Ceramics are nonmetallic, inorganic, amorphous solids and are mostly metallic oxides. They have poor tensile strength, and are brittle. They can be either crystalline or noncrystalline. Many ceramics are workable in extremely low (cryogenic) temperature range, while many others are able to sustain high temperatures. Silica, silicates, NaCI, rock salt, MgO, CaF2, glasses, CsCl etc. are some examples of ceramics.
Mechanical Properties of Ceramics
Mechanical properties of ceramics are as under:
Strength: They are much stronger in compression than in tension. Therefore, they are suitable for compressive load applications. Brickwork in a building is one such example.
High temperature resistance: They can sustain high temperatures. Their resistance to abrasion and chemical attack, and rigidity is also high at higher temperatures.
Brittleness, ductility and malleability: They are brittle in nature. They have negligible ductility and poor malleability. Therefore, they are not suitable for tensile load applications.
Melting point: They generally possess a high melting point. For example the melting point of alumina (Al203) is 1700°C, of silica (Si02) is 2000°C, and of zirconia (Zr02) is 3200°C. Thermal conductivity and Thermal expansion are poor in them. Therefore, they are suitable for thermal and electrical insulation purposes.
Stress-strain profile: Generally, they are linearly elastic materials. Their stress-strain curve is a straight line up to fracture point.
Hardness: They are very hard materials. Their hardness normally lies in the range of 500 BHN to 2000 BHN.
Creep behavior: Due to high melting point, ceramics do not normally creep up to workable high temperatures. Therefore, they are good creep resistant materials. Sialon (alloy of Si3N4 and Al203) is a recent material used for making gas turbine blades which are workable up to 1300°C.
Propagation of crack: Under the applied loads when stress concentration is created, the cracks are formed in ceramics. These cracks propagate under tension but are not affected by compressive loads. The compressive load is transmitted across the cracks. Fracture mode. Ceramics fail in brittle mode. Griffith theory for brittle fracture for glasses, also holds good for other ceramics.
Electrical Properties of Ceramics
Ceramics are widely used for insulation purposes in electric power transmission, electrical machines/equipments/devices etc. Mica, porcelain, glass, micanite, glass bonded mica, asbestos, glass tape etc. are commonly used (ceramic) materials. Some important electrical properties of ceramics are as under:
Dielectric constant: Ceramics have high values of dielectric constant. It is 8 for mica, 7 for soda-lime glass, and 6 for porcelain. That is why they are used for insulation purposes in electric motors, alternators, transformers etc.
Dielectric strength of ceramics is high. It is defined as the voltage required per unit thickness to cause a breakdown in material. Dielectric strength of mica is 100 MV/m. This is pretty good strength that makes the mica as a very good insulating material.
Dielectric loss: A good electrical insulator should have a low value of dielectric loss. It is characterized by power factor tan δ, where δ is the loss angle. Value of dielectric loss in mica is only 0.0005 which further supplements its use as a good insulating material.
Characteristics of Good Ceramic Material
A good ceramic material should also possess the following electrical properties.
- High thermal stability
- High abrasion resistance
- High hardness
- High mechanical strength
- High volume resistivity
- Least water absorptivity
- Least effect of oil, and acid and alkalies
Volume resistivity of porcelain is of the order of 109 to 1012 ohm-m and water absorptivity is up to 0.5% only. This makes the porcelain a very favourable insulating material.
Other Behavior of Ceramics
- Imperfections in ceramics are mostly of Frenkel’s defect and Schottky defect types.
- Their destructive testing can be done in a manner almost similar to as described in earlier article.
- Their non-destructive testing may be performed by the techniques discussed in earlier article.
The phase diagrams of ceramics are based on the concepts explained in earlier article.
Applications of Ceramics
Ceramics are widely used materials. They find use in almost all fields of engineering ranging from mechanical, civil and electrical to computer, biomedical and nuclear. Important applications of ceramics are as under:
- As firebricks and fireclay for lining of ovens and furnaces
- As artificial limbs, teeth etc. in biomedical/medical field
- As insulators (dielectrics) in electrical transmission and distribution
- As crockery in domestic uses, sanitary wares etc.
- Components of chemical processing vessels
- As radiation shield for nuclear reactor
- As ferroelectric crystals and Piezoelectric crystals to generate ultrasonic waves for non-destructive testing and other purposes
- As nuclear fuel elements, moderators, control rods etc.
- As magnetic materials in electrical machines, devices etc.
- As cutting tools such as BORAZON (CBN), in explosive forming etc.
- As powerful superconducting magnets to make magnetic rail for wheel-less train (leviated train).
- As ferrites in memory cores of computers.
- As miniature capacitors for electronic circuits.
- As garnet in microwave isolators and gyrators.
- As components of sonar device that helps to locate a small object in large volume (e.g. searching of a black box from within the ocean).
- As zirconia (Zr02) thermal barrier coating to protect superalloy made aircraft turbine engine.
- Ceramic armor to protect military vehicles from ballistic projectiles.
- As electronic packaging to protect ICs, Aluminium nitride (A/N), SiC, boron nitride (BN) are used as substrate materials for this purpose.
Processing of Ceramics
Due to high melting temperatures, hardness and brittleness, the ceramics cannot be fabricated using conventional forming techniques as adopted for metals. Therefore, various other techniques shown in Figure, are adopted for their processing/fabrication. A brief explanation of these processes is as follows.
Glass Forming Processes
Glass products are produced by heating the raw materials above their melting points. As these products have to be homogeneous and pore free, a though mixing of raw ingredients and their complete melting is essential.
To provide porosity in glass products, small gas bubbles are deliberately produced and blown into them in molten state. If porous spots are to be made in colors, the powders of desired colors are also blown along with the gas. Different glass forming processes are used to produce glass products of vivid varieties. These are:-
- Drawing process to form long piece of rods, sheets, tubes etc.
- Pressing process to form thick-walled plates, dishes etc.
- Blowing process to form bulbs, jars,-bottles etc.
- Fibre forming process to form glass fibers.
Drawing process is similar to a continuous hot rolling method in which the glass sheet is drawn from molten glass over the rolls. To provide surface finish and flatness to the sheet, the glass sheet is floated on a bath of molten tin at elevated temperature. The sheet is then slowly cooled and heat-treated by annealing.
Pressing process is accomplished by pressing the glass piece in a graphite-coated cast iron mould. The shape of mould has to be in conformity with the desired shape of product. The mould is heated also to obtain an even surface of the sheet.
Fiber forming is a kind of drawing operation. It is a sophisticated process in which the molten glass contained in a platinum heating chamber, is drawn through several small orifices provided at the chamber base.
Blowing of a glass piece kept in a blow mould is performed by the pressure created from air blast. It can be done manually or by an automated system. The ‘press and blow’ technique used for producing a glass bottle is shown in Figure.
In it a raw glass (gob) is placed in the mould which is pressed mechanically and then blown by compressed air pressure. The contour of the mould has to be prepared in conformity with contour of the bottle desired.
Particulate Forming Processes
Particulate forming processes are generally adopted for manufacturing of clay-based products, fireclay, and refractories. Different particulate forming processes are used to produce vivid varieties of products as given below.
- Powder pressing to form firebricks, refractory abrasives, electronic ceramics, magnetic ceramics etc.
- Slip casting to form sanitary ware, scientific laboratory ware, decorative objects, intricate articles etc.
- Hydroplastic forming to form tiles, pipes, bricks, solid and hollow blocks etc.
- Tape casting to form ceramic tapes for ICs and multilayered capacitors etc.
Powder Pressing: This processing is similar to powder metallurgy technique which is employed for metals. In powder pressing the ceramic in powdered form is mixed with water and binder (organic or inorganic), and is pressed into a mould (or die) of desired shape. The pressing may be done in two different ways viz. 1. Compaction (when pressure alone is applied), and 2. Sintering (when pressure and temperature both are applied).
By compaction the void space is minimized resulting in a dense mass. In sintering process the powder is heated first and then compacted hot. Thus the powder aggregate is compacted at elevated temperature. By doing so, the pores between powder particles become smaller and more spherical in shape. Simultaneously, it results in heat treatment of the product also.
The ceramic powder is prepared by mixing coarse and fine particles of different cross-sections (spherical, triangular, angular etc.,) in appropriate proportions. The binder is used to lubricate the powder particles and binds them in a coherent mass.
Slip Casting: This is a forming process for producing clay-based products. Casting of the product is performed from the ‘slip’ which is used as a raw material. The ‘slip’ is a material comprising of a suspension of clay and/or other nonplastic materials in water.
The ‘slip’ is poured into a porous mould (generally made of plaster of paris) where the water content of ‘slip’ is absorbed into the mould and solid layer of clay is left over. The sequential process of slip casting to manufacture a clay item is shown in Figure. Its details are self-explanatory.
Hydroplastic Forming: Hydroplastic forming refers to a kind of extrusion process (used to manufacture metallic pipes and rods etc.) in which a stiff ceramic mass (highly plastic state of clay mixed with water) is forced through a die/orifice of desired cross-sectional geometry. The stiff ceramic mass is forced through the orifice by means of an ‘auger’ driven by an electric motor. The hollow pieces such as pipes and hollow bricks are made by piercing ‘rod and inserts’ placed within the orifice.
Drying and Firing: The ceramic articles prepared by slip casting or hydroplastic forming contain some water and/or liquid. Therefore, they possess poor strength and high porosity. Hence, they are dried to remove water/liquid, and are heat treated at high temperatures i.e. fired to enhance the strength and density.
These articles may be dried in a conventional way or by microwave energy. During drying, the clay-based ceramic experiences some shrinkage. The firing is normally done between 900°C to 1400°C temperatures.
Tape Casting: This refers to such a casting process in which thin sheets of flexible tape are produced. Thin sheets are prepared from the ‘slip’ that consists of suspended ceramic particles in organic liquid. The tape is formed by pouring the slip onto a flat surface of glass, or stainless steel. Thickness of tape normally varies between 0.1 mm to 2 mm. Tape casted ceramics are used to manufacture integrated circuits (ICs) and multi-layered capacitors.
Types of Ceramics
There are various types of ceramics. Main among them are:
- Plain concretes
- Rocks and stones
- Prestressed concretes
- Reinforced cement concrete (RCC)
- Clay and clay products—bricks and tiles
- Abrasives etc.
Carbides of tungsten, titanium, zirconium, silicon, alkali halides and silicon nitride (Si3N4) are some important ceramic materials of present time.
Refractories are ceramic materials of specific nature which are capable of withstanding high temperatures. Commercial refractories are made of complex solid oxides of elements such as silicon, aluminium, magnesium, calcium and zirconium etc.
Refractories confine the heat in ovens and furnaces by preventing heat loss to the atmosphere. Refractories are uneffected by high temperatures. They are expected to resist mechanical abrasion, infusion of molten metals, slag or metallic vapours; and also the action of superheated steam, sulphurous oxide, chlorine and other gases.
One of the most widely used refractories is based on alumina-silica composition, varying from nearly pure silica to nearly pure alumina. Other common refractories are silica, magnesite, forsterite, dolomite, silicon carbide and zircon.
Refractoriness: It is the ability of a material to withstand the action of heat without appreciable deformation or softening. Refractoriness of a material is measured by its melting point. Some refractories, like fireclay and high alumina brick, soften gradually over certain range of temperatures.
Types of Refractories: According to chemical behaviour, the refractories are classified as follows.
(a) Acidic refractories: They readily combine with the bases, and are therefore termed acidic. Silica is their chief constituent. Important acid refractories are quartz, sand and silica brick etc.
(b) Basic refractories: They consist mainly of basic oxides without free silica and resist the action of bases. The most common basic refractories are magnesite, dolomite etc.
(c) Neutral refractories: They consist of substances which do not combine with either acidic or basic oxides. With increasing alumina content, silica-alumina refractories may gradually change from an acidic to a neutral type. Examples are silicon carbide, chromite and carbon.
Properties of Refractories: Some main properties of refractories are as under:
- They are chemically inactive at elevated temperatures.
- They are impermeable to gases and liquids.
- They have long life without cracking or spalling.
- They are capable of withstanding high temperatures, thermal shocks, abrasion and rough usage.
- Temperature variations cause minimum possible contraction and expansion in them.
- They are able to resist fluxing action of slags and corrosive action of gases.
- They are good heat insulators.
- They have low electrical conductivity.
Silica and silicates
The structures of silica and silicates consist of repeating units of silicate tetrahedron. Each unit has a silicon cation at the center of tetrahedron accompanied with four oxygen anions at the four corners. Three-dimensional network of tetrahedra is formed in the structure of silica.
Electrical neutrality is maintained in silica due to the arrangement of silicon cation and oxygen anions. The effective number of silicon is (1 x 1) =1 per tetrahedral unit and (4 x 1/2) = 2 for oxygen.
Crystalline and non-crystalline forms of silica: Silica exists in both the crystalline and non-crystalline forms having same coordination between silicon and oxygen. Quartz is an example of crystalline form and silica glass of noncrystalline form. Quartz exhibits piezoelectric effect. Silica yields silicates when oxides dissolve in them; Glass of different types are examples of silicates.
Configuration of minerals: Instead of a single silicon cation at the centre of tetrahedron, other cations may be introduced to obtain different mineral structures. Based on the sharing of corners, these minerals may be classified as
- island such as olivine and hemimorphite,
- single chain such as enstatite,
- double chain such as asbestos,
- ring chain such as beryl,
- sheet such as mica, talc and clay.
Glass is an inorganic fusion product, cooled to a condition in which crystallization does not occur. Silica is a perfect glass forming material. It has a very high melting point.
To lower its fusion point and viscosity, some basic metal oxides are added to it. If sodium oxide is added to silica, the mixture obtained is called sodium di-silicate (Na20 . 2 Si02). If calcium oxide is added to it, soda lime glass will be obtained.
Glasses are highly resistant (chemically) to most corrosive agents. Borosilicate and high silica glasses have much higher chemical resistance. Fused silica used in construction of chemical plants are even more chemically resistant.
Besides silica; oxides of boron, vanadium, germanium and phosphorous are the other constituents of glass. Elements and compounds such as tellurium, selenium and BeF2 can also form the glasses. The oxide components added to a glass may be sub-divided as
(a) Glass formers, (b) intermediates, and (c) modifiers. They are grouped on the basis of functions performed by them within the glass.
Glass formers and network formers include oxides such as Si02, B203, P205, V205 and Ge02. They form the basis of random three-dimensional network of glass.
Intermediates include Al203, Zr02, PbO, BeO, TiO3 and ZnO. These oxides are added in high proportions for linking-up with the basic glass network to retain structural continuity.
Modifiers include MgO, Li02, BaO, CaO, SrO, Na20 and K20. These oxides are added to modify the properties of glasses.
The other additions in glass are the fluxes which lower-down the fusion temperature of glass, and render the molten glass workable at reasonable temperatures. But fluxes may reduce the resistance of glass against chemical attack. Devitrified glass is undesirable since the crystalline areas are extremely weak and brittle. Stablizers are therefore added to this type of glass to overcome these problems.
Types of Glasses
Commercial glasses may be of the following four types. 1. Soda lime glass. 2. Lead glass. 3. Borosilicate glass. 4. High silica glass. Chemical compositions of these glasses are given in the following Table.
|Component||Soda lime glass (%)||Lead glass (%)||Borosilicate glass (%)||High silica glass (%)|
|SiO2||70 – 75||35 – 58||73 – 82||96|
|Na2O||12 – 18||5 – 10||3 – 10||–|
|K2O||0 – 1||9 – 10||0.4 – 1||–|
|CaO||5 – 14||0 – 6||0 – 1||–|
|PbO||–||15 – 40||0 – 1||–|
|B2O3||–||–||5 – 20||3|
|Al2O3||0.5 – 2.5||0 – 2||2 – 3||–|
|MgO||0 – 4||–||–||–|
Soda Lime Glasses: Soda lime glasses mainly contain oxides of sodium and calcium, and silica. Its compositional formula is Na20Ca0.6 SiO2. Small amount of alumina and magnesium oxides are added to it to improve the chemical resistance and durability of the glass. Soda lime glasses are cheap, resistant to water and devitrification.
They are widely used as window glass, in electric bulbs, bottles and tablewares where high temperature resistance and chemical stability are not essentially required.
Lead Glasses: Lead glasses contain 15 to 40% lead oxide. They are also known as flint glasses. They are used to make high quality tableware, optical devices, neon sign tubing etc.
Glasses having a high lead content, up to 80%, have relatively low melting points, high electrical resistivity and high refractive index.
They are used for extra dense optical glasses, windows and protective shields to protect against X-ray radiations.
Borosilicate Glasses: Borosilicate glasses contain mainly silica and boron oxide. Small amounts of alumina and alkaline oxide are also added to them. They have low coefficient of thermal expansion and high chemical resistance.
They are used in scientific piping, gauge glasses, laboratory ware, electrical insulators and domestic items. This glass is known as Pyrex glass by trade name.
High Silica Glasses: High silica glasses containing up to 96% silica are made by removing alkalies from a borosilicate glass. They are much more expensive than other types of glasses.
They have very low thermal expansion and high resistance to thermal shock. High-silica glasses are mainly used where high temperature resistance is required.
Other Glasses: Photochromatic and zena glasses are also in common use. Photochromatic glasses are used in making lens for goggles. Silver chloride is mixed in ordinary glass to make them. Zena glass is used to make chemical containers.
Reinforced Cement Concrete (RCC)
Concrete has high compressive strength but poor tensile strength. It is liable to crack when subjected to tension. Hence to provide sufficient tensile strength, reinforcements in the form of steel bars are provided. The combination of concrete and steel reinforcement results in a material known as Reinforced Cement Concrete (RCC).
It contains favourable properties of both the concrete and the steel. Steel is used as reinforcing material due to its high tensile strength and elasticity, and because its thermal coefficient is neary equal to that of concrete.
Ingredients of RCC: Various ingradients of RCC are 1. Cement 2. Fine aggregate (or coarse sand) 3. Coarse aggregate 4. Water 5. Reinforcement
Cement used should be according to the specifications of BIS. Generally sand is used as fine aggregate. Sand should be coarse with finess modulus 2.90 to 3.20. Coarse aggregate may stone grit, gravel or some other inert material. It should be hard, durable and free from other injurious materials such as saltpeter, salts etc.
Generally crushed stone grit, granite tips, stone ballast is used for this purpose. For RCC work, the size of coarse aggregate varies from 2 cm to 3 cm. Fresh drinkable water which is free from impurities like sulphates, chlorides etc. should be used for RCC.
Grades and Strength of RCC: Concrete mix in RCC works is classified into the following categories:
- M10……….. 1:3:6 mix
- M15 …………1:2:4 mix
- M20 …………1:1:5:3 mix
- M25 …………1:1:2: mix
- M30 …………1:4:8: mix, 1:5:10 mix, 1:6:12 mix
- M35 …………Post-tensioned prestressed concrete
- M40 ………… Post-tensioned prestressed concrete
The mix 1 : 3 : 6 indicates proportions of cement, aggregate and sand respectively.
M10 and M20 grade of concrete is used in mass concrete work. M15 is generally used for all purposes of general nature such as RCC slabs, and flooring.
M20 and M25 is used for thin RCC members such as precast members, shed roofs, thin shells etc.
M35 and M40 are used in prestressed concrete structures i.e for post-tensioned and pre-tensioned concrete members respectively. Strength of cement depends on the water cement ratio.
Reinforcing Materials: Following reinforcing materials are commonly used in RCC works:
- Mild steel bars (Plain bars)
- High Yield Strength Deformed (HYSD) bars
- Twisted bars
- Steel wires (or Tendons)
The main drawback of mild steel bars (plain bars) is their poor bond stress. Deformed bars (HYSD bars) are more suitable in comparison to plain bars. Cold worked twisted bars are used to improve the bond between concrete and reinforcing materials.
These bars increase the yield stress by about 50% and thus save on reinforcing material by 33%. These bars have greater bond strength and hence are very useful for water retaining structures such as retaining walls, dams etc. The size of bars varies from 5 mm to 50 mm.
Steel wires with high elasticity are used in prestressed concrete members. They have a diameter of about 6 mm and are known as tendons. They are also used in the form of wire mats.
Advantages of RCC: RCC has many advantages. It can be moulded into any desired shape due to its plastic properties. It is fire resistant, damp proof, rigid, durable and impermeable to moisture. Its structures can bear the shocks of earthquake effectively. Maintenance of RCC work is less while its appearance is good.
Chemically Bonded Ceramics
Dramatic new materials of today and tomorrow; the Chemically Bonded Ceramics (CBC) are high-performance, low-cost materials. CBC materials are mostly silicates, aluminates and phosphates. Their low cost results from processing temperatures far below the 1100°C needed for producing conventional ceramics. Chemically bonded ceramics represent a new technology that produces high-performance ceramics components at a lower cost.
CBCs have 10 to 20 times the tensile strength of concrete; are about 32 times cheaper than aluminium, 20 times cheaper than steel, and 4 to 6 times less expensive than plastics. CBCs have opened a new materials arena where formed shapes are used in the aerospace, automotive, appliance, electrical, electronics, and construction industries.
Applications of Chemically Bonded Ceramics (CBC): Great growth in CBCs is assured, not only due to their low cost and high performance, but also due to a worldwide movement away from materials that are petroleum-based, toxic, or flammable. Currently the following CBCs are in use.
- CBC armour brake linings,
- CBC roofing,
- CBC wall panels,
- CBC flooring,
- CBC electrical fixtures are in use.
CBC materials may soon be used in following applications.
- Fireproof sound dampeners for auto engines and electric motor,
- engine blocks and parts,
- aircraft and rocket parts,
- medical prostheses,
- cryogenic containers and
- containers for superconducting materials.
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