Ceramics

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Ceramics
Thermal Coatings
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History and Background

• Applications date into antiquity - earthenware,
  pottery, clay product, bricks, etc
• More modern uses Transparent glass, structural
  glass, refractories
• Advanced uses Thermal barrier coatings,
  structural ceramics, composite armor,
  electronics, glass-ceramics
• Ceramics can be Amorphous or Crystalline
• Atomic structure contains strong Ionic Bonds

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What are they?

• A compound of metallic and nonmetallic elements,
  for which the inter atomic
• bonding is predominantly ionic.
• They tend to be oxides, carbides, etc of metallic
  elements.
• The mechanical properties are usually good high
  strength, especially at elevated temperature.
• However, they exhibit low to nil-ductility, and
  have low fracture toughness.

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Crystalline Ceramics

• As with plastics, the amorphous ceramics tend to
  be transparent
• The structural ceramics tend to be crystalline
  and show greater strength, as well as stability
  at high temperature

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AX Structure - CsCl

• Cl-

Note This is not a BCC structure.
Cs
Simple Cubic Crystal
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AX Structure - NaCl
2- FCC interpenetrating lattices.
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Try it!
Compute the theoretical density of Rock Salt
based on its crystal structure.
For this NaCl structure, the crystal lattice
parameter is a 2 ( r Na r Cl -), where r
is ionic radius.
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AX structure - Si C
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Summary of most common ceramic crystal structures
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Silicate Ceramics

• Silica, SiO2
• Many polymorphsQuartz CrystobaliteTridymite
• Low density Quartz 2.65g/cm3

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Crystalline Crystabolite
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Carbon

• Pure carbon has many polymorphs with vastly
  varying properties. It also exists in the
  amorphous state.
• Diamond Is similar to ZnS in structure
• Graphite is considered to be a crystalline
  ceramic
• Fullerenes, C60, are a newly discovered polymorph
  - with interesting properties.

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Diamond

• AX type crystal structure similar
• to that of ZnS.
• Each carbon atom is covalently bonded to four
  other C atoms in a diamond-cubic crystal
  structure.
• The material is optically transparent and
  extremely hard (hardest natural material known)
  and durable.
• In engineering applications, cruder or industrial
  forms of diamond, that are much less expensive
  than the gemstone forms, are used as abrasives,
  indentors, and coatings (especially thin films)
  for a variety of applications.

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Graphite

• Layers of hexagonally arranged and
• covalently bonded C atoms.
• Between layers, weaker Van der Walls
• bonds are active, giving easy slip
• on the 0001 crystallographic planes.
• Excellent as a dry lubricant, relatively high
  strength at elevated temperatures, high thermal
  and electrical conductivity, low thermal
  expansion, resistance to thermal shock, and good
  machinability.
• Usage electrodes, heating elements, crucibles,
  casting molds, rocket nozzles, and other
  applications.

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Fullerenes, C60

• Molecular form of carbon with a
• hollow spherical structure resembling
• a geodesic dome (soccer ball.)
• Called buckyballs after R. Buckminister
• Fuller, who pioneered the geodesic dome.
• Discovered in 1985 and have since been found to
  occur naturally in several sources.
• In the solid crystalline state, C60 molecules
  pack together in a FCC unit cell arrangement with
  a lattice parameter a1.41 nm.
• The pure solid material density is about 1.65
  g/cm3 and it is relatively soft and is
  non-conducting since it has no free electrons.

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Properties of Buckyballs

• When alkali metal anions, most notably K, are in
  the structure (usually 3 per C60 molecule), the
  resulting molecular material (K3C60) displays the
  characteristics of a metal. In fact, K3C60 is
  considered to be the first molecular metal ever
  encountered.
• K3C60 buckyballs and similar molecular materials
  become super conducting (practically no
  electrical resistance) at about 18K (relatively
  high temperature for this phenomenon)
• Applications in low-power consumption,
  low-pollution, magnetic-levitation and propulsion
  devices for mass transit systems.
• Other synthetic ceramic materials have been
  developed that display superconductivity at even
  higher temperatures (up to 100K) above the
  temperature of liquid nitrogen, a relatively
  inexpensive and abundant coolant.

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Try It!

• Calcualte the theoretical density of pure C60
  based on a FCC unit cell as shown

a1.41 nm
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Defects in Crystalline Ceramics

• Vacancy
• Interstitial
• Dislocation
• Grain Boundary

Cation Interstitial Anion Vacancy Cation
Vacancy Schotky Defect Frenkel Defect
Electro- neutrality
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Mechanical Properties

• Brittle Materials, hard to perform a Tension
  Test.
• Flexural Test (Bend) is often substituted.
• Obtain Flexural Strength (Modulus of Rupture),
  Stiffness (Modulus of Elasticity), and Ductility.
• Strength is often good, Stiffness my be high, but
  Ductility and affected properties are poor.
• In crystalline ceramics, dislocation motion is
  difficult because of the need to maintain
  electro-neutrality. Consequently plastic
  deformation is restricted.

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Flexural Test Configuration
Rectangular Circular
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Stress-Strain Behaviour
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Mechanical Properties of Various Ceramics

• a Sintered with about 5 porosity

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Hardness of Ceramics
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Relative Hardness
B4C, SiC WC, Al2O3
Glass
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Effect of Porosity on Stiffness
Where Eo is the theoretical modulus of elasticity
with no porosity, and P is the volume fraction of
porosity.
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Effect of Porosity on Strength

• Where so is the theoretical modulus of rupture
  with no porosity, P is the volume fraction of
  porosity, and n is an empirical material constant

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Fracture Toughness
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Amorphous Ceramics - Glasses (Na20, Ca0, K2O, etc)

• The viscosity of the material at ambient
  temperature is relatively high, but as the
  temperature increases there is a continuous
  decrease in viscosity.
• When the viscosity has decreased to the point
  that the ceramic is a fluid, it is considered to
  have melted.
• At ambient temperature while it is still solid,
  it is said to be in the glassy condition.
• There is no distinct melting temperature (Tm) for
  these materials as there is with the crystalline
  materials.
• The glass transition temperature, Tg, is used to
  define the temperature below which the material
  is a solid and defines a practical upper limit
  on service temperature.

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Specific volume of amorphous and crystalline
ceramics.
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Viscous Behaviour in Amorphous Ceramics
t shear stress h viscosity of material

• Plastic deformation does not occur by
  dislocation motion in amorphous or
  non-crystalline ceramics, such as glass.
• Deformation is by viscous flow rate of
  deformation proportional to applied stress.

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Ceramic Phase Diagrams

• Note They are similar to metal alloy systems -
  except the temperatures are generally higher.

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Binary Eutectic Ceramic Alloy
Spinel