ceramics part 1: structure and properties...ceramics: crystal structures ax–type crystal...
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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
31
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