Knives and Steel 1
Knives and Steel 2
Observations about Knives and
Steel
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Knives and Steel
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Some knives can’t keep their cutting edges
Some knives bend while others break
Making good knives involves heat treatment
Some steel is stainless and doesn’t rust
Some stainless steel is poorly suited to knives
Turn off all electronic devices
Knives and Steel 3
4 Questions about Knives and
Steel
1.
2.
3.
4.
Why do some knives bend and others break?
Why does a good knife require heat treatment?
Why is some stainless steel unsuitable for knives?
Why are some good knives made of alloy steels?
Knives and Steel 4
Question 1
Q: Why do some knives bend and others break?
A: Their steel respond differently to stress.

Iron and soft steels contain ferrite crystals
y
(bcc)
( )
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Hardened steels contain martensite crystals
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Knives and Steel 5
Layers of atoms can slip across one another
These lowlow-strength materials bend when overstressed
Distorted crystals are resistant to slip
These high
high--strength steels break when overstressed.
Knives and Steel 6
Question 2
Question 3
Q: Why does a good knife require heat treatment?
A: Hardening steel often involves thermal effects.
Q: Why is some stainless steel unsuitable for knives?
A: Those steels are austenitic at room temperature.

Hot iron and steel contain austenite crystals
y
(fcc))
(fcc
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Carbon is much more soluble in austenite than ferrite
 Heating and cooling redistributes the carbon in steel
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Slow cooling lets carbon precipitate → ferrite
Fast cooling traps carbon → martinsite
Stainless steels contain chromium and nickel
Together, these additions stabilize austenite
Basic “18
“18--8” stainless is austenitic at room temp
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Cannot be hardened by carbon and heat treatment
Austenitic iron, steel, or stainless steel is nonnon-magnetic
Special stainless can be martensitic at room temp
1
Knives and Steel 7
Knives and Steel 8
Question 4
Summary about Knives and Steel
Q: Why are some good knives made of alloy steels?
A: Alloying can produce hard, rustrust-proof stainless

Martensitic stainless isn’t completely
p
y rustrust-proof
p
Alloying can precipitationprecipitation-harden rustrust-proof steel
Deforming the steel can workwork-harden the steel
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Knives and Steel 9
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Slip in its crystals allows steel to deform
Limiting slip hardens the steel
Ordinary steel is hardened by carbon and heat
Ordinary stainless steel is rustrust-proof but soft
Alloy stainless steel are hard and rustrust-proof
Knives and Steel 10
Observations about Windows and
Glass
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Windows and Glass
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Glass is made from sand somehow
Glass is typically transparent
Glass can be formed into any shape
Molten glass is a viscous liquid
As molten glass cools, it thickens into a solid
Broken glass has sharp, irregular edges
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Knives and Steel 11
5 Questions about Windows and
Glass
1.
2.
3.
4.
5.
Why does molten sand freeze into glass?
Why isn’t ordinary glass made only from sand?
How is flat window glass made?
How is cooking glass different from window glass?
Why does tempered glass break into tiny pieces?
Knives and Steel 12
Question 1
Q: Why does molten sand freeze into glass?
A: Crystallization of molten quartz takes too long.

Quartz sand (silicon
Q
(
dioxide)) is a network former

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Silicon and oxygen atoms attach via covalent bonds
Liquid silica freezes into a disordered, glassy solid
2
Knives and Steel 13
Knives and Steel 14
Question 2
Q: Why isn’t ordinary glass made only from sand?
A: Quartz melts at too high a temperature.
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Pure qquartz melts at 1723 °C and is ultraviscous
Soda (sodium oxide) and lime (calcium oxide)
act as fluxes, lowering the melting temperature
 weaken the network and lower its viscosity
 Soda
Soda--limelime-silica glass is more practical than quartz glass
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Knives and Steel 15
Question 3
Q: How is flat window glass made?
A: Liquid glass solidifies on a pool of molten tin.
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Glass is less dense than liquid
q tin
Glass and tin don’t mix
Tin is liquid over a very broad temperature range
Liquid glass can solidify on liquid tin
Knives and Steel 16
Question 4
Q: How is cooking glass different?
A: Cooking glass exhibits less thermal expansion

Cookingg glass
g
contains boron oxide
Question 5
Q: Why does tempered glass break into tiny pieces?
A: It’s core is under tension and can shred itself

Borosilicate glass expands less with temperature
 It is less likely to break during temperature changes

Chemically resistant glass contains aluminum oxide
“Crystal glass” contains lead oxide
 X-ray absorbing glass contains barium oxide
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Knives and Steel 17
Summary about Windows and
Glass
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Specialty glasses include other oxides

Glasses are made from network formers
Network formers can freeze into glassy solids
Practical glasses are often mixtures of oxides
Flat window glass is made by floating it on tin
Tempered
p
gglass has a compressed
p
surface layer
y
Glass breaks by tearing
A compressed surface layer is hard to tear
 Tempered glass is very difficult to break
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The core of tempered glass is under tension

When tempered glass breaks, its core shreds itself
Knives and Steel 18
Plastics
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Knives and Steel 19
Knives and Steel 20
Observations about Plastics
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They can take almost any shape
They can be clear, translucent, or opaque
They can tear or shatter
They can be hard, soft, elastic, fiberous
They can form by mixing chemicals
They can form by evaporating solvents
Knives and Steel 21
6 Questions about Plastics
1.
2.
3.
4.
5.
6.
How do plastics differ from ordinary molecules?
How does temperature affect plastics?
Why are some plastics clear, others translucent?
Why are some plastics unable to melt?
How do plastics form from simpler chemicals?
Why are some plastics so strong?
Knives and Steel 22
Question 1
Question 2
Q: How do plastics differ from ordinary molecules?
A: Plastics consist of giant molecules
Q: How does temperature affect plastics?
A: Thermal energy allows local and distant mobility.

Plastic molecules are enormous
Many are long linear chains
 Others are branched or networked
 They can become entangled
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Knives and Steel 23
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Plastics can exhibit five distinct mobilityy regimes
g
With increasing temperature, plastics go through:
Glassy solid: not even local mobility
Leathery solid: some local mobility
 Elastic solid: local mobility, but not long
long--range mobility
 Rubbery flow: some long
long--range mobility
 Liquid flow: extensive longlong-range mobility
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Knives and Steel 24
Local Mobility
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Local mobility is governed by molecular adhesion
Some plastic molecules cling together tightly
Acrylic plastics (Plexiglas, Lucite)
Polystyrene
P l
(Styrofoam,
(S
f
plastic
l i cups))
 PET and PETE (Mylar, soda bottles, plastic cups)
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Long--Range Mobility
Long
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Long-range mobility is governed by reptation
LongThermal energy causes chain motion
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Others cling weakly
Polyethylenes (milk jugs, grocery store bags)
 Natural rubber
 Silicones
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Some plastics stay tangled
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Chain motion is called reptation
R
Reptation
i allows
ll
chains
h i to di
disentangle
l themselves
h
l
Polyethylenes (jugs, bags)
Other plastics disentangle
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Chicle (chewing gum)
Silicones
4
Knives and Steel 25
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Knives and Steel 26
Plasticizers
Question 3
Plastics can be softened by chemical plasticizers
Q: Why are some plastics clear, others translucent?
A: Some are partly crystalline, others all amorphous
small molecules that are compatible with the plastics
 go into solution in the plastics (or vice versa)
 increase
in r
llocall and
nd llonglong
n -range
r n mobilities
m biliti
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Examples of plasticized plastics:
Solvent-based glues and paints
SolventWet hair, fabrics, paper, noodles, bread
 Vinyl upholstery fabrics
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Knives and Steel 27
Some p
plastics are all amorphous
p
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They are homogenous throughout
Light is undisturbed; they’re clear
Other plastics are partly crystalline
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They are inhomogenous
Light scatters at boundaries; translucent
Knives and Steel 28
Question 4
Question 5
Q: Why are some plastics unable to melt?
A: Their molecules are crosslinked in one network
Q: How do plastics form from simpler chemicals?
A: Molecular chain reactions assemble them.

Crosslinks tack p
polymer
y
chains to one another
Reptation cannot disconnect or disentangle them
 They remain in the elastic regime
 They can’t flow, so they don’t melt
 They are “thermosets
“thermosets”” (set shapes at all temperatures)
 Meltable plastics are “thermoplastics” (variable shapes)
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Knives and Steel 29
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Q: Why are some plastics so strong?
A: If all the molecules work together, they’re strong
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Aligning
g g polymer
p y
chains into fiber gives
g
strength
g
Organizing those chains can yield extreme strength
Liquid crystal fibers are naturally organized
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Monomers are small building block molecules
Monomers bind together in chains to form polymers
Plastics can have one monomer or several
Plastics can be linear or branched
Plastics can be orderly or more complicated
Knives and Steel 30
Question 6
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Most p
plastics begin
g as monomer molecules
Summary about Plastics
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Plastics consist of giant molecules
Temperature affects local and longlong-range mobility
Entanglements limit longlong-range mobility
Crosslinks can prevent longlong-range mobility
Aramids,, Kevlar
Aramids
Melt--drawn fibers are organized during formation
Melt
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Spectra
5