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Ceramics

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Ceramics

An inorganic compound consisting of a metal

(or semi-metal) and one or more nonmetals

Important examples:

Silica-silicon dioxide (SiO2

), the main ingredient

in most glass products

Alumina-aluminum oxide (Al2O3), used in various

applications from abrasives to artificial bones

Hydrous aluminum silicate (Al2Si2O5(OH)4) - morecomplex compounds such as the main ingredient

in most clay products

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Ceramic Phase Diagrams

MgO-Al2O3 diagram:

Adapted from Fig.

10.24, Callister &

Rethwisch 3e.

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Properties of Ceramic Materials

High hardness, electrical and thermal

insulating, chemical stability, and high melting

temperatures

Brittle, virtually no ductility - can cause

problems in both processing and performance

of ceramic products

Some ceramics are translucent, window glass

(based on silica) being the clearest example

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Categories of Ceramics

1. Traditional ceramics - clay products

such as pottery, bricks, common

abrasives, and cement

2. New (advanced) ceramics - more

recently developed ceramics based onoxides, carbides, etc., with better

mechanical or physical properties than

traditional ceramics3. Glasses- based primarily on silica and

distinguished by their noncrystalline

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Glasses Clay

products

Refractories Abrasives Cements New

ceramics

-optical-composite

reinforce-containers/household

-whiteware-structural

-bricks forhigh T(furnaces)

-sandpaper-cutting-polishing

-composites-structural

-enginerotorsvalvesbearings

-sensorsAdapted from Fig. 13.7 and discussion in

Section 13.4-10, Callister & Rethwisch 3e.

Classification of Ceramics

Ceramic Materials

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Why So Much SiO2in Glass?

Because SiO2is the best glass former

Silica is the main component in glass products, usually

comprising 50% to 75% of total chemistry

It naturally transforms into a glassy state upon coolingfrom the liquid, whereas most ceramics crystallize upon

solidification

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Other Ingredients in Glass

Sodium oxide (Na2O)

Calcium oxide (CaO)

Aluminum oxide (Al2

O3

)

Magnesium oxide (MgO)

Potassium oxide (K2O)

Lead oxide (PbO) Boron oxide (B2O3)

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Glass Additives

Act as flux (promoting fusion) during heating

Increase fluidity in molten glass for processing

Improve chemical resistance against attack by acids, basic

substances, or water Add color

Alter index of refraction for optics

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Silicate Ceramics

Most common elements in earths crust are Si & O

SiO44-

tetrahedron used to describe crystal structure SiO2 (silica) polymorphicforms are quartz, crystobalite, & tridymite

The strong Si-O bonds lead to a high melting temperature (1710C)

for this material(important for casting)

Si4+

O2-

Adapted from Figs.

3.10-11, Callister &

Rethwisch 3e crystobalite

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Quartz is crystalline

SiO2: corner oxygen atoms shared

Basic Unit: Glass is noncrystalline (amorphous)

Fused silica is SiO2to which noimpurities have been added

Other common glasses contain

impurity ions such as Na+, Ca2+,

Al3+, and B3+

(soda glass)

Adapted from Fig. 3.41,

Callister & Rethwisch 3e.

Glass Structure

Si04 tetrahedron4-

Si4+

O2-

Si4+

Na+

O2-

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Bonding of adjacent SiO44-

accomplished by the sharing ofcommon corners, edges, or faces

Silicates

Mg2SiO4 Ca2MgSi2O7

Adapted from Fig.

3.12, Callister &

Rethwisch 3e.

Presence of cations such as Ca2+, Mg2+, & Al3+

1. maintain charge neutrality, and

2. ionically bond SiO44-to one another

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Layered Silicates Layered silicates (e.g., clays, mica, talc)

SiO4 tetrahedra connectedtogether to form 2-D plane

A net negative charge is associated witheach (Si2O5)

2-unit

Negative charge balanced byadjacent plane rich in positively chargedcations

Second planar sheet has excess cations

Bonding within the sheets is strong &

intermediate ionic/covalent

Adjacent sheets loosely bound to each

other by weak physical bonds

Adapted from Fig.

3.13, Callister &

Rethwisch 3e.

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Kaolinite clay alternates (Si2O5)2-layer with Al2(OH)4

2+layer

Layered Silicates (cont)

Note: Adjacent sheets of this type are loosely bound to

one another by van der Waals forces.

Adapted from Fig. 3.14,

Callister & Rethwisch 3e.

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Mechanical Properties of Ceramics Theoretically, the strength of ceramics should

be higher than metals because their covalentand ionic bonding types are stronger than

metallic bonding

But metallic bonding allows for slip, the

mechanism by which metals deform plastically

when stressed

Bonding in ceramics is more rigid and does not

permit slip under stress

The inability to slip makes it much more

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Defects in Ceramics

Ceramics contain the same imperfections in

their crystal structure as metals - vacancies,

displaced atoms, interstitials, and microscopic

cracks

Internal flaws like cracks tend to concentrate

stresses, especially tensile, bending, or impact

Hence, ceramics fail by brittle fracture much more

readily than metals Strength is much less predictable due to random

imperfections and processing variations

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Effect of Porosity

Porosity has a negative influence on elasticproperties and strength

E = E0(1-1.9P+0.9P2)

s

fs= s

0

exp(-nP)

10 vol% porosity will decrease flexural

strength by 50% from measured value of

nonporous material.

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Concentration of Stress at Crack Tip

Adapted from Fig. 9.8(b),

Callister & Rethwisch 3e.

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Ceramics in Compression

Defects that limit the tensile strength ofceramic materials are not as operative when

compressive stresses are applied

Ceramics are substantially stronger in

compression than in tension

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Strengthening of Ceramics

Make starting materials more uniform

Decrease grain size in polycrystalline

ceramic products

Minimize porosity

Introduce compressive surface stresses

Use fiber reinforcement

Heat treat

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Ph i l P ti f C i

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Physical Properties of Ceramics

Densitymost ceramics are lighter than

metals but heavier than polymers Melting temperatures - higher than for most

metals

Some ceramics decompose rather than melt Electrical and thermal conductivities - lower

than for metals; but the range of values is

greater, so some ceramics are insulators while

others are conductors

Thermal expansionless than metals,

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Oxides

Insulators Semiconductors Metals

(-cm)-1

~ 10-8~ 10-20 ~ 103

Adapted from Fig. 7.1 R. E. Hummel Electronic Properties of Materials, 1993

SiO2

YBa2Cu3O7

ZnO

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Guide to Processing Ceramics

Processing of ceramics can be divided into

two basic categories:

1. Molten ceramics - major category of molten ceramics

is glassworking (solidification processes)2. Particulate ceramics - traditional and new ceramics

(particulate processing)

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