chapter material scince
TRANSCRIPT
-
8/12/2019 Chapter material scince
1/43
Engineering MaterialsMECH 220
(Ch. 12)
-
8/12/2019 Chapter material scince
2/43
P2
Topics
Atomic bonding and crystalline structure of ceramics
Common ceramic materials
Point defects in ceramics
Mechanical properties
Topics Covered 12.1-12.5, 12.7, 12.8, 12.9, 12.11
-
8/12/2019 Chapter material scince
3/43
P3
Ceramicsare compounds of metallic and
non metallicelementswith totally ionic or
predominantly ionic bonds. Ceramicmaterials may have a crystalline or partly
crystalline structure (charge, size),or may
be amorphous (e.g., a glass).
The word "ceramic" comes from the Greek
word (keramikos), burnt stuff".
China clay, bricks, porcelain, tiles, glass etc...
-
8/12/2019 Chapter material scince
4/43
P4 Al2Si2O5(OH)4.
http://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Aluminium -
8/12/2019 Chapter material scince
5/43
P5
-
8/12/2019 Chapter material scince
6/43
P6
Structural- bricks, pipes, floor, roof tiles.
Refractories - such as gas fire radiants, steel and glass
making crucibles.
Whitewares - including tableware, cookware, wall tiles,pottery products and sanitary ware.
Engineering - Such items include tiles used in the Space
Shuttle program, gas burner nozzles, ballistic protection,
nuclear fuel uranium oxide pellets, biomedical implants,coatings of jet engine turbine blades, ceramic disk brake,
missile nose cones, bearing (mechanical).
Categories of Ceramic Materials
-
8/12/2019 Chapter material scince
7/43
P7
Si3N4 bearing and partsClassification of engineering ceramics
-
8/12/2019 Chapter material scince
8/43
P8
Classification of engineering ceramics
-
8/12/2019 Chapter material scince
9/43
P9
The Porsche Carrera GT's carbon-
ceramic (silicon carbide) composite disc
brake
http://en.wikipedia.org/wiki/File:PCCB_Brake_Carrera_GT.jpg
-
8/12/2019 Chapter material scince
10/43
P10
AerospaceEngines components; Airframes; Missile nose-cones; Space Shuttle tiles;
ballistic shields; Rocket nozzles.
AutomotiveHeat shield; Exhaust heat management devices
Biomedical
hip prosthesis ceramic head; Artificial bone; Dentistry applications, teeth;Biodegradable splints; Implant materials.
ElectronicsCapacitors; Integrated circuit packages; Transducers; Insulators
OpticalOptical fibers; Laser amplifiers; Lenses; Infrared heat-seeking devices
http://en.wikipedia.org/wiki/File:Hip_prosthesis.jpghttp://en.wikipedia.org/w/index.php?title=Ceramic_engineering&action=edit§ion=18http://en.wikipedia.org/w/index.php?title=Ceramic_engineering&action=edit§ion=19http://en.wikipedia.org/w/index.php?title=Ceramic_engineering&action=edit§ion=17http://en.wikipedia.org/wiki/File:Hip_prosthesis.jpg -
8/12/2019 Chapter material scince
11/43
Atomic bonding in ceramics
Ceramicsconsist of two or more chemical elements
Depending on the difference in electronegativity, atomicbonding can be predominantly ionicor covalent
P11
SiC: Covalent
CaF2: Ionic
-
8/12/2019 Chapter material scince
12/43
Crystal structure of Ionicceramics
Most common ceramics are crystalline
The crystalline structure is such that: The structure is electrically neutral (equal number of + andcharges)
Each cation (+ve) has as many closest anion (-ve) neighbors as
possible
The number of closest neighbors of an atom is the
coordination number
P12
Coordination number = 2 Coordination number = 3 Coordination number = 4
Stable structures: cation-
anion contact
Linear
Planar triangle
Tetrahedral
-
8/12/2019 Chapter material scince
13/43
Crystal structureIonic ceramics
A cation is smaller than an anion:
The bigger the cation the higher the number of possiblenearest neighbors the coordination number depends on
Effect of ion charge on the radius (e.g. Fe2+, Fe3+)
P13
1c
a
r
r
rc / ra
Cation radius
Anion radius
-
8/12/2019 Chapter material scince
14/43
P14
Example: for coordination number 3?rc/ r
a
Answer = 0.155
(1) Geometrical
considerati
ons
(2) hard sphere
model
Computation of Minimum Cation-Anion Radius Ratio
-
8/12/2019 Chapter material scince
15/43
P15
Example: for coordination number 6?rc / ra
Answer = 0.414
-
8/12/2019 Chapter material scince
16/43
16
Determine minimum rcation/ranionfor an octahedral site(C.N. = 6)
a=2ranion
2ranion 2rcation= 2 2ranion
ranion rcation= 2ranion rcation= ( 21)ranion
arr 222 cationanion =
414.012
anion
cation==
r
r
Solution
-
8/12/2019 Chapter material scince
17/43
Example problem 12.2
Based on table 12.3, what is the crystal structure for FeO?
P17
Ionic radius (nm)
0.053
0.077
0.0690.100
0.140
0.181
0.133
Cation
Anion
Al3+
Fe2+
Fe3+Ca2+
O2-
Cl-
F-
0.0770.55
0.140
c
a
r
r= =
Based on Table 12.2 C.N.= 6
-
8/12/2019 Chapter material scince
18/43
P18
Crystal Structures where cationand anion have the same charge
-
8/12/2019 Chapter material scince
19/43
-
8/12/2019 Chapter material scince
20/43
Rock salt (or NaCl) crystalline structure
Each cation has six anion closest neighbors => CN = 6
The anions have the same
coordination number as cations
rc/rA=0.414-0.732
Cations and anions are
arranged in FCCcells
(interpenetratingFCC cells)
Examples: NaCl, MgO, FeO
P20
-
8/12/2019 Chapter material scince
21/43
Cesium chloride structure (CsCl)
Each cation has 8 anion nearest neighbors (cubic site,
CN=8 for both ions)
Anions occupy the corners of a cube, a cation occupies the
center
What type of structure is this one??BCC or FCC??
The anions and cations occupy simple
cubic lattice sites
P21
-
8/12/2019 Chapter material scince
22/43
Zinc blende (sphalerite, ZnS) crystalline
structure Each cation has 4 anion nearest neighbors (tetrahedralsite, CN = 4)
Anions occupy the corners and
faces of a cubic cell (???)
Cations occupy tetrahedral positions
Examples: ZnS, SiC
P22
Cations
Anions
Source: wikipedia
-
8/12/2019 Chapter material scince
23/43
P23
Crystal Structures where cationand anion have different charge
-
8/12/2019 Chapter material scince
24/43
P24
AmXpType Crystal Structure
If the charges on the cation and
anion are not same a compoundexists with a formula AmXpwhere
m and/or p 1
Example: Calcium fluorite CaF2,
the rc/raratio is about 0.8 and
coordination number 8. Ca ions
are at the center of the cube and F
ions at the corners.
Fluorite structure: CeO2, ZrO2, UO2
-
8/12/2019 Chapter material scince
25/43
ABX3-type structure, twocations
A and B for 3 anions X Example: BaTiO3(Barium
Titanate)
The smaller cation has room to
move within the cell interesting
electric and magnetic properties
At high temperature (>120 C),
BaTiO3belongs to the cubic
system
P25
AmBnXpType Crystal Structure
Perovskite structure
What is the number of Ba2+, Ti4+, O2-ions in the unit cell?????
-
8/12/2019 Chapter material scince
26/43
Theoretical Density computation from Unit cell dat
The density is given by
n: number of formula units (ex. NaCl, MgO, ) within unit cell
M: atomic mass of formula unit
Ac: atomic mass of a cation within unit cell
Aa: atomic mass of an anion within unit cell
Vc: volume of unit cell
NA: Avogadros constant
P26
//=
c a AA
c c
n A A N n M N
V V
=
-
8/12/2019 Chapter material scince
27/43
Example problem 12.3 (Self Study)
Determine the density of NaCl
ANa= 22.99 amu, ACl= 35.45 amu, NA= 6.022 x 1023 atoms/mol
Rock salt structure
both Na, Cl occupy FCC lattice sites
One FCC cell 4 atoms
Edge length:
From table 12.3:
P27
23 33
4 22.99 35.45 / 6.022 10= 2.14 g/cm
a
=
2 2Cl Na
a r r =
0.181 , 0.102Cl Na
r nm r nm = =
-
8/12/2019 Chapter material scince
28/43
Common ceramics - Silicates
Silicates are ceramics consisting of silicon and oxygen.
These are the most common chemical elements on earth The simplest form of silicates is SiO4
4-
SiO2is silica. It has a number of polymorphic
formsthat include quartz, crystobalite, & tridymite
Glass is an amorphous form of SiO2
The Si-O bonds are strong & covalent (directional) high
melting temperature~1700 C
P28
-
8/12/2019 Chapter material scince
29/43
29
Bonding of adjacent SiO4
4-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, and
2. ionically bond SiO44-
to one another
-
8/12/2019 Chapter material scince
30/43
The structure of glass
Glass is amorphous (non-crystalline)
Basic unit is SiO44-
Common glass can be Pure fused silica
Fused silica with impurities, such as Na+, Al3+
P30
Quartz is crystalline
SiO2:
-
8/12/2019 Chapter material scince
31/43
31
Layered Silicates
Layered silicates (e.g., clays, mica, talc)
SiO4 tetrahedra connectedtogether to form 2-D plane
A net negative charge is associated witheach (Si2O5)
2-unit
Negative charge balanced byadjacent plane rich in positively chargedcations
Adapted from Fig. 12.13,
Callister & Rethwisch 8e.
-
8/12/2019 Chapter material scince
32/43
32
Kaolinite clay alternates (Si2O5)2-layer with Al2(OH)4
2+
layer
Layered Silicates (cont.)
Note: Adjacent sheets of this type are loosely bound to one another by van derWaals forces.
Adapted from Fig. 12.14, Callister &
Rethwisch 8e.
OH-, O2-
Th f b
-
8/12/2019 Chapter material scince
33/43
The case of carbon
Carbon is not a ceramic, but graphite is somet imes classified
as a ceramic
Graphite has a number of polymorphs, including
Diamond: has the crystalline structure of Zinc blende
Fullerenes (C60): has a sphere-like structure containing 60 carbon atoms
Carbon nanotubes (CNT): tubular form, impressive mechanical properties
P33
GraphiteDiamond
C60
CNT
Spherical cluster
Di d
-
8/12/2019 Chapter material scince
34/43
Diamond
Carbon organized in tetrahedrons
Strong bonds hardest known material Very high thermal conductivity (> Cu)
Can exist as large single crystals
(gemstones!)
Small crystals are used forgrinding/cutting other materials
P34
weak van der Waals forces between layersplanes slide easily over one another -- good lubricant
Graphite
P i t d f t i i
-
8/12/2019 Chapter material scince
35/43
Point defects in ceramics
Point defects include vacancies, interstitial defects, and
substitutional defects Vacancies: missing cations or anions
Interstitial defect: extra cation inside interstice
Anions are too large for interstitial sites
P35
Cation interstiti
Cation vacancy
Anion vacancy
Electroneutralityshould be applied
[+]=[-]
S i l t f i t d f t
-
8/12/2019 Chapter material scince
36/43
Special types of point defects
Frenkel defect: cation vacancy AND cation interstitial
Schottky defect: cation vacancy AND anion vacancy
P36
Shottky
Defect:
Frenkel
Defect
Neutrality of electriccharge is preserved
Schottky
defect
Frenkel
defect
Stoichiometry is preserved:ratio of cations/anions is the samewith the formula ratio
Non stoichiometry: Fe1-XO instead of FeO (Fe2+, Fe3+states)
C ti th b f d f t
-
8/12/2019 Chapter material scince
37/43
Computing the number of defects
Number of Frenkel defects
Number of Schottky defects
P37
Nfr
=Nexp Qfr2kT
Ns =Nexp Qs
2kT
Activation energy
Number of lattice sites(potential defects)
Boltzmanns constant
Temperature (K)
C i Ph Di
-
8/12/2019 Chapter material scince
38/43
38
Ceramic Phase Diagrams
MgO-Al2
O3
diagram (as an example):
Adapted from Fig. 12.25,
Callister & Rethwisch 8e.
Spinel phase for a
range of
compositions
Low solubility of
Al2O3 to MgO
below 1400oC
MgO insoluble to
Al2O3
2 eutectic points
-
8/12/2019 Chapter material scince
39/43
F t f i (F t h f i )
-
8/12/2019 Chapter material scince
40/43
P41
Fracture of ceramics (Fractography of ceramics)Crack propagation study and microscopic features of the
fracture surface
Crack propagates until a critical velocity is reached (e.g. for glass is
half of the speed of sound) and this is repeated until a family of cracks
is formed.
Fl l ( b d) t th
-
8/12/2019 Chapter material scince
41/43
Flexural (or bend) strength
Stress-strain response of ceramics is difficult to determine using
a tensile experiment because of brittleness (fail after 0.1% ofstrain)Bending (flexure) test is applied instead
Flexural strength is the stress at fracture using a flexure test Specimen (rectangular of circular geometry), 3 or 4-point technique
For a beam of rectangular cross-section:
For a beam of circular cross-section:
Ff: Load at fracture, L: length of beam, R: radius, b: width of beam, d: height
P42
232
f
fsF Lbd
=
3
f
fs
F L
R
=
Compression state
Tension state
-
8/12/2019 Chapter material scince
42/43
P43
Influence of porosity on mechanical properties
-
8/12/2019 Chapter material scince
43/43
Influence of porosity on mechanical properties
Elastic stiffness (Youngs modulus) decreases with
increasing porosity (pores due to powder ceramic fabrication,forming, etc.)
Flexural strength decreases exponentially with porosity (pores
reduce the actual cross sectional area where load is applied
+they act as stress concentrators)
20 1 0.9 0.9E E P P=
Stiffness of non-porous material Volume fraction of pores
0 exp( )fs nP =
Flexural strength of non-porous material Material parameter