MECHANICAL PROPERTIES & APPLICATIONS OF CERAMICS

properties• Properties are the way the material responds to the

environment and external forces.• Mechanical properties – response to mechanical forces,

strength, etc.• Electrical and magnetic properties - response electrical

and magnetic fields, conductivity, etc.• Thermal properties are related to transmission of heat

and heat capacity.• Optical properties include to absorption, transmission

and scattering of light.• Chemical stability in contact with the environment -

corrosion resistance.

Brittle fracture

• Ceramic almost always fracture when applied tensile load (Tr)

• Simple fracture separation of body into two/more pieces in response to imposed stress (constant/change with t) at T < Tm

• Stress : tensile, compressive, shear

fracture• Any fracture process involves two steps 1)crack formation

(nucleation) and 2) propagation—in response to an imposed stress.

• 2 modes mechanism of fracture (ability to form plastic deformation before fracture): ductile & brittle

• Plastic deformation temporary change shape of material when a sufficient load is applied-self-reversing after the force is removed (low stress)

• Ductile exhibit plastic deformation• Brittle no plastic deformation

. • Ductile fracture is characterized by extensive plastic

deformation in the vicinity of an advancing crack. • the process proceeds relatively slowly as the crack

length is extended. • Such a crack is often said to be stable resists any

further extension; unless there is an increase in the applied stress.

• For brittle fracture, cracks may spread extremely rapidly, with very little accompanying plastic deformation.

• cracks may be said to be unstable once started, will continue spontaneously without an increase in magnitude of the applied stress.

• Stress raisers minute surface, interior cracks, internal pores, grain corners; impossible to be eliminated (moisture, contaminants)

• Direction of crack is very nearly perpendicular to direction of tensile stress flat structure surface

• Crack propagation successive & repeated breaking of atomic bonds cleavage

• 2 type of crack propagation: transgranular & intergranular

• Transgranular (transcrystalline) cracks pass through the fracture grain

• Intergranular crack propagation is along boundaries

• Under some circumstances, fracture occurs by slow propagation of crack stress are static in nature; static fatigue/delayed fracture

• After nucleation, & during propagation, a crack accelerates until critical/terminal velocity achieved;

• Then a crack may branch, process that may successively repeated family of crack

• The site of nucleation can often be traced back to the point where a set of cracks converges or comes together.

• Furthermore, rate of crack acceleration increases with increasing stress level;

• correspondingly, degree of branching also increases with rising stress.

• For example, from experience we know that when a large rock strikes (and probably breaks) a window, more crack branching results [i.e., more and smaller cracks form (or more broken fragments are produced)] than for a small pebble impact.

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Mechanical Behavior - Ceramics

• The stress-strain behavior of brittle ceramics is not usually obtained by a tensile test.1. It is difficult to prepare and test specimens with

specific geometry.2. It is difficult to grip brittle materials without

fracturing them.3. Ceramics fail after roughly 0.1% strain;

specimen have to be perfectly aligned.

The Bend Test for Brittle Materials Bend test - Application of a force to the center

of a bar that is supported on each end to determine the resistance of the material to a static or slowly applied load.

Flexural strength or modulus of rupture -The stress required to fracture a specimen in a bend test. (modulus of rupture or bend strength) is the stress at fracture.

Flexural modulus - The modulus of elasticity calculated from the results of a bend test, giving the slope of the stress-deflection curve.

Imperfections in ceramics• Atomic defects involving host atoms may exist • Vacancies & interstitial are possible, for each ion• NaCl : Na & Cl each interstitials & vacancies• Because the atoms exist as charged ions, when defect

structures are considered, conditions of electroneutrality must be maintained.

• Electroneutrality the state when there are equal numbers of positive and negative charges from the ions.

consequently, defects in ceramics do not occur alone.

• in AX materials, defect is a cation vacancy –anion vacancy pair known as a Schottky defect

• created by removing one cation and one anion from the interior of the crystal

• Since both cations and anions have the same charge, and since for every anion vacancy there exists a cation vacancy, the charge neutrality of the crystal is maintained.

• Defect of a cation–vacancy and a cation–interstitial pair a Frenkel defect

• formed by a cation leaving its normal position and moving into an interstitial site.

• There is no change in charge because the cation maintains the same positive charge as an interstitial.

• The ratio of cations to anions is not altered by the formation of either a Frenkel or a Schottky defect

• If no other defects are present, the material is said to be stoichiometric.

• Stoichiometry as a state for ionic compounds wherein there is the exact ratio of cations to anions predicted by the chemical formula.

• For example, NaCl is stoichiometric if the ratio of Na+ ions to Cl− ions is exactly 1:1.

• A ceramic compound is nonstoichiometric if there is any deviation from this exact ratio

• Nonstoichiometry may occur for some ceramic materials in which two valence (or ionic) states exist for one of the ion types.

• Iron oxide (FeO) can be present in both Fe2+ and Fe3+ states; depends on temperature and the ambient oxygen pressure.

• The formation of an Fe3+ ion disrupts the electroneutrality of the crystal by introducing an excess +1 charge, which must be offset by some type of defect.

• This may be accomplished by the formation of one Fe2+ vacancy (or the removal of two positive charges) for every two Fe3+ ions that are formed

Impurities in ceramics• Type: solid solutions of both substitutional and

interstitial• For an interstitial, the ionic radius of the impurity

must be relatively small in comparison to the anion. • A substitutional impurity will substitute for the host

ion (c/a) to which it is most similar in an electrical sense:

• if the impurity atom normally forms a cation in a ceramic material, it most probably will substitute for a host cation.

• For example, in NaCl, impurity Ca2+ and O2− ions would most likely substitute for Na+ and Cl− ions, respectively.

• To achieve any appreciable solid solubility of substituting impurity atoms, the ionic size and charge must be very nearly the same as those of one of the host ions

• For an impurity ion having a charge different from the host ion for which it substitutes, the crystal must compensate for this difference in charge so that electroneutrality is maintained with the solid.

• One way this is accomplished is by the formation vacancies or interstitials of both ion types

Deformation• Tr ceramic suffer fracture• Require high T• involves stretching of the bonds, but the atoms do not

slip past each other. • Reversible & nonrev.• Elastic & plastic deformation

F

bonds stretch

return to initial

1. Initial 2. Small load 3. Unload Deformation

planes still sheared

F

elastic + plastic

bonds stretch & planes shear

plastic

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Plastic Deformation• From an atomic perspective, plastic deformation

corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors.

• After removal of the stress, the large number of atoms that have relocated, do not return to original position.

• Permanent deformation is accomplished by means of a process called slip, which involves the motion of dislocations.

• A structure that has plastically deformed, or experienced a permanent change in shape, may not be capable of functioning as intended

• Measure resistance where a small indenter is forced into the surface of a material permanently • The depth or size of indentation is measured • corresponds to a hardness number• Large hardness number means: --resistance to plastic deformation or cracking in compression. --better wear properties• The softer the material, the larger & deeper the indentation lower hardness number

e.g., 10mm sphere

apply known force (1 to 1000g)

measure size of indent after removing load

dDSmaller indents mean larger hardness.

Hardness

APPLICATIONS

Glasses

• Non crystalline silicates with other oxide (e.g. CaO, Na2O, K2O & Al2O3)

• Optical transparency & relative ease to fabricated

Glass Ceramics

• Glass can be transformed to crystalline by high T heat treatment crystallisation

• Product: glass-ceramics (fine-grained polycrystalline)

• Process involves nucleation & growth stage• A nucleation agent (frequently TiO2) is addded

to promote crystallization; This shift the begin and end transformation curves to shorter time

Properties of Glass-Ceramics• relatively high mechanical

strengths• low coeff of thermal

expansion (to avoid thermal shock)

• Relative high T capability• Good dielectric properties

(for electronic packaging application)

• Good biological compatibility

• Transparent /opaque

• Ease to be fabricated in mass production

• commonly used as ovenware, tableware, oven window, rangetop

Clay Products

• Very popular products (abundant, inexpensive, easy to be formed)

• Contain nonplastic ingredient affect the change (drying & firing) & characteristics

• structural clay products: brick, tiles, pipes (applications in which structural integrity is important)

• Whitewares: porcelain, pottery, tableware, sanitary ware

Refractories

• Properties: endure at high T, capacity to remain inert in severe enviroment, provide thermal insulation

• Common product bricks• Application: metal refining, glass manufacturing,

metallurgical heat treatment, power generation• Type of refractories:

Abrasives• Used for wear, grind or

cut softer material grinding wheel, coated abrasive & loose grain

• Properties: hardness/wear resistance, tough

• Silicon carbide, tungsten carbide, aluminium oxide, silica sand, diamonds

Cements

• Characteritic: form paste when mixed with water subsequently set & hardens

• Chemically binds aggregate into single structure at Tr