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