new traditional ceramics - university of babylon · 2011. 6. 8. · high feldspar content gives...

Introduction to Ceramics: 4: Examples

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Traditional Ceramics! Different compositions are

used for different applications

! Liquid phase sintering with liquid content controlled by feldspar content

! High feldspar content gives lower firing temperature and high translucency

! High clay content is easier to form into green bodies

Flint

Clay

Feldspar


4 - 2Compositions 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


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Applications! Dental porcelains need good translucency - high

feldspar and low clay! Low glass content ceramics (high clay) are harder

and more resistant to chemical attack! China 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 product

! Triaxial porcelains are insensitive to precise composition of clay mix. Thus easy to use in industry


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Microstructure At Sintering Temperature! Feldspar melts and

dissolves alumino-silicate particles in the clays

! At 1200°C equilibrium is liquid plus mullite

! However, diffusion is very slow in the solid particles and equilibrium is never reached under practical firing conditions


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Sintered Microstructure! Equilibrium never fully

achieved! Feldspar relicts are

filled with a mixture of glass and Mullite


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Pottery Microstructures! Large quantities of

liquid form! High viscosity of liquid

- slow diffusion! Solution rings around

partially dissolved quartz particles

! Regions of Mullite needle formation within glass


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Engineering Ceramics: Alumina! Alumina (Al2O3) is an

important engineering ceramic! Properties

Hardness 16 GPaToughness 4.0 MPa¦mModulus 380 GPaStrength 200 - 300 MPa

! Fired at 1400 - 1800 °C –depending on purity


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Alumina Microstructures! Solid state sinter or can

liquid phase sinter at low purity (“debased alumina”)

! Because of the very high processing temperatures grain growth can occur

! Suppress grain growth by adding MgO dopant. This speeds grain boundary diffusion and pins boundaries

Debased alumina (about 95% Al2O3), with remnant glassy phase(about 1500ºC sintering)

“Pure” alumina (about 99% Al2O3), with MgO used to stabilise grain growth (about 1650ºC sintering)


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Engineering Ceramics: Zirconia

! Zirconia exists in 3 different crystal structures! a) monoclinic at low

temperature! b) tetragonal at intermediate

temperature! c) cubic at high temperature


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Stabilised Zirconia! Adding MgO stabilises the

cubic phase to lower temperatures

! cubic-tetragonal transformation is diffusional

! tetragonal to monoclinic transformation is martensitic with a 6% increase in volume

! High MgO or CaO: can get metastable cubic form at room temp

! ~2.5% Y2O3 : can get metastable tetragonal form at room temp


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Partially Stabilised Zirconia (PSZ)! Add about 10% MgO! Sinter the material in the

cubic phase! Lower 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 temperature! Remaining c-phase does not

get time to transform


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PSZ: Microstructure

Small tetragonal ZrO2precipitates in a cubic matrixNote how precipitates are aligned showing orientation relation between t and c phases 100nm


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Transformation Toughening

! The t/m transformation can occur instantaneously and the volume increase in pure zirconia causes premature fracture

! By dispersing the t-phase in an inert matrix we can suppress the transformation by the elastic constraint of the surrounding medium

! Transformation now needs a critical dilation stress to remove constraint

" This dilation can be supplied by the hydrostatic component of the stress field ahead of the crack - stress induced transformation

" The particles expand on transformation and induce a crack face compression in their wake


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PSZ: Properties

! Stabiliser: 8 - 14 at%CaO, MgO, Y2O3

! Have to get aging time right to get best t-precipitate size for optimum toughness and strength

! Hardness 10 - 14 GPa! Modulus 170 - 210 GPa

Strength 440 - 720 MPaToughness6 - 20 MPam1/2


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Silicon Nitride, Si3N4

! High melting temperature and covalent bonding make it difficult to sinter

! Oxide on surface of powder (SiO2) forms oxynitride glass and liquid phase sintering is possible

! Can form reaction bonded Si3N4 to near net shape but porosity results in reduced properties


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Si3N4: Crystal Structures 1! Si atom is surrounded by 4 N to form SiN4 tetrahedra

similar in size to SiO4 tetrahedra in silicates! N corners are shared by 3 tetrahedra! Bonding is intermediate ionic-covalent - about 70%

covalent! 2 crystal structures α and β are known, both are

hexagonal! α phase is harder than β


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Si3N4 Crystal Structures 2! α and β forms are distinguished by the stacking

sequence of Si-N layers in the structure! α form has β stacking plus a glide operator

β-Si3N4 α-Si3N4

ABAB repeat ABCD repeat


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Reaction Bonded Si3N4

! Use reaction 3Si + 2N2 --> Si3N4

! React solid Si with N2 gas - endothermic reaction so energy expensive

! Forms very pure Si3N4 with no glassy phases at grain boundaries

! Forms porous ceramic (pores needed to ensure N2 transport)


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Reaction Bonding! Volatile oxide SiO is

formed in reducing conditions

! SiO 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


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Hot Pressed/Sintered Silicon Nitride! Covalent nature of Si3N4 hinders sintering! If 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 N2 atmosphere


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Sintering Aids! Usually add 2-3% of a metal oxide e.g. MgO, Al2O3,

Ln2O3 or Y2O3. These combine with a surface SiO2layer on the Si3N4 powder

! This forms a low melting point oxynitride glass which aids liquid phase sintering and solidifies to a grain boundary glass

! Sintering is usually in range 1550-1800°C so starting α-Si3N4 powder transforms to β-Si3N4


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α and β Phases! β-Si3N4 is stable at high temperatures above

1420°C! Both phases can be stabilised by impurities – e.g.

oxygen impurities stabilise α-Si3N4

! Si2+ and other dopants result in both phases being capable of existing over very wide range of conditions


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Tough Microstructures! By exploiting the α−β

transformation we can grow elongated grains by heat treating after sintering

! Certain sintering aids (Y2O3) promote elongated β-grains

! Elongated grains deflect cracks and increase toughness


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Silicon Nitride: Typical Properties! Hard and strong ceramic

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

! Also has excellent thermal shock resistance! high thermal conductivity and toughness, relatively low

elastic moduli


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SIALON

! SiAlON - ceramic alloy! Si3N4 is built from SiN4 tetrahedra! AlO4 tetrahedra have the same size! If we substitute one Al3+ for each Si4+ and each N3- by

an O2-, charge neutrality is preserved! Crystal structure not too distorted! Composition Si6-zAlzOzN8-z for β-Sialon


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α-SIALON! α-Sialon is also a substituted Si3N4 structure! However now 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 interstices

! Formula Mx(Si12-pAlp)(OnN16-p)! By adjusting cation dopant and Al and O levels it is

possible to form 2-phase α−β sialons


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β′ -SIALON! β’-sialon has significant

AlO present! Surface oxide tends to

form mullite and not SiO2

! Forms grain boundary glasses more easily and is thus easier to sinter

! Negative deviation from Raoult’s law so lower than expected N2vapour pressure above it – atmospheric nitrogen strongly suppresses decomposition reaction

Si3N4 ⇔ 3Si + 2N2


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Sialon Phase Equilibria

" Phase diagram shows extent of β’-sialon solid solubility and other sialon phases


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Applications of Si3N4 & SIALON

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

new traditional ceramics - university of babylon · 2011. 6. 8. · high feldspar content gives...

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