MME 101

Materials Engineering
Fundamentals

Ceramic Processing
 Types of Ceramics and Their Processing

• Since ceramic materials have relatively high melting temperatures, casting

them is normally impractical.

• The brittleness of these materials precludes deformation.

• The as mined raw materials usually have to go through a milling or grinding

operation in which particle size is reduced, this is followed by screening to
yield a powdered product having a desired range of particle sizes.
Overview of Ceramics Particulate Processing

• Traditional ceramics are made from minerals occurring in nature

Products include pottery, porcelain, bricks, and cement
• New ceramics are made from synthetically produced raw materials
Products include cutting tools, artificial bones, nuclear fuels, and
substrates for electronic circuits
The starting material for all of these items is powder
The powders are usually mixed with water to temporarily bind the particles
together and achieve the proper consistency for shaping
After shaping, the green parts are fired (sintered), whose function is the same
as in powder metallurgy
• Preparation of the Raw Material
• Shaping processes require the starting material to be a plastic paste • This
paste is comprised of fine ceramic powders mixed with water
• The raw ceramic material usually occurs in nature as rocky lumps, and
reduction to powder is the purpose of the preparation step in ceramics
processing

Comminution Reducing particle size in ceramics processing by use of

mechanical energy in various forms such as impact, compression etc

– Two general types of comminution operations: • Crushing • Grinding

Crushing Reduction of large lumps from
the mine to smaller sizes for
subsequent further reduction
• Several stages may be required (e.g.,
primary crushing, secondary crushing),
• Crushing of minerals is accomplished
by compression against rigid surfaces
or by impact against surfaces in a rigid
constrained motion

• Jaw Crusher Large jaw go back and

forth to crush lumps against a hard,
rigid surface
• Roll Crusher Ceramic lumps are
squeezed between rotating rolls
Grinding In the context of comminution, grinding refers to the operation of
reducing the small pieces after crushing to a fine powder
• Accomplished by abrasion, impact, and compaction by hard media such as
balls or rolls • Examples of grinding include: • Ball mill • Roller mill • Impact
grinding
• Ball Mill Hard spheres mixed with stock are rotated inside a large cylindrical
container; the mixture is carried up the container wall as it rotates, and then
pulled back down by gravity for grinding action
Ingredients of Ceramic Paste for Shaping
• Clay (hydrous aluminum silicates) - usually the main ingredient because of
ideal forming characteristics when mixed with water
• Water – creates clay-water mixture with suitable plasticity for shaping
• Non-plastic raw materials, such as alumina and silica - reduce shrinkage in
drying and firing but also reduce plasticity of the mixture during forming
• Other ingredients, such as fluxes that melt (vitrify) during firing and promote
sintering, and wetting agents to improve mixing of ingredients

Shaping Processes
• Slip casting • The clay-water mixture is a slurry
• Plastic forming methods • The clay is plastic
• Semi-dry pressing • The clay is moist but has low plasticity
• Dry pressing • The clay is basically dry (less than 5% water) and has no
plasticity”
• Four categories of shaping processes used for ceramics, compared to
water content and pressure required to form the clay
Slip Casting A suspension of ceramic powders in water, called a slip, is
poured into a porous plaster of paris mold so that water from the mix is
absorbed into the plaster to form a firm layer of clay at the mold surface •
The slip composition is 25% to 40% water
Two principal variations:
• Drain casting - the mold is inverted to drain excess slip after a semi-solid layer
has been formed, thus producing a hollow product
• Solid casting - to produce solid products, adequate time is allowed for entire
body to become firm
The steps in a solid and drain slip
casting using a porous mold
Overview of Plastic Forming • The starting mixture must have a plastic
consistency, with 15% to 25% water • Variety of manual and mechanized
methods • Manual methods use clay with more water because it is more
easily formed • More water means greater shrinkage in drying • Mechanized
methods generally use a mixture with less water so starting clay is stiffer

Plastic Forming Methods

• Hand modeling (manual method)
• Jiggering (mechanized method)
• Extrusion (mechanized method)
Hand Modeling Creation of the ceramic product by manipulating the mass of
plastic clay into the desired geometry
• Handmolding - similar only a mold or form is used to define portions of the
part geometry
• Handthrowing on a potter's wheel is another refinement of handcraft
methods
Potter's wheel = a round table that rotates on a vertical spindle
• Jiggering Similar to potter's wheel methods, but hand throwing is replaced
by mechanized techniques
Extrusion Compression of clay through a die orifice to produce long sections
of uniform cross-section, which are then cut to required piece length •
Equipment utilizes a screw-type action to assist in mixing the clay and
pushing it through die opening
• Products: hollow bricks, shaped tiles, drain pipes, tubes, and insulators
Also used to make the starting clay slugs for other ceramics processing
methods such as jiggering
Semi-dry Pressing Uses high pressure to overcome the clay’s low plasticity
and force it into a die cavity

Dry Pressing Process sequence is similar to semi-dry pressing - the main

distinction is that the water content of the starting mix is typically below 5%
• Dies must be made of hardened tool steel or cemented carbide to reduce
wear since dry clay is very abrasive
• No drying shrinkage occurs, so drying time is eliminated and good
dimensional accuracy is achieved in the final product
• Typical products: bathroom tile, electrical insulators, refractory brick, and
other simple geometries
 Drying and Firing
• A ceramic piece that has been formed retains significant porosity and
insufficient strength for most practical applications.

• It may still contain some liquid which was added to assist in the forming
operation.

• This liquid is removed in a drying process; density and strength are

enhanced as a result of a high temperature heat treatment or firing
procedure.

• A body that has been formed and dried but not fired is termed green.

• Drying and firing techniques are critical because defects that render the
ware useless (distortion and cracks) may be introduced during the
operation.

• These defects normally result from stresses that are set up from nonuniform
shrinkage.
 Drying
• As a clay based ceramic body dries, it experiences some shrinkage.
• In the early stages of drying the clay particles are virtually surrounded by
and separated from one another by a thin film of water.
• As drying progresses and water is removed, the interparticle separation
decreases, which is manifested as shrinkage.

Several stages in the removal of water from between clay particles

during the drying process. (a) Wet body (b) Partially dry body (c)
Completely dry body
 Drying
During drying it is critical to control the rate of water removal. Drying at interior
regions of a body is accomplished by the diffusion of water molecules to the
surface where evaporation occurs.
If the rate of evaporation is greater than the rate of diffusion, the surface will
dry more rapidly than the interior.
Other factors also influence shrinkage. One of these is body thickness,
nonuniform shrinkage and defect formation are more pronounced in thick
pieces than in thin ones.
Firing

Heat treatment process that sinters the ceramic material.

After drying, a body is usually fired at a temperature between 900 and 1400C;
• Performed in a furnace called a kiln

• Bonds are developed between the ceramic grains, and this is accompanied by
densification and reduction of porosity • Additional shrinkage occurs in the
material in addition to that which has already occurred in drying.
• In the firing of ceramics, some complex reactions occur.
One of these is vitrification, the gradual formation of a liquid phase that flows into
and fills some of the pore volume and acts as a binder.
The degree of vitrification depends on firing temperature and time, as well as the
composition of the body.
The temperature at which the liquid phase forms is lowered by the addition of
fluxing agents. This fused phase flows around the remaining unmelted particles
and fills in the pores.
Upon cooling, this fused phase forms a glassy matrix that results in a dense,
strong body.
For a powder compact, microstructural
changes that occur during firing.
(a) After pressing many of the powder
particles touch one another.
(b) During the initial sintering stage,
necks form along the contact
regions between adjacent particles;
in addition, a grain boundary forms
within each neck and every
interstice between particles
becomes a pore.
(c) As sintering proceeds, the pores
change size and shape, becomes
smaller and more spherical in
shape.
• Glazing Application of a ceramic surface coating to make the piece more
impervious to water and enhance its appearance
• The usual processing sequence with glazed ware is: • Fire the piece once
before glazing to harden the body of the piece • Apply the glaze • Fire the
piece a second time to harden the glaze