6. processing of ceramics

ME 312 Manufacturing Technology

Vikrant Sharma, Mechanical Engineering Department. FET. MITS

PROCESSING OF CERAMICS

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 ME 312 Manufacturing Technology

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

Education