Materials Science Chapter 12 1

Chapter 12 Ceramics

• Structures of ceramic materials:How do they differ from that of metals?

• Point defects:How are they different from those in metals?

• Impurities:How are they accommodated in the lattice and howdo they affect properties?

• Mechanical Properties:What special provisions/tests are made for ceramicmaterials?

• How do we classify ceramics?

• What are some applications of ceramics?

• How is processing different than for metals?

Materials Science Chapter 12 2

CERAMIC BONDING• Bonding:

--Mostly ionic, some covalent.--% ionic character increases with difference in

electronegativity.

He -

Ne -

Ar -

Kr -

Xe -

Rn -

Cl 3.0

Br 2.8

I 2.5

At 2.2

Li 1.0

Na 0.9

K 0.8

Rb 0.8

Cs 0.7

Fr 0.7

H 2.1

Be 1.5

Mg 1.2

Sr 1.0

Ba 0.9

Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

C 2.5Si

1.8

F 4.0

Ca 1.0

Table of Electronegativities

CaF2: large

SiC: small

• Large vs small ionic bond character:

Materials Science Chapter 12 3

IONIC BONDING & STRUCTURE• Charge Neutrality:

--Net charge in thestructure shouldbe zero.

--General form: AmXp

m, p determined by charge neutrality• Stable structures:

--maximize the # of nearest oppositely charged neighbors.

- -

- -+

unstable

- -

- -+

stable

- -

- -+

stable

CaF2: Ca2+cation

F-

F-

anions+

Materials Science Chapter 12 4

• Coordination # increases withIssue: How many anions can you

arrange around a cation?

rcationranion

rcationranion

Coord #

< .155 .155-.225 .225-.414 .414-.732 .732-1.0

ZnS (zincblende)

NaCl (sodium chloride)

CsCl (cesium

chloride)

2 3 4 6 8

COORDINATION # AND IONIC RADII

Materials Science Chapter 12 5

EX: PREDICTING STRUCTURE OF FeO• On the basis of ionic radii, what crystal structure

would you predict for FeO?

Cation

Al3+

Fe2+

Fe3+

Ca2+ Anion

O2-

Cl-

F-

Ionic radius (nm)

0.053

0.077

0.069

0.100

0.140

0.181

0.133

• Answer:

rcationranion

=0.0770.140

= 0.550

based on this ratio,--coord # = 6--structure = NaCl

Materials Science Chapter 12 6

AmXp STRUCTURES

rcationranion

=0.1000.133

≈ 0.8• Consider CaF2 :

• Based on this ratio, coord # = 8 and structure = CsCl. • Result: CsCl structure w/only half the cation sites

occupied.

• Only half the cation sitesare occupied since#Ca2+ ions = 1/2 # F- ions.

Materials Science Chapter 12 7

DEFECTS IN CERAMIC STRUCTURES• Frenkel Defect

--a cation is out of place.

• Shottky Defect--a paired set of cation and anion vacancies.

Shottky Defect:

Frenkel Defect

• Equilibrium concentration of defects ~ e−QD /kT

Materials Science Chapter 12 8

IMPURITIES• Impurities must also satisfy charge balance

• Ex: NaCl Na+ Cl-

• Substitutional cation impurity

• Substitutional anion impurityinitial geometry Ca2+ impurity resulting geometry

Ca2+

Na+

Na+Ca2+

cation vacancy

initial geometry O2- impurity

O2-

Cl-

anion vacancy

Cl-

resulting geometry

Materials Science Chapter 12 9

MEASURING ELASTIC MODULUS• Room T behavior is usually elastic, with brittle failure.• 3-Point Bend Testing often used.

--tensile tests are difficult for brittle materials.F

L/2 L/2

δ= midpoint deflection

cross section

R

b

d

rectangle circular

• Determine elastic modulus according to:

E = F

δ

L3

4bd3= F

δ

L3

12πR4

rectangle cross

section

circularcross

section

Fx

linear-elastic behaviorδ

F

δslope =

Materials Science Chapter 12 10

MEASURING STRENGTH• 3-point bend test to measure room T strength.

FL/2 L/2

cross section

R

b

d

rect. circ.

location of max tension

• Flexural strength:

rectangle

σfs = σmfail = 1.5FmaxL

bd2= FmaxL

πR3x

FFmax

δmaxδ

• Typical values:Material σfs(MPa) E(GPa)

Si nitrideSi carbideAl oxideglass (soda)

700-1000550-860275-550

69

30043039069

circular

Materials Science Chapter 12 11

MEASURING ELEVATED T RESPONSE• Elevated Temperature Tensile Test (T > 0.4 Tmelt).

ε

time

creep test

σ

σx

slope = εss = steady-state creep rate.

• Generally,

εssceramics < εss

metals << εsspolymers. . .

Materials Science Chapter 12 12

• Properties:--Tmelt for glass is moderate, but large for other ceramics.--Small toughness, ductility; large moduli & creep resistance

• 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

TAXONOMY OF CERAMICS

Materials Science Chapter 12 13

APPLICATION: REFRACTORIES• Need a material to use in high temperature furnaces.• Consider Silica (SiO2) - Alumina (Al2O3) system.• Phase diagram shows:

mullite, alumina, and crystobalite (made up of SiO2)tetrahedra as candidate 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

Materials Science Chapter 12 14

APPLICATION: DIE BLANKS

tensile force

AoAddie

die

• Die blanks:--Need wear resistant properties!

• Die surface:--4 µm polycrystalline diamond

particles that are sintered on to acemented tungsten carbidesubstrate.

--polycrystalline diamond helps controlfracture and gives uniform hardnessin all directions.

Materials Science Chapter 12 15

APPLICATION: CUTTING TOOLS• Tools:

--for grinding glass, tungsten,carbide, ceramics

--for cutting Si wafers--for oil drilling

bladesoil drill bits

• Solutions:--manufactured single crystal

or polycrystalline diamondsin a metal or resin matrix.

--optional coatings (e.g., Ti to helpdiamonds bond to a Co matrixvia alloying)

--polycrystalline diamondsresharpen by microfracturingalong crystalline planes.

coated singlecrystal diamonds

polycrystallinediamonds in a resinmatrix.

Materials Science Chapter 12 16

APPLICATION: SENSORS• Ex: Oxygen sensor: ZrO2

• Principle: Make diffusion of ionsfast for rapid response.

• Approach:Add Ca impurity to:--increase O2- vacancies--increase O2- diffusion

• Operation:--voltage difference

produced whenO2- ions diffusebetween external and referencesgases.

A Ca2+ impurity

removes a Zr4+ and a

O2- ion.

Ca2+

reference gas at fixed oxygen content

O2-

diffus

ion

gas with an unknown, higher oxygen content

-+voltage difference produced!

sensor

Materials Science Chapter 12 17

CERAMIC FABRICATION METHODS-I

PARTICULATE FORMING

CEMENTATIONGLASSFORMING

• Fiber drawing: • Pressing: Gob

Parison mold

Pressing operation

wind up• Blowing:

suspended Parison

Finishing mold

Compressed air

Materials Science Chapter 12 18

GLASS STRUCTURE• Basic Unit: • Glass is amorphous

• Amorphous structureoccurs by adding impurities(Na+,Mg2+,Ca2+, Al3+)

• Impurities:interfere with formation ofcrystalline structure.

Si04 tetrahedron4-

Si4+

O2-

• Quartz is crystallineSiO2:

Si4+

Na+

O2-

(soda glass)

Materials Science Chapter 12 19

GLASS PROPERTIES

• Specific volume (1/ρ) vs Temperature (T):

Glass (amorphous solid)

T

Specific volume

Liquid (disordered)Supercooled

Liquid

Crystalline (i.e., ordered) solid

TmTg

• Crystalline materials: --crystallize at melting temp, Tm

--have abrupt change in specificvolume at Tm

• Glasses: --do not crystallize--specific volume varies smoothly

with T--Glass transition temp, Tg

• Viscosity: --relates shear stress &

velocity gradient:--has units of (Pa-s)

τ = η

dvdy

velocity gradient

dvdy

τ

τglass dv

dy

Materials Science Chapter 12 20

GLASS VISCOSITY VS T AND IMPURITIES

• Viscosity decreases with T• Impurities lower Tdeform

Vis

co

sity

[P

a ⋅

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

Materials Science Chapter 12 21

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:

--Result: surface crack growth is suppressed.

further cooledbefore cooling surface cooling

tensioncompression

compressionhot hot

cooler

cooler

Materials Science Chapter 12 22

• Milling and screening: desired particle size

GLASS FORMING

CEMENTATIONPARTICULATEFORMING

• Mixing particles & water: produces a "slip"

--Hydroplastic forming:extrude the slip (e.g., into a pipe)

• Form a "green" component

hollow component

pour slip into mold

absorb water into mold “green

ceramic”

pour slip into mold

drain mold

“green ceramic”

• Dry and Fire the component

--Slip casting:

solid component

ram billet

container

containerforce

die holder

die

Ao

Adextrusion

CERAMIC FABRICATION METHODS-IIA

Materials Science Chapter 12 23

• Clay is inexpensive• Adding water to clay

--allows material to shear easilyalong weak van der Waals bonds

--enables extrusion--enables slip casting

• Structure ofKaolinite Clay:

weak van der Waals bonding

charge neutral

charge neutral

Si4+

Al3+

-OHO2-

Shear

ShearFEATURES OF A SLIP

Materials Science Chapter 12 24

DRYING AND FIRING

wet slip partially dry “green” ceramic

• Drying: layer size and spacing decrease.

• Firing:--T raised to (900-1400 oC)--vitrification: glass forms from clay and flows between

SiO2 particles.Si02 particle (quartz)

glass formed around the particle

micrograph of porcelain

70µm

Materials Science Chapter 12 25

• Sintering: useful for both clay and non-clay compositions.• Procedure:

--grind to produce ceramic and/or glass particles--inject into mold--press at elevated T to reduce pore size.

• Aluminum oxide powder:--sintered at 1700 oC

for 6 minutes.

GLASS FORMING

CEMENTATIONPARTICULATEFORMING

15µm

CERAMIC FABRICATION METHODS-IIB

Materials Science Chapter 12 26

PARTICULATE FORMING

GLASS FORMING

CEMENTATION

• Produced in extremely large quantities.• Portland cement:

--mix clay and lime bearing materials--calcinate (heat to 1400 oC)--primary constituents:

tri-calcium silicatedi-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