CEMM 470 CERAMICs - University of Illinois at Chicago

Ceramics

J. Ernesto Indacochea

University of Illinois at Chicago
Civil & Materials Engineering Dept.

Introduction

Ceramics are inorganic and nonmetallic materials.

Compounds between metallic and nonmetallic elements
with totally ionic or predominantly ionic with some
covalent character.

Involves processing at elevated temperatures.

Traditional ceramics: china, porcelain, bricks, tiles,
glasses. Clay raw material.

New generation “engineering or advanced ceramics”:
electronic, computers, aerospace.

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Introduction
General Classification

Ceramic Materials

Glasses Clay products Refractories Abrasives Cements Advanced
ceramics

Glasses Glass Structural Whiteware Fireclay Silica Basic Special
ceramics

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Introduction
Functional Classification

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Introduction
Functional Classification

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Introduction

Oxides, carbides, nitrides or borides
Glasses:

non crystalline silicates.
Windows, lenses, containers

Clays:

aluminosilicates. Widely used as ceramic raw material
Bricks, pottery, china

Refractories:

stable @ high-T; inert in tough environments
Furnace lining

Abrasives:

hardness, wear, toughness.
Diamond, SiC, WC

Wear, cutting

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Introduction

Industrial Applications:

Microstructures are engineered to improve some of the
properties of ceramics, e.g. Fracture toughness.
Will continue to establish a prominent niche in our
advance technologies.

Valve train parts & precombustion chambers. Inserts for Ceramic turbochargers
piston, liner & exhaust port insulation.
UIC 7
J. E. Indacochea

Introduction UIC 8

Industrial Applications: Automotive

Spark plugs, water pump seals,
catalytic converter.
Heat engine:

Higher operating temperatures
⇒ Better fuel efficiency
Lower frictional forces &
ability to operate with no
cooling system
Excellent wear & corrosion
resistance
Lower densities ⇒ Decreased
engine weight

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Introduction

Industrial Applications: Aerospace

Coating of metal heat engine parts
⇒ improved wear &/or high
temperature damage.
Their low densities ⇒ lighter
turbine blades vs superalloys
Materials considered: Si3N4, SiC
and ZrO2
Draw back: disposition to brittle &
catastrophic failure.

J. E. Indacochea Helicopter gas turbine 9

UIC

Introduction UIC 10

Industrial Applications: Electronics

Packaging of integrated circuits --
(substrate):

electrically insulating.
low dielectric characteristics.
thermally conductive.

Aluminum oxide:

standard bearer.
low thermal conductivity & poor
electrical conductivity.

Aluminum nitride

☺ good thermal & electrical properties.
bonding with metals: poor.
payoff for metal pattern to stick: Mo
paste + additive @ 1600C or special
direct Cu bonding.

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Structure of Ceramics

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Ceramic Structures

Ceramic crystal structures are
often more complex since
they are composed of at least
two different elements.

bonding: ranges from purely
ionic to totally covalent.

The degree of ionic character
depends on the
electronegativity of the atoms

{ }% ⎣⎡ − )2 ⎤
Ionic character = 1− exp ( 0.25 ) ( X A − X B ⎦ ×100

where XA and XB are the elctronegativities of the two elements UIC 12

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Ceramic Structures

for most ceramics the bonding is predominantly ionic

composed of electrically charged ions: “cations” and
“anions”.

The crystal structure is influenced by:

magnitude of the electrical charge of cation and
anion.

relative sizes of the ions involved.

The crystal must be electronically neutral. The chemical
formula reflects this charge balance.

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Ceramic Structures

Anions are larger than cations, hence the rc/ra <1.
Each ion wants to have as many of the other ions as
nearest neighbors.
The stability of the crystal structure is influenced by the
ion contact.

The most common coordination numbers for ceramics are
4, 6, and 8.

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Ceramic Structures

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Ceramic Structures

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Ceramic Crystal Structures

A number of ceramic structures may be considered in
terms of close-packed plane of ions, as well as unit cells.
Usually, the close-packed planes are composed of large
anions.
Two types of interstitials:

tetrahedral: coordination number 4
octahedral: coordination number 6

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Ceramic Crystal Structures

the ceramic crystal structures depend on:

the stacking of the close-packed anion layers (FCC
and HCP arrangement possible).

way in which interstitial sites are filled with cations.
Spinel structure [AmBnXp] MgAl2O4: the O2– form an
FCC lattice, Mg2+ fill tetrahedral sites, and Al3+ occupy
octahedral sites.

Magnetic ceramics (ferrites) have a spinel-like structure.
Magnetic characteristics affected by the occupancy of
the interstitial positions.

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Ceramic Density

The theoretical density of a crystalline ceramic is given by:

ρ = n' (∑ A C + ∑ A A )

VC ⋅ N A

n’ = total number of each anions and cations in unit cell
AC = atomic weight of all cations
AA = atomic weight of all anions
VC = unit cell volume
NA = Avogadro’s number

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Ceramic Density

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Carbon Structures

Carbon is an element that exists in various polymorphic
forms, as well as in the amorphous state.
Carbon does not fall within any one of the traditional
metal, ceramic, polymer classification schemes.
Sometimes, it is classified as a ceramic.
C6: 1s22s22p2 ? 1s22s12p3! (hybridized state)
leads to all states being equal & to tetrahedral bonding.

109.5º angle between
bonds

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Carbon Structures

Diamond:

It is a metastable carbon polymorph at room temperature
and atmospheric pressure.
The sp3 hybridization requires 4C atoms as neighbors to
form a closed shell configuration.
But each of the 4 neighbors will require 4 other neighbors,
and so on.

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Diamond UIC 23

Zinc blende, in which carbon atoms
occupy all positions (both Zn & S).
called the diamond cubic crystal
structure.
found in other Group IVA elements
in the periodic table: Ge, Si and
gray Sn, below 13°C (55°F).
the hardest known material and has
a very low electrical conductivity
due to its crystal structure and the
strong interatomic covalent bonds.
unusually high thermal conductivity
for a nonmetallic material.

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Diamond

It is optically transparent and has a high index of
refraction.
diamonds are utilized to grind or cut other softer
materials.
Techniques to produce synthetic diamonds have been
developed beginning in the mid-1950s.
Over the last several years, diamond in the form of thin
films has been produced.

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Properties of Ceramics

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Properties of Ceramics

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CERAMICS: Processing

the constituent oxide for ceramics have high melting
points.

oxides are never molten and cast as metals.

oxide particles are bonded through a process known as
sintering or powder metallurgy.

constituent powders have to be mixed from different raw
materials at compositions described by phase diagrams.

to produce mullite (Al2O3 + SiO2): mix 3 molecular
weights of Al2O3 + 2 molecular weight SiO2.

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Ceramics: Processing

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Ceramics: Processing

(a) Uniaxial powder compaction
showing the die-punch assembly
during different stages. Typically,
for small parts these stages are
completed in less than a minute.

(b) Microstructure of a barium
magnesium tantalate (BMT)
ceramic prepared using compaction
and sintering.

(c) Different diffusion mechanisms
involved in sintering. The grain
boundary and bulk diffusion (1, 2
and 5) to the neck contribute to
densification. Evaporation-
condensation (4) and surface
diffusion (3) do not contribute to
densification.

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Ceramics: Processing

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Ceramics: Processing

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CERAMICS: Processing

Important steps in manufacturing of ceramic
bodies:

Batching and Preparation of Powders:

They must be in powder form.
Traditional ceramics are beneficiated from minerals.
Most common raw materials: silica, clay, fluxes, refractory
materials.
In advanced ceramics raw materials are high purity
chemically processed.
Mix particle sizes improves densification.
Powders are prepared into states that are compatible with
fabrication process: spray drying for dry pressing; slip casting
controlled of viscosity.

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CERAMICS: Processing

• Forming:

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CERAMICS: Processing

Drying and Firing:
The “green” ceramic form or compacted powder is
heated to:

dry the ceramic,
burn the added binders and plasticizers,
vitrify or bond the particles, and
densify the body.

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CERAMICS: Processing

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CERAMICS: Processing

Drying and Firing

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CERAMICS: Processing

Drying and Firing

Stages in the vitrification
and densification of a
ceramic body during
sintering; (a) loose powder
(“green” compact), (b)
initial bonding stage, (c)
intermediate stage-grain
boundaries form, (d) final
stage-- densification and
elimination of pores along
grain boundaries.

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CERAMICS: Processing

(a) (b)

Densification of ceramic occurs when the pores remained along grain
boundaries in (a), but does not occur when the pores are within the
grains in (b).

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CERAMICS: Processing

•Drying and Firing -- Sintering

Mass transport
mechanism for the
growth of the
necked region. It
includes: 1)
evaporation-
condensation, 2)
surface diffusion,
and 3) the bulk
transport
mechanisms:
volume and grain
boundary
diffusion and
plastic flow.

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Imperfections in Ceramics

Both vacancies and interstitials are likely for either ion, but
high concentration of anion interstitials is improbable!
Defect structure is the term used to describe types and
concentrations of atomic defects in ceramics.

Conditions of electroneutrality needs to be maintained in the
defect structure defect in ceramics usually occur in pairs:

Frenkel defect: cation vacancy/cation interstitial, no change
in charge. Cation maintains its charge as interstitial.
Schottky defect: found in AX ceramics. Cation
vacancy/anion vacancy.

The ratio of cations to anions is not altered by presence of
either defect material maintains its Stoichiometry ( exact
cation to anion ratio as predicted by the chemical formula).

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Imperfections in Ceramics

Nonstoichiometry: deviation from exact ratio. It occurs in
ceramics where two valence (ionic) states exist for one of
the two ions.

Wustite (Iron oxide, FeO):

Fe ion can be present in two
states depending on temperature
and oxygen partial pressure.

For every Fe3+ ion form there will be an excess 1+ charge
to maintain electroneutrality for every two Fe3+ cations

formed a vacancy of Fe2+ crystal no longer
stoichiometric because of one more O ion than Fe ion. Yet

structure remains electrically neutral,

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Impurities in Ceramics

Solid solution in ceramics exist as in metals.
Both substitutional and interstitial types may occur.
For interstitial solid solution the ionic radius of the
impurity must be small compared to the host anion.
Charge neutrality must be maintained.

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Ceramic Phase Diagrams

These diagrams have configurations similar to metal-metal systems and
are interpreted in the same way

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