Transcript Ceramics (Chapter 11 – reference)

*See notes at bottom
Chapter 11 - Ceramics:
Inorganic, non-metallic compounds formed by heat. Examples:
Ball Bearing: Silicone Nitride, Si3N4
or Alumina Oxide or Zirconia
Porcelain high voltage
insulator
Rocket Nozzle: Silicone Nitride,
Si3N4
Knife: Zirconium dioxide,
ZrO2
Dental
Disk brake, silicone
carbide
Ceramic inserts,
cutting tools,
tungsten carbide
See HO for Common
Types
III. Types of Ceramics:
I. Overview of ceramics:
Characterized by:
1. Compounds between metallic and non-metallic elements (i.e. Si and O, Si
and N, Al and O, etc..)
2. Frequently oxides, nitrides and carbides (i.e. silicon carbide – SpinWorks)
3. Can be crystalline or amorphous
4. Very strong covalent (sharing of electrons) or ionic (transfer of electrons)
bonds.
5. Properties include:
•
Strong but brittle
•
Low fracture toughness
•
Good insulators of electricity BUT good conductor of heat (i.e.
comparable to metals have reasonably high thermal conductivity, k) –
this is unique to ceramics.
•
Excellent high temp properties
•
Low coef of thermal expansion
I. Nature of ceramics:
Example: Aluminum Oxide, Al2O3
Covalent = strong
Electrons tied up
II. Properties of Ceramics
• Benefits:
– High chemical resistance
– High melting point and therefore high operating T.
– Extremely hard and stiff (i.e. 180 E6 psi)
– Good electrical insulator (electrons tied up).
Exception = superconductors
– Good thermal conductor (high K like metals)
– But, low thermal expansion and good thermal stability
– Good creep
– High modulus
– High compressive strength
II. Properties of Ceramics
•
Shortcomings:
– Low tensile strength and BRITTLE. Sut = Suc/10. Do not readily slip like metals
but fracture.
– Catastrophic failure
•
The bond is ionic and covalent. A material held by either type of bond will tend to fracture before any
plastic deformation can occur!
– Low fracture toughness (1/10 to 1/100 KIC compared to metals)
•
Materials tend to be porous and microscopic imperfections act as stress concentration decreasing the
toughness further.
– Elongation = 0%
– Low fatigue strength
– Large statistical spread and less predictable than metals (size, shape and
location of internal flaws is likely to differ from part to part)
– Prone to thermal shock
– Hard to machine and form
– Cost 8X more than metals
– See Table 8.2
Optimized selection using charts
Search
area
1
E M
ρ
2
3
Results
E1/2
ρ M
E1/3  M
ρ
22 pass
Material 1
Material 2
Material 3
etc...
Ranked
by Index E1/2 /ρ
2230
2100
1950
Electrical
Resistance:
Ceramics =
good electrical
insulators, but…
Selection: one-property indices
Good thermal conductors!
Good conductors:
metals and ceramics
Good insulators:
polymer foams, cork,
wood, cardboard….
III. Types of Ceramics:
(see HO 6 – 10):
a) Structural (clay) and whiteware: bricks, pipes, floor
and roof tiles, dinner ware, chinca, etc…
b) Refractory ceramics: kiln lingings, high T capability
most are based on silicates (sand)
c) Glass, amorphous ceramic, most based on silica,
SiO2
– Annealed glass
Go to HO pg 6!!
– Tempered glass
– Laminated glass
III. Types of Ceramics:
d)
(see HO 6 – 10):
Technical or engineering ceramics – 3 categories:
i. Oxides (alumina, zirconia) semiconductors
ii. Non-oxide (carbides, borides, nitrides) i.e. tungsten
carbide cutting tools, silicone nitride ball bearings,
silicone carbide furnace inserts.
iii. Cermits or composites – combination of metals and
ceramics (powder metallurgy), combines high
strength and hardness, thermal characteristic of
ceramics with toughness of metals)
V. How to Strengthen:
(see HO 4,5):
a) Flame polishing to reduce surface cracks
b) Close surface cracks use in compression
(or compress with metal band)
c) Atom gun to fill in surface cracks (fires
ions into cracks)
d) Reduce crystal size
e) Laminate or anneal (glass)
f) Combine materials to increase toughness
(cermits)
V. Manufacturing – see HO
• In situ – cement – mix
powder with water
• Sintering based
methods
WATCH!!!!
http://www.youtube.com/watch?v=69Y0VuOYqkU
http://ceramicinjectionmolding.com/