite samples used to generate the XRD patterns in Fig. 4. Well-formed faces and clay booklets are evident in the well-crystallized sample.

feldspars ( 1:6).28,31 (The alka1i:alumina ratio of nepheline syenite is equivalent to that of the feldspars.)

(3) Fillers Fillers are generally the coarsest-particle-size fraction of a

porcelain body and tend to serve several functions. The coarse particle size provides resistance to cracking during drying and forms a skeletal network during firing to mitigate pyroplastic

Fig. 6. sample.

SEM photomicrograph of a -400 mesh potash feldspar

IV. Processing

Depending on the industry, commercial porcelain processing is either dry or wet.35 In many processes, dry processing is sufficient, particularly in the consumer dinnerware market and a portion of the tile market. In the electrical insulator and commercial dinnerware market, wet processing is required to obtain the mixing homogeneity necessary to maintain strength and integrity in the finished products. Obviously, industries that rely on slip ~ a s t i n g ~ ~ , ~ ~ (such as the sanitaryware industry) and those that use spray-dried granulate for dry p r e ~ s i n g ~ ~ . ~ ~ (such as the low-tension electrical insulator industry and the porcelain tile industry) must use wet processing.

(1) Dry-Processing Route Conceptually, dry processing is the simplest processing

route, by which the clay, feldspar, and quartz are mixed directly with tap water.35.39,40 The amount of water varies with the specific surface area of the batch and with the impurity and dispersant levels (in the less-common case of dispersant use). The amount of water is usually -18 wt% but can be as high as 21 wt%. Mixing is accomplished using a muller-mixer ma- chine that imparts large amounts of shear to the powder-water mixture, breaking down agglomerates and producing a homogeneous body suitable for plastic formng. (The dry-processing route is closest to that used by the porcelain industry prior to automation and by artisans for hand throwing and building.)

(2) Wet-Processing Route In the wet-processing route, the clays are slurried together

using local tap water as the dispersion medium; sometimes the water contains dispersants. In some cases, the clay slurries are allowed to age (usually on the order of 24 h) before the introduction of the nonplastics (i.e., the feldspar or nepheline syenite and the quartz or alumina). After the addition of the


Fig. 7. SEM photomicrograph of a -325 mesh quartz sample.

nonplastics, the suspensions sometimes are allowed to age again, during which time the viscosity is adjusted through the addition of polymeric additives or soluble salts. Table VII contains a list of the commonly used polymeric41 and soluble salt additives. Mixing of the raw materials in the slurry form generally yields a more-consistent batch. A detailed discussion of the behavior of clays within the suspension process and within a whiteware batch is given below. Also, as noted in Table VIII, clays contribute -95% of the surface area of a typical batch; consequently, the colloidal behavior of the clays dominates the rheology of the porcelain batch.

(3) Colloidal Behavior of Clays The effective Hamaker constant (Aeff) of a particle sus-

pended in a liquid dictates the magnitude of the van der Waals attractive forces between the particles.4247 In the case of kaolinite, A,, is almost an order of magnitude larger than that of silica and a factor of 7 lower than that of alumina (see Table IX).44,47 Therefore, kaolinite particles inherently are easier to disperse than alumina; i.e., a lower zeta-potential is required for suspension stability, allowing a broad pH range for suspension ~ tab i l i t y .4~3~~ Kaolinite particles have zeta-potentials that are negative over a broad pH range,48-50 and the zeta-potential presumably lies between that of alumina and s i l i ~ a . ~ ' , ~ ~

It is well established that clays are negatively charged over a broad pH range, and much research has been conducted to understand the dispersion process for clays. Johnson and Nor- ton12 are considered by many as the architects of a majority of the dispersion science currently applied to clay systems. They have conducted an extensive series of studies to determine the effect of different salts-including LiOH, NaOH, KOH, Ba(OH),, Sr(OH),, Na,SiO,, and Na,CO,-on the dispersion of kaolinite. Their conclusions are that ". . . (a) the charge on the kaolinite particles controls the degree of deflocculation and is governed by the type of cation and (b) the stability of the system is controlled by the anion of the medium and is gov-

Table VIII. Comparison of the Specific Surface Areas of the Different Components of a Typical Whiteware Batch'

Specific Typical Component 5Ulfdce batch Surface area contribution

Raw material area (m2/g) (dry wt7c) (per component) (a) Kaolin 25.0 25 6.25 51.2 Ball clay 27.0 20 5.40 44.2 Quartz 0.9 25 0.23 1.9 Feldspar 1.1 30 0.33 2.7

Batch 12.2 m2/g 95.4% clay 'As shown, the clay contributes -95% of the total specific surface area, dominating

the system. (The surface area values were obtained via nitrogen-gas adsorption.)

Table IX. Effective Hamaker Constants for Kaolinite, Silica, and Aluminat

Material Effective Hamaker constant (J) Reference

Kaolini te 1.8 x 10-20 44 Fused silica 0.83 x 47 Quartz 1.7 x 47 ~ A l 2 0 , 5.32 x 10- l~ 47

'The van der Waals attractive potential are greatest for alumina, least for fused silica, and intermediate for kaolinite. Therefore, the zeta-potential necessary to disperse kaolinite is lower than that necessary to disperse alumina.

erned by the type of anion preferentially adsorbed' ' (emphasis existing). These conclusions are mostly correct: the charge on the kaolinite particle governs the degree of deflocculation, and, as illustrated in their work, that charge is governed by pH; the type and preferential adsorption of the anion controls deflocculation. The adsorption of the Cog- or SiOg- ion is not rec- ognized or discussed, and the OH- ion is assumed to be solely responsible for charge generation, with the Cog- or SiOZ- ions forming weak acids in the suspension medium. Recent zeta- potential data, however, show conclusively that the SiOg- ion preferentially adsorbs on the kaolinite particle, substantially increasing the net negative charge on the particle, as illustrated in Fig. 8.49

The cation does not control the charge on the particle; instead, the cation causes flocculation rather than deflocculation because of compression of the double layer as a counterion. Johnson and Norton's observations are correct that the higher- valent cations are undesirable for developing deflocculated systems, not because the lower-valent cations deflocculate the system, but because the higher-valent cations cause flocculation, following the Schulze-Hardy rules for compression of the double layer and the critical coagulation c~ncen t r a t ion .~~ The concentration of divalent counterions necessary to cause flocculation can be a factor of 50-100 lower than necessary for monovalent counterions-the specific ratio is dictated by the zeta-potential. The contribution of ionic strength to the stability of clay suspensions is illustrated in Fig. 9 for different salts and dispersant levels.54

Part of the confusion regarding the role of the ionic species

Table VII. Common Polymeric Additives and Soluble Salts Used in the Whitewares Industry to Control Suspension Behavior and Processing Performance

Additive name Nominal composition Role Common use

Na-PAA Na-PMAA Sodium silicate Sodium chloride Calcium chloride Magnesium chloride Magnesium sulfate PEG CMChIethocel PVA

Na-poly(acry1ic acid) Na-poly(methacry1ic acid) Na,SiO, NaCl CaC1, MgC1, MgSO4 Poly(ethy1ene glycol) Carboxy methyl cellulose Poly(viny1 alcohol)

Dispersant Dispersant Dispersant Flocculant Flocculant Flocculant Flocculant Plasticizer Binder Binder

Slips, plastic bodies Slips, plastic bodies Slips, plastic bodies Slips, plastic bodies Slips, plastic bodies Slips, plastic bodies Slips, plastic bodies Dry pressing (with PVA) Improve glaze adherence Dry pressing

10 Journal of the American Ceramic Society-Car9 and Senapati Vol. 81, No. 1

n .

" \ /O 4 0.05

-30

Kaolin Background Ionic Strength: 10 mM NaCl

0

a

I

0 2 4 6 a 10 12

PH

Fig. 8. Effect of Na,SiO, on the zeta-potential of a kaolin over a range of pH values against a background ionic strength of l0mM NaC1. Increase in the negative zeta-potential indicates the specific adsorption of the SiOg- ion on the clay particle surface. Na,SiO, was added at a level of 0.05 mg/m', based on the specific surface area of the kaolin. HCI was used to adjust the pH to 2.2 prior to the test, then NaOH was used to increase pH.

on colloidal behavior may result from the model of the kaolinite particle. Because of its crystal structure, kaolinite exhibits unique colloidal behavior in water. When a clay particle is dispersed in water, one surface acts similar to silica while the other acts similar to alumina. From a pH of -2 (the isoelectric point of silica) to a pH of -9 (the isoelectric point of alu- m i r ~ a ) , ~ ~ the two sides of the kaolinite particle are oppositely charged-the silica surface is negatively charged and the alumina surface is positively charged. This observation regarding the complex duality of kaolinite surface charge is highly con-

troversial, because it opposes the commonly held opinion that clay particles have negatively charged basal planes with positively charged

The agglomeration of negatively charged gold particles to the edges of clay platelets, as observed via transmission electron microscopy (TEM), forms the basis for the charge distribution on clay particles-e.g., that the edges must be positively charged.56 The lack of particles agglomerated to the faces of the kaolinite particles has been used to argue that the faces are, therefore, negatively charged. However, the pH of the suspension used to prepare the TEM samples is unclear. The mineralogy of kaolinite would dictate that one surface behaves similar to silica and the other behaves similar to a l~mina .~ ' Part of the problem may lie further in the large amount of literature published by clay mineralogists investigating the behavior of 2: 1 sheet silicates, such as those in the montmorillonite family; only a few references in the clay mineralogy literature, such as Gi i~en ,~ ' make the distinction between the basal plane surface chemistry characteristics of kaolinite versus other sheet silicates.

Direct confirmation regarding the dual surface nature of kaolinite is provided via atomic force microscopy (AFM) studies in conjunction with surface modeling. These results confirm the presence of silanol and hydroxide surfaces on kaolinite particles.58

Indirect evidence occurs in the dispersant-clay interactions. The amount of poly(acry1ic acid) (PAA) dispersant necessary to reach the minimum in a viscosity versus dispersant level curve, as illustrated in Fig. 10, is -50% of the amount to entirely coat the surface of an alumina powder.49 The amount of dispersant necessary to reach the minimum viscosity value for the alumina particles is consistent with the amount of poly- (methacrylic acid) (PMAA) necessary to coat the particle surface, as demonstrated by Cesarano et al.59 The observations regarding PAA and PMAA are consistent with observations on Na,SiO,. The adsorption measurements of PAA on various kaolinites indicate substantial differences in adsorbed amounts and a strong dependence on CaCl,

CaCI, Effect - A I -

8

No

Effect

1 0 2 lo-' 1 00 10' l o 2 103

Ionic Concentration (mM)

Fig. 9. Effect of cation levels on the viscosity of a typical whiteware batch showing that both a Na+-salt and a Ca2+-salt lead to coagulation of the suspension and the corresponding increase in viscosity if added at a sufficient level. Effect of salt additions is evident regardless of the dispersant

January 1998 Porcelain-Raw Materials, Processing, Phase Evolution. and Mechanical Behavior

0

0 v

v v v

0

a Kaolin

0

v Alumina

lo- I 0.0 0.2 0.4 0.6 0.8

Dispersant Level (mg/m2)

Fig. 10. Comparison of viscosity versus Na-PAA dispersant level for 30 vol% alumina and kaolin suspensions at pH of 8.5 (+0.2), corrected for the specific surface area of the powders (9.6 and 21.4 m2/g, re- ~pectively).~~ Data indicate that -50% as much dispersant is necessary to reach the minimum in the viscosity curve for kaolin compared to that for alumina.

The charge on a clay particle is attributed to ionic substitution in the tetrahedral layer (specifically, A13+ for Si4+) and octahedral layer (Fez+ or Mg2+ for A13+). Although this substitution most certainly occurs, it is below the level necessary to maintain the relatively large surface charges associated with clay particles and is inconsistent with the chemical analysis data. A majority of the surface charge on the silica surface of kaolinite particles probably is due to the ionization of surface silanol groups.60 The question of the colloidal nature of kaolinite and the mechanisms responsible for suspension stability are the research areas most important for the advancement of porcelain science.

Stability, in the colloidal sense used here, refers to cre- ating an environment in which the interparticle potentials are sufficiently large to prevent f l o c c ~ l a t i o n . ~ ~ ~ ~ ~ ~ ~ ~ In traditional whiteware terminology, a stable suspension is one that remains suspended. Because of the extremely broad particle-size distribution in a typical whiteware body, a suspension stable from the colloidal aspect would be undesirable, because it would lead to mass segregation of the particles because of its size. As a result, the body suspension never is deflocculated to the minimum in the viscosity curve during manufacture of porcelains.

(4) Suspension Rheology and Plasticity In addition to the particle-particle interactions, three other

factors contribute to the rheology and plasticity of porcelain systems. These are, ranked in order of importance, particle concentration, particle size and distribution, and particle morphology.61,62 The characterization of suspension rheology is now commonplace, and each whiteware industry (i.e., dinnerware, sanitaryware, etc.) has a preferred method. Historically, the measurement of plasticity has been difficult, and more than 30 different methods for quantifying plasticity have been suggested. BlooF3 provides an overview of the various methods that have been used. The spectrum of tests that have been reviewed ranges from simply judging the feel of the body by hand to methods that require sample preparation and long testing times, such as the triaxial test in soil mechanics. Further- more, these tests fall in two basic categories: unconstrained and constrained, of which constrained testing is most representative of actual forming methods. Despite the many attempts that have been made, there continues to be a need to develop a method that universally quantifies p l a s t i ~ i t y . ~ ~ , ~ ~

Bloor also reviews some of the theories and mechanisms used to explain why plasticity occurs in various materials as

11

t / \ j + 2 1 Ball Clay-Kaolin 1 j

Pressure Dependence ((kPa/kPa)xlOO)

Fig. 11. High-pressure shear rheology of ball clay, kaolin, 2:1 mixture of the clays, and a standard porcelain body (30% ball clay, 14% kaolin, 33% feldspar, and 23% quartz) showing the dependence of rheology on water content. Water contents (dry weight basis) are denoted by the values on the plot next to the data points.77

well as the limitations involved. Several theories have been advanced to explain plasticity in clay systems, including the presence of montmorillonites and illites (Grim65), colloidal and physical properties of the clays (Hauser and Johnson66), water films (Norton and Johnson67 and East68), stretched-mem- brane theory (Norton69), and surface tension ( S c h ~ a r t z ~ ~ and Kingery and Franc171). All of these theories are useful to explain a portion of the observed plastic behavior, but a unifying theory needs to be developed.

Consequently, in many cases, the description of plasticity is expressed in intangible terms (poor, fair, etc.); i.e., good plasticity implies a rheological behavior that is applied easily to a process and bad plasticity implies rheological behavior that is nonideal. What results is only a qualitative description of plasticity in reference to the process in which it is applied, with terms such as workability, cohesion, and stickiness being applied to describe the behavior of a plastic mass. There- fore, plasticity becomes a property of a material and workability a quality endowed by the process itself. Because plastic forming places a body in shear under pressure, a better description of plasticity may be the extent and character of shear behavior in a body under constraining pressure.

Building on the examples from soil mechanics72 and the work of J e ~ ~ i k e , ~ ~ Onoda and J a n n e ~ , ~ ~ and Kirby,75 a modified direct shear test has been developed using an annular design.34,76*77 This test allows plastic samples to be tested over a range of constraining pressures, over a relatively short time frame (-1.5 h), producing results similar to the direct shear test^.^^,^^ Adopting an annular design minimizes problems associated with nonuniform compaction and allows continuous shearing of the sample. Another advantage of the test is that a shear-yield stress is measured at several pressures on a single sample, thus allowing the sample cohesion (from a soil mechanics viewpoint) and the shear dependence on pressure to be calculated. The test is performed within the pressure range normally observed for plastic-forming processes, such as extrusion, but is entirely independent of the forming process, allowing a broad range of samples to be evaluated. As illustrated in Fig. 11, the data indicate that the water contents necessary to obtain the maximum cohesion strength correlate well with the saturation of the pore space with water.77 Also, the effect of nonplastic additions to the clay body shows that a significantly lower moisture level is necessary for a typical whiteware batch compared to individual clay samples.

Within the manufacturing environment, plastic forming is accomplished primarily through ram pressing, jiggering,78 ex-

12 Journal of the American Ceramic Socieg-Curty and Senaputi Vol. 81, No. 1

h

i

t r u ~ i o n , ~ ~ , ~ ~ and, to an extent, dry pressing, although the latter does not necessarily require that the body behaves plastically. Common to all of these forming methods is the application of pressure and shear to a plastic mass to form the desired product. This applied stress state causes the plastic mass to perma- nently deform, compact (facilitating water loss from the piece, if required), and flow during forming.

(5) Aging Aging is a common phenomena within porcelain bodies,

referring to the change in rheology or plasticity within the body over time. Historically, this change has been attributed to variations in the organic le~el~~-~~--particularly bacteria-over time, which would logically lead to changes in particle-particle interactions. Several different strains of bacteria have been identified, and their contribution to the aging of the body is u n d i s p ~ t e d . ~ ' ~ ~ ~ However, with most modern industrial processes using potable water from water-treatment plants, the potential for bacteria growth within the body is limited, as is the potential for biologically driven aging processes in commercial processes.

The chemistry of the suspension medium can change over time because of the dissolution of the raw materials, particularly the clays and the fluxes, leading to increased cation levels. Specifically, the kaolinite particles do not appear to dissolve; however, the impurity ions present in the clays as other clay species or as other mineral phases are the apparent source of the cations. Chemical analysis of the water from specific dissolution studies, as illustrated in Fig. 12, and filter press efflu- ent shows elevated cation levels, which slowly increase with time.86 Also, systems containing montmorillonites have high cation exchange capacities, allowing monovalent and divalent cations to be specifically adsorbed within the clay structure, depending on the ionic concentration of the suspension medium. Increasing cation levels cause compression of the charged double layer, leading to coagulation of the particles and significant changes in the rheology of the suspensions and the plastic bodies. It is not currently understood whether it is better to age the suspension or the plastic body.

(6) Plastic Body Preparation Plastic bodies are prepared in the wet process by the dewa-

tering of the body suspension using a filter press. The body suspension is pumped under pressure into the filter press, and

Monovalent Cations via Nepheline Syenite 4

I " " " " " " " " '

Aging time (hour)

0 20 40 60 80 100 120 140

Fig. 12. Combined monovalent (Na+ and K+) and divalent (Ca2+ and Mg2+) cation concentrations in solution for nepheline syenite and a ball clay, obtained from a 25 vol% aqueous suspension over the course of one week. Nepheline syenite represents the highest measured dissolution levels for monovalent cations, whereas the ball clay exhibits the highest dissolution level of divalent cations. Divalent cation dissolution level for nepheline syenite was at the detection limit and, therefore, not shown.86

the excess water is removed through filter membranes. The rate of filtration is dictated by the solids loading of the suspension, the degree of dispersion, and the particle size d i s t r i b ~ t i o n . ~ ~ , ~ ~ If the body is too well dispersed, the filter-pressing process is lengthy, particle segregation may occur throughout the cake, and the resulting body probably is too stiff for proper forming. If the body is highly flocculated, the filter-pressing process is very rapid, but the plastic body possesses too much water, and shape retention following the forming process is problematic. The rheology of the suspension feeding the filter press is controlled by the use of dispersants and, coagulants such as CaC1, (Table VII). Each manufacturing facility follows unique guide- lines that have been developed through years of practical ex- perience.

The initial suspension is filter pressed in processes that rely on slip casting for fabrication, and the filter press cakes are redispersed to make the casting slip. This process serves two purposes: it removes the soluble species, resulting in greater control over the rheology, and it provides a more economical means of storing the body prior to its use in the production process.

(7) Drying

Clay-based systems tend to be the most forgiving of all ceramics systems because of their intrinsic plasticity. Drying can be grouped into microscopic models and macroscopic control necessities. Both of these approaches have been addressed in detail in the literature, and excellent reviews are available e l s e ~ h e r e . ~ ~ . ~ ~ Of potential importance for the drying of whitewares is the new, so-called airless drying, in which steam is used to assist the drying process, eliminating meniscus formation and substantially shortening the drying ~ y c l e . ~ ' The con- cept of airless drying has not yet been applied to the drying of porcelains.

V. Firing Temperature, time, and atmosphere in the kiln affect chemi-

cal reactions and microstructural development in the porcelain body and, consequently, are important in the fired properties of porcelain. Fast firing of porcelain bodies has gained wide rec- ognition and application in the whiteware i n d ~ s t r y , ' ~ , ~ ~ - ~ ~ reducing production costs by efficient use of energy in the firing process. The fast firing of porcelain wares requires knowledge of chemical reactions occurring during the process and of microstructural development.

( I ) Reactions throughout the Firing Process The sequence of chemical reac\ions during the firing of por-

celain bodies depends on the type of raw materials in the body, but, for a typical clay-quartz-feldspar system, the basic reaction steps-ignoring the removal of nonchemically bound species, such as water and organics-can be outlined as follows:

The crystal structure of kaolinite contains hydroxyl groups, and the dehydroxylation of these groups to form metakaolin (A1,03.2Si0,) occurs at -550C.9"'00 The chemical equation representing this process is

(1)

-550'C A1,03~2Si0,-2H,0 _j Al,03.2Si0, + 2H,O?

This is observed in typical analytical studies, such as differen- tial thermal analysis (DTA) and thermogravimetry (TGA). DTA-TGA curves for kaolinite are presented in Fig. 13. These measurements also are important in the design of fast-firing schedules. 15,92-95 Dehydroxylation kinetics, believed to be first order, yields a dehydroxylation rate directly proportional to the surface area of k a ~ l i n . ~ ~ , ' ~ ~ Furthermore, the dehydroxylation process is an endothermic process that is accompanied by a reorganization of octahedrally coordinated aluminum in kaolinite to a mostly tetrahedrally coordinated aluminum in metakaolin.


100 200 300 400 500 600 700 800 900 1000 1100

Temperature ("C)

TGA

Well Crystallized Kaolinite

Poorly Crystallized Kaolinite

80 ' 100 200 300 400 500 600 700 800 900 1000 1100

Temperature ("C)

Fig. 13. DTA-TGA curves for a well-crystallized and poorly crystallized kaolinite sample (from Figs. 4 and 5) . Dehydroxylation process initiates at a slightly lower temperature in the poorly crystallized sample, compared to the well-crystallized sample. Similarly, the exothermic reaction associated with the spinel crystallization also is shifted to a slightly lower temperature.

(2) The a- to @-quartz inversion occurs at 573C. Because of the relatively great flexibility of the packed particle network, the quartz inversion is of little consequence during the heating cycle.

(3) Sanidine, the homogeneous, high-temperature, mixed- alkali feldspar, forms within 700-10000C.'0' The formation temperature apparently is dependent on the sodium:potassium ratio.

(4) Metakaolin transforms to a spinel-type structure and amorphous free silica at -950"-1000"C,'0~-106 as shown by the following chemical eq~ation:~'

-950-1 000C 3(A1,03~2Si0,) A 0.282A18(A1,,,,, @2.66)032

+ 6Si0, or

-950-1000C 3(A1,0,.2Si02) A 0.562Si8(A1,,~,, @5,33)032

+ 3Si0, where @ represents a vacancy. A y-alumina-type phase (0.282A18(A11,,,3@2~66)03~) and an aluminosilicate spinel (0.562Si,(A1,0,6,@,,,,)03~) are the predicted reaction products. The silica reaction product is amorphous. The exact structure of the spinel phase continues to be controversial, and the literature presents conflicting evidence regarding the existence of

either phase. The details regarding the two phases are discussed in the microstructure section. Furthermore, the exact role of the spinel phase in the reaction sequence and the microstructure development has not been clearly established.

The amorphous silica liberated during the metakaolin decomposition is highly reactive, possibly assisting eutectic melt formation at -990C, as suggested by S ~ h u l l e r . ' ~ ~ . ' ~ ~ Lundin'O' suggests instead that amorphous silica transforms directly to cristobalite at -1050"C, but the general lack of cristobalite in modem commercial porcelain bodies suggests that the former scenario is more plausible.

As illustrated in Fig. 1, a eutectic melt of potash feldspar with silica starts appearing at -990C. The eutectic temperature is dependent on the type of feldspar: for potash feldspar, the eutectic melt forms at -990C; for soda feldspar, the eutectic melt forms at 1050"C, according to the K,O-Al,O,- SiO, and Na,O-Al,O,-SiO, phase diagrams, respec t i~e ly . '~~ The lower liquid formation temperature in the potash feldspar systems is beneficial for reducing the porcelain firing temperature. Also, the presence of albite in potash feldspar can reduce the liquid formation temperature by as much as 60C.101 As the temperature continues to increase, porosity is eliminated via viscous-phase sintering.

Porcelain bodies generally contain two different mullite evolution paths: primary and secondary. The exact source and temperatures for the formation of these two different types of mullites continue to be debated. However, the spinel phase,

(5)

(6)

(7)

14 Journal of the American Ceramic Society-Carty and Senapati Vol. 81, No. 1

being a nonequilibrium unstable phase, certainly transforms to mullite above 1075C. The chemical reactions describing the conversion to mullite are97

0.282A1,(All,,,, @2.66)032 - 3A120,.2Si02 + 4Si0, or

-1075C

. " I _ - 0.562Si8(A1,,~,, $5,33)032 - 3A120,.2Si02 + 4Si0, Early studies on the aluminosilicate crystal structures indicate the mullite structure to be identical to the sillimanite structure, with each having orthorhombic crystal lattices.102 Later studies, however, indicate that the stable form of aluminosilicates formed at higher temperatures (>lOOO"C) and atmospheric pressure is mullite, but the literature also presents unresolved issues as to the nature of the melting of mullite, i.e., whether it melts congruently or incongruently. ' Furthermore, additional work on mullitel'o has indicated that chemical homogeneity is essential to the formation of mullite at lower temperatures. On the atomic level, mullite formation can start at 980C, but homogeneity on the nanometer scale can delay mullite formation to temperatures as high as 1300C.

(8) At -1200"C, the melt becomes saturated with silica- quartz dissolution ends, and quartz-to-cristobalite transformation begins.

(9) Above 12OO0C, mullite crystals grow as prismatic crystals into the remains of the feldspar grains (referred to as feldspar relicts by Lundin'O').

As the body starts to cool, pyroplastic deformation and relaxation within the glass phase prevent the development of residual stresses until the glass transition temperature is reached. As the body cools below the glass transition temperature, residual stresses are developed because of thermal expansion mismatch between the glass and the included crystalline phases (i.e., mullite and quartz, and in some cases, alumina and cristobalite).

Cooling through the quartz inversion (573C) results in a quartz particle volume decrease of 2%,29,102 which can produce sufficient strain to cause cracking of the glassy matrix and the quartz grains. The cracking severity is dictated by the quartz particle size and the cooling rate. The effect of free quartz on the porcelain strength is discussed below.

Finally, the p- to a-cristobalite inversion at 225"- 250C is similar to the quartz inversion, but it produces a larger volumetric change (-5%); with a higher activation energy bar- rier, the transformation is less severe than that of q ~ a r t z . ~ ~ , ' ~ ~

(2) Metakaolinite Formation The microstructure development in any clay-based product

is triggered by the conversion of kaolin to metakaolin. One of the earliest works in this field by LeChatlier114 concluded, on the basis of thermal analysis and acid dissolution tests, that the free alumina on dissolution comes from the molecular transformation of kaolinite. Several researchers have shown that kaolin transforms to metakaolin at -50O"-55O0C, but previous studies115-119 have suggested a noncrystalline or amorphous structure for metakaolin. Brindley and Nakahira (BN),96,97 on basis of XRD studies, first proposed a crystal structure for metakaolin. The BN model for metakaolin maintains the a and b kaolinite lattice parameters in metakaolin, but the c-axis pe- riodicity disappears, leading to diffuse XRD patterns. BN also have indicated that the octahedral aluminum hydroxide layer of kaolinite is likely to be changed more than the tetrahedral silica layer during the dehydroxylation process and that the structure of metakaolin allows the kaolinite layers to collapse to -0.63 nm, in agreement with the measured densities of kaolin and metakaolin. The proposed BN metakaolin structure is illustrated in Fig. 14(a). Most of the metakaolin models published since the BN mode1120-126 ha ve been modifications of it. On the basis of computer simulation and nuclear magnetic reso- nance (NMR) studies, MacKenzie et d. lo4 have proposed that

(10)

(1 1)

(12)

a fully satisfactory metakaolin structure should account for the presence of 11%-12% residual hydroxyl groups (based on the initial kaolinite hydroxyl level), the cell parameters should be consistent with spectroscopy data, and the model should not contravene bonding rules. The BN model shows no hydroxyl groups, whereas the MacKenzie model, shown in Fig. 14(b), does incorporate hydroxyl groups in the A1-0 layers and sat- isfies most of the deficiencies of the other models. In the current authors' opinion, controversies such as the coordination of aluminum (more specifically, the ratio of fourfold to sixfold coordinated aluminum), the exact dimensions of the c param- eter, and the amount of hydroxyl groups in the A1-0 layers continue to be far from resolved, and further characterization and structural modeling is necessary.

The structure of the spinel phase that forms at -980C by the decomposition of metakaolinite also is controversial. Some re- s e a r c h e r ~ ~ ~ ~ , ' ~ ~ * ~ ~ ~ believe it to be an aluminum silicon spinel, whereas ~ t h e r ~ ~ ~ ~ J ~ ~ , ~ ~ ~ believe that it is a y-alumina phase. Okada et aZ.Io5 suggest that the decomposition product of metakaolin is a y-alumina phase, but containing silica according to chemical analysis. In summary, the structure of the spinel phase remains unclear. (3) Mullite Formation

Unresolved questions regarding the structure and identity of the spinel phase also have led to difficulties in understanding the formation of mullite in porcelain bodies. LundinIo1 has concluded through electron microscopy studies of porcelain bodies that the concentration gradients and diffusion rates of alkalis are the two most important factors affecting mullite formation. Comer'30 has demonstrated that mullite can preferentially orient on the surface of kaolinite relicts (presented in Fig. 15), and Lundin has proposed that the mullite in the clay relicts serves as a seed for the crystallization of the mullite needles in the feldspar relicts. The mullite formed in the clay relicts is generally termed the primary m ~ l l i t e , ~ ~ ~ , ~ ~ ~ because it forms at a lower temperature and is a product of the clay minerals. As alkali diffuses out of feldspar at higher temperatures, secondary mullite nucleates and grows. An electron micrograph of secondary mullite in a feldspar relict is shown in Fig. 16.131 Studies on the morphology of mullite have indicated that primary mullite occurs mostly in the form of scaly crystals,112.113 whereas secondary mullites are mostly needle- shaped crystal^.^^^^^^^ It is believed that the secondary mullites are formed from the recrystallization and dissolution of aluminosilicates in the melt.

Johnson and P a ~ k ' , ~ have suggested that impurities, such as Fe,03 and TiO,, affect the kinetics and morphology of mullite formation and that the exsolution of silica facilitates mullite formation in kaolin. In another study, Pask and T ~ m s i a ' ~ ~ have postulated that the heating rate is crucial to the crystal structure of mullite. Slow heating rates lead to formation of spinel at lower temperatures, which then reacts with silica by a diffusion-nucleation mechanism to form orthorhombic mullite. Fast heating rates lead to tetragonal mullite formation, with the aluminum ion in sixfold coordination at temperatures as low as 980C. The tetragonal mullite then converts to the desirable orthorhombic mullite at higher temperatures. Further studies involving high-temperature dynamic XRD are required to understand the mullite formation kinetics, morphology, and crystal structures in porcelain bodies.

Another source for mullite may be from the dissolution of alumina that has been substituted for quartz within a porcelain body. Figure 17 shows the appearance of mullite surrounding alumina grains and the formation of mullite needles into the glass phase in a commercial dinnerware body fired at cone 9.I3l TEM is necessary to verify that the surrounding phase is mullite and that alumina particle dissolution is the basis for additional mullite formation. (4) Cristobalite Formation

Lundinlol has indicated that, in a porcelain body with excess quartz, the cristobalite phase begins to form when the rate of

'

January 1998 Porcelain-Raw Materials, Processing, Phase Evolution. and Mechanical Behavior

I \ ' \ \

A I

' *+-.4 Y

I Y B

0 @ Oxygen i o n s , v a r i o u s p o s i t i o n s

A l u m i n u m S i l i c o n

o=AL o= 0 0= OH

0 = Si o=o

Fig. 14. Meta-kaolin models from (a) Bnndley and Nakahira96,97 and (b) Mackenzie et

transition of quartz to cristobalite exceeds the rate of quartz dissolution. The transformation of quartz to cristobalite is a solid-state reaction starting from the surface of the quartz grain. Schneider et al. 134 have shown that the transformation of quartz to cristobalite can be accelerated by high crystal defect density and small particles. It also is probable that the cristobalite may form as a devitrification product from the glass phase as the melt becomes saturated with silica. The potential for cristobalite formation becomes less likely, however, under the modem trend to shorter firing schedules and lower firing temperatures.

VI. Mechanical Properties

There are three major theories that have been developed to explain the strength of porcelains.

(1) Mullite Hypothesis One of the oldest theories on the strength of porcelain, the

mullite hypothesis, first proposed by Zoellner, 135 posits the strength of a porcelain body as solely dependent on the feltlike interlocking of fine mullite needles. Later versions of this hy- potheses have indicated that the strength increases with in-

creasing mullite ~ o n t e n t . ' ~ ~ - ' ~ ~ At higher temperatures, the mullite needles coarsen, leading to a smaller number of larger needles. The larger needles do not interlock as efficiently as the smaller ones, resulting in decreased strength. Hence, firing temperature and generating the correct amount of properly sized mullite needles are vital in achieving the desired strength. Furthermore, secondary mullite, because of its acicular morphology and smaller needle diameter, might increase strength more than primary mullite.

(2) Matrix Reinforcement Hypothesis The difference in thermal expansion coefficients between the

matrix (glassy phase, in the case of porcelain) and dispersed particles (such as quartz and alumina) or crystalline phases formed during firing (such as mullite and cristobalite) produces strong compressive stresses on the glassy phase. Such induced "thermal" compressive stresses due to thermal expansion m i s - match lead to strength improvements in the porcelain bodies. This idea of matrix reinforcement, widely referenced in the literature as the prestressed theory of strength improvement in porcelain,139 is discussed most often in regard to quartz particles in a glassy matrix. For a single spherical particle in an isotropic medium, the differences in thermal expansion coef-

16 Journal of the American Ceramic Society-Car9 and Senapati Vol. 81, No. 1

Fig. 17. SEM photomicrograph of a polished and HF-etched commercial dinnerware sample illustrating the apparent dissolution of alumina grains (denoted A) and mullite formation around the particles with mullite needles growing into the glassy matrix.131

Fig. 15. Replica illustrating a kaolinite particle fired to 1200C with preferred orientation of mullite crystals on the particle surface. I3O This represents primary mullite formed during the firing of a porcelain.

Fig. 18. Polished, but unetched, porcelain sample showing cracking associated with quartz grains (denoted Q). Also visible in the photomicrograph are alumina particles (denoted A).131

Fig. 16. SEM photomicrograph of secondary mullite formed within a feldspar relict in a commercial dinnerware porcelain sample. Sample was polished, then etched using a 10% HF solution for 20 s.l3I

ficients can lead to radial and tangential stresses. The total stress, P, on the particle can be given by140

AaAT 1 - l + u m 1-2vp

2Em E p +-

where Aa is the difference in expansion coefficients between the glassy matrix and the particle, AT the cooling range of the matrix-particle system, vm and up the Poisson's ratios; Em and Ep the elastic moduli, and the subscripts m and p the matrix and the particle, respectively.

The nature of cracks in porcelain bodies is dependent on the expansion coefficients of the matrix and the particle. If the particles contract more than the matrix, P is negative, resulting in circumferential cracking around the particles. This is true for quartz particles in the feldspathic glass matrix of the porcelain body. The stress generation and associated cracking due to the presence of quartz particles tend to be severe because of the rapid displacive phase tradsformation of quartz during cooling.

If the matrix contracts more than the particle, then P is positive, resulting in radial cracks emapating from the particles, which could easily connect and cause deleterious strength. There is no evidence of particles or crystalline phases having lower expansion than the glassy phase in porcelain bodies; hence, the second effect can be ignored.

Warshaw and SeiderI4l have demonstrated that particle size is crucial to crack development in the porcelain body. The electron micrographs of larger quartz grains (50-150 pm) exhibit continuous peripheral fracture at or near the grain bound- aries and interconnected matrix fractures: those between 25 and 50 pm exhibit less-severe pgipheral fracture and rare matrix fractures, and those


to some observed cracks on surfaces because of stress release. Warshaw and Seider also have shown that alumina particles in porcelain bodies increase strength. The current authors support the idea, suggested first by Hasselman and F ~ l r a t h , ~ ~ that this is due to dispersion strengthening of the matrix and not to matrix reinforcement.

There is an increasing trend among porcelain manufacturers to use nepheline syenite instead of feldspar as a flux. Holm- ~ t r o m ~ ~ . ~ has attributed this trend to several factors. First, nepheline syenite contains less free quartz than feldspar. Sec- ond, nepheline syenite results in a glassy matrix whose thermal expansion is closer to that of the residual quartz crystals, thus reducing the potential for cracking due to the quartz inversion. Kristoffersson et al. 44 have noted similar effects of nepheline syenite on strength increase of porcelain bodies.

(3) Dispersion-Strengthening Hypothesis The dispersion-strengthening hypothesis proposes that the

dispersed particles limit the size of Griffith flaws, leading to increased strength. Hasselman and Fulrath also have studied the effect of alumina spheres in glass on the strength of glass- ceramic composites, with the expansion coefficient of the glass matched to that of alumina. The results of strength and fracture studies indicate that the strength of the glass-ceramic compos- ite is a function of the volume fraction of the dispersed phase at low volume fractions; at high volume fractions of the dispersed phase, the strength is dependent on the volume fraction and the particle size of the dispersed phase. Maity et ~ 1 . ~ ~ ~ 3 ~ ~ ~ have shown that the replacement of some of the quartz by sillimanite and some of the feldspar by cordierite increases strength. They have hypothesized that strength increase is a dispersion-strengthening effect, because sillimanite and cordierite act as dispersant solids in the glassy matrix. B l ~ d g e t t ~ ~ and Harada et aZ.148 also have indicated strength increase in porcelain bodies with the addition of alumina particles. Like- wise, they support a strengthening mechanism because of stronger particles, as observed when zircon is added to porcelain bodies.

In porcelain bodies, the thermal expansion coefficients of the glass matrix rarely match those of the dispersed particles; hence, there is always a strengthening effect because of matrix reinforcement. Furthermore, interlocking mullite needles always are formed because of the firing temperature and kinetics. Hence, a universal theory of strength in porcelain bodies should account for all the above mechanisms for strengthening.

(4) Overall Strength Considerations Intrinsic flaw size is perhaps the predominant factor affect-

ing the strength of porcelain bodies and depends very much on the microstructure. Although the simplest intrinsic flaw in a ceramic can be a pore, the presence of a glassy phase tends to generate nearly spherical pores in a highly dense matrix, thus improving the strength of porcelains. In the absence of pores, strength is dictated mostly by the presence of preexisting cracks. The typical strength-controlling factors in multiphase polycrystalline ceramics are thermal expansion coefficients of the phases, elastic properties of the phases, volume fraction of different phases, particle size of the crystalline phase, and phase transformations. All of these factors are present in porcelain systems and are dependent on the extent of the firing process. In terms of strength reduction, phase transformation is probably the most important because of the presence of quartz and cristobalite.

(5) Effect of Quartz and Cristobalite on Strength The presence of cristobalite, instead of quartz, in the porce-

lain body often produces an increase in strength, despite the fact that cristobalite experiences a larger displacive transformation than quartz during cooling. The increase in strength of cristobalite porcelains can be explained via three perspectives: the cristobalite grains tend to be much smaller than the quartz

grains because of being crystallized either from the glass phase or by the direct conversion of quartz; the cristobalite is formed at the expense of quartz, consequently reducing the quartz content of the body; and the cristobalite inversion temperature is lower, 225-25OoC, compared to 573C for the quartz inversion, leading to lower strain development during the cooling process. The lower inversion temperature, however, also may pose a potential cyclic fatigue problem for porcelains that may be heated and cooled during use (such as commercial dinnerware).

VII. Conclusions

Because of the complex interplay between raw materials, processing routes and approaches, and the kinetics of the firing process, porcelains represent some of the most complicated ceramic systems. The introduction of automated processing schemes has required the development of fundamental understanding, but, despite the immense amount of existing and ongoing research, this understanding remains limited. Major challenges to the development of porcelains and the improvement of commercial manufacturing processes remain, including in-depth understanding of the colloidal nature of the raw materials necessary for improved control; a reliable means of quickly quantifying and understanding plasticity in clay systems; the means to reliably predict the microstructure and phase evolution in porcelain systems, whether composed of three primary components or several raw materials; and theoretical explanations regarding the roles of the crystalline phases on firing, consistent with the observed mechanical properties. From a research perspective, the study of porcelains continues to present multiple, complex research challenges, offering significant opportunities for growth and development well into the foreseeable future.

Acknowledgments: The authors would like to thank J. Howles, M. Tindale, and C. Perry for their assistance with the XRD patterns and for their help preparing several of the figures.

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