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Imperfections in Ceramics

Atomic Point Defect Schottky, Frenkel

Impurities Interstitial, Substitutional

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Atomic Point Defect

Frenkel Defect--a cation is out of place.

Shottky Defect--a paired set of cation and anion vacancies.

Equilibrium concentration of defects kT / QDe~

Adapted from Fig. 12.21, Callister 7e. (Fig. 12.21 is from W.G.Moffatt, G.W. Pearsall, and J.Wulff, The Structure and Properties of Materials , Vol. 1,Structure , John Wiley and Sons,Inc., p. 78.)

Shottky

Defect:

Frenkel

Defect

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Impurities

Impurities must also satisfy charge balance = Electroneutrality

Ex: NaCl

Substitutional cation impurity

Na + Cl-

initial geometry Ca 2+ impurity resulting geometry

Ca 2+

Na+

Na +Ca 2+

cationvacancy

Substitutional anion impurity

initial geometry O 2- impurity

O 2-

Cl -

anion vacancy

Cl -

resulting geometry

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Mechanical Properties

Strength of ceramics vary greatly but they are generallybrittle . Tensile strength is lower than compressive strength.

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Mechanism of Deformation

Covalently bonded ceramics : Exhibit brittle fracture due to

separation of electron-pair bonds without their subsequentreformation. Ionically bonded ceramics: Single crystal show considerable plastic

deformation. Polycrystalline ceramics are brittle. Example: NaCl crystal

Slip in {100} familyof planes is rarely

observed as same

charges come into contact.

Cracking occurs at grain boundaries.

Figure 10.44

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Factors Affecting Strength

Failure occurs mainly from surface defects. Pores gives rise to stress concentration and

cracks. Pores reduce effective cross-sectional area.

Flaw size is related to grain size . Finer size ceramics have smaller flaws and henceare stronger.

Composition , microstructure , surface condition ,

temperature and environment also determinestrength.

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Elastic Modulus

Room T behavior is usually elastic, with brittle failure.

3-Point Bend Testing often used.--tensile tests are difficult for brittle materials.

Adapted from Fig. 12.32,Callister 7e.

F L/2 L/2

d = midpointdeflection

cross section

R

b

d

rect. circ.

Determine elastic modulus according to:F

x

linear-elastic behavior

F

slope =E =

F

L3

4bd 3 =F

L3

12 R 4

rect.cross

section

circ.cross

section

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Creep

time

Elevated Temperature Tensile Test ( T > 0.4 T m).

creep test

slope = ss = steady-state creep rate.

x

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Toughness of Ceramic

Ceramics have low strength .

Research has been conducted to improve toughness. Hot pressing with additives and reaction bonding improve

toughness.

K IC values obtained by four point bend test .

aY K f IC =

f = fracture stress (MPa)

a = half size of target internal flaw

Y = dimensionless constant

Figure 10.46

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Transformation Toughening of Partially Stabilized ZrO 2

Transformation of Zirconia combined withsome other refractory oxides (MgO) can

produce very high toughness ceramics.

ZrO 2 exists in 3 structures. Monoclinic Up to 1170 0C

Tetragonal 1170 2370 0C

Cubic above 2370 0C

Adding 10% mol of MgO stabilizes cubicform so that it can exist in metastable statein room condition.

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If a mixture of ZrO 2 9 mol% MgO is sintered at about

1800 0C and rapidly cooled, it will be in metastable state. If reheated to 1400 0C and

held for sufficient timetetragonal structure

precipitates. Under action of stress,

this tetragonal structuretransforms to monoclinic

increasing volume andhence retarding crack growth.

Figure 10.47a

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Thermal Properties of Ceramics

Low thermal conductivity and high heatresistance.

Many compounds are used as industrialrefractories.

For insulating refractories, porosity is desirable. Dense refractories have low porosity and high

resistance to corrosion and errosion. Aluminum oxide and MgO are expensive and

difficult to form and hence not used asrefractories.

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Porosity

Exist between powder particles due toforming processes.

Heat treatment will eliminate porosity butsome residual porosity will remain.

Influence the elastic properties and strength. Porosity reduce strength because:

Pores reduce cross sectional area Act as stress concentrator

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Hardness

High hardness Suitable for abrasive materials

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Electrical Properties

Basic properties of dielectric:

Dielectric constant :- Q = CV

Q = Charge

V = Voltage

C = Capacitance

C = 0A/d 0 = permeability of free space

= 8.854 x 10 -12 F/m When the medium is not free space C = K 0A/d Where K is dielectric constant of the

material between the plates

Figure 10.35

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Dielectric Strength and Loss Factor

Dielectric strength is measure of ability of material to hold

energy at high voltage.Defined as voltage gradient at which failure

occurs.

Measured in volts/mil.

Dielectric loss factor: Current leads voltage by 90 degreeswhen a loss free dielectric is between plates of capacitor.

When real dielectric is used, current leads voltage by 90 0 where is dielectric loss angle .

Dielectric loss factor = K tan measure of electric energy lost .