introduction to ceramics

transcript

Name: Fayza Shemsu

Jimma Institute of Technology

Department of Materials Science and

Engineering

TITLE: CERAMICS

CERAMICS

Thermal properties of ceramics Mechanical properties of ceramics Electrical properties of ceramics

Outline

Introduction Atomic bonding in ceramics Ceramics crystal structure Defects in ceramics General properties of ceramics

Classification of ceramics Electronic ceramics Processing of ceramics

The word ‘ceramic’ is originated from Greek word

“keromikos”, which means ‘burnt stuff’. Ceramics are compounds of metallic and non-metallic

elements.

Introduction

Are wide-ranging group of materials whose

ingredients are clays, sand and feldspar.

Are Inorganic non-metallic materials obtained by

the action of heat and subsequent cooling.

Always composed of more than one element (e.g., Al2O3,

NaCl, SiC, SiO2)

Bonds are partially or totally ionic, and can have

combination of ionic and covalent bonding

Generally hard and brittle

Generally electrical and thermal insulators

Can be optically opaque, semi-transparent, or Transparent

• Periodic table with ceramics compounds indicated by a combination of one or more metallic elements (in light color) with one or more nonmetallic elements (in dark color).

Atomic Bonding in Ceramics Bonding:

Degree of ionic character may be large or small:

SiC: small

CaF2: large

Can be ionic and/or covalent in character. % ionic character increases with difference

in electronegativity of atoms.

Ceramic Crystal Structures

Crystal structure is defined by -Magnitude of

the electrical charge on each ion.

Oxide structures

– oxygen anions larger than metal cations

– close packed oxygen in a lattice (usually FCC)

– cations fit into interstitial sites among oxygen ions

Stable ceramic crystal structures: anions surrounding a

cation are all in contact with that cation.

For a specific coordination number there is a critical or

minimum cation anion radius ratio rC/rA for which this

contact can be maintained.

Two interpenetrating FCC lattices

NaCl, MgO, LiF, FeO have this crystal structure

Rock Salt Structure Cesium Chloride Structure

Examples of crystal structures in ceramics

Zinc Blende Structure: typical for compounds where covalent bonding dominates. C.N. = 4

ZnS, ZnTe, SiC have this crystal structure

Fluorite (CaF2):

FCC structure with 3 atoms per lattice point

Silicate Ceramics• Most common elements on earth are Si & O

• SiO2 (silica) polymorphic forms are quartz, crystobalite, &

tridymite

• The strong Si-O bonds lead to a high melting temperature

(17100C) for this material 12

Adapted from Figs. 12.9-10, Callister & Rethwisch 8e crystobalite

Defects in Ceramics

• Vacancies -- vacancies exist in ceramics for both cations and anions

• Interstitials

Adapted from Fig. 12.20, Callister & Rethwisch 8e. (Fig. 12.20 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.)

Cation Interstitial

Cation Vacancy

Anion Vacancy

interstitials exist for cations interstitials are not normally observed for anions

because anions are large relative to the interstitial sites

Point Defects in Ceramics Point defects in ionic crystals are charged. The Coulombic forces are very large and any charge

imbalance has a strong tendency to balance itself. To maintain charge neutrality several point defects can be created:

Shottky Defect a paired set of cation and anion vacancies.

Shottky Defect:

Frenkel Defect

Frenkel Defect a cation vacancy-cation interstitial pair.

Low ductility– Very brittle– High elastic modulus

Low toughness– Low fracture toughness– Indicates the ability of a crack or flaw to produce a catastrophic failure

Low density– Porosity affects properties

High strength at elevated temperatures

General Properties of ceramics

Thermal properties

1)Thermal expansion The coefficients of thermal

expansion depend on the bond

strength between the atoms that

make up the materials.

Strong bonding (diamond,

silicon carbide, silicon nitrite) →

low thermal expansion

coefficient

Weak bonding ( stainless steel)

→ higher thermal expansion

coefficient in comparison with

fine ceramics

Comparison of thermal expansion coefficient between metals and fine ceramics

generally less than that of metals such as steel or copper

ceramic materials, in contrast, are used for thermal insulation due to their low thermal conductivity (except silicon carbide, aluminium nitride)

2)Thermal conductivity

A large number of ceramic materials are sensitive to thermal shock

Some ceramic materials → very high resistance to thermal shock is despite of low ductility (e.g. fused silica, Aluminium titanate )

The thermal stresses responsible for the response to temperature stress depend on:-geometrical boundary conditions-thermal boundary conditions-physical parameters (modulus of elasticity, strength…)

3)Thermal shock resistance

STRESS-STRAIN BEHAVIOR of selected materials

thermoplastic

http://www.keramvaerband.de/brevier_engl/5/5_2.htm

Mechanical Properties of Ceramics

Elastic modulus

The elastic modulus E [GPa] of almost all oxide and non-oxide ceramics is consistently higher than that of steel.

This results in an elastic deformation of only about 50 to 70 % of what is found in steel components.

http://www.keramverband.de/brevier_engl/5/3/4/5_3_4.htm

Material Class Vickers Hardness (HV) GPa

Glasses 5 – 10

Zirconias, Aluminium Nitrides 10 - 14

Aluminas, Silicon Nitrides 15 - 20

Silicon Carbides, Boron Carbides

20 - 30

Cubic Boron Nitride CBN 40 - 50

Diamond 60 – 70 >

Some typical hardness values for ceramic materials are provided below:

The high hardness of technical ceramics results in favourable wear resistance.

Ceramics are thus good for tribological applications. http://www.dynacer.com/hardness.htm

Hardness

http://www.subtech.com/dokuwiki/doku.php?id=fracture_toughness22

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Toughness

Material KIc (MPa-m1 / 2)

Metals

Aluminum alloy (7075) 24

Steel alloy (4340) 50Titanium alloy 44-66Aluminum 14-28CeramicsAluminum oxide 3-5Silicon carbide 3-5Soda-lime-glass 0.7-0.8Concrete 0.2-1.4PolymersPolystyrene 0.7-1.1Composites

Mullite fiber reinforced-mullite composite 1.8-3.3

Porosity can be generated through the appropriate selection of raw materials, the manufacturing process, and in some cases through the use of additives.

This allows closed and open pores to be created with sizes from a few nm up to a few µm.

http://www.ucl.ac.uk/cmr/webpages/spotlight/articles/colombo.htm

Change in elastic modulus with the amount of porosity in SiOC ceramic foams obtained from a preceramic polymer

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Porosity

Electrical properties of ceramic

• Most of ceramic materials are dielectric. (materials, having very low electric conductivity, but supporting electrostatic field).

• Dielectric ceramics are used for manufacturing capacitors, insulators and resistors.

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Superconducting properties

• Despite of very low electrical conductivity of most of the

ceramic materials, there are ceramics, possessing

superconductivity properties (near-to-zero electric resistivity).

• Lanthanum (yttrium)-barium-copper oxide ceramic may be

superconducting at temperature as high as 138 K.

• This critical temperature is much higher, than

superconductivity critical temperature of other

superconductors (up to 30 K).

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Ceramics are classified in many ways. It is due to

divergence in composition, properties and applications.

Based on their composition, ceramics are:

1.Classification –Ceramics

Carbides

Nitrides

Sulfides

Fluorides

CERAMICS

Oxides

Nonoxides

Composite

Oxide Ceramics:

Oxidation resistant

chemically inert

electrically insulating

generally low thermal conductivity

slightly complex manufacturing

low cost for alumina

more complex manufacturing

higher cost for zirconia.

zirconia

• Non-Oxide Ceramics:

Low oxidation resistance

extreme hardness

chemically inert

high thermal conductivity

electrically conducting

difficult energy dependent

manufacturing and high cost.

Silicon carbide cermic foam filter (CFS)

• Ceramic-Based Composites:

low and high oxidation

resistance (type related)

variable thermal and electrical

conductivity

complex manufacturing processes

high cost.

Ceramic Matrix Composite (CMC) rotor

Based on their engineering applications ,ceramics

are classified in to two groups as: traditional and

advanced ceramics.

Traditional ceramics–most made up of clay,

silica and feldspar

Advanced ceramics–these consist of highly

purified aluminum oxide(Al2O3), silicon carbide

(SiC) and silicon nitiride (Si3N4)

2.Classification –Ceramics

Classification of ceramics

The older and more generally known types

(porcelain, brick, earthenware, etc.)

Based primarily on natural raw materials of

clay and silicates

Applications; building materials (brick, clay pipe, glass)

household goods (pottery, cooking ware)

manufacturing ( abbrasives, electrical

devices, fibers)

Traditional Ceramics

1) Clay Ceramics

Made from natural clays and mixtures of clays and added crystalline ceramics.

These include:

Whitewares Crockery Floor and wall tiles Sanitary-ware Electrical porcelain Decorative ceramics

Whitewares Structural Clay Products

Whiteware: Bathrooms

2)Refractories Firebricks for furnaces and ovens.

Have high Silicon or Aluminium oxide content.

3)Amorphous Ceramics (Glasses)

Main ingredient is Silica (SiO2) If cooled very slowly will form crystalline structure. If cooled more quickly will form amorphous structure

consisting of disordered and linked chains of Silicon and Oxygen atoms.

This accounts for its transparency as it is the crystal boundaries that scatter the light, causing reflection.

Glass can be tempered to increase its toughness and resistance to cracking.

Three common types of glass: Soda-lime glass - 95% of all glass, windows

containers etc. Lead glass - contains lead oxide to improve

refractive index Borosilicate - contains Boron oxide, known as

Pyrex.

Glass ContainersLeaded Glass

4)Abrasives Natural (garnet, diamond, etc.) Synthetic abrasives (silicon carbide, diamond, fused

alumina, etc.) are used for grinding for cutting Si waferspolishing for oil drilling

lapping, or pressure blasting of materials.

Two Kyocera ceramic knives (Y:ZrO2)

oil drill bits

5) Cements Used to produce concrete roads, bridges, buildings,

have been developed over the past half century.

Include artificial raw materials, exhibit specialized

properties, require more sophisticated processing

Advanced ceramics are also referred to as “special,”

“technical,” or “engineering” ceramics.

They exhibit superior mechanical properties,

corrosion/oxidation resistance, or electrical, optical, and/or

magnetic properties.

Advanced Ceramics

laser host materials

piezoelectric ceramics

ceramics for dynamic random access

memories (DRAMs), often produced in small

quantities with higher prices.

as thermal barrier coatings to protect metal

structures, wearing surfaces

Engine applications :Si3N4, SiC, Zirconia

(ZrO2), Alumina (Al2O3))

Advanced ceramics include newer materials such as

Engine Components

Rotor (Alumina)

Gears (Alumina)

Turbocharger

Ceramic Rotor

Ceramic Si3N4 bearing parts

Radial rotor made from Si3N4 for a gas turbine engine

The Porsche Car silicon carbide disk brake

Structural ceramics

Silicon Carbide

Automotive Components in Silicon Carbide

Chosen for its heat and wear resistance

Body armour and other components chosen for their ballistic properties.

Ceramics in the field Biomaterials

Metallic framework

Angry gums

Ceramic framework

Why ceramics ?

Dental implant

The first use of ceramics in the electrical industry

took advantage of their stability when exposed to

extremes of weather and to their high electrical

resistivity, a feature of many siliceous materials.

Ceramics with higher resistivities also had high

negative temperature coe cients of resistivity, fficontrasting with the very much lower and positive

temperature coe cients characteristic of metals.ffi

Electronic Ceramics

Ferroelectric

Pyroelectric

Piezoelectric

Dielectric Dielectric Property

Piezoelectricity

Pyroelectricity

Ferroelectricity

Piezoelectricity Mechanical and electrical energy conversion phenomena,

discovered by France Scientist Pierre and Jacques Curie

brother in 1880.

They showed that crystals of tourmaline, quartz, topaz, cane

sugar, and Rochelle salt generate electrical polarization from

mechanical stress.

Piezoelectric Material will generate electric potential when

subjected to some kind of mechanical stress.

Crystal Structure of piezoelectric ceramics

A traditional piezoelectric ceramic is a mass of perovskite crystals.

Pervoskite structure,

Each crystal consists of a small tetravalent metal ion, usually titanium or zirconium, in a lattice of larger divalent metal ions, usually lead or barium, and O2- ions

with the chemical formula as ABO3

e.g. : BaTiO3, , CaTiO 3

Above the Curie point each perovskite crystal in the fired ceramic element exhibits a simple cubic symmetry

At temperatures below the Curie point, however, each crystal has tetragonal or rhombohedral symmetry and a dipole moment.

Applications of piezoelectric ceramics

Piezoelectric ceramics used as the resonator and filter in

communication system with frequency lower than 100MHz。The ceramic filter and resonator are made of high stability

piezoelectric ceramics that functions as a mechanical resonator.

The frequency is primary adjusted by the size and thickness of the

ceramic element.

Typical application includes telephones, remote controls and radios.

Processing of ceramics

powder compact or“green”

ceramic

Forming

Sintering ordensification or

firing

T 2Tm/3

Ceramic powder processing route: synthesis of

powder , followed by fabrication of green product

which is then consolidated to obtain the final

product.

Synthesis of powder involves

1)Ceramic powder processing

crushing,

grinding

Separating impurities

blending different powders.

Grinding refers to the reduction of small pieces after crushing to fine powder

Accomplished by abrasion, impact, and/or compaction by hard media such as balls or rolls

Examples of grinding include: Ball mill Roller mill Impact grinding

Ball mill Roller mill

Grinding

Green component can be manufactured in

different ways:

Green component is then fired/sintered to get

final product.

tape casting slip casting extrusion injection molding and cold-/hot-compaction.

Shaping Processes

Slip casting

• A suspension of ceramic powders in water , slip, is poured into a porous plaster mold .

• Water from the mix is absorbed into the plaster to form a firm layer of clay at the mold surface

http://global.kyocera.com/fcworld/first/process06.html

Raw materials are mixed with resin to provide the necessary fluidity degree.

Then injected into the molding die The mold is then cooled to harden the binder and produce a "green"

compact part (also known as an unsintered powder compact).

Drying process• Water must be removed from clay piece before

firing• Shrinkage is a problem during drying. Because

water contributes volume to the piece, and the volume is reduced when it is removed.

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• Sintering step is still very much required• Driving force for sintering–reduction in total surface area

and thus energy. • Functions of sintering are the same as before:

1. Bond individual grains into a solid mass2. Increase density3. Reduce or eliminate porosity

Sintering of Ceramics

Finishing Operations for Ceramics

• Parts made of ceramics sometimes require finishing, with one or more of the following purposes: 1. Increase dimensional accuracy 2. Improve surface finish3. Make minor changes in part geometry

• Finishing usually involves abrasive processes – Diamond abrasives must be used to cut the

hardened ceramic materials

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