Kutz M. Handbook of materials selection

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1106 COMPOSITES FABRICATION PROCESSES

progressively welded into position. This technique holds great promise for the

future but has yet to be perfected as a viable commercial process. The commin-

gled tows may also be used as feedstock for pultruding. Again, this is a devel-

oping technology of considerable promise but is currently not competitive with

thermoset-based pultruding.

The commingling principle is also being developed using other thermoplastics

as the matrix. There would appear to be considerable potential in the use of

polyamide ﬁbers, especially PA11 or PA12, which have melting points of about

200

⬚C and low melt viscosity. These matrices in combination with carbon ﬁbers

offer better mechanical properties and elevated temperature performance than

the E-glass/polypropylene materials at a cost that is attractive for automotive

and similar applications. There has also been development using high-

performance aromatic thermoplastics such as PEEK with carbon reinforcement.

Such materials are intrinsically much more expensive and need to be processed

at temperatures of about 400

⬚C and are only of interest where high performance

is essential. However, the development of the commercial commingled systems

means that an installed plant could be used for processing the more exotic com-

binations in the future. Overall this is a technology with great potential.

15.4 Fiber–Powder Combinations

Another technique, which has enjoyed some commercial success, is to combine

reinforcement ﬁbers with thermoplastic polymer powders in a towlike material.

The reinforcing ﬁbers are passed through a ﬂuidized bed or a slurry of suspended

polymer powder. The ﬁber bundles are teased open, using gas or liquid jets so

that powder is entrapped between the ﬁbers. The whole bundle is then encap-

sulated in a thin-walled tube of the polymer formed by wrapping a tape of

polymer foil around the bundle. The tape is then welded using a hot gas jet or

an ultrasonic welding device. The resultant product is a ﬂexible tube containing

the reinforcement ﬁbers and the entrapped powder particles. On heating and

pressing, both the foil tube and the powder particles are melted and ﬂow to

inﬁltrate the ﬁbers and form the matrix of the composite. This material may also

be woven, although not as conveniently as the commingled ﬁbers, and may also

be used as tow placement, ﬁlament winding, and pultrusion feedstock.

16 SRIM OF MONOMER PRECURSOR THERMOPLASTICS

In general, one of the key attractions of thermoplastics composites is that there

are no chemical reactions during the ﬁnal processing stages. There is, however,

a small group of materials that may be processed by in situ polymerization of

monomeric precursors to produce thermoplastic matrices. These are mainly the

polyamides, which are polymerized by ring-opening chemistry. These are no-

tably PA11 and PA12. The monomers, also known as lactams, are liquids of

very low, waterlike, viscosity. They are quite unstable, and addition of small

proportions of an initiator or catalyst will induce a rapid molecular ring opening

followed by addition polymerization, forming the thermoplastic. The reactions

are very fast and may be completed in periods of less than one minute. The

process is, thus, eminently suitable for use in a SRIM-type process, where dry

preforms of the reinforcement are placed in the mold, and the liquid precursor

is then injected. The equipment is basically similar to that used for conventional

17 INJECTION-MOLDED SHORT-FIBER-REINFORCED THERMOPLASTICS 1107

RIM or SRIM, but a mixing head of different design is needed as only a small

proportion of initiator/catalyst must be mixed with the monomer. This is the

critical operation. Provision must also be made to prevent overheating due to

the reaction exotherm. The very low viscosity of the monomer ensures rapid and

complete inﬁltration, and the rapid polymerization allows fast cycling. This is a

very specialized technology applicable only to this one class of polymer, at least

at the present time. Carbon ﬁbers are particularly well suited for this process as

a strong interface bond is formed with the polyamide. This combination provides

considerable potential for mass production of higher performance components.

17 INJECTION-MOLDED SHORT-FIBER-REINFORCED

THERMOPLASTICS

17.1 General

Short-ﬁber-reinforced thermoplastics (SFRTPs), are often classiﬁed as reinforced

plastics rather than composites, but since they are one of the most important

groups of commodity materials, it is useful to include them in the present dis-

cussion. The total quantity of SFRTPs manufactured each year is comparable

with that of the basic E-glass/unsaturated polyester GRP materials. Together

they account for more than 80% of the composites market. They are injection-

moldable materials consisting of thermoplastics matrices with discontinuous ﬁ-

bers, normally in the length range of 10 mm down to

⬍0.1 mm. The ﬁbers are

much shorter than those in the other composites systems discussed in the pre-

vious sections. The ﬁber loading is also lower than in most other composites,

rarely more than V

⬇ 0.2, although the ﬁber content is often expressed as a

weight percentage. Thus, a polyamide with 30 wt % of E-glass would have an

equivalent V

of about 0.18. The very short ﬁbers and low ﬁber loading are

necessary to allow the compounds to be easily injection molded and to achieve

a good surface ﬁnish. Somewhat better mechanical properties may be realized

at the expense of surface ﬁnish if longer ﬁbers are used at higher loadings.

SFRTPs are often formulated with particulate ﬁllers, e.g., talc or chalk, and

sometimes rubber additives as well as the ﬁbers to obtain the optimum balance

of properties and cost.

17.2 Principal Systems

Fiber-reinforced grades of most of the commodity and engineering thermoplas-

tics are offered by the polymer suppliers and compounders. The most important

commercial systems are those based on polypropylene, the polyamides, the ac-

etal resins (Delrin), and the thermoplastic polyester family. In all of these ma-

terials the principal ﬁber used is E-glass. Glass-reinforced grades of the higher

performance thermoplastics such as polyphenylene sulﬁde and PEEK are also

offered, and carbon ﬁber is also used with polyamides and the higher perform-

ance materials.

17.3 Compounding

Short-ﬁber-reinforced thermoplastics are invariably precompounded. There are

two principal methods. In the ﬁrst either continuous ﬁber rovings or rovings

precut into short lengths (e.g., 10 mm) are fed into a screw extruder through

1108 COMPOSITES FABRICATION PROCESSES

Standard fiber-reinforced

thermoplastic molding pellet:

Random very short fibers.

Long-fiber molding

pellet.

Fibers aligned and

full length of pellet

≈

10 mm

4 - 5 mm

Fig. 46 Illustration of two types of injection-molding pellet for short-ﬁber-reinforced thermo-

plastics. The standard pellet is formed by extruding a compounded mixture of polymer and ﬁ-

ber. It contains very short ﬁbers in a random orientation distribution. The long-ﬁber pellet is

made by a cross-head extruder or a variation on the pultrusion principle. The ﬁbers are oriented

along the full length of the pellet that are typically cut into 10 mm lengths.

which the plasticized polymer is also transported. The ﬁber and polymer are

vigorously mixed as the blend passes through the screw and are ﬁnally extruded

through a spaghetti die, and the extrudate cut into short pellets. The ﬁbers are

broken into short lengths during this process. The output is cylindrical granules,

typically 4 mm diameter and length. The granules contain a semirandom array

of very short ﬁbers, typically less than 1 mm long. These granules are used as

feedstock for the injection-molding process.

In the alternative process, continuous ﬁber rovings are impregnated with the

polymer, either in a specially adapted extruder with a cross-head die or by a

variant of the pultrusion process. The output is a continuous rod of about 4 mm

diameter. These rods are then chopped to lengths of about 10 mm, and this

material used as feedstock for the injection molders. The distinction between the

two compounding processes is that, in the former, the granules contain very

short ﬁbers, while in the latter the ﬁbers are the full length of the pellets. Further

ﬁber breakage occurs during injection molding, but the moldings made with the

long-ﬁber granules will contain ﬁbers of much greater average length. This in-

duces superior stiffness, strength, and toughness, but surface ﬁnish may be

poorer. A schematic of the structure of both types of molding pellet are shown

in Fig. 46.

17.4 Injection Molding

Modern injection-molding machines are built around screw plasticizers that can

be rotated and reciprocated. The charge of compounded granules is gravity fed

from a conical hopper into the feed section of the screw. From here the charge

is carried forward by the action of the rotating screw. The charge passes down

the heated barrel, where it is melted and subjected to a strong mixing action,

which homogenizes the melt. The plasticized melt builds up at the end of the

17 INJECTION-MOLDED SHORT-FIBER-REINFORCED THERMOPLASTICS 1109

Feed hopper for reinforced

thermoplastics pellets

Heated barrel

Screw

Mold opening &

closing mechanism

Ejected part

Nozzle with shut-off

valve

Screw rotation and

reciprocation mechanisms

Water cooled

matched dies

Fig. 47 Schematic of an injection-molding machine for reinforced thermoplastics. The pellets

are charged into the hopper, from which they are gravity fed into the entry section of a screw

pump. The charge is carried forward and melted in the heated barrel, while the screw retracts

under the pressure of the melted charge at its tip. When sufﬁcient charge has accumulated,

the screw is forced forward and the nozzle valve opened allowing the molten charge

to be injected into the cold mold.

screw, against a closed discharge valve. The screw is forced back in the barrel

by the pressure built up against the valve. When sufﬁcient charge has been

accumulated, the screw rotation is stopped, the valve is opened, and the screw

thrust forward by a hydraulic ram, which injects the charge into the mold. The

mold is relatively cool and the pressure is maintained (the dwell) until the ma-

terial in the gate has frozen. At this point screw rotation is restarted and the

next charge prepared. Meanwhile when the material in the mold has cooled

sufﬁciently, the mold may be opened and the part ejected. The whole cycle is

completed in times ranging from as little as 10 s to a few minutes. The critical

rate determining factor is usually the length of time required for the charge

to cool in the mold prior to ejection. This will be longer for larger moldings

and thicker sections where heat transfer through the mold is less efﬁcient. A

schematic of the essential parts of an injection-molding machine is shown in

Fig. 47.

The operation of the machine must be carefully controlled to assure that the

speciﬁed mechanical properties are obtained, that the surface ﬁnish is acceptable,

and that the optimum production rate is achieved. The main controls are the

barrel and mold temperatures, the screw backpressure, the injection pressure and

rate, and the dwell time. A higher barrel temperature will increase the maximum

plasticization rate, usually expressed in kilograms/hour. Too high a temperature

may lead to degradation of the polymer. A lower mold temperature will speed

up the cooling cycle but often leads to poor surface ﬁnish. Better surface ﬁnish

usually entails a compromise, and mold temperatures are typically held at 40–

100

⬚C with polypropylene and polyamides. This increases cooling time and,

hence, the total cycle time. The screw backpressure regulates the pressure in the

melt reservoir prior to injection. A higher backpressure gives more thorough

mixing but also breaks the ﬁbers into shorter lengths. This compromises

mechanical properties but tends to improve surface ﬁnish. Faster injection rates

result in better mold ﬁlling due to the shear thinning (pseudoplastic) behavior

1110 COMPOSITES FABRICATION PROCESSES

of polymer melts, where apparent viscosity decreases with increased shear rate.

There is also adiabatic heating, where the injected charge is heated through the

shear work performed during injection. Overheating can lead to burning and

degradation. Finally, higher pressure maintained during the dwell period tends

to compensate for the shrinkage that occurs as the polymer freezes. In this way

a sharper impression of the mold and better control of dimensions is ensured.

When molding SFRTPs, the principal concerns are to control the ﬁber length

distribution and the ﬁber orientation distribution to ensure that optimum per-

formance is obtained. The ﬁber length distribution is affected mainly by the

choice of feedstock, long or short ﬁber, and the backpressure and injection rate

settings. The orientation distribution is ﬂow dominated. Convergent ﬂow leads

to ﬁber orientation in the ﬂow direction, while divergent ﬂow has the opposite

effect. The ﬂow pattern is mainly determined by the geometry of the part and

the placing of the injection point(s) or gate(s). Comprehensive suites of software

(e.g., Moldﬂo) are available that allow the ﬂow in the mold to be visualized so

that optimum gate placement may be determined at the tool design stage.

BIBLIOGRAPHY

Recommendations for further reading are listed below under the principal section

headings used in this chapter.

Basic Principles for Processing

Bader, M. G., and C. Lekakou, Processing for Laminated Structures, in Composites Engineering

Handbook, P. K. Mallick (Ed.), Marcel Dekker, New York, 1997, pp. 371–480.

Gutowski, T. G. (Ed.), Advanced Composites Manufacturing, Wiley, New York, 1997.

Kelly, A., and S. T. Mileiko (Eds.), Fabrication of Composites, Vol. 4, Handbook of Composites, A.

Kelly and Yu N. Rabotnov (Eds.), Elsevier Science, Amsterdam and New York, 1983.

Manson, J-A. E., M. D. Wakeman, and N. Bernet, Composite Processing and Manufacturing—An

Overview, in Comprehensive Composite Materials, A. Kelly and C. Zweben (Eds.), Vol. 2, R.

Talreja and J-A. E. Manson (Eds.), Elsevier Science, Oxford, UK, 2000, pp. 577–608.

Contact Molding

Cripps, D., T. J. Searle, and J. Summerscales, ‘‘Open Mold Techniques for Thermoset Composites,’’

in Comprehensive Composite Materials, A. Kelly and C. Zweben (Eds.), Vol. 2, R. Talreja and

J-A. E. Manson (Eds.), Elsevier Science, Oxford, UK, 2000, pp. 737–761.

Autoclave Processing of Prepreg

Seferis, J. C., R. W. Hillermeier, and F. U. Buehler, ‘‘Prepregging and Autoclaving of Thermoset

Composites,’’ in Comprehensive Composite Materials, A. Kelly and C. Zweben (Eds.), Vol. 2,

R. Talreja and J-A. E. Manson (Eds.), Elsevier Science, Oxford, UK, 2000, pp. 701–736.

Resin Film Inﬁltration (RFI)

Cripps D., T. J. Searle, and J. Summerscales, ‘‘Open Mold Techniques for Thermoset Composites,’’

in Comprehensive Composite Materials, A. Kelly and C. Zweben (Eds.), Vol. 2, R. Talreja and

J-A. E. Manson (Eds.), Elsevier Science, Oxford UK, 2000, pp. 737–761.

Resin Transfer Molding (RTM)

Advani, S. G., and E. M. Sozer, ‘‘Liquid Molding of Thermoset Composites,’’ in Comprehensive

Composite Materials, A. Kelly and C. Zweben (Eds.), Vol. 2, R. Talreja and J-A. E. Manson

(Eds.), Elsevier Science, Oxford, UK, 2000, pp. 807–844.

Rudd, C. D., A. C. Long, K. N. Kendall, and C. G. E. Mangin, Liquid Molding Technologies,

Woodhead Publishing, Cambridge, England, 1997.

Verpoest, I., ‘‘Composite Preforming Techniques,’’ in Comprehensive Composite Materials, A. Kelly

and C. Zweben (Eds.), Vol. 2, R. Talreja and J-A. E. Manson (Eds.), Elsevier Science, Oxford,

UK, 2000, pp. 623 –669.

BIBLIOGRAPHY 1111

Filament Winding and Tow Placement and Pultrusion

Peters, S. T., and Yu M. Tarnopol’skii, ‘‘Filament Winding,’’ in Composites Engineering Handbook,

P. K. Mallick (Ed.), Marcel Dekker, New York, 1997, pp. 515–540.

Sohl, C., ‘‘Continuous Molding of Thermoset Composites,’’ in Comprehensive Composite Materials,

A. Kelly and C. Zweben (Eds.), Vol. 2, R. Talreja and J-A. E. Manson (Eds.), Elsevier Science,

Oxford, UK, 2000, pp. 845–852.

Sumerak, J. E., ‘‘The Pultrusion Process for Continuous Automated Manufacture of Engineered Com-

posite Proﬁles,’’ in Composites Engineering Handbook, P. K. Mallick (Ed.), Marcel Dekker, New

York, 1997, pp. 549–578.

Press Molding and Sheet and Bulk Molding Compounds

Revellino, M., L. Saggese, and E. Gaiero, ‘‘Compression Molding of SMCs,’’ in Comprehensive

Composite Materials, A. Kelly and C. Zweben (Eds.), Vol. 2, R. Talreja and J-A. E. Manson

(Eds.), Elsevier Science, Oxford, UK, 2000, pp. 763–805.

Glass Mat Thermoplastics (GMT)

Wakeman, M. D., and C. D. Rudd, ‘‘Compression Molding of Thermoplastic Composites,’’ in Com-

prehensive Composite Materials, A. Kelly, and C. Zweben (Eds.), Vol. 2, R. Talreja, and

J-A. E. Manson (Eds.), Elsevier Science, Oxford, UK, 2000, pp. 915–963.

Thermoplastic Matrix Composites

Gibson, A. G., ‘‘Continuous Molding of Thermoplastic Composites,’’ in Comprehensive Composite

Materials, A. Kelly and C. Zweben (Eds.), Vol. 2, R. Talreja, and J-A. E. Manson (Eds.), Elsevier

Science, Oxford, UK, 2000, pp. 979–998.

SRIM of Monomer Precursor Thermoplastics

Bourban P-E., ‘‘Liquid Molding of Thermoplastic Composites,’’ in Comprehensive Composite Ma-

terials, A. Kelly and C. Zweben (Eds.), Vol. 2, R. Talreja, and J-A. E. Manson (Eds.), Elsevier

Science, Oxford, UK, 2000, pp. 965–977.

Injection-Molded Short-Fiber Reinforced Thermoplastics

Bader, M. G., and A. R. Hill, ‘‘Short Fiber Composites,’’ in Materials Science & Technology, R. W.

Cahn, P. Haasen, and E. J. Kramer (Eds.), Vol. 13, T. W. Chou (Ed.), VCH, New York, 1993,

pp. 291 –338.

Brooks, R., ‘‘Injection Molding Based Techniques,’’ in Comprehensive Composite Materials, A. Kelly,

and C. Zweben (Eds.), Vol. 2, R. Talreja, and J-A. E. Manson (Eds.), Elsevier Science, Oxford,

UK, 2000, pp. 999 –1028.

1113

CHAPTER 35

ADVANCED CERAMICS PROCESSING

Lisa C. Klein

Department of Ceramic and Materials Engineering

Rutgers University

Piscataway, New Jersey

1 OVERVIEW OF CERAMIC

PROCESSING 1113

1.1 Microstructure 1114

1.2 Motivation for Improvements 1114

1.3 Manufacturing versus

Processing 1115

1.4 Back to Basics 1115

2 CONVENTIONAL PROCESSING 1115

3 ADVANCED PROCESSING 1117

3.1 New Energy Sources 1117

3.2 New Shapes 1120

3.3 New Precursors 1123

4 SUMMARY AND PROJECTIONS 1127

BIBLIOGRAPHY 1127

1 OVERVIEW OF CERAMIC PROCESSING

Processing of ceramics is carried out through the consolidation of loose powders

to form polycrystalline objects. The majority of ceramics are sintered, using

solid-state sintering or liquid-phase sintering [See, e.g., Rahaman (1995),

Barsoum (1997)]. Generally, sintering occurs at temperatures below the melting

point for single-phase ceramics or below their liquidus for multiphase ceramics,

although a liquid phase may be present temporarily during sintering. The dis-

tinction that sets the majority of ceramics apart from the minority is the fact that

the majority is sintered below the melting temperature by solid-state transport.

The minority are those processed above the melting temperature, in particular,

glasses and glass-ceramics. Therefore, the key to successful processing of ce-

ramics is ﬁnding energy sources that can drive the transport processes.

In most cases, the source of energy is surface energy. The elimination of

surface energy, where surfaces of loose powders are converted to grain bound-

aries between particles, is what accounts for most of the energy available for

sintering. There are countless examples of ceramic products where the conver-

sion of this energy has resulted in a successful outcome. Nevertheless, there can

always be improvements in ceramic processing. Therefore, the focus of this

chapter is on new sources of energy to drive the consolidation and better, more

efﬁcient uses of this energy to reach the goal of a well-consolidated, polycrys-

talline ceramic.

HandbookofMaterialsSelection.EditedbyMyerKutz

1114 ADVANCED CERAMICS PROCESSING

Fig. 1 Industrial issues relevant to ceramics.

1.1 Microstructure

When it comes to describing the outcome of processing in ceramics, the focus

of that description is the microstructure (Allen and Thomas, 1999). To put this

in perspective, the terms we use in describing microstructure need to be deﬁned.

As such, microstructure is described in terms of grains, grain boundaries, and

porosity. For single-phase material, this is straightforward, but far more compli-

cated for multiphase materials. Because the starting point for processing is loose

powders or particles, the ﬁrst feature in the microstructure is described as necks,

which are the contacts between particles. Next, we distinguish between a single

crystal, which is a ‘‘perfect’’ crystal without interruption, and a grain, which is

a single crystal of ﬁnite size. Once there is consolidation within a powder com-

pact, what was once called a particle becomes a grain. The regions of disorder

between these grains become grain boundaries. In the grain boundaries, we often

ﬁnd a second phase, which is the remains of a liquid phase.

Another way of looking at the microstructure is focusing on the porosity,

which is the collection of voids. Initially, the porosity is interconnected. Grad-

ually, material transport ﬁlls in the voids, bringing more area of grains into

contact. Triple-grain junctions are the typical location of closed porosity. In the

end, the polycrystalline solid takes on its ﬁnal microstructure through grain

growth and coarsening. In short, the goal of sintering is to turn the loose powders

into a well-consolidated polycrystalline solid.

With these terms, the features of the microstructure can be quantiﬁed. Quan-

tities such as pore volume, percent theoretical density, and grain size can be

used to draw the correlations we are familiar with in materials science. No

materials science student can graduate without learning the structure–

processing–properties–performance paradigm known as SP

. The structure of

the material determines the properties, and the structure, in turn, is determined

by processing.

1.2 Motivation for Improvements

An alternative way to view ceramic processing is in terms of industrials needs.

Suppose there is a need, that can be satisﬁed by a ceramic product. The engi-

neering factors involved are shown in Fig. 1. The role of processing weighs

more heavily on the manufacturing side. Because issues of cost and reproduci-

bility balance the function of the product, the eventual implementation of the

ceramic product may hinge on whether or not the product can be made in a

practical way.

2 CONVENTIONAL PROCESSING 1115

Fig. 2 Process-induced changes that link raw material to ﬁnished part.

1.3 Manufacturing versus Processing

Is ceramic manufacturing comparable in scale to metals fabrication or compa-

rable in automation to pharmaceuticals production? The easy answer would be

no. What efforts have been made to introduce continuous production, automa-

tion, and continuous improvement into ceramic manufacturing? How much in-

telligent processing is used in ceramic manufacture? By intelligent processing,

we mean that we can insert diagnostics in the ceramic process that allow us to

evaluate physical properties during the process. The goal of intelligent manu-

facturing is achieved when we can correct the process to stay within acceptable

values of our physical properties before we reach the ﬁnal product.

These questions about intelligent manufacturing are important to the health

of ceramic manufacture in a setting where at times more than one material or

design satisfy the industrial needs. Clearly, there are some ceramic products that

cannot be replaced by other material systems, but there are other ceramic prod-

ucts, such as glass containers, that are seriously threatened by other materials,

such as plastics. To answer these questions about ceramic manufacture, and, in

particular, about manufacture of advanced ceramics, requires a careful look at

individual segments of the industry. Moreover, they are useful questions to keep

in mind, as new processing technologies are introduced.

In terms of manufacturing a product that meets customer requirements, the

technical issues include quantitative speciﬁcations of properties, process im-

provement and ‘‘zero defects.’’ The nontechnical issues include government reg-

ulation and availability of investment for capital and product development.

Overall, the goal of manufacturing is to produce a high-quality product rapidly,

in an economical way, and in an environmentally acceptable manner.

1.4 Back to Basics

To improve processing means understanding the fundamental mechanisms dic-

tated by thermodynamics and kinetics. What goes on in ceramic processing is

listed in Fig. 2, connecting raw material to ﬁnished part. In taking raw material

to ﬁnished part, how many of these changes can be followed in real time? How

many of these changes are predictable, reproducible, and what is the accuracy

of the measurement that tells us that they are reproducible? These questions are

the real challenges of ceramic processing and ceramic manufacture.

2 CONVENTIONAL PROCESSING

As with any materials process, the unit operations we evaluate are powder prep-

aration, forming, consolidation, and sintering. For each step, it is necessary to

1116 ADVANCED CERAMICS PROCESSING

Table 1 Examples of Structural and Functional Ceramics, Principally Single-Phase,

Produced by Sintering

Ceramic Property Application

Structural

ZrO

High-temperature strength Furnace lining

Wear resistance Bearings

SiC Corrosion resistance Heat exchanger

TiC Hardness Cutting tools

Functional

BaTiO

Ferroelectricity Capacitor

Electronic insulation Substrates for electronic packaging /

Spark plugs

BaO

䡠 6Fe

Hard magnet Ceramic magnets

(Zn)Fe

Soft magnet Transformer core

LiNbO

Nonlinear optics Optical switches

Fig. 3 Unit operations in manufacturing ceramic part.

understand the mechanisms and the extent to which we can model the steps with

thermodynamics or atomistics. A key question at each step is: What needs to be

measured and how can it be measured? Does the instrumentation exist that al-

lows us to follow a ceramic process in a continuous way? Are we measuring

the parameter directly or indirectly? When a process seems to be going out of

control, how can we ﬁx it and can we ﬁx it without going back to the beginning?

The steps in the fabrication of polycrystalline ceramics are divided between

those before ﬁring and those during and after ﬁring. Many excellent treatments

of ceramic processing have appeared over the last few years. A comprehensive

handbook on the subject treats each step in detail (Schneider, 1991). For con-

ventional ceramics (e.g., whitewares), ceramics produced in large volume (e.g.,

abrasives), and ceramics used at high temperatures (e.g., refractories), there are

textbooks that cover the processing and the applications [see, e.g., Reed (1995),

West and Hench (1990), Richerson (1992)]. Table 1 lists typical single-phase

sintered ceramics and their uses.

Some of the common steps in ceramic manufacture are shown in Fig. 3.

Rather than review what is now well known, this discussion will cover pro-

cesses that are new. These processes may not be suitable for production of all

ceramics in widespread use. Instead, these processes are variations of conven-

tional ways, in some cases, and radically new ways, in other cases, for making

ceramics. In all cases, the advantages of these processes over old methods will

be highlighted and evaluated.