ceramic

1. CERAMIC
CERAMIC
MATERIALS
MATERIALS
What are Ceramics?
What are Ceramics?
Ceramics
Ceramics are
are inorganic, non
inorganic, non-
-metallic
metallic and
and crystalline materials
crystalline materials that are
that are
typically produced using clays and other minerals from the earth
typically produced using clays and other minerals from the earth or
or
chemically processed powders
chemically processed powders
Ceramics are crystalline and are compounds formed between metall
Ceramics are crystalline and are compounds formed between metallic
ic
and non
and non-
-metallic elements such as aluminium and oxygen (alumina
metallic elements such as aluminium and oxygen (alumina-
-
Al
Al2
2O
O3
3 ), silicon and nitrogen (silicon nitride
), silicon and nitrogen (silicon nitride-
- Si
Si3
3N
N4
4) and silicon and
) and silicon and
carbon (silicon carbide
carbon (silicon carbide-
-SiC).
SiC).
Glasses
Glasses are non
are non-
-metallic, inorganic but
metallic, inorganic but amorphous
amorphous. They are often
. They are often
considered as belonging to ceramics.
considered as belonging to ceramics.
Characteristics of Ceramics
Characteristics of Ceramics
Ceramics
Ceramics
Low density
Low density
High T
High Tm
m
High elastic modulus
High elastic modulus
Brittle
Brittle
Non
Non-
-reactive
reactive
Goff electrical and
Goff electrical and
thermal insulators
thermal insulators
High hardness and
High hardness and
wear resistance
wear resistance
Metals
Metals
High density
High density
Medium to high T
Medium to high Tm
m
Medium to high elastic
Medium to high elastic
modulus
modulus
Ductile
Ductile
Reactive (corrode)
Reactive (corrode)
Good electrical and
Good electrical and
thermal conductors
thermal conductors
Polymers
Polymers
Very low density
Very low density
Low Tm
Low Tm
Low elastic modulus
Low elastic modulus
Ductile and brittle
Ductile and brittle
Structure of Ceramics
Structure of Ceramics
Ceramics
Ceramics exhibit ionic, covalent bonding or a combination of the two
exhibit ionic, covalent bonding or a combination of the two
(like in Al
(like in Al2
2O
O3
3)
)
Type of bonding strongly influences the crystal structure of cer
Type of bonding strongly influences the crystal structure of ceramics
amics
l
lCeramics crystallise in two main groups:
Ceramics crystallise in two main groups:
1.
1. Ceramics with simple crystal structure (e.g; SiC, MgO)
Ceramics with simple crystal structure (e.g; SiC, MgO)
2.
2. Ceramics with complex crystal structures based on silicate SiO
Ceramics with complex crystal structures based on silicate SiO4
4
(known as silicates)
(known as silicates)

2. Ionic bonding: metallic ions + nonmetallic ions
Cations Anions
Stable structure
Coordination Number: RC/RA
RC/RA = 0.155
6
• Bonding:
-- Mostly ionic, some covalent.
-- % ionic character increases with difference in
electronegativity.
Adapted from Fig. 2.7, Callister 7e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical
Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by
Cornell University.
• Large vs small ionic bond character:
SiC: small
CaF2: large
Ceramic Bonding
Ceramic crystal structure considerations
Ceramic crystal structure considerations
Charge Neutrality
Charge Neutrality
The bulk ceramic must remain electrically neutral
The bulk ceramic must remain electrically neutral
For example, the compound MgO
For example, the compound MgO2
2 does not exist
does not exist
Mg
Mg+2
+2
O
O-
-2
2
: net charge / molecule = 1(+2) + 2(
: net charge / molecule = 1(+2) + 2(-
-2) =
2) = -
-2
2
must MgO
must MgO
Coordination Number (CN) : The number of atomic or ionic
Coordination Number (CN) : The number of atomic or ionic
nearest neighbors.
nearest neighbors.
Depends on atomic size ratio
Depends on atomic size ratio
CN increases as the
CN increases as the R
RC
C/R
/RA
A increases
increases
CN determines the possible crystal structure,
CN determines the possible crystal structure,
Thus, CN determines the physical properties
Thus, CN determines the physical properties

3. Cs+
Cl-
= Na+
= Cl-
Examples of AX type structure
Examples of AX type structure
Rock Salt Structure
10
MgO and FeO also have the NaCl structure
O2- rO = 0.140 nm
Mg2+ rMg = 0.072 nm
Adapted from Fig.
12.2, Callister 7e.
Each oxygen has 6 neighboring Mg2+
MgO and FeO
11
AX–Type Crystal Structures include NaCl, CsCl, and zinc blende
939
.
0
181
.
0
170
.
0
Cl
Cs =
=
−
+
r
r
Adapted from Fig.
12.3, Callister 7e.
Cesium Chloride structure:
Each Cs
+ has 8 neighboring Cl
-
AX Crystal Structures
12
Zinc Blende structure
Adapted from Fig.
12.4, Callister 7e.
Ex: ZnO, ZnS, SiC
AX Crystal Structures

4. 13
Fluoride structure
• Calcium Fluoride (CaF2)
• cations in cubic sites
• UO2, ThO2, ZrO2, CeO2
Adapted from Fig.
12.5, Callister 7e.
AX2 Crystal Structures
14
• Perovskite
Ex: complex oxide
BaTiO3
Adapted from Fig.
12.6, Callister 7e.
ABX3 Crystal Structures
15
We know that ceramics are more brittle than metals.
Why?
• Consider method of deformation
slippage along slip planes
in ionic solids this slippage is very difficult
too much energy needed to move one anion past
another anion
Mechanical Properties
Our focus is HERE !!!
Our focus is HERE !!!
ceramics
ceramics
ceramics
ceramics
ceramics
ceramics
ceramics
ceramics
Clay Products
Clay Products
Glasses
Glasses Refractories
Refractories
Abrasives
Abrasives
Cements
Cements Eng.
Eng.
Ceramics
Ceramics
Glasses
Glasses Glass
Glass-
-
ceramics
ceramics

5. Engineering ceramics are generally
Engineering ceramics are generally
classified into the following:
classified into the following:
Structural ceramics,
Structural ceramics,
Industrial wear parts, bioceramics,
cutting tools, engine components
Electrical and Electronic ceramics,
Electrical and Electronic ceramics,
Capacitors, insulators, substrates, IC
packages, piezoelectrics, magnets,
superconductors
Ceramic coatings,
Ceramic coatings,
Industrial wear parts, cutting tools,
engine components
Chemical processing environmental
Chemical processing environmental
ceramics
ceramics
Filters, membranes, catalysts
Bioceramics
Cutting tools
Coating
Engine parts
Silicate Ceramics
Silicate Ceramics
Most common elements on
earth are Si O
Si
O
Si
Si-
-O Tetrahedron
O Tetrahedron
The strong Si-O bond leads to a
strong, high melting material
(1710ºC)
20
Combine SiO4
4- tetrahedra by having them share
corners, edges, or faces
Cations such as Ca2+, Mg2+, Al3+ act to neutralize
provide ionic bonding
Mg2SiO4 Ca2MgSi2O7
Adapted from Fig.
12.12, Callister 7e.
Silicates

6. Two most common silicate ceramics are:
Two most common silicate ceramics are:
Silica and silica glasses
Silica and silica glasses
1.
1. Silica (SiO
Silica (SiO2
2)
)
If the tetrahedra are arranged in a
If the tetrahedra are arranged in a
regular and ordered manner, a
regular and ordered manner, a
crystalline structure is formed. Silica
crystalline structure is formed. Silica
have 3 different types: quartz,
have 3 different types: quartz,
crystobalite and tridymite
crystobalite and tridymite
O
Silica
Silica
Si
Si
Si-
-O Tetrahedron
O Tetrahedron Silicate Ceramics
Silicate Ceramics
2.
2. Silica Glasses
Silica Glasses
If the tetrahedra are randomly arranged, a non
If the tetrahedra are randomly arranged, a non-
-
crystalline structure, known as
crystalline structure, known as Glass
Glass is formed.
is formed.
O
Silica glasses is a dense form of amorphous
silica
- Charge imbalance corrected with
“counter cations” such as Na+
-Borosilicate glass is the pyrex glass
used in labs
-better temperature stability less
brittle than sodium glass
23
• Silica gels - amorphous SiO2
Si4+ and O2- not in well-ordered
lattice
Charge balanced by H+ (to form
OH-) at “dangling” bonds
very high surface area 200 m2/g
SiO2 is quite stable, therefore
unreactive
makes good catalyst support
Adapted from Fig.
12.11, Callister 7e.
Amorphous Silica
Other oxides may also be incorporated into a
Other oxides may also be incorporated into a
glassy SiO
glassy SiO2
2 network in different ways:
network in different ways:
1.
1. Network formers:
Network formers: form glassy structures
form glassy structures
(B
(B2
2O
O3
3)
)
2.
2. Network modifiers:
Network modifiers: added to terminate (break
added to terminate (break
up) the network (CaO, Na
up) the network (CaO, Na2
2O). These are
O). These are
added to silica glass to lower its viscosity (so
added to silica glass to lower its viscosity (so
that forming is easier)
that forming is easier)
3.
3. Network intermediates:
Network intermediates: these oxides cannot
these oxides cannot
form glass network but join into the silica
form glass network but join into the silica
network and substitute for Si.
network and substitute for Si.

7. 25
• Carbon black – amorphous –
surface area ca. 1000 m2/g
• Diamond
tetrahedral carbon
hard – no good slip planes
brittle – can cut it
large diamonds – jewellery
small diamonds
often man made - used for
cutting tools and polishing
diamond films
hard surface coat – tools,
medical devices, etc.
Adapted from Fig.
12.15, Callister 7e.
Carbon Forms
26
• layer structure – aromatic layers
weak van der Waal’s forces between layers
planes slide easily, good lubricant
Adapted from Fig.
12.17, Callister 7e.
Carbon Forms
27
• Fullerenes or carbon nanotubes
wrap the graphite sheet by curving into ball or tube
Buckminister fullerenes
Like a soccer ball C60 - also C70 + others
Adapted from Figs.
12.18 12.19,
Callister 7e.
Carbon Forms
28
• Frenkel Defect
-a cation is out of place.
• Shottky Defect
--a paired set of cation and anion vacancies.
• Equilibrium concentration of defects
kT
/
QD
e
~ −
Adapted from Fig. 12.21, Callister
7e. (Fig. 12.21 is from W.G.
Moffatt, G.W. Pearsall, and J.
Wulff, The Structure and
Properties of Materials, Vol. 1,
Structure, John Wiley and Sons,
Inc., p. 78.)
Shottky
Defect:
Frenkel
Defect
Defects in Ceramic Structures

8. Mechanical Properties of Ceramics
Mechanical Properties of Ceramics
Ceramics have inferior mechanical properties compared to metals,
Ceramics have inferior mechanical properties compared to metals, and this
and this
has limited their applications
has limited their applications
The main limitation is that ceramics fail in
The main limitation is that ceramics fail in “
“brittle
brittle”
” manner with little or no
manner with little or no
plastic deformation.
plastic deformation.
Fracture strength of ceramics are significantly lower than predi
Fracture strength of ceramics are significantly lower than predicted by
cted by
theory because of the presence of very small cracks in the mater
theory because of the presence of very small cracks in the material (stress
ial (stress
concentrators).
concentrators).
Lack of ductility in ceramics is due to their strong ionic and c
Lack of ductility in ceramics is due to their strong ionic and covalent bonds.
ovalent bonds.
Mechanical Properties of Ceramics
Mechanical Properties of Ceramics
Ceramics have excellent compressive strength (used in cement and
Ceramics have excellent compressive strength (used in cement and
concrete in foundations for structures and equipment)
concrete in foundations for structures and equipment)
The principles source of fracture in ceramics is surface cracks,
The principles source of fracture in ceramics is surface cracks, porosity,
porosity,
inclusions and large grains produced during processing.
inclusions and large grains produced during processing.
Testing ceramics using the usual tensile testing is not possible
Testing ceramics using the usual tensile testing is not possible, so a
, so a
transverse bending test is used and a
transverse bending test is used and a modulus of rupture
modulus of rupture (MOR) is
(MOR) is
determined.
determined.
Strength of ceramics can only be described by statistical method
Strength of ceramics can only be described by statistical methods and it is
s and it is
dependent on specimen size.
dependent on specimen size.
Flexural
strength, σfs= 2
2
3
bd
L
Ff
Rectangular cross section
= 3
3
R
L
Ff
π
Circular cross section
Material Symbol
Transverse
rupture
strength
(MPa)
Compressive
strength
(MPa)
Elastic
modulus
(GPa)
Hardness
(HK)
Poisson’s
ratio (n)
Density
(kg/m
3
)
Aluminum
oxide
Al2O3 140–240 1000–2900 310–410 2000–3000 0.26 4000–4500
Cubic boron
nitride
CBN 725 7000 850 4000–5000 — 3480
Diamond — 1400 7000 830–1000 7000–8000 — 3500
Silica, fused SiO2 — 1300 70 550 0.25 —
Silicon
carbide
SiC 100–750 700–3500 240–480 2100–3000 0.14 3100
Silicon
nitride
Si3 N4 480–600 — 300–310 2000–2500 0.24 3300
Titanium
carbide
TiC 1400–1900 3100–3850 310–410 1800–3200 — 5500–5800
Tungsten
carbide
WC 1030–2600 4100–5900 520–700 1800–2400 — 10,000–15,000
Partially
stabilized
zirconia
PSZ 620 — 200 1100 0.30 5800
The properties vary widely depending on the condition of the material (crack size)

9. Failure of ceramics occurs mainly from
Failure of ceramics occurs mainly from
structural defects; surface cracks, porosity,
structural defects; surface cracks, porosity,
inclusions and large grains during processing.
inclusions and large grains during processing.
Porosity in ceramics acts as stress
Porosity in ceramics acts as stress
concentrators: crack forms and propagates
concentrators: crack forms and propagates
leading to failure.
leading to failure.
Once cracks start to propagate, they will
Once cracks start to propagate, they will
continue to grow until fracture occurs.
continue to grow until fracture occurs.
Porosity also decrease the cross
Porosity also decrease the cross-
-sectional
sectional
area over which a load in applied: lower the
area over which a load in applied: lower the
stress a material can support.
stress a material can support.
Factors Affecting Strength of Ceramics
Factors Affecting Strength of Ceramics
Strength of ceramics is thus determined by many factors:
Strength of ceramics is thus determined by many factors:
1.
1. Chemical composition
Chemical composition
2.
2. Microstructure
Microstructure -
- In dense ceramics materials, no large pores, the flaw is
In dense ceramics materials, no large pores, the flaw is
related to grain size. Finer grain size ceramics, smaller flaws
related to grain size. Finer grain size ceramics, smaller flaws size at the
size at the
boundaries, hence stronger than large grain size.
boundaries, hence stronger than large grain size.
3.
3. Surface condition
Surface condition
4.
4. Temperature and environment (failure at RT, usually due to larg
Temperature and environment (failure at RT, usually due to large flaws).
e flaws).
Toughening Mechanisms of Ceramics
Toughening Mechanisms of Ceramics
Fracture strength or toughness of ceramics can be improved only
Fracture strength or toughness of ceramics can be improved only by
by
mechanisms that influence the crack propagation (ceramics always
mechanisms that influence the crack propagation (ceramics always
contain cracks).
contain cracks).
There are various methods used to improve the toughness of
There are various methods used to improve the toughness of
ceramics:
ceramics:
1.
1. Transformation toughening
Transformation toughening
2.
2. Microcrack induced toughening
Microcrack induced toughening
3.
3. Crack deflection
Crack deflection
4.
4. Crack bridging
Crack bridging
1.
1. Transformation Toughening: e.g.
Transformation Toughening: e.g. Partially Stabilised Zirconia (PSZ)
Partially Stabilised Zirconia (PSZ)
Zirconia (ZrO
Zirconia (ZrO2
2) exists on 3 different crystal structures:
) exists on 3 different crystal structures:
Melt Cubic Tetrag
Melt Cubic Tetragonal Monoclinic
onal Monoclinic
Transformation toughening is achieved by stabilising the tetrago
Transformation toughening is achieved by stabilising the tetragonal
nal
structure at room temperature by adding other oxides such as: Mg
structure at room temperature by adding other oxides such as: MgO, CaO,
O, CaO,
and Y
and Y2
2O
O3
3 to zirconia.
to zirconia.
If cubic ZrO
If cubic ZrO2
2 is stabilised, so it retains cubic structure at RT called
is stabilised, so it retains cubic structure at RT called fully
fully
stabilised zirconia.
stabilised zirconia.
If tetragonal ZrO
If tetragonal ZrO2
2 is stabilised, it called as PSZ.
is stabilised, it called as PSZ.
Mixture of ZrO
Mixture of ZrO2
2-
-9 mol %MgO is sintered at 1800
9 mol %MgO is sintered at 1800o
oC, then rapidly cooled to
C, then rapidly cooled to
RT become metastable cubic structure. The materials is reheated
RT become metastable cubic structure. The materials is reheated at
at
1400
1400o
oC for sufficient time, a fine metastable precipitate with tetrag
C for sufficient time, a fine metastable precipitate with tetragonal
onal
structure known as PSZ formed.
structure known as PSZ formed.
As the crack propagates, it creates a local stress field that in
As the crack propagates, it creates a local stress field that induces
duces
transformation of the tetragonal structure to the monolithic (or
transformation of the tetragonal structure to the monolithic (or monoclinic)
monoclinic)
structure in that region.
structure in that region.
1150
1150 o
o
C
C
2370
2370 o
o
C
C
2680
2680 o
o
C
C
•
• This transformation is accompanied by a volume expansion, causin
This transformation is accompanied by a volume expansion, causing a
g a
compressive stress locally and in turn a
compressive stress locally and in turn a squeezing effect on the crack and
squeezing effect on the crack and
enhancing the fracture toughness also
enhancing the fracture toughness also significantly extends the reliability
and lifetime of products made with stabilized zirconia.
Precipitate around
crack is monoclinic
ZrO2-MgO
Matrix is cubic ZrO2-
MgO
Precipitate is
tetragonal ZrO2-MgO

10. Single crystals of the cubic phase of
zirconia are commonly used as diamond
simulant in jewelery.
The cubic phase of zirconia also has a very
low thermal conductivity, which has led to
its use as a thermal barrier coating or TBC
in jet and diesel engines to allow operation
at higher temperatures.
Stabilized zirconia is used in oxygen
sensors and fuel cell membranes because
it has the ability to allow oxygen ions to
move freely through the crystal structure at
high temperatures.
2.
2. Micro
Micro-
-crack Induced Toughening:
crack Induced Toughening:
Microcracks are purposely introduced by internal stresses during
Microcracks are purposely introduced by internal stresses during
processing of the ceramics tend to blunt the tip of the propagat
processing of the ceramics tend to blunt the tip of the propagating crack and
ing crack and
thus reduce the stress concentration at the crack tip.
thus reduce the stress concentration at the crack tip.
This micro
This micro-
-crack will interfere the crack tip propagation.
crack will interfere the crack tip propagation.
3.
3. Crack Deflection and Crack Bridging
Crack Deflection and Crack Bridging
This is achieved by reinforcing the ceramics: produce ceramic ba
This is achieved by reinforcing the ceramics: produce ceramic based
sed
composites (CMC)
composites (CMC)
• The high hardness of some ceramic materials makes them
useful as abrasive materials for cutting, grinding, polishing
e.g. Al2O3 and SiC, diamonds
Ceramic as abrasive materials
• MEMS – mechanical devices that integrated with large
number of electrical elements on a substrate of Silicon –
e.g. for microsensors
• Current research on ceramic materials to replace silicon,
because ceramic are tougher, more refractory and more
inert e.g. silicon carbonitrides (silicon carbide-silicon
nitrides alloys)
Advanced ceramics

11. Properties of Glasses
Properties of Glasses
Glasses posses properties not found in other engineering materia
Glasses posses properties not found in other engineering materials.
ls.
Combination of transparency, ability to transfer light, hardness
Combination of transparency, ability to transfer light, hardness at room
at room
temperature, a sufficient strength and corrosion resistance to m
temperature, a sufficient strength and corrosion resistance to most
ost
environments. These make glasses important for many applications
environments. These make glasses important for many applications: vehicle
: vehicle
glazing, lamps, electronic industry, laboratory apparatus.
glazing, lamps, electronic industry, laboratory apparatus.
Deformation of glass varies with temperature:
Deformation of glass varies with temperature:
At high temperatures: viscous flow
At high temperatures: viscous flow
At low temperatures: elastic and brittle
At low temperatures: elastic and brittle
At intermediate temperatures: visco
At intermediate temperatures: visco-
-elastic
elastic
Heat Treatment of Glasses
Heat Treatment of Glasses
•
• Glasses can be rendered more fracture resistance by introducing
Glasses can be rendered more fracture resistance by introducing compressive
compressive
stresses on the glass surface. This is followed by glass temperi
stresses on the glass surface. This is followed by glass tempering
ng
1.
1. Glass Annealing
Glass Annealing
Used to reduce internal residual stresses, which weaken the glas
Used to reduce internal residual stresses, which weaken the glass and may lead to
s and may lead to
fracture.
fracture.
The glass is heated to the annealing temperature, then slowly co
The glass is heated to the annealing temperature, then slowly cooled to RT
oled to RT
2.
2. Glass Tempering
Glass Tempering
Used to strengthen glass by inducing compressive stresses at the
Used to strengthen glass by inducing compressive stresses at the surface.
surface.
Tempering is achieved by heating the glass to a temperature T
Tempering is achieved by heating the glass to a temperature Tg
g, then rapidly
, then rapidly
cooled to room temperature.
cooled to room temperature.
The surface of the glass cools first and contracts; later the ce
The surface of the glass cools first and contracts; later the centre cools and attempts
ntre cools and attempts
to contract but is prevented from doing so by the rigid and stro
to contract but is prevented from doing so by the rigid and strong surface.
ng surface.
This produces high tensile stresses in the centre but compressiv
This produces high tensile stresses in the centre but compressive stresses at the
e stresses at the
surface.
surface.
This tempering treatment increases the strength of the glass bec
This tempering treatment increases the strength of the glass because applied tensile
ause applied tensile
stresses must surpass the compressive stresses on surface before
stresses must surpass the compressive stresses on surface before fracture occurs.
fracture occurs.
Tempered glass has higher impact resistance than annealed glass
Tempered glass has higher impact resistance than annealed glass and about 4x
and about 4x
stronger than annealed glass.
stronger than annealed glass.