2. Ceramics
2
Ceramics materials are inorganic and nonmetallic
materials that consist of metallic and nonmetallic
elements bonded together primarily by ionic and/or
covalent bonds.
Good electrical and heat insulation property.
Hard, brittle, and lesser ductility and toughness than
metals.
High chemical stability and high melting temperature.
3.
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Traditional Ceramics: Basic components (Clay and
Silica).
Example:
- Glass
- Bricks
- Tiles
Engineering Ceramics: Pure compounds (Al2O3, SiC).
Example:
- Gas turbine engine (Silicon Carbide, SiC)
- Spark plug insulator material(Alumina, Al2O3)
4. Mixture of Ionic and Covalent Types.
Note:
Depends on electronegativity difference.
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5. Processing of Ceramics
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Produced by compacting powder or particles into shapes and
heated to bond particles together.
The basic steps in the processing of ceramics are:
1.
2.
3.
Material preparation:
- Particles and binders and lubricants are (sometimes ground) and
blend wet or dry.
Forming or casting:
- Formed in dry, plastic or liquid conditions.
- Cold forming process is predominant.
- Pressing, slipcasting and extrusion are the common forming
processes.
Thermal treatment
6. Forming Or Casting
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PRESSING
Dry Pressing:
- Simultaneous uniaxial compaction and shaping of power along
with binder.
- Wide variety of shapes can be formed rapidly and accurately.
7.
Isolatic pressing:
- Ceramic powder is loaded into a flexible chamber and pressure is
applied outside the chamber with hydraulic fluid.
Examples: Spark plug insulators, carbide tools.
Hot pressing:
-Ceramics parts of high density and improved mechanical
properties are produced by combining the pressing and firing
operations.
- Both unaxial and isostatic methods are used.
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8. SLIP CASTING
Slip is poured into porous mold and liquid portion is partially
absorbed by mold.
Layer of semi-hard material is formed against mold surface.
Excess slip is poured out of cavity or cast as solid.
Alternatively, a solid shape maybe made by allowing the casting to
continue until the whole mold cavity is filled. This called s solid
casting.
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Powdered ceramic material and a liquid mixed to prepare a stable
suspension (slip).
The material in mold is allowed to dry and then fired.
9. Figure 11.25 Slip casting of ceramic shapes:
a) Drain casting in porous plaster of paris mold
b) Solid casting
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10. EXTRUSION
Single cross sections and hollow shapes of ceramics can be
produced by extrusion.
Plastic ceramic material is forced through a hard steel or alloy die
by a motor driven augur.
Examples:
- refractory brick
- sewer pipe
- hollow tubes.
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11. Thermal Treatment
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Drying:
- Parts are dried before firing to remove water from ceramic body.
- Usually carried out at or below 1000C.
Sintering:
- Small particles are bonded together by solid state diffusion
producing dense coherent product.
- Carried out at higher temperature but below MP. Longer the
sintering time, larger the particles are.
Vitrification:
- During firing, glass phase liquefies and fills the pores.
- Upon cooling liquid phase of glass solidifies and a glass matrix
that bonds the particles is formed.
12. Mechanical Properties of Ceramics
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Strength of ceramics vary greatly but they are generally
brittle.
Tensile strength is lower than compressive strength.
Many ceramic materials are hard and have low impact
resistance due to their ionic-covalent bonding.
13. Ceramics Deformation
Covalently bonded ceramics:
- Exhibit brittle fracture due to separation of electron-pair bonds
without their subsequent reformation.
Ionically bonded ceramics:
- Single crystal show considerable plastic deformation.
- Polycrystalline ceramics are brittle.
Example:
NaCl crystal slip in {100} family of planes is rarely observed as
same charges come into contact. Cracking occurs at grain
boundaries
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14. Ceramics Fatigue Failure
Fatigue fracture in ceramics is RARE due to absence of plastic
deformation.
Straight fatigue crack in has been reported in alumina after 79,000
compression cycles.
Ceramics are hard and can be used as abrasives.
Examples:- Al2O3, SiC.
By combining ceramics, improved abrasives can be developed.
Example:- 25% ZrO2 + 75% Al2O3
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Figure 11.37:
Optical micrograph of fatigue cracking of polycrystalline
alumina under cyclic compression.
15. Question 1
a.
b.
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What are factors affecting the strength of the ceramic materials?
What are important properties for industrial abrasives?
16. Thermal Properties of Ceramics
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Low thermal conductivity and high heat resistance.
Many compounds are used as industrial refractories
which are materials that resist the action of hot
environment.
For insulating refractories, porosity is desirable.
Dense refractories have low porosity and high
resistance to corrosion and errosion.
Aluminum oxide and MgO are expensive and difficult to
form and hence not used as refractories.
17. Question 2
Define the following defect terms:
a.
b.
Ceramic thermal shock
c.
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Ceramic creep
Ceramic static fatigue
18. Glass
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Glass material is a ceramic material that is made from
inorganic materials at high temperature and
distinguished from other ceramics in that its constituents
are heated to fusion and then cooled to a rigid condition
without crystallization.
Up on cooling, it transforms from rubbery material to
rigid glass.
Some of the glass properties are transparency, strength,
hardness and corrosion resistance.
19. 19
Figure 11.41:
Solidification of crystalline and glassy (amorphous) materials showing
changes in specific volume.
Tg is the glass transition temperature of the glassy material.
Tm is the melting temperature of the crystalline material.
20. Viscous Deformation of Glass
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Viscous above Tg and viscosity decreases with increase
in temperature.
η* = η0eQ/RT
Where;
Q = Activation energy
η* = Viscocity of glass (PaS)
η0 = preexponential constant (PaS)
21. Question 3
A 96 percent silica glass has a viscosity of 1013 P at its annealing
point of 9400C and a viscosity of 108 P at its softening point of
14700C. Calculate the activation energy in kilojoules per mole for the
viscous flow of this glass in this temperature range.
(Answer : 382 kJ/mol)
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22. Figure 11.44:
Effect of temperature on the viscosities of various types of glasses. Number
of curves refer to different compositions.
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1.
2.
3.
4.
Working point: 103 PaS – glass fabrication can be carried out
Softening point: 107 PaS – glass flows under its own weight.
Annealing point: 1012 PaS – Internal stresses can be relieved..
Strain point: 10 13.5 PaS – glass is rigid below this point.
23. Glass Forming Method
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Forming sheet and plate glass:
- Ribbon of glass moves out of
furnace and floats on a bath of
molten tin.
- Glass is cooled by molten tin.
- After it is hard, it is removed
and passed through a long
annealing furnace.
24. Blowing, Pressing and Casting
Blowing:
Air blown to force molten glass into molds.
Pressing:
Optical and sealed beam lenses are pressed by a plunger into a
mold containing molten glass.
Casting:
Molten glass is cast in open mold.
Centrifugal casting:
Glass globs are dropped into spinning mold.
Glass first flows outward towards wall of mold and then upward
against the mold wall.
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26. Tempered Glass
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Glass is heated into near softening point and rapidly
cooled.
Surface cools first and contracts.
Interior cools next and contracts causing tensile stresses
in the interior and compressive stress on the surface.
Tempering strengthens the glass.
Examples:
Auto side windows and safety glasses.
27. Figure 11.47:
Cross section of tempered glass.
(a) After surface has cooled from high temperature near glass-softening temperature
(b) After center has cooled.
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28. Question 4
Define the following terms:
a.
b.
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Annealing glass material
Tempering glass material
29. Optical properties of Glass
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Refractive index
Reflectance
Transparency
Translucency
Opticity
Colour
30. Transparency & Refractive Index
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When photons are transmitted through a transparent material,
they loose some energy and speed and the direction changes.
Refractive Index =
C (Velocity of light in vacuum)
V (velocity of light in a medium)
31.
If light passes through one media to another:
n
n
'
Sin
'
Sin
Total internal reflection if angle φ > φc
If light passes from media of high refractive index to a media of
low refractive index, φ’ = 900 at φ = φc
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32. Reflectance of Light
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For a particular wavelength λ :
(Reflected fraction) λ +(Absorbed fraction) λ + (transmitted fraction) λ = 1
33.
Reflection of light from a glass surface:
n 1
Fraction of light reflected = R =
n 1
R = reflectivity (φi=900)
n = refractive index.
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2
34. Question 5
a.
Calculate the reflectivity of ordinary incident light from the polished
flat surface of a silicate glass with a reflective index of 1.46.
(Answer: 3.5%)
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35.
Absorption of light by glass plate: Light intensity decreases as
light path decreases.
I
I0
I
I0
α
t
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e
t
= Fraction of light exiting
= Fraction of light entering
= linear absorption coefficient.
= thickness
36. Question 6
a.
Ordinary incident light strikes a polished glass plate 0.5 cm thick
that has a refractive index of 1.5 . What fraction of light is absorbed
by the glass as the light passes between the surfaces of the plate?
Given = 0.03cm-1.
(Answer: 1.5%)
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37. Translucency
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Translucency also called translucence or translucidity.
Translucency is a super-set of transparency, allows light to pass
through but does not necessarily follow Snell's law.
The photons can be scattered at either of the two interfaces where
there is a change in index of refraction or internally.
In other words, a translucent medium allows the transport of light
while a transparent medium not only allows the transport of light but
allows for the image formation.
38. Opacity
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Opacity is the degree to which light is not allowed to travel through
which the opposite property of translucency.
40.
Light is in form of waves and consist of particles called photons.
ΔE = hν = hC/λ
ΔE = Energy
λ = wavelength
ν = frequency
C = speed of light = 3 x108 m/s
H = plank’s constant = 6.62 x 10-34 J.s
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41. Question 7
a.
A photon in a ZnS semiconductor drop from an impurity energy
level at 1.38 eV below its conduction band to its valence band.
What is the wavelength of the radiation given off by the photon in
the transition? If visible, what is the colour of the radiation? ZnS
has an energy band gap of 3.54 eV.
(Answer: 574.7nm)
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42. References
A.G. Guy (1972) Introduction to Material Science,
McGraw Hill.
J.F. Shackelford (2000). Introduction to Material
Science for Engineers, (5th Edition), Prentice Hall.
W.F. Smith (1996). Priciple to Material Science and
Engineering, (3rd Edition), McGraw Hill.
W.D. Callister Jr. (1997) Material Science and
Engineering: An Introduction, (4th Edition) John
Wiley.
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