Ceramic Materials
Types of Ceramics
• Ceramics consist of metallic and non metallic elements joint
by ionic or covalent bonds
Characteristic properties:
– High hardness,
– High brittleness,
– High melting point,
– Low thermal and electrical
conductivity,
– Good chemical and thermal
stability,
– High compressive strength.
a) Glasses
i. Ceramic Glasses: Non crystalline glasses - silica base.
Applications:
– Pottery,
– Bricks,
– Tiles,
– Cooking ware,
– Soil pipes to glasses,
– Refractory,
– Magnets, electrical devices,
– Abrasives or wear resistant
materials.
ii. Glass Ceramics: Crystalline glasses – also silica base.
b) Crystalline ceramics: Most of ceramics except glasses.
• Clay based ceramics; white ware, bricks, tiles, pottery, etc.
• Advanced ceramics; oxides, nitrides, carbides, etc.
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Processing of Glasses / Ceramics
As general, processing may be considered in two categories,
1. Solidification processing for Glasses,
2. Particulate processing for crystalline ceramics.
Glasses
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Ceramic Glasses
Glasses
Glass structures may be
1. Ceramic Glasses: Non crystalline has short range ordered
(SRO) structure;
2. Glass Ceramics: Crystalline long range order (LRO)
crystalline structures.
√ Non crystalline materials with random network (short range ordered
(SRO)) structure below glass transition temperature.
√ Best example; window glass.
√ Main constituents: fused silica (pure SiO2)
Ceramic Glasses
Ceramic Glasses
Glass Ceramics
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Modifiers
The modifiers, such as Sodium oxide (Na2O) break up the glassy network
and reduce the ability to form a glass. Melting range considerable decreases
and thus processing becomes easier. But not good for high Temperature
applications.
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Processing of Glasses
Important
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Process Sequence in Glassworking
At high Temperatures; viscosity of glasses can be controlled
Processing techniques;
a) Glass products (single products)
•Pressing
•Press and Blow
•Drawing of fibers
b)Sheet and plate manufacturing (continuous products)
The typical process sequence in glassworking:
(1) preparation of raw materials and melting,
(2) shaping,
(3) heat treatment.
•Hot rolling
•Floating on molten tin bath
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Spinning
Spinning of the back sections of cathode ray tubes for TVs and
computer monitors:
(1) Gob of glass dropped into mold;
(2) Rotation of mold to cause spreading of molten glass on
mold surface.
Pressing
(1) glass gob is fed into mold from furnace; (2) pressing into shape by
plunger; and (3) plunger is retracted and finished product is removed
(symbols v and F indicate motion (velocity) and applied force).
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Press-and-Blow
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Drawing of Glass
Continuous glass fibers of
small diameter (lower limit ~
0.0025 mm) are produced by
pulling strands of molten glass
through small orifices in a
heated plate made of a platinum
alloy.
Press-and-blow forming sequence: (1) molten gob is fed into mold
cavity; (2) pressing to form a parison; (3) the partially formed parison,
held in a neck ring, is transferred to the blow mold, and (4) blown into
final shape.
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Rolling of Flat Plate
Float Process
Starting glass from melting furnace is squeezed through
opposing rolls whose gap determines sheet thickness, followed
by grinding and polishing for parallelism and smoothness
Hot Rolling
Molten glass flows onto surface of a molten tin bath, where it
spreads evenly across the surface, achieving a uniform thickness
and smoothness - no grinding or polishing is needed
Floating
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Glass transition (fictive) temperature
Glass temperature (fictive or glass transition temperature)- The
point where solid glass start to behave as an undercooled liquid.
Manufacturing properties of glasses
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• During crystallization, an abrupt
change in the density occurs.
• The change in slope indicates
the formation of a glass.
• Glass does not have melting T
(Tm).
• Glass ceramics (Crystalline
materials) show a fixed Tm and
not a Tg.
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High Temperature Prop of Glasses
Melting range: Glass in a liquid form, viscosity 50-500
poise.
Working range: Glass can be deformed to desired
geometries, viscosity 104-107 poise.
Annealing range: To relieve residual stresses, viscosity
1013 poise.
Strain point, Glass in completely rigid state, viscosity
1015 poise.
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Annealing of Glass
τ
η=
dν dz
η = ηo exp
Qn
RT
Heating to elevated temperature and holding to eliminate
stresses and temperature gradients; then slow cooling to
suppress stress formation, then more rapid cooling to room
temperature
• Annealing temperatures are ~ 500°C
• Annealing has the same function in glassworking as in
metalworking – to relieve stresses
• Annealing is performed in tunnel-like furnaces, called lehrs, in
which the products flow slowly through the hot chamber on
conveyors
η:
viscosity Poise (Pa s g/cm.)
τ:
shear stress,
dυ/dz: velocity gradient.
ηwater: 0.01poise at 20oC
ηglycerin: 15 poise
•Qn activation energy for viscosity: T η .
•Amount of modifiers (breaks up the structure) Qn , η .
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Tempered Glass
Important
• Heating the glass to the working range (above Tg),
• Quenching of surfaces by air jets; the surfaces cool first, contract
and harden while interior is still plastic.
• As the internal glass cools and contracts compressive stresses
on surfaces; toughness extremely increases.
• Surface becomes resistant to scratching and breaking.
• Products: windows for tall buildings, all-glass doors, safety
glasses, and other products requiring toughened glass.
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Processing of Glasses / Ceramics
As general, processing may be considered in two categories:
1. Solidification processing for Glasses
2. Particulate processing for crystalline ceramics:
Clay based ceramics:
• Powder preparation
• Forming/Shaping
• Drying
• Sintering
Particulate ceramics
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Advanced ceramics:
• Powder preparation
• Forming/Shaping
• Sintering
Special methods:
• Hot pressing
• Hot isostatic pressing
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Clay based
crystalline ceramics
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Particulate processing
Shaping of Clay based Ceramics
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(a) Article
(b) Microstructure
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Steps
Preparation of raw materials,
Shaping,
Drying, and
Firing (sintering)
Hand modeling
Jiggering
Plastic pressing
Extrusion
Slip casting
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Extrusion
Jiggering (Sııvama)
Movie- jiggering
(1) wet clay slug is placed on a convex mold;
Movie-tempered
glasses
(2) batting; and (3) a jigger tool imparts the final product shape.
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Pressing
Movie- pressing
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Slip Casting
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Drain Casting
Important
Slip; a suspension of ceramic powders in liquid,
Slip is poured into a porous plaster of Paris mold so that water
from the mix is absorbed into the plaster to form a firm layer
of ceramic at the mold surface.
• The slip composition is 25% to 40% liquid
• Two principal variations:
– Drain casting - the mold is inverted to drain excess slip
after a semi-solid layer has been formed, thus producing a
hollow product
– Solid casting - to produce solid products, adequate time
is required.
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Movie- slip casting
(1)
(2)
(3)
(4)
slip is poured into mold cavity,
water is absorbed into plaster mold to form a firm layer,
excess slip is poured out,
part is removed from mold and trimmed.
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Drying Rate and Volume Reduction
Drying of Clay based Ceramics
Drying process occurs in two stages:
• Stage 1 - drying rate is rapid and constant as water evaporates
from the surface into the surrounding air and water from the
interior migrates by capillary action to the surface to replace it
– This is when shrinkage occurs, with the risk of warping
and cracking
• Stage 2 - moisture content has been reduced to where the
ceramic grains are in contact
– Little or no further shrinkage occurs
Typical drying rate curve and associated volume reduction
for a ceramic body. Drying rate in second stage is depicted
as a straight line; it is sometimes concave or convex.
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Drying
Large dimensional changes can
causes residual stresses and
therefore cracking during drying
Types of water in the structure
• Interparticle water
• Pore water.
During drying
• Drying of interparticle moisture
decrease in volume. High
risk for crack or break.
• Dimensional change continues
until particles touches each
other.
• Drying of pore moisture No
dimensional changes. Min. risk
for cracking.
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After shaping and drying, a sintering / firing is
needed to gain the ceramic a strength of Clay
based Ceramics. It will be discussed later with
the advanced ceramics.
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Advanced
crystalline ceramics
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Applications
• Ceramics are used in a wide range of technologies such as:
– Refractory, spark plugs, dielectrics capacitors in electronics,
sensors/actuators, abrasives, biomaterials, magnets, magnetic
recording media, etc.
• Example: The space shuttle makes use of ~25,000 reusable, lightweight,
highly porous ceramic tiles that protect the aluminum frame from the heat
generated during re-entry into the Earth’s atmosphere.
©
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Shaping of the advanced
Ceramics
Important
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Starting Materials for shaping
• The starting material for all of ceramic items is powder
• For traditional ceramics, the powders are usually mixed
with water to temporarily bind the particles together and own
proper features for shaping into green state
• For new ceramics, substances other than water may be used
as binders during shaping
• After shaping, the green parts are fired (sintered):
– To effect a solid state reaction which bonds the material
into a hard solid mass
Producing a green ceramic (green body)
Pressing (shown)
Slip casting (shown)
Tape casting
Extrusion (shown)
Injection (shown)
Cold Isostatic pressing
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Tape Casting
Cold isostatic pressing
Tape casting – Used to make thin sheets of ceramics using a ceramic slurry
• Minimize the porosity in the
shaped ceramics
• Thus maximize the density in the
sintered part
•The ceramic slurry used in both tape casting and slip casting
•Slurry consists of also binders, plasticizers, etc.
•The slurry is cast in to a mold in slip casting
•The slurry is coated on a carrier film (plastic substrate) in tape
casting.
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After shaping, sintering (firing) is
required to get strengthened ceramics
to provide the bonding between the
particles…
No melting in the particles involved in
sintering..
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Sintering / Firing
Heating to the temperatures of 0.7-0.9 Tm to bond the ceramic
particles together with diffusion (no melting!!).
Bonding of ceramic particles
Directly bonding by diffusion
Liquid phase sintering (ceramic bonds) by a glassy phase
and reduce porosity.
Reaction bonding with a chemical reaction between grains
Elimination of pores from the body: With grain boundary
diffusion and/or bulk diffusion,
Volume shrinkage and densification: Due to the reduce in
pore size or elimination of pores,
Increase in rigidity and strength.
Film!
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Important
Direct bonding of the particles during
sintering. Crystalline advanced ceramics
Liquid phase sintering (ceramic
bond): bonding of particles with a
glassy phase. In clay based ceramics.
Not desired in crystalline ceramics
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Pores
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Treatment of Fracture in Ceramics
σ critical
σ=
2 Eγ
=
πa
K
f πa
a ⇒ σmax R ⇒ σmax a
: The length of the surface crack (or one
half the length of an internal crack).
γ : The surface energy (per unit area).
KIC : Fracture toughness
σ : Allowable max. stress which does not
cause brittle or sudden fracture
K1 ≤ K1C
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Failure in Ceramics
Factors effecting the brittle fracture;
1. Small cracks/flaws,
2. Porosity,
3. Glassy phases,
4. Grain size,
5. Foreign inclusions.
Equations point out that:
1. The mechanical properties on the size of flaws present in the ceramic.
2. Manufacturing processes important to minimize the flaw for improving
the strength of ceramics.
3. The flaws are most important in tensile stresses acting on the material.
Compressive stresses try to close the cracks so no problem.
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• Both crystalline and non-crystalline ceramics are very brittle
due to the presence of flaws or porosities.
Any flaw or crack or imperfection limits tensile strength, causes
stress concentration and magnification of applied stress.
• Ceramics with a small grain size are stronger than those with
coarse-grain sizes. (Finer grain sizes help reducing stresses
developed at grain boundaries due to anisotropic expansion and
contraction).
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Porosity in Ceramics
Apparent porosity - The percentage of a ceramic body that
is composed of interconnected porosity (open to surface)
True porosity - The percentage of a ceramic body that is
composed of both closed and interconnected porosity.
Bulk density - The mass of a ceramic body per unit volume,
including closed and interconnected porosity.
Special Methods:
Both sintering and pressure involved.
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Hot pressing
• Applying a uniaxial pressure and temperature during sintering
to minimize the porosity in the sintered part.
• Almost 100% densities can be obtained.
• But the geometries that can be obtained are very limited.
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Hot isostatic pressing
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• Applying the hydrostatic pressure and temperature
during sintering to minimize the porosity in the part.
• Almost 100% densitiy can be riched.
• Many different geometries can be obtained.
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