Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering

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Silicate Glasses The silicate glasses are the most widely used. Fused silica, formed

from pure SiO

, has a high-melting point, and the dimensional changes during heating

and cooling are small (Figure 15-6). Generally, however, the silicate glasses contain

additional oxides (Table 15-5). While oxides such as silica behave as glass formers,an

intermediate oxide (such as lead oxide or aluminum oxide) does not form a glass by

itself but is incorporated into the network structure of the glass formers. A third group

of oxides, the modiﬁers, break up the network structure and eventually cause the glass

to devitrify, or crystallize.

Modified Silicate Glasses Modiﬁers break up the silica network if the oxygen-to-

silicon ratio (O:Si) increases signiﬁcantly. When Na

O is added to silica glass, for ex-

ample, the sodium ions enter holes within the network rather than becoming part of the

network. However, the oxygen ion that enters with the Na

O does become part of the

network (Figure 15-7). When this happens, there are not enough silicon ions to combine

with the extra oxygen ions and keep the network intact. Eventually, a high O:Si ratio

causes the remaining silica tetrahedra to form chains, rings, or compounds, and the

silica no longer transforms to a glass. When the O:Si ratio is above about 2.5, silica

glasses are di‰cult to form; above a ratio of three, a glass forms only when special

precautions are taken, such as the use of rapid cooling rates.

Modiﬁcation also lowers the melting point and viscosity of silica, making it possible

to produce glass at lower temperatures. The e¤ect of Na

O additions to silica is shown

in Figure 15-8. As you can see, the addition of Na

O produces eutectics with very low

melting temperatures. Adding CaO, which reduces the solubility of the glass in water,

further modiﬁes these glasses. The example that follows shows how to design a glass.

Figure 15-6

The expansion of quartz. In addition

to the regular—almost linear—

expansion, a large, abrupt expansion

accompanies the a-tob-quartz

transformation. However, glasses

expand uniformly.

TABLE 15-5 9 Division of the oxides into glass

formers, intermediates, and modifiers

Glass Formers Intermediates Modifiers

TiO

SiO

ZnO MgO

GeO

PbO

CaO

PbO

BeO Na

CHAPTER 15 Ceramic Materials480

Figure 15-7 The effect of Na

O on the silica glass network. Sodium oxide is a modifier,

disrupting the glassy network and reducing the ability to form a glass.

Figure 15-8

The SiO

-Na

O phase diagram.

Additions of soda (Na

O) to silica

dramatically reduce the melting

temperature of silica by forming

eutectics.

EXAMPLE 15-2

Design of a Glass

We produce good chemical resistance in a glass when we introduce B

into

silica. To assure that we have good glass-forming tendencies, we wish the O:Si

ratio to be no more than 2.5, but we also want the glassware to have a low-

melting temperature to make the glass-forming process easier and more eco-

nomical. Design such a glass.

SOLUTION

Because B

reduces the melting temperature of silica, we would like to add

as much as possible. We also, however, want to assure that the O:Si ratio is no

more than 2.5, so the amount of B

is limited. As an example, let us deter-

mine the amount of B

we must add to obtain exactly an O:Si ratio of 2.5.

Let f

be the mole fraction of B

added to the glass, and ð1  f

Þ be the

mole fraction of SiO

15-5 In organic Glasses 481

O ions



ðf

Þþ 2

O ions

SiO



ð1  f

Si ion

SiO



ð1  f

¼ 2:5

þ 2  2f

¼ 2:5  2:5f

or f

¼ 0:143

Therefore, we must produce a glass containing no more than 14.3 mol% B

In weight percent:

wt% B

ðf

Þð69:62 g=molÞ

ðf

Þð69:62 g=molÞþð1  f

Þð60:08 g=molÞ

 100

wt% B

ð0:143Þð69:62Þ

ð0:143Þð69:62Þþð0:857Þð60:08Þ

 100 ¼ 16:2

Glasses are manufactured into useful articles at a high temperature with viscosity con-

trolled so that the glass can be shaped without breaking. Figure 15-9 helps us under-

stand the processing in terms of the viscosity ranges.

1. Liquid range. Sheet and plate glass are produced when the glass is in the molten

state. Techniques include rolling the molten glass through water-cooled rolls or ﬂoating

Figure 15-9

The effect of temperature

and composition on the

viscosity of glass.

CHAPTER 15 Ceramic Materials482

the molten glass over a pool of liquid tin (Figure 15-10). This ﬂoat-glass process pro-

duces an exceptionally smo oth surface on the glass. The development of the ﬂoat-glass

process was a genuine breakthrough in the area of glass processing. Basic ﬂoat-glass

composition has been essentially unchanged for many years (Table 15-6). Glasses for

automotive applications contain small amounts of iron oxide to control absorption of

light and infrared radiation.

Some glass shapes, including large optical mirror s, are produced by casting the

molten glass into a mold, then assuring that cooling is as slow as possible to minimize

residual stresses and avoid cracking of the glass part. Glass ﬁbers may be produced by

drawing the liquid glass through small openings in a platinum die [Figure 15-11(c)].

Typically, many ﬁbers are produced simultaneously for a single die.

2. Working range. Shapes such as those of containers or light bulbs can be formed

by pressing, drawing, or blowing glass into molds (Figure 15-11). A hot gob of liquid

Figure 15-10 Techniques for manufacturing sheet and plate glass: (a) rolling and (b) floating

the glass on molten tin.

TABLE 15-6 9 Compositions of typical glasses (in weight percent)

Glass SiO

CaO Na

MgO PbO Others

Fused silica 99

Vycor

96 4

Pyrex

81 2 4 12

Glass jars 74 1 5 15 4

Window glass 72 1 10 14 2

Plate glass/Float glass 73 1 13 13

Light bulbs 74 1 5 16 4

Fibers 54 14 16 10 4

Thermometer 73 6 10 10

Lead glass or crystal 67 6 17 10% K

Optical flint 50 1 19 13% BaO, 8% K

O, ZnO

Optical crown 70 8 10 2% BaO, 8% K

E-glass fibers 55 15 20 10

S-glass fibers 65 25 10

15-5 In organic Glasses 483

glass may be preformed into a crude shape (parison), then pressed or blown into a

heated die to produce the ﬁnal shape. The glass is heated to the working range so that

the glass is formable, but not ‘‘runny.’’

3. Annealing range. Some glass parts may be annealed to reduce residual stresses

introduced during forming. Large glass castings, for example, are often annealed and

slowly cooled to prevent cracking. Some glasses may be heat treated to cause devitriﬁ-

cation, or the precipitation of a crystalline phase from the glass.

Tempered glass is produced by quenching the surface of plate glass with air, causing

the surface layers to cool and contract. When the center cools, its contraction is re-

strained by the already rigid surface, which is placed in compression (Figure 15-12). Tem-

pered glass is capable of withstanding much higher tensile stresses and impact blows

than untempered glass. Tempered glass is used in car and home windows, shelving for

refrigerators, ovens, furniture, and many other applications where safety is important.

Laminated glass, consisting of two annealed glass pieces with a polymer (such as poly-

vinyl butyral or PVB) in between, is shatterproof and used to make car windshields.

Glass Compositions Pure SiO

must be heated to very high temperatures to obtain

viscosities that permit economical forming. Most commercial glasses are based on silica

(Table 15-6); modiﬁers such as soda (Na

O) are added to break down the network

structure and form eutectics with low melting temperatures, whereas lime (CaO) is

added to reduce the solubili ty of the glass in water. The most common commercial glass

Figure 15-11 Techniques for forming glass products: (a) pressing, (b) press and blow process,

and (c) drawing of fibers.

Figure 15-12

Tempered glass is cooled

rapidly to produce

compressive residual

stresses at the surface.

CHAPTER 15 Ceramic Materials484

contains approximately 75% SiO

, 15% Na

O, and 10% CaO and is known as soda-

lime glass.

Borosilicate glasses, which contain about 15% B

, have excellent chemical and

dimensional stability. Their uses include laboratory glassware (Pyrex

) and containers

for the disposal of high-level radioactive nuclear waste. Calcium aluminoborosilicate

glass—or E-glass—is used as a general-purpose ﬁber for composite materials, such as

ﬁberglass. Aluminosil icate glass, with 20% Al

and 12% MgO, and high-silica

glasses, with 3% B

, are excellent for high-temperature resistance and for protection

against heat or thermal shock. The S-glass, a magnesium aluminosilicate, is used to

produce high-strength ﬁbers for composite materials. Fused silica, or virtually pure

SiO

, has the best resistance to high temperature, thermal shock, and chemical attack,

although it is also expensive.

Special optical qualities can also be obtained, including sensitivity to light. Photo-

chromic glass, which is darkened by the ultraviolet portion of sunlight, is used for sun-

glasses. Photosensitive glass darkens permanently when exposed to ultraviolet light;

if only selected portions of the glass are exposed and then immersed in hydroﬂuoric

acid, etchings can be produced. Polychromatic glasses are sensitive to all light, not just

ultraviolet radiation. Similarly, nanosized crystals of semiconductors such as cadmium

sulﬁde (CdS) are nucleated in silicate glasses in a process known as striking. These

glasses exhibit lively colors and also have useful optical properties.

15-6 Glass-Ceramics

Glass-ceramics are crystalline materials that are derived from amorphous glasses. Usu-

ally, glass-ceramics have a substantial level of crystallinity (@>70–99%). The formation

of glass-ceramics was discovered serendipitously by Don Stookey. With glass-ceramics,

we can take advantage of the formability of glass. Also a product that contains very

low porosity can be obtained by producing a shape with conventional glass-forming

techniques, such as pressing or blowing. Glass, however, has poor creep resistance. We

then crystallize the glass using heterogeneous nucleation by such oxides as TiO

and/or

ZrO

. These oxides react with the glass and with each other and provide the nuclei that

ultimately lead to glass crystallization. Phase separation of glasses plays an important

role in formation of the nuclei. In some commercial glass-ceramics (e.g., Visionware

)

nano-sized crystallites are formed and the resultant material remains optically trans-

parent.

The ﬁrst step in producing a glass-ceramic is to assure that crystallization does not

occur during cooling from the forming temperature. A continuous and isothermal

cooling transformation diagram, much like the CCT and TTT diagrams for steels, can

be seen for silicate-based glasses. Figure 15-13(a) shows a TTT diagram for a glass. If

glass cools too slowly, a transformation line is crossed ; nucleation and growth of the

crystals begin, but in an uncontrolled manner. Addition of modif ying oxides to glass,

much like addition of alloying elements to steel, shifts the transformation curve to

longer times and prevents devitriﬁcation even at slow cooling rates. As noted in pre-

vious chapters, strictly speaking we should make use of CCT (and not TTT) diagrams

for this discussion.

Nucleation of the crystalline phase is controlled in two ways. First, the glass con-

tains agents, such as TiO

, that react with other oxides and form phases that provide

the nucleation sites. Second, a heat treatment is designed to provide the appropriate

number of nuclei; the temperature should be relatively low in order to maximize the rate

15-6 Glass-Ceramics 485

Figure 15-13 Producing a glass-ceramic: (a) Cooling must be rapid to avoid the start of

crystallization. Isothermal and continuous cooling curves for lunar glass. (b) The rate of

nucleation of precipitates is high at low temperatures, whereas the rate of growth of the

precipitates is high at higher temperatures. (c) A typical heat-treatment profile for glass-

ceramics fabrication, illustrated here for Li

O-Al

-SiO

glasses. (d) A large mirror blank of

the Zerodur

made by Schott Glass. (Source: Photo Courtesy of Schott Glass.)

CHAPTER 15 Ceramic Materials486

of nucleation [Figure 15-13(b)]. However, the overall rate of crystallization depends on

the growth rate of the crystals once nucleation occurs; higher temperatures are required

to maximize the growth rate. Consequently, a heat-treatment schedule similar to that

shown in Figure 15-13(c) for the Li

O-Al

-SiO

glass-ceramics (Pyroceram

) can be

used. The low-temperature step provides nucleation sites, and the high-temperature step

speeds the rate of growth of the crystals; as much as 99% of the part may crystallize.

This special structure of glass-ceramics can provide good mechanical strength and

toughness, often with a low coe‰cient of thermal expansion and high-t emperature

corrosion resistance. Perhaps the most important glass-ceramic is based on the Li

O-

-SiO

system. These materials are used for cooking utensils (Corning Ware

)

and ceramic tops for stoves. Other glass-ceramics are used in communication, com-

puter, and optical applications. Figure 15-13(d) shows the processing of a large

telescope mirror being prepared using the Zerodur

glass-ceramic materia l. This

glass-ceramic is especially suited for telescope mirror blanks because it has a very small

coe‰cient of thermal expansion (Chapter 2).

15-7 Processing and Applications of Clay Products

Crystalline ceramics are often manufactured into useful articles by preparing a shape, or

compact, composed of the raw materials in a ﬁne powder form. The powders are then

bonded by chemical reaction, partial or complete vitriﬁcation (melting), or sintering.

Clay products form a group of traditional ceramics used for producing pipe, brick,

cooking ware, and other common products. Clay, such as kaolinite, and water serve as

the initial binder for the ceramic powders, which are typically silica. Other materials,

such as feldspar [(K, Na)

O  Al

 6SiO

] serve as ﬂuxing (glass-forming) agents dur-

ing later heat treatment.

Forming Techniques for Clay Products The powders, clay, ﬂux, and water are mixed

and formed into a shape (Figures 15-3 and 15-4). Dry or semi-dry mixtures are me-

chanically pressed into ‘‘green’’ (unbaked) shapes of su‰cient strength to be handled.

For more uniform compaction of complex shapes, isostatic pressing may be done; the

powders are placed into a rubber mold and subjected to high pressures through a gas or

liquid medium. Higher moisture contents permit the powders to be more plastic or

formable. Hydroplastic forming processes, including extrusion, jiggering, and hand

working, can be applied to these plastic mixes. Ceramic slurries containing large

amounts of organic plasticizers, rather than water, can be injected into molds.

Still higher moisture contents permit the formation of a slip, or pourable slurry,

containing ﬁne ceramic powder. The slip is poured into a porous mold. The water in the

slip nearest to the mold wall is drawn into the mold, leaving behind a soft solid which

has a low-moisture content. When enough water has been drawn from the slip to pro-

duce a desired thickness of solid, the remain ing liquid slip is poured from the mold,

leaving behind a hollow shell (Figure 15-4). Slip casting is used in manufacturing

washbasins and other commercial products. After forming, the ceramic bodies—or

greenware—are still weak, contain water or other lubricants, and are porous, and sub-

sequent drying and ﬁring are required.

Drying and Firing of Clay Products During drying, excess moisture is removed and

large dimensional changes occur. Initially, the water between the clay platelets—the

interparticle water—evaporates and provides most of the shrinkage. Relatively little

15-7 Proce ssing and Applications of Clay Products 487

dimensional change occurs as the remaining wa ter between the pores evaporates. The

temperature and humidity are carefully controlled to provide uniform drying through-

out the part, thus minimizing stresses, distortion, and cracking.

The rigidity and strength of a ceramic part are obtained during ﬁring. During

heating, the clay dehydrates, eliminating the hydrated water that is part of the kaolinite

crystal structure, and vitriﬁcation begins (Figure 15-14). Impurities and the ﬂuxing

agent react with the ceramic particles (SiO

) and clay, producing a low-melting-point

liquid phase at the grain surfaces. The liquid helps eliminate porosity and, after cooling,

changes to a rigid glass that binds the ceramic particles. This glassy phase provides a

ceramic bond, but it also causes additional shrinkage of the entire ceramic body.

The grain size of the ﬁnal part is determined primarily by the size of the original

powder particles. Furthermore, as the amount of ﬂux increases, the melting temperature

decreases; more glass forms, and the pores become rounder and smaller. A smaller ini-

tial grain size accelerates this process by providing more surface area at which vitri-

ﬁcation can occur.

Applications of Clay Products Many structural clay products and whitewares are

produced using these processes. Brick and tile used for construction are pressed or ex-

truded into shape, dried, and ﬁred to produce the ceramic bond. Higher ﬁring temper-

atures or ﬁner original particle sizes produce more vitriﬁcation, less porosity, and higher

density. The higher density improves mechanical properties but reduces the insulating

qualities of the brick or tile.

Earthenware are porous clay bodies ﬁred at relatively low temperatures. Little

vitriﬁcation occurs; the porosity is very high and interconnected, and earthenware

ceramics may leak. Consequently, these products must be covered with an impermeable

glaze.

Higher ﬁring temperatures, which provide more vitriﬁcation and less porosity,

produce stoneware. The stoneware, which is used for drainage and sewer pipe, contains

only 2% to 4% porosity. Ceramics known as china and porcelain require even higher

ﬁring temperatures to cause complete vitriﬁcation and virtually no porosity.

15-8 Refractories

Refractory materials are important components of the equipment used in the produc-

tion, reﬁning, and handling of metals and glasses, for constructing heat-treating furna-

ces, and for other high-temperature processing equipment. The refractories must surviv e

at high temperatu res without being corroded or we akened by the surrounding environ-

Figure 15-14

During firing, clay and other fluxing

materials react with coarser particles to

produce a glassy bond and reduce porosity.

CHAPTER 15 Ceramic Materials488

ment. Typical refractories are composed of coarse oxide particles bonded by a ﬁner

refractory material. The ﬁner material melts during ﬁring, providing bonding. In some

cases, refractory bricks contain about 20 to 25% apparent porosity to provide improved

thermal insulation.

Refractories are often divided into three groups—acid, basic, and neutral—based

on their chemical behavior (Table 15-7).

Acid Refractories Common acidic refractories include silica, alumina, and ﬁreclay

(an impure kaolinite). Pure silica is sometimes used to contain molten metal. In some

applications, the silica may be bonded with small amounts of boron oxide, which melts

and produces the ceramic bond. When a small amount of alumina is added to silica, the

refractory contains a very low-melting-point eutectic microconstituent (Figure 15-15)

and is not suited for refractory applications at temperatures above about 1600



C, a

temperature often required for steel making. However, when larger amounts of alumina

are added, the microstructure contains increasing amounts of mullite, 3Al

 2SiO

which has a high melting temperature. These ﬁreclay refractories are generally relatively

weak, but they are inexpensive. Alumina concentrations above about 50% constitute

the high-alumina refractories.

TABLE 15-7 9 Compositions of typical refractories (weight percents)

Refractory SiO

MgO Fe

Acidic

Silica 95–97

Superduty firebrick 51–53 43–44

High-alumina firebrick 10–45 50–80

Basic

Magnesite 83–93 2–7

Olivine 43 57

Neutral

Chromite 3–13 12–30 10–20 12–25 30–50

Chromite-magnesite 2–8 20–24 30–39 9–12 30–50

(From Ceramic Data Book, Cahners Publishing Co., 1982.)

Figure 15-15

A simplified SiO

-Al

phase

diagram, the basis for alumina

silicate refractories.

15-8 Refractories 489

Askeland D.R., Fulay P.P. Essentials of Materials Science &amp; Engineering

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Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering