foundations of materials science and engineering third edition · used as reinforcement in...

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CHAPTER

11

Ceramics

1

The Porsche Carrera GT's carbon-

ceramic (silicon carbide) disc brake

Ceramic Si3N4 bearing parts

Radial rotor made from Si3N4 for a gas turbine engine

http://en.wikipedia.org/wiki/Disc_brake

Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi


Introduction

• Ceramics are inorganic solids composed of compound that contain

nonmetallic elements.

• Bounded by mixture of ionic and covalent types, depends on

electronegativity difference.

• Brittle, and lesser ductility and toughness than metals.

• High chemical stability and high melting temperature.

• Good electrical and heat insulation property.

Figure Relative fracture toughness of engineering materials

2



Simple Ionic Arrangements

• Packing of Ions depends upon

Relative size of ions.

Need to balance electron charges.

• If the anion does not touch the

cation, then the arrangement is

unstable.

• Radius ratio = rcation/ranion

• Critical radius ratio for stability

for coordination numbers 8,6 and

3 are >0.732, >0.414 and > 0.155

respectively.

Unstable

Stable

3



Cesium Chloride Crystal Structure

• CsCl is ionically bonded with radius ratio = 0.94 and

CN = 8.

• Eight chloride ion surround a central cesium cation at

the ( ½ , ½ , ½ ) position.

• CsBr, TlCl and TlBr have similar structure.

4



Sodium Chloride Crystal Structure

• Highly Ionically bonded

with Na+ ions occupying

interstitial sites between

FCC and Cl- ions.

• Radius ratio = 0.56, CN =

6.

• MgO, CaO, NiO and FeO

have similar structures.

5



Silicate Structures

• Silicate (SiO44-) is building block of silicates.

• 50% Ionic and 50% covalent.

• Many different silicate structures

can be produced.

• Island structure: Positive ions

bond with the oxygen of SiO44-

tetrahedron.

• Chain/ring structure: Two

corners of each SiO44- tetrahedron

bonds with corners of other

tetrahedron (a).

6



Sheet Structures of Silicates

• Sheet structure: Three corners of same planes of silicate

tetrahedron bonded to the corners of three other silicate

tetrahedra (b).

• Each tetrahedron has one

unbounded oxygen and hence

chains can bond with other

type of sheets.

• If the bondings are weak,

sheets slide over each other

easily.

7



Silicate Networks

• Silica: All four corners of the SiO44- tetrahedra share

oxygen atoms.

• Basic structures: Quartz, tridynute and cristobarlite.

• Important compound

of many ceramic and

glasses.

• Feldspars: Infinite 3D

networks.

• Some Al3+ Ions replace

Si4+ Ions Net negative charge.

8



• Figure Spectrum of ceramics uses

The Nature of Ceramics



Groups of Ceramics

• Traditional Ceramics

– Basic components (Clay, Silica and feldspar).

– Examples : glasses, bricks, tiles (used in construction industries

and electrical porcelain

• Engineering Ceramics

– Pure compounds (Al2O3, SiC, Si3N4)

– Examples : SiC for high temp experiment , automotive gas turbine

engine

– Aluminum oxide in support base for integrated circuit chips in a

thermal-conduction module.



Traditional Ceramics

Roofing tiles Bricks

Tiles Sewer pipe



Engineering Ceramics

• Alumina (Al2O3): Aluminum oxide is doped with

magnesium oxide, cold pressed and sintered.

Uniform structure. Used for electric applications.

• Silicon Nitride (Si3N4): Compact of silicon powder is

nitrided in a flow of nitrogen gas.

Moderate strength and used for parts of advanced engines.

• Silicon Carbide (SiC): Very hard refractory carbide,

sintered at 21000C.

Used as reinforcement in composite materials.

• Zirconia (ZrO2): Polymorphic and is subject to cracking.

Combined with 9% MgO to produce ceramic with high

fracture toughness.

12



Processing of Ceramics

• Produced by compacting

powder or particles into

shapes and heated to bond

particles together.

• Material preparation: Particles

and binders and lubricants are

(sometimes ground) and blend

wet or dry.

• The blending of the

ingredients with water is

common practice.

• Raw materials are grind dry

along with binders and other

additives

• Combination of wet and dry

process

13

Some ceramic fabrication process



Ceramic Forming Method

• Forming: Formed in dry, plastic or liquid

conditions.

• Cold forming process is predominant.

• Common forming processes :

1) Pressing

2) Slip casting

3) Extrusion



1- Fig 11.26 Dry Pressing of Ceramic Particles

(a) and (b) are filling, (c) pressing , (d) ejection

Application : manufacturing of insulating parts, magnetic ceramics,

and capacitors

Dry pressing is defined as the simultaneous uniaxial

compaction and shaping of a granular powder along with small

amounts of water or binder in a die.



Isostatic Processing of Spark Plug Insulator

Application : Ceramics parts such as refractories, bricks , spark plug

Insulators, crucibles and bearings

Isostatic pressing: Ceramic powder is loaded into a flexible

chamber and pressure is applied outside the chamber with hydraulic

fluid.



Figure 11.24 Stages of Spark Plug Insulator Manufacture

Stages of spark plug insulator manufacture by the isostatic processing

Method (a) Pressed blank (b) Turned (ground) insulator (c ) Fired

Insulator (d) Glazed and decorated finished insulator ( e) Cross section

of assembled automotive spark plug showing position of insulator.




• 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.

18

2- Slip Casting

( a) drain casting (b) solid casting




• 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 resulting in a long product ( rods, bars, hollow

tubes, pipes)

• Examples: Sewer pipe, hollow tubes, heat exchanger tubes

19

Extrusion



Ceramic Products

Moulded ceramics Extruded ceramics

Extruded ceramics filter



Thermal Treatments

• Drying: Parts are dried before firing to remove water from ceramic

body.

Usually carried out at or below 1000C.

• Sintering: Small particles are bonded together by solid state diffusion

producing dense coherent product. Sintering occurs by diffusion of

atoms through the microstructure

Carried out at higher temperature but below MP.

Longer the sintering time, larger the particles are.

• Vetrification: During firing, glass phase liquefies and fills the pores.

Upon cooling liquid phase of glass solidifies and a glass matrix

that bonds the particles is formed.

21



Mechanical Properties of Ceramics

• Strength of ceramics vary greatly but they are generally

brittle.

• Tensile strength is lower than compressive strength.

22



Factors Affecting Strength

• Failure occurs mainly from surface defects.

• Pores gives rise to stress concentration and cracks.

• Pores reduce effective cross-sectional area.

• Flaw (fault, or other imperfection) size is related to grain

size.

• Finer size ceramics have smaller flaws and hence are

stronger.

23



Fatigue Failure

• Fatigue fracture (The failure or rupture of a plastic article

under repeated cyclic stresses) in ceramics is rare due to

absence of plastic deformation.

• Straight fatigue crack in has been reported in alumina after

79,000 compression cycles.

• Ceramics are hard and can be used as abrasives (sheets and

wheels)

• Examples:- Al2O3, SiC.

• By combining ceramics, improved abrasives can be

developed.

• Example:- 25% ZrO2 + 75% Al2O3

• Note: An abrasive is a material that is used to shape or finish a

workpiece. 24



Thermal Properties of Ceramics

• Low thermal conductivity and high heat resistance.

• Many compounds (aluminum oxide and magnesium oxide )

are used as industrial refractories (A refractory material is

one that retains its strength at high temperatures).

• Dense refractories have low porosity and high resistance to

corrosion and errosion and to penetration by liquids and

gases

• For insulating refractories, porosity is desirable.

25



Glasses

• Combination of transparency, strength, hardness and

corrosion resistance.

• Glass is an inorganic product of fusion that has cooled to a

rigid condition without crystallization.

• Glass does not crystallize

up on cooling.

• Up on cooling, it transforms

from rubbery material to

rigid glass.

26



Structure of Glasses

• Fundamental subunit of glass is SiO44- tetrahedron.

• Si 4+ ion is covalently ionically bonded to four oxygen

atoms.

• In cristobalite, Si-O tetrahedron are joined corner to corner

to form long range order.

• In simple silica glass, tetrahedra are joined corner to corner

to form loose network.

Simple silica glass Cristobailite 27



Types of Glasses

• Soda lime glass: Very common glass (90%).

71-73% SiO2, 12-14% Na2O, 10-12% CaO.

Easier to form and used in flat glass and containers.

Used for windowpanes, and glass containers (bottles and jars) for beverages, food, and some commodity items

• Borosilicate glass: Alkali oxides are replaced by boric oxide in silica glass network. (silica and boron oxide )

Known as Pyrex glass and is used for lab equipments and piping.

• Lead glass: Lead oxide acts as network modifier and network former.

Low melting point – used for solder sealing.

Used in radiation shields, optical glass and TV bulbs.

28



Table 11.9 Types and Composition of Glasses



Soda Lime Glass

Milk bottles

Ordinary glass is soda lime glass



Borosilicate Glass



Lead Glass

Lead glass observation windows

Against radiation



Forming Methods for Glasses

• Blowing: Air blown to force molten glass into molds.

• Pressing: Flat items such as optical and sealed beam lenses

are pressed by a plunger into a mold containing molten

glass.

• Casting: Molten glass is cast in open mold.

33



Blowing of glass



Pressing of glass



Casting of Glass

Cast glass from art glass glass objects are cast by

directing molten glass into a

mold where it solidifies

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