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Kwame Nkrumah University of

Science & Technology, Kumasi, Ghana

MSE 351

Engineering Ceramics I

Ing. Anthony Andrews (PhD)Department of Materials Engineering

Faculty of Mechanical and Chemical Engineering

College of Engineering

Website: www.anthonydrews.wordpress.com www.knust.edu.gh

Course Objectives1. Differentiate between traditional and advanced ceramics.

2. Recognize and describe common ceramic crystal structures.

3. Understand the basics of ceramic processing, including sintering theory and grain growth.

4. Basic understanding of the types of commercial refractory materials, their manufacturing methods, their properties, and their characteristics during usage.

5. Understand the basics of the properties of advanced ceramic materials

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Outline

Week Topic

1 Introduction

2-3 Crystal structures of ceramics

4 Defects in ceramics

5-6 Mechanical properties of ceramics

7 Mid-semester exams

8 Ceramic phase diagrams

9-10 Refractories

11-12 Ceramic processing

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Prerequisite

• Principles of Materials Science I & II

• Phase Transformations

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Recommended Books

1. P. Hloben (2000) Refractory materials – major industrial

applications

2. W. D. Callister Jr. and D.G. Rethwisch (2010) Materials

science and engineering: An introduction, 8th edition

3. D. R. Askerland, P.P. Fulay, and W.J. Wright (2010) The

science and engineering of materials

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Course Assessment

Quizzes (Attendance) 10

Mid-Semester Exam 20

End Semester Exam 70

Total 100

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Introduction• Ceramics are compounds between inorganic and nonmetallic

elements.

– 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

• Involves processing at elevated temperatures (high Tm).

• Keramikos (burnt stuff in Greek) → desirable properties of

ceramics are normally achieved through a high-temperature heat

treatment process (firing).

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Characteristics of Ceramics

• Generally hard and brittle

• Generally electrical and thermal insulators

– Exceptions: graphite, diamond, AlN… and others)

• Can be optically opaque, semi-transparent, or transparent

• Generally low density

• Low to medium tensile strength

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Groups of Ceramics

• Traditional ceramics – based on clay (china, bricks, tiles,

porcelain), glasses.

• New ceramics (Advanced) for electronic, computer, aerospace

industries.

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General Classification of Ceramics

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Classification of Ceramics

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Classification of Ceramics

• Glasses:

– Primarily silica (SiO2) but non-crystalline!

– glassware, lenses, fiberglass, windows, borosilicate, glass

Pyrex, etc.

• Glass-ceramics:

– Glasses that have been transformed from a noncrystalline state

into a crystalline state through high temperature heat treatment.

(30-90% crystallinity)

– Pyroceram, Corning Ware.

• Crystalline ceramics:

– Clay, clay products, pottery, china, plumbing fixtures,

refractory ceramics

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Common Ceramic Materials

• Non-silicate ceramics

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Common Ceramic Materials

• Silicate ceramics

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Common Ceramic Materials

Electrical porcelain

Steatite porcelain

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Natural Ceramic Materials

• Stone is one of the oldest construction materials – very

cheap

– Limestone (CaCO3), Sandstone (SiO2), Granite

(aluminosilicates)

• Behavior similar to all brittle ceramic materials

• Use in compression only!

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Cement and Concrete• Used on an enormous scale in the construction industry

• Only brick and timber rival in volume (then steel)

• Very cheap - about 1/10th the cost per volume of steel

• Mixtures of lime (CaO), silica (SiO2) and alumina (Al2O3) which

hydrate (react with water) to form solids.

• Can be cast to shape.

• Relatively easy to manufacture from raw materials

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Glass• Enormous tonnages used - about the same as Al

– Up to 80% of the surface area of a modern building may be

glass

– Not load bearing

• Load bearing applications in vehicle windows, pressure

vessels, vacuum chambers

• Inert glass coatings used in chemical & food industries

(glazes)

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Traditional Ceramics• Include pottery, porcelain, tiles, structural and refractory bricks

• Made from clays which are molded in a plastic state and then

fired

• Consist of a glassy phase which melts and “glues” together a

complex polycrystalline multiphase body

• Raw material: Clays

– Kaolinite: Al2(Si2O5)(OH)4

– Montmorrilonite: Al5(Na,Mg)(Si2O5)6(OH)4

– Feldspar: K2O.Al2O3.8SiO2

– Quartz: SiO2

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Engineering Ceramics

• Traditional ceramics

– weak due to many pores and cracks.

– elastic moduli are low due to glassy phases present.

• Engineering ceramics

– pure fully dense ceramics with fewer cracks and higher

intrinsic elastic modulus.

– Contain more of pure compounds of oxides, carbides and

nitrides.

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Engineering Ceramics

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Engineering Ceramics - Alumina

• Refractory tubing

• High purity crucibles for high

temp

• High quality electrical applications

(low dielectric loss and high

resistivity)

• Spark plug insulator

Alumina tubes

Microstructure of sintered,

powdered Al2O3 doped with

MgO

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Engineering Ceramics – Silicon

Nitride

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Engineering Ceramics – Silicon

Carbide

• Hard refractory carbide.

• Form skin of SiO2 at high

temp.

• Resistance to oxidation at

high temp.

• Can be sintered 2100oC with

0.5-1%B.

• Fibrous reinforcement in

ceramic matrix composite

material.

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Ceramic Products

• Clay construction products - bricks, clay pipe, and building tile

• Refractory ceramics - ceramics capable of high temperature

applications such as furnace walls, crucibles, and molds

• Cement used in concrete - used for construction and roads

• Whiteware products - pottery, stoneware, fine china, porcelain,

and other tableware, based on mixtures of clay and other minerals

• Glass - bottles, glasses, lenses, window pane, and light bulbs

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Ceramic Products

• Glass fibers - thermal insulating wool, reinforced plastics

(fiberglass), and fiber optics communications lines

• Abrasives – Al2O3 and SiC

• Cutting tool materials - WC, Al2O3, and cBN

• Ceramic insulators - applications include electrical transmission

components, spark plugs, and microelectronic chip substrates

• Magnetic ceramics – example: computer memories

• Nuclear fuels based on uranium oxide (UO2)

• Bioceramics - artificial teeth and bones

Advanced Ceramics: Sensors, Valves, Dielectrics, Space shuttle, Spark plugs,

Magnetic recording media,

Glasses: Optical Composite, Porous ceramic tiles

Cements: Composites structural Clay: Whiteware Bricks

Refractories: Bricks for high T

Abrasives: Sandpaper Polishing

Heat Engines

Excellent wear

Corrosion resistance

Low frictional losses

Heat resistant

Low density

Brittle, Difficult to

machine

Si3N4, SiC, & ZrO2

Ceramic Armor

Al2O3, B4C, SiC &

TiB2, B6O

Hard materials

Electronic Packaging

Boron nitride (BN)

Silicon Carbide (SiC)

Aluminum nitride (AlN)

Good expansion

Good heat transfer

coefficient

Poor electrical

Conductivity

CERAMICS MATERIALS

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Ceramic Structures

• Ceramic crystal structures are

more complex since they are

composed of at least two

different elements.

• Bonding: ranges from purely

ionic to totally covalent.

• The degree of ionic character

depends on the electronegativity

of the atoms

where XA and XB are the elctronegativities of the two elements

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Atomic Bonding in Ceramics

• Degree of ionic character may be large or small:

SiC: small

CaF2: large

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Ceramic Crystal Structures

• Similar to metal crystal structures but with one important

difference…

– In ceramics, the lattice sites are occupied by IONS

• CATIONS: positively charged ions (the smaller of the two)

– electron loss TO the more electronegative atom

– cations are usually metals, from the left side of the periodic table

• ANIONS: negatively charged ions (the larger of the two)

– electron gain FROM the more electropositive atom

– anions are usually non-metals, from the right side of the periodic table

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Ceramic Crystal Structures

1. Charge neutrality

– the bulk ceramic must remain electrically neutral

– this means the NET CHARGE must sum to ZERO

2. Coordination number (CN)

– CN ≡ number of nearest-neighbor atoms

– as rc/ra increases, the cation’s CN also increases

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Charge Neutrality

Ex: The compounds “MgO2” or “Cs2Cl” do not exist. Why not?

• Consider the element’s electronic valence states:

1. Mg: Mg2+ (divalent), O: O2- (divalent)

• net charge per MgO2 molecule = 1(2+) + 2(2-) = -2

• this is a net negative charge, which is not allowed

2. Cs: Cs1+ (monovalent), Cl: Cl1- (monovalent)

• net charge per Cs2Cl molecule = 2(1+) + 1(1-) = -1

• this is a net positive charge, which is not allowed.

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Charge Neutrality

Examples of allowable ceramic stoichiometry:

• Mn - O: Mn2+O2- net charge = 1(2+) + 1(2-) = 0

• Mn - F: Mn2+F2- net charge = 1(2+) + 2(1-) = 0

• Fe - O: Fe2+O2- net charge = 1(2+) + 1(2-) = 0

• Fe - O: Fe3+2O

2-3 net charge = 2(3+) + 3(2-) = 0

• Ti - O: Ti4+O2-2 net charge = 1(4+) + 2(2-) = 0

• Si - O: Si4+O2-2 net charge = 1(4+) + 2(2-) = 0

• Al - O: Al3+2O

2-3 net charge = 2(3+) + 3(2-) = 0