CERAMICS

Part 1: Structure and Properties

MSE 206-Materials Characterization I

Lecture-7

Classification of Materials

Compounds between metallic and nonmetallic elements;

i.e CaF2, MgO, NaCl, Al2O3, SiO2, Si3N4, ZnS, SiC

Ceramics

Bonding:

-- Can be ionic and/or covalent in character.

-- % ionic character increases with difference in

electronegativity of atoms.

Ceramics comes from Greek word keramikos, means ‘’burnt

stuff’’

SiC: small

CaF2: large

Ceramics

• Degree of ionic character may be large or small:

Ceramics

PROPERTIES:

Hard and brittle with low toughness and ductility

Good electrical and thermal insulator because of absence of

conduction electrons

High melting point and high chemical stability

We know that ceramics are more brittle than

metals. Why?

• Consider method of deformation

– slippage along slip planes

• in ionic solids (ceramics) this slippage is very difficult

• too much energy needed to move one anion past another anion

TYPES OF CERAMICS:

1-) Traditional ceramic materials (glasses, refractories, cement

and concrete, bricks and tiles)

2-) Advanced ceramic materials (consist of pure or nearly pure

compounds such as Al2O3, SiC, Si3N4.

Ceramics

Structural: bioceramics, cutting tools, engine components, armour.

Electrical: Capacitors, insulators, magnets and superconductors

2-ADVANCED CERAMICS

1-TRADITIONAL CERAMICS

Brake disc SiC engine components SiC body armour Cutting tools

Whiteware

Ceramics

Gas turbine rotor, Si3N4

Ceramics

Ceramics: Crystal Structures

The ionic ceramics’ crystal structure can be thought of as

being composed of electrically charged ions:

- The metallic positively charged ions are called cations.

(give valence electrons) - The non-metallic ions are negatively charged and called anions

(accept electrons)

i.e. CaF2:

Ca2+ ion: has two positive charges (cation)

F- ion: has one negative charge (anion)

Ionic radii of the cations and anions:

- Metallic elements give up electrons

- Non metallic elements accept electrons

Cations are smaller than

anions

- -

- - +

Ceramics: Crystal Structures

AX–Type Crystal Structures

include NaCl, CsCl, and zinc blende

AX2 Crystal Structures

UO2, ThO2, ZrO2, CeO2

Fluorite structure

ABX3 Crystal Structures

• Perovskite structure

Ex: complex oxide

BaTiO3

Most common elements on earth are Si & O

Depending on the arrangement of SiO44- different silicate

structures arise.

SiO2 (silica) polymorphic forms are quartz, crystobalite, &

tridymite

The strong Si-O bonds lead to a high melting temperature

(1710ºC) for this material

Si4+

O2-

crystobalite

Silicate Ceramics: Silica (SiO2)

SiO44-

• Basic Unit: Si0 4 tetrahedron 4-

Si 4+

O 2 -

Si 4+

Na +

O 2 -

Sodium silicate glass

(soda glass)

Silica Glasses

1) CRYSTALLINE 2) AMORPHOUS (NON-CRYSTALLINE)

No impurities are added

Fused silica is SiO2

Network modifiers are

added, i.e. CaO, Na2O

Used for containers, windows Borosilicate glass is the pyrex glass used in labs

better temperature stability & less brittle than sodium glass

Complex structures are formed by sharing of oxygen atoms of

SiO44- on the corners, edges, or faces

Mg2SiO4 Ca2MgSi2O7

Adapted from Fig.

12.12, Callister &

Rethwisch 8e.

Presence of cations such as Ca2+, Mg2+, & Al3+

1. maintain charge neutrality, and

2. ionically bond SiO44- to one another

Silicates: Simple Silicates

Addition of Mg Ca, Mg

14

• Layered silicates (e.g., clays, mica, talc)

– SiO4 tetrahedra connected together to form 2-D plane

• A net negative charge is associated with each (Si2O5)

2- unit

• Negative charge balanced by adjacent plane rich in positively charged cations

Adapted from Fig.

12.13, Callister &

Rethwisch 8e.

Silicates: Layered Silicates

• Kaolinite (Al2(Si2O5)(OH)4) clay alternates (Si2O5)2- layer with

Al2(OH)42+ layer

Note: Adjacent sheets of this type are loosely bound to one another by van der Waal’s forces.

Adapted from Fig. 12.14,

Callister & Rethwisch 8e.

Silicates: Layered Silicates

Diamond

- Hardest material known

- Covalently bonded

tetrahedral carbon

• hard – no good slip planes

• brittle – can cut it

Adapted from Fig.

12.15, Callister 7e.

CARBON: Diamond

Diamond cubic structure

- High thermal conductivity and optically transperant

- large diamonds – jewelry

- small diamonds-often man made - used for cutting

tools and polishing

- diamond films- hard surface coat – tools, medical

devices, etc.

– In layers, between carbon atoms the bond type is

covalent; Between layers weak van der Waal’s

forces

– planes slide easily, good lubricant

– Electrical conductivity is high in crystallograhic

directions parallel to the hexagonal sheets

– High thermal conductivity

– Good machinability Adapted from Fig.

12.17, Callister 7e.

CARBON FORMS: Graphite

Applications: heating elements for furnaces, casting molds, rocket

nozzles, electrical contacts

– wrap the graphite sheet by curving into ball or tube

– Buckminister fullerenes

• Like a soccer ball C60 - also C70 + others

CARBON FORMS: Nanotubes and Fullerenes

C60

Carbon nanotube

- Extremely strong and stiff

(tensile strength : 50-200 GPa, elastic modulus : 1 TPa)

- Very low density

- May behave as conductor or semiconductor

Mechanical Properties of Ceramics

The main drawback is catastrophic fracture in a brittle manner

with very low energy

σ

ε

X X

ceramics

metals

Mechanical Properties of Ceramics

Metals

Crystalline

Metallic bond (non-directional)

Dislocations move under relatively low stresses

Ceramics

Crystalline (covalently or ionically bonded) or Non-crystalline

1-In covalently bonded ceramic:

- Bond is directional,

- limited slip system,

- Brittle fracture due to seperation of electron-pair bonds

2- In Ionically bonded ceramics:

- Limited slip systems

- Cracking occurs at the grain boundaries

3-In Non-crystalline ceramics:

-Deform by viscous flow (rate of deformation is proportional to stress)

-Atoms or ions slide past one another by breaking and reforming of interatomic

bonds.

Mechanical Properties of Ceramics

Elastic modulus and Flexural Strength

Stress-strain behavior is not determined by a tensile test (difficult to shape

and grip the ceramic)

Transverse bending test, in which the ceramic material is bent until fracture

- Rod specimen having either a circular or rectengular cross-section

- Three or four point loading technique

F L/2 L/2

d = midpoint

deflection

cross section

R

b

d

rect. circ.

22

F L/2 L/2

δ = midpoint

deflection

cross section

R

b

d

rect. circ.

• Determine elastic modulus according to:

F x

linear-elastic behavior d

F

d slope =

E = F

d

L 3

4 bd 3 =

F

d

L 3

12 p R 4

rect.

cross

section

circ.

cross

section

Mechanical Properties of Ceramics

Elastic modulus

F L/2 L/2

δ= midpoint deflection

cross section

R

b

d

rect. circ.

location of max tension

rect.

s fs

= 1.5Ff L

bd 2

= Ff L

pR3

x

F Ff

d fs

d

Mechanical Properties of Ceramics

Flexural Strength

The stress at fracture using this flexure test is konown as the flexural strength,

modulus of rupture, or bend strength

Mechanical Properties of Ceramics

Mechanical Properties of Ceramics

Factors Effecting the Strength

Surface cracks

Voids (porosity)

Inclusions

Large grains

EFFECT OF POROSITY: Pores act as stress risers. When pores reach a critical value,

a crack forms and propogates.

Mechanical Properties of Ceramics

Brittle Fracture of ceramics

The measured fracture strengths of ceramics are smaller than predicted

Flaws in ceramics act as stress riser

SURFACE CRACK INTERNAL CRACK

σm : maximum stress at the crack tip

σ0 : magnitude of the nominal applied tensile stress

ρt : radius of curvature of the crack tip

a : length of a surface crack or half of the length of an internal crack

sm = 2so(a/t)1/2

Maximum stress at the crack tip;

Mechanical Properties of Ceramics

Fracture toughness

σ : applied stress

a : crack length

Y : dimensionless parameter and its value depends on

both crack and specimen sizes and geometries, and load

application.

KIc = Yspa

Y=1 for aplate of infinite width having a through-thickness crack

Y= 1.1 for aplate of semi-infinite width having an edge crack

lentgh of a.

Ability of material to resist fracture when a crack is present

Ceramics have relatively low fracture toughness

Mechanical Properties of Ceramics

Hardness

Hardness is important when the material is going to be used in

abrasive or grinding action

Mechanical Properties of Ceramics

Abrasive ceramics that are used to grind or cut away other materials:

- High hardness or wear resistance is needed

- High degree of tougness is also needed to ensure that the abrasive particles

don’t easily fracture

The abresive wear resistance (AWR) of a cutting tool

AWR α KIC3/4 H1/2

KIC : Fracture toughness

H : hardness number

Al2O3-SiC composites are used primarily for cutting tool applications

Mechanical Properties of Ceramics

Material Density, ρ

(g/cm3)

Hardness vickers

(GPa)

Elastic Modulus

(GPa)

Fracture toughness

(MPa(m)-1/2)

Sintered Alumina 3.98 18.5 440 3.8

Al2O3-30%TiC (hp) 4.26 21.1 420 4.0

Al2O3-30%TiC (lps) 4.26 22 4.3

HP-Si3N4 3.26 21.6 317 5.0

Al2O3-SiC (lps) 17.8 7.9

Sintered SiALON 3.4 19.6 300 7.7

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e

time

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

creep test

s

s

slope = e ss = steady-state creep rate .

x

Mechanical Properties of Ceramics