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Ceramics

Callister: Chapters 12, 13

Structure, Properties, Applications and Processing Techniques

of:

Silicates

Glass - Ceramics

Traditional Ceramics

Advanced (Engineering) Ceramics

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Ceramics

Ceramics are compounds of metallic and non-metallic elementsbonded ionically (some are partially covalent).

This type of atomic bonding means that most ceramics have:

High Youngs Modulus

High Melting Point

Low C.T.E.

Strong (high yield strength)

Brittle

Very low ductility means that ceramics are very sensitive tointernal cracks and flaws

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Crystal Structure

Since ceramics are comprised of at least two elements, their

crystal structures are often more complicated than metals.

How the cations (+ve) and anions (-ve) fit together depends on:

1. Maintaining electrical neutrality

according to the chemical formula (e.g. NaCl, Al2O3, )

2. The relative sizes of the ions

Stable structures require that the anions and cations touch

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Coordination Numbers

The cations coordinationnumberdepends on theratio of the radii:

a

c

r

r

Linear

Triangular

Tetrahedral

Octahedral

Cubic

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AX Crystal Structures

NaCl (Rock Salt) structure

The Rock Salt structure is formed from two,interwoven FCC structures.

The coordination number of both ions is 6(octahedral)

Other common ceramics with this structure:

MgO, MnS, LiF, FeO

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AX Crystal Structures

Cesium Chloride structure

The CsCl structure is based onthe BCCstructure.

It is not BCC because two different atoms

are involved)

The coordination number of both ions is 8

(cubic)

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AX Crystal Structures

Zinc Blende (ZnS) structure

The Zinc Blende structure is an FCC-basedlattice with zinc anions in 4 of the eight

tetrahedral sites

The coordination number of both ions is 4

(tetrahedral)

Other common ceramics with this structure:

ZnTe, SiC

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Silicate Ceramics

The bulk of soils, rocks, clays, and sand are silicateceramics

Silica (SiO2) is a covalently bonded tetrahedral molecule

The bonds are directional and strong.

Rather than discussing unit cells, silicates are describedaccording to the arrangement of the tetrahedra.

They can form one-, two-, and three-dimensional structures

-4

4SiO

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Crystalline Silica

Silica (SiO2) is the simplest of the sil icates

It is a three-dimensional network formed when everyoxygen atom is shared by two, adjacent tetrahedra

Electrical neutrality is maintained (Si:O = 1:2)

There are three polymorphs of crystalline silica

Quartz, cristobalite, and tridymite

Neither form is closed packed Density of silica is low.

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Silica Glasses

If molten silica is cooled relatively quickly, it is possible toprevent the formation of a crystalline structure.

Fused Silicais still made up of the SiO4 tetrahedra, but not alloxygen atoms are shared between two tetrahedra.

There is short-range, but not long-range order.

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Silica Glasses

To change the properties of the glass, other oxides are often

added.

Network Formersfit in with the SiO2 tetrahedra. They areadded to change the properties of the solidified glass.

e.g. 12% B2O3 is added to silica to make Pyrex. The additionlowers the forming temperature without changing the thermal

expansion coefficient.

Network Modifiersdo not fit in with the silicanetwork. They make it easier to form a glass(as opposed to crystalline silica).

e.g. most glass (windows, food containers,)contain up to 15% Na2O to make it easier to forma glass

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The Silicates

Depending on how many oxygenatoms are shared, silicates can

form a wide variety of structures.

Additional cations are often

required to maintain chargeneutrality (e.g. Ca2+, Mg2+, Al3+)

Clays are layeredsilicates.

Each sheet is covalently boundtogether,

but adjacent sheets are weakly

bound by van der Waals forces

Just add water

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Carbon

Carbon exists in a variety of forms:

Diamond

Graphite

Fullerenes

The different forms have very different propertiesbecause theyhave different structures.

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Diamond

Diamond Cubic crystal structure(similar to zinc blende)

Each carbon atom is covalentlybonded to 4 others in a tetrahedron.

Very hard/strong

Low electrical conductivity

High thermal conductivity

Most industrial quality diamond isman-made.

Knives, machine tools, etc.

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Graphite

Graphite has a layered structure.

Each atom is covalently bonded to threeothers in the layer.

The fourth bonding electron contributes to

van der Waals bonding between thelayers.

The properties of graphite are directional.

StrongWeak

Weak

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Fullerenes

Named after R. Buckminster Fuller,inventor of the geodesic dome.

Two forms discovered so far

C60, Buckyballs

Again, three covalent and one vander Waals bond

C60 molecules pack together in

an FCC arrangement

Carbon Nanotubes

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Strength of Ceramics

Very low fracture toughness of ceramics means that failure isalmost always due to flaws in the part.

Therefore, the design strengths of ceramic materials are

described using statistics.

26 MPam2024 Aluminum

55 MPamTi-6Al-4V

5 MPamSi3N4

1.7 MPamAl2O3

99 MPam4340 Steel

Fracture Toughness KICMaterial

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Fracture Statistics

Nominally identicalsamples may fail at very

different stresses

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Three-Point Bend Test

It is difficult to perform tensile

tests on brittle materials. They often crush in the

grips.

The three-point bend test

avoids this problem, but hasits own drawbacks

The maximum tensile stressis only seen by the materialon the bottom surface,

directly under the plunger.

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Three-Point Bend Test

The results from a 3-point bend testappear similar to those from a tensile

test.

These particular results illustrate the

effect that structure has on properties:

Crystalline Al2O3 is stiffer and stronger

than amorphous glass

Note the very small strains

0.2% is not even on the scale!

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Three-Point Bend Test

The stress is calculated

according to the formulashown for a rectangularcross-section

The fracture stressdetermined in this test is

known as:

Flexural Strength

Modulus of Rupture

The flexural strength is oftenhigher than the tensile

strength

Statistics: The biggest flaw may not see the highest stress

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Processing Ceramics

All ceramics have short-range atomic order, some have long-range order.

Crystalline ceramics have short and long-range order

Glasses have short-range order

Ceramic-glasses have a combination of crystalline and glassycomponents

Deformation of crystalline ceramics is due to dislocation motion,

HOWEVER:

The complex crystal structure and strong atomic bonding make

dislocation motion exceedingly difficult.

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Processing Glasses

Deformation of non-crystalline ceramics is due to Viscous Flow

dydv

AF

dydv

=

=

Viscosity is a measure of how difficultit is to shear (e.g. stir) a liquid.

Water has a low viscosity

Molasses has a high viscosity

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Liquid

Processing Glasses

When a crystalline material

solidifies, there is a step change involume at the melting temperature.

Temperature

SpecificVolume

Glasses do not really solidify inthe traditional sense.

The molecules pack closer andcloser together, becoming an

increasingly denser liquid.

The slight change in slope occurs when the

molecules are essentially unable to flow.

This is the Glass Transition Temperature.

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Ceramic Glasses

Strain Point Brittle

Annealing Point

Residual stresses removed

Softening point

Can be handled withoutdeformation

Working Point

Easily deformable

Melting Point

True liquid

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Fabricating Glasses

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Fabricating Glasses

How do you get perfectly flat, parallel sided plate glass for

windows?

The molten glass is floated on top of molten tin (Tm =231C)

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Tempered Glass

The fracture properties of glass can be altered by

Laminating

Tempering

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Tempered Glass

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Clay Products

Clay is an aluminosilicate(i.e. Al2O3 and SiO2) with a variety ofimpurities (usually various other oxides)

Common clay products include:

Building bricks, tiles, sewer pipes

Pottery, porcelain, china

Clay products are made from various proportions of:

Clay and a flux material (e.g feldspar)

Sheet silicate structure

Filler materials (typically crushed quartz)

Crystalline

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Fabricating Clay Products

Clay is mixed with water to form a plastic body and formed to the

desired shape

The wet body is then dried and fired.

Drying removes water from the clay a controlled rate

Firing vitrifies the clay (turns it into a glass)

Degree of vitrification depends on firing temperature

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Microstructure of Clay Products

Crystalline particles surrounded by a glassy matrix with somepores

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Fabricating Crystalline Ceramics

The melting temperature of most ceramics is too high for casting

to be a practical option.

Engineering ceramics in general are made from powders

Powders are compacted / consolidated to form a green body

Methods include various pressing techniques and tape casting.

The green body is then sintered at elevated temperatures (often

under pressure) to bond the powders

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Microstructure of Crystalline Ceramics

Sintered crystalline grains with porosity

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Properties of Crystalline Ceramics

Crystalline ceramics are the Engineering ceramics

High melting points

Strong

Hard

Brittle

Good corrosion resistance

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Properties of Glasses

Like crystalline ceramics, glasses are

hard,

brittle

corrosion resistant

Unlike crystalline ceramics, glasses:

lower melting temperatures

can be easily deformed at high temperatures

are not porous

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Properties of Clay Products

Clay products are:

Hard Brittle

Corrosion resistant

They generally have some porosity, but this can be minimized by

increasing the firing temperature

Their high temperature creep properties are better than glasses butnot as good as crystalline ceramics.