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CERAMICSDefinition:

A compound of metallic and non-metallic elements prepared by the action of heatand subsequent cooling.

There are two general categories of ceramic;

Traditional ceramics – tiles, brick, sewer pipe, pottery

Industrial ceramics (engineering, high-tech, or fine ceramics) – turbine,semiconductors, cutting tools

The structure of ceramics is maybe crystalline or partly crystalline structure, or maybe amorphous.

Generally atoms in ceramics are covalent or ionic bonded and the much stronger ismetallic bonds.

The hardness and thermal and electrical resistance in ceramics are better than inmetals.

The grain size influences the structure of ceramics (finer grain size has give higherstrength and toughness).

The oldest materials to make ceramics is clay (fine-grained sheet l ike structure) i.e.kaolinite (a white clay of silicate of aluminum with alternating weakly bonded layersof silicon and aluminum ions).

The other common materials are flint (a rock composed of very fine grained SiO₂)and feldspar (a group of crystalline minerals of aluminum silicate and potassium,calcium or sodium).

Porcelain is a white ceramic made of kaolin, quartz, and feldspar used mostly inkitchen appliance and bath ware.

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Mechanical properties

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Physical properties Most ceramics have low specific gravity. They also have very high melting or decomposition temperatures. The thermal conductivity of ceramics decrease with increasing

temperature and porosity because air is a poor thermal conductor. k = kₒ (1 – P) k o = thermal conductivity at zero porosity P = the porosity as a fraction of the total volume Thermal shock or thermal fatigue may be caused by internal

stresses formed during thermal expansion and thermal conductivity. Thermal cracking or spalling (a small piece or layer from the surface

break off) will not occur when combine with lower thermal

expansion and high thermal conductivity. Anisotropy of thermal expansion •that varies with different direction

which lead to cracking.

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Alumina Also called corundum or emery

Most widely used

Used in pure form or as raw material

High hardness and moderate strength

Alumina + other oxides are used as refractorymaterials for high-temp applications

Suitable as electrical and thermal insulation,cutting tools/abrasives, etc.

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Zirconia Good toughness, good resistance to thermal

shock, wear and corrosion

Have low friction coefficient Used in hot extrusion die, grinding

beads/dispersion media for aerospace coatings,etc.

Have thermal stability and low thermal

conductivity

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Carbides

Made of tungsten and titanium,silicon

Examples : Tungsten carbide (WC),titanum carbide (TiC), silicon carbide (SiC)

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Nitrides

Cubic boron nitride (CBN)

Titanum nitride (TiN)

Silicon nitride (Si3N4)

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Glass Amorphous solid

Super-cooled liquid (cooled at a rate too high for

crystal formation) Content •more than 50% silica (glass former)

Types of commercial glasses •̈ sodalime glass (mostcommon), lead alkali glass, borosilicate glass,aluminosilicate glass, 96% silica glass, fused silica

glass Thermal classification - hard (greater heat, e.g.,

borosilicate) or soft glass (e.g., soda lime glass •̈lampworking)

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Mechanical properties

Perfectly elastic andbrittle

Bulk form has strength+/- 140MPa

Strength measurement

→ bending Static fatigue (same

with ceramics)

Physical properties

Low coefficient ofthermal expansion

High electrical resistivity

Dielectric strength

CTE lower than metals

and plastic, mayapproach zero

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Glass ceramics

High crystalline microstructure

Stronger than glass

Shaped and then heat treated

Treatment •process known asdevitrification(recrystallization of glass)

Near •zero coefficient of thermalexpansion, high thermal shock resistance

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Graphite

Crystalline form of carbon •layeredstructure

Basal planes or sheets of close packed Catoms

Weak when sheared along the layers

Also known as lampblack •(pigment

High electrical and thermal conductivity

Good resistance to thermal shock andhigh temperature

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Types of graphite Fibers •- important use in reinforced plastics and

composite materials

Foams - high service temperature, chemicalinertness, low coefficient of thermal expansionand electrical properties

Carbon foams - graphitic or non-graphiticstructures

Buckyballs - carbon molecules in the shape ofsoccer balls. Also called fullerents, chemicallyinert,and act like solid lubricant particles

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Diamond Diamond-Like Carbon (DLC) •developed as

diamond film coating

Can be coated with Ni, Cu, or Ti for improvedperformance

Cutting tools materials (single or polycrystalline)

Abrasive in grinding

Dressing of grinding wheels (abrasive sharpening)

Dies for wire drawing

Cutting tools and dies coating

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