CERAMICSkeramikos - burnt stuff in Greek - desirable properties of

ceramics are normally achieved through a high temperature heat

treatment process (firing or sintering).

Usually a compound between metallic and nonmetallic elements

Always composed of more than one element (e.g., Al2O3, NaCl, SiC, SiO2)

Bonds are partially or totally ionic, can have combination of ionic and

covalent bonding

Generally hard and brittle

Generally electrical and thermal insulators

Can be optically opaque, semi-transparent, or transparent

Traditional ceramics – based on clay (china, bricks, tiles, porcelain), glasses.

Structural ceramics – “New ceramics” for electronic, computer, aerospace

industries.

Generally low density (compared to steel)

• 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.

Classification of Ceramics

armor

• 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

Ao

Addie

die

• Die blanks:-- Need wear resistant properties!

• Die surface:-- 4 mm 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 single

crystal diamonds

polycrystalline

diamonds in a resin

matrix.

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.

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

Mechanical Property Data Sheet

(6) creep curves and stress rupture data at high temperatures.

The mechanical property data that one would like to have available for a

particular ceramic ideally include a considerable list as follows:

(1) Elastic moduli as a function of temperature;

(2) average strength and Weibull parameters, both as a function of

temperature;

(3) values of toughness as a function of temperature

(4) data characterizing slow crack propagation for each

temperature of possible use

(5) data on cyclic fatigue behavior for a range of amplitudes of the

time-varying applied stress intensity factor and the static component

Mechanical Properties of Ceramic Materials in Common Use

Wachtman, J. B., Cannon, W. R., Matthewson, M. J., Mechanical Properties of Ceramics

Hardness Conversion

Room-Temperature Properties for Various Ceramic Materials

Meyers, M. A. and Chawla, K. K., Mechanical Behavior of Materials,

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.

sfs

=1.5Ff L

bd 2

=Ff L

pR3Si nitride

Si carbide

Al oxide

glass (soda)

250-1000

100-820

275-700

69

304

345

393

69

Material sfs (MPa) E(GPa)

xF

Ff

dfs

d

• Room T behavior is usually elastic, with brittle failure.

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

--tensile tests are difficult for brittle materials.

Dislocations in CeramicsDislocations do exist in ceramics.

If the temperature or lateral confinement of the

material is sufficiently high, ductile behavior can be

observed; in this case, dislocations play an important

role.

Dislocation observed in a [021]-oriented Mo5SiB2 single crystal deformed at 1500

°C. The thin foil was cut parallel to slip planes, i.e. (010) [Ihara, 2002]

However, at room temperature, due to ionic bondig of different atoms,

dislocation motion is not a factor.

Meyers, M. A. and Chawla, K. K., Mechanical Behavior of Materials,

Strength of Ceramics

Remember: theoretical strength is E/10

We would expect strength of ceramic materials at the

order of 10 GPa.

In practice, strength of ceramic materials are much lower.

Also, there is usually considerable variation and scatter

in the fracture strength

The frequency distribution of observed

fracture strengths for a silicon nitride material.

Strength of Ceramics

The frequency distribution of

observed fracture strengths for a

silicon nitride material.

Strength-Defect Size relation

A ceramic material will fail by brittle fracture.

Fracture will occur if the right hand side is greater than

the fracture toughness, KIc

aY

Kc

fp

s =

Therefore, strength of a ceramic material depends

on the size of the typical defect present in the structure.

Defect size Strength

Example: Consider polycrystalline alumina samples

with two grain sizes: 0.5 and 50 μm. During cooling, the

thermal expansion mismatch produces cracks that

have approximate dimensions equal to the grain-

boundary facets. If KIc = 4 MPa m1/2, determine the

tensile strength of each sample.

Solution: We assume that the flaw size, i.e., 2a, is

equal to the grain size. Then

Effect of PorosityMany ceramic fabrication techniques start from powder.

Pores or void spaces will exist between the powder

particles. During the ensuing heat treatment, much of this

porosity will be eliminated but some residual porosity will

remain

Effect of Porosity

The influence of porosity on the modulus of elasticity and

flexural strength for aluminum oxide at room temperature.

For small P E=E0(1-bP)

Statistical analysis of strength

Cumulative probability of

flexural strengths for three

ceramics

Strength distribution

of a brittle and a ductile solid.

Statistical analysis of strength

Probability of survival

Meyers, M. A. and Chawla, K. K., Mechanical Behavior of Materials,

m: Weibull modulus; so: Weibull strength

Statistical analysis of strength

A Weibull plot for a steel, a conventional alumina, and a

controlled-particle-size (CPS) alumina. Note that the

slope (Weibull modulus m)→∞ for steel. For CPS

alumina, m is double that of conventional alumina.

Statistical analysis of strength

If we process alumina carefully -- say, by using a

controlled particle size -- the value of m increases. By

a controlled particle size, we mean a monosize

powder that enhances packing, less use of a binder

material (which produces flaws after sintering), more

uniform shrinkage, etc.

Statistical analysis of strength

Effect of Specimen Size

Effect of Specimen SizeProbability of failure is a function of the specimen size.

Size

Similarly

Tensile test Four point bending Three point bending

s s s

L L L

s s s

probability of larger defects strength

volume effected by maximum stress strength

Increasing sf

Strengthening Mechanismsa) Introduce compressive residual surface stresses

b) Eliminate surface defects

c) Second phase (composites)

d) Phase transformation

e) Proof testing

Factors by Which Glasses Can Be Strengthened by Various Treatments

Wachtman, J. B., Cannon, W. R., Matthewson, M. J., Mechanical Properties of Ceramics

a) Introduce compressive residual surface stresses

Thermal Tempering

Annealed Tempered

Ion exchange (Chemical Tempering)

Chemical tempering principle and the residual stress profile in a chemically

tempered glass sheet

https://www.corning.com/gorillaglass/worldwide/en/technology/how-it-s-made.html

(J. Rösler · H. Harders · M. Bäker)

b) Eliminate surface defects

b) Eliminate surface defects

Schematic of a controlled-grind process to ensure removal

of coarse flaws. Flaw depth is assumed to be 3 the particle size.

Fibers have small diameters.

Fibes are fabricated very carefully to avoid all possible defects.

Small defect size high strength

b) Eliminate surface defects: Fibers

d) Phase transformation