Transcript Slide 1

West Virginia University
Ceramics
Mechanical & Aerospace Engineering
West Virginia University
Taxonomy of Ceramics
Glasses
Clay Refractories
products
Abrasives Cements
Advanced
ceramics
-optical
-whiteware -bricks for -sandpaper -composites engine
high T
-composite -bricks
-cutting
-structural
-rotors
(furnaces) -polishing
reinforce
-valves
-containers/
-bearings
Adapted from Fig. 13.1 and discussion in
household
Section 13.2-6, Callister 7e.
-sensors
• Properties:
-- Tm for glass is moderate, but large for other ceramics.
-- Small toughness, ductility; large moduli & creep resist.
• Applications:
-- High T, wear resistant, novel uses from charge neutrality.
• Fabrication
-- some glasses can be easily formed
-- other ceramics can not be formed or cast.
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Application: Refractories
• Need a material to use in high temperature furnaces.
• Consider the Silica (SiO2) - Alumina (Al2O3) system.
• Phase diagram shows:
mullite, alumina, and crystobalite as candidate refractories.
2200
T(°C)
3Al2O3-2SiO2
2000
Liquid
(L)
1800
mullite
alumina + L
mullite
+L
crystobalite
+L
1600
1400
mullite
+ crystobalite
0
20
alumina
+
mullite
Adapted from Fig. 12.27,
Callister 7e. (Fig. 12.27
is adapted from F.J. Klug
and R.H. Doremus,
"Alumina Silica Phase
Diagram in the Mullite
Region", J. American
Ceramic Society 70(10),
p. 758, 1987.)
40
60
80
100
Composition (wt% alumina)
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Application: Die Blanks
• Die blanks:
die
-- Need wear resistant properties!
Ao
die
Courtesy Martin Deakins, GE
Superabrasives, Worthington,
OH. Used with permission.
Ad
tensile
force
Adapted from Fig. 11.8 (d),
Callister 7e.
• Die surface:
-- 4 mm polycrystalline diamond
particles that are sintered onto a
cemented tungsten carbide
substrate.
-- polycrystalline diamond helps control
fracture and gives uniform hardness
in all directions.
Courtesy Martin Deakins, GE
Superabrasives, Worthington,
OH. Used with permission.
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Application: Cutting Tools
• Tools:
-- for grinding glass, tungsten,
carbide, ceramics
-- for cutting Si wafers
-- for oil drilling
• Solutions:
-- manufactured single crystal
or polycrystalline diamonds
in a metal or resin matrix.
-- optional coatings (e.g., Ti to help
diamonds bond to a Co matrix
via alloying)
-- polycrystalline diamonds
resharpen by microfracturing
along crystalline planes.
oil drill bits
blades
coated single
crystal diamonds
polycrystalline
diamonds in a resin
matrix.
Photos courtesy Martin Deakins,
GE Superabrasives, Worthington,
OH. Used with permission.
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Application: Sensors
• Example: Oxygen sensor ZrO2
• Principle: Make diffusion of ions
Ca 2+
fast for rapid response.
• Approach:
Add Ca impurity to ZrO2:
A Ca 2+ impurity
removes a Zr 4+ and a
O2- ion.
-- increases O2- vacancies
-- increases O2- diffusion rate
• Operation:
-- voltage difference
produced when
O2- ions diffuse
from the external
surface of the sensor
to the reference gas.
sensor
gas with an
unknown, higher
oxygen content
O2diffusion
+
reference
gas at fixed
oxygen content
-
voltage difference produced!
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Glass & Vitreous Ceramics
Glass
Vitreous Ceramics
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High-performance Engineering Ceramics
Diamond: used as cutting tools, rock drills, abrasive, etc.
Generic High-performance Ceramics
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Cement and Concrete
Cement: Mixture of lime (CaO), silica (SiO2) and
alumina (Al2O3), which sets when mixed with water
Concrete: Sand and stones held together by cement.
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Natural Ceramics
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Structure of Ceramics - Ionic
NaCl Structure
MgO Structure:
Can be thought of as an
fcc packing with Mg
ions in octahedral
holes.
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Structure of Ceramics - Ionic
ZrO2: fcc packing of
Zr with O in the
tetrahedral holes
Al2O3: c.p.h packing of
O with Al in 2/3 of the
octahedral holes
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Structure of Ceramics – Simple Covalent
Diamond: each atom
has four neighbors.
SiC: diamond structure
with half the atoms
replaced by Si.
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Structure of Ceramics – Simple Covalent
Cubic SiO4: diamond structure with an SiO4
tetrahedron on each atom site.
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Structure of Ceramics
Microstructure of Ceramics
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Mechanical Properties of Ceramics
Elastic Moduli
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Mechanical Properties of Ceramics
Strength, Hardness & Lattice Resistance
Normalised Hardness of Pure Metals, Alloys and Ceramics
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Mechanical Properties of Ceramics
Dislocation Movement
Dislocation motion in covalent solids is intrinsically difficult
because the interatomic bonds must be broken and reformed
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Mechanical Properties of Ceramics
Dislocation Movement
Dislocation motion in ionic solids is easy on some planes, but
hard on others. The hard systems usually dominate.
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Mechanical Properties of Ceramics
Fracture Strength of Ceramics
The design strength of a ceramic is determined by fracture
toughness and lengths of the microcracks it contains.
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Mechanical Properties of Ceramics
Fracture Strength of Ceramics
(a) Tensile test measures tensile strength, TS
(b) Bend test measures modulus of rupture, r, typically 1.7 TS
(c) Compression test measures crushing strength, c, typically
15TS
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Mechanical Properties of Ceramics
- Fracture Strength
In compression, many
flaws propagate stably
to give general crashing
In tension the largest
flaw propagates unstably
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Mechanical Properties of Ceramics
Thermal Shock Resistance
ET = TS
Where E is the Young’s modulus,  is the coefficient of
expansion and TS is tensile strength. T represents the
ceramic’s thermal shock resistance
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Ceramic Fabrication Methods-I
PARTICULATE
FORMING
GLASS
FORMING
CEMENTATION
• Pressing:
Gob
Pressing
operation
plates, dishes, cheap glasses
--mold is steel with
graphite lining
Parison
mold
• Fiber drawing:
Compressed
air
• Blowing:
suspended
Parison
Finishing
mold
Mechanical
Adapted from Fig. 13.8, Callister, 7e. (Fig. 13.8 is adapted from
C.J. Phillips,
Glass: The Miracle Maker, Pittman Publishing Ltd., London.)
wind up
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Sheet Glass Forming
• Sheet forming – continuous draw
– originally sheet glass was made by “floating” glass on a pool of mercury
Adapted from Fig. 13.9, Callister 7e.
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Glass Structure
• Basic Unit:
4Si0 4 tetrahedron
Si 4+
O2-
• Quartz is crystalline
SiO2:
• Glass is amorphous
• Amorphous structure
occurs by adding impurities
(Na+,Mg2+,Ca2+, Al3+)
• Impurities:
interfere with formation of
crystalline structure.
Na +
Si 4+
O2-
(soda glass)
Adapted from Fig. 12.11,
Callister, 7e.
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Glass Properties
• Specific volume (1/r) vs Temperature (T):
• Crystalline materials:
Specific volume
Liquid
(disordered)
Supercooled
Liquid
• Glasses:
Glass
(amorphous solid)
Crystalline
(i.e., ordered)
Tg
-- crystallize at melting temp, Tm
-- have abrupt change in spec.
vol. at Tm
Tm
solid
T
-- do not crystallize
-- change in slope in spec. vol. curve at
glass transition temperature, Tg
-- transparent
- no crystals to scatter light
Adapted from Fig. 13.6, Callister, 7e.
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Glass Properties: Viscosity
• Viscosity, h:
-- relates shear stress and velocity gradient:

dy
glass

dv
dv
dy
dv
h
dy
velocity gradient
h has units of (Pa-s)
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Glass Viscosity vs. T and Impurities
• Viscosity decreases with T
• Impurities lower Tdeform
• soda-lime glass: 70% SiO2
balance Na2O (soda) & CaO (lime)
• borosilicate (Pyrex):
13% B2O3, 3.5% Na2O, 2.5% Al2O3
Viscosity [Pa  s]
• Vycor: 96% SiO2, 4% B2O3
• fused silica: > 99.5 wt% SiO2
10 14
10 10
10 6
10 2
1
200
strain point
annealing range
Tdeform : soft enough
to deform or “work”
Tmelt
600 1000 1400 1800 T(°C)
Adapted from Fig. 13.7, Callister, 7e.
(Fig. 13.7 is from E.B. Shand, Engineering
Glass, Modern Materials, Vol. 6, Academic
Press, New York, 1968, p. 262.)
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Heat Treating Glass
• Annealing:
--removes internal stress caused by uneven cooling.
• Tempering:
--puts surface of glass part into compression
--suppresses growth of cracks from surface scratches.
--sequence:
before cooling
hot
surface cooling
further cooled
cooler
hot
cooler
compression
tension
compression
--Result: surface crack growth is suppressed.
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Ceramic Fabrication Methods-IIA
GLASS
FORMING
PARTICULATE
FORMING
CEMENTATION
• Milling and screening: desired particle size
• Mixing particles & water: produces a "slip"
Ao
• Form a "green" component
container
--Hydroplastic forming:
force
extrude the slip (e.g., into a pipe)
--Slip casting:
pour slip
into mold
absorb water
into mold
“green
ceramic”
solid component
pour slip
into mold
ram
bille
t
container
drain
mold
die holder
extrusion
Ad
Adapted from
Fig. 11.8 (c),
Callister 7e.
die
“green
ceramic”
Adapted from Fig.
13.12, Callister 7e.
(Fig. 13.12 is from
W.D. Kingery,
Introduction to
Ceramics, John
Wiley and Sons,
Inc., 1960.)
hollow component
• Dry and fire the componentMechanical & Aerospace Engineering
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Clay Composition
A mixture of components used
(50%) 1. Clay
(25%) 2. Filler – e.g. quartz (finely ground)
(25%) 3. Fluxing agent (Feldspar)
binds it together
aluminosilicates + K+, Na+, Ca+
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Features of a Slip
Shear
• Clay is inexpensive
• Adding water to clay
-- allows material to shear easily
along weak van der Waals bonds
-- enables extrusion
-- enables slip casting
• Structure of
Kaolinite Clay:
Adapted from Fig. 12.14, Callister 7e.
(Fig. 12.14 is adapted from W.E. Hauth,
"Crystal Chemistry of Ceramics", American
Ceramic Society Bulletin, Vol. 30 (4), 1951,
p. 140.)
charge
neutral
weak van
der Waals
bonding
charge
neutral
4+
Si
3+
Al
OH
2O
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Drying and Firing
• Drying: layer size and spacing decrease.
Adapted from Fig.
13.13, Callister 7e.
(Fig. 13.13 is from
W.D. Kingery,
Introduction to
Ceramics, John
Wiley and Sons,
Inc., 1960.)
wet slip
partially dry
“green” ceramic
Drying too fast causes sample to warp or crack due to non-uniform shrinkage
• Firing:
--T raised to (900-1400°C)
--vitrification: liquid glass forms from clay and flows between
SiO2 particles. Flux melts at lower T.
Si02 particle
(quartz)
micrograph of
porcelain
glass formed
around
the particle
70mm
Adapted from Fig. 13.14,
Callister 7e.
(Fig. 13.14 is courtesy H.G.
Brinkies, Swinburne
University of Technology,
Hawthorn Campus,
Hawthorn, Victoria,
Australia.)
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Ceramic Fabrication Methods-IIB
GLASS
PARTICULATE
CEMENTATION
FORMING
FORMING
Sintering: useful for both clay and non-clay compositions.
• Procedure:
-- produce ceramic and/or glass particles by grinding
-- place particles in mold
-- press at elevated T to reduce pore size.
• Aluminum oxide powder:
-- sintered at 1700°C
for 6 minutes.
Adapted from Fig. 13.17, Callister 7e.
(Fig. 13.17 is from W.D. Kingery, H.K.
Bowen, and D.R. Uhlmann, Introduction
to Ceramics, 2nd ed., John Wiley and
Sons, Inc., 1976, p. 483.)
15Mechanical
mm
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Powder Pressing
Sintering - powder touches - forms neck & gradually neck thickens
– add processing aids to help form neck
– little or no plastic deformation
Uniaxial compression - compacted in single direction
Isostatic (hydrostatic) compression - pressure applied by
fluid - powder in rubber envelope
Hot pressing - pressure + heat
Adapted from Fig. 13.16, Callister 7e.
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Tape Casting
• thin sheets of green ceramic cast as flexible tape
• used for integrated circuits and capacitors
• cast from liquid slip (ceramic + organic solvent)
Adapted from Fig. 13.18, Callister 7e.
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Ceramic Fabrication Methods-III
GLASS
PARTICULATE
CEMENTATION
FORMING
FORMING
• Produced in extremely large quantities.
• Portland cement:
-- mix clay and lime bearing materials
-- calcinate (heat to 1400°C)
-- primary constituents:
tri-calcium silicate
di-calcium silicate
• Adding water
-- produces a paste which hardens
-- hardening occurs due to hydration (chemical reactions
with the water).
• Forming: done usually minutes after hydration begins.
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Applications: Advanced Ceramics
Heat Engines
• Advantages:
– Run at higher temperature
– Excellent wear & corrosion
resistance
– Low frictional losses
– Ability to operate without a
cooling system
– Low density
• Disadvantages:
– Brittle
– Too easy to have voidsweaken the engine
– Difficult to machine
• Possible parts – engine block, piston coatings, jet engines
Ex: Si3N4, SiC, & ZrO2
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Applications: Advanced Ceramics
• Ceramic Armor
– Al2O3, B4C, SiC & TiB2
– Extremely hard materials
» shatter the incoming projectile
» energy absorbent material underneath
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Applications: Advanced Ceramics
Electronic Packaging
• Chosen to securely hold microelectronics & provide heat transfer
• Must match the thermal expansion coefficient of the
microelectronic chip & the electronic packaging material.
Additional requirements include:
– good heat transfer coefficient
– poor electrical conductivity
• Materials currently used include:
» Boron nitride (BN)
» Silicon Carbide (SiC)
» Aluminum nitride (AlN)
• thermal conductivity 10x that for Alumina
• good expansion match with Si
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