Mechanical & Aerospace Engineering West Virginia University Ceramics.
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Transcript of Mechanical & Aerospace Engineering West Virginia University Ceramics.
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Mechanical & Aerospace Engineering
West Virginia University
Ceramics
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• 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.
Glasses Clay products
Refractories Abrasives Cements Advanced ceramics
-optical -composite reinforce -containers/ household
-whiteware -bricks
-bricks for high T (furnaces)
-sandpaper -cutting -polishing
-composites -structural
engine -rotors -valves -bearings
-sensors
Adapted from Fig. 13.1 and discussion in Section 13.2-6, Callister 7e.
Taxonomy of Ceramics
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• 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.
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.)
Application: Refractories
Composition (wt% alumina)
T(°C)
1400
1600
1800
2000
2200
20 40 60 80 1000
alumina +
mullite
mullite + L
mulliteLiquid
(L)
mullite + crystobalite
crystobalite + L
alumina + L
3Al2O3-2SiO2
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tensile force
AoAddie
die
• Die blanks: -- Need wear resistant properties!
• Die surface: -- 4 m 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.
Adapted from Fig. 11.8 (d), Callister 7e. Courtesy Martin Deakins, GE
Superabrasives, Worthington, OH. Used with permission.
Application: Die Blanks
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• Tools: -- for grinding glass, tungsten, carbide, ceramics -- for cutting Si wafers -- for oil drilling
bladesoil drill bits• Solutions:
coated singlecrystal diamonds
polycrystallinediamonds in a resinmatrix.
Photos courtesy Martin Deakins,GE Superabrasives, Worthington,OH. Used with permission.
Application: Cutting Tools
-- 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.
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• Example: Oxygen sensor ZrO2
• Principle: Make diffusion of ions fast for rapid response.
Application: Sensors
A Ca2+ impurity
removes a Zr4+ and a
O2- ion.
Ca2+
• Approach: Add Ca impurity to ZrO2:
-- increases O2- vacancies
-- increases O2- diffusion rate
reference gas at fixed oxygen content
O2-
diffusion
gas with an unknown, higher oxygen content
-+voltage difference produced!
sensor• Operation: -- voltage difference produced when
O2- ions diffuse from the external surface of the sensor to the reference gas.
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Mechanical & Aerospace Engineering
West Virginia University
Glass & Vitreous Ceramics
Vitreous Ceramics
Glass
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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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Mechanical & Aerospace Engineering
West Virginia University
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 & Aerospace Engineering
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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 & Aerospace Engineering
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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 & Aerospace Engineering
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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 15TS
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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
ET = 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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• Pressing:
GLASSFORMING
Adapted from Fig. 13.8, Callister, 7e. (Fig. 13.8 is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.)
Ceramic Fabrication Methods-I
Gob
Parison mold
Pressing operation
• Blowing:
suspended Parison
Finishing mold
Compressed air
plates, dishes, cheap glasses
--mold is steel with graphite lining
• Fiber drawing:
wind up
PARTICULATEFORMING
CEMENTATION
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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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• Quartz is crystalline SiO2:
• Basic Unit: • Glass is amorphous• Amorphous structure occurs by adding impurities
(Na+,Mg2+,Ca2+, Al3+)• Impurities: interfere with formation of crystalline structure.
(soda glass)
Adapted from Fig. 12.11, Callister, 7e.
Glass Structure
Si04 tetrahedron4-
Si4+
O2-
Si4+
Na+
O2-
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• Specific volume (1) vs Temperature (T):
• Glasses: -- do not crystallize -- change in slope in spec. vol. curve at
glass transition temperature, Tg
-- transparent - no crystals to scatter light
• Crystalline materials: -- crystallize at melting temp, Tm
-- have abrupt change in spec. vol. at Tm
Adapted from Fig. 13.6, Callister, 7e.
Glass Properties
T
Specific volume
Supercooled Liquid
solid
T m
Liquid(disordered)
Crystalline (i.e., ordered)
T g
Glass (amorphous solid)
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Glass Properties: Viscosity
• Viscosity, : -- relates shear stress and velocity gradient:
has units of (Pa-s)
y
v
d
d
velocity gradient
dv
dy
glass dvdy
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• Viscosity decreases with T• Impurities lower Tdeform
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.)
Glass Viscosity vs. T and Impurities
fused silica
96% silica
soda-lime
glass
Vis
cosi
ty [P
a s
]
1
102
106
1010
1014
200 600 1000 1400 1800 T(°C)
Tdeform : soft enough to deform or “work”
annealing range
Pyrex
Tmelt
strain point
• fused silica: > 99.5 wt% SiO2
• soda-lime glass: 70% SiO2 balance Na2O (soda) & CaO (lime)
• Vycor: 96% SiO2, 4% B2O3
• borosilicate (Pyrex): 13% B2O3, 3.5% Na2O, 2.5% Al2O3
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• Annealing: --removes internal stress caused by uneven cooling.
• Tempering: --puts surface of glass part into compression --suppresses growth of cracks from surface scratches. --sequence:
Heat Treating Glass
further cooled
tensioncompression
compression
before cooling
hot
surface cooling
hotcooler
cooler
--Result: surface crack growth is suppressed.
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• Milling and screening: desired particle size• Mixing particles & water: produces a "slip"• Form a "green" component
• Dry and fire the component
ram billet
container
containerforce
die holder
die
Ao
Adextrusion--Hydroplastic forming: extrude the slip (e.g., into a pipe)
Adapted from Fig. 11.8 (c), Callister 7e.
Ceramic Fabrication Methods-IIA
solid component
--Slip casting:
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
pour slip into mold
drain mold
“green ceramic”
pour slip into mold
absorb water into mold “green
ceramic”
GLASSFORMING
PARTICULATEFORMING
CEMENTATION
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Clay CompositionA 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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• Clay is inexpensive• Adding water to clay -- allows material to shear easily along weak van der Waals bonds -- enables extrusion -- enables slip casting
• Structure ofKaolinite 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.)
Features of a Slip
weak van der Waals bonding
charge neutral
charge neutral
Si4+
Al3+
-OHO2-
Shear
Shear
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• 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.)
Drying and Firing
Drying too fast causes sample to warp or crack due to non-uniform shrinkagewet slip partially dry “green” ceramic
• Firing: --T raised to (900-1400°C) --vitrification: liquid glass forms from clay and flows between SiO2 particles. Flux melts at lower T.
Adapted from Fig. 13.14, Callister 7e.(Fig. 13.14 is courtesy H.G. Brinkies, Swinburne University of Technology, Hawthorn Campus, Hawthorn, Victoria, Australia.)
Si02 particle
(quartz)
glass formed around the particle
micrograph of porcelain
70 m
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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.)
Ceramic Fabrication Methods-IIB
15 m
GLASSFORMING
PARTICULATEFORMING
CEMENTATION
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Powder PressingSintering - powder touches - forms neck & gradually neck thickens
– add processing aids to help form neck
– little or no plastic deformation
Adapted from Fig. 13.16, Callister 7e.
Uniaxial compression - compacted in single direction
Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope
Hot pressing - pressure + heat
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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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• 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.
Ceramic Fabrication Methods-III
GLASSFORMING
PARTICULATEFORMING
CEMENTATION
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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 voids-
weaken 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