What are Ceramics? · 2/8/2011 · CERAMIC MATERIALS What are Ceramics? Ceramics are inorganic, non -metallic and crystalline materials that are typically produced using clays and

CERAMIC CERAMIC

MATERIALSMATERIALS

What are Ceramics?What are Ceramics?

�� Ceramics Ceramics are are inorganic, noninorganic, non--metallicmetallic and and crystalline materialscrystalline materials that are that are

typically produced using clays and other minerals from the earthtypically produced using clays and other minerals from the earth or or

chemically processed powderschemically processed powders

�� Ceramics are crystalline and are compounds formed between metallCeramics are crystalline and are compounds formed between metallic ic

and nonand non--metallic elements such as aluminium and oxygen (aluminametallic elements such as aluminium and oxygen (alumina--

AlAl22OO33 ), silicon and nitrogen (silicon nitride), silicon and nitrogen (silicon nitride-- SiSi33NN44) and silicon and ) and silicon and

carbon (silicon carbidecarbon (silicon carbide--SiC). SiC).

�� GlassesGlasses are nonare non--metallic, inorganic but metallic, inorganic but amorphousamorphous. They are often . They are often

considered as belonging to ceramics.considered as belonging to ceramics.

Characteristics of CeramicsCharacteristics of Ceramics

CeramicsCeramics

�� Low densityLow density

�� High THigh Tmm

�� High elastic modulusHigh elastic modulus

�� BrittleBrittle

�� NonNon--reactivereactive

�� Goff electrical and Goff electrical and

thermal insulatorsthermal insulators

�� High hardness and High hardness and

wear resistancewear resistance

MetalsMetals

�� High densityHigh density

�� Medium to high TMedium to high Tmm

�� Medium to high elastic Medium to high elastic

modulusmodulus

�� Ductile Ductile

�� Reactive (corrode)Reactive (corrode)

�� Good electrical and Good electrical and

thermal conductorsthermal conductors

PolymersPolymers

�� Very low densityVery low density

�� Low TmLow Tm

�� Low elastic modulusLow elastic modulus

�� Ductile and brittleDuctile and brittle

Structure of CeramicsStructure of Ceramics

�� CeramicsCeramics exhibit ionic, covalent bonding or a combination of the two exhibit ionic, covalent bonding or a combination of the two

(like in Al(like in Al22OO33))

�� Type of bonding strongly influences the crystal structure of cerType of bonding strongly influences the crystal structure of ceramicsamics

�� llCeramics crystallise in two main groups:Ceramics crystallise in two main groups:

1.1. Ceramics with simple crystal structure (e.g; SiC, MgO)Ceramics with simple crystal structure (e.g; SiC, MgO)

2.2. Ceramics with complex crystal structures based on silicate SiOCeramics with complex crystal structures based on silicate SiO44

(known as silicates)(known as silicates)

Ionic bonding: metallic ions + nonmetallic ions

Cations Anions

Stable structureCoordination Number: RC/RA

RC/RA = 0.155

6

• Bonding:-- Mostly ionic, some covalent.

-- % ionic character increases with difference in electronegativity.

Adapted from Fig. 2.7, Callister 7e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical

Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 byCornell University.

• Large vs small ionic bond character:

SiC: small

CaF2: large

Ceramic Bonding

Ceramic crystal structure considerationsCeramic crystal structure considerations

�� Charge NeutralityCharge Neutrality

�� The bulk ceramic must remain electrically neutralThe bulk ceramic must remain electrically neutral

�� For example, the compound MgOFor example, the compound MgO22 does not existdoes not exist

MgMg+2+2 & O& O--22: net charge / molecule = 1(+2) + 2(: net charge / molecule = 1(+2) + 2(--2) = 2) = --22

must MgOmust MgO

�� Coordination Number (CN) : The number of atomic or ionic Coordination Number (CN) : The number of atomic or ionic

nearest neighbors.nearest neighbors.

�� Depends on atomic size ratioDepends on atomic size ratio

�� CN increases as the CN increases as the RRCC/R/RAA increasesincreases

�� CN determines the possible crystal structure, CN determines the possible crystal structure,

�� Thus, CN determines the physical propertiesThus, CN determines the physical properties

Cs+

Cl-

= Na+

= Cl-

Examples of AX type structureExamples of AX type structure

Rock Salt Structure

10

MgO and FeO also have the NaCl structure

O2- rO = 0.140 nm

Mg2+ rMg = 0.072 nm

Adapted from Fig.

12.2, Callister 7e.

Each oxygen has 6 neighboring Mg2+

MgO and FeO

11

AX–Type Crystal Structures include NaCl, CsCl, and zinc blende

939.0181.0

170.0

Cl

Cs==

−

+

r

r

Adapted from Fig. 12.3, Callister 7e.

Cesium Chloride structure:

Each Cs+

has 8 neighboring Cl-

AX Crystal Structures

12

Zinc Blende structure


Ex: ZnO, ZnS, SiC

AX Crystal Structures

13

Fluoride structure

• Calcium Fluoride (CaF2)

• cations in cubic sites

• UO2, ThO2, ZrO2, CeO2


AX2 Crystal Structures

14

• Perovskite

Ex: complex oxide

BaTiO3


ABX3 Crystal Structures

15

We know that ceramics are more brittle than metals. Why?

• Consider method of deformation

� slippage along slip planes

�in ionic solids this slippage is very difficult

�too much energy needed to move one anion past another anion

Mechanical Properties

Our focus is HERE !!!Our focus is HERE !!!

ceramicsceramicsceramicsceramicsceramicsceramicsceramicsceramics

Clay ProductsClay Products

GlassesGlasses RefractoriesRefractories

AbrasivesAbrasives

CementsCements Eng. Eng.

CeramicsCeramics

GlassesGlasses GlassGlass--

ceramicsceramics

Engineering ceramics are generally Engineering ceramics are generally

classified into the following: classified into the following:

�� Structural ceramics, Structural ceramics,

� Industrial wear parts, bioceramics, cutting tools, engine components

�� Electrical and Electronic ceramics,Electrical and Electronic ceramics,

� Capacitors, insulators, substrates, IC

packages, piezoelectrics, magnets,

superconductors

�� Ceramic coatings, Ceramic coatings,

� Industrial wear parts, cutting tools,

engine components

�� Chemical processing & environmental Chemical processing & environmental

ceramicsceramics

� Filters, membranes, catalysts

Bioceramics

Cutting tools

Coating

Engine parts

Silicate CeramicsSilicate Ceramics

� Most common elements on earth are Si & O

Si

OSiSi--O TetrahedronO Tetrahedron

The strong Si-O bond leads to a strong, high melting material (1710ºC)

20

� Combine SiO44- tetrahedra by having them share

corners, edges, or faces

� Cations such as Ca2+, Mg2+, & Al3+ act to neutralize & provide ionic bonding

Mg2SiO4 Ca2MgSi2O7


Silicates

Two most common silicate ceramics are:Two most common silicate ceramics are:

Silica and silica glassesSilica and silica glasses

1.1. Silica (SiOSilica (SiO22))

�� If the tetrahedra are arranged in a If the tetrahedra are arranged in a

regular and ordered manner, a regular and ordered manner, a

crystalline structure is formed. Silica crystalline structure is formed. Silica

have 3 different types: quartz, have 3 different types: quartz,

crystobalite and tridymitecrystobalite and tridymite

O

Silica Silica

Si

SiSi--O TetrahedronO Tetrahedron Silicate CeramicsSilicate Ceramics

2.2. Silica GlassesSilica Glasses

�� If the tetrahedra are randomly arranged, a nonIf the tetrahedra are randomly arranged, a non--crystalline structure, known as crystalline structure, known as GlassGlass is formed. is formed.

O

Silica glasses is a dense form of amorphous silica

- Charge imbalance corrected with “counter cations” such as Na+

-Borosilicate glass is the pyrex glass used in labs

-better temperature stability & less brittle than sodium glass

23

• Silica gels - amorphous SiO2

� Si4+ and O2- not in well-ordered lattice

� Charge balanced by H+ (to form OH-) at “dangling” bonds

�very high surface area > 200 m2/g

� SiO2 is quite stable, therefore

unreactive

�makes good catalyst support


Amorphous Silica

Other oxides may also be incorporated into a Other oxides may also be incorporated into a glassy SiOglassy SiO22 network in different ways:network in different ways:

1.1. Network formers: Network formers: form glassy structures form glassy structures (B(B22OO33))

2.2. Network modifiers: Network modifiers: added to terminate (break added to terminate (break up) the network (CaO, Naup) the network (CaO, Na22O). These are O). These are

added to silica glass to lower its viscosity (so added to silica glass to lower its viscosity (so that forming is easier)that forming is easier)

3.3. Network intermediates: Network intermediates: these oxides cannot these oxides cannot form glass network but join into the silica form glass network but join into the silica

network and substitute for Si. network and substitute for Si.

25

• Carbon black – amorphous –surface area ca. 1000 m2/g

• Diamond

� tetrahedral carbon

�hard – no good slip planes

�brittle – can cut it

� large diamonds – jewellery

� small diamonds

�often man made - used for cutting tools and polishing

� diamond films

�hard surface coat – tools, medical devices, etc.

Adapted from Fig.

12.15, Callister 7e.

Carbon Forms

26

• layer structure – aromatic layers

� weak van der Waal’s forces between layers

� planes slide easily, good lubricant

Adapted from Fig.

12.17, Callister 7e.

Carbon Forms

27

• Fullerenes or carbon nanotubes

� wrap the graphite sheet by curving into ball or tube

� Buckminister fullerenes

�Like a soccer ball C60 - also C70 + others

Adapted from Figs. 12.18 & 12.19,

Callister 7e.

Carbon Forms

28

• 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

Defects in Ceramic Structures

Mechanical Properties of CeramicsMechanical Properties of Ceramics

�� Ceramics have inferior mechanical properties compared to metals,Ceramics have inferior mechanical properties compared to metals, and this and this

has limited their applicationshas limited their applications

�� The main limitation is that ceramics fail in The main limitation is that ceramics fail in ““brittlebrittle”” manner with little or no manner with little or no

plastic deformation. plastic deformation.

�� Fracture strength of ceramics are significantly lower than prediFracture strength of ceramics are significantly lower than predicted by cted by

theory because of the presence of very small cracks in the matertheory because of the presence of very small cracks in the material (stress ial (stress

concentrators). concentrators).

�� Lack of ductility in ceramics is due to their strong ionic and cLack of ductility in ceramics is due to their strong ionic and covalent bonds. ovalent bonds.

Mechanical Properties of CeramicsMechanical Properties of Ceramics

�� Ceramics have excellent compressive strength (used in cement andCeramics have excellent compressive strength (used in cement and

concrete in foundations for structures and equipment)concrete in foundations for structures and equipment)

�� The principles source of fracture in ceramics is surface cracks,The principles source of fracture in ceramics is surface cracks, porosity, porosity,

inclusions and large grains produced during processing.inclusions and large grains produced during processing.

�� Testing ceramics using the usual tensile testing is not possibleTesting ceramics using the usual tensile testing is not possible, so a , so a

transverse bending test is used and a transverse bending test is used and a modulus of rupturemodulus of rupture (MOR) is (MOR) is

determined. determined.

�� Strength of ceramics can only be described by statistical methodStrength of ceramics can only be described by statistical methods and it is s and it is

dependent on specimen size. dependent on specimen size.

Flexural

strength, σfs= 22

3

bd

LFfRectangular cross section

= 3

3

R

LFf

π

Circular cross section

Material Symbol

Transverse

rupture

strength

(MPa)

Compressive

strength

(MPa)

Elastic

modulus

(GPa)

Hardness

(HK)

Poisson’s

ratio (n)

Density

(kg/m3)

Aluminum

oxide

Al2O3 140–240 1000–2900 310–410 2000–3000 0.26 4000–4500

Cubic boron

nitride

CBN 725 7000 850 4000–5000 — 3480

Diamond — 1400 7000 830–1000 7000–8000 — 3500

Silica, fused SiO2 — 1300 70 550 0.25 —

Silicon

carbide

SiC 100–750 700–3500 240–480 2100–3000 0.14 3100

Silicon

nitride

Si3 N4 480–600 — 300–310 2000–2500 0.24 3300

Titanium

carbide

TiC 1400–1900 3100–3850 310–410 1800–3200 — 5500–5800

Tungsten

carbide

WC 1030–2600 4100–5900 520–700 1800–2400 — 10,000–15,000

Partially

stabilized

zirconia

PSZ 620 — 200 1100 0.30 5800

The properties vary widely depending on the condition of the material (crack size)

�� Failure of ceramics occurs mainly from Failure of ceramics occurs mainly from structural defects; surface cracks, porosity, structural defects; surface cracks, porosity, inclusions and large grains during processing. inclusions and large grains during processing.

�� Porosity in ceramics acts as stress Porosity in ceramics acts as stress concentrators: crack forms and propagates concentrators: crack forms and propagates leading to failure. leading to failure.

�� Once cracks start to propagate, they will Once cracks start to propagate, they will continue to grow until fracture occurs.continue to grow until fracture occurs.

�� Porosity also decrease the crossPorosity also decrease the cross--sectional sectional area over which a load in applied: lower the area over which a load in applied: lower the stress a material can support. stress a material can support.

Factors Affecting Strength of CeramicsFactors Affecting Strength of Ceramics

Strength of ceramics is thus determined by many factors:Strength of ceramics is thus determined by many factors:1.1. Chemical compositionChemical composition2.2. Microstructure Microstructure -- In dense ceramics materials, no large pores, the flaw is In dense ceramics materials, no large pores, the flaw is

related to grain size. Finer grain size ceramics, smaller flaws related to grain size. Finer grain size ceramics, smaller flaws size at the size at the boundaries, hence stronger than large grain size.boundaries, hence stronger than large grain size.

3.3. Surface conditionSurface condition4.4. Temperature and environment (failure at RT, usually due to largTemperature and environment (failure at RT, usually due to large flaws).e flaws).

Toughening Mechanisms of CeramicsToughening Mechanisms of Ceramics

�� Fracture strength or toughness of ceramics can be improved only Fracture strength or toughness of ceramics can be improved only by by

mechanisms that influence the crack propagation (ceramics alwaysmechanisms that influence the crack propagation (ceramics always

contain cracks).contain cracks).

�� There are various methods used to improve the toughness of There are various methods used to improve the toughness of

ceramics:ceramics:

1.1. Transformation toughening Transformation toughening

2.2. Microcrack induced tougheningMicrocrack induced toughening

3.3. Crack deflectionCrack deflection

4.4. Crack bridgingCrack bridging

1.1. Transformation Toughening: e.g. Transformation Toughening: e.g. Partially Stabilised Zirconia (PSZ)Partially Stabilised Zirconia (PSZ)

�� Zirconia (ZrOZirconia (ZrO22) exists on 3 different crystal structures:) exists on 3 different crystal structures:

Melt Cubic TetragMelt Cubic Tetragonal Monocliniconal Monoclinic

�� Transformation toughening is achieved by stabilising the tetragoTransformation toughening is achieved by stabilising the tetragonal nal structure at room temperature by adding other oxides such as: Mgstructure at room temperature by adding other oxides such as: MgO, CaO, O, CaO, and Yand Y22OO33 to zirconia. to zirconia.

�� If cubic ZrOIf cubic ZrO22 is stabilised, so it retains cubic structure at RT called is stabilised, so it retains cubic structure at RT called fully fully stabilised zirconia.stabilised zirconia.

�� If tetragonal ZrOIf tetragonal ZrO22 is stabilised, it called as PSZ.is stabilised, it called as PSZ.

Mixture of ZrOMixture of ZrO22--9 mol %MgO is sintered at 18009 mol %MgO is sintered at 1800ooC, then rapidly cooled to C, then rapidly cooled to RT become metastable cubic structure. The materials is reheated RT become metastable cubic structure. The materials is reheated at at 14001400ooC for sufficient time, a fine metastable precipitate with tetragC for sufficient time, a fine metastable precipitate with tetragonal onal structure known as PSZ formed. structure known as PSZ formed.

�� As the crack propagates, it creates a local stress field that inAs the crack propagates, it creates a local stress field that induces duces transformation of the tetragonal structure to the monolithic (ortransformation of the tetragonal structure to the monolithic (or monoclinic) monoclinic) structure in that region. structure in that region.

1150 1150 ooCC2370 2370 ooCC2680 2680 ooCC

•• This transformation is accompanied by a volume expansion, causinThis transformation is accompanied by a volume expansion, causing a g a

compressive stress locally and in turn a compressive stress locally and in turn a squeezing effect on the crack and squeezing effect on the crack and

enhancing the fracture toughness alsoenhancing the fracture toughness also significantly extends the reliability

and lifetime of products made with stabilized zirconia.

Precipitate around

crack is monoclinic

ZrO2-MgO

Matrix is cubic ZrO2-

MgO

Precipitate is tetragonal ZrO2-MgO

Single crystals of the cubic phase of

zirconia are commonly used as diamond

simulant in jewelery.

The cubic phase of zirconia also has a very

low thermal conductivity, which has led to

its use as a thermal barrier coating or TBC

in jet and diesel engines to allow operation

at higher temperatures.

Stabilized zirconia is used in oxygen

sensors and fuel cell membranes because

it has the ability to allow oxygen ions to

move freely through the crystal structure at

high temperatures.

2.2. MicroMicro--crack Induced Toughening:crack Induced Toughening:

�� Microcracks are purposely introduced by internal stresses duringMicrocracks are purposely introduced by internal stresses during

processing of the ceramics tend to blunt the tip of the propagatprocessing of the ceramics tend to blunt the tip of the propagating crack and ing crack and

thus reduce the stress concentration at the crack tip. thus reduce the stress concentration at the crack tip.

�� This microThis micro--crack will interfere the crack tip propagation.crack will interfere the crack tip propagation.

3.3. Crack Deflection and Crack BridgingCrack Deflection and Crack Bridging

�� This is achieved by reinforcing the ceramics: produce ceramic baThis is achieved by reinforcing the ceramics: produce ceramic based sed

composites (CMC)composites (CMC)

• The high hardness of some ceramic materials makes them

useful as abrasive materials for cutting, grinding, polishing e.g. Al2O3 and SiC, diamonds

Ceramic as abrasive materials

• MEMS – mechanical devices that integrated with large

number of electrical elements on a substrate of Silicon –e.g. for microsensors

• Current research on ceramic materials to replace silicon, because ceramic are tougher, more refractory and more inert e.g. silicon carbonitrides (silicon carbide-silicon

nitrides alloys)

Advanced ceramics

Properties of GlassesProperties of Glasses

�� Glasses posses properties not found in other engineering materiaGlasses posses properties not found in other engineering materials. ls.

�� Combination of transparency, ability to transfer light, hardnessCombination of transparency, ability to transfer light, hardness at room at room

temperature, a sufficient strength and corrosion resistance to mtemperature, a sufficient strength and corrosion resistance to most ost

environments. These make glasses important for many applicationsenvironments. These make glasses important for many applications: vehicle : vehicle

glazing, lamps, electronic industry, laboratory apparatus. glazing, lamps, electronic industry, laboratory apparatus.

�� Deformation of glass varies with temperature:Deformation of glass varies with temperature:

�� At high temperatures: viscous flowAt high temperatures: viscous flow

�� At low temperatures: elastic and brittleAt low temperatures: elastic and brittle

�� At intermediate temperatures: viscoAt intermediate temperatures: visco--elasticelastic

Heat Treatment of GlassesHeat Treatment of Glasses•• Glasses can be rendered more fracture resistance by introducing Glasses can be rendered more fracture resistance by introducing compressive compressive

stresses on the glass surface. This is followed by glass temperistresses on the glass surface. This is followed by glass temperingng

1.1. Glass AnnealingGlass Annealing

�� Used to reduce internal residual stresses, which weaken the glasUsed to reduce internal residual stresses, which weaken the glass and may lead to s and may lead to fracture. fracture.

�� The glass is heated to the annealing temperature, then slowly coThe glass is heated to the annealing temperature, then slowly cooled to RToled to RT

2.2. Glass TemperingGlass Tempering

�� Used to strengthen glass by inducing compressive stresses at theUsed to strengthen glass by inducing compressive stresses at the surface. surface.

�� Tempering is achieved by heating the glass to a temperature > TTempering is achieved by heating the glass to a temperature > Tgg, then rapidly , then rapidly cooled to room temperature. cooled to room temperature.

�� The surface of the glass cools first and contracts; later the ceThe surface of the glass cools first and contracts; later the centre cools and attempts ntre cools and attempts to contract but is prevented from doing so by the rigid and stroto contract but is prevented from doing so by the rigid and strong surface. ng surface.

�� This produces high tensile stresses in the centre but compressivThis produces high tensile stresses in the centre but compressive stresses at the e stresses at the surface. surface.

�� This tempering treatment increases the strength of the glass becThis tempering treatment increases the strength of the glass because applied tensile ause applied tensile stresses must surpass the compressive stresses on surface beforestresses must surpass the compressive stresses on surface before fracture occurs.fracture occurs.

�� Tempered glass has higher impact resistance than annealed glass Tempered glass has higher impact resistance than annealed glass and about 4x and about 4x stronger than annealed glass.stronger than annealed glass.

Documents

What are Ceramics? · 2/8/2011 · CERAMIC MATERIALS What are Ceramics? Ceramics are inorganic, non -metallic and crystalline materials that are typically produced using clays and