INTRODUCTION:
Ceramics is from the Greek word KERAMOS, which means burnt stuff. Ceramics are those containing phases that are compounds of metallic and non-metallic elements. Earlier the term ceramics was applied to products made from natural earth that had been exposed to high temperatures .Now a days the ceramics have even been used as lubricants (Molybdenum disulphide) and fuels (Uranium oxide for nuclear reactors and Boron hydride for solid propellant rockets). Most of the ceramic materials are silicates, aluminates, oxides, carbides, borides, nitrides and hydrides.
CHARACTERISTICS OF CERAMIC MATERIAL
Ceramic materials are stable at relatively high temperatures.They are generally hard ,brittle materials that can withstand compressive load even at high temperatures. They are chemically inert and susceptible to thermal shocks.
STRUCTURE OF CERAMIC MATERIALS:
Most ceramics materials phases like metals have crystal structures. Ceramic crystals are formed by either a pure ionic bond (as in sodium chloride), a pure covalent bond (as in silicon carbide), or by bonds that process the ionic as well as covalent characteristics as in SiO2 and MgO.
The chemical bonds in ceramics are generally ionic bonds, which make ceramic materials relatively highly, stable. They are more resistant to chemical reaction and also posses high hardness, high melting point and low electrical conductivity at room temperatures.
The crystal structure of ceramic materials is invariably more complex as compared to those of metals. Since atoms of different sizes and electronic configurations are assembled together.
They are classified into two different ways.
1. Functional classification indicating particular industries and industrial applications. The table below shows groups and examples for function classification.
GROUPS EXAMPLES
1. Abrasive Alumina, Carborundum
2. Pure oxide ceramics MgO, Al2O3, Si O2.
3. Fired clay products Bricks, tiles, porcelain etc.
4. In-organic glasses Window glass, lead glass
5. Cementing material Portland cement, lime etc
6. Minerals Quartz, Calcite etc.
7. Rocks Granite, sandstone, etc.
8. Refractors Silica bricks, magnesite etc
2. Structural classification indicating the structural criteria. The table below shows the group and examples for structural classification.
GROUP EXAMPLES
1. Crastilline ceramics Single phase like MgO or multiphase from
the MgO to Al2O3 binary system.
2. Non crystalline ceramics Natural and synthetic in-organic glasses
(Eg.Window glass)
3. Glass bonded ceramics Fired-clay products-crystalline phases are
held in glassy matrix.
4. Cements Crystalline or crystalline and non-crystalline
phases.
ABRASIVES
These are the ceramic materials, which are having uniform hardness to a greater extent. They may be natural (e.g. diamond, garnet, flint etc.) or synthetic (e.g. silicon carbide, aluminium oxide). The types and uses (applications) of abrasives are as shown in the table.
TYPES USES /APPLICATIONS
1. Diamond Bricks and concrete saws, wiredrawing dies,
drills for drilling hard rocks, grinding wheel
dressing etc.
2. Garnet and flint Sand paper.
3. Emery Polishing metals.
4. Rottenstone and pumice Wood finishing.
5. Wall nut shells Cleaning aircraft engine parts.
6. Sandstone Sharpening –woodworking tools.
7. Quartz Fine grinding of glass.
8. Tripoli Fine grinding of brass, aluminium & precious metals
9. Silicon carbide Grinding wheels, heating element in electrically
Heated industrial furnaces, polishing wax and pastes.
10. Aluminium oxide (Al2O3) For polishing cast iron, high luster finishing of
Stainless steel, floor sand machines.
11. Carbides of W and BO Boron carbide is used in polishing pastes.
(Modern abrasives)
WHITE WARE
White ware is a product of clay. White ware becomes white after the high temperature firing. White ware includes porcelain,potters ,table ware ,china, sanitary ware etc.
Porcelain is glazed or unglazed vitreous ceramic white ware which is used for technical purposes. Typical porcelain products are electrical, chemical mechanical,structural and thermal ware.
Pottery is a generic name for all fire clay ware. They are soft, non-vitreous and capable of absorbing more than 3% moisture.
China is glazed or unglazed or vitreous (hard and non-absorbent) white ware, which is used for non- technical purposes. The products are of dinnerware and works of arts.
GLASS
Glass is an inorganic product of fusion of one or more oxides of silicon
boron, calcium, magnesium, sodium, etc. cooled to a rigid material without crystallization.
Table below shows the types, properties and uses of glasses.
S.no.
|
Types
|
Properties
|
Uses
|
1.
|
Soda –lime glass
|
Moderate corrosion resistance, withstands high temperature (upto 860°F)
|
Windows, bottles
|
2.
|
Lead glasses
|
Thermal property, corrosion resistance, co-efficient of expansion depends on the lead percentage, possess high electrical resistivity.
|
Neon-sign tubing, electric light bulb stems.
|
3.
|
Borosilicate glass
|
Possess resistance to thermal shock.
|
Glass- to-metal sealing purposes, telescopes, mirrors, electric tubes.
|
4.
|
Aluminosilicate glass
|
Possess resistance to corrosion to thermal shocks, good corrosion resistance.
|
Power tubes, travelling wave tubes, thermometers, stovetop cookware, combustion tubes.
|
5.
|
Fused silica glasses
|
Highly transparent, withstands high temperatures (upto 1650°F)
|
Laboratory optical systems, delay lines in radar installation.
|
6.
|
Recrystal glasses
|
Posses high hardness, impact strength, thermal stability.
|
Refrigerators, Microwave Ovens, cooking dishes,
|
7.
|
Foam glasses
|
Light weight
| |
8.
|
Fibre glasses
|
Non –flamable, conduct neither heat nor electric current, poor conductors of sound and chemically inactive. Possess high tensile stress.
|
Automobile and aircraft parts.
|
BRICKS AND TILES
Bricks and tiles are structural clay products, which are made either by the mud or soft mud process. The plasticity depends on the amount of water used, as salt glazing is a commonly used technique in the manufacture of structural clay products.
The different types of bricks are building brick, facing brick and paving bricks. The different types of tiles are hollow tiles, roofing tiles and drain tile.
CEMENTS AND CONCRETES
CEMENTS
Cement is material for bonding solids together. They may be organic (Rubber cement) or inorganic (Portland cement). As it is inert towards water is absolutely irreplaceable all parts and submarine structure, for construction of dams, for the erection of buildings in dam places etc. Portland cement is cementing material, which is used in to day’s life. This is currently produced by a partial fusion of a mixture of calcareous and argillaceous materials such as clay and limestone. And oxides of silicon, aluminium, iron and often magnesium. A small amount of gypsum is added to the cement powder to prevent the cement from taking a “flash set” when mixed with water.
CONCRETE
In combination with water, cements weld together aggregates of crushed stones, gravels and sand in to a coherent and solid mass called concrete. This is less expensive and stronger than Portland cement. Structures made of concrete bases on iron beams and rods are called reinforced concrete structure.
Water is added to concrete to produce hydrates and to lubricate the aggregates to make more workable. If the optimum water – cement ratio will not be followed, then there will be a formation of porosity which weakens the concrete strength. Asphalt paints or salts of fatty acids are used to minimize or avoid the porosity of concrete even when it is made by optimum water –cement ratio. Concrete is widely used to built vaults, arches, bridges, cisterns tanks, etc.
DIFFERENT TYPES OF CEMENTS
Natural cements, artificial cements, quick setting cements, rapid hardening cements, sulphate resisting cements, low heat cement, high alumina cement, coloured cement, blast furnace slag cement, hydrographic cement, etc.
REFRACTORIES
Refractors are ceramic materials that can withstand usually high heat as well as abrasion and the corrosive effects of acids and the alkaline. They can withstand high temperatures (550-2150°c) without being fused. They also posses low thermal co-efficient of expansion, chemically inert, withstands high pressure and insulating for heat and electric current.
Refractors are grouped into 3 categories. And they are as shown below.
S.NO.
| CATEGORY | PROPERTY | USES | ||
1.
|
Acid refractors:
E.g. Silica, Alumina, Aluminium silica, Silicate.
|
Withstands high temperatures (1700-1900°c)
|
Used for arched roofs of steel and glass making furnaces.
| ||
2.
|
Basic refractors:
E.g. Magnesia
|
Withstands high temperatures (2800°c) and also resistance to attack slag.
|
Used in steel making open-hearth furnaces.
| ||
3.
|
Neutral refractors:
E.g.: chromite, graphite
|
Withstands high temp (2180°c-3000°c)
|
Used in construction of furnaces.
| ||
INSULATORS
Electric insulators are generally ceramic materials and they prevent the flow of electric current throw it. They contain only covalent, ionic and or molecular bonds .An insulator or Dielectric has a specific resistance in the range of 108to1020 Ω-cm. They have wide forbidden energy gap between conduction band and valence band. The bonds in insulated materials can breakdown under high electrical voltages and conducts, but usually they break down is a surface phenomenon. They are glazed to make them non-absorbent. Examples of insulating materials are: -
(a) Ceramic insulators: - Asbestos, calcium silicate, rock wool, haydite, vermiculite, fiberglass, perlite, etc.
(b) Plastic insulators: - Porcelain and Mylar.
PROPERTIES OF CERAMIC MATERIALS
Mechanical properties
(a) Hardness and resistance to wear
Certain ceramics like carborandum and nitrite have great hardness and resistances to wear which are very useful for grinding wheels and cutting tools. Certain ceramic materials having hardness and resistance to wear are being used for tape recorder (pick-up and play back) heads.
(b) Tensile strength
Theoretically ceramics possess high tensile strength but in practice it is quite low. This is because, of stress concentration due to porosity. Ceramics generally fail due to this low tensile strength.
(c) Compressive strength
As compared to tensile strength ceramics possess high compressive strength. For example compressive strength of alumina ranges from 19500 to 3500 kg/cm2. Bricks, cements and glass are always used in compression rather than in tension.
(d) Transverse strength
Transverse strength is difficult to ascertain in ceramic materials. Therefore these materials are not used where transverse strength is an important criterion. Transverse strength of alumina is 3500kg/cm2.
(e) Fracture strength:
Ceramic materials do face impact loading under certain conditions. Impact value of stoneware is 1.1-1.3N-m and for vitrified ware 0.7-1.2N-m.
(f) Torsion strength:
It is seldom considered as a critical property of ceramics since tensile and cantilever requirements will show the torsional strength of the material.
(g) Shear strength: -
High shear strength and low fracture are general characters of ceramic material with the exception of a few materials such as clay. Thus they commonly fail non-ductility i.e. in a brittle manner or fracture.
(h) Modulus of elasticity:
The value of modulus of elasticity for ceramic material ranges from 7x 1010 to 40x1010 N/m2.
(i) Plastic deformation:
The ceramic material does not permit plastic deformation since they have greater resistance to slip (Resistance to slip is more than that in metals).
ELECTRICAL PROPERTIES
Electrical properties of ceramics depend on the composition, texture, density, size, temperature and time.
(a) Electrical insulation: -
Ceramic materials are used in electric circuit both as electrical insulators and as its functional parts. Porcelain, Steatite, forsterite, and alumina are often used as electrical insulators.
(b) Di-electric property: -
The Di-electric is an other name for insulator because material separating two charged bodies is known as Di-electric. Ceramic material has good Di-electric capacity and that is why they are used as cores in transformers, and Di-electric medium in condensers. Glass, porcelain, alumina, quartz, mica good Di-electric materials.
(c) Electrical conductivity: -
Ceramics are practically non-conductors at lower temperature, particularly in cold conduction since they do not possess movement of free electron at this temperature. But some ceramics such as Iron oxide, Nickel, Cobalt oxide and Barium titanate conduct electricity quite well at room temperature which constitute a very special class of semi-conductors (N.T.C&P.T.C resistors).
THERMAL PROPERTIES:
Ceramics possess favorable thermal properties at high temperature and under oxidizing condition.
- Thermal capacity:
The specific heat of fire clay bricks is 0.25 at 1000°C and 0.297 at 1400°C. Carbon bricks have specific heat of about 0.812 at 200°C and 0.412 at 1000° C.
- Thermal conductivity:
Since there are no enough free electrons, the heat is conducted by Phonon conductivity and the interaction of the lattice vibration while at high temperature conduction takes place by the transfer of radiant energy. Thermal conductivity decreases with impurity and porosity.
- Thermal shock:
Ceramics have better thermal shock resistance property. Lithium compounds are used in ceramics to reduce the thermal expansion and to provide excellent thermal shock resistance. Common ceramic materials graded in order of decreasing thermal shock resistance are hot pressed silicon nitride, fused silica, cordierite, zircon, silicon carbide, berylliea, alumina, porcelain and stearite.
Applications of Ceramic Materials
- The application of ceramic materials in the aerospace industries includes the use of alumina ceramics for missile and rocket nose cones. Silicon carbide or molybdenum di-silicide for rocket nozzles etc.
- Enriched uranium-di-oxide is used as the fuel element in nuclear power generation plant.
- Laser material (part of ceramics) are used for machining, welding, cutting etc.
- Piezo electric materials such as barium titanite and leas zirconate-titanate are used in photograph, cartridges and ultra sonic devices.
COMMENTS