Download Introduction to Ceramics

Introduction to
Ceramics
Types of synthetic materials
Metals
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High density
Medium to high
melting point
Medium to high
elastic modulus
Reactive
Ductile
Polymers
•Very low
density
•Low melting
point
•Low elastic
modulus
•Very reactive
•Ductile and
brittle types
Ceramics
•Low density
•High melting point
•Very high elastic
modulus
•Unreactive
•Brittle
Types of Ceramic
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Glasses
Traditional ceramics -clay based
Engineering ceramics
Cement and concrete
Rocks and Minerals
Ceramic Composites
Covalent or ionic , interatomic bonding, Often compounds; usually oxides
New “engineering ceramics” can also be carbides, nitrides and borides
Ceramics data
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Hard brittle solids – no unique failure strength because it
depends on crack size
Data can vary markedly from manufacturer to manufacturer
Strength may depend on history after manufacture (surface
damage)
Some data are invariant - structure insensitive e.g. Melting point,
Density, Elastic Modulus
Others are highly structure sensitive e.g. Tensile Strength,Fracture
Toughness,Thermal conductivity,Thermal Expansion Coefficient
Natural Ceramic Materials
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Stone is one of the oldest construction materials Very
durable
(The Pyramids and Stonehenge!) Very cheap
Limestone (CaCO3)
Sandstone (SiO2)
Granite
(aluminosilicates)
Behaviour similar to all brittle ceramic materials
Cement and Concrete
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Used on an enormous scale in the construction industry Only
brick and timber rival in volume (then steel)
Very cheap - about one tenth the cost per volume of steel
Mixtures of lime (CaO), silica (SiO2) and alumina (Al2O3)
which hydrate (react with water) to form solids.
Can be cast to shape.
Relatively easy to manufacture from raw materials
Glass
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Enormous tonnages used - about the same as
aluminium. Up to 80% of the surface area of a modern
building may be glass (not load bearing)
Load bearing applications in vehicle windows, pressure
vessels, vacuum chambers
Inert glass coatings used in chemical & food
industries (glazes)
Typical Glasses and Applications
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Soda-lime Glass
70% SiO2, 10% CaO, 15%Na2O, 5% MgO / Al2O3: Windows, bottles etc.
Low melting/softening point, easily formed
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Borosilicate Glass (Pyrex)
80% SiO2, 13% B2O3, 4% Na2O, 3% Al2O3: Cooking and chemical
glassware. High temperature strength, low coefficient of thermal expansion
(CTE), good thermal shock resistance
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LAS Glass-Ceramic
60% SiO2, 20% Al2O3 , 20% Li2O, + TiO2 (nucleating agent): cooker tops,
ceramic composites. Heat treatment causes glass to crystallise to form
crystal/amorphous composite with greater creep resistance and very low CTE
– hence excellent thermal shock resistance
Traditional Ceramics (“whitewares”)
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Pottery, porcelain, tiles, structural and refractory bricks
are still made by processes very similar to those of 2000
years ago
Made from clays which are moulded in a plastic state
and then fired
Consist of a glassy phase which melts and “glues”
together a complex polycrystalline multiphase body
Trad. Ceramics: Raw Materials
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Clays: complex hydrous aluminosilicates e.g.
Kaolinite: Al2(Si2O5)(OH)4
Montmorrilonite Al5 (Na,Mg) (Si2O5)6(OH)4
Feldspars (low melting point): K2O.Al2O3.8SiO2
Quartz sand / “Flint” (cheap, high m.p.): SiO2
Engineering Ceramics
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Traditional ceramics are weak because they
contain many pores and cracks. Their elastic
moduli are low because of the glassy phases
present.
“Engineering ceramics” have been developed:
are pure, fully dense ceramics with many fewer
cracks and higher intrinsic elastic modulus.
Advanced ceramics
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Electronic ceramics
Magnetic ceramics
Superconducting ceramics
Structural or engineering ceramics
Bioceramics
Ceramic – ceramic composites
Other ceramic composites
Fabrication of ceramic shapes
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Because of their high melting point, hardness and
brittleness, ceramic components cannot be made by the
manufacturing routes used with metals and polymers.
Incongruent melting
Main method is Sintering or firing
Starts with powder.
Powder handling and powder processing are required.
How ceramics are made
powder
processing
shaping
machining
Green body
sintering
Dense or porous body
Major steps
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Powder Synthesis
Powder Handling
Green Body Formation
Sintering of Green Body
Final Machining and Assembly
Ceramic Powder
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typically in the size range 0.5 - 5.0 µm
Natural materials such as clays are weathered mineral powders of
this size mixed with water
Traditional ceramics are made from treated mixtures of clays
Engineering ceramic powders are synthesized( different
techniques)
Eg. Al2O3 and ZrO2 are precipitated from ore sands (bauxite
and zircon)
SiC and Si3N4 are made by reaction e.g. the reduction of sand by
coke
Chemical Methods of powder
preparation
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Precipitation
Sol –gel
Hydrothermal
heating
Evaporation and oxidation
Powder Handling
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Often a liquid suspension stage followed by drying
to give solid for compaction.
Need to control the forces between particles in
suspension so that they repel each other until adhesion
is required
Premature particle bonding leads to agglomeration
Dried powders are then compacted to form a
“greenbody” before firing. This must have some
interparticle strength to hold shape
Agglomeration
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Ceramic powders agglomerate because of Van der Waals surface
forces
Agglomerated powders do not fill space efficiently
May get voids in final product
Control by forming emulsion in fluid, usually water - “ceramic
slips”
In “engineering ceramics”, surfactants added, pH controlled.
Process ceramic slips and then remove fluid
Evaporate, Air dry, Spray dry, Freeze dry
Sintering
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Sintering is the conversion of a ceramic green
body into a solid by heating.
Process consists of mass transfer deforming the
ceramic powder, filling interparticle voids and
causing overall shrinkage of the compact
Process is thermally activated and controlled by
diffusion.
Sintering stages
Sintering Driving Forces
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Sintering is driven by reduction in surface energy
Two surfaces (green body) replaced by one
(lower energy) grain boundary (sintered solid).
Driving Force for sintering
Driving force is approximately surface energy
/volume of particle
Ε/V = γ(4πr2)/(4πr3/3) = 3γ/r
 A typical ceramic has a surface energy of 1Jm-2
 Thus driving force for a 1µm diameter ceramic
powder is = 3MJm-3
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microstructure
Progress of Sintering
and microstructure
development
Structure and properties of ceramics
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Polymeric materials
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Metallic
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Ceramics
covalent
“ sea of electrons”
ionic
Solids and Crystal structure
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Crystalline – arrangement of atoms in a crystalline solid is
represented by a three dimensional space lattice which is described by unit
cell. Depending on the axial length and angles, there are seven crystal systems
such as cubic, tetragonal, orthorhombic, rhombohedral , hexagonal,
monoclinic and triclinic
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Non crystalline or amorphous
Phase
Allotropy elements or compounds can exhibit
two or more phases in the solid state
Eg. Alumina, α, γ etc.
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Phase change and its implication
Zirconia exists in 3 different crystal
structures
 a) monoclinic at low temperature
 b) tetragonal at intermediate
temperature
 c) cubic at high temperature
High MgO or CaO: can get
metastable cubic form at
room temp
~2.5% Y2O3 : can get
metastable tetragonal form
at room temp
Imperfections in crystals
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Point ( Schottky and Fenkel defects)
line
or plane defects ( Grain Boundary)
Line dislocation
Plane dislocation, grain boundary
evolution and microstructure
Single
crystal
Ceramics are Polycrystalline
Strength of ceramics
Tensile fracture stress σF is controlled by the defects
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σF = KIc/ α √ aπ
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KIc - Fracture toughness
α – geometrical factor (~1)
a – size of biggest crack under stress
Toughness, Crack Size and Strength
toughness
Hardbrittle
Dectile tough
stress
Dectile soft
strain
Why poor mechanical properties
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Plastic flow by dislocation motion is very difficult in covalent
and ionic materials
High yield stress and hardness
Very limited plastic flow at crack tips – low fracture
toughness
Compression: strong (high yield stress); may flow or
propagate shear cracks (crushing).
Tension: weak (low toughness); always fail by brittle
fracture
One material different
Applications
A simple example
Aluminium Oxide
Preparation : aluminium salt
aluminium hydroxide
aluminiu oxide, Al2 O3
Spark plug
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The spark plug is connected to thousands of
volts generated by the ignition coil. As the
electrons are pushed in from the coil, a voltage
difference appears between the center electrode
and side electrode. No current can flow because
the fuel and air in the gap is an insulator, but as
the voltage rises further, it begins to change the
structure of the gases between the electrodes
High temperature furnace
Cutting tool
Integrated Circuit, ICs
Alumina-Transparent Window
One material different
Applications
reading
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• Introduction to the class of ceramic materials: traditional ceramics, engineering
ceramics, glasses and ceramic composites.
• Interatomic bonding and crystal structures found in ceramics.
• Structure of glasses - random network model.
• Brittle nature of ceramics.
• Fabrication of ceramics: powder synthesis, powder processing, sintering and reaction
sintering.
• Microstructures, mechanical properties and applicatio.ns
Reading List
• “Engineering Materials 2”, M.F. Ashby and D.r.H. Jones, Chapters 15-20.
• “Introduction to Ceramics”, W.D. Kingery, H.K. Bowen and D.R. Uhlmann.
• “Ceramic Science for Materials Technologists”, I.J. McColm.
• “Ceramic Microstructures”, W.E. Lee and W.M. Rainforth
• “Materials Science and Technology – volume 11 - Structure and Properties of
Ceramics”, edited by M.V. Swain
• “Mechanical Behaviour of Ceramics”, R.W. Davidge
• "An Introduction to the Mechanical Properties of Ceramics", D.J. Green