Ceramics
description
Transcript of Ceramics
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Fundamental Engineering Materials (MA1002)
Associate Professor Sandy Chian School of Mechanical & Aerospace Engineering
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Text Book • William D. Callister, David G.Rethwisch “Materials Science and Engineering – An IntroducMon” 8th EdiMon, John Wiley and Sons, 2011
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Course Learning ObjecBves • To understand the Materials around us:
– Understand the fundamental building blocks of Materials [(Chapters 1 & 2)]
– The inter-‐relaBonship between • these building blocks (atoms), • their arrangements (structures and processes) and • their impact on properBes (applicaBons) [(Chapters 3 – 9)]
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Topics to be Introduced Ceramics (Chapter 12)
Polymers (Chapters 14, 15) Composites (Chapter 16)
• What are these materials? • What are their structures and properBes? • What are they used for?
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Review QuesBons on Bondings • Important to recap the different types primary and secondary bonds in materials. – Refer to Professor Shearwood’s notes if needed.
• See Ceramic Review QuesBon Set 1 MA1002-‐Fundamental Engineering-‐On Line Course Materials 5
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Review QuesBon Set (1)
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Ceramics
Structures & ProperBes
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What are Ceramics?
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Common items associated with the term Ceramics: Porcelain, chinawares, bricks, crystal vase, floor Mles, oven ware, etc.….. Materials known for their high compressive strengths, heat resistance BUT briWle
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Atoms in Ceramics • Ceramics are inorganic, non-‐metallic materials consisMng of
– Metal atoms (e.g. Iron, Aluminum, Calcium, etc )and – non-‐metal atoms (e.g. Oxides (O), Nitrides (N), and Carbides (C)) elements
• Refer to Periodic Table for examples of Metals and Non-‐Metals
• These metallic and non-‐metallic elements are bonded together primarily by ionic or covalent bonds or both
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Periodic Table
MA1002-‐Fundamental Engineering-‐On Line Course Materials 11 Figure 2.6: Callister & Rethwisch, 8th ediMon
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Examples of Ceramics Oxide based Ceramics
– Aluminum oxides (Al2O3)
– Silicon dioxide (SiO2)
– Sodium Silicate (Na2SiO3)
Non-‐oxide based Ceramics – Silicon nitride (Si3N4)
– Tungsten carbide (WC)
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Comparison with Metals Metals • Crystalline • Metallic bond • Free electrons • Good thermal/electrical conducMvity • Opaque • High tensile strength • Low shear strength • PlasMc flow • High density • Moderate hardness
Ceramics • Crystalline/Amorphous • Ionic/Covalent • CapMve electrons • Poor thermal/electrical conducMvity • Transparent/Opaque • Poor tensile strength • High shear strength • None • IniMal low density • High hardness
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See Review QuesBon set (2) on Molecular Structures
• The concept of Crystal and Amorphous structures were covered in previous lectures (Prof Shearwood & Prof Tan) and its Mmely to recap them at this juncture before proceeding.
• ObjecMve is to re-‐cap what the terms “crystalline” and “amorphous” structures mean and how these properMes and bonding influence material behaviours.
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Review QuesBon Set (2)
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Ceramic Materials Outline • Firstly we shall introduce Ceramics in terms of their ApplicaMon classificaMons …..
• Following that we will explore their various Structures and Morphologies
• And finally, how these structures & morphologies affect their properMes (and applicaMons)
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2 Main ClassificaBons of Ceramics in Terms of their ApplicaBons
• TradiBonal (ConvenMonal) Ceramics – Clay based products
• Porcelains, tableware, pokeries, etc.
• Fine Ceramics – Structural Ceramics
• Used for their mechanical and engineering properMes
– FuncBonal Ceramics • ApplicaMons other than mechanical strength such as electrical, opMcal and or
magneMc properMes
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ConvenBonal Ceramics
versus
Fine Ceramics
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TradiBonal (ConvenBonal) Ceramics • Made from 3 Principal Components:
• Clay: Consists of hydrated aluminum silicates with small amounts of other oxides such as TiO2, Fe2O3, MgO, CaO, Na2O and K2O
– FuncMon is to provide workability of the materials before firing hardens it and consMtutes the major body material.
• Silica: (Also known as flint or quartz) has a high melBng component and its funcMon is to provide high temperature properMes
• Potash feldspar: Basic composiMon K2O/Al2O3.6SiO2 – has a low melBng temperature and makes a glass when the
ceramic mix is fired and bonds the refractory components together.
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PlasBcity of clay allows shaping of tradiBonal ceramic product
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hkps://www.youtube.com/watch?v=aCkIgAcj644
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Fine Ceramics
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Structural (Engineering) Ceramics • Refers mainly to ceramics with excepBonal mechanical properBes (i.e.
high tensile load, compressive strength, wear resistance, etc.)
• Mainly pure compounds or nearly pure compounds of predominantly oxides, carbides or nitrides.
• Examples of important engineering ceramics: – Alumina (Al2O3) – Silicon nitride (Si3N4) – Silicon carbide (SiC) – Zirconia (ZrO2)
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FuncBonal Ceramics • Refers to ceramics that have special properBes such as electrical, magneMc, dielectrical, opMcal, etc.
• Examples include: – Barium Mtanate: mechanical transducer, ceramic capacitors
– Bismuth stronMum calcium copper oxide: High temperature super conductors
– Lead zirconate Mtanate (PZT): Ultrasonic transducer
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FuncBonal Ceramics
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Ni-‐LaNbO4 La-‐(Sr)MnO3 Fibre opMcs
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Advanced Ceramics
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HydroxyapaMte for bone implants
Space shukle shield
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Ceramics in Space Engineering
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Ceramic Review QuesBon Set (3)
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Structure & Morphology
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4 ClassificaBons of Ceramics Morphology • Polycrystalline ceramics
– Alumina, Silicon carbide, silica
• Glass – Amorphous
• non-‐crystalline
– Glass Ceramics • Small crystalline precipitates in glass matrix • (Schok’s Ceran™ ceramic cooktop panels)
– Porcelain • Large crystals in glass matrix • Heterogeneous microstructures
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General ProperBes of Ceramics • Hard and brikle with low toughness and ducMlity
• Low thermal and electrical conducMvity
• High melMng point and chemical resistance
• Low thermal coefficient of thermal expansion
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ProperBes (Density)
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Lower than metals
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ProperBes (Mechanical)
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Similar to metals
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ProperBes ( Toughness)
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Lower than metals
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ProperBes (Electrical)
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insulators
Lower conducBvity than metals
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What Determines Ceramic ProperBes? • Chemical composiBons
– Types of atoms and bondings
• Micro-‐ and Nano-‐ Structures
• Defects – Point, line, or planar defects – Refer to Prof. Shearwood’s notes on defects
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Bonding in Ceramics • Can be ionic and/or covalent.
– Many ceramics exhibit both bonds
– Ionic character of the bond depends on the electronegaMviMes of the atoms present
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Percentage Ionic Character • Greater the difference in electronegaBvity of the atoms, the more ionic the bond.
• Percentage Ionic Character:
where XA and XB are the electronegaBviBes for the respecMve elements
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% Ionic character = {1- exp[- (0.25)(XA - XB )2 ]} *100
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Adapted from Fig. 2.7, Callister & Rethwisch 8e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd ediMon, Copyright 1939 and 1940, 3rd ediMon. Copyright 1960 by Cornell University.)
• Degree of ionic character may be large or small:
Ionic Characteristics in Ceramics
SiC: small
CaF2: large
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Is Si-‐C bond Ionic or Covalent? • ElectronegaMvity values of Si and C are 1.8 and 2.5 respecMvely
• Si-‐C bond is 88.5% covalent in character MA1002-‐Fundamental Engineering-‐On Line Course Materials 43
% ionic character = 100 {1-exp[-0.25(XSi − XC )2 ]} =11.5%
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Review QuesBon Set (4) • Sample calculaMons for the students to try ….
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Ceramic Structure Stability • Two important condiMons for ceramic crystal structure stability: – Charge Neutrality – Maximum contact between CaBons and Anions • Determined by the Co-‐OrdinaBon Number (CN)
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• Bulk ceramic must remain electrically neutral – i.e. the Net Charge must be Zero – Determined by the molecular formula (AmXp, where m and p values to achieve charge neutrality) • “m” and “p” are associated with the valencies of atoms X and A respecMvely.
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Charge Neutrality
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Does MgO2 exists? • Consider the element’s electronic valence state:
– Mg: Mg2+ (divalent) – O: O2-‐ (divalent)
• Net charge per MgO2 molecule = 1(2+) + 2(2-‐) = -‐2 – i.e. a net negaBve charge, which is Not Allowed
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Allowable Ceramic Stoichiometry • Mn-‐O: Mn2+O2-‐; Net charge = 1(2+)+1(2-‐) = 0
• Mn-‐F2: Mn2+F2-‐; Net charge = 1(2+)+2(1-‐) = 0
• Ti-‐O2: Ti4+O2-‐2; Net charge = 1(4+)+2(2-‐) = 0
• Al2-‐O3: Al3+2O2-‐3; Net charge = 2(3+)+3(2-‐) = 0
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Significance of Cation-Anion Contact
“Stable” and “Unstable” structures
Adapted from Fig. 12.1, Callister & Rethwisch 8e.
-‐ -‐ -‐ -‐ +
unstable
-‐ -‐ -‐ -‐ +
stable
-‐ -‐
-‐ -‐ +
stable
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CaBon & Anions not in contact
CaBon & Anions in contact
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• The maximum contact between caMons and anions is determined by the relaMve size of the ions.
• Co-‐ordinaBon number (CN) & Ionic Radii – Dependent on radius raBo of caBon to anion (rc / ra)
– For a given co-‐ordinaMon number, there is a criBcal (minimum) radius raBo rc / ra , for which this caBon-‐anion contact is established.
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Co-‐OrdinaBon Number
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• On the basis of ionic radii, what crystal structure would you predict for FeO?
• Answer:
550014000770
anion
cation
...
rr
=
=
based on this raMo, -‐-‐ C.N = 6 because
0.414 < 0.550 < 0.732
-‐-‐ crystal structure is NaCl Data from Table 12.3, Callister & Rethwisch 8e.
Sample Problem: Predicting the Crystal Structure of FeO
Ionic radius (nm) 0.053 0.077 0.069 0.100
0.140 0.181 0.133
CaBon
Anion
Al 3+
Fe 2 +
Fe 3+
Ca 2+
O 2-‐
Cl -‐
F -‐
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Ionic Radius Ratio, Coordination Number and Crystal Structure
ZnS (zinc blende)
NaCl (sodium chloride)
CsCl (cesium chloride)
Adapted from Table 12.2, Callister & Rethwisch 8e.
2
r caBon r anion
Coord No.
< 0.155
0.155 -‐ 0.225
0.225 -‐ 0.414
0.414 -‐ 0.732
0.732 -‐ 1.0
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4
6
8
linear
triangular
tetrahedral
octahedral
cubic
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Types of Crystal Structures AX AX2
ABX3 (not covered)
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AX-‐Type Crystal Structures • Ceramics with equal numbers of caBons and anions.
• Several different crystal structures possible, which are named axer a common material that assumes the parMcular structure – Rocksalt (NaCl) :-‐ Octohedral – Caesium Chloride (CsCl) :-‐ Cubic – Zinc Blende (ZnS) :-‐ Tetrahedral
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Rock Salt Structure i.e. NaCl (sodium chloride or rock salt) structure
rNa = 0.102 nm
rNa/rCl = 0.564 (bet 0.414 and 0.732) (i.e. CN = 6)
∴ anion (Cl-) prefers octahedral sites
Adapted from Fig. 12.2, Callister & Rethwisch 8e.
rCl = 0.181 nm
4 Na+ and 4 Cl-‐ ions per unit cell
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Octahedral Sites
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CsCl Structure 939.0
181.0170.0
Cl
Cs ==−
+
r
r
Adapted from Fig. 12.3, Callister & Rethwisch 8e.
∴ Since 0.732 < 0.939 < 1.0, Hence CN = 8 and cubic
sites preferred
So each Cs+ has 8 neighbor Cl-
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Each unit cell of CsCl has 1-‐Cs ion and 8-‐Cl ions
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Zn Blende (ZnS) Crystal Structure
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All corners and faces are occupied by sulfur ions (4 per unit cell)
Zn atoms fill interior tetrahedral posiMons
Most highly covalently bonded compounds exhibit this type of crystal structure
rZn2+/rS2-‐ = 0.402, hence CN = 4
4 ZnS molecules per unit cell
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Tetrahedral Sites
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AmXp Type Crystal Structures • Where charges on CaMons and Anions are not the same where “m” and/or “p” ≠ 1 – i.e. AX2
• Common crystal structure is found in fluorite (CaF2)
• Other compounds include ZrO2 (cubic), UO2, PuO2 and ThO2
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CaF2 Crystal Structure • Molecular formula of Calcium
Fluorite (CaF2) • No of Ca2+/unit cell = 4 • No of F-/unit must be 8
• Ca2+ cations in FCC (CN=8) • F- anions in tetrahedral (CN = 4)
• i.e. Each Ca2+ co-ordin. with 8 F- Each F- co-ordin with 4 Ca2+
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fluorine
calcium
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Density Computations for Ceramics
A
AC )(NV
AAn
C
Σ+Σ"=ρ
Number of formula units/unit cell
Volume of unit cell Avogadro’s number
= sum of atomic weights of all anions in formula unit
€
ΣAA
€
ΣAC = sum of atomic weights of all caBons in formula unit
Formula unit = All the ions that are included in the chemical formula unit. For example: BaTiO3 = one Barium, one Ti, and three Oxygen.
MA1002-‐Fundamental Engineering-‐On Line Course Materials
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(n’): Formula Units/Unit Cell
MA1002-‐Fundamental Engineering-‐On Line Course Materials 67
No. of Na+ per unit cell = 4
To maintain charge neutrality in NaCl, there must also be 4 Cl-‐
Hence there are 4 molecules of NaCl per unit cell, i.e. n’ = 4
Refer to Example problem 12.3 in Callister’s 8th EdiBon (page 463)
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What about n’ for CaF2 ?
MA1002-‐Fundamental Engineering-‐On Line Course Materials 68
No. of Ca2+/unit cell = 4
There must be 4 molecules of CaF2 per unit cell of CaF2
n’ for CaF2 is therefore 4
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Summary (1) • Interatomic bonding in ceramics ranges from purely ionic
to totally covalent
• Predominantly Ionic – Metallic caMons are posiMvely charged; Non-‐metallic anions are negaMvely charged
– Crystal structure is determined by (a) the charge magnitude on each ion, and (b) the radius of each type of ion
MA1002-‐Fundamental Engineering-‐On Line Course Materials 69
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Summary (1) • Many of the simpler crystal structures are described in terms of
unit cells: – Rock Salt (AX) – Cesium chloride (AX) – Zinc blende (AX) – Calcium Fluorite (AmXp)
• Some crystal structures may be generated from the stacking of close-‐packed planes of anions; caMons fill intersMMal tetrahedral and/or octahedral posiMons that exists between adjacent planes.
MA1002-‐Fundamental Engineering-‐On Line Course Materials 70
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Ceramics Review QuesBons 5 • Re-‐cap on calculaMng unit cell dimensions and unit cell volumes, etc…
• PracMce calculaMon on co-‐ordinaMon number and determinaMon of crystal structures.
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 72
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Common, Abundant Ceramics
MA1002-‐Fundamental Engineering-‐On Line Course Materials 73
Silicates
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Silicate Ceramics • Silicate ceramics consist of silicon and oxygen atoms (ions) bonded
together in various arrangements. – Silicon and oxygen are the two most abundant elements found in the earth’s
crust
• Commonly found in naturally occurring minerals like clay, feldspars and micas
• Useful engineering materials due to low cost, availability, and special properMes – ConstrucMon materials as glass, portland cement, and brick – Electrical insulators
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Structure of Sand • Common name is Silicon Oxide (SiO2) or silica • In reality there is no discrete molecules of SiO2 in sand but a network of bonds: – Each silicon atom is bonded to 4 oxygen atoms – Each oxygen is bonded to 2 silicon atoms
• Structure of sand is formed by bonding of structural repeaMng units of SiO4
4-‐ ions (orthosilicates)
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Silicate Ceramics
MA1002-‐Fundamental Engineering-‐On Line Course Materials 76
Si4+
O2-
Adapted from Figs. 12.9-‐10, Callister & Rethwisch 8e
Tetrahedral structure (i.e. Orthosilicate Ion)
Basic building block of silicate ceramics is the Tetrahedral SiO44-‐
Each singly bonded oxygen has a single negaBve charge
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Various Silicate Structures • Different silicate structures can be formed depending on how the tetrahedra are linked together through their verBces.
• Tetrahedra are linked through a shared oxygen atom
MA1002-‐Fundamental Engineering-‐On Line Course Materials 77
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78
Bonding of adjacent SiO44- accomplished by the sharing of
common corners, edges, or faces
Silicates
Adapted from Fig. 12.12, Callister & Rethwisch 8e.
Mg2SiO4 Ca2MgSi2O7
MA1002-‐Fundamental Engineering-‐On Line Course Materials
Example of silicate
compounds: Na2Ca2Si3O9
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Single Strand Silicate Structure
MA1002-‐Fundamental Engineering-‐On Line Course Materials 79
Single strand silicate formed by tetrahedra linked through a shared oxygen atom.
RepeaBng unit = Si2O64-‐ or as simplest formula SiO3
2-‐
Example of single strand silicate structure: enstaBte, MgSiO3 which consist of rows of single-‐strand silicate chains with Mg2+ ions between the strands to maintain charge neutrality
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Other of Silicate Structures • Ordered Structures
– 2-‐D Layered sheet – 3-‐D Crystalline (Network)
• Disordered structured – Glass
MA1002-‐Fundamental Engineering-‐On Line Course Materials 80
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81
2-D Layered Silicates • Tetrahedra connected to 3 others to form
2-D sheet structure • Repeating unit for layered silicate sheet
is (Si2O5)2-
• Negative charge balanced by
adjacent plane rich in positively charged cations
• Examples: Talc, mica, clay
• Layered structure offers lubricity MA1002-‐Fundamental Engineering-‐On Line Course Materials
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82
• Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)42+ layer
Layered Silicates (cont.)
Adjacent sheets of this type are loosely bound to one another by van der Waal’s forces.
Adapted from Fig. 12.14, Callister & Rethwisch 8e.
MA1002-‐Fundamental Engineering-‐On Line Course Materials
NegaMvely charged layer
PosiMvely charged layer
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 83
Forms a Network Structure which gives SiO2 its strength and high melBng point.
i.e. similar to Diamond
3-‐D Network Crystalline Silicate Quartz (Silica) is a crystalline silicate
containing pure silicon dioxide (SiO2)
Diamond
Quartz
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84
Glass is non-‐crystalline (amorphous) contains impurity ions such as Na+, Ca2+, Al3+, and B3+ (which are network disruptors)
(soda glass)
Adapted from Fig. 12.11, Callister & Rethwisch 8e.
Glass Silicate
Si 4+
Na +
O 2 -‐
MA1002-‐Fundamental Engineering-‐On Line Course Materials
Network is disrupted & losing
3D regularity
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Summary (2) • Silicate structures are more conveniently represented in terms of interconnecBng SiO4
4-‐ tetrahedra.
• Complex structures may result when other caMons (e.g Ca2+, Mg2+, Al3+) and anions (e.g. OH-‐) are added.
• Silicate ceramics include crystalline silica (Quartz), layered silicates and non-‐crystalline silica glasses
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 86
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Carbon HybridisaMon of Carbon
Graphite Diamonds
MA1002-‐Fundamental Engineering-‐On Line Course Materials 87
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88
Structure of Carbon
• Atomic number = 6 • No. of proton = 6 • No. of electrons = 6 • Atomic mass = 12g/mole
• Electronic configuraBon – 1s2 – 2s2 2p2
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89
Various Structural Forms of Carbon
MA1002-‐Fundamental Engineering-‐On Line Course Materials
Sheet-‐like (Graphite)
Fullerene (Ball)
Diamond (Network)
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90
Methane • Simplest organ compound of carbon.
• Molecular structure : CH4
• All the C-‐H bonds are the same.
• Important QuesBon: – How can carbon with two kinds of orbitals (2s and 2p) forms 4 idenBcal bonds with hydrogen (1s)?
MA1002-‐Fundamental Engineering-‐On Line Course Materials
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Carbon-‐Hydrogen Covalent Bond
MA1002-‐Fundamental Engineering-‐On Line Course Materials 91
Unhybridised C: 1S2, 2S2 2P2
C H
2S1
2S1
1S1 2P1 2P1
Expect at two different bond lengths when the C forms covalent bond with 4 hydrogen
atoms to form methane
BUT all C-‐H covalent bonds are the same length
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ResoluBon to QuesBon? • Have to find a way of resolving how electrons with different orbitals in C (i.e. 2s and 2p) combine to form equivalent orbitals.
• Need to introduce the concept of HybridisaBon.
MA1002-‐Fundamental Engineering-‐On Line Course Materials 92
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HybridisaBon • HybridisaMon relates to a combinaMon of atomic orbitals of a
single atom. A mathemaMcal concept introduced by Linus Pauling.
• For Carbon, one s orbital and three p orbitals can combine or hybridise to form 4 equivalent (sp3) atomic orbitals with tetrahedral orientaBon.
• Other hybridisaMon of Carbon involving combinaMon of different
orbitals are also possible, i.e. sp2 and sp MA1002-‐Fundamental Engineering-‐On Line Course Materials 93
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 94
Other types of hybridisaBon and the resultant shapes of the orbital
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95
sp3 Hybrid
MA1002-‐Fundamental Engineering-‐On Line Course Materials
S
p
sp3
4 sp3
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96
Methane Structure
MA1002-‐Fundamental Engineering-‐On Line Course Materials
109.5o
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97
Ethane Structure
MA1002-‐Fundamental Engineering-‐On Line Course Materials
sp3 hybridised
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98
Other Carbon HybridisaBon • One 2s-‐orbital and three 2p-‐orbitals can be hybridised into the following states:
– Four sp3 (as in carbon in methane, C-‐H)
– Three sp2 hybrid orbitals with one 2p-‐orbital unhybridised (e.g. alkene, -‐C=C-‐)
– Two sp hybrid orbitals with two 2p-‐orbital unhybridised (e.g. alkyne, -‐C≡C-‐)
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99
sp2 HybridisaBon
MA1002-‐Fundamental Engineering-‐On Line Course Materials
sp2
3 sp2
s
p
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100
Ethylene Structure
MA1002-‐Fundamental Engineering-‐On Line Course Materials
120o
sp2 hybridised
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Double Bonds
MA1002-‐Fundamental Engineering-‐On Line Course Materials 101 hkp://en.wikipedia.org/wiki/Double_bond
The double bonds in ethylene C=C is due to (1) sp2-‐sp2 overlap (sigma bond) and (2) overlapping of the unhybridised p-‐orbitals of both carbons (pi-‐
bond)
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102
sp HybridisaBon
MA1002-‐Fundamental Engineering-‐On Line Course Materials
2 sp orbitals
s
p
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103
Ethyne Structure
MA1002-‐Fundamental Engineering-‐On Line Course Materials
sp -‐ hybridised
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CΞC Triple Bond • The triple bond between the two carbon atoms are due to:
• (1) sp – sp overlap (sigma bond) • (2) py – py overlap (pi bond) • (3) pz – pz overlap (pi bond)
MA1002-‐Fundamental Engineering-‐On Line Course Materials 104
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Insert separate video on the impact of hybridisaMon on shape
of atoms and molecules
MA1002-‐Fundamental Engineering-‐On Line Course Materials 105
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 106
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Summary 3 • The hybridisaMon state of carbon determines the type of bond formed: – sp3 => Single Bond (all sigma bonds) – sp2 => Double Bond (one sigma + one pi-‐bond) – sp => Triple Bond (one sigma + two pi-‐bonds)
– The overlapping pi-‐orbitals to form pi-‐bonds give rise to bond sBffness and enables electrons in pi-‐orbitals to delocalise.
MA1002-‐Fundamental Engineering-‐On Line Course Materials 107
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Allotropes of Carbon
108
a = Diamond b = Graphite c = Lonsdaleite d = Buckyball (C60) e = C540 f = C70 g = Amorphous carbon h = Single walled carbon nanotube (Buckytube)
MA1002-‐Fundamental Engineering-‐On Line Course Materials
Atoms of the element are bonded differently together.
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Graphite • A polymorphic form of Carbon
• Although not a compound of a metal and a non-‐metal, it is someMmes considered a ceramic material.
• Graphite has a layered structure in which the carbon atoms in the layers are strongly covalently bonded in hexagonal arrays
• The layers are weakly bonded by secondary bonds so allowing the layers to slide past each other. Hence giving graphite its lubricaMng properMes.
109 MA1002-‐Fundamental Engineering-‐On Line Course Materials
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110
Structure of Graphite – layered structure – parallel hexagonal arrays of carbon
atoms
– weak van der Waal’s forces between layers – planes slide easily over one another
Adapted from Fig. 12.17, Callister & Rethwisch 8e.
MA1002-‐Fundamental Engineering-‐On Line Course Materials
Line indicates VDW
forces not primary bonds
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C-‐HybridisaBon in Graphite
MA1002-‐Fundamental Engineering-‐On Line Course Materials 111
Adapted from NTU.AC.UK Unhybridised p-‐orbital overlaps. Graphite layer is flat. Electrons are delocalised in the p-‐orbital, i.e. electron conducBve
overlappingpi-‐bond
sigma-‐bond
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112
Structure of Diamond – Tetrahedral bonding of carbon – Network structure
• hardest material known • very high thermal conductivity but
electrical insulator – large single crystals – gem
stones – small crystals – used to grind/
cut other materials – diamond thin films
• hard surface coatings – used for cutting tools, medical devices, etc.
Adapted from Fig. 12.15, Callister & Rethwisch 8e.
MA1002-‐Fundamental Engineering-‐On Line Course Materials
sp3-‐sp3 sigma bond
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Show model of Graphite and Diamond
The impact of hybridisaMon on shape of Molecules
MA1002-‐Fundamental Engineering-‐On Line Course Materials 113
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 114
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Diamond versus Graphite
• Diamond – Carbon is sp3-‐hybridised – Pyramidal (tetrahedral) structure – All electrons are Mghtly held by the
carbon atoms – Non-‐electron conducMve – OpMcally clear due to high
refracMve index – Structure is very rigid – Good thermal conductor (800
Mmes beker than graphite)
• Graphite – Carbon has 3 sp2 and one p-‐orbital – Flat hexagonal structure – Electrons in the p-‐orbital are delocalised
and are electron conducMve – The p-‐orbital absorbs electrons/phonons,
hence opMcally opaque – Sheet structure held together by weak
VDW forces. Appears as smooth and sox as layers can slide.
– Bonds in each graphite sheet are very strong.
– Good thermal conductor but less than diamond
MA1002-‐Fundamental Engineering-‐On Line Course Materials 115
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116
Other Polymorphic Forms of Carbon 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.
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 117
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 118 hkp://www.youtube.com/watch?v=xVZRGcg-‐BXI
Buckyball
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MA1002-‐Fundamental Engineering-‐On Line Course Materials 119
Carbon Nanotubes
hkp://www.youtube.com/watch?v=-‐OKyTmM_faA
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Summary 4 • The hybridisaMon state of carbon determines the nature of the bond present: – Single (sp3) – Tetrahedral (Diamond) – Double (sp2) – Planar (Graphite) – Triple (sp) -‐ Linear
MA1002-‐Fundamental Engineering-‐On Line Course Materials 120
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Summary 4 • Carbon may exist in several polymorphic forms: Diamond,
Graphite, Fullerenes, and Nanotubes
• Each of these material has its own unique properMes: – Diamond (hardness, high thermal conducMvity) – Graphite (high-‐temperature chemical stability, good lubricity) – Fullerenes (electrically insulaMve, conducMve or semi-‐conducMve)
– Carbon nanotubes (extremely strong and sMff, electrically conducMve or semi-‐conducMve)
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Review QuesBon Set (6)
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ImperfecBons in Ceramics
MA1002-‐Fundamental Engineering-‐On Line Course Materials 124
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Causes of ImperfecBon • Structural imperfecMon has significant impact on the properMes of the material/product
• ImperfecMon could be due to: – Manufacturing processes giving rise to porosity, cracks, etc
– Inherent structural defects due to atoms arrangement creaMng point defects.
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Atomic Level Point Defects • ImperfecMon at atomic level is due to the introducBon of caBons and/or anions with different charges OR size as addiMves or impuriMes in the ceramics.
– The necessity to maintain charge neutrality gives rise to several point defects in ceramics
MA1002-‐Fundamental Engineering-‐On Line Course Materials 126
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127
• Vacancies -‐-‐ vacancies exist in ceramics for both caBons and anions • IntersBBals -‐-‐ intersMMals exist for caBons -‐-‐ intersMMals are not normally observed for anions because of their size
Adapted from Fig. 12.20, Callister & Rethwisch 8e. (Fig. 12.20 is from W.G. Moffak, G.W. Pearsall, and J. Wulff, The Structure and Proper=es of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.)
Point Defects in Ceramics (i)
CaMon IntersMMal
CaMon Vacancy
Anion Vacancy
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128
• Frenkel Defect -‐ a caBon vacancy-‐caBon intersBBal pair. • ShoWky Defect -‐ a paired set of caBon and anion vacancies.
Adapted from Fig.12.21, Callister & Rethwisch 8e. (Fig. 12.21 is from W.G. Moffak, G.W. Pearsall, and J. Wulff, The Structure and Proper=es of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.)
Point Defects in Ceramics (ii)
MA1002-‐Fundamental Engineering-‐On Line Course Materials
These defects occur in pairs to maintain charge neutrality
ShoWky Defect
Frenkel Defect
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Example of Point Defect FormaBon
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Case (1): SubsBtuBonal CaBon
Pure NaCl
Ca 2+
AddiMon of Ca2+ impurity
Each Ca2+ (divalent) will replace 2 Na+ (monovalent)
to maintain charge neutrality
with impurity
Ca 2+
caBon vacancy
One Ca2+ occupy the displaced Na+ caBon and to maintain charge neutrality another Na+ has to be removed, i.e. caBon vacancy
Na + Cl -‐
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Case (2): SubsBtuBonal Anion
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Pure NaCl
2-‐ O impurity
O
Each O2-‐ (divalent) will replace 2 Cl-‐ (monovalent) to maintain
charge neutrality
One O2-‐ occupy the displaced Cl-‐ anion and to maintain charge neutrality another Cl-‐ has to be removed, i.e. anion vacancy
Cl-‐ vacancy
NaCl with oxygen impurity
Oxygen anion
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Case (3): OxidaBon of Fe2+ to Fe3+
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Fe2+
O2-‐ Before Fe2+ oxidaBon
What happens when 2 Fe2+ oxidised
to form Fe3+
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SoluBon
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(Col A)
Original of +’ve charges (i.e. Fe2+)
(Col B)
Original –’ve charges
(O2-‐)
Col A + Col B
NeW Charge
Before OxidaBon
(Col C)
Final +’ve charges aser
Fe2+ oxidaBon
(Col D)
Final –’ve charges
Col C + Col D
NeW Charge
Aser oxidaBon
10(2+) = 20+
10(2-‐) = 20-‐
0 8(2+) + 2(3+) = 22+
10(2-‐) = 20-‐
2+
One neW 2+ charge forms for every 2 Fe2+ undergoing oxidaBon to Fe3+
To maintain charge neutrality two possible scenarios:
(a) Create a Fe2+ vacancy (b) Create a O2-‐ intersBBal OR X
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Fe2+ vacancy
Fe3+
Fe2+
O2-‐
Point Defect aser Fe2+ to Fe3+ transformaBon
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Porosity in Ceramics • Ceramics are oxen processed from powder and involved
compacMon under very high pressure and temperature.
• Pores and voids between parMcles are present during compacMon, and during heat treatments much of these pores are eliminated. Oxen some pores sMll remain.
• Porosity in ceramics significantly reduces their elasMc property and strength.
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Summary 5 • Atomic point defects, intersMMals and vacancies for each anion and caMon type are possible.
• Electrical charges are associated with atomic point defects in ceramic materials, defects someMmes occur in pairs (e.g. Frenkel and Schokky) in order to maintain charge neutrality.
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Summary 5 • In stoichiometric ceramic the raMo of caMons to anions is the same as predicted
by the chemical formula.
• Non-‐stoichiometric materials are possible in cases where one of the ions may exist in more than one ionic state, e.g. Fe(1-‐x)O for Fe2+ and Fe3+
• AddiMon of impurity atoms may result in the formaMon of subsBtuBonal or intersBBal solid soluMons. For subsMtuMonal, an impurity atom will subsMtute for that host atom to which it is most similar in an electrical sense.
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Review QuesBon Set (7)
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Mechanical ProperBes of Ceramics
Why are Ceramics BriWle?
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Crystalline Ceramics • At room temperature (or sub-‐ambient) most ceramics fracture before the onset of plasMc deformaMon.
• Most crystalline ceramics, which are predominantly ionic bonding, have very few slip systems along which dislocaMons may move.
• Porous nature of ceramics MA1002-‐Fundamental Engineering-‐On Line Course Materials 140
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Effect of Bonds in Ceramics on PlasBc DeformaBon
• Crystalline (ionic) ceramics – electrostaMc repulsion between ions when brought into close proximity of each other limits the amount slip.
• Crystalline (covalent) ceramics – Strong covalent bonds between atoms – Very limited slip system – Complex dislocaMon structure
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Crystalline (Ionic) Ceramics
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(a): Atoms are held by ionic bonds, i.e. each ion is surrounded by oppositely charged ions
(b): Any aWempt by the ions to slip past one another in response to the applied force is faced with strong repulsive coulombic forces. This makes slipping very difficult and the material responds by breaking. This is briWle failure.
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DeformaBon in Non-‐Crystalline Ceramics
• No plasMc deformaMon can occur by dislocaBon moBon due to absence of regular atomic structure.
• Material deforms by viscous flow
• In viscous flow, the material response to an applied shear stress, by sliding atoms or ions past each other by the breaking and re-‐forming of interatomic bonds.
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Impact of Porosity of Tensile Strength • Pores
– Reduces the cross-‐secMonal area across which load is applied
– Acts as stress concentrator • For an isolated spherical pore, an applied tensile stress is amplified by a factor of 2
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Effects of Porosity
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σ fs =σ o exp −nP( )E = Eo 1−1.9P + 0.9P2( )
Modulus of ElasBcity Flexural Strength
Eo = Modulus of non-‐porous material σ0 and n are experimental constants; P = volume fracMon
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Summary 6 • Microcracks in ceramics are hard to control and tensile
stresses are amplified resulMng in low fracture strengths (flexural strengths)
• Fracture strengths varies according to size of crack-‐iniBaBng flaws, and vary from sample to sample.
• Stress amplificaMon does not occur in compression, hence ceramics are stronger in compression.
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Summary 6 • Brikleness is due to the limited operable slip systems and
dislocaBon moBon
• PlasMc deformaMon for non-‐crystalline is by viscous flow. ViscosiMes of many non-‐crystalline ceramics are very high.
• Many ceramics have residual pores and this reduces their modulus of elasMcity and flexural strengths.
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Review QuesBon Set (8)
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