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Ceramics
Callister: Chapters 12, 13
Structure, Properties, Applications and Processing Techniques
of:
Silicates
Glass - Ceramics
Traditional Ceramics
Advanced (Engineering) Ceramics
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Ceramics
Ceramics are compounds of metallic and non-metallic elementsbonded ionically (some are partially covalent).
This type of atomic bonding means that most ceramics have:
High Youngs Modulus
High Melting Point
Low C.T.E.
Strong (high yield strength)
Brittle
Very low ductility means that ceramics are very sensitive tointernal cracks and flaws
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Crystal Structure
Since ceramics are comprised of at least two elements, their
crystal structures are often more complicated than metals.
How the cations (+ve) and anions (-ve) fit together depends on:
1. Maintaining electrical neutrality
according to the chemical formula (e.g. NaCl, Al2O3, )
2. The relative sizes of the ions
Stable structures require that the anions and cations touch
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Coordination Numbers
The cations coordinationnumberdepends on theratio of the radii:
a
c
r
r
Linear
Triangular
Tetrahedral
Octahedral
Cubic
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AX Crystal Structures
NaCl (Rock Salt) structure
The Rock Salt structure is formed from two,interwoven FCC structures.
The coordination number of both ions is 6(octahedral)
Other common ceramics with this structure:
MgO, MnS, LiF, FeO
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AX Crystal Structures
Cesium Chloride structure
The CsCl structure is based onthe BCCstructure.
It is not BCC because two different atoms
are involved)
The coordination number of both ions is 8
(cubic)
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AX Crystal Structures
Zinc Blende (ZnS) structure
The Zinc Blende structure is an FCC-basedlattice with zinc anions in 4 of the eight
tetrahedral sites
The coordination number of both ions is 4
(tetrahedral)
Other common ceramics with this structure:
ZnTe, SiC
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Silicate Ceramics
The bulk of soils, rocks, clays, and sand are silicateceramics
Silica (SiO2) is a covalently bonded tetrahedral molecule
The bonds are directional and strong.
Rather than discussing unit cells, silicates are describedaccording to the arrangement of the tetrahedra.
They can form one-, two-, and three-dimensional structures
-4
4SiO
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Crystalline Silica
Silica (SiO2) is the simplest of the sil icates
It is a three-dimensional network formed when everyoxygen atom is shared by two, adjacent tetrahedra
Electrical neutrality is maintained (Si:O = 1:2)
There are three polymorphs of crystalline silica
Quartz, cristobalite, and tridymite
Neither form is closed packed Density of silica is low.
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Silica Glasses
If molten silica is cooled relatively quickly, it is possible toprevent the formation of a crystalline structure.
Fused Silicais still made up of the SiO4 tetrahedra, but not alloxygen atoms are shared between two tetrahedra.
There is short-range, but not long-range order.
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Silica Glasses
To change the properties of the glass, other oxides are often
added.
Network Formersfit in with the SiO2 tetrahedra. They areadded to change the properties of the solidified glass.
e.g. 12% B2O3 is added to silica to make Pyrex. The additionlowers the forming temperature without changing the thermal
expansion coefficient.
Network Modifiersdo not fit in with the silicanetwork. They make it easier to form a glass(as opposed to crystalline silica).
e.g. most glass (windows, food containers,)contain up to 15% Na2O to make it easier to forma glass
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The Silicates
Depending on how many oxygenatoms are shared, silicates can
form a wide variety of structures.
Additional cations are often
required to maintain chargeneutrality (e.g. Ca2+, Mg2+, Al3+)
Clays are layeredsilicates.
Each sheet is covalently boundtogether,
but adjacent sheets are weakly
bound by van der Waals forces
Just add water
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Carbon
Carbon exists in a variety of forms:
Diamond
Graphite
Fullerenes
The different forms have very different propertiesbecause theyhave different structures.
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Diamond
Diamond Cubic crystal structure(similar to zinc blende)
Each carbon atom is covalentlybonded to 4 others in a tetrahedron.
Very hard/strong
Low electrical conductivity
High thermal conductivity
Most industrial quality diamond isman-made.
Knives, machine tools, etc.
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Graphite
Graphite has a layered structure.
Each atom is covalently bonded to threeothers in the layer.
The fourth bonding electron contributes to
van der Waals bonding between thelayers.
The properties of graphite are directional.
StrongWeak
Weak
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Fullerenes
Named after R. Buckminster Fuller,inventor of the geodesic dome.
Two forms discovered so far
C60, Buckyballs
Again, three covalent and one vander Waals bond
C60 molecules pack together in
an FCC arrangement
Carbon Nanotubes
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Strength of Ceramics
Very low fracture toughness of ceramics means that failure isalmost always due to flaws in the part.
Therefore, the design strengths of ceramic materials are
described using statistics.
26 MPam2024 Aluminum
55 MPamTi-6Al-4V
5 MPamSi3N4
1.7 MPamAl2O3
99 MPam4340 Steel
Fracture Toughness KICMaterial
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Fracture Statistics
Nominally identicalsamples may fail at very
different stresses
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Three-Point Bend Test
It is difficult to perform tensile
tests on brittle materials. They often crush in the
grips.
The three-point bend test
avoids this problem, but hasits own drawbacks
The maximum tensile stressis only seen by the materialon the bottom surface,
directly under the plunger.
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Three-Point Bend Test
The results from a 3-point bend testappear similar to those from a tensile
test.
These particular results illustrate the
effect that structure has on properties:
Crystalline Al2O3 is stiffer and stronger
than amorphous glass
Note the very small strains
0.2% is not even on the scale!
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Three-Point Bend Test
The stress is calculated
according to the formulashown for a rectangularcross-section
The fracture stressdetermined in this test is
known as:
Flexural Strength
Modulus of Rupture
The flexural strength is oftenhigher than the tensile
strength
Statistics: The biggest flaw may not see the highest stress
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Processing Ceramics
All ceramics have short-range atomic order, some have long-range order.
Crystalline ceramics have short and long-range order
Glasses have short-range order
Ceramic-glasses have a combination of crystalline and glassycomponents
Deformation of crystalline ceramics is due to dislocation motion,
HOWEVER:
The complex crystal structure and strong atomic bonding make
dislocation motion exceedingly difficult.
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Processing Glasses
Deformation of non-crystalline ceramics is due to Viscous Flow
dydv
AF
dydv
=
=
Viscosity is a measure of how difficultit is to shear (e.g. stir) a liquid.
Water has a low viscosity
Molasses has a high viscosity
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Liquid
Processing Glasses
When a crystalline material
solidifies, there is a step change involume at the melting temperature.
Temperature
SpecificVolume
Glasses do not really solidify inthe traditional sense.
The molecules pack closer andcloser together, becoming an
increasingly denser liquid.
The slight change in slope occurs when the
molecules are essentially unable to flow.
This is the Glass Transition Temperature.
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Ceramic Glasses
Strain Point Brittle
Annealing Point
Residual stresses removed
Softening point
Can be handled withoutdeformation
Working Point
Easily deformable
Melting Point
True liquid
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Fabricating Glasses
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Fabricating Glasses
How do you get perfectly flat, parallel sided plate glass for
windows?
The molten glass is floated on top of molten tin (Tm =231C)
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Tempered Glass
The fracture properties of glass can be altered by
Laminating
Tempering
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Tempered Glass
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Clay Products
Clay is an aluminosilicate(i.e. Al2O3 and SiO2) with a variety ofimpurities (usually various other oxides)
Common clay products include:
Building bricks, tiles, sewer pipes
Pottery, porcelain, china
Clay products are made from various proportions of:
Clay and a flux material (e.g feldspar)
Sheet silicate structure
Filler materials (typically crushed quartz)
Crystalline
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Fabricating Clay Products
Clay is mixed with water to form a plastic body and formed to the
desired shape
The wet body is then dried and fired.
Drying removes water from the clay a controlled rate
Firing vitrifies the clay (turns it into a glass)
Degree of vitrification depends on firing temperature
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Microstructure of Clay Products
Crystalline particles surrounded by a glassy matrix with somepores
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Fabricating Crystalline Ceramics
The melting temperature of most ceramics is too high for casting
to be a practical option.
Engineering ceramics in general are made from powders
Powders are compacted / consolidated to form a green body
Methods include various pressing techniques and tape casting.
The green body is then sintered at elevated temperatures (often
under pressure) to bond the powders
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Microstructure of Crystalline Ceramics
Sintered crystalline grains with porosity
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Properties of Crystalline Ceramics
Crystalline ceramics are the Engineering ceramics
High melting points
Strong
Hard
Brittle
Good corrosion resistance
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Properties of Glasses
Like crystalline ceramics, glasses are
hard,
brittle
corrosion resistant
Unlike crystalline ceramics, glasses:
lower melting temperatures
can be easily deformed at high temperatures
are not porous
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Properties of Clay Products
Clay products are:
Hard Brittle
Corrosion resistant
They generally have some porosity, but this can be minimized by
increasing the firing temperature
Their high temperature creep properties are better than glasses butnot as good as crystalline ceramics.