Ceramics brief study

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CERAMICS

• Introduction

• Forming processes

• Applications



Introduction

Ceramics are non-metallic, inorganic materials. Most of the ceramics are

compounds of metallic and non-metallic elements for which interatomic bonds are

either Ionic Bonds or Covalent bonds. Some times more predominantly ionic and having

covalent characteristics. The term Ceramics is derived from a Greek word “Keramikos”,

which means “Burnt Stuff”, indicating its characteristics of manufacturing as these are

normally heat treated to high temperatures called as “Firing”.

Up until past sixty to seventy years the most of the important materials in this

class were termed as the “traditional Ceramics” as those primary raw materials were

Clays; products considered to be china, porcelain, bricks, tiles and in addition glasses

and high temperature ceramics. OF late the significance of these materials were studied

and year by year the term ceramics got a broader sense of meaning in engineering

application.

Classification:

Ceramics are classified on the bases of its characteristics of which behaves inEngineering application . Ceramics are classified as follows.

Ceramics

Fig: Classification of Ceramics on the basis of Applications.

Glass Cla Products Refractories Abrassives

1. Structural Clay

Products

2. White Wares

1. Fireclay

2. Silica

3. Basic

4. Special

Cements and

Advanced

Ceramics

1. Glasses

2. Glass

Ceramics



GLASSES:

The glasses are Familiar group of ceramics; containers, lenses etc. As they are

mentioned to be a non-crystalline silicates containing other oxides, notably CaO, Na2O, K2O and

Al2O3, which influences he properties of glasses. A typical soda lime Glass consists of

approximately 70 wt. % SiO2, the balance being mainly Na2O(Soda), and CaO (lime). The

composition of several common glass materials are contained in the following Table with the

consideration of prime aspects of optical transparency and ease of manufacture.

Glasses type Composition (wt.%) Characteristics

and Applications

SiO2 Na2O CaO Al2O3 B2O3 other

Fused silica >99.5 High melting Temp.Very low coeff. Of

Expansion

(thermally shock

resistant)

96% silica (vycor) 96 4 Thermally shock

and chemically

resistant –

Laboratory ware

Borosilicate

(Pyrex)

81 3.5 2.5 13 Thermally shock chemically inert –

oven wareContainer

(soda lime)

74 16 5 1 4MgO Low melting

temperature,

Easily worked

also durable

Fiber Glass 55 16 15 10 4MgO Easily drawn in to

fibers-glasss-resin

composites

Optical Flint 54 1 37PbO,

8K2O

High density and

high index of

refraction- optical

lenses

Glass -ceramics 43.5 14 30 5.5 6.5TiO2

0.5

As2O3

Easily fabricated,

strong, resists

thermal shocks-

ovenwareTable: composition and characteristics of some of the common commercial glass types



Crystalline structure of Glasses:

Commercial glasses are based on silica. They are made of the same SiO4 tetrahedra on

which the crystalline silicates are based, but they are arranged in a non-crystalline, or

amorphous, way. The difference is shown schematically in Fig. In the glass, the tetrahedra link

at the corners to give a random (rather than a periodic) network. Pure silica forms a glass with a

high softening temperature (about 1200°C). Its great strength and stability, and its low thermal

expansion, suit it for certain special applications, but it is hard to work with because its viscosity

is high.

Glass Ceramics

Most of the inorganic glasses are made to transform from a non-crystalline state to onethat is crystalline state by the proper high temperature heat treatment. This process is called

crystallization, and the product is fine grained polycrystalline material which s often called a

Glass-ceramic.

The formation of these small glass ceramic grains is in a sense a phase transformation,

which involves nucleation and growth stages. As a consequence the kinetics (i.e., the rate) of

crystallization may be described using the same principles that were applied to phase

transformation for the metal systems. Dependence of degree of transformation on the

temperature and the time may be expressed using isothermal transformations and the

continuous cooling transformation.

CASE STUDY: Continuous cooling phase transformation of Lunar Glass:



Fig: The Continuous cooling transformation diagram for the Crystallization of Lunar Glass (35.5 wt.% SiO 2, 14.3

wt.% TiO2, 3.7 wt.% Al2O3, 23.5 wt.% FeO, 11.6 wt.% MgO, 11.1 wt.% CaO, and 0.2wt% Na 2O). Also

superimposed on this plot are two cooling curves “1” & “2”.

The continuous cooling transformation diagram for the crystallization of a lunar glass is

presented as in figure the begin and end on transformation curves on this plot have the same

general shapes is in iron carbon alloy of eutectoid composition. Also included are twocontinuous cooling curves are represented as 1 & 2.bthe cooling rate represented by the curve

2 is much greater than that for the curve 1. As also noted on this plot, for the continuous

cooling path represented by the curve 1 crystallization begins at this intersection with the

upper curve and progresses as the time increases and the temperature decreases continuously;

upon crossing the lower curve as the original glass has crystallized. The other cooling curve just

misses the nose of the crystallization start curve. It represents that minimum cooling rate less

than this some Glasses ceramic is 100°C/min. A nucleating agent (frequently Titanium Dioxide)

is often used to the glasses to promote the crystallization. The presence of the nucleating agent

shifts the ‘begin’ and ‘end’ transformation curves to short times.



PROPERTIES: Glass materials have been damaged to have the following characteristcs

1. High mechanical strength

2. Low coefficient of thermal expansion to avoid thermal shocks

3. Relatively high temperature capabilities

4. Good dielectric properties for electron packaging application

5. Good biological compatibility

6. good optical characteristics

7. good bondage resistance in fibres of glass in composites

8. good corrosion resistance

9. ease of sand-witching properties with high cohesion

10. good electrical insulating

APPLICATION: some common application of glasses and glasses ceramics

Some commercial names of glass manufacturing under common names as follows

1. Pyroceram

2. Corning wares

3. Cercor

4. Vision

Applications:

1. Ovenware

2. Table ware

3. Oven windows

4. Rangetops5. Electrical insulation

6. Substrates for printed circuits boards

7. Architectural cladding

8. For heat exchangers and regenerators.

CLAY PRODUCTS (VIREOUS PRODUCTS):

One of the most widely used ceramic raw materials is clay. This inexpensive ingredient,

found naturally in great abundance, often is used as mined without any upgrading of quality.

Another reason for its popularity lies in the ease with which clay form a plastic mass that is very

amenable to shaping. The formed piece is dried to remove some of the moisture. After which it

is fired at an elevated temperature to improve its mechanical strength.

Potters have been respected members of society since ancient times. Their products

have survived the ravages of time better than any other; the pottery of an era or civilization

often gives the clearest picture of its state of development and its customs. Modern pottery,

porcelain, tiles, and structural and refractory bricks are made by processes which, though

automated, differ very little from those of 2000 years ago. All are made from clays, which are

formed in the wet, plastic state and then dried and fired. After firing, they consist of crystalline

phases (mostly silicates) held together by a glassy phase based, as always, on silica (SiO 2). The

glassy phase forms and melts when the clay is fired, and spreads around the surface of the



inert, but strong, crystalline phases, bonding them together. The important information is

summarized in table

Most of the clay-based products fall within two broad classifications; the structural clay

products and the white-wares.

• Structural clay products include building bricks, tiles, and sewer pipes.

• White wares ceramics become white after the high temperature firing. The white -wares

are porcelain, pottery, table ware, china and plumbing fixtures (sanitary ware). In

additions to clay, many of these products also contain nonplastic ingredient, which

influence he the changes that take place during the drying and firing processes.

REFRACTORIES:

Another important class of ceramics that are utilized in large tonnages is the

“Refractories Ceramics”. The salient properties of these materials include

1. Withstand high temperatures without decaying

2. Zero Decomposition of any change in environmental parameters

3. Very high melting points

4. Capacity to remain unreactive

5. Inert to many chemicals

6. Inert when exposed to many changes in environments

7. Thermal insulation

POROSITY: porosity is the one microstructural variable that must be controlled to produce a suitable

refractory brick. Strength, load carrying capacity and resistance to attack by corrosive materials all

increase with reduction of porosity. At the same time thermal insulation characteristics and resistance tothermal shock are diminished.



The performance of a refractory ceramic to a large degree, depends on its composition.

On this basis there are several classifications but namely

1. Fireclay refractories

2. Silica refractories

3. Basic refractories

4. Special refractories

Some five common refractories and its composition are given in following table

Refractory types Composition (wt. %) Apparent

porosity %

Al2O3 SiO2 MgO Cr2O3 Fe2O3 CaO TiO2

Fire clay 25-45 70-50 0-1 0-1 0-1 1-2 10-25

High Alumina

fireclay

90-50 10-45 0-1 0-1 0-1 1-4 18-25

Silica 0.2 96.3 0.6 2.2 25

Periclase 1.0 3.0 90.0 0.3 3.0 2.5 22

Periclase Chrome

ore

9.0 5.0 73.0 8.2 2.0 2.2 21

FIRE CLAY REFRACTORIES:

• Main ingredients silica and alumina mixtures

• Silica in the form of “cristobalite” about wt. % of 70 to 50

• alumina in the form of “Mullite” about the wt.% 25 to 45

• for this composition operating temperature is 1587°C, for hich the formation is possible

without liquid phase

• above 1587°C liquid phase present will be depending upo the compostion

• Upgrading alumina content will increase the maximum service temperature allowing

formation of small amount of liquid.

• Thermal Resistance is not only the criteria of these refractories but also the dimensional

stabilities higher temperatures.

• Application

Principally used in furnace constructions

Furnace inside liners and bricks

Thermal insulators where the high temperature exposures

SILICA REFRACTORIES:

• It consists of small concentrations of alumina by the wt.% of 0.2-0.1%, and the silica by

the wt.% of 95-97%.

• The oprating temperature of this eutectic composition of alumina is 7.7% is very near to

silica extremity is 1600°C.

• Application:

High temperature & high load bearing capacity applications

Roofing of steel and glass making furnaces

Used as containments where the high resistant to Acidic Slags formations.



BASIC REFRACTORIES:

• Ingredients of these refractories are Perclase or Magnesia( MgO)

•

They may also contain calcium, chromium, iron compounds • Application:

Basic refractories are highly used where the basic slag formations is more such as

Steel making process in Open Hearth Furnaces.

Some high heating chambers of sintering process.

SPECIAL REFRACTORIES:

• These are yet other ceramics materials which include high purity of oxides which may

contain little porosity. They may include Alumina, silica, magnesia, beryllia, zirconia,

mullite and some carbide compounds

• Applications : Crucibles of steel industry

• Carbide inclusion may be used in electrical resistance heating furnaces.

ABRASIVES:

• Abrasives are another form of ceramics which is having following characteristics

high wear resistance,

high hardness

toughness to shock loads to take care of abrasive particles don’t fracture

high grindablity

high temperature resistance due to frictional load.

Refractorness is also essential• Common abrasives are made of powder metallurgy process

• Common abrasives are diamond(natural & synthetic), silicon carbide, tungsten

carbide,aluminium oxide or corundum and silica sand

• Abrasives are used inseveral forms as bonded to griding wheels, coated abrasives or as

loose grains

CEMENTS:

• Cements are also a type of ceramics which include lime or calcium substrates which

upon affinity towards water, becomes paste then a gel form which formed is then

settled for setting then become hard solid structure with strong bods by Cohesive

forces, which is known property of ceramic.

• Some of common cement is ortland cement which include the major contents like tri

calcium silicate or dicalcium silicate. Which is when hydrated then becomes a srong

cementinous gell which then setting done upon which it becomes more solid.



ADVANCED CERAMICS:

• Some of advanced ceramics are made for the unusual properties Dielectric property

Piezoelectric ceramics

High refractive indices

• Some applications are MEMS, Optical fibers, Ceramic Ball bearings, etc



CERAMICS FORMING PROCESSES

The surface area of fine powders is enormous. A cupful of alumina powder with a

particle size of 1_m has a surface area of about 103 m2. If the surface energy of alumina is 1

Jm−2

, the surface energy of the cupful of powder is 1 kJ. This energy drives sintering. When the

powder is packed together and heated to a temperature at which diffusion becomes very rapid

(generally, to around 2/3Tm), the particles sinter , that is, they bond together to form small

necks which then grow, reducing the surface area, and causing the powder to densify. Full

density is not reached by this sort of sintering, but the residual porosity is in the form of small,

rounded holes which have only a small effect on mechanical strength. Figure shows, at a

microscopic level, what is going on. Atoms diffuse from the grain boundary which must form at

each neck (since the particles which meet there have different orientations), and deposit in the

pore, ending to fill it up. The atoms move by grain boundary diffusion (helped a little by latticediffusion, which tends to be slower). The reduction in surface area drives the process, and the

rate of diffusion controls its rate. This immediately tells us the two most important things we

need to know about solid state sintering:

• Fine particles sinter much faster than coarse ones because the surface area (and thus

the driving force) is higher, and because the diffusion distances are smaller.

• The rate of sintering varies with temperature in exactly the same way as the diffusion

coefficient. Thus the rate of densification is given by:

Here ρ is the density, a is the particle size, C and n are constants, Q is the activation energy for

sintering, R is the gas constant and T is the absolute temperature. n is typically about 3.



• The very specific character of ceramics high temperature stability makes

conventional fabrication routes unsuitable for ceramic processing.



• Inorganic glasses though make use of lower melting temperatures. Most other

ceramic products are manufactured through powder processing

• Typical ceramic processing route: powder synthesis–green component(casting, extrusion, compaction) – sintering/firing

Figure: Flow route of powder synthesis



Processing ceramics –Glasses

• Most of them are silica-soda-lime variety.

• Raw materials are heated to an elevated temperature where melting

occurs.

• Glass melt is processed by different route to form different products:

• Pressing– to form shapes like plates table wares and dishes

• Blowing–used to produce objects like jars, bottles, light bulbs.

• Drawing–to form lengthier objects like tubes, rods, whiskers, etc.



Ceramic powder processing• Ceramic powder processing route: synthesis of powder, followed by

fabrication of green product which is then consolidated to obtain the finalproduct.

• Synthesis of powder involves crushing, grinding, separating impurities,

blending different powders.

• Green component can be manufactured in different ways: tape casting, slip

casting, extrusion, injection molding and cold-/hot-compaction.

• Green component is then fired/sintered to get final product.

Ceramic powder processing -Casting

Slurry of ceramic powder is processed via casting routes

Tape casting, and slip casting.

Tape casting– also known as doctor blade process used for making thin

ceramic tapes. In this slurry of ceramic powder + binders + plasticizers is spread



over plastic substrate. Tape is then dried using hot air. Later tape is subjected to

binder burn out and sintering.

Slip casting–here aqueous slurry of ceramic powder is poured into plaster of

Paris mold. As water begins to move out due to capillary action, thick mass builds

along mold wall. It is possible to form solid piece by pouring more slurry.

Ceramic powder processing –Extrusion & Injection molding

Extrusion–viscous mixture of ceramic particles, binder and other additives

is fed through an extruder where continuous shape of green ceramic is

produced. Then the product is dried and sintered.

Injection molding–it is similar to the process used for polymer processing.

Mixture of ceramic powder, plasticizer, thermoplastic polymer, and

additives is injected into die with use of a extruder. Then polymer is burnt

off, before sintering rest of the ceramic shape. It is suitable for producing

complex shapes.

Extrusion and Injection molding are used to make ceramic tubes, bricks,

and tiles.

Ceramic powder processing –Compaction

Ceramic powder is compacted to form green shapes of sufficient strength to

handle and to machine.

Basis for compaction – application of external pressure from all directions.

In cold iso- static pressing (CIP), pressure is applied using oil/fluid, then

green product is subjected to sintering.

In hot iso– static pressing (HIP), pressure is applied at high temperatures

thus compaction and sintering occurs simultaneously. It is expensive buthas certain advantages.



Figure: normal ceramics forming processes by powder metallurgy technique



Figure: Typical Cemented Carbide Forming Technique through powder metallurgy.







Figure: Composite Ceramic Liners of Steel Melting Hearth of Furnace

Figure: Ceramic liners inside the steel furnace crucible

Figure: ceramic hexa bricks inside the furnace

Hot tuyeres liners

Rubber composite ceramics bricks

arrangements for the hoppers.



Blast Furnace roofing …

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Ceramics brief study