Introduction to Engineering Materials ENGR2000 Chapter 12...

Introduction to Engineering MaterialsENGR2000

Chapter 12: Structures and Properties of CeramicsDr. Coates

Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com)

12.1 Introduction

• Ceramics– Compounds between metallic & non-metallic elements– Predominantly Ionic bonds– Some have covalent bonds– Greek word keramikos which means ‘burnt stuff’– Traditional ceramics

• china, porcelain, bricks, tiles, glasses, etc.

12.2 Crystal Structures

• Crystals composed of electrically charged ions• Cations

– Positively charged– Metallic ions

• Anions– Negatively charged– Non-metallic ions

• Concepts influencing the crystal structure– Crystal must be electrically neutral– Cations & anions are of different sizes– Cations & anions may have different magnitudes of

electrical charge

• Cations are usually smaller than anions

• Each cation prefers to have as many nearest-neighbor anions as possible.

• Each anion prefers to have as many nearest-neighbor cations as possible.

1/ AC rr

Stable ceramic crystal structures

• Anions surrounding a cation are all in contact withthat cation.

Coordination number– # of anion nearest neighbors

for a cation• The CN is related to

• For a specific CN, there is a critical or minimumradius ratio for which this contact is established.

Ac rr /

Example 12.1

• Show that the minimum cation-to-anion radiusratio for the coordination number 3 is 0.155.

155.01866.01

866.0/1

/2330cos

ACACAA

rrrrrrrr

rrAOrAP

AX-Type Crystal Structures

• Equal number of cations & anions• AX compounds• A denotes the cation• X denotes the anion

Rock salt (NaCl) structure

• Two interpenetrating FCC lattices – one composedof cations and the other of anions

732.0/414.0

AC rrClCNNaCN

Rock salt (NaCl) structure

• Common ceramic materials with this crystalstructure– NaCl, MgO, MnS, LiF, FeO

Cesium Chloride Structure

• Note that this is not a BCC crystal structure(different kinds of ions)

8 ClCNCsCN

Zinc Blende (ZnS) Structure

• Common ceramic materials with this crystalstructure– ZnS, ZnTe, SiC

4 XCNACN

AmXp-Type Crystal Structures

• Consider the AX2- type CaF2 (flourite) structure

• Predicts a structure similar to the CsCl• There are ½ as many Ca ions as there are F ions.

tablefrom88.0/

CNrr AC

Flourite (CaF2) Crystal Structure

• Only ½ the center cube positions are occupied byCa ions

Example 12.2

• On the basis of ionic radii, what crystal structurewould you predict for FeO?

structure crystalsalt rock -

6:12.2 Table From

550.0140.0077.0

compound. type-AXan is FeO

XCNACN

Example

• On the basis of ionic radii, what crystal structurewould you predict for CaF2?

Ca of #F of #

F of #21Ca of #

8:12.2 Table From

752.0133.0100.0

compound. type-AXan is CaF

FCNFCN

Ceramic Density Computations

number. sAvogadro' theis and volumecellunit theis

unit formula in the anions all of wtsatomic of sum theis

unit formula in the cations all of wtsatomic of sum theis cellunit e within thunits formula ofnumber theis ' where

':data cellunit fromdensity lTheoretica

Formula unit

• All the ions that are included in the chemicalformula.– A formula unit of BaTiO3 consists of one barium ion,

one titanium ion and three oxygen ions.

Example 12.3

• On the basis of crystal structure, compute thetheoretical density for NaCl. How does thiscompare with its measured density?

molgAA

molgAAn

/45.35

/99.224'

124111'

g/cm2.16density Measured

/14.2 '

:density lTheoretica

102.0:12.3 tableFrom

nmrnmr

12.3 Silicate Ceramics

• Silicates are materials composed primarily of Siand O, the two most abundant elements in theearth’s crust.– Soils, rocks, clay & sand

Silicate CeramicsMost 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(1710ºC) for this material

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

Bonding of adjacent SiO44- accomplished by the sharing of

common corners, edges, or faces

Silicates

Mg2SiO4 Ca2MgSi2O7

Adapted from Fig. 12.12, Callister & Rethwisch 8e.

Presence of cations such as Ca2+, Mg2+, & Al3+

1. maintain charge neutrality, and2. ionically bond SiO4

4- to one another

Layered Silicates

• Layered silicates (e.g., clays, mica, talc)– SiO4 tetrahedra connected

together to form 2-D plane

• A net negative charge is associated witheach (Si2O5)2- unit

• Negative charge balanced byadjacent plane rich in positively chargedcations

• Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)42+ layer

Layered Silicates (cont.)

Note: Adjacent sheets of this type are loosely bound to one another by van der Waal’s forces.

• Quartz is crystallineSiO2:

• Basic Unit: Glass is noncrystalline (amorphous)• Fused silica is SiO2 to which no

impurities have been added• Other common glasses contain

impurity ions such as Na+, Ca2+, Al3+, and B3+

(soda glass)Adapted from Fig. 12.11, Callister & Rethwisch 8e.

Glass Structure

Si0 4 tetrahedron4-

Si4+Na +

12.4 Carbon

• Carbon exists in various polymorphic forms aswell as in the amorphous form.

Polymorphic Forms of Carbon

Diamond– tetrahedral bonding of

carbon• hardest material known• very high thermal conductivity

– large single crystals – gemstones

– small crystals – used togrind/cut other materials

– diamond thin films• hard surface coatings – used

for cutting tools, medicaldevices, etc.

Polymorphic Forms of Carbon (cont)Graphite– layered structure – parallel hexagonal arrays of carbon

– weak van der Waal’s forces between layers– planes slide easily over one another -- good lubricant

Polymorphic Forms of Carbon (cont)Fullerenes and Nanotubes

• Fullerenes – spherical cluster of 60 carbon atoms, C60

– Like a soccer ball• Carbon nanotubes – sheet of graphite rolled into a tube

– Ends capped with fullerene hemispheres

Adapted from Figs. 12.18 & 12.19, Callister & Rethwisch 8e.

Carbon Nanotubes

• Tube diameters ~ 100 nm• Each nanotube is a single molecule composed of

millions of atoms• Extremely strong & stiff as well as ductile• Tensile strengths ~ 50 Gpa to 200 Gpa• Elastic modulus ~ 1000 Gpa• Low density

12.5 Imperfections in CeramicsAtomic point defects

• Vacancies– Anion vacancy– Cation vacancy

• Interstitials– Anion interstitial– Cation interstitial

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

• Interstitials-- interstitials exist for cations-- interstitials are not normally observed for anions because anions

are large relative to the interstitial sites

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.)

Point Defects in Ceramics (i)

Cation Interstitial

Cation Vacancy

Anion Vacancy

• Frenkel Defect-- a cation vacancy-cation interstitial pair.

• Shottky Defect-- a paired set of cation and anion vacancies.

• Equilibrium concentration of defects

Adapted from Fig.12.21, Callister & Rethwisch 8e. (Fig. 12.21 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.)

Point Defects in Ceramics (ii)

Shottky Defect:

Frenkel Defect

/kTQDe

Why are anion interstitials highly improbable?

• Anion is relatively larger• Has to fit into a small interstitial position

• No, they occur in pairs• Electro neutrality must be maintained

Can defects in ceramics occur alone?

Stoichiometry

• A state for ionic compounds wherein there is theexact ratio of cations to anions as predicted by thechemical formula.

• NaCl is stoichiometric if the ratio of Na+ ions toCl- ions is exactly 1:1

The ratio of cations to anions is not altered by the formation of a Frenkel or Schottky defect.

- Material is stoichiometric

Non stoichiometric materials

• FeO – iron is present as Fe2+ or Fe3+

Impurities in Ceramics

• Substitutional solid solution– Substitutional impurity will substitute for the host ion

to which it is most similar in an electrical sense

Anion substitutional

Cation substitutional

Impurities in Ceramics

• Interstitial solid solution– impurity must be relatively small

12.7 Ceramic Phase DiagramsThe Al2O3 – Cr2O3 system

• Substitutional solid solution– Al3+ substitutes for Cr3+– Similarly charged ions– Similar ionic radii (0.053 nm and 0.062 nm)– Al2O3 and Cr2O3 have similar crystal structures

Isomorphous system

The MgO – Al2O3 system

• Limited solubility– Differences in charge in Mg2+ and Al3+

– Differences in radii (0.072 nm and 0.053 nm)

Eutectic points

Intermediate phase

Mechanical Properties

Brittle fracture - before any plastic deformation

Slope=elastic modulus

Elastic modulus of metals, ceramics & polymers

• Ceramics– high elastic modulus– diamond, graphite (covalent bonds)

• Metals– high elastic modulus

• Polymers– low elastic modulus– weak secondary bonds between chains

Increasing E

Plastic deformation in metals, ceramics & polymers

• Polymers– strain hardening due to chain alignment

• Metals– strain hardening

• Ceramics– almost no plastic deformation

Increasingtendency forplastic deformation

12.9 Stress-Strain Behavior

Mechanical properties of metals & ceramics

Metals• high elastic modulus• significant plastic

deformation• high fracture stress• stress-strain curves for

tension & compression arenearly identical

Ceramics• higher elastic modulus• no significant plastic

deformation• higher fracture stress• stress-strain curves depend

on the load (tension orcompression)

12.10 Mechanisms of Plastic DeformationCrystalline ceramics

• Plastic deformation occurs by motion ofdislocations

• Dislocation motion is impeded• Hence ceramics are hard & brittle

12.11 Miscellaneous Mechanical Considerations

• Porosity– Initial material is in the form of a powder for some

ceramic fabrication techniques– Residual pores or void spaces remain after the

fabrication process

material. nonporous the of modulus elastic theis

porosityfraction volume theis 9.09.11

Hardness

• Ceramics are the hardest known materials– utilized when an abrasive or grinding action is required

Resistance to environmental failure of metals, ceramics & polymers

• Ceramics– high resistance to environmental attack

• Metals• Polymers

– moisture effectsIncreasingresistance

Example

• Which material has the highest elastic modulus?• Which material has the highest ductility?• Which material has the highest toughness?• Which material does not exhibit

any significant plastic deformationprior to fracture?

Mechanical Properties

Ceramic materials are more brittle than metals.Why is this so?

• Consider mechanism of deformation– In crystalline, by dislocation motion– In highly ionic solids, dislocation motion is difficult

• few slip systems• resistance to motion of ions of like charge (e.g., anions) past one

another

• Room T behavior is usually elastic, with brittle failure.• 3-Point Bend Testing often used.

-- tensile tests are difficult for brittle materials.Adapted from Fig. 12.32, Callister & Rethwisch 8e.

Flexural Tests – Measurement of Elastic Modulus

FL/2 L/2

d = midpoint deflection

cross section

rect. circ.

• Determine elastic modulus according to:F

linear-elastic behaviord

slope =3

4bdLFE

d (rect. cross section)

12 RLFEd

(circ. cross section)

• 3-point bend test to measure room-T flexural strength.

Flexural Tests – Measurement of Flexural Strength

FL/2 L/2

d = midpoint deflection

cross section

rect. circ.

location of max tension

• Flexural strength: • Typical values:

Data from Table 12.5, Callister & Rethwisch 8e.

Si nitrideSi carbideAl oxideglass (soda-lime)

250-1000100-820275-700

30434539369

Material sfs (MPa) E(GPa)

LFffs s (rect. cross section)

(circ. cross section)3RLFf

SUMMARY• Interatomic bonding in ceramics is ionic and/or covalent.• Ceramic crystal structures are based on:

-- maintaining charge neutrality-- cation-anion radii ratios.

• Imperfections-- Atomic point: vacancy, interstitial (cation), Frenkel, Schottky-- Impurities: substitutional, interstitial-- Maintenance of charge neutrality

• Room-temperature mechanical behavior – flexural tests-- linear-elastic; measurement of elastic modulus-- brittle fracture; measurement of flexural modulus

Introduction to Engineering Materials ENGR2000 Chapter 12...

Documents

Transcript of Introduction to Engineering Materials ENGR2000 Chapter 12...

Supplementary Materials - Materials

Introduction to Engineering Materials ENGR2000 Chapter 9 ...engineering.armstrong.edu/cameron/ENGR2000_phasediagrams2.pdfProperties of the different phases • α-ferrite is relatively

Robotics boe bot - Georgia Southern University …engineering.armstrong.edu/cameron/Robotics_boe_bot.pdfMicrosoft PowerPoint - Robotics_boe_bot.pptx Author cameron Created Date 11/10/2010

Materials Wrought materials production - European … · Materials – Wrought materials production Table of contents 5 Wrought materials production ...

Introduction to Engineering Materials ENGR2000 …engineering.armstrong.edu/cameron/ENGR2000...Introduction to Engineering Materials ENGR2000 Chapter 10: Phase Transformations in Metals

Impression Materials Impression Materials DH 363 Dental Materials I.

Introduction to Engineering Materials ENGR2000 Chapter 5 ...engineering.armstrong.edu/cameron/ENGR2000_diffusion.pdf · • The wear resistance of a steel gear is to be improved by

Introduction to Engineering Materials ENGR2000 Chapter 9 ...engineering.armstrong.edu/cameron/ENGR2000_phasediagrams1.pdf · •Tie lines and lever rule still apply. ... calculate

Huber Engineered Materials | Inorganic Materials | Advanced … · 2019-01-15 · Huber Engineered Materials | Inorganic Materials | Advanced Industrial Raw Materials

Commentary: The Materials Project: A materials genome ... · APL MATERIALS 1, 011002 (2013) Commentary: The Materials Project: A materials genome approach to accelerating materials

AS: Use of Maths Algebra & Graphs Practice sheet ...engineering.armstrong.edu/cameron/rearranging-formulas-extra...© Nuffield Foundation 2004 AS Use of Maths Scheme of Work Wizard

Materials Science and Engineering - 2017-2018 Berkeley ...guide.berkeley.edu/departments/materials-science-engineering/materials... · Materials Science and Engineering 1 Materials

Mechanisms Presentation - Georgia Southern …engineering.armstrong.edu/cameron/murphy's law in action.pdfWedge Cam Follower • Typically the followers for the wedge cam can be one

Resistant Materials Resistant Materials Resistant ... Materials Resistant Materials Resistant Materials Resistant Materials Resistant MaterialsName: ... 13. You need to keep this booklet

ENGR2000 Fluid Mechanics Semester 1, 2017ctl.curtin.edu.au/teaching_learning_services/unit_outline_builder/... · l Munson, B.R, Young, ... Fluid Mechanics, 5th Edition (or higher),

Introduction to Engineering Materials ENGR2000 Chapter 14 ...engineering.armstrong.edu/cameron/ENGR2000_polymers I.pdf · Introduction to Engineering Materials ENGR2000 Chapter 14:

Force System Resultants Moments, Couples, and Force …engineering.armstrong.edu/cameron/Moment of a Force I.pdf · 4.1 Introduction to Moments The tendency of a force to rotate a

Mechanisms - Georgia Southern University-Armstrong …engineering.armstrong.edu/cameron/Mechanisms[1].pdf · internally combustible engine; two slider crank mechanisms in the form

Introduction to Engineering Materials ENGR2000 Chapter 4 ...engineering.armstrong.edu/cameron/ENGR2000_imperfections.pdf · 2. Crystal structure – same for metals of both atom types.

Introduction to Engineering Materials ENGR2000 …engineering.armstrong.edu/cameron/ENGR2000_failure.pdf · 8.3 & 8.4 Ductile & Brittle Fracture Extremely soft materials like gold