glass ceramic technology

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Wolfram Holand
I
vo clar Viva den
t
A
G
and
George
Beall
Co
m
ing
Incorporated
Published by The American Ceramic Society,
735
Ceramic Place, Westerville,
OH
43081
The American Ceramic Society
735
Ceramic Place
Westerville, Ohio
4308
1
0
2002
by The American Ceramic Society.
All

rights reserved.
Printed in the United States of America.
05 04 03 02
5
4 3 2
1
ISBN:
1-57498-107-2
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Introduction

xi
History

xv
)1/
Principles
of
Designing Glass-Ceramic Formation

1
1.1 ADVANTAGES
OF
GLASS-CERAMIC FORMATION

1
1.1.1 Processing Properties

3
1.1.2 Thermal Properties

3
1.1.3 Optical Properties

3
1.1.4 Chemical Properties

3
1.1.5 Biological Properties

4
1.1.6 Mechanical Properties

4
1.1.7 Electrical and Magnetic Properties

4
1.2 FACTORS
OF
DESIGN

5
1.3 CRYSTAL STRUCTURES AND MINERAL PROPERTIES

5

1.3.1 Crystalline Silicates

6
1.3.1.
1
Nesosilicutes

6
1.3.1.2
Sorosilicotes

7
1.3.7.3 Cyc/osi/icotes

7
1.3.7.4 lnosilicates

8
7.3.
I
.
5
Phyllosilicotes

8
1.3.1.6 Tectosilicotes

9
1.3.2 Phosphates

32
1.3.2.1
Apatite

32
1.3.2.2
Orthophosphotes
ond
Diphosphates

35
1.3.2.3. Metophosphotes

37
1.4 NUCLEATION

38
1.4.1 Homogeneous Nucleation

40
1.4.2 Heterogeneous Nucleation

42
V
GLASS-CERAMIC
TECHNOLOGY
1.4.3
1.4.4
Kinetics
of

Homogeneous and Heterogeneous Nucleation

43
Examples for Applying the Nucleation Theory in the
Development
of
Glass-Ceramics

46
1.4.4.
1
Volume Nucleation

46
1.4.4.2 Surface Nucleation

53
1
.
4.4.3
Temperature-Time-Transformation Diagrams

55
1.5 CRYSTALGROWTH

57
1 S.1 Primary Growth

59
1 S.2 Anisotropic Growth

60
1.5.3 SurfaceGrowth

66
1 S.4 Dendritic and Spherulitic Crystallization

69
1.5.4.7 Phenomenology

69
1.5.4.2 Dendritic and Spherulitic Crystallization Applications

71
1
S.5
Secondary Grain Growth

72
121
Composition Systems
for
Glass-Ceramics

75
2.1 ALKALINE AND ALKALINE-EARTH SILICATES

75
2.1.1
50,-Li,

0
(Lithium Disilicate)

75
2.7.1.
1
Stoichiometric Composition

75
2.1.1.2
Nonstoichiometric Compositions

77
Si0,- Boo (Sanbornite)

84
2.1.2.
I
Stoichiometric Barium-Disilicate

84
2
.
1
.
2.2
Multicomponent Glass-Ceramics

85
2.2 ALUMlNOSlllCATES

86
2.1.2
2.2.1 50,-AI,O, (Mullite)

86
2.2.2 Si0 Al.0 li.
0
(p-Quartz Solid Solution. p-Spodumene Solid Solution)

88
2.2.2.
1
pQuartz
Solid
Solution
Glass-Ceramics

89
2.2.2.2
p-Spodumene Solid
Solution
Glass-Ceramics

94
2.2.3 90,-Al,O,-No,
0
(Nepheline)

97

2.2.4 Si0,-A1,03-Cs,
0
(Pollucite)

100
2.2.5 SiO,-AI,O,- Mg0 (Cordierite. Enstatite)

104
2.2.5.
1
Cordierite Glass-Ceramics

104
2.2.5.2 Enstatite Glass-Ceramics

108
50,-AI,O,- Coo (Wollastonite)

110 2.2.6
Contents
2.2.7
SiO,.AI,O,.Zn 0.Mg0 (Spinel. Gahnite)

112
2.2.7.
I
Spinel Gluss-Cemmic
without
p-Quartz

112
2.2.7.2
p-Quurtz-Spine1 Glusderumics

114
50,-AI,O,-
Coo
(Slag
Sital)

115
90,-AI,O,-K,
0
(Leucite)

119
2.2.8
2.2.9
2.3 FLOUROSlllCATES

124
2.3.1
Si0,.R(lll),0,.Mg0.R(ll)0.R(l),
0.F
(Mica)

124
2.3.7.
I
Alkuline Phlogopite Gloss-Cerumics

125
2.3.
I
.
2
Alkuli-free Phlogopite Glusderumics

129
2.3.1.3
Alkuline-free Tetrusilicic Micu Gluss-Cerumic

131
50,-AI,O,-Mg0-CaO-Zr0,-
F
(Mica, Zirconia)

132 2.3.2
2.3.3
2.3.4
50,-CaO-R,
O-F
(Canasite)

134
SiO,-MgO-CoO-R(I),
O-F
(Amphibole)

140

2.4 SlllCOPHOSPHATES

145
2.4.1 50,-CaO-Na,O-P,O, (Apatite)

145
2.4.2.
2.4.3
2.4.4
2.4.5
2.4.6
SiO,.MgO.CoO-P,O,.
F
(Apatite. Wollastonite)

145
Si0,-MgO-Na,O-K,O-Ca O-P,O, (Apatite)

147
Si0,-AI,O3-Mg0-Ca0-Na,O-K~0-P~O
5-F
(Mica. Apatite)

148
SiO2-Mg0-CaO-Ti0,-P,
0,
(Apatite. Magnesium Titanate)

152
SiO,-AI,O,-CaO-No,O-K, O-P,O, (Apatite. leucite)

154
2.4.6.
I
Monolithic Glusderumics

156
2.4.6.2
Sintered Gluderumics

160
Si0,-AI,O3-Ca0-No,0-P~O
,-F
(Needlelike Apatite)

161
2.5 IRON SlllCATES

161
2.4.7
2.5.1 SiO,-Fe,O,- Coo

161
SiO,.AI,O,.Fe,O,.R(I) O.R(Il)O (Basalt)

165
2.5.2
2.5.3
SiO,.AI,O,.FeO.Fe,O,.K,
0

(Mica. Ferrite)

162
2.6. PHOSPHATES

167
2.6.1 P,O,-
COO
(Metaphosphates)

167
2.6.2 P,O,-CaO-TiO,

171
P,O,-No, O-BOO and P.0 Ti0 WO.

172
2.6.3.7
P.O I
u.O-Eu0
System

172
2.6.3
2.6.3.2
P205-Ti02-W03
System

173
GLASS-CERAMIC

TECHNOLOGY
U
2.6.4
P,05-AI,0,- COO (Apatite).
P.0
COO
(Metaphosphate)

173
2.6.4.7
Pz05-A/z0,-
CaO
(Apatite)

173
2.6.4.2
Pz05-Ca0 (Me faphosphate)

175
2.6.5 P,O,-B,O,-SiO,

175
2.6.6 P,O,-50,-Li,O-ZrO,

178
2.6.6.
I
Glass-Ceramics Containing
I6
wt%

ZrO,

180
2.6.6.2
G/ass-Ceramics Containing
20
wt%
ZrO,

180
2.7 OTHERS SYSTEMS

183
2.7.1 Perovskite-Type Glass-Ceramics

183
2.7.7.1
SiO,-Nb,O,-Na,
O-(BaO)

183
2.7.1.2
SiOz-A/~03-li02-Pb0

184
2.7.1.3
SiO~-A/~03-K,0-~~,0 ,-Nb2O5

186
2.7.2. Ilmenite-Type (Si0,-Al,03-Li,O-Ta,O,

)
Glass-Ceramics

187
2.7.3
B203-BaFe12019 (Barium Hexaferrite)
or
(BaFe,,O,,
)
Barium Ferrite

187
2.7.4
Si02-AI,0,-Ba0-Ti0, (Barium Titanate)

188
2.7.5
Bi203-Sr0-CaO-Cu0

190
131
Microstructure Control

191
3.1 SOLID-STATE REACTIONS

191
3.1.1 lsochemichal Phase Transformation

191

3.1.2 Reaction Between Phases

192
3.1.3 Exsolution

192
3.1.4 Use of Phase Diagrams to Predict Glass-Ceramic Assemblages

193
3.2 MICROSTRUCTURE

194
3.2.1 Nanocrystalline Microstructures

194
3.2.2 Cellular Membrane Microstructures

196
3.2.3 Coast-and-Island Microstructure

198
3.2.4 Dendritic Microstructures

200
3.2.5 Relict Microstructures

204
3.2.6 House-of-Cards Microstructures

205

3.2.6.
I
Nucleation Reactions

206
3.2.6.2
Primary Crystal Formation
and
Mica Precipitation

206
3.2.7 Cabbage-Head Microstructures

208
Contents
R
3.2.8 Acicular Interlocking Microstructures

213
3.2.9 Lamellar Twinned Microstructures

216
3.2.1
0
Preferred Crystal Orientation

218
3.2.1 1
Crystal Network Microstructures

219
3.2.1 2
Nature as an Example

219
3.3 CONTROL
OF
KEY
PROPERTIES

221
3.4 METHODS AND MEASUREMENTS

222
3.4.1
3.4.2
3.4.3
3.4.4
3.4.5
Chemical System and Crystalline Phases

222
Determination of Crystal Phases

223
Kinetic Process of Crystal Formation

224
Design
of

Microstructure

226
.Mechanical, Optical, Electrical, Chemical. and Biological Properties

227
Applications
of
Glass-Ceramics

229
4.1 TECHNICAL APPLICATIONS

229
4.1.1 Radomes

229
4.1.2
Photosensitive and Etched Patterned Materials

229
4.1.2.1 fotoform@
and
fotoceram@

230
4.1.2.2
foturan@

232

4.1.2.3 Additional Products

235
4.1.3 Machinable Glass-Ceramics

236
4.7.3.1
MACOP
and
DKOP

236
4.1.3.2 KtronitTM

240
4.1.3.3 PhotoveelTM

241
Magnetic Memory Disk Substrates

241
Liquid Crystal Displays

245
4.2 CONSUMER APPLICATIONS

247
4.1.4
4.1.5
4.2.1

4.2.2
p-Spodumene Solid Solution Glass-Ceramics

247
p-Quartz Solid Solution Glass-Ceramics

248
4.3 OPTICAL APPLICATIONS

253
4.3.1. Telescope Mirrors

253
4.3.7.
1
Requirements for Their Development

253
4.3.
1
.
2
ZeroduP Glass-Ceramics

253
I
I
GLASS-CERAMIC
TECHNOLOGY
4.3.2

4.3.3
Integrated Lens Arrays

255
Applications for Luminescent Glass-Ceramics

258
4.3.3.
I
Cr-Doped Mullite for Solar Concentrators

258
4.3.3.2 Rare-Earth-Doped Oxyfluorides for Upconversion and Amplification

260
4.3.4 Optical Components

263
4.3.4.
1
Glass-Ceramics for fiber Bragg Grating Athermalizatian

263
4.3.4.2 Glass-Ceramic ferrule for Optical Connectors

272
4.4 MEDICAL AND DENTAL GLASS-CERAMICS

272
4.4.1

Glass-Ceramics
for
Medical Applications

274
4.4.1.7 CERABONE@

274
4.4.1.2 CERAVlflP

276
4.4.1.3 BIOVERIP

276
4.4.2
Glass-Ceramics for Dental Restoration

4.4.2.
I
Requirements for Their Development

4.4.2.2
DICOP

4.4.2.3 IPS EM PRESS@ Glass-Ceramic

4.4.2.4 IPS Empress@ Cosmo Glass-Ceramic for Dental Core Buildups
.
4.4.2.5 IPS EmpresP2 Gloss-Ceramic

4.4.2.6. IPS d.SIGN@ Glass-Ceramic

4.4.2.7 Pro CAP

4.4.2.8 IPS ERlS for
E2

4.5 ELECTRICAL AND ELECTRONIC APPLICATIONS


277

277

279

282

287

291

301

307

308

309
4.5.1 Insulators

309
4.5.2 Electronic Pockaging

310
4.5.2.
I
Requirements for Their Development

310
4.5.2.2 Properties and Processing

311
4.5.2.3 Applications

312
4.6 ARCHITECTURAL APPLICATIONS

313
4.7 COATINGS AND SOLDERS

317
Epilogue

319
Appendix

311
Credits

333
References

337
Index

361
odern science and technology constantly require new materials with special
properties to achieve breathtaking innovations.
This
development centers on
the improvement of scientific and technological fabrication and working procedures.
That means rendering them faster, economically more favorable, and better in quality.
At
the same time, new materials are introduced to improve our general quality of life,
especially in human medicine and dentistry and daily life (e.g., housekeeping).
Among all these new materials, one group plays a very special role:
glass-ceramic materials.
Glass-ceramics
off
er the possibility of com bining the special properties of con-
ventional sintered ceramics with the distinctive characteristics of glasses. It is, howev-
er, possible to develop modern glass-ceramic materials with features unknown thus far
in either ceramics
or
glasses
or
in other materials such as metals
or
organic polymers.

Furthermore, developing glass-ceramics demonstrates the advantage of combining var-
ious remarkable properties in one material.
A
few examples may illustrate this statement.
As
will be shown in the book,
glass-ceramic materials consist of at least one glass phase and at least one crystal
phase. Processing of glass-ceramics is carried out by controlled crystallization of a base
glass. The possibility of generating such a base glass bears the advantage of benefit-
ing from the latest technologies in glass processing, such as casting, pressing, rolling,
or
spinning, which may also be used in the fabrication of glass-ceramics
or
formation
of a sol-gel-derived base glass.
By precipitating crystal phases in the base glass, however, new exceptional
characteristics are achieved. Among these, for example, are the machinobility of glass
ceramics resulting from mica crystallization and the minimum thermal expansion
of chinaware, kitchen hot plates,
or
scientific telescopes as a result of p-quartz-
p-spdumene crystallization.
w
n
G
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G
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Another new field consists of glass-ceramic materials used as biomaterials
in restorative dentistry
or
in human medicine. New high-strength, metal-free glass-
ceramics will be presented for dental restoration. These are examples that demonstrate
the versatility of material development in the field of glass-ceramics.
At
the same time,
however, they clearly indicate how complicated
it
is to develop such materials and what
kind of simultaneous, controlled solid-state processes are required for material develop-
ment to be beneficial.
We intend
this
book to make an informative contribution to all those who
would like to know more about new glass-ceramic materials and their scientific-tech-
nological background
or
who want to use these materials and benefit from them.

It
is
therefore a book for students, scientists, engineers, and technicians. Furthermore
this
monograph
is
intended to serve as a reference for all those interested in natural
or
med-
ical science and technology, with special emphasis on glass-ceramics as new materials
with new properties.
As a result of this basic idea, the first three chapters, "Principles of Designing
G
I
a
ss-C
e
r
a m
ic
F
o
r
m a
t
i
o
n
,
"

"
CO
m position Systems for
G
I
ass-Ce
r
a m
i
cs,
"
a n
d
"Microstructural Control," satisfy the requirements of a scientific-technological text-
book. These three chapters supply in-depth information on the various types of glass-
ceramic materials. The scientific methods of material development are clearly pointed
out, and direct parallels to the applications in Chapter
4
can be drawn easily. Chapter
4
focuses on the various possibilities of glass-ceramic materials in technical, consumer,
optical, medical, dental, electrical, electronic, and architectural applications, as well as
uses for coating and soldering.
This
chapter
is
organized like a reference book.
Based on its contents,
this
book may be classified somewhere between

technical monograph, textbook, and reference book. It contains elements of all three
categories and thus will appeal to a broad readership. As the contents of the book are
arranged along various focal points, readers may approach the book in a
differentiated manner.
For
instance, engineers and students of materials science and
technology will follow the given structure of the book, beginning at Chapter
1.
By
contrast, dentists
or
dental technicians may want to read Chapter
4
first, where they
can find details on the application of dental glass-ceramics. Thus,
if
they want to know
Introduction
xiii
0
more details on the material (e.g., microstructure, chemical composition, crystals, etc.),
they will then read Chapters
1,
2,
or
3.
We carry out
scientific-technological
work on
two

continents, namely the
United States and Europe. Since we are in close contact to scientists of Japan in Asia,
the thought arose to analyze and illustrate the field of glass-ceramics under the aspect
of glass-ceramic technology worldwide.
Moreover, we, who have worked in the field of development and application
of glass-ceramic materials for several years
or
even decades, have the opportunity to
introduce
our
results to the public. We can, however,
also
benefit from the results of our
colleagues, in close cooperation with other scientists and engineers.
The authors would like to thank the following scientists who helped with this
book by. providing technical publications on the topic of glass-ceramic research and
development:
T.
Kokubo,
Y.
Abe, M. Wada, and
T.
Kasuga, from Japan,
1.
Petzoldt, W. Pannhorst from Germany,
I.
Donald from the
U.K.
E.
Zanotto from Brazil.

Special thanks
go
to
V.
Rheinberger (Liechtenstein) for supporting the book
and for numerous scientific discussions; M. Schweiger (Liechtenstein) and
his
team
for the technical and editorial advice;
R.
Nesper (Switzerland)
for
the support in
presenting crystal structures;
S.
Fuchs (South Africa) for the translation into English;
and
1.
Pinckney (USA) for the reading and editing of the manuscript.
This page intentionally left blank
lass-ceramics are ceramic materials formed through the controlled
nucleation and crystallization of glass. Glasses are melted, fabricated to
shape, and thermally converted to a predominantly crystalline ceramic. The basis of
controlled internal crystallization lies in efficient nucleation that allows the development
of fine, randomly oriented grains generally without voids, microcracks,
or
other
porosit)l. The glass-ceramic process, therefore, is basically a simple thermal process as
illustrated in fig. 1-1.
It occurred to Reamur

(1
739)
and to many others since then that a dense
ceramic made via the crystallization of glass objects would be highly desirable.
It
was
not until about
35
years ago, however, that this idea was consummated. The inven-
tion of glass-ceramics took place in the mid-1
950s
by the famous glass chemist and
inventor
S.D.
Stookey.
It
is useful to examine the sequence of events leading to the
discovery of these materials. (Table H-1
1.
At
the time, Stookey was not interested primarily in ceramics. He was pre-
occupied with precipitating silver particles in glass to achieve a permanent photo-
graphic image. He was studying lithium silicate compositions as host glasses because
a)
b)
c)
Fig.
H-1
on nuclei, (c) glass-ceramic microstructure.
From glass

to
glass-ceramic. (a) nuclei formation,
(b)
crystal growth
xV1
6
LASS-
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G
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n
Table
H-1
Invention
of
Glass-Ceramics
(S.D.
Stookey
1950s).
Photosensitive silver precipitation in Li,O-SiO, glass;

furnace overheats; Li,Si,O, crystallizes on Ag nuclei; first
g lass-ceram ic.
Sample accidentally dropped; unusual strength.
Near-zero-thermal-expansion crystal phases described in
Li,O-Al,O,-SiO, system (Hummel, Roy).
TiO, tried as nucleation agent based on its observed
precipitation in dense thermometer opals.
Aluminosilicate glass-ceramic (e.g. Corning Ware@)
developed.
he found he could chemically precipitate silver in alkali silicate glasses, and those
containing lithium had the best chemical durability.
To
develop the silver particles,
he normally heated glasses previously exposed to ultraviolet light just above their
glass transition temperature at around
450°C.
One night the furnace accidentally
over-heated to
850°C
and on observation
of
the thermal recorder, he expected to
find a melted pool of glass. Surprisingly, he observed a white material that had not
changed shape. He immediately recognized the material as a ceramic showing no
distortion from the original glass article.
A
second serendipitous event then occurred.
He accidentally dropped the sample and
it
sounded more like metal than glass. He

then realized that the ceramic he had produced had unusual strength.
On contemplating the significance of
this
unplanned experiment, Stookey
recalled that lithium aluminosilicate crystals had been reported with very low
thermal expansion characteristics; in particular, a phase, 6-spodumene, had
been described by Hummel
(1951)
as having a near-zero thermal expansion
characteristic. He was well aware of the significance of even moderately low
expansion crystals in permitting thermal shock in otherwise fragile ceramics. He
realized that
if
he could nucleate these and other
low
coefficient of thermal
expansion phases in the same way as he had lithium disilicate, the discovery would
be far more meaningful. Unfortunately, he soon found that silver
or
other colloidal
History
metals are not effective in nucleation of these aluminosilicate crystals. Here he
paused and relied on his personal experience with specialty glasses. He had at one
point worked on dense thermometer opals. These are the white glasses that
compose the dense, opaque stripe in a common thermometer. Historically,
this
effect had been developed by precipitation of crystals of high refractive index such as
zinc sulfide
or
titania. He, therefore, tried adding titania as a nucleating agent in

aluminosilicate glasses and discovered
it
to be amazingly effective. Strong and
thermal shock resistant glass-ceramics were then developed commercially within a
year
or
two
of this work as well-known products such as rocket nose cones and
CORNINGWARE@ cookware (Stookey
1959).
In summary, a broad materials advance had been achieved from a mixture
of serendipitous events controlled by chance and good exploratory research related
to a practical concept, albeit unrelated to a specific vision of any of the eventual
products. Knowledge of the literature, good observation skills, and deductive reasoning
were clearly evident in allowing the chance events to bear fruit.
Without the internal nucleation process as a precursor to crystallization,
devitrification
is
initiated at lower energy surface sites.
As
Reaumur was painfully
aware, the result is an icecube-like structure (Fig. H-2)) where the surfaceoriented
crystals meet in a plane of weakness. Flow of the uncrystallized core glass in
response to changes in bulk density during crystallization commonly forces the
original shape to undergo grotesque distortions. On the other hand, because crystal-
a)
b)
a
Fig.
H-2

Crystallization
of
glass without internal nucleation.
n
XvIII
G
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G
Y
lization can occur uniformly and at high viscosities, internally nucleated glasses can
undergo the transformation from glass to ceramic with little
or
no deviation from
the original shape.
To consider the advantages of glass-ceramics over their parent glasses,
one must consider the unique features of crystals, beginning with their ordered
structure. When crystals meet, structural discontinuities
or

grain boundaries are
produced. Unlike glasses, crystals also have discrete structural plans that may
cause deflection, branching,
or
splintering of cracks. Thus the presence of cleavage
planes and grain boundaries serves to act as an impediment for fracture
propagation. This accounts for the better mechanical reliability of finely crystallized
glasses. In addition, the spectrum of properties in crystals
is
very broad compared
with that of glasses. Thus some crystals may have extremely low
or
even negative
thermal expansion behavior. Others, like sapphire, may be harder than any glass,
and crystals like mica might be extremely
soft.
Certain crystalline families also may
have unusual luminescent, dielectric,
or
magnetic properties. Some are semi-
conducting
or
even, as recent advances attest, may be superconducting at liquid
nitrogen temperatures. In addition,
if
crystals can be oriented, polar properties
like piezoelectricity
or
optical polarization may be induced.
In recent years, another method of manufacture of glass-ceramics

has proven technically and commercially viable.
This
involves the sintering and
crystallization of powdered glass. This approach has certain advantages over body-
crystallized glass-teramics. Firstly, traditional glass-ceramic processes may be used,
e.g., slip casting, pressing, and extruding. Secondly, because of the high flow rates
before crystallization, glass-ceramic coatings on metals
or
other ceramics may be
applied by using this process. Finally, and most importantly,
is
the ability to use
surface imperfections in quenched frit as nucleation sites.
This
process typically
involves milling a quenched glass into fine 3-15 pm particle diameter particulate.
This
powder
is
then formed by conventional ceramming called forming techniques in
viscous sintering to full density just before the crystallization process
is
completed.
Figure H-3 shows transformation of a powdered glass compact (Fig. H-3a) to a
dense sintered glass with some surface nucleation sites (Fig. H-3b) and finally to a
History
Fig.
H-3
Glass-ceramics from powdered glass. (a) powdered glass compact,
(b)

densification and incipient crystallization, (c) frit-derived glass-ceramic.
highly crystalline frit-derived glass-ceramic (Fig. H-3c). Note the similarity in
structure between the internally nucleated glass-ceramic in Fig.
H-1
c.
The first
commercial exploitation of frit-derived glass-ceramics was the devitrifying frit
solder glasses for sealing television bulbs. Recently, the technology has been
applied to cofired multilayer substrates for electronic packaging.
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1.1
ADVANTAGES
OF
GLASS-CERAMIC FORMATION
Glass-ceramics have been shown to feature favorable thermal, chemical,
biological, and dielectric properties, generally superior to metals and organic
polymers in these areas. Moreover, glass-ceramics also demonstrate consider-
able advantages over inorganic materials, such as glasses and ceramics. The
large variety of compositions and the possibility of developing special
microstructures should be noted in particular.
It
goes without saying that
these advantageous properties assure the favorable characteristics of the glass-
ceramic end products.
As
the name clearly indicates, glass-ceramics are classified between inor-
ganic glasses and ceramics.
A
glass-ceramic may be highly crystalline or may
contain substantial residual glass.

It
is composed of one or more glassy and
crystalline phases. The glass-ceramic is produced from a base glass by con-
trolled crystallization. The new crystals produced in this way grow directly in
the glass phase, and at the same time slowly change the composition of the
remaining glass.
The synthesis of the base glass represents an important step in the devel-
opment of glass-ceramic materials. Many different ways of traditional melt-
ing and forming as well as sol-gel, chemical vapor deposition, and other
means of production of the base glasses are possible. Although the develop-
ment of glass-ceramics is complicated and time-consuming, the wide spec-
trum of their chemical synthesis is useful for achieving different properties.
The most important advantage of the glass-ceramic formation, however, is
the wide variety of special microstructures. Most types of microstructures
that form in glass-ceramics cannot be produced in any other material. The
glass phases may themselves demonstrate different structures. Furthermore,
they may be arranged in the microstructure in different morphological ways.
Crystal phases possess an even wider variety of characteristics. They may
demonstrate special morphologies related to their particular structures
as
well
as
considerable differences in appearance depending on their mode of growth.
PRINCIPLES
OF
DESIGNING
GLASS-CERAMIC
FORMATION
0
All

these different ways of forming microstructures involve controlled
nucleation and crystallization,
as
well
as
the choice of parent glass composition.
Glass-ceramics demonstrating particularly favorable properties were devel-
oped on the basis of these
two
key advantages, that is, the variation of the
chemical composition and of the microstructure. These properties are listed
in Tables
1-1
and
1-2,
and are briefly outlined below:
Table
1-1
Particularly Favorable Properties
of
Glass-Ceramics
Processing properties
Rolling, casting, pressing, spin casting, press-and-blow
method, drawing are possible
Limited and controllable shrinkage
No porosity in monolithic glass-ceramics
Thermal properties
Expansion can be controlled as desired, depending on the temperature,
with zero or even negative expansion being coefficients of thermal
expansion possible

High-temperature stability
Optical properties
Translucency or opacity
Photo-induction is possible
Pigmentation
Opalescence, fluorescence
Chemical properties
Resorbability or high chemical durability
Biological properties
Biocompatibility
Bioactivity
Mechanical properties
Mac hinability
High strength and toughness
Electrical and magnetic properties
Isolation capabilities (low dielectric constant and
loss,
high resistivity and
Ion conductivity and superconductivity
Ferromagnetism
breakdown voltage)
Advantages
of
Glass-Ceramic Formation
1.1.1
Processing Properties
The research on the discovery of suitable base glasses revealed that the tech-
nology used in the primary shaping of glass could also be applied to glass-ceram-
ics. Therefore, bulk glasses are produced by rolling, pressing, casting, spin cast-
ing, or by press-blowing a glass melt or by drawing a glass rod or ring from the

melt. The thin-layer method is also used to produce thin glass sheets, for exam-
ple. In addition, glass powder or grains are transformed into glass-ceramics.
1.1.2
Thermal Properties
A
particular advantage in the production of glass-ceramics is that products
demonstrating almost zero shrinkage can be produced. These specific ma-
terials are produced on a large scale for industrial, technological, and domes-
tic applications (e.g., kitchenware).
1.1.3
Optical Properties
Since glass-ceramics are nonporous and usudly contain a glass-phase, they
demonstrate a high level of translucency and in some cases even high trans-
parency. Furthermore, it is also possible to produce very opaque glass-ceram-
ics, depending on the type of crystal and the microstructure of the material.
Glass-ceramics can be produced in virtually every color. In addition, photo-
induced processes may be used to produce glass-ceramics and to shape high-
precision and patterned end products.
Fluorescence, both visible and infrared, and opalescence in glass-ceramics
are also important optical characteristics.
1.1.4
Chemical Properties
Chemical properties, ranging from resorbability to chemical stability, can
be controlled according to the nature of the crystal, the glass phase or the
Particularly Favorable Combinations
of
Properties of Glass-Ceramics (Selection)
Mechanical property (machinability)
+
thermal properties

(tem perat ure resistance)
Thermal property (zero expansion
+
temperature resistance)
+
chemical durability
Mechanical property (strength)
+
optical property
(translucency)
+
favorable processing properties
Strength
+
Translucency
+
biological properties
+
favorable processing properties
PRINCIPLES
OF
DESIGNING
GLASS-CERAMIC FORMATION
0
nature of the interface between the crystal and the glass phase.
As
a result,
resorbable or chemically stable glass-ceramics can be produced. The
microstructure in particular also permits the combination of resorbability of
one phase and chemical stability of the other phase.

1
.
1.5
Biological Properties
Biocompatible glass-ceramics have been developed for human medicine
and for dentistry in particular. Furthermore, bioactive materials are used in
implantology.
1.1.6
Mechanical Properties
Although the highest flexural strength values measured for metal alloys
have not yet been achieved in glass-ceramics, it has been possible to achieve
flexural strengths of up to
500
MPa. The toughness of glass-ceramics has also
been considerably increased over the years.
As
a result, values of more
than
3
MPa-mo*5 have been reached. No other material demonstrates these
properties together with translucency and allows itself to be pressed or cast,
without shrinking or pores developing, as in the case of monolithic glass-
ceramics.
The fact that glass-ceramics can be produced as machinable materials rep-
resents an additional advantage. In other words, by first processing the glass
melt, a primary shape is given to the material. Next, the glass-ceramic is pro-
vided with a relatively simple final shape by drilling, milling, grinding, or saw-
ing. Furthermore, the surface characteristics of glass-ceramics, for example,
roughness, polishability, luster, or abrasion behavior can also be controlled.
1.1

.7
Electrical and Magnetic Properties
Glass-ceramics with special electrical or magnetic properties can also be
produced. The electrical properties are particularly important if the material
is used for isolators in the electronics or micro-electronics industries.
It
must
also be noted that useful composites can be formed by combining
glass-ceramics with other materials, for example, metal. In addition, glass-
ceramics demonstrating high ion conductivity and even superconductivity
have been developed. Furthermore, magnetic properties in glass-ceramics
were produced similarly to those in sintered ceramics. These materials are
processed according to methods involving primary shaping of the base glasses
followed
by
thermal treatment for crystallization.