Processing of ceramic

Shaping (Forming)
Ceramic products must be fabricated into useful shapes. Many shaping
methods are used for ceramic products and these can be grouped into
categories, which are not necessarily independent.
1.Powder compaction: dry pressing, hot pressing, cold isostatic pressing,
etc. is simply the pressing of a free flowing powder. The powder may be
dry pressed (i.e., without the addition of a binder) or pressed with the
addition of a small amount of a suitable binder. The pressure is applied
either uniaxially or isostatically. The choice of pressing method depends
on the shape of the final product. We make simple shapes by applying
the pressure uniaxially; more complex shapes require isostatic pressing.
2. Plastic forming: using pressure to shape the green ceramic Plastic
forming consists of mixing the ceramic powder with a large volume
fraction of a liquid to produce a mass that is deformable (plastic) under
pressure. Such processes were developed and used originally for clay and
have since been adapted to polymeric materials. For traditional clay
based ceramics the liquid is mainly water. For ceramic systems that are
not based on clay, an organic may be used in place of, or in addition to,
water. The binders are often complex and contain multiple components to
achieve the required viscosity and burn-out characteristics.
The major methods that are included in each of the above categories and
the types of shape that can be produced. First some of the words:
Binder is a component that is added to hold the powder together while we
shape the body.
Slurry is a suspension of ceramic particles in a liquid.
Plasticizer is the component of a binder that keeps it soft or pliable; it
improves the rheological properties.
Green is a ceramic before it is fired. Brown, white, or gray potter’s clays
are well known green ceramics.
Slip is the liquid-like coating used to form the glaze when fired.
Some of the shaping methods produce a ceramic compact that is strong
enough to be handled and machined; however, it is not fully dense and
the bonds between the grains are not strong. This is called the “green”
state and represents a transition state between the loose powder and the
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high-density sintered product. Other shaping methods, those that involve
the use of high temperatures and pressures, can directly produce a very
dense sintered product.
BINDERS AND PLASTICIZERS
It is often necessary to add a binder to the ceramic powder. The binder
has two functions. In some shaping methods ,such as extrusion, the
binder provides the plasticity necessary for forming. The binder also
provides the dry (green)shape with strength sufficient to survive the
handling process between shaping and sintering.
One of the most important requirements for the binder is that we must be
able to eliminate it from the compact during the firing process without
any disruptive effect: polymers are thus often ideal binders.
In pottery, the binder is often water that is present in sufficient quantity to
make the clay easily shaped with the shape being retained during firing.
The idea is that we then add a plasticizer to optimize the rheology of the
Material, these processes are not exclusive to ceramics but are general to
powder processing.
Debinding
The process of binder removal. The organic additives and other
processing aids as the binder must be removed from the green body prior
to sintering. the remove of binder completely without disrupting the
particle packing or producing any new microstructural defects in the
green body. Debinding can be a critical step in ceramic processing,
especially in the case of the forming methods where the binder content in
the green body is relatively high.
Debinding can be accomplished by three methods:
(1) extraction by capillary flow into a porous surrounding material.
(2) solvent extraction.
(3) thermal decomposition, but by far the most commonly used method
is thermal decomposition (referred to as thermal debinding).
HOT PRESSING
Hot pressing is the pressing at high temperatures.
The main difference is that in hot pressing the die assembly is contained
within a high temperature furnace . During hot pressing the ceramic
powders may sinter together to form a high-density component.We can
summarize the advantages of this process.
_ The powder does not have to be of the highest quality.
_ Large pores that are caused by nonuniform mixing are easily removed.
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_ We can densify at temperatures lower (typically half the melting
temperature of the material) than those needed for conventional
pressureless sintering.
_ Extensive grain growth or secondary recrystallization does not occur
when we keep the temperature low during densification.
_ We can densify covalently bonded materials such as B4C, SiC, and
Si3N4 without additives.
The principal disadvantage is also important.
_ Dies for use at high temperatures are expensive and do not generally
last long.
FINAL MACHINING
Ideally the shaping and forming processes that are employed would
produce the ceramic component in the desired shape with the specified
dimensional tolerances and with an acceptable surface finish. However,
in many cases this is not the situation and some final machining(after
firing/sintering) of the ceramic is necessary. Generally final machining is
required to:
_ Meet dimensional tolerances.
_ Improve the surface finish.
_ Remove surface flaws.
Ceramic materials are difficult to machine because they are hard and
brittle. The tooling costs are high because diamond tools are likely to be
required or if conventional tools are used the tool life is very short. Also
the time required to machine ceramics is long because if high tensile
loads are applied to the ceramic part it might fracture.
Mechanical approaches to machining ceramics include the following:
_ Grinding uses tools in which abrasive particles are embedded in a
softer matrix such as glass, rubber, or polymer resin, or even a metal (as
for WC in Co).
_ Lapping uses loose abrasive particles placed on a soft cloth.
_ Sandblasting uses abrasive particles accelerated by compressed air and
directed through a nozzle at high velocity.
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_ Water-jet machining uses a high-pressure (?400 MPa pumping
pressure) water jet to transport the abrasive particles to the ceramic
surface.
ADDITIVES AND CERAMIC FORMING
In the forming of ceramics, the use of certain additives, sometimes in
concentrations as low as a fraction of a percent by weight, is often vital
for controlling the characteristics of the feed material, for achieving the
desired shape, and for controlling the packing uniformity of the green
body. In methods such as casting and injection molding, the selection of
suitable additives forms one of the most vital parts of the forming
process.
The additives are either organic or inorganic in composition. Organic
additives, which can be synthetic or natural in origin, find greater use in
the forming of advanced ceramics because they can be removed almost
completely prior to the sintering step. Organic additives can also be
synthesized with a wide variety of compositions so they provide a large
number of chemicals for specialized applications. Inorganic additives
cannot generally be removed after the forming step and are used in
applications, particularly in the traditional ceramics industry, where the
residues do not have an adverse effect on the properties of the final
product.
The number of additives used in a given forming process should be kept
to a minimum because the potential for undesirable interactions between
the components increases with their number. However, small amounts of
other additives are sometimes used to serve special functions:
surfactants added primarily to reduce the surface tension of liquids .
Lubricants are commonly used in die compaction, to reduce the friction
between the particles themselves or between the particles and the die
walls.
Homogenizers such as cyclohexanone are sometimes used to increase the
solubility of the components, thereby improving the homogeneity of the
mixture.
Solvents provide fluidity for the powder during forming, serve as
solvents for dissolving the additives to be incorporated into the powder.