Materials Technology
Ceramic
Dr Jasmi Hashim
INTRODUCTION
Keramikos - burnt stuff in Greek
Desirable properties of ceramics are normally achieved through a
high –temperature heat treatment process (firing).
Bonds are partially or totally ionic, or combination of ionic & covalent
Generally hard and brittle
Generally electrical and thermal insulators
Can be optically opaque, semi-transparent, or transparent
High chemical stability and high melting temperature
Example of traditional ceramic products
Example of engineering ceramic products
Properties Metal Ceramic Polymer
Density High Low Very low
Melting Temperature Medium to high High Low
Modulus Elasticity Medium to high Very high Low
Ductility Ductile Brittle Ductile to brittle
Typical Mechanical Properties
Brittle: Kic in ~ 1 - 12 MPam1/2 (Al alloys have KIc - 25 - 50 MPam1/2).
High elastic moduli - greater than metals
High compressive strengths is typically ten times the TS
Very high hardness
Application of ceramics
The compressive strength is typically ten times the TS. In
structures, design must be done for compressive loads.
The transparency of light of many ceramics ;
Optical applications (windows, photographic cameras,
telescopes, etc)
Silicate glasses (non crystalline silicates (SiO2 containing other
oxides) ; windows, containers, lenses, fibreglass etc..
Good thermal insulators ; used in oven, the exterior tiles of the
Space Shuttle
Good electrical insulation ; used to support conductor in
electrical and electronic application
Good chemical properties; applications in reactive applications
1. Traditional Ceramics
– based on clay (china, bricks, tiles, porcelain), glasses.
2. Engineering Ceramics:
- Pure compounds of metal + O, Metal + C, Metal + N
Oxide eg: Al2O3, ZrO
Carbide eg : TiC, WC, SiC
Nitride eg : Si3N4, , TiN, CBN
Ceramic Materials
Glasses Clay Refractories Abrasives Cements Advanced
products ceramics
-optical -whiteware -bricks for -sandpaper -composites -engine rotors
-composite -structural high T -cutting -structural valves
reinforce (furnaces) -polishing -bearings
-containers/ household -sensors
CERAMIC PROCESSING
1. POWDER Ball mill,blending,
crushing etc
PREPARATION + additives
2. FORMING (Green Body)
Slip Casting Hydro plastic forming
Pressing (Compaction) -drain casting Tape
Uniaxial, CIP, HIP Casting -Injection
-solid casting -extrusion
3. DRYING & SINTERING
4. FINISHING
Cleaning, sizing, machining,
glazing
1. POWDER PREPARATION
+ ADDITIVES
Water minimum 4% for dry processing ,up to 12%
for wet processing., binder, Lubricant , Wetting
agent, Plasticizer
Blending
Blending : Mixing powder of same chemical composition but different size
Mixing : combining powders of different chemistries.
Blending and mixing are accomplished y mechanical means
Lubricants : to reduce the particle-die friction
Binders : to achieve enough strength before sintering
2. FORMING (Green Body)
i. Dry Pressing
• Dry Pressing:
Simultaneous uniaxial compaction and shaping of power along with
binder.
Wide variety of shapes can be formed rapidly and accurately.
• Isolatic pressing (CIP, HIP):
Ceramic powder is loaded into a flexible chamber and pressure is applied
outside the chamber with hydraulic fluid.
Examples: Spark plug insulators, carbide tools.
After J. S. Reed and R. B Runk, “ Ceramic Fabrication Process,” vol 9: “1976, p.74.
Cold Isostatic Pressing (CIP)
Slip Casting
• Powdered ceramic material and a liquid mixed to prepare a stable suspension
(slip).
• Slip is poured into porous mold and liquid portion is partially absorbed by mold.
• Layer of semi-hard material is formed against mold surface.
• Excess slip is poured out of cavity or cast as solid.
• The material in mold is allowed to dry and then fired.
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Tape Casting
• Thin sheets of green ceramic cast as flexible tape (0.2 to 2 mm)
• Used for integrated circuits and capacitors
• Slip = suspended ceramic particles + organic liquid
(contains binders, plasticizers)
Extrusion
• Single cross sections and hollow shapes of ceramics can be produced by
extrusion.
• Plastic ceramic material is forced through a hard steel or alloy die by a
motor driven augur.
• Examples: Refractory brick, sewer pipe, hollow tubes.
Injection Moulding
Ceramic powder + binder
Binder : TP plastic spt PP, LDPE, ethylene vinyl acetate
3. FIRING ( Drying and Sintering)
wet body partially dry completely dry
• Drying: • Firing:
- as water is removed – interparticle -- heat treatment between
spacings decrease 900-1400ºC
– shrinkage . -- vitrification: liquid glass forms
Drying too fast causes sample to from clay and flux – flows
warp or crack due to non-uniform between SiO2 particles. (Flux
shrinkage lowers melting temperature).
Typical heat treatment
cycle in sintering
Schematic cross section
of a continuous
sintering furnace
Sintering
Sintering occurs during firing of a piece that has been powder pressed
-- powder particles coalesce and reduction of pore size
Traditional Ceramics
• Made up of clay, silica and fledspar.
• Clay: Provide workability and hardness.
• Silica: Provide better temperature resistance and MP.
• Potash Fledspar: Makes glass when ceramic is fired.
5. FINISHING OPERATIONS
To further improve the properties of sintered product :
i. Re- pressing : also called coining and sizing
ii. Forging, grinding
iii. Heat treatment : quenching and tempering
iv. Machining : milling, grinding, ultra sonic, laser, electrical-
discharge machining (EDM)
v. Glazing, plating, painting
Glass : Effect of temperature
Viscous Deformation of glasses.
1. Working point: 103 PaS
– glass fabrication can be carried out
2. Softening point: 107 PaS
– glass flows under its own weight.
3. Annealing point: 1012 PaS
– Internal stresses can be relieved.
4. Strain point: 10 13.5 PaS
– glass is rigid below this point.
Units are Pa-s, or Poises (P)
1 P = 0.1 Pa-s
Viscosity of water at room temp is ~ 10-3 P
Viscosity of typical glass at room temp >> 1016 P
Question :
A piece of silica glass having the
viscosity of 1013 P at 940oC (annealed
point) and the viscosity of 108 P at
1470oC (softening point). Calculate the
activation energy for viscous flow in
this temperature range for this glass
Solution :
Annealing point of glass,
Tap = 1213K, ap = 1013P
Softening point of glass,
Tsp = 1743K, sp = 108 P,
R = 8.314 J/(mol.K)
Use Equation: = o e + Q/RT
Q = 8.32 x 105 j/mol
GLASS PROCESSING
1. Sheet Glass Forming
• Sheet forming – continuous casting
Sheets are formed by floating the molten glass on a pool of molten tin
• Forming sheet and plate glass:
Ribbon of glass moves out of furnace and floats on a bath of molten tin.
• Glass is cooled by molten tin.
• After it is hard, it is removed and passed through a long annealing furnace.
HEAT TREATMENT OF GLASS
1. Thermal Tempering
• Glass is heated into near softening point and rapidly cooled.
• Surface cools first and contracts.
• Interior cools next and contracts causing tensile stresses in the
interior and compressive stress on the surface.
• suppresses growth of cracks from surface scratches
• Tempering strengthens the glass.
• Examples: Auto side windows and safety glasses.
2. Chemically Strengthened Glass
Special treatment increases chemical resistance of glasses.
Example:- Sodium alumino silicate glasses are immersed in a bath of
potassium nitrate at 500C for 6 to 10 hours
- Large potassium ions are induced into surface causing compressive stress
- Compressive layer is much thinner than that in thermal tempering.
- Used for supersonic aircraft glazing and lenses.
3. Laminated glass
Laminate Strengthening method ; two pieces of flat glass are
assembled with a thin sheet of tough plastic
(eg . Polyninyl butyral, PVB) between them.
When the glass is broken, its pieces are held together by the plastic
sheet.
4. Annealing
When ceramic materials is cooled from an elevated temp. internal
stresses (thermal stresses,) is introduced as a result of the
difference in cooling rate and thermal contraction between the
surface and interior region.
Method :
The glassware is heated to the annealing point, then slowly
cooled to room temperature
References
• K. G. Budinski and M.K. Budinski, Engineering Materials, Properties and
Selection, 9th Ed, Pearson and Prentice Hall, 2010
• J. F. Shackelford, Introduction to Materials Science for Engineers, 6th Ed.,
Pearson and Prentice Hall, 2005.
• J.A. Schey, Introduction to Manufacturing Process, 3rd Ed., Mc Graw Hill,
2000.
• J. A. Jacobs and T. F. Kilduff, Engineering Materials and Technology,
Structure, Processing, Properties and Application, 5th ed., Pearson &
Prentice Hall, 2005.
• G. E. Dieter, Mechanical Metallurgy, McGraw-Hill, 3rd ed., 1991
• W. D. Callister, Mater. Science and Engineering – An Introduction, 8th ed.,
John Wiley & sons, Inc, 2011
• W. F. Smith, Principles of Materials Science and Engineering, 4rd ed.,
McGraw Hill, 2007.
