Mechanical Behavior of Materials ME 2105 Dr. R. Lindeke, Ph.D.
Ceramic Their Properties and Material Behavior Engr 2110 Dr. R. Lindeke.
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Transcript of Ceramic Their Properties and Material Behavior Engr 2110 Dr. R. Lindeke.
Ceramic Their Properties and Material Behavior
Engr 2110
Dr. R. Lindeke
• 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
• Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity (remember!?!).
Adapted from Fig. 2.7, Callister 7e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 byCornell University.
• Large vs small ionic bond character:
Ceramic Bonding
SiC: small
CaF2: large
Ceramic Crystal Structures
Oxide structures– oxygen anions much larger than metal cations– close packed oxygen in a lattice (usually FCC)– cations in the holes of the oxygen lattice
• The same ideas apply to all “ceramics”• Principles of Ceramic Architecture:
– Size relationships Cation to Anion– Electrical Neutrality of the overall structure– Crystallographic Arrangements– Stoichiometry Must Match
Silica Glass
• A “Dense form” of amorphous silica– Charge imbalance corrected with “counter
cations” such as Na+
– Borosilicate glass is the pyrex glass used in labs
• better temperature stability & less brittle than sodium glass
Noncrystalline Silicates + oxides (CaO, Na2O, K2O, Al2O3)
GLASSES – transparent and easily shaped
E.g. Soda lime glass = 70wt% SiO2 + 30% [Na2O (soda) and CaO(lime)
Table 13.1
9
• Specific volume (1) vs Temperature (T):
Glass (amorphous solid)
T
Specific volume
Liquid (disordered)Supercooled
Liquid
Crystalline (i.e., ordered) solid
TmTg
• Glasses: --do not crystallize --spec. vol. varies smoothly with T --Glass transition temp, Tg
• Crystalline materials: --crystallize at melting temp, Tm
--have abrupt change in spec. vol. at Tm
• Viscosity: --relates shear stress & velocity gradient: --has units of (Pa-s)
dvdy
velocity gradient
dvdy
glass dv
dy
Adapted from Fig. 13.5, Callister, 6e.
GLASS PROPERTIES
10
• Viscosity decreases with T increase• Impurities lower Tdeform
Vis
cosi
ty [
Pa
s]
1
102
106
1010
1014
200 600 1000 1400 1800 T(°C)
Tdeform: soft enough to deform or “work”
annealing range
fused silica
96% silica
Pyrex
soda-lime
glass
from E.B. Shand, Engineering Glass, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.
GLASS VISCOSITY VS T AND IMPURITIES
Important Temperatures
• Viscosity decreases with T• Impurities lower Tdeform
•Melting point = viscosity of 10 Pa.s•Working point= viscosity of 1000 Pa.s•Softening point= viscosity of 4x107Pa.s
Temperature above which glass cannot be handled without altering dimensions)
•Annealing point= viscosity of 1012 Pa.s.•Strain point = viscosity of 3x1013Pa.s
Fracture occurs before deformation
– Combine SiO44- tetrahedra by having them
share corners, edges, or faces
– Cations such as Ca2+, Mg2+, & Al3+ act to neutralize & provide ionic bonding
Silicates
Mg2SiO4 Ca2MgSi2O7
Layered Silicates
• Layered silicates (clay silicates)– SiO4 tetrahedra connected
together to form 2-D plane
• (Si2O5)2-
• So need cations to balance charge
=
• Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)42+ layer
Layered Silicates
Note: these sheets loosely bound by van der Waal’s forces
Adapted from Fig. 12.14, Callister 7e.
Layered Silicates
• Can change the counterions – this changes layer spacing– the layers also allow absorption of water
• Micas: KAl3Si3O10(OH)2
• Bentonite– used to seal wells– packaged dry
– swells 2-3 fold in H2O
– pump in to seal up well so no polluted ground water seeps in to contaminate the water supply.
– Used in bonding Foundry Sands and Taconite pellets
Carbon Forms• Carbon black – amorphous –
surface area ca. 1000 m2/g
• Diamond
– tetrahedral carbon
• hard – no good slip planes
• brittle – can cleave (cut) it
– large diamonds – jewelry
– small diamonds
• often man made - used for cutting tools and polishing
– diamond films
• hard surface coat – cutting tools, medical devices, etc.
• layer structure – aromatic layers
– weak van der Waal’s forces between layers– planes slide easily, good lubricant
Carbon Forms - Graphite
Carbon Forms – Fullerenes and Nanotubes
• Fullerenes or carbon nanotubes– wrap the graphite sheet by curving into ball or tube– Buckminister fullerenes
• Like a soccer ball C60 - also C70 + others
Adapted from Figs. 12.18 & 12.19, Callister 7e.
• Frenkel Defect --a cation is out of place.
• Shottky Defect --a paired set of cation and anion vacancies.
• Equilibrium concentration of defects kT/QDe~
Adapted from Fig. 12.21, Callister 7e. (Fig. 12.21 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.)
Defects in Ceramic Structures
Shottky Defect:
Frenkel Defect
Mechanical Properties
We know that ceramics are more brittle than metals. Why?
• Consider method of deformation– slippage along slip planes
• in ionic solids this slippage is very difficult• too much energy needed to move one anion
past another anion (like charges repel)
• Room T behavior is usually elastic, with brittle failure.• 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials!
Adapted from Fig. 12.32, Callister 7e.
Measuring Elastic Modulus
FL/2 L/2
d = midpoint deflection
cross section
R
b
d
rect. circ.
• Determine elastic modulus according to:
Fx
linear-elastic behavior
F
slope =
E =F
L3
4bd3=
F
L3
12R4
rect. cross section
circ.cross section
• 3-point bend test to measure room T strength.
Adapted from Fig. 12.32, Callister 7e.
Measuring Strength
FL/2 L/2
d = midpoint deflection
cross section
R
b
d
rect. circ.
location of max tension
• Flexural strength: • Typ. values:
Data from Table 12.5, Callister 7e.
rect.
fs 1.5Ff L
bd 2
Ff L
R3Si nitrideSi carbideAl oxideglass (soda)
250-1000100-820275-700
69
30434539369
Material fs (MPa) E(GPa)
xF
Ff
fs
Mechanical Issues:
• Properties are significantly dependent on processing – and as it relates to the level of Porosity:
• E = E0(1-1.9P+0.9P2) – P is fraction porosity fs = 0e-nP -- 0 & n are empirical values
• Because the very unpredictable nature of ceramic defects, we do not simply add a factor of safety for tensile loading
• We may add compressive surface loads • We often choose to avoid tensile loading at all – most
ceramic loading of any significance is compressive (consider buildings, dams, brigdes and roads!)
• 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
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
• 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.
• 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.
Alternative Energy – Titania Nano-Tubes"This is an amazing material architecture for water photolysis," says Craig Grimes, professor of electrical engineering and materials science and engineering. Referring to some recent finds of his research group (G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, C. A. Grimes, Enhanced Photocleavage of Water Using Titania Nanotube-Arrays, Nano Letters, vol. 5, pp. 191-195.2005 ), "Basically we are talking about taking sunlight and putting water on top of this material, and the sunlight turns the water into hydrogen and oxygen. With the highly-ordered titanium nanotube arrays, under UV illumination you have a photoconversion efficiency of 13.1%. Which means, in a nutshell, you get a lot of hydrogen out of the system per photon you put in. If we could successfully shift its bandgap into the visible spectrum we would have a commercially practical means of generating hydrogen by solar
energy.
• 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
Sheet Glass Forming
• Sheet forming – continuous draw– originally sheet glass was made by “floating”
glass on a pool of mercury – or tin
Adapted from Fig. 13.9, Callister 7e.
Modern Plate/Sheet Glass making:
Image from Prof. JS Colton, Ga. Institute of Technology
• 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.
• 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
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+
• 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
• 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
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
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
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.
• 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-IIIGLASS
FORMING PARTICULATE
FORMINGCEMENTATION
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
Applications: Advanced Ceramics
• Ceramic Armor– Al2O3, B4C, SiC & TiB2
– Extremely hard materials • shatter the incoming projectile• energy absorbent material underneath
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