Chapter 12 Ceramics - University of Houston

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 how
do they affect properties?

• Mechanical Properties:

What special provisions/tests are made for ceramic
materials?

• How do we classify ceramics?

• What are some applications of ceramics?

• How is processing different than for metals?

Materials Science Chapter 12 1

CERAMIC BONDING

• Bonding:

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

electronegativity.

• Large vs small ionic bond character:

H CaF2: large He
2.1 -
Li Be SiC: small C
1.0 1.5 2.5 F Ne
4.0 -
Na Mg Si
0.9 1.2 1.8 Cl Ar
3.0 -
K Ca Ti Cr Fe Ni Zn As
0.8 1.0 Br Kr
1.5 1.6 1.8 1.8 1.8 2.0 2.8 -
Rb Sr
0.8 1.0 I Xe
2.5 -
Cs Ba
0.7 0.9 At Rn
2.2 -
Fr Ra
0.7 0.9 Table of Electronegativities

Materials Science Chapter 12 2

IONIC BONDING & STRUCTURE

• Charge Neutrality: CaF2: cCaati2o+n+ F-
anions
--Net charge in the F-
structure should
be zero.

--General form: AmXp

m, p determined by charge neutrality

• Stable structures:

--maximize the # of nearest oppositely charged neighbors.

-- -- --
+ + +

-- -- --

unstable stable stable

Materials Science Chapter 12 3

COORDINATION # AND IONIC RADII
rcation
• Coordination # increases with ranion

Issue: How many anions can you

arrange around a cation?

rcation Coord # ZnS
ranion 2 (zincblende)
< .155

.155-.225 3 NaCl
.225-.414 4 (sodium
.414-.732 6 chloride)

.732-1.0 8 CsCl
(cesium
Materials Science Chapter 12 chloride)

4

EX: PREDICTING STRUCTURE OF FeO

• On the basis of ionic radii, what crystal structure
would you predict for FeO?

Cation Ionic radius (nm) • Answer:
Al3+ 0.053
Fe2+ 0.077 rcation = 0.077
Fe3+ 0.069 ranion 0.140
Ca2+ 0.100
= 0.550

Anion 0.140 based on this ratio,
0.181 --coord # = 6
O2- 0.133 --structure = NaCl
Cl-
F-

Materials Science Chapter 12 5

AmXp STRUCTURES

• Consider CaF2 : rcation = 0.100 ≈ 0.8
ranion 0.133

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

occupied.

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

Materials Science Chapter 12 6

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 7

IMPURITIES

• Impurities must also satisfy charge balance
• Ex: NaCl Na+ Cl-

• Substitutional cation impurity cation
vacancy
Ca2+

Na+

Na+ Ca2+
resulting geometry
initial geometry Ca2+ impurity
anion vacancy
• Substitutional anion impurity

O2-

initial geometry Cl- Cl- resulting geometry
O2- impurity
Materials Science 8
Chapter 12

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.

cross section F

L/2 L/2

dR

rectabngle circula δ= midpoint
deflectio
• DeteFrmirne elastic modulus according to: n

x E=F L3 = F L3
slope = F δ 4bd3 δ 12πR4

δ rectangle circular

δ cross cross
section section
linear-elastic behavior
Materials Science
Chapter 12 9

MEASURING STRENGTH

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

cross section F

L/2 L/2

dR

b

rect. circ.

location of max tension

• Flexural strength: • Typical values:

σ mfail 1.5FmaxL FmaxL Material σfs(MPa) E(GPa)
Fx bd2 πR3
σfs = = = Si nitride 700-1000 300
rectangle circular
Fmax Si carbide 550-860 430

Al oxide 275-550 390

glass (soda) 69 69

δ Chapter 12 10

Materials Scienceδmax

MEASURING ELEVATED T RESPONSE

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

creep test

ε x

σ

slope = . = steady-state creep rate

εss

σ

time

• Generally,
ε. scesramics < ε.smsetals << ε. pssolymers

Materials Science Chapter 12 11

TAXONOMY OF CERAMICS

Glasses Clay Refractories Abrasives Cements Advanced

products ceramics

-optical -whiteware -bricks for -sandpaper -composites engine
-composite -bricks high T -cutting -structural -rotors
(furnaces)
reinforce -polishing -valves
-containers/ -bearings
-household

• Properties: -sensors

--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.

Materials Science Chapter 12 12

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.

2200 3Al2O3-2SiO2
T(°C)
Liquid mullite
2000 (L)
alumina + L

1800 mullite alumina
+L +
crystobalite
mullite mullite
+ L 1600 + crystobalite

14000 20 40 60 80 100
Composition (wt% alumina)
Materials Science
Chapter 12 13

APPLICATION: DIE BLANKS

• Die blanks: Ao die Ad tensile
force
--Need wear resistant properties!
die

• Die surface:

--4 µm polycrystalline diamond
particles that are sintered on to a
cemented tungsten carbide
substrate.

--polycrystalline diamond helps control
fracture and gives uniform hardness
in all directions.

Materials Science Chapter 12 14

APPLICATION: CUTTING TOOLS

• Tools: blades
coated single
--for grinding glass, tungsten, crystal diamonds
carbide, ceramics
polycrystalline
--for cutting Si wafers diamonds in a resin
--for oil drilling matrix.

• Solutions: 15

--manufactured single crystal
or polycrystalline diamonds oil drill bits
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.

Materials Science Chapter 12

APPLICATION: SENSORS

• Ex: Oxygen sensor: ZrO2 Ca2+
• Principle: Make diffusion of ions

fast for rapid response.

• Approach: A Ca2+ impurity
removes a Zr4+ and a
Add Ca impurity to:
O2- ion.
--increase O2- vacancies

--increase O2- diffusion

• Operation: sensor

--voltage difference gas with an O2- reference
unknown, higher diffus gas at fixed
produced when oxygen content ion oxygen content
O2- ions diffuse
between external
and references

gases. voltage diff+erence -

produced!

Materials Science Chapter 12 16

CERAMIC FABRICATION METHODS-I

GLASS PARTICULATE CEMENTATION
FORMING FORMING
• Pressing:
• Fiber drawing:
Gob
Pressing
Parison operation
mold
Compressed wind up
• Blowing: air
17
suspended Finishing
Parison mold

Materials Science Chapter 12

GLASS STRUCTURE

• Basic Unit: • Glass is amorphous
• Amorphous structure
Si044- tetrahedron
occurs by adding impurities
Si4+
O2- (Na+,Mg2+,Ca2+, Al3+)

• Impurities:

interfere with formation of
crystalline structure.

• Quartz is crystalline Na+
SiO2: Si4+

O2-

(soda glass)

Materials Science Chapter 12 18

GLASS PROPERTIES

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

Specific volume • Crystalline materials:

Supercooled Liquid --crystallize at melting temp, Tm
Liquid (disordered)
--have abrupt change in specific
Glass volume at Tm
(amorphous solid)
• Glasses:
Crystalline
(i.e., ordered) solid --do not crystallize

Tg Tm T --specific volume varies smoothly

with T

• Viscosity: --Glass transition temp, Tτg

--relates shear stress & τ = η dv glass dy dv
velocity gradient: dy dv dy

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

Materials Science Chapter 12 19

GLASS VISCOSITY VS T AND IMPURITIES

• Viscosity decreases with T
• Impurities lower Tdeform

Viscosity [Pa ⋅ s]fu9ss6oeg%ddlPaassys-iirlsllieiimccxaae
1014 annealing range
1010

106 Tdeform: soft enough 20
to deform or “work”
102
1 600 1000 1400 1800 T(°C)
200
Chapter 12
Materials Science

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:

before cooling surface cooling further cooled

hot cooler compression

hot tension
cooler compression

--Result: surface crack growth is suppressed.

Materials Science Chapter 12 21

CERAMIC FABRICATION METHODS-IIA

GLASS PARTICULATE CEMENTATION
FORMING FORMING

• Milling and screening: desired particle size

• Mixing particles & water: produces a "slip"
• Form a "green" component
Ao container die holder
--Hydroplastic forming: force
ram billet extrusion Ad
extrude the slip (e.g., into a pipe)
container die
--Slip casting:
pour slip absorb water
into mold into mold “green pour slip drain “green
into mold mold ceramic”
ceramic”

solid component hollow component
• Dry and Fire the component
Materials Science
Chapter 12 22

FEATURES OF A SLIP
Shear

• Clay is inexpensive charge
• Adding water to clay neutral

--allows material to shear easily weak van
along weak van der Waals bonds der Waals
bonding
--enables extrusion
--enables slip casting Si 4+
Al 3+
• Structure of charge OH-
Kaolinite Clay: neutral O2-

Materials Science Shear

Chapter 12 23

DRYING AND FIRING

• Drying: layer size and spacing decrease.

wet slip partially dry “green” ceramic

• Firing:

--T raised to (900-1400 oC)

--vitrification: glass forms from clay and flows between

SiO2 particles. Si02 particle

(quartz)

micrograph of glass formed
porcelain around
the particle

Materials Science 70µm 24
Chapter 12

CERAMIC FABRICATION METHODS-IIB

GLASS PARTICULATE CEMENTATION
FORMING FORMING

• 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.

15µm

Materials Science Chapter 12 25