6. processing of ceramics

ME 312 Manufacturing Technology
Vikrant Sharma, Mechanical Engineering Department. FET. MITS

Introduction:
The metallic and nonmetallic materials are not suitable for certain engineering
applications- for example
 an electrical insulator to be used at high temperature
 floor tiles to resist spills, scuffing and abrasion
 a transparent baking dish
For such applications we need materials that have properties such as
high temperature strength; hardness; inertness to chemicals, food; resistance
to wear and corrosion. Few of such materials are ceramics, glass, graphite,
diamond etc.
Vikrant Sharma , FET. MITS

For purposes of organization, we classify ceramic materials into three basic
types:
(1) traditional ceramics—silicates used for clay products such as pottery and
bricks, common abrasives, and cement
(2) new ceramics—more recently developed ceramics based on nonsilicates
such as oxides and carbides, and generally possessing mechanical or physical
properties that are superior or unique compared to traditional ceramics; and
(3) glasses—based primarily on silica and distinguished from the other
ceramics by their noncrystalline structure.

Ceramics :
Ceramics are compound of metallic and non metallic elements.
The word ceramic traces from the Greek keramos meaning potter’s clay and
keramikos meaning clay products.
Among the oldest of the raw materials used for making ceramics is clay. The
most commonly given example is kaolin (from Kao-ling, a hill in China)
 Other major raw materials for ceramics that are found in nature are flint (a
rock composed of very fine-grained silica, SiO2 and feldspar ( a group of
crystalline minerals consisting of aluminum silicates plus potassium, calcium
or sodium)

Shaping Ceramics:
(Adapted from Fundamentals of Modern Manufacturing M. P. Groover)

Preparation of Powders:

Shaping Processes:
The shaping processes can be divided according to the consistency of the
mixture:
(1) slip casting, in which the mixture is a slurry with 25% to 40% water;
(2) plastic-forming methods that shape the clay in a plastic condition at 15% to
25% water;
(3) semi-dry pressing, in which the clay is moist (10% to 15% water) but has
low plasticity; and
(4) dry pressing, in which the clay is basically dry, containing less than 5%
water. Dry clay has no plasticity.

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.
 Used for large parts, complex shapes; low equipment cost.
 Low production rate, limited dimensional accuracy.

Plastic Forming:
 Plastic forming, also called soft, wet forming, can be carried out by various
methods, such as extrusion, injection molding, or molding and jiggering.
(Adapted from Manufacturing Engineering and Technology, S. Kalpakjian)

Semi dry pressing:

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:
 The next step in ceramics processing is to dry and fire the part to give it the
proper strength and hardness.
 Water plays an important role in most of the traditional ceramics shaping
processes. Thereafter, it serves no purpose and must be removed from the
body of the clay piece before firing.
 Shrinkage is a problem during this step in the processing sequence because
water contributes volume to the piece, and when it is removed, the volume is
reduced.
As water is removed from the wet clay, the volume of the piece shrinks.

The drying process occurs in two stages:
• Stage 1 - drying rate is rapid and constant as water evaporates from the
surface into the surrounding air and water from the interior migrates by capillary
action to the surface to replace it.
• Stage 2 - the moisture content has been reduced to where the ceramic grains
are in contact.

 Firing also called sintering, involves heating the part to an elevated
temperature in a controlled environment.
 A process similar to the sintering employed in powder metallurgy.
 Firing gives the ceramics part its strength and hardness.
 This improvement in properties results from,
a) development of strong bond between the complex oxide particles in the
ceramics, and
b) reduced porosity.

6. processing of ceramics

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6. processing of ceramics