Chapter 12 - Structures and Properties of Ceramics

8/4/2019 Chapter 12 - Structures and Properties of Ceramics

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Course Description: The course covers the fundamental knowledge of engineering ceramics,

polymeric materials, composite materials and construction materials, as well ascorrosion/degradation of these materials. The materials microstructure,processing, mechanical behaviour and applications are emphasized. Materialsselection is discussed with case studies.

Prerequisites: MECH 141 Engineering Materials I or equivalent course

Textbook: Materials Science and Engineering, eighth edition,by William D. Callister, Jr. and David G. Rethwisch

Supplementary Textbooks (Reserved in the Library):Engineering Materials 2: An introduce to Microstructures, Processing and

Design, second edition, 1998 by M. F. Ashby and D. R. H. Jones, Butterworth-Heinemann, Oxford

Engineering Materials: Properties and Selection, 6th edition, 1999, by K.G.Budinski and M.K. Budinski, Prentice Hall, N.J.

Materials Selection in Mechanical Design, 1st edition, by M.F. Ashby,

Butterworth-Heinemann, Oxford, 1993

MECH3420 (MECH 242) ENGINEERING MATERIALS II


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Instructor: Tong-Yi ZHANG, Chair Professor

(Tel. 7192, Rm: 2546, E-mail: [email protected])

Teaching Assistants:Miss Yijing SUN (Tel. 7199, Rm: 2125, E-mail: [email protected])Mr. Yu Xuan TEOH (Tel. 7461, Rm: 4599, E-mail: [email protected])

Lectures: Monday & Wed 10:30-11:50 am (Room 3008) Tutorials: Tuesday 6:00-6:50 pm (Room 2306)

Grading Policy: Homework and quizzes: 20%Midterm exam: 35%Final exam: 45%

All homework assignments should be handed in by the same day of theweek after the tutorial class. A late submission bears a penalty of 10%per day of the mark.

Mid-term exam is scheduled on 24 October (Monday).


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1. Engineering Ceramics (3 weeks)

Ceramic structures

Mechanical properties

Processing and applications

2. Polymers (3 weeks)

Polymer structures Mechanical and thermomechanical behavior

Processing and applications

3. Composites (2 weeks) Particle-reinforced composites

Fiber-reinforced composites

Structural composites

Lecture Contents


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4. Corrosion and Degradation of Materials (2 weeks) Corrosion and oxidation of metals

Corrosion of ceramics & degradation of polymers

5. Electrical Properties (2 weeks) Electrical conductivity Semiconduction Electrical properties of metals, ceramics and

polymers

6. Construction Materials (2 weeks) Cements and concrete Wood

Lecture Contents


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Chapter 12

Structures and Properties ofCeramics

Department of Mechanical Engineering

Hong Kong University of Science & Technology


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Outline of the Chapter

Why Ceramics? Applications?

Understanding the ceramicstructures

Mechanical Properties of Ceramics


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Part I

Why Ceramics and Their Applications?


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Characteristics of Ceramics

Ceramic = Keramikos in Greek meaning burnt stuff

General characteristics Inorganic and non-metallic

Outstanding hardness

High mechanical strength

High dimensional stability

Resistance to wear

Other distinctive characteristics in applicationspecific ceramics Piezoelectric ceramics dimensional change in response

to an electric field, or vice versa

Biocompatible ceramic coating bone implantation coating

Department of Mechanical EngineeringHong Kong University of Science & Technology


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Highlights of Ceramic Applications

Kitchenware

High performance electronic substrates

Knock sensorTo detect theknocking of enginedue to gasolinequality or difference

in operatingconditions

Cutting blade



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Part II

Understanding the ceramic structures


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Ceramic StructuresIntroduction (1)

Composed of at least two electrically charged

elements: cations (metallic) and anions(nonmetallic)

Bonding: mostly ionic, some covalent

The closer the atoms are together, the greater thedegree of covalency

Degree of ionic character is determined by

electro-negativity of the atoms



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Percentage of Ionic Character

% Ionic Character (IC) ={1exp [-(0.25)(xA-xB)2]} x 100% (Eq.2.10)

where xA & xB are the electronegativities of the

respective elements e.g. CaF2 Ca ~ 1.0; F ~ 4.0 (see Table 12.1)

IC = {1exp [-(0.25)(1-4)2]} x 100% = 89.46%



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Factors determining the crystal structure

Magnitude of the electrical charge of thecomponent ions electrically neutral

Relative size of the cations and anions

Stable ceramics: anions surrounding a cation are all incontact with that cation



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Coordination number:

number of the nearestneighbors or touching atoms

Common coordination

numbers of ceramicmaterials: 4,6,8



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rcationranion

Coord #

< .155

.155-.225

.225-.414

.414-.732

.732-1.0

ZnS(zincblende)

NaCl(sodiumchloride)

CsCl(cesium

chloride)

2

3

4

6

8

Coordination number increases with rcation/ranion



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Example 12.2Q. On the basis of ionic radii, what crystal structure

would you predict for FeO?

Cation

Al3+

Fe2+

Fe3+

Ca2+

Anion

O2-Cl-

F-

Ionic radius (nm)

0.053

0.077

0.069

0.100

0.1400.181

0.133

rcationranion

0.077

0.140

0.550

Based on this ratio,Coordination No = 6

From Table 12.3,



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Ceramic StructuresAX Type (1)

Containing equal numbers of cations and

anions 3 structure types

Rock salt structure: Coordination no. = 6

Cesium chloride structure: Coordination no. = 8

Zinc blende structure: Coordination no. = 4



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Coordination no.=6

FCC structure

Typical examples:

NaCl, MgO, MnS,LiF & FeO

Rock Salt Structure



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Coordination no.=8

Similar to BCCstructure, but notexactly, due to the

involvement of twodifferent kinds of atoms

Typical example:

CsCl

Cesium Chloride Structure



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Coordination no.=4

Tetrahedrallycoordinated

Typical examples:ZnS, ZnTe and SiC

Zinc Blende Structure



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Ceramic StructuresAm

Xp

Type

The absolute charge ofeach cation does notequal that of each anion

Florite (CaF2): rcation/ranion =0.100/0.133 0.8

Coordination no.=8 Only half the cation sites

are occupied since #Ca2+

ions = F-

ions



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Ceramic StructuresAm

Bn

Xp

Type

Containing more

than one type ofcation

Perovskite

Crystalline Structure

Typical Example:BaTiO

3



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Crystal Structures from the Close Packing

of Anions (1)

In ceramics, the close-packed planes of

anions are stacking on one another to form aFCC (Face-Centered Cubic) or HCP(Hexagonal Close-Packed) structure

The small cations may reside (or stay) at thesmall interstitial sites in-between the planesof anions



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of Anions (2)

Tetrahedral Position: Four atoms surrounding one site

Octahedral Position: Six atoms surrounding one site



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of Anions (3)

Two tetrahedral sites per anion 1/4 x 4 x 2 = 2



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of Anions (4)

One octahedral site per anion 1/6 x 3 x 2 = 1



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Octahedral Position

An octahedral

position for an(interstitial) atom isthe space in the

interstices between 6regular atoms thatform an octahedra


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of Anions (3)

This is a rock salt crystal structure, in which the cationsreside in octahedral position with 6 neighbor anions

{111} plane



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Ceramic density computations


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Silicate ceramics

Silicates composed of Si & O2 with Si-O covalent bonds: mostabundant on earth (e.g. soils, rocks, clays, sand)

Basic unit: (SiO4)4- tetrahedron, can be combined into differentarrangements for various silicate structures, like forsterite (Mg2SiO4)and akermanite (Ca2MgSi2O7)

Basic Unit Silicate ion structuresbased on the Basic Unit



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Silica: SiO2 (1)

Silica may form a crystalline3D network, in which atomsare periodically arrangedand every corner oxygen inthe (SiO4)

4- tetrahedron isshared by adjacenttetrahedrons

Electrically neutral andstable atomic structure

Three primary polymorphiccrystalline forms are quartz,cristobalite and tridymite.


Cristobalite crystal


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Silica (2)

Quartz Cristobalite Tridymite

They all possess the same chemical composition, SiO2, but theyhave different structures and appearances

Low density, e.g. quartzs room density is 2.65g/cm3

High melting point (1710C)



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Silica Glasses

Non-crystalline, highdegree of atomicrandomness

Adding of CaO & Na2O(network modifiers) can

lower the melting pointand viscosity of a glasseasier for shapechanging

Applications: containers,windows



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Layered Silicates A layered structure is

produced by repeating thetetrahedral unit of (Si2O5)

2-

In a common layered

silicates, kaolinite, the silicatetrahedral layer iselectrically neutralized by anadjacent Al2(OH)4

2+

Bonding within this two-layered sheet is strong andintermediate ionic-covalent,whereas bonding between

the adjacent sheets is weak,van der Waals.

+ve

+ve

-ve

-ve

-ve



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CarbonDiamond (1)http://en.wikipedia.org/wiki/Diamond#Crystal_structure

Carbon exists in variouspolymorphic forms, like

diamond, graphite, fullerenesand Carbon nanotube

Diamond structure

Metastable crystal structure

at ambient temperature andpressure

A variant of the zinc blende

Totally covalent carbon-carbon bonds

Also called diamond cubiccrystal



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CarbonDiamond (2) Hardness: the hardest known

material

Clarity: optical transparent over alarger range of wavelengths (from

the ultraviolet into the far infrared) Thermal Conductivity: conducts

heat better than anything - fivetimes better than the second bestelement, Silver!

Melting Point: possess thehighest melting point (3820K)! Lattice Density: The atoms of

Diamond are closely packed. Diamond or/and diamond-like thin

film coatings are used to enhancehardness and wear properties ofother materials



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CarbonGraphite (1)

Composed of layers ofhexagonally arranged

carbon atoms Coexistence of strong

covalent bonds betweencarbon atoms and a

weak van der Waalsbond between layers

The weak interplanebond allows easy

interplanar cleavage -excellent lubricationproperties



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CarbonGraphite (2)

Characteristics High electrical conductivity

parallel to the hexagonalsheets

high strength and goodchemical stability at hightemperatures

high thermal conductivity low coefficient of thermal

expansion high resistance to thermal

shock

Applications: heating elements; electrodes

for arc welding; metallurgicalcrucible; casting moulds; high-temp refractories; insulation inrocket nozzle



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Carbon - Fullerenes Hollow spherical cluster of 60

carbon atoms (carbon-cagemolecules)

Each C60 molecule consists of

groups of carbon atoms toform hexagon (x20) andpentagon (x12)

In the solid state, the C60 unitsform a crystalline structure andpack together in a FCC array

Pure crystalline solid of C60 iselectrically insulating. Whenpotassium (3K+ ions) is added,

the resulting material K3C60has a very high electricalconductivity, withcharacteristics of a metal.

Composed of both hexagonand pentagon, in whichnotwo pentagon share acommon side



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CarbonNanotube (1)

Multi-wall carbon nanotubeDepartment of Mechanical EngineeringHong Kong University of Science & Technology

A single sheet of graphite rolling into atube with the end cap of C60 fullerenehemisphere

Excellent mechanical properties Strongest and stiffest fibers ever

known (E = 0.3 1.2 TPa; UTS =50-200GPa)

Thermal properties Good thermal conductor at room

temp. (higher than metals) High decomposition temp. >600 )

Negligible CTE Electrical conductivities

Variable electrical conductivity

(metal to semiconductor to insulator)depending fabrication process andstructure

Low density (1.30~1.80 g/cm3) Excellent chemical stability (Acid, base)


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CarbonNanotube (2)

Patterning of the carbon multi-walled

nanotubes (Dai & Mau, Adv Mater,2001)


Single-walled nanotube (SWNT) fabricatedby Phy Dept, HKUST

Potential Applications: LightEmitting Diode (LED) and flatpanel display, nano-scaletransistor, nanoelectronic

devices, electrical conductingmaterial, structural composites.


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Multi-Wall Carbon Nanotubes synthesized by thermal

decomposition of hydrocarbons in the presence of catalysts andpurified in nitric acid.

I f i i C i (1) A i P i


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Imperfections in Ceramics (1)Atomic Point

Defects

2 types of Atomic PointDefects

Frenkel Defect: cation-vacancy + cation-interstitial

Schottky Defect: cation-vacancy + anion vacancy

Condition of the Defects:

Electroneutrality shouldbe maintained


E ilib i b f F k l d


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Equilibrium numbers of Frenkel and

Schottky defects Equilibrium number of vacancies, Nv

kT

QNN vv exp (4.1)

N: the total number of atomic sitesQv: the formation energy of a vacancy

k: Boltzmanns constant

T: the absolute temperature in kelvins

E ilib i b f F k l d


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Equilibrium numbers of Frenkel and

Schottky defects Equilibrium number of Frenkel defects, Nfr,

and Schottky defects, Ns

N: the total number of lattice sites

Qfr: the formation energy of a Frenkel defectQs: the formation energy of a Frenkel defect

k: Boltzmanns constant

T: the absolute temperature in kelvins

kT

QNN

fr

fr exp

kT

QNN ss exp


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I f ti i C i (2)


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Imperfections in Ceramics (2)

Stoichiometry (Valence defects) Stoichiometry: a state for ionic compounds

wherein there is the exact ratio of cations toanions as predicted by the chemical formula

Non-stoichiometry one of the ion types in

the ceramic showing two valence (or ions)states


Imperfections in Ceramics Non


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Imperfections in CeramicsNon-

stoichiometry

Example for non-stoichiometry

Both Fe2+ and Fe3+ canexist, depending ontemperature and the

ambient oxygen pressure The electrically neutrality

is always maintained, butthe ratio of cation and

anion would not as shownin the chemical formula


Imp rf ti n in C r mi (3) Impurities


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Imperfections in Ceramics (3)Impurities

Substitution impurity

will substitute for thehost ion to which it isthe most similar in

an electrical sense Interstitial: impurity

ionic radius must be

small compared tothe anion



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Ceramic Phase Diagrams


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Ceramic Phase Diagrams

Experimentally or/and theoretically

determined It is frequently the case that the two

components are compounds that share a

common element, often oxygen Ceramic phase diagram may have

configurations similar to metal-metal systems

Examples: Al2O3-Cr2O3 , MgO-Al2O3 , ZrO2-CaO ,SiO2-Al2O3


Ceramic Phase Diagrams Al O Cr O


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Ceramic Phase DiagramsAl2O3-Cr2O3 Same form as the

isomorphous Cu-Ni phasediagram

Single liquid, single solid

and two-phase solid-liquidphases

Al2O3 and Cr2O3 have thesame crystal structure

Al and Cr have the samecharge and similar radii(0.053 and 0.062 nm,respectively)

Al2O3-Cr2O3 is also called asSapphire, which is used ashard bearings in watchesand scientific instruments, ordinnerware, artware, andwithin the tile industries


Ceramic Phase Diagrams MgO Al O (1)


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Ceramic Phase Diagrams MgO-Al2O3 (1)

Similar to Pb-Mg phasediagram

An intermediate phase

(spinel: MgO-Al2O3 =MgAl2O4) there is a rangeof composition over whichspinel is stable

Limited solubility of Al2O

3in MgO below 1400Cdue to the differences incharge and radii of theMg2+ and Al3+ ions

(0.072nm, 0.053nm). Two eutectic reactions

(on either side of spinel at2000C)


Ceramic Phase Diagrams Eutectic and


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Ceramic Phase Diagrams Eutectic and

Eutectoid Reactions Eutectic reaction Upon cooling, a liquid

phase transforms isothermally and reversiblyinto two intimately mixed solid phases

Eutectoid reaction Upon cooling, a solid

phase transforms isothermally and reversiblyinto two intimately mixed, new solid phases


Ceramic Phase Diagrams ZrO CaO


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Ceramic Phase Diagrams ZrO2-CaO

One eutectic (2250C and 23wt%CaO), two eutectoid (at 1000 and2.5wt% CaO; and at 850C and

7.5wt% CaO) reactions Three different crystal structures:

tetragonal (T), monoclinic (M) andcubic (C).

Pure ZrO2 experiences T - M phase

transformation at about 1150C, with alarge volume change (shrinkage) andcrack formation

Partially stabilized ZrO2 (PSZ) byadding 3-7wt% CaO (or Y2O2, MgO):Upon cooling the C + T phases are

retained, without causing crackformation


Ceramic Phase Diagrams SiO2-Al2O3


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Ceramic Phase Diagrams SiO2 Al2O3

Silica and Alumina areprime refractory

materials They are not mutually

soluble in one another An intermediate

compound (mulite:3Al2O3-2SiO2) forms at72wt% Al2O3

A single eutectic exists(at 1587C and 7.7 wt%Al2O3 )



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Part III




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Brittle Fracture At room temperature, ceramics exhibit brittle fracture without

plastic deformation in tension.

If crack growth in crystalline ceramics is along a specificcrystallographic plane, which is called cleavage.

Maximum stress at the crack tip:

Fracture Toughness

whereY = dimensionless parameter = applied stressa = surface crack lengtht = crack tip radius

)a

(2=1/2

t

max

aY=KIc


Mechanical Properties of Ceramic


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p

Brittle Fracture: Observations (1)#1:The measured fracture strengthsof ceramics


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Poling direction

Specimen Silicone oil

Loading

High Voltage

Experimental set-up

Bending strength (MPa)

0 20 40 60 80 100 120 140

0

10

20

0

10

0

10

20 3.3 kV/cm

Proba

bilitydensity(

%)

0

10

20 6.7 kV/cm

0

10

2010 kV/cm

0

10

20 16 kV/cm

20 kV/cm

Zirconate Titanate Ceramics

Microflaws play an important role


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c o aws p ay a po ta t o e

On average, small samples have highfracture strengths than large samples

because the possibility to have alarge size flaw in a small sample islower than in a large sample.

Ceramics appear to be stronger in

bending than in tension because thelargest flaw may not be near thesurface.

Mechanical Properties of Ceramic


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p

Brittle Fracture: Observations (2)#3:Brittle ceramics display much higher strengths in compression than

in tensionExplanation:For compression, no stress amplification associated with

flaws

#4:How can the fracture strength of a brittle ceramic be increased?

Explanation:By imposing residual compressive stresses at its

surface by thermal tempering

#5:Why tensile tests are not employed for brittle ceramics?Explanation:

1.Difficult to grip brittle samples2.Difficult to prepare samples with specific dimensions

3.Brittle ceramics fail at about 0.1% strain


Stress-strain BehaviorFlexural Strength (1)


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A linear relationship between stressand strain for most ceramics

Brittle at room temperature: elasticlimit = fracture strength

Flexural strength:

Rectangular cross section

Circular cross section


3R

LFffs

22

3

bd

LFffs

Stress-strain BehaviorFlexural Strength (2)


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Size effects: Higherpossibility of bigger flaws

in larger size specimens High specific elastic

moduli for most

engineering ceramics(Modulus: 70~500GPa )


Plastic DeformationCrystalline Ceramics


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Plastic deformation is difficult to occur by the

motion of dislocations For ionic predominant crystalline ceramics

Very few slip systems

Slip is difficult in ions of like charge because ofelectrostatic repulsion

For covalent predominant crystalline ceramics

Strong covalent bonds

Limited number of slip systems

Complex dislocation structures


Plastic DeformationNon-crystalline ceramics


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Plastic deformation is notinduced by dislocation motionbecause of the irregular

atomic structure. Viscous flow occurs instead

of dislocation slip: atoms orions slide past one another inshear by the breaking and

reforming of interatomicbonds.

The viscosity of glassdecreases with increasingtemperature as theinteratomic bond strength isdiminished.

dy/d

F/A

dy/d

Viscosity:


Effects of Porosity on Mechanical Properties


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Pores or void spaces existbetween powder particles infabricated ceramics

Any residual pores aredeleterious because Reduced cross-sectional area Pores act as the stress raisers

Elastic Modulus of ceramicscontaining pores of volume

fraction, P:E = E0 (1 1.9P + 0.9 P2 )

Flexural strength decreasesexponentially with the volumefraction (P) as

fs = 0 exp (-nP)

Elastic Modulus

Flexural Strength


Hardness


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Hardness


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Hardest known material -beneficial mechanicalproperties of ceramics that can

be utilised for abrasion andgrinding other materials.

Creep


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Ceramics creep when subjected to stresses at elevatedtemperatures

Time-deformation creep behaviour is similar to metals.


Creep strain at a given load and temperature. Effects of stress and temperatureon the creep behavior

Documents

Chapter 12 - Structures and Properties of Ceramics