Chapter 11: Structural Materials Metals, Ceramics, and Glasses

경상대학교 Ceramic Design Lab.


ISSUES TO ADDRESS...

• How are metal alloys classified and how are they used?

• What are some of the common fabrication techniques?

• How do properties vary throughout a piece of material

that has been quenched, for example?

• How can properties be modified by post heat treatment?

Chapter 11: Structural Materials –

Metals, Ceramics, and Glasses


Taxonomy of Metals Metal Alloys

Steels

Ferrous Nonferrous

Cast Irons Cu Al Mg Ti

<1.4wt%C 3-4.5 wt%C

Steels <1.4 wt% C

Cast Irons 3-4.5 wt% C

Fe 3 C

cementite

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g

austenite

g +L

g +Fe3C a

ferrite a +Fe3C

L+Fe3C

d

(Fe) Co , wt% C

Eutectic:

Eutectoid: 0.76

4.30

727°C

1148°C

T(°C) microstructure: ferrite, graphite cementite


Steels

Low Alloy High Alloy

low carbon <0.25 wt% C

Med carbon 0.25-0.6 wt% C

high carbon 0.6-1.4 wt% C

Uses auto struc. sheet

bridges towers press. vessels

crank shafts bolts hammers blades

pistons gears wear applic.

wear applic.

drills saws dies

high T applic. turbines furnaces V. corros. resistant

Example 1010 4310 1040 43 40 1095 4190 304

Additions none Cr,V Ni, Mo

none Cr, Ni Mo

none Cr, V, Mo, W

Cr, Ni, Mo

plain HSLA plain heat

treatable plain tool

austenitic stainless

Name

Hardenability 0 + + ++ ++ +++ 0 TS - 0 + ++ + ++ 0 EL + + 0 - - -- ++

increasing strength, cost, decreasing ductility


Refinement of Steel from Ore

Iron Ore Coke

Limestone

3CO + Fe2O3 2Fe +3CO2

C + O2 CO2

CO2 + C 2CO

CaCO3 CaO+CO2 CaO + SiO2 + Al2O3 slag

purification

reduction of iron ore to metal

heat generation

Molten iron

BLAST FURNACE

slag air

layers of coke

and iron ore

gas refractory

vessel


Ferrous Alloys

Iron containing – Steels - cast irons

Nomenclature AISI & SAE

10xx Plain Carbon Steels

11xx Plain Carbon Steels (resulfurized for machinability)

15xx Mn (10 ~ 20%)

40xx Mo (0.20 ~ 0.30%)

43xx Ni (1.65 - 2.00%), Cr (0.4 - 0.90%), Mo (0.2 - 0.3%)

44xx Mo (0.5%)

where xx is wt% C x 100

example: 1060 steel – plain carbon steel with 0.60 wt% C

Stainless Steel -- >11% Cr


TABLE 11.1 AISI–SAE Designation System for Carbon and Low-Alloy Steels


TABLE 11.1 (continued) AISI–SAE Designation System for Carbon and Low-Alloy Steels


Low-carbon ste el: less than about 0.25 wt% C, no martensite, cold work strengthening, Ferrite and pearlite microstrure

y = 275 MPa, =415-550 Mpa, 25% EL

High-strength, low-alloy (HSLA) steel contain other alloying elements such as Mn, P, Si, Ni, Mo in

combined concentration as high as 1 wt% Most may be strengthened by heat treatment, giving tensile

strength in execes of 480 Mpa.

Medium-carbon ste el: between about 0.25 and 0.60 wt% C, may be heat-treated by austenitizing, quenching, and then tempering to improve

their mechanical properties , They ate most often utilized in the tempered condition, having microsturtures of tempered martensite.

High-carbon steel: between about 0.60 and 1.4 wt% C, are the highest, strongest, and yet least ductile of the carbon steels. They are almost

always used in a hardened and tempered condition, as such, are specially wear resistant and capable of holding a sharp cutting edge. Alloys

containing Cr, V, Mo, W combine with carbon to form very hard and wear-resistant carbide compounds (e.g., Cr23C6, V4C3, and WC)


Stainless Steels

• Alloy steels containing at least 10% Cr are SS.

• Contain sufficient amount of Cr that they are considered high alloy.

• Corrosion resistance is imparted by the formation of a passivation

layer characterized by:

– Insoluble chromium oxide film on the surface of the metal - (Cr2O3) .

– Develops when exposed to oxygen and impervious to water and air.

– Layer is too thin to be visible

– Quickly reforms when damaged

– Susceptible to sensitization, pitting, crevice corrosion and acidic envir

onments.

– Passivation can be improved by adding nickel, molybdenum and vana

dium.


General Properties of Stainless Steels

• Electrical Resistivity

– Surface & bulk resistance is

higher than that for plain-

carbon steels

• Thermal Conductivity

– About 40 to 50 percent that of

plain-carbon steel

• Melting Temperature

– Plain-carbon:1480-1540 °C

– Martensitic: 1400-1530 °C

– Ferritic: 1400-1530 °C

– Austenitic: 1370-1450 °C

• Coefficient of Thermal

Expansion

– Greater coefficient than plain-

carbon steels

• High Strength

– Exhibit high strength at room and

elevated temperatures

• Surface Preparation

– Surface films must be removed

prior to welding


SS can also be classified by crystal structure (austenitic, ferritic, marten

sitic)

•Best corrosion resistance (CR): Austenitic (25% Cr)

•Middle CR : Ferritic (15% Cr),

•Least CR: Martensitic (12% Cr), but strongest

• Over 150 grades of SS available, usually categorized into 5 series

containing alloys w/ similar properties.

• AISI classes for SS:

– 200 series = chromium, nickel,manganese (austenitic)

– 300 series = chromium, nickel (austenitic)

– 400 series = chromium only (ferritic)

– 500 series = low chromium <12% (martensitic)

– 600 series = Precipitation hardened series (17-7PH, 15-5PH)


200/300 Series SS (Austenitic):

• Most common SS (roughly 70% of total SS production)

• Used for flatware, cookware, architecture, automotive, etc.

• 0.15% C (max), 16% Cr (min) and Ni or Manganese

• Austenitic, High strength, best corrosion resistance. High temp capability

up to 1200 F. non-magnetic, good ductility and toughness, not hardenable

by heat treatment, but they can be strengthened via cold working, best

corrosion resistance but most expensive, corrosive in hydrochloric acid.

• General use where corrosion resistance is needed.

• Typical alloy 18% Cr and 10% Ni = commonly known as 18/10 stainless

• Also have low carbon version of Austenitic SS (316L or 304L) used to avoid

corrosion problem caused by welding, L = carbon content < 0.03%


400 Series SS (Ferritic):

• Ferritic, Automotive trim, chemical processing, blades, knives,

springs, ball bearings, surgical instruments. Can be heat

treated!

• Contain between 10.5% and 27% Cr, little Ni and usually

molybdenum.

– Common grades: 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni

• Magnetic (high in Fe content) and may rust due to iron

content.

• Lower strength vs 300 series austenitic grades

• Cheap


500 Series SS (Martensitic):

• Not as corrosion resistant as the other classes but extremely

strong and tough as well as machineable and can be

hardened via heat treat.

• High strength structural applications (Su up to 300 ksi) –

nuclear plants, ships, steel turbine blades, tools, etc.

• Magnetic


600 Series Precipitation Hardening Martensitic SS:

• Have corrosion resistance comporable to 300 series austentic

grades but can be precipitation hardened for increased

strength!

• Key: High strength + corrosion resistance BOTH.

• Why? Aerospace industry – defense budgets determined 2%

of GDP spent dealing with corrosion so developed high

strength corrosion resistant steel to replace alloy steels.

• Lockheed-Martin Joint Striker Fighter – 1st aircraft to use PH

SS for entire airframe.

• Common Grades: – 630 grade = 17-4 PH (17% Cr, 4% Ni),

– 17-4 PH,

– 15-5 PH


Cast Iron

• Ferrous alloys with > 2.1 wt% C

– more commonly 3 - 4.5 wt%C

• low melting (also brittle) so easiest to cast

• Cementite decomposes to ferrite + graphite

Fe3C 3 Fe (a) + C (graphite)

– generally a slow process


Fe-C True Equilibrium Diagram

Graphite formation

promoted by

• Si > 1 wt%

• slow cooling

1600

1400

1200

1000

800

600

400 0 1 2 3 4 90

L

g +L

a + Graphite

Liquid +

Graphite

(Fe) Co , wt% C

0.6

5 740°C

T(°C)

g + Graphite

100

1153°C g

Austenite 4.2 wt% C

a + g


Types of Cast Iron

Gray iron

• graphite flakes

• weak & brittle under tension

• stronger under compression

• excellent vibrational dampening

• wear resistant

Ductile iron

• add Mg or Ce

• graphite in nodules not flakes

• matrix often pearlite - better

ductility


Types of Cast Iron

White iron

• <1wt% Si so harder but brittle

• more cementite

Malleable iron

• heat treat at 800-900ºC

• graphite in rosettes

• more ductile


Ductile Iron

Malleable Iron Gray Iron

White Cast Iron

White cast iron Gray iron Ductile iron Malleable iron

Ferrous metallurgy


Production of Cast Iron


Limitations of Ferrous Alloys

1) Relatively high density

2) Relatively low conductivity

3) Poor corrosion resistance


Nonferrous Alloys

NonFerrous Alloys

• Al Alloys -lower r : 2.7g/cm3

-Cu, Mg, Si, Mn, Zn additions -solid sol. or precip. strengthened (struct.

aircraft parts & packaging)

• Mg Alloys -very low r : 1.7g/cm3

-ignites easily - aircraft, missiles

• Refractory metals -high melting T -Nb, Mo, W, Ta • Noble metals

-Ag, Au, Pt - oxid./corr. resistant

• Ti Alloys -lower r : 4.5g/cm3

vs 7.9 for steel -reactive at high T - space applic.

• Cu Alloys Brass: Zn is subst. impurity (costume jewelry, coins, corrosion resistant) Bronze : Sn, Al, Si, Ni are subst. impurity (bushings, landing gear) Cu-Be : precip. hardened for strength


Metal Fabrication

• How do we fabricate metals?

– Blacksmith - hammer (forged)

– Molding - cast

• Forming Operations

– Rough stock formed to final shape

Hot working vs. Cold working

• T high enough for • well below Tm

recrystallization • work hardening

• Larger deformations • smaller deformations


• Casting (Cast iron)

• Wrought processing

– Rolling

– Extrusion

– Forming

– Stamping

– Forging

– Drawing

• Joining

– Welding

– Brazing

– Soldering

• Powder metallurgy

• Hot isostatic pressing

• Superplastic forming

• Rapid solidification

• Additive manufacturing

Major processing methods for metals


FORMING

roll

A o

A d roll

• Rolling (Hot or Cold Rolling)

(I-beams, rails, sheet & plate)

A o A d

force

die

blank

force

• Forging (Hammering; Stamping)

(wrenches, crankshafts)

often at

elev. T

Metal Fabrication Methods - I

ram billet

container

container

force die holder

die

A o

A d extrusion

• Extrusion

(rods, tubing)

ductile metals, e.g. Cu, Al (hot)

tensile force

A o

A d die

die

• Drawing

(rods, wire, tubing)

die must be well lubricated & clean

CASTING JOINING


FORMING CASTING JOINING

Metal Fabrication Methods - II

• Casting- mold is filled with metal

– metal melted in furnace, perhaps alloying elements added. Then cast in a mold

– most common, cheapest method

– gives good production of shapes

– weaker products, internal defects

– good option for brittle materials


• Sand Casting

(large parts, e.g.,

auto engine blocks)


• trying to hold something that is hot

• what will withstand >1600ºC?

• cheap - easy to mold => sand!!!

• pack sand around form (pattern) of desired shape

Sand Sand

molten metal



plaster

die formed

around wax

prototype

• Sand Casting

(large parts, e.g.,

auto engine blocks)

• Investment Casting

(low volume, complex shapes

e.g., jewelry, turbine blades)


Investment Casting

• pattern is made from paraffin.

• mold made by encasing in plaster of paris

• melt the wax & the hollow mold is left

• pour in metal

wax


Sand Sand

molten metal


plaster

die formed

around wax

prototype

• Sand Casting

(large parts, e.g.,

auto engine blocks)

• Investment Casting

(low volume, complex shapes

e.g., jewelry, turbine blades)


wax

• Die Casting

(high volume, low T alloys)

• Continuous Casting

(simple slab shapes)

molten

solidified


Sand Sand

molten metal


CASTING JOINING

Metal Fabrication Methods - III

• Powder Metallurgy

(materials w/low ductility)

pressure

heat

point contact

at low T

densification

by diffusion at

higher T

area contact

densify

• Welding

(when one large part is

impractical)

• Heat affected zone:

(region in which the

microstructure has been

changed).

piece 1 piece 2

fused base metal

filler metal (melted) base metal (melted)

unaffected unaffected heat affected zone

FORMING


Powder Metallurgy


Weakening by:

•Porosity (produced by casting, welding, or powder metallurgy)

•Annealing

•Hot-working

•Heat-affected zone (welding)

•Phase transformations (e.g., tempered martensite)

Strengthening by:

•Cold working

•Alloying (e.g., solution hardening)

•Phase transformations (e.g., martensitic)

General “Rules of Thumb” for properties of processed materials


Variation of the mechanical properties of copper–nickel alloys with composition


•The mechanical properties of two brass alloys vary with the degree of cold work

•Note the general tradeoff between strength and ductility.


Ceramics and Glasses

Three main categories:

- Crystalline Ceramics

- Glasses

- Glass-ceramics


• Properties: -- Tm for glass is moderate, but large for other ceramics.

-- Small toughness, ductility; large moduli & creep resist.

• Applications: -- High T, wear resistant, novel uses from charge neutrality.

• Fabrication -- some glasses can be easily formed

-- other ceramics can not be formed or cast.

Glasses Clay

products

Refractories Abrasives Cements Advanced

ceramics

-optical - composite

reinforce

- containers/

household

-whiteware - bricks

-bricks for

high T

(furnaces)

-sandpaper - cutting

- polishing

-composites - structural

engine - rotors

- valves

- bearings

-sensors

Adapted from Fig. 13.1 and discussion in

Section 13.2-6, Callister 7e.

Taxonomy of Ceramics


• Need a material to use in high temperature furnaces.

• Consider the Silica (SiO2) - Alumina (Al2O3) system.

• Phase diagram shows: mullite, alumina, and crystobalite as candidate refractories.

Application: Refractories

Composition (wt% alumina)

T(°C)

1400

1600

1800

2000

2200

20 40 60 80 100 0

alumina +

mullite

mullite + L

mullite Liquid

(L)

mullite + crystobalite

crystobalite + L

alumina + L

3Al2O3-2SiO2


ZrO2-CaO diagram:


• Example: Oxygen sensor ZrO2 • Principle: Make diffusion of ions

fast for rapid response.

Application: Sensors

A Ca 2+ impurity

removes a Zr 4+ and a

O 2 - ion.

Ca 2+

• Approach: Add Ca impurity to ZrO2: -- increases O2- vacancies

-- increases O2- diffusion rate

reference gas at fixed oxygen content

O 2-

diffusion

gas with an unknown, higher oxygen content

- + voltage difference produced!

sensor • Operation: -- voltage difference

produced when

O2- ions diffuse

from the external

surface of the sensor

to the reference gas.


TABLE 11.4 Compositionsa of Some Silicate Ceramics


Amorphous Silica


• Quartz is crystalline

SiO2:

• Basic Unit: • Glass is amorphous

• Amorphous structure occurs by adding impurities

(Na+,Mg2+,Ca2+, Al3+)

• Impurities: interfere with formation of

crystalline structure.

(soda glass)

Glass Structure

Si0 4 tetrahedron 4-

Si 4+

O 2 -

Si 4+

Na +

O 2 -


Glasses

Network modifiers: Oxides that breakup the

glass network.

Added to glass to increase workability.

Examples:- Na2O, K2O, CaO, MgO.

Oxygen atom enters network and other

ion stay in interstices.

Intermediate oxides: Cannot form glass

network by themselves but can join into an

existing network.

Added to obtain special properties.

Examples: Al2O3, Lead oxide.

Soda lime glass: Very common glass (90%). 71-73% SiO2,

12-14% Na2O, 10-12% CaO. Easier to form and used in

flat glass and container


TABLE 11.5 Compositions of Some Silicate Glasses


TABLE 11.6 Compositions of Some Glass-Ceramics


• Fusion casting

• Slip casting

• Sintering

• Hot isostatic pressing (HIP)

• Glass forming

• Controlled devitrification

• Sol-gel processing

• Biomimetic processing

• Self-propagating high temperature synthesis (SHS)

Some major processing methods for ceramics

and glasses


• Pressing:

GLASS

FORMING

Ceramic Fabrication Methods-I

Gob

Parison mold

Pressing operation

• Blowing:

suspended Parison

Finishing mold

Compressed air

plates, dishes, cheap glasses

--mold is steel with

graphite lining

• Fiber drawing:

wind up

PARTICULATE

FORMING

CEMENTATION


The high degree of flatness achieved in modern architectural plate glass is the result of the float glass process in which the layer of glass is drawn across a bath of molten tin.

Flat glass process developed by the Pilkington Glass Company


• Quartz is crystalline

SiO2:

• Basic Unit: • Glass is amorphous

• Amorphous structure occurs by adding impurities

(Na+,Mg2+,Ca2+, Al3+)

• Impurities: interfere with formation of

crystalline structure.

(soda glass)

Glass Structure

Si0 4 tetrahedron 4-

Si 4+

O 2 -

Si 4+

Na +

O 2 -


• Milling and screening: desired particle size

• Mixing particles & water: produces a "slip" • Form a "green" component

• Dry and fire the component

ram bille

t container

container force

die holder

die

A o

A d extrusion --Hydroplastic forming: extrude the slip (e.g., into a pipe)

Ceramic Fabrication Methods-IIA

solid component

--Slip casting:

hollow component

pour slip

into mold

drain

mold “green

ceramic”

pour slip into mold

absorb water into mold

“green ceramic”

GLASS

FORMING

PARTICULATE

FORMING

CEMENTATION


•Slip casting of ceramics (“slip” = powder-water mixture)

•Much of the water absorbed into the porous mold.


Clay Composition

A mixture of components used

(50%) 1. Clay

(25%) 2. Filler – e.g. quartz (finely ground)

(25%) 3. Fluxing agent (Feldspar)

binds it together

aluminosilicates + K+, Na+, Ca+


• Clay is inexpensive

• Adding water to clay -- allows material to shear easily

along weak van der Waals bonds

-- enables extrusion

-- enables slip casting

• Structure of

Kaolinite Clay:

Features of a Slip

weak van der Waals bonding

charge neutral

charge neutral

Si 4+

Al 3 +

- OH

O 2-

Shear

Shear


• Drying: layer size and spacing decrease.

Drying and Firing

Drying too fast causes sample to warp or crack due to non-uniform shrinkage

wet slip partially dry “green” ceramic

• Firing: --T raised to (900-1400°C)

--vitrification: liquid glass forms from clay and flows between

SiO2 particles. Flux melts at lower T.

Si02 particle

(quartz)

glass formed around the particle

micrograph of porcelain

70 mm


Sintering: useful for both clay and non-clay compositions.

• Procedure: -- produce ceramic and/or glass particles by grinding

-- place particles in mold

-- press at elevated T to reduce pore size.

• Aluminum oxide powder: -- sintered at 1700°C

for 6 minutes.

Ceramic Fabrication Methods-IIB

15 mm

GLASS

FORMING

PARTICULATE

FORMING

CEMENTATION


Powder Pressing

Sintering - powder touches - forms neck & gradually neck thickens – add processing aids to help form neck

– little or no plastic deformation Uniaxial compression - compacted in single direction

Isostatic (hydrostatic) compression - pressure applied by

fluid - powder in rubber envelope

Hot pressing - pressure + heat


Tape Casting

• thin sheets of green ceramic cast as flexible tape

• used for integrated circuits and capacitors

• cast from liquid slip (ceramic + organic solvent)


Porosity effect

2

01 1 9 0 9( . . )E E P P +

0exp( )

fnP

where P: 기공의 부피분율, E0: 기공이 없는 재료의 탄성율

where 0와 n은 실험상수


• Produced in extremely large quantities.

• Portland cement: -- mix clay and lime bearing materials

-- calcinate (heat to 1400°C)

-- primary constituents:

tri-calcium silicate

di-calcium silicate

• Adding water -- produces a paste which hardens

-- hardening occurs due to hydration (chemical reactions

with the water).

• Forming: done usually minutes after hydration begins.

Ceramic Fabrication Methods-III

GLASS

FORMING

PARTICULATE

FORMING

CEMENTATION


From Wikipedia, the free encyclopedia

“Biomimetics (also known as Bionics,

biognosis, biomimicry or bionical

creativity engineering) is the application

of biological methods and systems found

in nature to the study and design of

engineering systems and modern

technology”

"taking design ideas from nature"

“Biomimetic refers to human-made

processes, materials, devices, or systems

that imitate nature.”

Biomimetic processing


Advanced Materials, Functional Materials, Smart Materials, Intelligent Materials !! Conscious Materials ??!!!

Lotus effect: Self-clean water

repellent surfaces:

Structural colors:

Photonic Crystals

Bio-Adhesion:

nano-

velcro;

Geckel

glue

Low temp

ceramics

http://upload.wikimedia.org/wikipedia/commons/1/13/Lotus3.jpg

http://www.nano.ecs.soton.ac.uk/images/research/biomimetics/butterfly1_large.jpg


Gecko feet: biology

Millions of hairs

called setae

Fiber radius is

nanometer-scale

Adhesion due to

van der Waals

and capillary

forces

• Use electron beam lithography

and dry etching to create synthetic

polymer hairs

• Attach to flexible base

• Palm-sized piece can support a

human


drag reduction by shark skin

special alignment and grooved structure

of tooth-like scales embedded in shark skin

decrease drag and thus

greatly increase swimming proficiency

Airbus fuel consumption down 1½ %

when “shark skin” coating applied to aircraft

Friction-Reducing Sharkskin

Speedo's Fastskin FSII swimsuits made

their appearance at the Bejing Olympics

and may have helped US swimmer

Michael Phelps to his record eight gold

medals

http://images.google.com.au/imgres?imgurl=http://images.asme.org/MEMagazine/Articles/2008/April/17512.jpg&imgrefurl=http://memagazine.asme.org/articles/2008/april/Beyond_Sharkskin_Suit.cfm&usg=__hw0mtqDFo-wbrKuUI_OvK9Y4BdY=&h=356&w=300&sz=12&hl=en&start=40&um=1&tbnid=VVDkc8Jx9vaEpM:&tbnh=121&tbnw=102&prev=/images?q%3Dsharkskin%2Bsuit%26ndsp%3D21%26hl%3Den%26sa%3DN%26start%3D21%26um%3D1


Butterfly-Inspired Displays

By mimicking the way light reflects from the

scales on a butterfly's wings, the Qualcomm

company has developed Mirasol Displays

that make use of the reflected light principle

with an understanding of how human beings

perceive that light. Using an interferometric

modulator [IMOD] element in a two-plate

conductive system, the display uses near-

zero power whenever the displayed image is

static while at the same time offering a

refresh rate fast enough for video. Perfect

for 'smart' hand-held devices, already

deployed in many, and a battery-saver

extraordinaire!

http://www.mirasoldisplays.com/mems-displays/index-mobile-display-imod-technology.php




Apatite related biomaterials

Basics of biominralization of bones

1) Osteogenic differentiation of

mesenchymal stem cells (MSCs).

2) Organization of matrix proteins

secreted from osteoblasts.

3) Nucleation and crystal growth of

apatite minerals on matrix.

Fig. Biomimetic mineralization of apatite in fibrin gel


Self-propagating High-temperature Synthesis (SHS)

The SHS is a method for producing inorganic compounds by exothermic

reactions, usually involving salts or pure metals. A variant of this method is

known as solid state metathesis (SSM). Since the process occurs at high

temperatures, the method is ideally suited for the production of refractory

materials with unusual properties, for example: powders, metallic alloys, or

ceramics with high purity, corrosion–resistance at high–temperature or super-

hardnessity.

FIG. Laboratory setup for combustion synthesis. 1-reaction chamber;

2-sample; 3-base; 4-quart.z window; 5-tungsten coil; &power supply;

7-video camera; &video cassette recorder; 9-video monitor; 1 O-

computer with data acquisition board; 1 I-thermocouple; 12-vacuum

pump; 13-inert or reactant gas; 14-valve.


Mechanical Properties

We know that ceramics are more brittle than

metals. Why?

• Consider method of deformation

– slippage along slip planes

• in ionic solids this slippage is very difficult

• too much energy needed to move one anion past

another anion


Figure. The frequency distribution of observed

fracture strengths for a silicon nitride material

Figure. For brittle ceramic materials, schematic

representations of crack origins and configurations.


Figure. Schematic diagram that shows typical

features observed on the fracture surface of a

brittle ceramic.


Thermal Expansion

• Materials change size when heating.

)TT(L

LLinitialfinal

initial

initialfinal a

coefficient of

thermal expansion (1/K or 1/°C)

T init

T final L final

L init

• Atomic view: Mean bond length increases with T.

Bond energy

Bond length (r)

incre

asin

g T

T 1

r(T

5)

r(T

1)

T 5 bond energy vs bond length curve is “asymmetric”


Thermal Expansion: Comparison

• Q: Why does a

generally decrease

with increasing

bond energy?

Polypropylene 145-180 Polyethylene 106-198

Polystyrene 90-150 Teflon 126-216

• Polymers at room T

• Ceramics Magnesia (MgO) 13.5

Alumina (Al2O3) 7.6

Soda-lime glass 9 Silica (cryst. SiO2) 0.4

• Metals Aluminum 23.6 Steel 12

Tungsten 4.5 Gold 14.2

a(10-6/K) Material

Polymers have smaller

a because of weak

secondary bonds


• General: The ability of a material to transfer heat.

• Quantitative: temperature

gradient

thermal conductivity (J/m-K-s)

heat flux

(J/m2-s)

• Atomic view: Atomic vibrations in hotter region carry

energy (vibrations) to cooler regions.

T 2 > T 1 T 1

x 1 x 2 heat flux

Thermal Conductivity

dx

dTkq Fourier’s Law


Thermal Conductivity: Comparison in

cre

asin

g k

• Polymers Polypropylene 0.12 Polyethylene 0.46-0.50 Polystyrene 0.13 Teflon 0.25

By vibration/ rotation of chain molecules

• Ceramics Magnesia (MgO) 38 Alumina (Al2O3) 39 Soda-lime glass 1.7 Silica (cryst. SiO2) 1.4

By vibration of atoms

• Metals Aluminum 247 Steel 52 Tungsten 178 Gold 315

By vibration of atoms and motion of electrons

k (W/m-K) Energy Transfer Material


• Occurs due to: -- uneven heating/cooling

-- mismatch in thermal expansion.

• Example -- A brass rod is stress-free at room temperature (20°C).

-- It is heated up, but prevented from lengthening.

-- At what T does the stress reach -172 MPa?

Thermal Stress

)( roomthermal

room

TTL

La

T room

L room

T

L

100GPa 20 x 10-6 /°C

20°C Answer: 106°C -172 MPa

compressive keeps L = 0

E(thermal ) Ea(T Troom)


• Occurs due to: uneven heating/cooling.

• Ex: Assume top thin layer is rapidly cooled from T1 to T2:

Tension develops at surface

)( 21 TTE a

Critical temperature difference

for fracture (set = f)

a

ETT f

f racture21 )(

set equal

• Large thermal shock resistance when is large. a

E

kf

• Result: a

E

kff racture f orrate) (quench

Thermal Shock Resistance

Temperature difference that

can be produced by cooling:

kTT

rate quench)( 21

rapid quench

resists contraction

tries to contract during cooling T2

T1


• Application:

Space Shuttle Orbiter

• Silica tiles (400-1260C): --large scale application --microstructure:

Thermal Protection System

reinf C-C (1650°C)

Re-entry T Distribution

silica tiles (400-1260°C)

nylon felt, silicon rubber coating (400°C)

~90% porosity!

Si fibers

bonded to one

another during

heat treatment. 100 mm


Applications: Advanced Ceramics

Heat Engines

• Advantages: – Run at higher

temperature

– Excellent wear &

corrosion resistance

– Low frictional losses

– Ability to operate without

a cooling system

– Low density

• Disadvantages:

– Brittle

– Too easy to have voids-

weaken the engine

– Difficult to machine

• Possible parts – engine block, piston coatings, jet engines

Ex: Si3N4, SiC, & ZrO2



• Ceramic Armor

– Al2O3, B4C, SiC & TiB2

– Extremely hard materials

• shatter the incoming projectile

• energy absorbent material underneath



Electronic Packaging

• Chosen to securely hold microelectronics & provide heat transfer

• Must match the thermal expansion coefficient of the microelectronic chip & the electronic packaging material. Additional requirements include:

– good heat transfer coefficient

– poor electrical conductivity

• Materials currently used include: • Boron nitride (BN)

• Silicon Carbide (SiC)

• Aluminum nitride (AlN)

– thermal conductivity 10x that for Alumina

– good expansion match with Si


Optical Properties

Light has both particulate and wavelike properties

– Photons - with mass

hchE

m/s) 10 x (3.00 light of speed c

)sJ1062.6( constant sPlanck'

frequency

wavelength

energy

8

34

xh

E


Refractive Index, n

• Note: n = f ()

Typical glasses ca. 1.5 -1.7

Plastics 1.3 -1.6

PbO (Litharge) 2.67

Diamond 2.41

medium)inlightof(velocity

vacuum)inlightof(velocity

v

c

• Transmitted light distorts electron clouds.

• Light is slower in a material vs vacuum.

n = refractive index

+ no

transmitted light

transmitted light

+

electron cloud distorts

--Adding large, heavy ions (e.g., lead

can decrease the speed of light.

--Light can be

"bent"


Total Internal Reflectance

sin

sin

n

nn’(low)

n (high)

n > n’

1

c

'

1

angle critical

angle refracted

angle incident

c

i

i

reflected internally is light for

90 whenoccurs

ci

ic


Example: Diamond in air

• Fiber optic cables are clad in low n material for this reason.

5.2441.2

1sin

sin

1

sin

90sin

1

41.2

sin

sin

cc

ccn

n


• Incident light is either reflected, absorbed, or

transmitted: SRATo IIIII +++

Light Interaction with Solids

• Optical classification of materials:

single

crystal

polycrystalline

dense

polycrystalline

porous

Transparent Translucent

Opaque

Incident: I0

Absorbed: IA

Transmitted: IT

Scattered: IS

Reflected: IR


• Absorption of photons by electron transition:

• Metals have a fine succession of energy states.

• Near-surface electrons absorb visible light.

Optical Properties of Metals:

Absorption

Energy of electron

Planck’s constant

(6.63 x 10-34 J/s)

freq. of incident light

filled states

unfilled states

E = h required!

I o


Light Absorption

tI

I ln

0

a

t

I

I a e0 thicknesssample

cm][tcoefficienabsorptionlinear 1

a

t


• Reflectivity = IR/Io is between 0.90 and 0.95.

• Reflected light is same frequency as incident.

• Metals appear reflective (shiny)!

Optical Properties of Metals:

Reflection

• Electron transition emits a photon.

re-emitted

photon from

material surface

Energy of electron

filled states

unfilled states

E

IR “conducting” electron


Reflectivity, R

• Reflection

– Metals reflect almost all light

– Copper & gold absorb in blue & green => gold

color

tyreflectivi1

12

+

n

nR

17.0141.2

141.22

+

R

reflectedislightof%17

• Example: Diamond


Scattering

• In semicrystalline or polycrystalline

materials

• Semicrystalline – density of crystals higher than amorphous

materials speed of light is lower - causes light to

scatter - can cause significant loss of light

• Common in polymers

– Ex: LDPE milk cartons – cloudy

– Polystyrene – clear – essentially no crystals


• Absorption by electron transition occurs if h > Egap

• If Egap < 1.8 eV, full absorption; color is black (Si, GaAs)

• If Egap > 3.1 eV, no absorption; colorless (diamond)

• If Egap in between, partial absorption; material has a color.

Selected Absorption: Semiconductors

incident photon

energy h

Energy of electron

filled states

unfilled states

E gap

I o

blue light: h = 3.1 eV

red light: h = 1.7 eV


c hc

Eg

(6.62 x 1034 J s)(3 x 108m/s)

(0.67eV)(1.60 x 1019 J/eV) 1.85 mm

note : for Si Eg 1.1eV c 1.13 mm

Wavelength vs. Band Gap

If donor (or acceptor) states also available this provides other

absorption frequencies

Eg = 0.67 eV

Example: What is the minimum wavelength absorbed

by Ge?


• Color determined by sum of frequencies of -- transmitted light,

-- re-emitted light from electron transitions.

• Ex: Cadmium Sulfide (CdS) -- Egap = 2.4 eV,

-- absorbs higher energy visible light (blue, violet),

-- Red/yellow/orange is transmitted and gives it color.

Color of Nonmetals

• Ex: Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr2O3 -- Sapphire is colorless (i.e., Egap > 3.1eV)

-- adding Cr2O3 : • alters the band gap

• blue light is absorbed

• yellow/green is absorbed

• red is transmitted

• Result: Ruby is deep red in color.

40

60

70

80

50

0.3 0.5 0.7 0.9

Tra

nsm

itta

nce (

%)

ruby

sapphire

wavelength, (= c/)(mm)


Luminescence • Luminescence – emission of light by a

material

– material absorbs light at one frequency & emits at

another (lower) frequency.

activator level

Valence band

Conduction band

trapped states Eg

Eemission

How stable is the trapped state?

• If very stable (long-lived = >10-8 s) = phosphorescence

• If less stable (<10-8 s) = fluorescence

Example: glow in the dark toys. Charge them up by exposing them to the light. Reemit over time. -- phosphorescence


Photoluminescence

• Arc between electrodes excites mercury in lamp to higher

energy level.

• electron falls back emitting UV light (i.e., suntan lamp).

• Line inner surface with material that absorbs UV, emits visible

Ca10F2P6O24 with 20% of F - replaced by Cl

-

• Adjust color by doping with metal cations

Sb3+ blue

Mn2+ orange-red

Hg

uv

electrode electrode


Cathodoluminescence

• Used in T.V. set

– Bombard phosphor with electrons

– Excite phosphor to high state

– Relaxed by emitting photon (visible) ZnS (Ag+ & Cl-) blue

(Zn, Cd) S + (Cu++Al3+) green

Y2O2S + 3% Eu red

• Note: light emitted is random in phase & direction

– i.e., noncoherent


LASER Light

• Is non-coherent light a problem? – diverges – can’t keep tightly columnated

• How could we get all the light in phase? (coherent)

– LASERS • Light

• Amplification by

• Stimulated

• Emission of

• Radiation

• Involves a process called population inversion of energy states


Population Inversion

• What if we could increase most species to the excited

state?


LASER Light Production

• “pump” the lasing material to the excited state

– e.g., by flash lamp (non-coherent lamp).

– If we let this just decay we get no coherence.


LASER Cavity

“Tuned” cavity:

• Stimulated Emission – One photon induces the

emission of another

photon, in phase with the

first.

– cascades producing very

intense burst of coherent

radiation.

• i.e., Pulsed laser


Continuous Wave LASER

• Can also use materials such as CO2 or yttrium- aluminum-garret (YAG) for LASERS

• Set up standing wave in laser cavity –

– tune frequency by adjusting mirror spacing.

• Uses of CW lasers 1. Welding

2. Drilling

3. Cutting – laser carved wood, eye surgery

4. Surface treatment

5. Scribing – ceramics, etc.

6. Photolithography – Excimer laser


• Apply strong forward

bias to junction.

Creates excited state

by pumping electrons

across the gap-

creating electron-hole

pairs.

electron + hole neutral + h

excited state ground state photon of

light

Semiconductor LASER


Uses of Semiconductor LASERs

• #1 use = compact disk player

– Color? - red

• Banks of these semiconductor lasers are used as

flash lamps to pump other lasers

• Communications

– Fibers often turned to a specific frequency

(typically in the blue)

– only recently was this a attainable


Applications of Materials Science

• New materials must be developed to make new &

improved optical devices.

– Organic Light Emitting Diodes (OLEDs)

– White light semiconductor sources

• New semiconductors

• Materials scientists

(& many others) use lasers as tools.

• Solar cells


Solar Cells

• p-n junction: • Operation: -- incident photon produces hole-elec. pair.

-- typically 0.5 V potential.

-- current increases w/light intensity.

• Solar powered weather station:

polycrystalline Si

n-type Si

P-type Si p-n junction

B-doped Si

Si

Si

Si Si B

hole

P

Si

Si

Si Si

conductance electron

P-doped Si

n-type Si

p-type Si p-n junction

light

+ -

+ + +

- - -

creation of

hole-electron

pair


Optical Fibers

• prepare preform as indicated in Chapter 13

• preform drawn to 125 mm or less capillary fibers

• plastic cladding applied 60 mm


Optical Fiber Profiles

Step-index Optical Fiber

Graded-index Optical Fiber


• Steels: increase TS, Hardness (and cost) by adding

--C (low alloy steels)

--Cr, V, Ni, Mo, W (high alloy steels)

--ductility usually decreases w/additions.

• Non-ferrous:

--Cu, Al, Ti, Mg, Refractory, and noble metals.

• Fabrication techniques:

--forming, casting, joining.

Ceramic materials have covalent & ionic bonding.

• Structures are based on:

-- charge neutrality

-- maximizing # of nearest oppositely charged neighbors.

• Structures may be predicted based on:

-- ratio of the cation and anion radii.

Summary


• Defects

-- must preserve charge neutrality

-- have a concentration that varies exponentially w/T.

• Room T mechanical response is elastic, but fracture

is brittle, with negligible deformation.

• Elevated T creep properties are generally superior to

those of metals (and polymers).

• Fabrication Techniques: -- glass forming (impurities affect forming temp).

-- particulate forming (needed if ductility is limited)

-- cementation (large volume, room T process)

• Heat treating: Used to -- alleviate residual stress from cooling,

-- produce fracture resistant components by putting

surface into compression.

Chapter 11: Structural Materials Metals, Ceramics, and Glasses

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Transcript of Chapter 11: Structural Materials Metals, Ceramics, and Glasses