Chapter 12 -
Chapter 12,13: Ceramics
School of Mechanical Engineering Choi, Hae-Jin
Materials Science - Prof. Choi, Hae-Jin 1
Chapter 12 - 2
ISSUES TO ADDRESS...
• How do the crystal structures of ceramic materials differ from those for metals?
• What types of defects arise in ceramics?
• What types of failures exists in ceramics?
• How do we classify ceramics?
• What are some applications of ceramics?
Chapter 12 - 3
• atoms pack in periodic, 3D arrays Crystalline materials...
-metals -many ceramics -some polymers
• atoms have no periodic packing Noncrystalline materials...
-complex structures -rapid cooling
crystalline SiO2
noncrystalline SiO2 "Amorphous" = Noncrystalline Adapted from Fig. 3.40(b), Callister & Rethwisch 3e.
Adapted from Fig. 3.40(a), Callister & Rethwisch 3e.
Materials and Packing
Si Oxygen
• typical of:
• occurs for:
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 4
• Bonding: -- Can be ionic and/or covalent in character. -- % ionic character increases with difference in electronegativity of atoms.
Adapted from Fig. 2.7, Callister & Rethwisch 3e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.
• Degree of ionic character may be large or small:
Atomic Bonding in Ceramics
SiC: small CaF2: large
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 5
Ceramic Crystal Structures
Oxide structures – oxygen anions larger than metal cations – close packed oxygen in a lattice (usually FCC) – cations fit into interstitial sites among oxygen ions
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 6
Factors that Determine Crystal Structure 1. Relative sizes of ions – Formation of stable structures: --maximize the # of oppositely charged ion neighbors.
Adapted from Fig. 3.4, Callister & Rethwisch 3e.
- -
- - +
unstable
- -
- - +
stable
- -
- - +
stable 2. Maintenance of Charge Neutrality : --Net charge in ceramic should be zero. --Reflected in chemical formula:
CaF 2 : Ca 2+ cation
F -
F -
anions +
A m X p m, p values to achieve charge neutrality
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 7
• Coordination # increases with
Coordination # and Ionic Radii
Adapted from Table 3.3, Callister & Rethwisch 3e.
2
r cation r anion
Coord #
< 0.155
0.155 - 0.225
0.225 - 0.414
0.414 - 0.732
0.732 - 1.0
3
4
6
8
linear
triangular
tetrahedral
octahedral
cubic
Adapted from Fig. 3.5, Callister & Rethwisch 3e.
Adapted from Fig. 3.6, Callister & Rethwisch 3e.
Adapted from Fig. 3.7, Callister & Rethwisch 3e.
ZnS (zinc sulfide)
NaCl (sodium chloride)
CsCl (cesium chloride)
r cation r anion
To form a stable structure, how many anions can surround around a cation?
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 8
Computation of Minimum Cation-Anion Radius Ratio
• Determine minimum rcation/ranion for an octahedral site (C.N. = 6)
a = 2ranion
2ranion + 2rcation = 2 2ranion
ranion + rcation = 2ranion
rcation = ( 2 −1)ranion
arr 222 cationanion =+
414.012anion
cation =−=rr
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 9
• On the basis of ionic radii, what crystal structure would you predict for FeO?
• Answer:
550014000770
anion
cation
...
rr
=
=
based on this ratio, -- coord # = 6 because
0.414 < 0.550 < 0.732
-- crystal structure is NaCl Data from Table 3.4, Callister & Rethwisch 3e.
Example: Predicting the Crystal Structure of FeO
Ionic radius (nm) 0.053 0.077 0.069 0.100
0.140 0.181 0.133
Cation
Anion
Al 3+
Fe 2 +
Fe 3+
Ca 2+
O 2-
Cl -
F - Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 10
Rock Salt Structure Same concepts can be applied to ionic solids in general. Example: NaCl (rock salt) structure
rNa = 0.102 nm
rNa/rCl = 0.564 ∴ cations (Na+) prefer octahedral sites
Adapted from Fig. 3.5, Callister & Rethwisch 3e.
rCl = 0.181 nm
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 11
MgO and FeO
O2- rO = 0.140 nm
Mg2+ rMg = 0.072 nm
rMg/rO = 0.514 ∴ cations prefer octahedral sites
So each Mg2+ (or Fe2+) has 6 neighbor oxygen atoms
Adapted from Fig. 3.5, Callister & Rethwisch 3e.
MgO and FeO also have the NaCl structure
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 12
AX Crystal Structures
939.0181.0170.0
Cl
Cs ==−
+
r
r
Adapted from Fig. 3.6, Callister & Rethwisch 3e.
Cesium Chloride structure:
∴ Since 0.732 < 0.939 < 1.0, cubic sites preferred
So each Cs+ has 8 neighbor Cl-
AX–Type Crystal Structures include NaCl, CsCl, and zinc blende
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 13
AX2 Crystal Structures
• Calcium Fluorite (CaF2) • Cations in cubic sites
• UO2, ThO2, ZrO2, CeO2
• Antifluorite structure – positions of cations and anions reversed
Adapted from Fig. 3.8, Callister & Rethwisch 3e.
Fluorite structure
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 14
ABX3 Crystal Structures
Adapted from Fig. 3.9, Callister & Rethwisch 3e.
• Perovskite structure Ex: complex oxide BaTiO3
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 15
Density Computations for Ceramics
A
AC )(NV
AAn
C
Σ+Σ′=ρ
Number of formula units/unit cell
Volume of unit cell
Avogadro’s number
= sum of atomic weights of all anions in formula unit
ΣAA
ΣAC = sum of atomic weights of all cations in formula unit
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 16
Densities of Material Classes ρ metals > ρ ceramics > ρ polymers
Why?
Data from Table B.1, Callister & Rethwisch, 3e.
ρ (g/
cm )
3
Graphite/ Ceramics/ Semicond
Metals/ Alloys
Composites/ fibers Polymers
1
2
2 0
30 B ased on data in Table B1, Callister
*GFRE, CFRE, & AFRE are Glass, Carbon, & Aramid Fiber-Reinforced Epoxy composites (values based on 60% volume fraction of aligned fibers
in an epoxy matrix). 10
3 4 5
0.3 0.4 0.5
Magnesium
Aluminum
Steels
Titanium
Cu,Ni Tin, Zinc
Silver, Mo
Tantalum Gold, W Platinum
G raphite Silicon Glass - soda Concrete
Si nitride Diamond Al oxide
Zirconia
H DPE, PS PP, LDPE PC
PTFE
PET PVC Silicone
Wood
AFRE * CFRE * GFRE* Glass fibers
Carbon fibers A ramid fibers
Metals have... • close-packing (metallic bonding) • often large atomic masses Ceramics have... • less dense packing • often lighter elements Polymers have... • low packing density (often amorphous) • lighter elements (C,H,O) Composites have... • intermediate values
In general
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 17
Silicate Ceramics Most common elements on earth are Si & O
• SiO2 (silica) polymorphic forms are quartz, crystobalite, & tridymite
• The strong Si-O bonds lead to a high melting temperature (1710ºC) for this material
Si4+
O2-
Adapted from Figs. 3.10-11, Callister & Rethwisch 3e crystobalite
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 18
• Quartz is crystalline SiO2:
• Basic Unit: Glass is noncrystalline (amorphous) • Fused silica is SiO2 to which no impurities have been added • Other common glasses contain impurity ions such as Na+, Ca2+, Al3+, and B3+
(soda glass) Adapted from Fig. 3.41, Callister & Rethwisch 3e.
Glass Structure
Si0 4 tetrahedron 4-
Si 4+
O 2 -
Si 4+ Na +
O 2 -
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 19
Polymorphic Forms of Carbon Diamond
– tetrahedral bonding of carbon
• hardest material known • very high thermal
conductivity – large single crystals –
gem stones – small crystals – used to
grind/cut other materials – diamond thin films
• hard surface coatings – used for cutting tools, medical devices, etc.
Adapted from Fig. 3.16, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 20
Polymorphic Forms of Carbon (cont) Graphite
– layered structure – parallel hexagonal arrays of carbon atoms
– weak van der Waal’s forces between layers – planes slide easily over one another -- good
lubricant
Adapted from Fig. 3.17, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 21
Polymorphic Forms of Carbon (cont) Fullerenes and Nanotubes
• Fullerenes – spherical cluster of 60 carbon atoms, C60
– Like a soccer ball • Carbon nanotubes – sheet of graphite rolled into a tube
– Ends capped with fullerene hemispheres
Adapted from Figs. 3.18 & 3.19, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 12 - 22
Point Defects in Ceramics (i) • Vacancies -- vacancies exist in ceramics for both cations and anions • Interstitials -- interstitials exist for cations -- interstitials are not normally observed for anions because anions are large relative to the interstitial sites
Adapted from Fig. 5.2, Callister & Rethwisch 3e. (Fig. 5.2 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.)
Cation Interstitial
Cation Vacancy
Anion Vacancy
Chapter 12 - 23
Point Defects in Ceramics (ii)
• Frenkel Defect -- a cation vacancy-cation interstitial pair. • Shottky Defect -- a paired set of cation and anion vacancies.
• Equilibrium concentration of defects
Adapted from Fig. 5.3, Callister & Rethwisch 3e. (Fig. 5.3 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.)
Shottky Defect:
Frenkel Defect
/kTQDe−∝
Chapter 12 - 24
Imperfections in Ceramics • Electroneutrality (charge balance) must be maintained when impurities are present • Ex: NaCl Na + Cl - • Substitutional cation impurity
without impurity Ca 2+ impurity with impurity
Ca 2+
Na +
Na + Ca 2+
cation vacancy
• Substitutional anion impurity
without impurity O 2- impurity
O 2-
Cl -
an ion vacancy
Cl -
with impurity
Chapter 12 - 25
Brittle Fracture Surfaces • Intergranular (between grains) 304 S. Steel
(metal) Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p. 650. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.)
Polypropylene (polymer) Reprinted w/ permission from R.W. Hertzberg, "Defor-mation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.35(d), p. 303, John Wiley and Sons, Inc., 1996.
4 mm
• Intragranular (within grains)
Al Oxide (ceramic)
Reprinted w/ permission from "Failure Analysis of Brittle Materials", p. 78.
Copyright 1990, The American Ceramic
Society, Westerville, OH. (Micrograph by R.M.
Gruver and H. Kirchner.)
316 S. Steel (metal)
Reprinted w/ permission from "Metals Handbook", 9th ed, Fig. 650, p. 357.
Copyright 1985, ASM International, Materials
Park, OH. (Micrograph by D.R. Diercks, Argonne
National Lab.)
3 mm
160 mm
1 mm (Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p. 1119.)
Chapter 12 -
Brittle Fracture of Ceramics • Characteristic Fracture
behavior in ceramics – Origin point – Initial region (mirror) is flat
and smooth – After reaches critical
velocity crack branches • mist • hackle
26
Adapted from Figs. 9.14 & 9.15, Callister & Rethwisch 3e.
Chapter 12 - 27
Classification of Ceramics
Glasses Clay products
Refractories Abrasives Cements Advanced ceramics
-optical - composite reinforce - containers/ household
-whiteware - structural
-bricks for high T (furnaces)
-sandpaper - cutting - polishing
-composites - structural
-engine rotors valves bearings
-sensors Adapted from Fig. 13.7 and discussion in Section 13.4-10, Callister & Rethwisch 3e.
Ceramic Materials
Chapter 12 - 28
Ceramics Application: Die Blanks
tensile force
A o A d die
die
• Die blanks: -- Need wear resistant properties!
• Die surface: -- 4 µm polycrystalline diamond particles that are sintered onto a cemented tungsten carbide substrate. -- polycrystalline diamond gives uniform hardness in all directions to reduce wear.
Adapted from Fig. 14.2 (d), Callister & Rethwisch 3e.
Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.
Chapter 12 - 29
Ceramics Application: Cutting Tools
• Tools: -- for grinding glass, tungsten, carbide, ceramics -- for cutting Si wafers -- for oil drilling
blades oil drill bits
Single crystal diamonds
polycrystalline diamonds in a resin matrix.
Photos courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.
• Materials: -- manufactured single crystal or polycrystalline diamonds in a metal or resin matrix.
-- polycrystalline diamonds resharpen by microfracturing along cleavage planes.
Chapter 12 - 30
Refractories • Materials to be used at high temperatures (e.g., in high temperature furnaces). • Consider the Silica (SiO2) - Alumina (Al2O3) system. • Silica refractories - silica rich - small additions of alumina depress melting temperature (phase diagram):
Fig. 10.26, Callister & Rethwisch 3e. (Fig. 10.26 adapted from F.J. Klug and R.H. Doremus, J. Am. Cer. Soc. 70(10), p. 758, 1987.)
Composition (wt% alumina)
T(°C)
1400
1600
1800
2000
2200
20 40 60 80 100 0
alumina +
mullite
mullite + L
mullite Liquid
(L)
mullite + crystobalite
crystobalite + L
alumina + L
3Al2O3-2SiO2
Chapter 12 -
Advanced Ceramics: Materials for Automobile Engines
• Advantages:
– Operate at high temperatures – high efficiencies
– Low frictional losses – Operate without a cooling
system – Lower weights than
current engines
31
• Disadvantages:
– Ceramic materials are brittle
– Difficult to remove internal voids (that weaken structures)
– Ceramic parts are difficult to form and machine
• Potential candidate materials: Si3N4, SiC, & ZrO2 • Possible engine parts: engine block & piston coatings
Chapter 12 -
Advanced Ceramics: Materials for Ceramic Armor
32
Components: -- Outer facing plates -- Backing sheet Properties/Materials: -- Facing plates -- hard and brittle — fracture high-velocity projectile — Al2O3, B4C, SiC, TiB2 -- Backing sheets -- soft and ductile — deform and absorb remaining energy — aluminum, synthetic fiber laminates
Chapter 12 - 33
• Blowing of Glass Bottles:
GLASS FORMING
Adapted from Fig. 14.18, Callister & Rethwisch 3e. (Fig. 14.18 is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.)
Ceramic Fabrication Methods (i)
Gob
Parison mold
Pressing operation
Suspended parison
Finishing mold
Compressed air
• Fiber drawing:
wind up
PARTICULATE FORMING
CEMENTATION
-- glass formed by application of pressure -- mold is steel with graphite lining
• Pressing: plates, cheap glasses
Chapter 12 - 34
Sheet Glass Forming • Sheet forming – continuous casting
– sheets are formed by floating the molten glass on a pool of molten tin
Adapted from Fig. 14.19, Callister & Rethwisch 3e.
Chapter 12 - 35
• Quartz is crystalline SiO2:
• Basic Unit: Glass is noncrystalline (amorphous) • Fused silica is SiO2 to which no impurities have been added • Other common glasses contain impurity ions such as Na+, Ca2+, Al3+, and B3+
(soda glass) Adapted from Fig. 3.41, Callister & Rethwisch 3e.
Glass Structure
Si0 4 tetrahedron 4-
Si 4+
O 2 -
Si 4+ Na +
O 2 -
Chapter 12 - 36
• Specific volume (1/ρ) vs Temperature (T):
• Glasses: -- do not crystallize -- change in slope in spec. vol. curve at glass transition temperature, Tg -- transparent - no grain boundaries to scatter light
• Crystalline materials: -- crystallize at melting temp, Tm -- have abrupt change in spec. vol. at Tm
Adapted from Fig. 14.16, Callister & Rethwisch 3e.
Glass Properties
T
Specific volume
Supercooled Liquid
solid
Tm
Liquid (disordered)
Crystalline (i.e., ordered)
Tg
Glass (amorphous solid)
Chapter 12 - 37
Glass Properties: Viscosity
• Viscosity, η: -- relates shear stress (τ) and velocity gradient (dv/dy):
η has units of (Pa-s)
dydv /τ
=η
velocity gradient
dv dy
τ
τ
glass dv dy
Chapter 12 - 38
Visc
osity
[Pa-
s]
1 10 2
10 6
10 10
10 14
200 600 1000 1400 1800 T(°C)
Working range: glass-forming carried out
annealing point
Tmelt
strain point
• Viscosity decreases with T
Adapted from Fig. 14.17, Callister & Rethwisch 3e. (Fig. 14.17 is from E.B. Shand, Engineering Glass, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.)
Log Glass Viscosity vs. Temperature
• 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
Chapter 12 - 39
• Annealing: -- removes internal stresses caused by uneven cooling. • Tempering: -- puts surface of glass part into compression -- suppresses growth of cracks from surface scratches. -- sequence:
Heat Treating Glass
at room temp.
tension compression
compression
before cooling
hot
initial cooling
hot cooler
cooler
-- Result: surface crack growth is suppressed.
Chapter 12 - 40
• Mill (grind) and screen constituents: desired particle size • Extrude this mass (e.g., into a brick)
• Dry and fire the formed piece
ram billet
container
container force die holder
die
A o
A d extrusion Adapted from Fig. 14.2 (c), Callister & Rethwisch 3e.
Ceramic Fabrication Methods (iia)
GLASS FORMING
PARTICULATE FORMING
CEMENTATION
Hydroplastic forming:
Chapter 12 - 41
• Mill (grind) and screen constituents: desired particle size
• Slip casting operation
• Dry and fire the cast piece
Ceramic Fabrication Methods (iia)
solid component
Adapted from Fig. 14.22, Callister & Rethwisch 3e. (Fig. 14.22 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”
GLASS FORMING
PARTICULATE FORMING
CEMENTATION
Slip casting:
• Mix with water and other constituents to form slip
Chapter 12 - 42
Typical Porcelain Composition (50%) 1. Clay (25%) 2. Filler – e.g. quartz (finely ground) (25%) 3. Fluxing agent (Feldspar)
-- aluminosilicates plus K+, Na+, Ca+
-- upon firing - forms low-melting-temp. glass
Chapter 12 - 43
• Clay is inexpensive • When water is added to clay -- water molecules fit in between layered sheets -- reduces degree of van der Waals bonding -- when external forces applied – clay particles free to move past one another – becomes hydroplastic
• Structure of Kaolinite Clay:
Adapted from Fig. 3.14, Callister & Rethwisch 3e. (Fig. 3.14 is adapted from W.E. Hauth, "Crystal Chemistry of Ceramics", American Ceramic Society Bulletin, Vol. 30 (4), 1951, p. 140.)
Hydroplasticity of Clay
weak van der Waals bonding
charge neutral
charge neutral
Si 4+
Al 3 + - OH
O 2-
Shear
Shear
Chapter 12 - 44
• Drying: as water is removed - interparticle spacings decrease – shrinkage .
Adapted from Fig. 14.23, Callister & Rethwisch 3e. (Fig. 14.23 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 shrinkage wet body partially dry completely dry
• Firing: -- heat treatment between 900-1400°C -- vitrification: liquid glass forms from clay and flux – flows between SiO2 particles. (Flux lowers melting temperature). Adapted from Fig. 14.24, Callister & Rethwisch 3e.
(Fig. 14.24 is courtesy H.G. Brinkies, Swinburne University of Technology, Hawthorn Campus, Hawthorn, Victoria, Australia.)
Si02 particle (quartz)
glass formed around the particle
mic
rogr
aph
of p
orce
lain
70 µm
Chapter 12 - 45
Powder Pressing: used for both clay and non-clay compositions. • Powder (plus binder) compacted by pressure in a mold -- Uniaxial compression - compacted in single direction -- Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope -- Hot pressing - pressure + heat
Ceramic Fabrication Methods (iib)
GLASS FORMING
PARTICULATE FORMING
CEMENTATION
Chapter 12 - 46
Sintering
Adapted from Fig. 14.26, Callister & Rethwisch 3e.
Aluminum oxide powder: -- sintered at 1700°C for 6 minutes.
Adapted from Fig. 14.27, Callister & Rethwisch 3e. (Fig. 14.27 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.)
15 µm
Sintering occurs during firing of a piece that has been powder pressed -- powder particles coalesce and reduction of pore size
Chapter 12 - 47
Tape Casting • Thin sheets of green ceramic cast as flexible tape • Used for integrated circuits and capacitors • Slip = suspended ceramic particles + organic liquid
(contains binders, plasticizers)
Fig. 14.28, Callister & Rethwisch 3e.
Chapter 12 - 48
• Hardening of a paste – paste formed by mixing cement material with water • Formation of rigid structures having varied and complex shapes • Hardening process – hydration (complex chemical reactions involving water and cement particles)
Ceramic Fabrication Methods (iii)
GLASS FORMING
PARTICULATE FORMING
CEMENTATION
• Portland cement – production of: -- mix clay and lime-bearing minerals -- calcine (heat to 1400°C) -- grind into fine powder