Advanced Materials by Design Page: 43
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Ch. 2-Ceramics * 43
Table 2.4.-Common Processing Operations for
Advanced CeramicsProcess
Operation
Powder
preparation
Forming
Densification
FinishingExamples
SiC
SiN,
ZrO,
Glasses
Combustors, stators
Cutting tools
Tubing, honeycomb
Turbocharger rotors
Capacitors
Glass ceramics
AI,0,
Si,N,
Si,N., SiC, BN
Si,N., SiC
Diamond grinding
Etching
Laser, electron beam
Electric dischargeSOURCE Office of Technology Assessment 1988
Powder Preparation
Although most of the basic raw materials for
ceramics occur abundantly in nature, they must
be extensively refined or processed before they
can be used to fabricate structures. The entire
group of silicon based ceramics (other than sil
ica) does not occur naturally. Compositions of
silicon carbide, silicon nitride, and sialon (an al
loy of silicon nitride with aluminum oxide in
which aluminum and oxygen atoms substitute
into silicon and nitrogen lattice positions, respec
tively) must all be fabricated from gases or other
ingredients. Even minerals that occur naturally,
such as bauxite, from which alumina is made,
and zircon sands, from which zirconia is derived,
must be processed before use to control purity,
particle size and distribution, and homogeneity.
The crucial importance of powder preparation
has been recognized in recent years. Particle sizes
and size distributions are critical in advanced ce
ramics to produce uniform green (unfired) den
sities, so that consolidation can occur to produce
a fully dense, sintered, ceramic part.
Various dopants or sintering aids are added to
ceramic powders during processing. Sinterabil
ity can be enhanced with dopants, which con
trol particle rearrangement and diffusivities. These
dopants permit sintering at lower temperaturesSynthesis
Sizing
Granulating
Blending
Solution chemistry
Slip casting
Dry pressing
Extrusion
Injection molding
Tape casting
Melting/casting
Sintering
Reaction bonding
Hot pressing
Hot isostatic pressing
Mechanical
Chemical
Radiation
Electric75-7920 - 88 - 2
and/or faster rates. Dopants are also used to con
trol grain growth or achieve higher final densities.
The use of dopants, although providing many
beneficial results, can also have a detrimental in
fluence on the material properties. Segregation
of dopants at the grain boundaries can weaken
the final part, and final properties such as con
ductivity and strength may differ significantly from
those of the pure material.
Forming
Ceramic raw materials must be formed and
shaped before firing. The forming process often
determines the final ceramic properties. The im
portant variables in the forming step are particle
shape, particle packing and distribution, phase
distribution, and location of pores.
Forming processes for ceramics are generally
classified as either cold forming or hot forming.
The major cold forming processes include slip
casting, extrusion, dry pressing, injection mold
ing, tape casting, and variations of these. The
product of such processes is called a green body,
which may be machined before firing. The homo
geneity of the cold formed part determines the
uniformity of shrinkage during firing.
Hot forming processes combine into one step
the forming and sintering operation to produce
simple geometric shapes. These processes include
hot pressing (in which pressure is applied along
one direction) and hot isostatic pressing (HI Ping,
i n which pressure is applied to the ceramic from
all directions at once).
Densification
Sintering is the primary method for converting
loosely bonded powder into a dense ceramic
body. Sintering involves consolidation of the
powder compact by diffusion on an atomic scale.
Moisture and organics are first burned out from
the green body, and then, at the temperature
range at which the diffusion process occurs, mat
ter moves from the particles into the void spaces
between the particles, causing densification and
resulting in shrinkage of the part. Combined with
forming techniques such as slip casting, sinter
ing is a cost effective means of producing intri
cate ceramic components. Its drawback lies in
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United States. Congress. Office of Technology Assessment. Advanced Materials by Design, report, June 1988; [Washington D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc39896/m1/48/: accessed April 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.