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					West Virginia University




           Ceramics




                 Mechanical & Aerospace Engineering
              West Virginia University

               Taxonomy of Ceramics
Glasses        Clay Refractories            Abrasives Cements             Advanced
             products                                                     ceramics
 -optical     -whiteware -bricks for -sandpaper -composites engine
 -composite -bricks       high T                -cutting         -structural -rotors
  reinforce               (furnaces) -polishing                              -valves
 -containers/           Adapted from Fig. 13.1 and discussion in
                                                                             -bearings
  household             Section 13.2-6, Callister 7e.
                                                                             -sensors
• Properties:
  -- Tm for glass is moderate, but large for other ceramics.
  -- Small toughness, ductility; large moduli & creep resist.
• Applications:
  -- High T, wear resistant, novel uses from charge neutrality.
• Fabrication
  -- some glasses can be easily formed
  -- other ceramics can not be formed or cast.
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          Application: Refractories
• Need a material to use in high temperature furnaces.
• Consider the Silica (SiO2) - Alumina (Al2O3) system.
• Phase diagram shows:
  mullite, alumina, and crystobalite as candidate refractories.

             2200                3Al2O3-2SiO2
          T(°C)
                                     mullite
              2000        Liquid
                            (L)                    alumina + L
                                                           Adapted from Fig. 12.27,
              1800                                         Callister 7e. (Fig. 12.27
                                   mullite                 is adapted from F.J. Klug
     crystobalite                               alumina    and R.H. Doremus,
          +L                        +L             +       "Alumina Silica Phase
              1600                              mullite    Diagram in the Mullite
                                                           Region", J. American
                             mullite                       Ceramic Society 70(10),
                         + crystobalite                    p. 758, 1987.)
              1400
                     0   20    40    60      80    100
                              Composition (wt% alumina)
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              Application: Die Blanks
• Die blanks:                                              die       Ad
  -- Need wear resistant properties!             Ao                             tensile
                                                                                force
                                                           die
                                                                   Adapted from Fig. 11.8 (d),
        Courtesy Martin Deakins, GE                                Callister 7e.
        Superabrasives, Worthington,
        OH. Used with permission.




• Die surface:
  -- 4 mm polycrystalline diamond
     particles that are sintered onto a
     cemented tungsten carbide                           Courtesy Martin Deakins, GE
     substrate.                                          Superabrasives, Worthington,
                                                         OH. Used with permission.
  -- polycrystalline diamond helps control
     fracture and gives uniform hardness
     in all directions.
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              West Virginia University

           Application: Cutting Tools
• Tools:
 -- for grinding glass, tungsten,
    carbide, ceramics
 -- for cutting Si wafers
 -- for oil drilling

• Solutions:                               oil drill bits               blades
  -- manufactured single crystal
     or polycrystalline diamonds                                 coated single
                                                                 crystal diamonds
     in a metal or resin matrix.
  -- optional coatings (e.g., Ti to help
     diamonds bond to a Co matrix                                polycrystalline
                                                                 diamonds in a resin
     via alloying)
                                                                 matrix.
  -- polycrystalline diamonds
                                                    Photos courtesy Martin Deakins,
     resharpen by microfracturing                   GE Superabrasives, Worthington,
                                                    OH. Used with permission.
     along crystalline planes.
                                    Mechanical & Aerospace Engineering 5
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              Application: Sensors
• Example: Oxygen sensor ZrO2
• Principle: Make diffusion of ions             Ca 2+
   fast for rapid response.
• Approach:
   Add Ca impurity to ZrO2:                                   A Ca 2+ impurity
   -- increases O2- vacancies                              removes a Zr 4+ and a
   -- increases O2- diffusion rate                               O2- ion.

• Operation:
                                                        sensor
   -- voltage difference
                                     gas with an                     reference
      produced when                  unknown, higher                 gas at fixed
      O2- ions diffuse               oxygen content         O2-
                                                                     oxygen content
                                                        diffusion
      from the external
      surface of the sensor
      to the reference gas.                            +         -
                                          voltage difference produced!


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        Glass & Vitreous Ceramics

Glass




Vitreous Ceramics




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High-performance Engineering Ceramics

Diamond: used as cutting tools, rock drills, abrasive, etc.

Generic High-performance Ceramics




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              Cement and Concrete




Cement: Mixture of lime (CaO), silica (SiO2) and
alumina (Al2O3), which sets when mixed with water

Concrete: Sand and stones held together by cement.

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     Natural Ceramics




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     Structure of Ceramics - Ionic


                               MgO Structure:
                               Can be thought of as an
                               fcc packing with Mg
                               ions in octahedral
NaCl Structure                 holes.




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         Structure of Ceramics - Ionic




ZrO2: fcc packing of                Al2O3: c.p.h packing of
Zr with O in the                    O with Al in 2/3 of the
tetrahedral holes                   octahedral holes

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Structure of Ceramics – Simple Covalent




Diamond: each atom                 SiC: diamond structure
has four neighbors.                with half the atoms
                                   replaced by Si.

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Structure of Ceramics – Simple Covalent




  Cubic SiO4: diamond structure with an SiO4
  tetrahedron on each atom site.

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             Structure of Ceramics
Microstructure of Ceramics




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    Mechanical Properties of Ceramics


Elastic Moduli




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     Mechanical Properties of Ceramics

Strength, Hardness & Lattice Resistance




 Normalised Hardness of Pure Metals, Alloys and Ceramics

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     Mechanical Properties of Ceramics

Dislocation Movement




Dislocation motion in covalent solids is intrinsically difficult
because the interatomic bonds must be broken and reformed
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     Mechanical Properties of Ceramics

Dislocation Movement




Dislocation motion in ionic solids is easy on some planes, but
hard on others. The hard systems usually dominate.
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     Mechanical Properties of Ceramics

Fracture Strength of Ceramics




The design strength of a ceramic is determined by fracture
toughness and lengths of the microcracks it contains.


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            West Virginia University

     Mechanical Properties of Ceramics

 Fracture Strength of Ceramics




(a) Tensile test measures tensile strength, TS
(b) Bend test measures modulus of rupture, r, typically 1.7 TS
(c) Compression test measures crushing strength, c, typically
15TS
                             Mechanical & Aerospace Engineering
         West Virginia University

    Mechanical Properties of Ceramics
          - Fracture Strength
                                    In compression, many
                                    flaws propagate stably
                                    to give general crashing




In tension the largest
flaw propagates unstably


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     Mechanical Properties of Ceramics

Thermal Shock Resistance



       ET = TS


 Where E is the Young’s modulus,  is the coefficient of
 expansion and TS is tensile strength. T represents the
 ceramic’s thermal shock resistance


                           Mechanical & Aerospace Engineering
                            West Virginia University

                       Ceramic Fabrication Methods-I

          GLASS                               PARTICULATE                             CEMENTATION
         FORMING                               FORMING
      • Pressing:
                                                                  Pressing
                                  Gob
                                                                  operation
                                                                                       plates, dishes, cheap glasses
                                                                                         --mold is steel with
                Parison                                                                     graphite lining
                mold
                                                                                    • Fiber drawing:
                                                                Compressed
      • Blowing:                                                    air

       suspended
       Parison

                                        Finishing
                                          mold                                                        wind up
                                                             Mechanical
Adapted from Fig. 13.8, Callister, 7e. (Fig. 13.8 is adapted from C.J. Phillips,   & Aerospace Engineering 24
Glass: The Miracle Maker, Pittman Publishing Ltd., London.)
                 West Virginia University

                      Sheet Glass Forming
• Sheet forming – continuous draw
   – originally sheet glass was made by “floating” glass on a pool of mercury




    Adapted from Fig. 13.9, Callister 7e.



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           West Virginia University

                  Glass Structure
• Basic Unit:                 • Glass is amorphous
       4-                     • Amorphous structure
    Si0 4 tetrahedron           occurs by adding impurities
                Si 4+           (Na+,Mg2+,Ca2+, Al3+)
                   O2-
                              • Impurities:
                                interfere with formation of
                                crystalline structure.

• Quartz is crystalline
                                                      Na +
  SiO2:
                                                      Si 4+
                                                         O2-

                                                     (soda glass)
                                                     Adapted from Fig. 12.11,
                                                     Callister, 7e.

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                    West Virginia University

                               Glass Properties
• Specific volume (1/r) vs Temperature (T):
                                                    • Crystalline materials:
  Specific volume
                                                      -- crystallize at melting temp, Tm
                                                      -- have abrupt change in spec.
      Supercooled                 Liquid
         Liquid                (disordered)
                                                         vol. at Tm


     Glass                                          • Glasses:
(amorphous solid)
                                                       -- do not crystallize
                   Crystalline                         -- change in slope in spec. vol. curve at
                 (i.e., ordered)            solid
                                                           glass transition temperature, Tg
               Tg         Tm                 T         -- transparent
                                                            - no crystals to scatter light

   Adapted from Fig. 13.6, Callister, 7e.

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             Glass Properties: Viscosity
• Viscosity, h:
  -- relates shear stress and velocity gradient:


                             
                                      dy dv           dv
             glass               dv               h
                                        dy            dy
     
                          velocity gradient


                                              h has units of (Pa-s)




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                             West Virginia University

                     Glass Viscosity vs. T and Impurities
                                                   • soda-lime glass: 70% SiO2
• Viscosity decreases with T                         balance Na2O (soda) & CaO (lime)
• Impurities lower Tdeform                         • borosilicate (Pyrex):
                                                     13% B2O3, 3.5% Na2O, 2.5% Al2O3
                                                   • Vycor: 96% SiO2, 4% B2O3
                                                   • fused silica: > 99.5 wt% SiO2
Viscosity [Pa  s]




                 10 14                           strain point
                                                 annealing range
                     10 10

                     10 6                        Tdeform : soft enough
                                                 to deform or “work”
                     10 2
                                                 Tmelt    Adapted from Fig. 13.7, Callister, 7e.
                                                          (Fig. 13.7 is from E.B. Shand, Engineering
                       1                                  Glass, Modern Materials, Vol. 6, Academic
                      200    600 1000 1400 1800 T(°C)     Press, New York, 1968, p. 262.)


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                  Heat Treating Glass
• Annealing:
 --removes internal stress caused by uneven cooling.
• Tempering:
 --puts surface of glass part into compression
 --suppresses growth of cracks from surface scratches.
 --sequence:
 before cooling        surface cooling      further cooled
                          cooler                       compression
       hot                  hot                        tension
                          cooler                       compression

 --Result: surface crack growth is suppressed.




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         Ceramic Fabrication Methods-IIA

    GLASS                        PARTICULATE                  CEMENTATION
   FORMING                         FORMING
• Milling and screening: desired particle size
• Mixing particles & water: produces a "slip"
• Form a "green" component        Ao
                                         container                   die holder             Adapted from
   --Hydroplastic forming:              force
                                                    ram   bille       extrusion       Ad    Fig. 11.8 (c),
      extrude the slip (e.g., into a pipe)                t                                 Callister 7e.
                                                      container         die
   --Slip casting:
     pour slip   absorb water           pour slip     drain       “green
     into mold   into mold              into mold     mold        ceramic”        Adapted from Fig.
                             “green                                               13.12, Callister 7e.
                             ceramic”                                             (Fig. 13.12 is from
                                                                                  W.D. Kingery,
                                                                                  Introduction to
                                                                                  Ceramics, John
                                                                                  Wiley and Sons,
                                                                                  Inc., 1960.)
       solid component                  hollow component
• Dry and fire the component
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                 Clay Composition
A mixture of components used

(50%) 1. Clay
(25%) 2. Filler – e.g. quartz (finely ground)
(25%) 3. Fluxing agent (Feldspar)
         binds it together

                         aluminosilicates + K+, Na+, Ca+




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                             Features of a Slip
                                                                    Shear
• Clay is inexpensive
• Adding water to clay
  -- allows material to shear easily                                          charge
     along weak van der Waals bonds                                           neutral
  -- enables extrusion
  -- enables slip casting

                                                                         weak van
                                                                         der Waals
                 • Structure of                                          bonding
                                                                                4+
                 Kaolinite Clay:                      charge                  Si
                                                                                 3+
     Adapted from Fig. 12.14, Callister 7e.           neutral                 Al
     (Fig. 12.14 is adapted from W.E. Hauth,                                      -
     "Crystal Chemistry of Ceramics", American
                                                                              OH
                                                                                2-
     Ceramic Society Bulletin, Vol. 30 (4), 1951,                             O
     p. 140.)

                                                                 Shear
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                        Drying and Firing
• Drying: layer size and spacing decrease.
                                                                      Adapted from Fig.
                                                                      13.13, Callister 7e.
                                                                      (Fig. 13.13 is from
                                                                      W.D. Kingery,
                                                                      Introduction to
                                                                      Ceramics, John
                                                                      Wiley and Sons,
                                                                      Inc., 1960.)
             wet slip       partially dry        “green” ceramic
  Drying too fast causes sample to warp or crack due to non-uniform shrinkage
• Firing:
  --T raised to (900-1400°C)
  --vitrification: liquid glass forms from clay and flows between
    SiO2 particles. Flux melts at lower T.
                                                            Adapted from Fig. 13.14,
                                            Si02 particle   Callister 7e.
                                            (quartz)        (Fig. 13.14 is courtesy H.G.
                                                            Brinkies, Swinburne
 micrograph of                              glass formed    University of Technology,
 porcelain                                  around          Hawthorn Campus,
                                            the particle    Hawthorn, Victoria,
                                                            Australia.)


                          70mm    Mechanical & Aerospace Engineering 34
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        Ceramic Fabrication Methods-IIB

     GLASS              PARTICULATE           CEMENTATION
   FORMING                 FORMING
Sintering: useful for both clay and non-clay compositions.
• Procedure:
  -- produce ceramic and/or glass particles by grinding
  -- place particles in mold
  -- press at elevated T to reduce pore size.
• Aluminum oxide powder:
  -- sintered at 1700°C
     for 6 minutes.
                                              Adapted from Fig. 13.17, Callister 7e.
                                              (Fig. 13.17 is from W.D. Kingery, H.K.
                                              Bowen, and D.R. Uhlmann, Introduction
                                              to Ceramics, 2nd ed., John Wiley and
                                              Sons, Inc., 1976, p. 483.)


                              15 mm
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                     Powder Pressing
Sintering - powder touches - forms neck & gradually neck thickens
    – add processing aids to help form neck
    – little or no plastic deformation



Uniaxial compression - compacted in single direction
Isostatic (hydrostatic) compression - pressure applied by
  fluid - powder in rubber envelope

Hot pressing - pressure + heat
                                                        Adapted from Fig. 13.16, Callister 7e.




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                       Tape Casting
• thin sheets of green ceramic cast as flexible tape
• used for integrated circuits and capacitors
• cast from liquid slip (ceramic + organic solvent)




                              Adapted from Fig. 13.18, Callister 7e.
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        Ceramic Fabrication Methods-III
     GLASS          PARTICULATE           CEMENTATION
   FORMING             FORMING
• Produced in extremely large quantities.
• Portland cement:
  -- mix clay and lime bearing materials
  -- calcinate (heat to 1400°C)
  -- primary constituents:
      tri-calcium silicate
      di-calcium silicate
• Adding water
  -- produces a paste which hardens
  -- hardening occurs due to hydration (chemical reactions
      with the water).
• Forming: done usually minutes after hydration begins.
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  Applications: Advanced Ceramics
Heat Engines                              • Disadvantages:
• Advantages:                                 – Brittle
   – Run at higher temperature                – Too easy to have voids-
   – Excellent wear & corrosion                 weaken the engine
     resistance
                                              – Difficult to machine
   – Low frictional losses
   – Ability to operate without a
     cooling system
   – Low density



 • Possible parts – engine block, piston coatings, jet engines
    Ex: Si3N4, SiC, & ZrO2

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  Applications: Advanced Ceramics
• Ceramic Armor
   – Al2O3, B4C, SiC & TiB2
   – Extremely hard materials
      » shatter the incoming projectile
      » energy absorbent material underneath




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Applications: Advanced Ceramics
Electronic Packaging
• Chosen to securely hold microelectronics & provide heat transfer
• Must match the thermal expansion coefficient of the
  microelectronic chip & the electronic packaging material.
  Additional requirements include:
    – good heat transfer coefficient
    – poor electrical conductivity
• Materials currently used include:
         » Boron nitride (BN)
         » Silicon Carbide (SiC)
         » Aluminum nitride (AlN)
              • thermal conductivity 10x that for Alumina
              • good expansion match with Si




                                      Mechanical & Aerospace Engineering 41