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Module 4 (part one)

Ceramics

Introduction

Generally ceramics are inorganic compounds, composed of more than one element formed

from metallic (Al, Mg, Na, Ti, W, Si, B) and non-metallic (O, N, C) elements. For example,

alumina (Al2O3) is a ceramic made up of aluminum atoms and oxygen atoms. (e.g. NaCl,

SiC, SiO2).

Bonds are partially or totally ionic or the combination of ionic and covalent bonding.

Ceramics are typically characterized as possessing a high melting temperature (i.e.,

“refractory”), hard and brittle, electrical and thermal insulators.

Classification of ceramics

Traditional ceramics - clay based material (product like clay brick, glasses and tile,

portland cement etc).

Advanced ceramics - typically consist of pure or nearly pure compounds such as

aluminum oxide, silicon carbide, silicon nitride and zirconium oxide etc.

Advanced ceramics exhibits superior mechanical, electrical, optical, and magnetic properties

and corrosion or oxidation resistance. Applications: heat engines, cutting tools, die materials,

superconductors etc

Properties of ceramics

The properties of ceramic materials, like all materials, are dictated by the types of atoms present, the

types of bonding between the atoms, and the way the atoms are packed together. The bonding of

atoms together is much stronger in covalent and ionic bonding than in metallic. That is why, generally

speaking, metals are ductile and ceramics are brittle. Due to ceramic materials wide range of

properties, they are used for a multitude of applications.

In general, advanced ceramics have the following inherent properties:

Hard and wear resistant)

Resistant to high temperatures

Good corrosion resistance

Low thermal conductivity

Low electrical conductivity

Brittle (Resistant to plastic deformation)

Chemically inert

Non magnetic

Low tensile and fracture strength

However, some ceramics exhibit high thermal conductivity and/or high electrical conductivity.

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Ceramics usually have a combination of stronger bonds called ionic and covalent bonds.

Ionic and covalent bonds are stronger than metallic bond. The strength of an ionic bond

depends on the size of the charge on each ion and on the radius of each ion. These types of

bonds result in high elastic modulus and hardness, high melting points, low thermal

expansion, and good chemical resistance. On the other hand, ceramics are also hard and often

brittle, which leads to fracture.

In general, metals have weaker bonds than ceramics, which allows the electrons to move

freely between atoms. This type of bond results in the property called ductility, where the

metal can be easily bent without breaking, allowing it to be drawn into wire. The free

movement of electrons also explains why metals tend to be conductors of electricity and heat.

Applications

Ceramics offer many advantages compared to other materials. They are harder and stiffer

than steel; more heat and corrosion resistant than metals or polymers; less dense than most

metals and their alloys; and their raw materials are both plentiful and inexpensive. Ceramic

materials display a wide range of properties which facilitate their use in many different

product areas.

Advanced ceramics has excellent properties of high strength, high temperature, wear

resistance, corrosion resistance, high insulation, which metal, plastic and other materials don't

possess, and has been widely used in electronic, electrical, mechanical, aerospace, chemicals,

textiles and many other fields.

Aerospace: space shuttle tiles, thermal barriers, high temperature glass windows, fuel

cells

Used as cutting tool

Used in military –ceramic armour, structural components for ground, air and naval

vehicles, missiles, sensors

Automotive: catalytic converters, ceramic filters, airbag sensors, spark plugs, pressure

sensors, vibration sensors, oxygen sensors, safety glass windshields, piston rings

Computers: insulators, resistors, superconductors, capacitors, ferroelectric

components, microelectronic packaging

Consumer Uses: glassware, windows, pottery, magnets, dinnerware, ceramic tiles,

lenses, home electronics, microwave transducers

Ceramic crystal structure is a unique arrangement of atoms or molecules in a crystalline

liquid or solid. Since ceramics are composed of two or more elements, their crystal structures

tend to be more complex than those of metals.

Ceramic crystal structure is determined by the relative charge and relative size of the anion

and cation. Remember that in a ceramic, the cation gives up electrons to the anion, so the

anion is normally much larger.

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Crystal structure is defined by:

1. Magnitude of the electrical charge on each ion.

Crystal must be electrically neutral (total cation, anion charges must be equal).

Chemical formula of a compound indicates the ratio of cations to anions, or

composition that achieves this charge balance,

E.g. in CaF2: 2 F – ions (negative ions) and 1 Ca

2+ ion (positive ions).

2. Relative size of the cation and anion. The ratio of the atomic radii (rcation/ranion) dictates the

atomic arrangement. Stable structures have cation/anion contact.

(i.e. Crystal structure of the ceramic is determined by the coordination number)

Stable ceramic crystal structures: anions surrounding a cation are all in contact with that

cation otherwise unstable. For a specific coordination number there is a critical or minimum

cation anion radius ratio rC/rA for which this contact can be maintained.

The metallic ions, or cations, are smaller and positively charged since they give up their

valence electrons to the non-metallic, negatively charged ions, or anions. Usually compounds

between metallic ions (e.g. Fe, Ni, Al) – called cations and non-metallic ions (e.g. O, N, Cl) -

called anions. Cations (positive electric charge (e.g. Na+)) usually smaller than anions

(negative electric charge (e.g. Cl-). Each tries to maximize number of opposite neighbours.

Co ordination number

The Coordination Number (CN) is defined as the number of anions that can fit around a

cation. This number increases as the radius ratio increases. The number of anions that can

„fit‟ around a cation is related to the relative size difference between the ions, and this size

difference can be described using the radius ratio.When this number is small, then only a few

anions can fit around a cation. When this number is large, then more anions can fit around a

cation. When CN is 4, it is known as tetrahedral coordination; when it is 6, it is octahedral;

and when it is 8, it is known as cubic coordination.

Radius ratio

Radius ratio is the ratio of the ionic radius of the cation to the ionic radius of the anion in a cation-

anion compound. This is simply given by The radius ratio when the anions just start

to contact each other and the central cation

is critical (limiting/minimum) radius

ratio. As the size of a cation increases,

more anions of a particular size can pack

around it. This ratio can be determined by

simple geometrical analysis.

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LIMITING RADIUS RATIO FOR VARIOUS TYPES OF CRYSTAL STRUCTURE

r/R CN Structural

arrangement

Arrange of anions around the

cation Example

0.15 – 0.225 3 Trigonal Corners of equilateral triangle Boron

oxide

0.225 – 0.414 4 Tetrahedral Corners of a tetrahedron ZnS

0.414 – 0.73 6 octahedral Corners of an octahedron NaCl

0.732 – 1.000 8 Cubic corners of a cube CsCl

Calculation of radius ratio for different co-ordination number

Radius ratio for co-ordination number 3

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Radius ratio for co-ordination number 4

We can assume anion are at 4 corners of cube of size “ a”

rc= radius of cations

ra=radius of anions

face diagonal of cube =ra+ra

2ra = 2a

Body diagonal of cube 2ra+2rc =a 3

2ra+2rc= 3*2

2 ar

225.0a

c

r

rie

Radius ratio for co-ordination number 6

Radius ratio for coordination 8

Consider a cube having a side “a”= 2ra

Then the diagonal of that cube (room diagonal)

Diagonal of that cube given by a3

Also diagonal = 2ra+2rc

ie = ca rra 223

731.013

222*3

a

c

caa

r

r

rrr

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Ceramics structures types

Ceramics structure is based on chemistry and charge magnitude of cations and anions.

General form of ceramics structure is represented in the form “AmXp”

A represent metal

X represent non metal

m & p magnitude of cation and anions charge to make neutral structure

More than one type of atoms (cations, anions).

Complex structures, based on BCC, FCC, and HCP.

Standard type of crystal structures

1. AX - NaCl, CsCl, ZnS etc

2. AmXp - More complex structures: CaF2, UO2, Si3N4, etc.

3. AmBnXp - Yet more complex structures: BaTiO3, etc.

AX-Type Crystal Structure

Most common ceramics are made of equal number of cations and anions, and are referred to

as AX compounds (A-cation, and X-anion). These ceramics assume many different

structures, named after a common material that possesses the particular structure.

Examples for AX ceramics structure

Rock Salt Structure -NaCl, MgO, MnS, LiF, FeO

Cesium Chloride Structure (CsCl)

Zinc Blende Structure (ZnS, ZnTe, SiC)

1. AX-Rock Salt Structure: The rock salt structure is like two superimposed FCC structures.

Eg) NaCl

Rock salt structure: here the coordination number is 6, i.e. rc/ra= 0.414-0.732. This

structure can be viewed as an FCC of anions with cations occupying center of each edge and

the center of the cell. Thus it can be said that lattice is made of two interpenetrating FCC

lattices, one composed of cations, and the other of anions. E.g.: NaCl, MgO, FeO.

Structures are named based on the first mineral that is discovered to have the structure. (e.g.,

rock salt structure)

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2. Cesium Chloride Structure

Coordination number for both ions: 8

rC/rA=0.939

8 anions at cube corners and 1 cation at

center of cube, simple cubic (not BCC)

This is not BCC crystal structure

because (cation and anion - two

different ions are involved).

3. Zinc Blende structure (ZnS)

Coordination number: 4

“S” atoms: at cube corners and face

positions

Zn atoms: interior tetrahedral

positions, Each Zn atom is bonded to

four S atoms and vice versa. Often

highly covalent. eg. ZnS, ZnTe, SiC

AmXp-TYPE structure The number of cations charge and anions charge are not equal (m and p are not same). For

CaF2, show figure below. Same as CsCl, but half of the cation (Ca) sites are empty.

Structure where cations and anions are

not the same

rC/rA = 0.75

C.N. = 8

Example - (CaF2,UO2, PuO2 and ThO2)

AmBnXp-TYPE structure

StructuresIt is also possible for ceramic compounds to have more than one type of cation; for

two types of cations(represented by Aand B), their chemical formula may be designated as

AmBnXp. Barium titanate(BaTiO3), having both Ba2+and Ti4+cations, falls into this

classification.

Consists of more than one cation

charges

eg) BaTiO3, CoTiO3, SrTiO3

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Imperfections in material

The periodic nature of crystalline materials can be interrupted by imperfections. Imperfections in

ceramic crystals include point defects and impurities like in metals. It is important to have

knowledge about the types of imperfections that exist and the roles they play in affecting the

behavior of materials.

Types of Imperfections

Vacancy atoms

Interstitial atoms Point defects

Substitutional atoms

Dislocations ---- Line defects

Point Defects in Ceramics

Point defects include the Frenkel and Schottky defects. Frenkel or Schottky defects: no

change in cation to anion ratio →compound is stoichiometric.

Vacancies: vacancies exist in ceramics for both cations and anions

Interstitials: interstitials exist for cations (interstitials are not normally observed

for anions because anions are large relative to the interstitial sites)

Frenkel defect: a cation vacancy and a

cation interstitial or an anion vacancy

and anion interstitial. A Frenkel-defect

occurs when a host atom moves into a

nearby interstitial position to create a

vacancy-interstitial pair of cations.

Schottky defect: pair of anion and cation

vacancies. Schottky defect occurs

when a host atom leaves its position

and moves to the surface creating a

vacancy-vacancy pair.

Impurities in Ceramics

Impurities are atoms which are different from the host. Impurity atoms can exist as either

substitutional or interstitial solid solutions.

Phase diagram of ceramics

Phase diagrams map the number and types of phases of phases that are present, the

composition of each phase, and the microstructures that exist. The phase diagram is important in understanding the formation and control of the microstructure of poly phase ceramics.

Phase diagram of Al2O3 – Cr2O3

Alumina - Chrome (Al2O3 - Cr2O3) refractory oxides system has found many applications in

the industry. The Al2O3–Cr2O3 is a substitutional solid solution in which Al3+

substitutes for

Cr3-

and vice versa. It consisting of single liquid and single solid phase regions separated by a

two-phase solid–liquid region having the shape of a blade. It exists for all compositions

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below the melting point of Al2O3 where both aluminum and chromium ions have the same

charge as well as similar radii (0.053 and 0.062 nm, respectively). Furthermore, both Al2O3

and Cr2O3 have the same crystal structure.

Melting point of pure Al2O3 is about 2050 0C and melting point of pure Cr2O3 is about 2275

0C. The solidus temperatures of both components are above 2000

0C, there is no danger of

melting any of constituents.

The phae diagram interprets that at temperature below the solidus line, all mixtures of Al2O3 – Cr2O3

are solid solutions. Cr2O3 substitutes into Al2O3 crystal lattice and Al2O3 substitutes into Cr2O3

crystal lattice. At temperature above liquidus line all mixtures of Al2O3 – Cr2O3 are liquid and the

Al2O3 – Cr2O3 mix with each other. Between the liquidus and solidus lines, both liquid and solid

phase exits

Phase diagram of MgO - Al2O3

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There exists an intermediate phase, compound called spinel, which has the chemical formula

MgAl2O4 (or MgO–Al2O3). Even though spinel is a distinct compound (72 wt% Al2O3–28

wt% MgO), it is represented on the phase diagram as a single-phase field rather than as a

vertical line. There is a range of compositions over which spinel is a stable compound.

Furthermore, there is limited solubility of Al2O3 in MgO below about 1400 0C at the left-hand

extremity of figure which is due primarily to the differences in charge and radii of the Mg2+

and Al3+ ions. For the same reasons, MgO is virtually insoluble in Al2O3, as evidenced by a

lack of a terminal solid solution on the right-hand side of the phase diagram.

Pure alumina melts at 20540C

Pure magnesia melts at 28000C

Magnesia can dissolve up to 2% alumina at 20000C called periclase.

Mechanisms of plastic deformation of ceramic materials

In materials science, deformation is a change in the shape or size of an object due to an

applied force or a change in temperature. At room temperature most ceramic materials

suffer fracture before the onset of plastic deformation.

Deformation of Crystalline ceramics

For crystalline ceramics, plastic deformation occurs, by the motion of dislocations (slip),

which is difficult due to the structure and the strong local (electrostatic) potentials. There is

very little plastic deformation before fracture. One reason for the hardness and brittleness of

these materials is the difficulty of slip (dislocation motion). This is not a problem in metals,

since all atoms are electrically neutral.

Ceramics in which the bonding is highly covalent, slip is difficult and they are brittle for the

following reasons:

(1) the covalent bonds are relatively strong;

(2) there are also limited numbers of slip systems; and

(3) dislocation structures are complex.

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Deformation of non-crystalline ceramics

Non-crystalline ceramics, (common glass) deform by viscous flow (like very high-density

liquids). Viscosity decreases strongly with increases temperature. Plastic deformation does

not occur by dislocation motion for noncrystalline ceramics because there is no regular

atomic structure. Rather, these materials deform by viscous flow, the same manner in which

liquids deform; the rate of deformation is proportional to the applied stress.

In response to an applied shear stress, atoms or ions slide past one another by the breaking

and reforming of inter-atomic bonds. However, there is no prescribed manner or direction in

which this occurs, as with dislocations. Viscous flow on a macroscopic scale is demonstrated

in Figure.

Representation of the viscous flow of a liquid

or fluid glass in response to an applied shear

force

The characteristic property for viscous flow, viscosity, is a measure of a noncrystalline

material‟s resistance to deformation. For viscous flow in a liquid that originates from shear

stresses imposed by two flat and parallel plates, the viscosity )( is the ratio of the applied

shear stress )( and the change in velocity dv with distance dy in a direction perpendicular to

and away from the plates,

Mechanical properties of ceramics

Some mechanical properties of ceramics materials make its application wide in engineering

field.

Mechanical Properties

1. Ceramics posses great hardness and resistance to wear and can be used for grinding.

2. Ceramics posses good compressive strength.

3. Ceramic materials have low tensile strength.

4. They generally fail due to stress concentration on cracks, pores etc.

5. Most ceramics posses low fracture strength and fail in a brittle manner.

6. Values of Modulus of Elasticity for ceramics ranges from

*** N/m2 to ****N/m

2 .

This strong bonding also accounts for the less attractive properties of ceramics, such as low

ductility and low tensile strength. The absence of free electrons is responsible for making

most ceramics poor conductors of electricity and heat.

Ceramics are brittle.

The compressive strength is typically ten times the tensile strength. This makes

ceramics good structural materials under compression (e.g., bricks in houses, stone

blocks in the pyramids).

Other electrical, magnetic and chemical properties also make the ceramics unique in

application

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Application of advanced ceramics

Introduction

Up until the past 50 or so years, the most important ceramic materials were termed the

„„traditional ceramics,‟‟. Of late, significant progress has been made in understanding the

fundamental character of these materials and of the phenomena that occur in them that are

responsible for their unique properties. Consequently, a new generation of these materials has

evolved, and the term „„ceramic‟‟ has taken on a much broader meaning. Some of these, the

„„advanced ceramics,‟‟ have begun and will continue to establish a prominent niche in our

advanced technologies. In particular, electrical, magnetic, and optical properties and property

combinations unique to ceramics have been exploited in a host of new products. These new

materials have a rather dramatic effect on our lives; electronic, computer, communication,

aerospace, and a host of other industries rely on their use. Some of these will now be

discussed.

Heat engine applications

Advanced ceramic materials are just beginning to be used in automobile internal combustion

engines. The principal advantages of these new materials over the conventional metal alloys

include:

the ability to withstand higher operating temperatures, thereby increasing fuel

efficiency;

excellent wear and corrosion resistance - long life to engine

lower frictional losses; the ability to operate without a cooling system;

lower densities, which result in decreased engine weights.

Such engines are still in the developmental stage; however, ceramic engine blocks as well as

valves, cylinder liners, pistons, bearings, and other components have been demonstrated.

Furthermore, research is also being conducted on automobile gas turbine engines that employ

ceramic rotors, stators, regenerators, and combustion housings. On the basis of their desirable

physical and chemical characteristics mentioned above, advanced ceramic materials will, at

some future time, most certainly be utilized in jet aircraft engines. Materials presently under

consideration for use in ceramic heat engines include silicon nitride (Si3N4), silicon carbide

(SiC), and zirconia (ZrO2). The wear resistance and/or high-temperature deterioration

characteristics of some metal heat engine parts currently in use have been improved

significantly by using ceramic surface coatings.

The chief drawback to the use of ceramics in heat engines is their disposition to brittle and

catastrophic failure, due to their relatively low fracture toughnesses. Techniques are currently

being developed to enhance the toughness characteristics of these materials; these involve

ceramic-matrix composites. Furthermore, improved material processing techniques are

necessary so as to produce materials that have specific microstructures and, therefore,

uniform and reliable mechanical and corrosion-resistant characteristics at elevated

temperatures.

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Ceramic armor

Some of the new advanced ceramics are being used in armor systems to protect military

personnel and vehicles from ballistic projectiles. Most ceramic armor systems consist of one

or more outer ceramic facing plates that are combined with a ductile and softer backing sheet.

Ceramic armor materials include alumina (Al2O3), boron carbide (B4C), silicon carbide (SiC),

and titanium diboride (TiB2).

Upon the impact of missile or bullet, the solid ceramics takes the impact and becomes powder

which is intact with backing materials and penetrates through the very hard and wear resistant

ceramic powder takes away the impact completely.

The armor backing must absorb the remaining projectile kinetic energy by deformation and,

in addition, restrain continued penetration of projectile and ceramic fragments. Aluminum

and laminates of synthetic fibers embedded in a plastic matrix are commonly used.

Electronic packaging

The electronics Industry is continually looking for new materials to keep up with its ever

changing technologies. Of particular interest is the packaging of integrated circuits (ICs). For

some package designs, the ICs are mounted on a substrate material that must be electrically

insulating, have appropriate dielectric characteristics (i.e., low dielectric constant), as well as

dissipate heat generated by electrical currents that pass through the electronic components

(i.e., be thermally conductive). As the IC electronic components become packed closer

together, this dissipation of heat becomes an increasingly more critical consideration.

Aluminum oxide has been the standard-bearer substrate material; its chief limitation,

however, is a relatively low thermal conductivity. As a general rule, materials that are poor

electrical conductors are also poor thermal conductors, and vice versa. These include boron

nitride (BN), silicon carbide (SiC), and aluminum nitride (AlN). Currently, the most

promising substrate alternative is AlN, which has a thermal conductivity a factor of 10 better

than that for alumina.

Ceramics processing methods

The very specific character of ceramics high temperature stability and high hardness makes

conventional fabrication routes unsuitable for ceramic processing. Ceramic processes involve

forming, firing and finishing. Most other ceramic products are manufactured through powder

metallurgy processing. Powder is added with water and/or additives such as binders, followed by a

shape forming process. Other forming methods for ceramics processing include extrusion, slip

casting, pressing, tape casting and injection molding.

• Extrusion – viscous mixture of ceramic particles, binder and other additives is fed

through an extruder where continuous shape of green ceramic is produced. Then the

product is dried and sintered.

• Injection molding –Mixture of ceramic powder, plasticizer, thermoplastic polymer,

and additives is injected into die with use of an extruder. Then polymer is burnt off,

before sintering rest of the ceramic shape. It is suitable for producing complex shapes.

Extrusion and Injection molding are used to make ceramic tubes, bricks, and tiles

• Powder metallurgy process

• Slip casting (mentioned in PM syllabus)

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Question from old syllabus

Ceramics

1. What are the two basic classes of ceramic material and how does their processing

differ

(Traditional and advanced ceramics)

2. Explain the advantages of ceramics

3. Give functional classification of ceramics

4. What are the properties of ceramics

5. Write short note on ceramics

6. Classify ceramic materials. Discuss the steps of processing of ceramics

7. Discuss advantages and applications of ceramics

8. List the factors that you would account when replacing a metal component with a

ceramics

High temperature stability, wear resistance, hardness, insulating properties

9. How does porosity affect mechanical properties of ceramics and why

The porosity of a ceramic has a major effect on a ceramic's modulus of elasticity (low tensile strength), modulus of rupture (micro crack formation) and insulating properties.

The forming of ceramics from powders necessarily generates porosity. Porous ceramic can be used in place of metals, plastics or fibres providing equal or higher levels of performance and extending the useful life under harsh conditions. Porosity can be tailored but inherent with ceramics products. The most common way of lowering a ceramic's porosity is sintering.

10. Explain why mechanical properties of ceramics are generally higher than those metals

Atomic bonding is ionic and covalent which is stronger than metallic bonding where

slip is restricted. So high temperature stability, hardness, brittleness and insulating

properties is better than metals.

11. What are the imperfections in ceramics? Why it is brittle

Due to ionic and covalent bonds, slipping of bonds are difficult.

12. What are refractories?. How they are classified

Refractories are materials that can withstand high temperatures without softening or

deformation in shape. Refractories are mainly used for construction of lining in furnaces,

kilns, converters, etc. Clay refractories and non clay refractories. Based on chemical

composition it is classified into acidic, basic and neutral refractories.

13. What are the different processing technique of ceramics

14. Discuss the effects porosity of ceramics materials

15. What are cermets. How are they manufactured

16. Explain why ceramics are weaker in tension than compression

Because of its porosity, micro cracks forms resulting crack propagation in tension

17. Explain the modulus of elasticity at elevated temperature

Modulus of elasticity reduces with temperature

18. What type of finishing process used on ceramics

Many ceramics products require some type of finishing operation to obtain the specified

shape, size, surface finish or other. Finishing processes can be performed after the firing

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process. It often consists of some type of material removal or abrasive process such as

sawing, drilling, contouring, chemical machining, laser cutting, grinding or polishing.

19. What are the advantages of glass ceramics

• They have zero or very low porosity.

• It is possible for them to combine a variety of desired properties

20. Why water glass (sodium silicate) added to casting slip

In slip casting water glass(sodium silicate) makes the ceramic slurry (viscous clay-

water mixture ) into one so thin that it runs like water in the mould.

21. Classify the ceramics materials and discuss any two ceramics fabtication process

Question from Composites part

22. Differentiate thermosetting and thermoplastics(refer note for other questions)

23. Define the term polymer

Very high molecular-weight compound made up of a large number of simpler molecules

(called monomers) of the same kind.

24. Explain thermosetting plastics

25. Explain polymer matrix composites along with their properties and applications

26. Explain the application of MMC and CMC

27. What are the advantages of polymer matrix composite

28. Differentiate between thermosetting and thermoplastic with a suitable example

Syllabus for ceramic part

Ceramic Structures and properties: - coordination number and radius rations - AX, AmXp,

AmBmXp type crystal structures – imperfections in ceramics- phase diagrams of Al2O3 – Cr2O3

and MgO- Al2O3 only – mechanical properties – mechanisms of plastic deformation –

ceramic application in heat engine, ceramic armor and electronic packaging.

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General notes

Ceramics can be classified on different basis as follows

1. Functional Classification (application and products):

• Glasses

• Clay products

• Refractories

• Abrasives

• Cements

• Advanced ceramics

o Glasses products: based on SiO2

Containers and optical glasses etc

o Advanced Ceramics products: ceramics having improved toughness, wear

resistance, electrical properties, etc.

Cutting tool, sensor, abrasives, laser, bearing, superconductor

o Traditional Ceramics products: clay-based products

Porcelain, sanitary ware, Bricks, tiles and Refractories

2. Structural Classification: - Crystalline ceramics & Non-Crystalline ceramics.

3. Classification based on composition

• Oxides

• Carbides

• Nitrides

• Sulfides

• Fluorides etc

Examples

o Silicate Ceramics: presence of glassy phase in a porous structure

clay ceramics (with mullite – 3Al2O3 + 2SiO2)

silica ceramics (with cordierite 2MgO + 2Al2O3+. 2SiO)

o Oxide Ceramics: dominant crystalline phase, Al2O3, BaTiO3 etc

o Non-oxide Ceramics: carbon, SiC, BN, TiB2, sialon

o Glass-ceramics: partially crystallised glass SiO2-Li2O

4. Basically (engineering applications) ceramics are classified into two groups as

o Traditional and Engineering ceramics

i. Traditional ceramics most made up of clay, silica and feldspar

ii. Engineering ceramics–these consist of highly purified aluminium

oxide (Al2O3), silicon carbide (SiC) and silicon nitiride (Si3N4)

Electrical Properties of ceramics: Ceramics are often used for electric insulation.

1. Some ceramics conduct electrically well and are used as Semiconductors

2. Many ceramics have a dielectric constant value upto 100 and very low dielectric losses.

3. Some ceramics also exhibit piezoelectric properties and can transfer mechanical

deformations in to voltage changes.

Chemical Properties:-

1. Majority of ceramics are highly resistant to all chemicals and organic solvents.

2. Ceramics are completely resistant to oxidation even at high temperature.

3. Glazed porcelain ceramics is used for chemical vessels.

Optical Properties

1. Many types of glasses are used for windows and optical lenses.

2. They also find use in selective transmission or absorption of certain wavelengths.

Thermal Properties

Ceramic materials do not have enough electrons for bringing about thermal conductivity.

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Silicate Ceramics (glass)

As mentioned previously, the silica structure is the basic structure for many ceramics, as well

as glass. It has an internal arrangement consisting of pyramid (tetrahedral or four-sided) units.

Four large oxygen (0) atoms surround each smaller silicon (Si) atom. When silica

tetrahedrons share three corner atoms, they produce layered silicates (talc, kaolinite clay,

mica). Clay is the basic raw material for many building products such as brick and tile. When

silica tetrahedrons share four comer atoms, they produce framework silicates (quartz,

tridymite). Quartz is formed when the tetrahedra in this material are arranged in a regular,

orderly fashion. If silica in the molten state is cooled very slowly it crystallizes at the freezing

point. But if molten silica is cooled more rapidly, the resulting solid is a disorderly

arrangement which is glass.

Silicates are materials composed primarily of silicon and oxygen (soils, rocks, clays, sand,

and glass). Any material that has solidified and become rigid without forming a regular

crystal structure is known as glass.

• Generally term “glass” commonly applied to silicate based ceramic materials.

• The term glass describes a state of matter where the atoms/molecules are randomly

arranged,

• Composed mainly of silicon and oxygen, the two most abundant elements in earth‟s

crust (rocks, soils, clays, sand)

• the building blocks (SiO4 tetrahedra) are arranged randomly

• Si-O bonding is largely covalent, but overall SiO4 block has charge of –4

• Silica glasses • amorphous, a high degree of atomic randomness

2. There is no long range order, although the silicate tetrahedra are still linked together.

3. Can also be load-bearing (e.g., car window, container glass, vacuum equipment)

Department of Mechanical Engineering SSET

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