ceramics handout 1

Real Ceramics: oxides Oxides: 1

Traditional CeramicsDifferent compositions are used for different applicationsLiquid phase sintering with liquid content controlled by feldspar contentHigh feldspar content gives lower firing temperature and high translucencyHigh clay content is easier to form into green bodies

Flint

ClayFeldspar


Whitewares: ApplicationsDental porcelains need good translucency - high feldspar and low clayLow glass content ceramics (high clay) are harder and more resistant to chemical attackChina clays have large particles and give white products - ball clays have very fine particles but impurities give dark product. Ball clays make easier formed green bodies but give inferior productTriaxial porcelains are insensitive to precise composition of clay mix. Thus easy to use in industry


Compositions for Typical Traditional Ceramics (“whitewares”)

-95-5Dental Porcelain

2 CaCO335221031Hotel China

38 bone ash2215-25Bone China

21253023Semi-vitreous Whiteware

18342030Sanitary Ware

2 talc25351028Electrical Insulators

25251040Hard Porcelain

OtherFlint (SiO2)

Feld-Spar

Ball Clay

China Clay

Type


Microstructure at Sintering Temperature

Feldspar melts and dissolves alumino-silicate particles in the claysAt 1200°C equilibrium is liquid plus mulliteHowever, diffusion is very slow in the solid particles and equilibrium is never reached under practical firing conditions


Sintered MicrostructureEquilibrium never fully achievedFeldspar relicts are filled with a mixture of glass and Mullite


Pottery MicrostructuresLarge quantities of liquid formHigh viscosity of liquid -slow diffusionSolution rings around partially dissolved quartz particlesRegions of Mullite needle formation within glass


ALUMINAAlumina (Al2O3) is the most important engineering ceramic

Uses –insulators

thermalelectrical

wear partsbiomedical implantshigh-T parts

kiln furnitureengineering components

abrasive


Alumina Microstructures

Can liquid phase sinter at low purity (80% - 95% alumina, with added silicates, MgO) –“debased alumina”. Sintering temperatures 1400 - 1550 ºC.For high purity alumina, because of the very high processing temperatures (>1550), grain growth occurs.Suppress grain growth by adding MgO dopant (1% or less). This speeds grain boundary diffusion and pins boundaries.


Typical alumina propertiesYoung’s modulus: 380 - 400 GPa (good)

(anisotropic) (bad)

Hardness: 16 – 18 GPa (good)Fracture Toughness: 3.5 – 4.5 MPam1/2 (low)Bend Strength: 300 – 500 MPa (OK)Grain sizes:

1-2 µm (Hot pressed)3 – 25 µm (Sintered)

Thermal expansion co-eff: 9 x 10-6 K-1 (high)(anisotropic) (bad)

Thermal conductivity: 8 W m-1 K-1 (low)


Zirconia

Zirconia exists in 3 different crystal structures based on the fluorite structurea) monoclinic at low temperatureb) tetragonal at intermediate

temperaturec) cubic at high temperature

Cubic ⇔ tetragonal transformation is diffusional.Tetragonal ⇒ monoclinic transformation is martensitic with a 6% increase in volume

monoclinic tetragonal

cubic


Stabilised Zirconia

A number of dopants can be used to stabilise the cubic phase to lower temperaturesMost important are CaO, MgO, Y2O3 and CeO2

ZrO2 ZrO2 % MgO% CaO

Tem

pera

ture

(ºC

)

Tem

pera

ture

(ºC

)


Partially Stabilised Zirconia (PSZ)

Add about 10% MgOSinter the material in the cubic phaseLower temperature and heat treat (age) to nucleate small precipitates of t-phase

These are grown to below the critical size for t-m transformation

Cool to room temperatureRemaining c-phase does not get time to transform

ZrO2 % MgO

Tem

pera

ture

(ºC

)


PrecipitationPSZ microstructures are analagous to those formed in precipitating metal systemsToo much stabiliser tends to generate grain boundary precipitates which are too large to remain in t-phaseToo little stabiliser and firing temperatures are too high.

ZrO2wt% CaO

Tem

pera

ture

(ºC

)


Mg-PSZ MicrostructuresAge at 1400°C. After 4-5 hours tetragonal precipitates form and grow by conventional diffusionalprocesses as coherent spheroids along {001} cube planes.Below a well defined critical size of about 200 nm the t-particles are retained on cooling to room temperature.Optimum microstructures contain about 25% - 30% by volume of tetragonal phase.

100 nm


OVER-AGED MICROSTRUCTURESIn over-aged microstructures coherency is lost and particles spontaneously transform to monoclinic on coolingTwinned precipitates are visible, twinning helps relieve transformation strain mismatch


Toughness / Aging Time RelationPeak toughness in PSZ microstructure is obtained when optimum mean precipitate size is achieved.

Optimum size is just below that for spontaneous transformation.

However, in the overaged state microcrack toughening is possible and so toughness falls away more slowly than expected


Properties of PSZ

6 - 86 - 126 – 20 Fracture Toughness (MPam1/2)

650 – 1000400 - 650440 - 720Bend Strength (MPa)

180 – 220200 - 220170 - 210Young’s Modulus (GPa)

8 – 1214 - 1710 - 14Hardness (GPa)

5.0 – 103.0 – 4.52.5 – 3.6Wt% Stabiliser

Y - PSZCa - PSZMg - PSZ


Tetragonal Zirconia Polycrystals (TZP)

Addition of 2.5% Y2O3 results in a considerable expansion of the tetragonal phase field

Similar behaviour seen with CeO2additions


TZP MicrostructureSinter at 1400- 1550°C, much lower than 1800°C for PSZ

Potentially 100% of the microstructure is transformable giving tougher ceramics

Powders often contain small inclusions of cubic material


TZP propertiesFracture toughness in Y-TZP can be very highCritical grain size for transformation is a function of Yttria content

mol % Y2O3 mol % Y2O3

Crit

ical

Gra

in S

ize

(µm

)

Frac

ture

Tou

ghne

ss (M

Pam

1/2 )


Properties of TZP

6 - 306 - 15Fracture Toughness (MPam1/2)

500 – 800800 – 1300Bend Strength (MPa)

200 – 220140 - 200Young’s Modulus (GPa)

7 – 1010 – 12Hardness (GPa)

12 – 152 – 3Wt% Stabiliser

Ce – TZPY - TZP


Si3N4 Crystal StructureSi atom is surrounded by 4 N to form SiN4 tetrahedra similar in size to SiO4 tetrahedra in silicatesN corners are shared by 3 tetrahedraBonding is intermediate ionic-covalent - about 70% covalent2 crystal structures α and β are known; both are hexagonalα phase is harder than β


Si3N4 Crystal Structure

α and β forms are distinguished by the stacking sequence of Si-N layers in the structureα form has β stacking plus a glide operator

β-Si3N4 α-Si3N4

ABAB repeat ABCD repeat


Si3N4 - Phase Stabilityβ-Si3N4 is stable at high temperatures above 1420°CBoth phases can be stabilised by impurities - notably oxygen impurities stabilise α-Si3N4

The presence of Si2+ and other dopants results in both phases being capable of existing over a very wide range of conditions


Reaction Bonded Si3N4

Use reaction 3Si + 2N2 --> Si3N4React solid Si with N2 gas - endothermic reaction: energy expensiveForms porous ceramic: pores needed to ensure N2 transportForms very pure Si3N4 with no glassy phases at grain boundariesSlow process: may take 2 - 10 days to form componentMost commercial Si3N4 powder is made by grinding RBSN


Reaction BondingVolatile oxide SiO is formed in reducing conditionsSiO transports Si to gas phase reacts with N2 to deposit Si3N4

Both α and β phases formed depending on temperature(α if T < 1410°C)

3/2O2

3SiO+2N2

Si3N4

Si

3/2O2


Hot Pressed/Sintered Silicon NitrideCovalent nature of Si3N4 hinders sinteringIf we use very high temperatures to promote sintering tend to get thermal decomposition

Si3N4 --> 3Si + 2N2

Densification is usually achieved by Hot-Pressing in a N2atmosphere


Sintering AidsUsually add 2-3% of a metal oxide e.g. MgO, Al2O3, Ln2O3or Y2O3. These combine with a surface SiO2 layer on the Si3N4 powderThis forms a low melting point oxynitride glass which aids liquid phase sintering and solidifies to a grain boundary glassSintering is usually in range 1550-1800°C so starting α-Si3N4 powder transforms to β-Si3N4


α−β TransformationStarting α powder has SiO2coatingReacts with metal oxide additive to form sintering liquidAt sintering temperature β-phase precipitates out of the melt as αphase dissolves - dissolution reprecipitationFinal microstructure has elongated β-grains in glassy matrix


Si3N4: Tough MicrostructuresBy exploiting the α−β transformation we can grow elongated grains by heat treating after sinteringElongated grains deflect cracks and increase toughness by generating R-curve behaviour

0

2

4

6

8

10

0 100 200 300 400 500 600

Crack Size (µm)

Frac

ture

Tou

ghne

ss (M

Pam

1/2 )


Silicon NitrideHard and strong ceramic

SSN RBSNDensity (gcm-3) 3.2-3.9 2.2-3.2Hardness (GPa) 14-18 4-7Toughness MPa√m 3.4-8.2 1.5-3.6Modulus (GPa) 280-320 100-220Strength (MPa) 400-1000 190-400

Also has excellent thermal shock resistance


SIALON

Si3N4 is built from SiN4 tetrahedraAlO4 tetrahedra have the same sizeIf we substitute one Al3+ for a Si4+ and one O2- for an N3-, charge neutrality is preservedCrystal structure not too distortedSiAlON - ceramic alloyComposition Si6-zAlzOzN8-z for β-Sialon


Sialon Phase Diagram

Phase diagram shows extent of β’-sialon solid solubility and other sialon phasesβ`: Si6-zAlzOzN8-z

β`

Si2O4 Al2O3

Al2N4Si3N4

(Al2O3 – AlN)34


α-Sialonα-Sialon is also a substituted Si3N4 structureHere more Si is substituted by Al than N by O and so balancing cations (e.g Ca, Mg. Li Ce or Y) are accommodated in intersticesFormula Mx(Si12-pAlp)(OnN16-p)By adjusting cation dopant and Al and O levels it is possible to form 2-phase α−β sialons


Making Sialon β’-sialon has significant AlO presentSurface oxide tends to form mullite and not SiO2

Forms grain boundary glasses more easily and is thus easier to sinter.Presence of Al reduces

eutectic by about 200°C

Can produce ceramics by pressureless sinteringNegative deviation from Raoult’s law so low vapour pressure above it -suppresses decomposition reaction

Si3N4 ⇒ 3Si + 2N2


Sialon SynthesisTypically mix and mill powders of Si3N4 (>90%), Al2O3, AlN, SiO2 and MO As with Si3N4 , start with α powder and transform to β form on sinteringIdeally, choose mix to form a fully crystalline solid on transformation (transient liquid phase sintering)In practice, diffusion is too slow in N-containing glassy liquid: residual glass is always presentNormal commercial material uses MgO as sintering aid


Sialon MicrostructuresDense sintered sialon bodies contain significantly more glass than Si3N4. Also possible to form glass-ceramic bonded sialon. Y2O3 used as sintering aid. Post-sintering heat treatment crystallises g.b. glass to yttrium-aluminium garnet (YAG) plus reduced glass contentCrystallised YAG bonded sialons have much better creep resistance


Si3N4 & Sialon: High Temperature Properties

Generally good properties up to 1000°CDegradation increased if lots of g.b. glass presentRBSN properties retained to higher temperatures because there is no g.b. glass phase

Ben

d S

treng

th (M

Pa)

Temperature (ºC)


Applications of Silicon Nitride & Sialon

Cutting tools, grinding media, grit blasting nozzles, turbocharger rotors, crucibles, ball bearings


Silicon CarbideMost widely used non-oxide ceramicMostly used as an abrasiveVery high melting point and almost completely covalent structure make it very difficult to sinterSintering aids: B, C and Al but must sinter around 2000°CClosely related to diamond and silicon. Both C and Si are in sp3 hybridisation..


Structure of Silicon CarbideSiC occurs as 2 (groups of) phasesβ-SiC is stable at low temperatures below 1800°C. It is cubic and has the zincblende structure.α-SiC is stable above 1800 °C. It has a variety of faulted hexagonal structures based on wurtzite.6H, 2H and 15R are the most common polytypes - number gives stacking repeat sequence, letter gives lattice type.


SiC Powder Synthesis

Most SiC is made for abrasive applicationsAcheson process - carbothermalreduction of sand by coke

SiO2 + 3C ⇒ SiC + 2CO

Highly endothermic, requires very cheap energy resourceReaction occurs at 2200°C and produces α-SiC

SiC can be fabricated at lower temperatures using more finely divided SiO2 and CO gas or carbon black to give β-SiC

Other routes include direct reaction of Si and C or pyrolysis of SiO2 containing vegetable matter (e.g. rice hulls)


Sintered SiCThe covalent nature of SiC makes it very difficult to sinterSintering occurs at very high temperatures near to 2000°C

Must use very fine, sub-micron powdersC has low solubility in oxide glasses so liquid phase sintering is not favouredSintering aids are B, C or Al

Role of sintering aids is unclear - all are very strong reducing agents and possibly have a role in reducing the SiO2 coating on the powderSingle doping of Al or B leads to grain growth - doping with both Al and B simultaneously leads to equiaxed fine grained structures

Fine β-SiC powders sintered in the presence of Al, B, C or BeOcan transform to α-SiC and grow elongated grains analagous to Si3N4 transformation (but it’s tricky to do)

R-curve generating microstructures cannot be routinely fabricated


Reaction Bonded SiC (RBSC)

Direct reaction between Si and C can be exploited

Si + C ⇒ SiCTwo commercial reaction bonding processes:

Infiltrate SiC & C compact with liquid Si (“REFEL”)Form SiC compact bonded with phenolic resin and carbonise before infiltration.


RBSC MicrostructuresReaction occurs around the melting point of SiMicrostructures show interpenetrating networks of SiC and SiOriginal SiC is α phase. New SiC forms at low (-1400°C) temperatures: probably as β phase (though this is disputed as it grows epitaxially on original grits)

Reflected LightMicrograph

Backscatteredelectron image

Secondaryelectron image


SiC PropertiesRBSC SSC HPSC

Density (gcm-3) 3.15 - 3.25 3.1 - 3.15 3.20Hardness (GPa) 18 - 22 21 - 25 23 - 30Modulus (GPa) 280 - 390 410 450Strength (MPa) 350 - 540 430 640Toughness (MPa√m) 4 - 5 3 - 5 5 - 6


SiC: High Temperature Properties

At elevated temperatures SiC shows less of a degradation than Si3N4

RBSC retains good properties at high T - but only up to melting point of Si

Ben

d S

treng

th (M

Pa)

Temperature (ºC)


Applications of SiCAbrasives are (by far) most important application.More temperature stable than Si3N4- may be better for high temperature applications.Not as tough as Si3N4.High theraml conductivity.Very wear resistant. Not good as a cutting tool because of C reactivity with metals.

SiC Turbocharger Rotors

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industryreal ceramics

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high purity alumina

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alumina alumina al2o3