5 LOOKING INSIDE MATERIALS
Determining atomic and molecular dimensions
o Explain how an STM,
AFM and SEM work
o Determine resolution,
magnification and
atomic dimensions from
microscope data
o Estimate molecular size
from experimental data
•
Cotton wool
SEM 150x
Space shuttle tile
SEM 2000x
Looking inside glasses
1m
Looking inside woods
1m
human scale
cellular structure
human scale
1 mm
1 mm
strength
stiffness
thermal
cracks
cell wall fibre composite
fracture strength
1 m
1 m
flow
strength
stiffness
thermal
1 nm
1 nm
stiffness
thermal conduction
random atomic packing
cellulose fibres
strength
stiffness
1 pm
1 pm
semiconduction
optical
Si
thermal
Si
atoms
cellulose molecules
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
Looking inside polymers
Looking inside metals and ceramics
1m
1m
human scale
human scale
XX
XX
XX X
X
craze
1 mm
1 mm
fracture
strength
(ceramics)
strength
crack
precipitates
arrays of dislocations
1 m
1 m
flow
yield strength
(metals)
alloying element
1 nm
1 nm
tangled molecules
H
C
dislocation
H
C
atoms and electrons
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
C
H
H
H
C
H
C
C
H
H
H
H
H
H
C
C
C
H
H
H
molecules
1 pm
stiffness
electrical
optical
thermal
C
H
stiffness
thermal
optical
yield strength
(metals)
H
C
electrical
C
C
atoms
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
1 pm
World’s smallest advertisement: STM of xenon atoms
STM of iron on copper
STM of iron on copper: “The atomic corral”
Making the corral
STM of metal surface showing instrumentally-induced distortion of atom shapes
AFM
AFM image of gold 111
AFM of rhodium screw dislocations
Say hallo to “carbon monoxide man”
(STM image)
AFM of DNA strand
SEM
SEM of fruit fly head.
Be afraid........... Be very afraid..........
SEM of solar spider
Will he catch the fruit fly?
SEM of ant
SEM of snowflake
Fracture behaviour
o Learn how to calculate
fracture energy
o Distinguish between
strength and toughness
in terms of fracture
behaviour of materials
o Explain why metals are
tough
Energy stored in stretched material
Energy stored = area under
graph
= ½xFxe
= ½ x (k x e) x e
= ½ x k x e2
Fracture energy and tensile strength
energy to create
voids in material
energy to create
new surface area
energy of sound
in material
tensile
force
tensile
force
cross-sectional
area
energy to move atoms
energy of increased
around (e.g. slip)
lattice vibration
energy of
flying fragments
fracture energy =
tensile strength =
total energy used to fracture
specimen cross-sectional area
breaking force
specimen cross-sectional area
Large fracture energy = tough
Large tensile strength = strong
Strong and tough
STRONG
strong
brittle
1000
high tensile
steel
strong
tough
mild
steel
glass
bone
nylon
wood
100
BRITTLE
TOUGH
epoxy resin
polyester
10
weak
brittle
weak
tough
brick stone
1
1
10
100
10 000
1000
–2
fracture energy / J m
WEAK
100 000
Cracks and stress
h
Material is bent,
upper surface
stretched
lower surface
compressed.
no cracks
Contours of
equal tensile
stress. Largest
stress near
surface.
with a crack
Crack deflects
tensile stress.
Stress
concentrated
below crack.
very small area
at tip of crack:
stress very
large here
bond between atoms
at tip highly stressed
bond breaks
next bond is stressed
Under tensile stress, cracks propagate through materials.
Fracture surfaces in metals
Which shows ductile fracture,
and which shows brittle
fracture?
Fracture of CFRP in a tennis racquet
Metals are tough because they are ductile
Stopping cracks propagating
Metals
ductile metal flows, crack blunted
high stress
stress
stress reduced
stress
stress
stress
Metals resist cracking because they are ductile. Cracks are broadened and blunted. They do not propagate.
Composite materials
• Know the meaning of the
term composite material
• For a range of composite
materials (ferroconcrete,
bone, CFRP etc.), explain
how creating a
composite can improve
on the properties of the
individual components
Fibre-reinforced materials are tough because cracks can't propagate through the soft matrix
Fibre-reinforcement
stress
stress
strong fibre
soft matrix
sticks to fibres
stress
stress
one fibre breaks, stress taken
up by other fibres
Fibre-reinforced materials use a matrix to share stress amongst many strong fibres. The matrix also protects the fibres
from cracks forming.
Composite materials
• Investigate properties of
composite materials
based on ice and/or jelly
Metal microstructures
• Recognise the
various atomic-level
structural features
present in metals
Grain boundaries
A dislocation: an
incomplete row
of atoms
Metal microstructures
• Explain the effects that
micro structural features
have on the properties of
metals
Shaping and slipping
Atoms in gold are in a regular array: a crystal lattice. To shape the metal, one
layer must be made to slide over another.
dislocation
To slip, layer of
atoms must
move as a whole
Atoms can
move one
by one
All atoms move:
layer moves
One atom
moves:
dislocation
moves
atom
moves
dislocation
moves
Layer has
moved one
atomic spacing
in both a layer has slipped by one atomic spacing
Dislocation
reaches
edge of
crystal
Pure crystal
Alloy
dislocation pinned
dislocation
dislocation free to move: slip occurs easily
alloy atom pins dislocation: slip is more difficult
Alloys are generally less ductile than pure metals
Questions on modifying the properties of metals
1. Draw diagrams to illustrate the following:
(a) the pinning of a dislocation by a foreign atom
(b) a large substitutional impurity atom in a crystal
(c) an interstitial atom
2. What common effect(s) on the metal’s properties do all of the
modifications described in Q1 have?
3. How can excessive work hardening of a metal be reduced?
4. A metal contains large crystal grains. How could you change the
crystal grain size to create smaller grains?
5. Now try Questions 70X from Folio Views
Heat treatment of steel
• Investigate and explain
how various heat
treatments of steel can
affect its properties
Stiffness and elasticity
• Explain stiffness and
elasticity in metals,
ceramics and polymers
Ceramics versus metals
Ceramics have rigid structures
Covalent structures
example: silica (also diamond, carborundum)
oxygen atom
Covalent bonds share electrons between neighbour
atoms. These bonds are directional: they lock atoms
in place, like scaffolding.
The bonds are strong: silica is stiff
The atoms cannot slip: silica is hard and brittle
joins to others
Atoms are linked in a rigid giant structure
silicon atom
Metals have non-directional bonds
Metallic structures
example: gold
negative
electron 'glue'
Atoms in metals are ionised. The free electrons
move between the ions. The negative charge of the
electrons 'glues' the ions together. But the ions can
easily change places.
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The bonds are strong: metals are stiff
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The ions can slip: metals are ductile and tough
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Ions are held together, but can move
gold ion
Ionic structures
example: common salt
Ionic bonds pass electrons from one atom to another.
Because like charges repel and unlike charges attract,
the charged ions hold each other in place.
–
+
–
chlorine ion
+
–
+
sodium ion
The bonds are strong: salt crystals are stiff
–
+
–
The ions cannot slip: salt crystals are hard and brittle
Ions are linked in a rigid giant structure
Summary
Ceramics covalent or ionic
Metals
strong, rigid scaffolding,
stiff, hard, brittle
mobile strong electron glue,
stiff, ductile, tough
Explaining stiffness and elasticity
Metals
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stretching has to pull bonds apart
a metal is an array of
positive ions bonded
by negative electron
'glue'
gaps open up a little
Elastic extensibility ~ 0.1%
Stretching a metal stretches bonds — but not much.
Young modulus
~1011 — 1012 Pa
Explaining stiffness and elasticity
Polythene
bond
rotates
chains are
folded
bond
rotates
polythene is a
long flexible
chain molecule
which folds up
stretching can rotate some bonds,
making the folded chain longer
Elastic extensibility ~ 1%
Stretching polythene rotates bonds
Young modulus
~108 — 109 Pa
Explaining plasticity
Polythene
‘neck’
polythene strip 10 mm  100 mm
crystalline
amorphous
thin crystalline strip ‘pulled out
of’ wider region
new crystalline
region
Polythene is semi-crystalline. Think of polythene
When stretched plastically, the chains slip past
as like cooked spaghetti. In amorphous regions
each other. More of the material has lined-up
the chains fold randomly. In crystalline regions
chains. More of it is crystalline.
the chains line up.
Plastic extensibility > 100%
Rubber
sulphur cross-links
sulphur cross-links
In unstretched rubber, chains meander randomly
between sulphur cross-links.
In stretched rubber, the chain bonds rotate, and chains
follow straighter paths between cross-links. When let go,
the chains fold up again and the rubber contracts.
Elastic extensibility > 100%
Rubber stretches and contracts by chains uncoiling and coiling up again. Rubber is elastic, not plastic.
Comparisons of materials of different
classes(metals, ceramics, polymers)
See p112-3
Give an example of a material with
(a) giant covalent structure;
(b) an ionic structure;
(c) metallic structure
Explain why ceramics, salts and metals are all stiff, but
only metals are ductile and tough
Why are polymers generally much less stiff than metals?
How can some polymers be made stiffer?
Why does rubber get stiffer the more it is stretched?
Electrical conductivity
• Investigate and explain
the temperature
dependence of the
conductivity in metals,
semiconductors and
insulators
Conduction by metals and semiconductors
8
10
4
pure metal
alloy
logarithmic
scale
10
1
silicon
–4
10
•
•
copper.swf
nichrome.swf
–8
10
200
400
600
temperature / K
800
Metal and alloy
10
10
10
pure metal
linear
scale
8
pure metal
4
alloy
logarithmic
scale
1
10
10
silicon
–4
–3
200
5
400
600
800
temperature / K
alloy
0
200
400
600
800
temperature / K
Observation Metals conduct very well.
Explanation All the atoms in the metal are ionised. The ‘spare’ electrons are
free to move.
all atoms ionised
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‘soup’ of free
electrons
Observation The conductivity of a metal decreases a little as it gets warmer.
Explanation No more electrons become free to move. Moving electrons scatter
from the vibrating lattice – so move a little less freely as the temperature rises
and lattice vibrations increase (impurities and defects also reduce the conductivity).
Silicon
(part of temperature range only)
10
10
10
8
pure metal
4
alloy
logarithmic
scale
1
linear
scale
10
10
silicon
–4
–8
200
5
400
600
800
temperature / K
silicon
0
300
350
temperature / K
Observation Seminconductors conduct much less well than metals, much
more than insulators.
Explanation Only a few (1 in 1012) atoms are ionised. There are only these few
electrons free to move.
rare free
electrons
+
occasional
atoms ionised
+
Observation The conductivity of a pure semiconductor increases dramatically
as it gets warmer.
Explanation At higher temperatures, more atoms become ionised. The
conductivity increases because there are more charge carriers free to move.
Effects of extra lattice vibrations are much smaller.