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MME 131: Introduction to Metallurgy and Materials

Lecture 28

Processing of Ceramic Materials

AKMB Rashid Professor, MME Dept

BUET, Dhaka

Today’s Topics

Glass Forming Processes

Ceramics Forming Processes

Cementation Processes

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General Fabrication Routes

Cementation Processes

Pressing

Blowing

Drawing

Fibre Forming Hydroplastic

Forming

Slip Casting

Tape Casting

Hot Uniaxial Isostatic

Firing

Glass Forming Processes

Ceramic Particulate Forming Processes

Powder Forming

Drying The brittleness of ceramics

precludes deformation !

Glasses Forming Processes

To obtain good optical transparency the glass must

be homogeneous and pore free!

Homogeneity is achieved by complete melting and

mixing of the raw ingredients.

Proper adjustment of the viscosity of the molten

material is required in order to eliminated gas bubbles

generated to obtain minimum porosity.

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The viscosity - temperature relation

for glass forming processes

Viscosity - temperature

characteristics of glass

controls the forming

processes

Viscosity decreases with

temperature

Impurities or additives

lower Tdeform

The formability of a glass

is tailored to a large

degree by its composition

Commercial melting

and forming of glass

• Batching of the raw materials

• Mixing of the batch – Water typically added to

reduce dusting

• Melting the batch to a homogenous viscous liquid

• Conditioning (cooling) the melt to the forming temperature

• Forming operation – Float glass – Container glass – Fiber glass – Specialty glass

• Annealing

• Inspection

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For thick-walled pieces (e.g.,

plates and dishes)

Pressure application in a

graphite-coated cast iron mould

mould heated to ensure an

even surface finish

Pressed Glass Processing

Softened glass

Glass Forming – Pressing

Parison mould

Gob Pressing Operation

Suspended Parison

Finishing Mould

Compressed Air

The press-and-blow

technique for producing a

glass bottle

Either manual (for artworks)

of fully automated (jars,

bottles, light bulbs, etc.)

From raw gob of glass, a

parison (temporary shape),

is formed by mechanical

pressing in a mould

The piece is then inserted

into a finishing or blow

mould to finalize the shape

Glass Forming – Press and Blow

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Glass Forming – Drawing Processes

Used to form long form such as sheet,

rod, tubing, and fibers, which has

constant cross section

Usually the liquid glass is drawn

between the rolls (Foucault/Pittsburg

/Libby-Ownes process) to form sheet

Plate glass is flat glass that has

been ground and polished to produce

two perfectly plane and parallel faces

with a high quality optical finish.

The surface of a flat glass sheet is

flattened by grinding it between two

cast iron wheels with sand abrasive

and water lubricant.

Float Glass Process

Flatness and surface finish may be improved significantly by

floating the sheet on a bath of molten tin at an elevated

temperature (Float process)

Developed by Pilkington in 1959 in the UK , the float process

revolutionized the flat-glass industry. Over 90% of the world’s

window glass is produced now using the float process.

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Glass Fibre Process

Glass Forming

– Fibre Drawing

Uses sophisticated drawing

operations

Molten glass is contained in

a platinum heating chamber;

fibres are formed by drawing

the molten glass through

many small orifices at the

chamber base

The glass viscosity is

controlled by chamber and

orifice temperatures

Heat Treating Glass

Heat Treating Glasses – Annealing

Internal stresses (thermal stresses) are introduced when the glass is cooled from an

elevated temperature – a result of the difference in cooling rate and thermal

contraction between the surface and interior regions.

Thermal stresses weaken the material and may lead to fracture Thermal shock.

Thermal stresses can be reduced by cooling the piece at a sufficiently slow cooling

rate, then followed by an annealing heat treatment in which the glassware is heated

to the annealing point, and then slowly cooled to room temperature.

Heat Treating Glass – Tempering

The strength of a glass piece is enhanced by intentionally inducing compressive

residual surface stresses

When interior cools and contracts it draws the exterior into compression.

Compressive stresses on the surface with tensile stresses at interior regions formed.

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further cooledbefore cooling surface cooling

tension

compression

compression

hot hot

cooler

cooler

Result: Surface crack growth is suppressed

Thermal Tempering

Chemical Tempering

Cations with large ionic radius are diffused into the surface. This strains

the “lattice” inducing compressive strains and stresses.

Before cooling Surface cooling Further cooling

Ceramics Forming Processes Particulate Forming Processes

Ceramic powders Additions (liquids, organic additives)

Adaptation of the system to the shaping process

(grinding, mixing, dispersion, granulation, etc.)

Shaping

Drying (elimination of organic additives)

Firing (obtaining of sintered product)

General flow chart of the manufacturing of ceramic products

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Producing shapes from a mixture of powders and additives

that are deformable under pressure.

Mixture include:

Traditional Ceramics – Clay and water, defloculant, wetting agent, lubricant

Engineering Ceramics – Non-clay materials (pure oxides, nitrides, carbides etc),

25-50 vol.% organic additives with/without water to provide plasticity

Ceramic Forming - Hydroplastic Forming

Removal of organic materials prior firing is a major problem

Water-clay system – substantial shrinkage occurs during drying, increasing risk of

shrinkage cracks

Non-clay system – formation of flaw-free green part is a problem and extraction of

organics is also problematical

> Too rapid extraction – cracking, bloating, distortion

> Inadequate extraction – cracking, bloating, distortion during densification.

Clay minerals, when mixed with water, become highly plastic and

can be moulded without cracking

Green products have extremely low strength; the consistency

(water-clay ratio) is controlled so that the formed ware can be

maintained its shape during handling and drying

The raw materials are wet ball milled, screened to remove water

and form a plastic mass

The plastic mass is forced through the die using an auger and air is

removed using vacuum to enhance density

The cake thus formed is given the shape using many of the forming

processes e.g., extrusion, injection/compression moulding,

jiggering, etc.

The formed ware are then dried and sintered to obtain the finished

product

Common products formed in this way include: brick, tiles, furnace

tube (extrusion); turbine rotor blade, combustion nozzle (moulding);

cooking ware, electrical porcelain, refractories (jiggering)

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Mass

A slip is a suspension of clay-based materials in water, prepared by wet ball milling

the ingredients

The slip is poured in a porous mould (made of plaster of Paris), the water is

absorbed by the mould, leaving behind a solid layer on the mould wall the thickness

of which depends on the time

The process may continue until the entire mould cavity becomes solid (solid

casting), or draining the remaining slip when a solid of sufficient thickness is formed

(hollow casting)

As the casting dries and shrinks, it will pull away from the mould wall so that the

green product is removed easily.

The formed ware are then dried and

sintered to obtain the finished product

The nature of the slip (high sp. gravity,

high fluidity) and the mould (porosity,

moisture content) are extremely

important

Common products formed in this way

include: sanitaryware, ceramic vases,

art objects

Ceramic Forming - Slip Casting

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This process is used to make flat ceramic sheets having a thickness up to about

1 mm. The process was developed during the 1940s for capacitor dielectrics. The

production of ceramic capacitors is still one of the most important applications of

tape casting.

A slurry containing a powdered ceramic together with a complex mixture of solvents

and binders is spread onto a moving polymer sheet.

Ceramic Forming - Tape Casting

The thickness of the

deposited layer is

determined by the

height of the doctor

blade above the

polymer sheet.

Extensive shrinkage

occurs during drying

and firing of the tape

because of the large

volume fraction of

organics in the slurry.

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The powdered raw materials are mixed with suitable binders (e.g.,

polyvinyl alcohol, PVA) and then pressed into green compacts inside

a suitable die

The green compacts contain lots of porosity between the powders

The green compacts are then dried and sintered to obtain the

finished product

During sintering, diffusion of atoms at the boundary of particles takes

place and the powdered particles are diffusion bonded and the

degree of porosity is reduced to increase density of the product

Ceramic Forming - Particulate Forming

The quality of the finished products depends on time and

temperature of the sintering cycle

Almost all engineering ceramics are formed by particulate forming.

Common traditional ceramics formed by this method are tiles and

refractory bricks.

Filling mould Compaction Green part ejected,

then sintered

Uniaxial Compaction

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Drying

Before drying, clay particles

are virtually surrounded by and

separated from one another by

a thin film of water.

As drying progresses and

water is removed, the inter-

particle separation decreases,

and the clay piece shrinks.

It is critical to control the rate of water removal.

If the rate of evaporation at the surface is greater than the rate of diffusion of water

molecules to the surface, the surface will dry (shrink) more rapidly than the interior, with

a high probability of the formation of defects.

The rate of surface evaporation can be controlled by temperature, humidity, and rate of

airflow.

Factors that influence shrinkage: body thickness, water content, clay particle size.

After drying, a clay piece is usually fired at temperature between

900 – 1400C.

During firing, the density of the piece is further increased, so as the

strength.

Vitrification (formation of a liquid glass) occurs during firing that flows

into and fills some of the pore volume.

The degree of vitrification depends on: firing temperature, time, and

composition.

Fluxing agents are added to decrease the temperature at which

vitrification happens.

A glassy matrix is formed upon cooling, which gives rise to a dense,

strong body.

The final microstructure: the vitrified phase, unreacted particles (e.g.

quartz), and some porosity.

Firing

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• Strength, durability, and

density of the clay piece are

enhanced as the degree of

vitrification increases.

• Firing temperature

determines the extent of

vitrification – higher the

temperature, more vitrification.

• To achieve optically

translucent (i.e. high

vitrification), firing takes place

at very high temperatures.

• Complete vitrification is

avoided during firing since the

body might become too soft

and might collapse.

Controlled mixture of clay (Al2O3.2SiO2.2H2O) and chalk

(CaCO3) is fired in a kiln at 1500 C

Firing gives three products:

Clay + Chalk = C3A + C3S + C2S

When cement is mixed with water, hydrated cement paste

(h.c.p.) is formed

All cements harden by reaction, not by drying

Portland Cement

C – CaO

S – SiO2

A – Al2O3

H – H2O

Cementation Processes

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Hardening of Portland cement:

(a) Setting and hardening reaction

(b) Heat evolution

First Reaction

C3A + 6H = C3AH6 + Heat

• occurs in ~4 hours and set the cement

Second Set of Reaction

2C2S + 4H = C3S2H3 + CH + Heat

2C3S + 6H = C3S2H3 + 3CH + Heat

• starts in ~10 hours and completes in

~100 days or more to harden the

cement

• Tobomorite gel (C3S2H3) is the main

bonding material which occupies

~70% of the structure

MME 131: Lecture 32

Polymeric Materials

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