D 24: C

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29 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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D
AY

24: C
ERAMICS


Design Concepts Review


The Concept of Thermal Shock Resistance


Review of Selected Engineering Ceramics


Glass

Q
UICK

R
EVIEW

OF

D
ESIGN

C
ONCEPTS


Design to a minimum safe value is not possible!


We must design to an accepted probability of
failure.


Weibull statistics is used instead of normal
statistics in fitting data and computing
probabilities.


The Weibull modulus indicates the extent to
which the strength data is scattered. Want it as
high as possible.


The size of the object being designed must be
accounted for. Designs that work for small
components won’t work for large.

T
HE

I
MPORTANCE

OF

F
RACTURE

M
ECHANICS


Recall that fracture mechanics allows predictions
of whether a flaw in a known material will be
stable.



Brittle materials are very prone to cracking and
fracture.


Hence fracture mechanics is incorporated into
the design of engineering ceramics.

Ic
K
a
Y
K




T
HERMAL

S
TRESS


What is thermal stress? Mismatched materials
and uniform heating.

D
T > 0

a
1

<
a
2

a
1

a
2

Material 1 in compression. Material 2 in tension. Think about
what would happen if there were a surface crack.

T
HERMAL

S
HOCK


Consider uniform material and nonuniform
temperature. I took something hot out of the
oven and poured cold water on it.

Hot, in compression

Cold, in tension

Gradient is actually
gradual.

What would happen
to a crack about
right here?

COULD FAIL SUDENLY!

T
HERMAL

S
HOCK

R
ESISTANCE


To have thermal shock resistance what’s needed.

1.
Strength High

2.
Modulus of Elasticity Low

3.
Coefficient of Linear Expansion Low

4.
Thermal Conductivity High


T
WO

F
ORMULAS

F
OR

T
HERMAL

S
HCOK

R
ESISTANCE


Here’s the one in the notes.



1
f
R
E
 
a


Here’s another one that is sometimes used.

E
k
R
f
a


V
ALUES

OF

R


From Notes:

R
EVIEW

OF

THE

M
AJOR

F
AMILIES
:

A
LUMINA


Lowest cost Cheapest

Most Commonly used


excellent environmental resistance, room
temperature strength. Not really a high
temperature material


Alumina ceramic is the most widely used
material out of a variety of fine ceramics.This
material is applied to widely diversified
industrial field for its superb material
characteristics such as high electrical insulation,
high mechanical strength, high wear and
chemical resistance.


Uses: seals, liners, substrates for electronics,
electrical insulators, body armor plates.


R
EVIEW

OF

THE

M
AJOR

F
AMILIES
:

Z
IRCONIA


Toughest


Highest Coefficient of Thermal Expansion Close
to Steel


Best Low Temperature strength


Not really a high temperature material


Zirconia ceramic has high mechanical strength
and toughness at room temperature out of a
series of engineering fine ceramics. Zirconia was
the first material adopted to fine ceramic scissors
or knife application. Its excellent surface
smoothness has brought this material into parts
for pump products.

W
HY

IS

Z
IRCONIA

T
OUGH
?


Zirconia contains material that experiences a
martensite type transformation


when it sees
high enough stress.


Such as just in front of a growing microcrack.


The transformed material expands, and is
compressed by the unexpanded untressed
material in the vicinity.


The compression blunts the crack making it
harder for it to expand. Thereby toughening the
material.

R
EVIEW

OF

THE

M
AJOR

F
AMILIES
:

S
ILICON

C
ARBIDE


Best Strength at Highest Temperatures


Best Thermal Conductivity


Best Corrosion Resistance


Silicon carbide maintains its high mechanical
strength up to as high temperature as 1,400 C.
Typical application is part for mechanical seal
ring and pump due to higher chemical corrosion
resistance than other ceramics.


Uses: seals, rings for harsh environments, also
for high temperatures.


R
EVIEW

OF

THE

M
AJOR

F
AMILIES
:

S
ILICON

N
ITRIDE


Best Thermal Shock Resistance


Lowest Coefficient of Thermal Expansion


Silicon nitride exceeds other materials in thermal
shock resistance. This material does not
deteriorate at high temperature, therefore it's
appropriate for automotive engine and parts for
gas turbine, including turbocharger rotor, glow
plug of diesel engine and hot plug. It is expected
that the field range this material can be applied
to will widely expand.


A
PPLICATIONS

G
LASS


Glass is a very important family of ceramic
materials. Common, cheap, very, very useful.


Plates, containers, optical cables, fibers, art!


Glass is often based on SiO
2
, the oxide of silicon.


If you add other oxides, such as Na
2
O, or Na
2
CO
3
,
sodium carbonate, you get a much lower melting
point. Other additives are CaO and MgO to make
it more chemically stable.


This is soda
-
lime glass which is about 90% of the
glass produced.


S
TRUCTURE

AND

P
ROPERTIES

OF

GLASS


Glass is amorphous. (We briefly mentioned
amorphous metals.) Amorphous means that
there is no discernable crystal structure. The
consituents form a 3D network with strong
bonds.


The name “glass” is applied to all such solids be
they metal, ceramic or polymer.


One consequence is the transparency which is
such a desirable feature in glass. (Due to time
limits we will not dwell on optical properties.)


Another consequence in terms of mechanical
behavior is brittleness.

G
LASS

IS

A
MORPHOUS

G
LASS

T
RANSITION

T
EMPERATURE
,
T
G

T

Specific Vol

T
g

Solidification of a
crystalline material


Supercooled liquid


Glass

Note: slope
change

M.P.

G
LASS

T
RANSITION



Used not just for ceramic glasses but also for
polymers.


T
EMPERING

G
LASS



Heat to above Tg.


Cool suddenly with blast of air or quench bath.


Different cooling rates lead to the following
stress pattern: compression on the surface and
tension in the interior. Outside cools first and
rigidifies, holding inner portion in tension.




Tension

Compression

G
LASS

A
RMOR

It is fundamentally made by layering a
polycarbonate substance between ordinary
glass pieces in a process of lamination.

http://www.technical
-
discovery.com/2009/04/nature
-
of
-
bulletproof
-
glass.html


In the new paper, Ortiz and her colleagues, including MIT Dean
of Engineering Subra Suresh, report that the shell of the hot
vent gasotropod has several features that help dissipate
mechanical energy from a potential penetrating predatory
attack. Of particular importance is its tri
-
layered shell structure,
which consists of an outer layer embedded with iron sulfide
granules, a thick organic middle layer, and a calcified inner
layer.

Most other snail shells have a calcified layer with a thin organic
coating on the outside. In the scaly foot gastropod, simulations
suggest that the relatively thick organic middle layer can absorb
much energy during a penetrating attack. It may also help to
dissipate heat and thermal fluctuations exhibited near
hydrothermal vents.

http://www.rdmag.com/News/2010/01/Materials
-
Soldiers
-
may
-
one
-
day
-
defend
-
with
-
sea
-
snail
-
armor/