Materials Exam 2 – Flashcards

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question
elastic means _____
plastic means _____
answer
elastic means REVERSIBLE
plastic means PERMANENT
question
common states of stress:
- simple tension
- torsion
- simple compression
- bi-axial tension
- hydrostatic compression

match: pressurized tank, balanced rocks, cable, fish under water, drive shaft
answer
common states of stress:
- simple tension - CABLE
- torsion - DRIVE SHAFT
- simple compression - BALANCED ROCKS
- bi-axial tension - PRESSURIZED TANK
- hydrostatic compression - FISH UNDER WATER
question
units of stress
answer
units of stress:
N/m^2
lb/in^2
question
units of strain
answer
units of strain:
NONE
question
what is hooke's law?
answer

? = ? ?

 

? = stress 

? = modulus of elasticity (young's modulus) 

? = strain

question
Slope of stress strain plot (which is proportional to the elastic modulus) depends on what property?
answer

Slope of stress strain plot (which is proportional to the elastic modulus) depends on what property?

BOND STRENGTH

Steeper the slope, stronger the bond

 

question
What is yield strength?
answer
yield strength: ?y, stress at which noticeable plastic deformation has occurred
question
what is tensile strength?
answer
tensile strength: TS, MAXIMUM STRESS ON ENGINEERING STRESS-STRAIN CURVE
metals - occurs when noticeable necking starts
polymers - occurs when polymer backbone chains are aligned and about to break
question
what is ductility? how can we measure it?
answer
ductility is PLASTIC TENSILE STRAIN at FAILURE.
measure it by %EL or %RA
question
what is toughness?
answer

toughness is the ENERGY to BREAK a unit volume of material. approximate by the area under the stress-strain curve

 

 

question
What is hardness?
answer
hardness is RESISTANCE TO PERMANENTLY INDENTING THE SURFACE.
large hardness means:
- resistance to plastic deformation or cracking in compression.
- better wear properties
question
2 ways to measure hardness
answer
hardness measurement:
ROCKWELL
HB = BRINELL HARDNESS
question
Why do "true" stress and strain exist?
answer
Cross sectional area changes when sample is stretched
question
Stress measures _____
Strain measures _____
answer
Stress measures LOAD
Strain measures DISPLACEMENT
(both size independent measures)
question
What is hardening?
answer
Hardening is an INCREASE in YIELD STRENGTH
due to PLASTIC DEFORMATION.
question
Elastic modulus is a _____ property.
answer
Elastic modulus is MATERIAL property

• Critical properties depend largely on sample flaws (defects,
etc.). Large sample to sample variability.
question
Describe dislocation motion in metals.
answer
• Metals (Cu, Al):
Dislocation motion EASIEST
- non-directional bonding
- close-packed directions
for slip
question
Describe dislocation motion in covalent ceramics.
answer
• COVALENT Ceramics (Si, diamond)
Motion DIFFICULT
- directional (angular) bonding
question
Describe dislocation motion in ionic ceramics.
answer
• Ionic Ceramics (NaCl):
Motion DIFFICULT
- need to avoid nearest
neighbors of like sign (- and +)
question
Metals - plastic deformation occurs by ____ – an edge
dislocation (extra half-plane of atoms) slides over adjacent
plane half-planes of atoms.
answer
Metals - plastic deformation occurs by SLIP – an edge
dislocation (extra half-plane of atoms) slides over adjacent
plane half-planes of atoms.
question
If dislocations ___ ____, plastic deformation doesn't occur!
answer
If dislocations CAN'T MOVE, plastic deformation doesn't occur!
question
A dislocation moves along a slip plane in a slip direction
______ to the dislocation line
• The slip direction is the same as the _____ vector direction
answer
A dislocation moves along a slip plane in a slip direction
PERPENDICULAR to the dislocation line (This is on his notes but would anyone like to explain that to me)
• The slip direction is the same as the BURGERS vector direction
question
Slip System

– Slip plane - plane on which ______ slippage occurs
• Highest _____ densities (and large interplanar spacings)

– Slip directions - directions of _____ movement
• Highest _____ densities
answer
Slip System

– Slip plane - plane on which EASIEST slippage occurs
• Highest PLANAR densities (and large interplanar spacings)

– Slip directions - directions of DISLOCATION movement
• Highest LINEAR densities
question
What is meant by resolved shear stress? ?R
answer

What is meant by resolved shear stress? ?R


results from applied tensile stresses -- factors in the angles between the applied stress and the slip direction & slip plane normal

Condition for dislocation motion: ?R > ?CRSS (Critcally resolved shear stress)

?R = ?*cos?*cos?


?R maximum at ? = ? = 45 degrees

__________________________________________


? = applied tensile stress = F/A

? = angle between F and slip direction (Fs)

? = angle between F and slip plane normal (As)

question
For deformation to occur the _____ stress must
be greater than or equal to the ______ stress
answer
For deformation to occur the APPLIED stress must
be greater than or equal to the YIELD stress
question

SLIP MOTION in POLYCRYSTALS

• Polycrystals stronger than single crystals – ____ _____ are barriers to dislocation motion.

• Slip planes & directions (?, ?) change from one grain to another.

• ____ will vary from one grain to another.

• The grain with the ______ ?R yields first. • Other (less favorably oriented) grains yield later.

answer

SLIP MOTION in POLYCRYSTALS

• Polycrystals stronger than single crystals – GRAIN BOUNDARIES are barriers to dislocation motion.

• Slip planes & directions (?, ?) change from one grain to another.

• ?R will vary from one grain to another.

• The grain with the LARGEST ?R yields first. • Other (less favorably oriented) grains yield later.

question
Four Strategies for Strengthening:
answer
Four Strategies for Strengthening:
1: Reduce Grain Size
2: Form Solid Solutions
3: Precipitation Strengthening
4: Cold Work (Strain Hardening)
question
Why would we want to Reduce Grain Size
answer
1. Reduce Grain Size
- A strategy for STRENGTHENING
- Grain boundaries are barriers to slip.
- Barrier "strength“ increases with increasing angle of misorientation.
- Smaller grain size: more barriers to slip.
question
Why would we want to Form Solid Solutions
answer
2: Form Solid Solutions
- strategy for STRENGTHENING
- IMPURITY ATOMS DISTORT the lattice & generate lattice STRAINS.
- These strains can act as barriers to dislocation motion.
question
- Small impurities tend to concentrate at dislocations in regions of _____ strains
- Large impurities tend to concentrate at dislocations in regions of _____ strains.
- both ____ mobility of dislocations and ____ strength
answer
- Small impurities tend to concentrate at dislocations in regions of COMPRESSIVE strains
- Large impurities tend to concentrate at dislocations in regions of TENSILE strains.
- both REDUCE mobility of dislocations and INCREASE strength
question
Alloying increases _____ strength and _____ strength
answer
Alloying increases YIELD strength and TENSILE strength
question
Why is Precipitation Strengthening useful
answer
Precipitation Strengthening:

Hard precipitates are difficult to shear.
Ex: Ceramics in metals (SiC in Iron or Aluminum).
basically means it just cuts thru the precipitate

Either shears them or bows out around them
question
What are the 4 common forming operations that reduce the cross sectional area during Cold Work (Strain Hardening)
answer
• Cold worDeformation at room temperature (for most metals).
• Common forming operations reduce the cross-sectional area:
- forging
- rolling
- drawing
- extrusion

%CW = Ao-Ad/Ao x 100
question
How does the dislocation structure change during cold working?
answer
Dislocations entangle with one another during cold work.
• Dislocation motion becomes more difficult.
question
What happens to
- yield strength
- tensile strength
- ductility
as cold work is INCREASED
answer
as COLD WORK is INCREASED:

- YIELD STRENGTH (?y) INCREASES.
- TENSILE STRENGTH (TS) INCREASES.
- DUCTILITY (%EL or %AR) DECREASES.

can check values by looking at graph
question
Effect of heat treating after cold work?
answer
Heat treating after cold work...
• 1 hour treatment at Tanneal...
decreases TS and increases %EL.
• Effects of cold work are nullified!
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What are the 3 annealing stages?
answer
3 Annealing stages:
1. Recovery
2. Recrystallization
3. Grain growth
question
The 1st step of annealing, RECOVERY, does what?
answer
RECOVERY REDUCES the number of DISLOCATIONS
question
The 2nd step of annealing, RECRYSTALLIZATION, does what?
answer
RECRYSTALLIZATION forms new grains that:
- have LOW dislocation densities
- are SMALL in size
- consume and replace parent cold-worked grains
question
The 3rd step of annealing, GRAIN GROWTH, does what?
answer
GRAIN GROWTH:

at longer times, average grain size increases
- small grains shrink and ultimately disappear
- large grains continue to grow
question

TR = RECRYSTALLIZATION TEMPERATURE = temperature at which recrystallization just reaches completion in ___.

 

0.3Tm < TR < 0.6Tm

 

For a specific metal/alloy, TR depends on:

• %CW -- TR DECREASES with _____ %CW

• Purity of metal -- TR DECREASES with _____ purity

answer

TR = RECRYSTALLIZATION TEMPERATURE = temperature at which recrystallization just reaches completion in 1 HOUR.

 

0.3Tm < TR < 0.6Tm

 

For a specific metal/alloy, TR depends on:

• %CW -- TR DECREASES with INCREASING %CW

• Purity of metal -- TR DECREASES with INCREASING purity

question
STRENGTHENING IN METALS SUMMARY:

• Dislocations are observed primarily in ____ and _____.
• Strength is _____ by making dislocation motion difficult.
• Strength of metals may be increased by:
-- ______ grain size
-- solid _____ strengthening
-- _______ hardening
-- ____ working
• A cold-worked metal that is ____ treated may experience
recovery, recrystallization, and grain growth – its properties
will be altered.
answer
STRENGTHENING IN METALS SUMMARY:

• Dislocations are observed primarily in METALS and ALLOYS.
• Strength is INCREASED by making dislocation motion difficult.
• Strength of metals may be increased by:
-- DECREASING grain size
-- solid SOLUTION strengthening
-- PRECIPITATE hardening
-- COLD working
• A cold-worked metal that is HEAT treated may experience
recovery, recrystallization, and grain growth – its properties
will be altered.
question
Ductile fracture
– Accompanied by significant _____ deformation

Brittle fracture
– Little or no _____ deformation
– Catastrophic
answer
Ductile fracture
– Accompanied by significant PLASTIC deformation

Brittle fracture
– Little or no PLASTIC deformation
– Catastrophic
question
Ductile vs Brittle Failure (picture it)
answer
[image]
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Ductile fracture is usually ____ desirable than brittle fracture
answer
Ductile fracture is usually MORE desirable than brittle fracture
question
• Ductile failure:
-- [one/many] piece(s)
-- [small/large] deformation(s)

• Brittle failure:
-- [one/many] piece(s)
-- [small/large] deformation(s)
answer
• Ductile failure:
-- ONE piece
-- LARGE deformation

• Brittle failure:
-- MANY pieces
-- SMALL deformations
question
Brittle fracture surfaces:

1. ______ (between grains)
2. ______ (through grains)
answer
Brittle fracture surfaces:

1. INTERGRANULAR (between grains)
2. TRANSGRANULAR (through grains)
question
Moderately ductile failure stages
answer
[image]
question
TS engineering materials [>>/<<] TS perfect materials
answer
TS engineering materials << TS perfect materials
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DaVinci (500 yrs ago!) observed...
-- the longer the wire, the smaller the load for failure.
• Reasons:
--
--
answer
DaVinci (500 yrs ago!) observed...
-- the longer the wire, the smaller the load for failure.
• Reasons:
-- FLAWS CAUSE PREMATURE FAILURE
-- larger samples contain longer flaws!
question
_____ are stress concentrators!
answer
FLAWS are stress concentrators!
question
Cracks having [sharp/blunt] tips propagate easier than cracks having [sharp/blunt] tips

[Brittle/ductile] materials have sharp crack tips
[Brittle/ductile] materials have blunt crack tips
answer
Cracks having SHARP tips propagate EASIER than cracks having BLUNT tips

BRITTLE materials have sharp crack tips
DUCTILE materials have blunt crack tips
question
Energy balance on the crack:
- ______ _____ energy-
• Energy stored in material as it is elastically deformed.
• This energy is released when the crack propagates.
• Creation of new surfaces requires energy
answer
Energy balance on the crack:
- ELASTIC STRAIN energy-
• Energy stored in material as it is elastically deformed.
• This energy is released when the crack propagates.
• Creation of new surfaces requires energy
question
what will cause a crack to propagate?
answer
Crack propagates if crack-tip stress (?m) EXCEEDS a critical stress (?c)

the critical stress is that equation with a = half length of internal crack & specific surface energy & modulus of elasticity
question
How do we find out how much energy goes into failing a material? (Hint: A mechanical way of measuring toughness)
answer
Impact Testing

1 example is charpy testing (actually kinda neat, maybe watch the online lecture, idk if its important tho)

- severe testing case
- makes material more brittle
- decreases toughness
question
Toughness can change depending on ductile-to-brittle transition temperature (DBTT)
answer
Toughness can change depending on ductile-to-brittle transition temperature

there is a graph on the powerpoint

Higher temp = more ductile

Design strategy: stay above the DBTT (so things dont get brittle and break)
question
Fatigue = failure under applied ____ stress.

Stress varies with ____.

• Key points: Fatigue...
--can cause part failure, even though ?max < ?y.
--responsible for ~ 90% of mechanical engineering failures.
answer
Fatigue = failure under applied CYCLIC stress.

Stress varies with TIME.

• Key points: Fatigue...
--can cause part failure, even though ?max < ?y.
--responsible for ~ 90% of mechanical engineering failures.
question
Types of fatigue behavior:

S = STRESS AMPLITUDE

- Fatigue limit, Sfat:
--no fatigue if S [] Sfat

- For some materials, there is no fatigue limit! (Al)
answer
Types of fatigue behavior:

S = STRESS AMPLITUDE

- Fatigue limit, Sfat:
--no fatigue if S < Sfat

- For some materials, there is no fatigue limit! (Al)
question
Cracks grow incrementally:

-- crack grows faster as
• change in stress [increases/decreases]
• _____ gets longer
• loading freq. [decreases/increases].
answer
Cracks grow incrementally:

-- crack grows faster as
• change in stress INCREASES
• CRACK gets longer
• loading freq. INCREASES.
question
Improving fatigue life:

1.

2.
answer
Improving fatigue life:

1. Impose compressive surface stresses (to SUPPRESS SURFACE CRACKS FROM GROWING)

2. Remove stress concentrators (AVOID SHARP EDGES)
question
Primary Creep: slope (creep rate) _____ with time.

Secondary Creep: steady-state
i.e., constant slope (??/?t).

Tertiary Creep: slope (creep rate)
_____ with time, i.e. _____ of rate.
answer
Primary Creep: slope (creep rate) DECREASES with time.

Secondary Creep: steady-state
i.e., constant slope (??/?t).

Tertiary Creep: slope (creep rate)
INCREASES with time, i.e. ACCELERATION of rate.
question
Creep is temperature dependent

Occurs at elevated temperature, T > ___ Tm (in K)
answer
Creep is temperature dependent

Occurs at elevated temperature, T > 0.4 Tm (in K)
question
Secondary Creep:

Strain rate is _____ at a given T, ?

strain rate increases with _____ T, ?
answer
Secondary Creep:

Strain rate is CONSTANT at a given T, ?

strain rate increases with INCREASING T, ?
question
SUMMARY OF MATERIAL FAILURES:

• Sharp corners produce ____ stress concentrations
and premature failure.

• Engineering materials [as strong / not as strong] as predicted by theory

• Flaws act as stress concentrators that cause failure at
stresses lower than _____ values.

• Failure type depends on T and s :

-For simple fracture (noncyclic s and T < 0.4Tm), failure stress decreases with:
- ____ maximum flaw size,
- ____ T,
- ____ rate of loading.

- For _____ (cyclic s):
- cycles to fail decreases as Ds increases.

- For _____ (T > 0.4Tm):
- time to rupture decreases as s or T increases.
answer
SUMMARY OF MATERIAL FAILURES:

• Sharp corners produce LARGE stress concentrations
and premature failure.

• Engineering materials NOT AS STRONG as predicted by theory

• Flaws act as stress concentrators that cause failure at
stresses lower than THEORETICAL values.

• Failure type depends on T and s :
-For simple fracture (noncyclic s and T < 0.4Tm), failure stress
decreases with:
- INCREASED maximum flaw size,
- DECREASED T,
- INCREASED rate of loading.
(logical^)

- For FATIGUE (cyclic s):
- cycles to fail decreases as Ds increases.

- For CREEP (T > 0.4Tm):
- time to rupture decreases as s or T increases.
question
Ceramics:
compounds of ____ and _____ elements
answer
Ceramics:
compounds of METALLIC and NON-METALLIC elements

e.g. Al2O3, ZrO2, MgO, TiO2, SiC, WC, B4C, ZnS, TiB2 and carbon…
question
_______ ceramics:
- china, porcelain, bricks, tiles, glasses
- many based on clays

______ ceramics - oxides, carbides, nitrides
- used in electronics, computers, aerospace…
answer
TRADITIONAL ceramics:
- china, porcelain, bricks, tiles, glasses
- many based on clays

TECHNICAL ceramics - oxides, carbides, nitrides
- used in electronics, computers, aerospace…
question
Ceramic Structure/Composition:
• At least __ elements - sometimes more (BaTiO3, Ti3, SiC2)
? [more/less] complex structures than metals
• Determined by bonding:
- ranges from ~purely ____ to ~purely ____
- many exhibit combination of both
answer
Ceramic Structure/Composition:
• At least 2 elements - sometimes more (BaTiO3, Ti3, SiC2)
? MORE complex structures than metals
• Determined by bonding:
- ranges from ~purely IONIC to ~purely COVALENT
- many exhibit combination of both
question
Ceramic bonding:
-- Mostly _____ with varying degrees of ____.
-- % ionic character increases with difference in
_____ of atoms.
answer
Ceramic bonding:
-- Mostly IONIC with varying degrees of COVALENCY.
-- % ionic character increases with difference in
ELECTRONEGATIVITY of atoms.
question
(Ceramics) If Predominantly Ionic:

• Consider as electrically charged ____, not ____
• Positively charged ____ ions (cations), e.g. Ca2+
– Given up valence e to _____ anions
– Generally _____ than anions

• Negatively charged ______ ions (anions),e.g. F
– Accepted valence e from _____ cations
– Generally _____ than cations
answer
(Ceramics) If Predominantly Ionic:

• Consider as electrically charged IONS, not ATOMS

• Positively charged METALLIC ions (cations), e.g. Ca2+
– Given up valence e to NON-METALLIC anions
– Generally SMALLER than anions

• Negatively charged NON-METALLIC ions (anions),e.g. F
– Accepted valence e from METALLIC cations
– Generally LARGER than cations
question
Key factors influencing ceramic structure:
1.
2.
answer
Key factors influencing ceramic structure:
1. Magnitude of electrical charges
- crystal must be ELECTRICALLY NEUTRAL
- required charge balance dictates ratio of # cations to anions
2. Relative sizes (IONIC RADII) of cations to anions
- each cation prefers max # nearest neighbor anions and vise versa, this determines how ions pack in crystal
question
Non-metallic anions (-) much ____ than metallic cations (+)
• Pack like atoms in metals

______ metallic cations may thus fit into the spaces (i.e. the
_____ sites)
answer
Non-metallic anions (-) much LARGER than metallic cations (+)
• Pack like atoms in metals

SMALLER metallic cations may thus fit into the spaces (i.e. the INTERSTITIAL sites)
question
What are the two possible types of ceramic interstitial sites? Give some info on both.
answer
Tetrahedral Sites: 4 anion spheres as nearest neighbors
• 3 in 1 plane + 1 in adjacent plane surround site
• CN = 4
• rcation/ranion -> .225 - .414


Octahedral Sites: 6 anion spheres as nearest neighbors
• 3 each, in 2 planes
• CN = 6
• rcation/ranion -> .414 - .732
question
What determines which site the cations will occupy?
answer
What determines which site the cations will occupy?

1. Size of sites:
– does the cation fit in the site?

2. Stoichiometry:
– if all of one type of site is full, the remainder have to go into other types of sites

3. Bond Hybridization:
– % covalency
question
What determines which site the cations will occupy?

1. Size: Must form stable structures:
• ____ the # of oppositely charged ion neighbors.
• CN related to cation-anion radius ratio, CN ____ with increasing rcation/ranion
answer
What determines which site the cations will occupy?

1. Size: Must form stable structures:
• MAXIMIZE the # of oppositely charged ion neighbors.
• CN related to cation-anion radius ratio, CN INCREASES with increasing rcation/ranion
question
Crystal structure stoichiometry:

If, for a specific ceramic, each unit cell has 6 cations and these prefer the (octahedral) OH sites, then:
– __ will go into (octahedral) OH
– __ will go into (tetrahedral) TD
answer
Crystal structure stoichiometry:

If, for a specific ceramic, each unit cell has 6 cations and these prefer the (octahedral) OH sites, then:
– 4 will go into (octahedral) OH
– 2 will go into (tetrahedral) TD
question
List the AX-Type crystal structures

AX-Type means EQUAL # of cations and anions
answer
List the AX-Type crystal structures:


1. NaCl (ROCK SALT)
2. CsCl (Cesium Chloride)
question
Describe NaCl (ROCK SALT) structure:

- can be considered as 2 interpenetrating ___ lattices: one of cations and one of anions
- CN = __
- cations prefer ____ sites
answer
1. NaCl (ROCK SALT)
- can be considered as 2 interpenetrating FCC lattices: one of cations and one of anions
- CN = 6
- cations prefer OCTAHEDRAL sites
question
Describe CsCl (Cesium Chloride) structure:

- Each ion has __ neighbors
- ___ sites preferred
- like a ___ but not ___ since there are two different kind of ions.
answer
Describe CsCl (Cesium Chloride) structure

- Each ion has 8 neighbors
- CUBIC sites preferred
- like a BCC but not BCC since there are two different kind of ions.
question
Describe Zinc Blende (ZnS) structure:

- small Zn cations sit between __ S anions, CN = __
- Happens when .225<rc/ra<.0414
- Zn is in the ____ interstitial site
- __ of the possible ______ sites are occupied
- The bonding is usually highly ______
answer
Describe Zinc Blende (ZnS) structure:

- small Zn cations sit between 4 S anions, CN = 4
- Happens when .225<rc/ra<.0414
- Zn is in the TETRAHEDRAL interstitial site
- 4 of the possible TETRAHEDRAL sites are occupied
- The bonding is usually highly COVALENT (ZnS, ZnTe, SiC).
question
AmXp - Type Crystal Structures

Charges on cations and anions are not equal (m and/or p ? 1)

• Calcium Fluorite (CaF2)

• rc/ra ~ 0.8, CN = __

• Cations in ____ sites

• Similar to ____ but only half of cubic sites are occupied

• Unit cells consist of 8 “sub-cubes”
- e.g. UO2, ThO2, ZrO2, CeO2

• ______ structure – positions of cations and anions reversed
- e.g. Li2O, Na2O
answer
AmXp - Type Crystal Structures

Charges on cations and anions are not equal (m and/or p ? 1)

• Calcium Fluorite (CaF2)

• rc/ra ~ 0.8, CN = 8

• Cations in CUBIC sites

• Similar to CsCl but only half of cubic sites are occupied

• Unit cells consist of 8 “sub-cubes”
- e.g. UO2, ThO2, ZrO2, CeO2

• ANTIFLUORITE structure – positions of cations and anions reversed
- e.g. Li2O, Na2O
question
AmBnXp Crystal Structures

• >1 type of ___
• ____ structure

• Ex: complex oxide BaTiO3
• ____ crystal structure
• Ba2+ cations at corners
• Ti4+ cation in center
• O2- anions at each face
center
answer
AmBnXp Crystal Structures

• >1 type of CATION
• PEROVSKITE structure

• Ex: complex oxide BaTiO3
• CUBIC crystal structure
• Ba2+ cations at corners
• Ti4+ cation in center
• O2- anions at each face
center
question
Diamond (Similar to ___ ___)

- A _____ form of carbon

– ____ bonding of carbon
• hardest material known
• very ___ thermal conductivity

– 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.
answer
Diamond (Similar to ZINC BLENDE)

- A POLYMORPHIC form of carbon

– TETRAHEDRAL bonding of carbon
• hardest material known
• very HIGH thermal conductivity

– 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.
question
______:

- A POLYMORPHIC form of carbon
– Layered structure – parallel hexagonal arrays of carbon
atoms
-- Weak Van der Waal’s forces between layers
– Planes slide easily over one another -- good lubricant
answer
GRAPHITE:

- A POLYMORPHIC form of carbon
– Layered structure – parallel hexagonal arrays of carbon
atoms
-- Weak Van der Waal’s forces between layers
– Planes slide easily over one another -- good lubricant
question
2 more polymorphic forms of carbon:

• _____ – spherical cluster of 60 carbon atoms, C60
– Like a soccer ball
• _____ – sheet of graphite rolled into a tube
– Ends capped with fullerene hemispheres
answer
2 more polymorphic forms of carbon:

• 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
question
Point defects in ceramics:

• _____ exist in ceramics for both cations and anions
• ______ exist for cations, but are not normally observed for anions because anions are large relative to the _____ sites
answer
Point defects in ceramics:

• VACANCIES exist in ceramics for both cations and anions
• INTERSTITIALS exist for cations, but are not normally observed for anions because anions are large relative to the INTERSTITIAL sites (not saying its impossible, just far less likely)
question
• ____ Defect
-- a paired set of cation and anion vacancies.
• ____ Defect
-- a cation vacancy-cation interstitial pair.
answer
• SHOTTKY Defect
-- a paired set of cation and anion vacancies.
• FRENKEL Defect
-- a cation vacancy-cation interstitial pair.
question
_________ must be maintained when impurities are present
answer
ELECTRONEUTRALITY (charge balance) must be maintained
when impurities are present

When there is a SUBSTITUTIONAL cation or anion, the substitution must have equal charge to what is being taken away (so if 2 Na+ is replaced with 1 Ca2+, there will be a substitutional cation and a cation vacancy)
question
Diffusion of ____ is more complicated than in ____ due to the motion of two ionic species with opposite charges.

• Diffusion usually occurs by a ___ mechanism.
• Local charge neutrality must be maintained, so diffusion of an ion must be accompanied by the diffusion of an ion(s) with equal and opposite charge.
• Therefore diffusion is rate limited by the ____ moving ion.
answer
Diffusion of CERAMICS is more complicated than in METALS due to the motion of two ionic species with opposite charges.

• Diffusion usually occurs by a VACANCY mechanism.
• Local charge neutrality must be maintained, so diffusion of an ion must be accompanied by the diffusion of an ion(s) with equal and opposite charge.
• Therefore diffusion is rate limited by the SLOWER moving ion.
question
Ceramic materials are ____ brittle than metals: Why is this
so?

• Consider mechanism of deformation:
– In crystalline, by ____ motion
– In highly ionic solids, ___ motion is difficult
• few slip systems
• resistance to motion of ions of like charge (e.g., anions) past one another
answer
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
question
In general, why do ceramics fail?
answer
In general, ceramics fail due to BRITTLE FRACTURE before any plastic deformation occurs.

• Cracks propagate either transgranularly or intergranularly.

• Initiation sites are from small flaws that are present in the material which serve as stress risers.

• Because there is no mechanisms for plastic deformation, crack blunting cannot occur, leading to cracks propagating un-impeded above a certain stress.

Sometimes cracks can propagate slowly at stresses lower that
defined by the equation above (Kic = Y(delta)(sqrt(pi*a)) in moist environments leading to stress corrosion process at the crack tip.
question
Chalk cut from bulk material shows how different pieces could have different flaws.
• This leads to a statistical variation in the tensile strength.
• This distribution of cracks also creates a ____ dependence on strength since more cracks would be in a larger sample.
• A brittle rod will be stronger in ___ than in ____.
answer
Chalk cut from bulk material shows how different pieces could have different flaws.
• This leads to a statistical variation in the tensile strength.
• This distribution of cracks also creates a VOLUME dependence on strength since more cracks would be in a larger sample.
• A brittle rod will be stronger in BENDING than in TENSION. (tension is applied to the entire sample and cracks propagate in tension... with bending there is tension at some points, compression in others, so the tension is being applied to a smaller amount of the sample, making it less likely to have these flaws.
question
____ _____ Ps(Vo) is the fraction of identical samples, with volume Vo which survive loading to a tensile stress, (Delta)

who can you thank for this
answer
SURVIVAL PROBABILITY Ps(Vo) is the fraction of identical samples, with volume Vo which survive loading to a tensile stress, (Delta)

as stress increases, Survival probability decreases

THANKS WEIBULL
question
Modulus of rupture, DELTAr is larger than tensile strength for two reasons:
answer
Modulus of rupture, DELTAr is larger than tensile strength for two reasons:
• In bending half the beam is subjected to compression leading
to flaws closing up.
• The peak tensile stress is below the center load with stress
lower in other areas leading to a small volume being
subjected to high tensile stress.
question
Give 3 examples of applications for ceramics
answer
Give 3 examples of applications for ceramics
1. DIE BLANKS
2. CUTTING TOOLS
3. SENSORS
question
What are refractories?
answer
Refractories are materials to be used at HIGH TEMPERATURES.
question
Materials for Automobile engines:

Advantages:
– Operate at high temperatures – high efficiencies
– Low frictional losses
– Operate without a cooling system
– Lower weights than current engines

Disadvantages:
– Ceramic materials are brittle
– Difficult to remove internal voids (that weaken structures)
– Ceramic parts are difficult to form and machine


• Potential candidate materials: Si3N4, SiC, & ZrO2
• Possible engine parts: engine block & piston coatings
answer
Probably wont ask this directly as a question?, but it has important observations about applying ceramics:

Materials for Automobile engines:

Advantages:
– Operate at high temperatures – high efficiencies
– Low frictional losses
– Operate without a cooling system
– Lower weights than current engines

Disadvantages:
– Ceramic materials are brittle
– Difficult to remove internal voids (that weaken structures)
– Ceramic parts are difficult to form and machine


• Potential candidate materials: Si3N4, SiC, & ZrO2
• Possible engine parts: engine block & piston coatings
question
SUMMARY OF CERAMICS:

• Interatomic bonding in ceramics is ____ and/or ____.
• Ceramic crystal structures are based on:
-- maintaining ____ ____
-- cation-anion ____ ___ .
• Imperfections
-- Atomic point: vacancy, interstitial (cation), Frenkel, Schottky
-- Impurities: substitutional, interstitial
-- Maintenance of charge neutrality
• Room-temperature mechanical behavior – flexural tests
-- ____-____; measurement of elastic modulus
-- brittle fracture; measurement of ____ ____
answer
SUMMARY OF CERAMICS:

• 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.
question
what is a polymer?
answer
poly = many
mer = repeat unit
question
• Originally natural polymers were used
– Wood – Rubber
– Cotton – Wool
– Leather – Silk

• Oldest known uses
– Rubber balls used by Incas
– Woven cloth
answer
idk how to make this a question but its probably good to kno

• Originally natural polymers were used
– Wood – Rubber
– Cotton – Wool
– Leather – Silk

• Oldest known uses
– Rubber balls used by Incas
– Woven cloth
question
Polymers since WWII

• Most plastics, rubbers, fibers today are _____ polymers…
– Some of the first? ___ & ___
• Since WW II, synthetic polymers have revolutionized the
materials field:
– ____ to produce
– ____ can be controlled as desired
– Often superior to natural materials
– Supplanted metals & wood in some applications:
• lower cost & superior properties
• _____ concerns…recycling, resource usage &
conservation…sustainability
answer
Polymers since WWII

• Most plastics, rubbers, fibers today are SYNTHETIC polymers…
– Some of the first? BAKELITE & NYLON
• Since WW II, synthetic polymers have revolutionized the
materials field:
– CHEAP to produce
– PROPERTIES can be controlled as desired
– Often superior to natural materials
– Supplanted metals & wood in some applications:
• lower cost & superior properties
• ENVIRONMENTAL concerns…recycling, resource usage &
conservation…sustainability
question
Most polymers are ______
– i.e., made up of H and C
answer
Most polymers are HYDROCARBONS
– i.e., made up of H and C
question
Polymer composition:

• Strong covalent _____-molecular bonds
– 4 valence e- per C atom
– 1 valence e- per H atom
– Recall CH4…4 x single covalent bonds…

• Weaker hydrogen and van der Waals ____molecular
bonds:
– Thus polymers have ____ melting/boiling points
answer
Polymer composition:

• STRONG covalent INTRA-molecular bonds
– 4 valence e- per C atom
– 1 valence e- per H atom
– Recall CH4…4 x single covalent bonds…

• WEAKER hydrogen and van der Waals INTERmolecular
bonds:
– Thus polymers have LOW melting/boiling points
question
Saturated hydrocarbons
– Each carbon singly bonded to ___ other atoms.
– All covalent bonds are ____ bonds.
– Example:
• Ethane, C2H6
– No new atoms can be joined without removal of others already bonded.
answer
Saturated hydrocarbons
– Each carbon singly bonded to FOUR other atoms.
– All covalent bonds are SINGLE bonds.
– Example:
• Ethane, C2H6
– No new atoms can be joined without removal of others already bonded.
question
Unsaturated Hydrocarbons
• Share _ or _ pairs of electrons.
• Double & triple bonds somewhat _____ – can form new bonds
• Each carbon not bonded to 4 other atoms
– Other atoms or groups can bond to the original
molecule
answer
Unsaturated Hydrocarbons
• Share 2 or 3 pairs of electrons.
• Double & triple bonds somewhat UNSTABLE – can form new bonds
• Each carbon not bonded to 4 other atoms
– Other atoms or groups can bond to the original
molecule

Double bond found in ethylene or ethene - C2H4
Triple bond found in acetylene or ethyne - C2H2
question
Polymer molecules are very large – _____
– ____ bonds within each molecule
– Backbone = string of __ atoms singly bonded to each other
– Remaining 2 valence electrons are used for side-bonds to atoms and radicals
– ex. Ethylene (C2H4) is a gas at room temp but can be a polymer building block
answer
Polymer molecules are very large – MACROMOLECULES
– COVALENT bonds within each molecule
– Backbone = string of C, CARBON atoms singly bonded to each other
– Remaining 2 valence electrons are used for side-bonds to atoms and radicals
– ex. Ethylene (C2H4) is a gas at room temp but can be a polymer building block
question
Chemistries other than pure HC are possible:
– Polymerize (CF2=CF2) --> PTFE (Teflon®)
– Fluorocarbons
– PVC which is similar to ethylene where 1 in 4 H replaced
with Cl

General forumla:

-CH2-CHR-

R = ____ or _____
answer
Chemistries other than pure HC are possible:
– Polymerize (CF2=CF2) --> PTFE (Teflon®)
– Fluorocarbons
– PVC which is similar to ethylene where 1 in 4 H replaced
with Cl

General forumla:

-CH2-CHR-

R = ATOMS or GROUPS
question
Chain Repeat Units
– If all the same: _____
– If different: _____
answer
Chain Repeat Units
– If all the same: HOMOPOLYMERS
– If different: COPOLYMERS
question
_______ polymerization

– Monomer unit daisy-chained 1 at a time to form linear
macromolecule
– 3 stages: initiation, propagation & termination

- Chain forms via sequential addition of monomer units
– Active site (unpaired e-) transfers to each successive end
monomer

• Termination:
– Active ends of 2 propagating chains may link: ____
– Dead chains are possible, e.g. via H atom: _____
answer
FREE RADICAL polymerization

– Monomer unit daisy-chained 1 at a time to form linear
macromolecule
– 3 stages: initiation, propagation & termination

- Chain forms via sequential addition of monomer units
– Active site (unpaired e-) transfers to each successive end
monomer

• Termination:
– Active ends of 2 propagating chains may link: COMBINATION
– Dead chains are possible, e.g. via H atom:
DISPROPORTIATION
question
________ polymerization

– Intermolecular reactions involving >1 monomer species giving off a low MW condensate (e.g. H2O)
– E.g. Nylon 6,6
answer
CONDENSATION polymerization

– Intermolecular reactions involving >1 monomer species giving off a low MW condensate (e.g. H2O)
– E.g. Nylon 6,6
question
______ ______ Polymer (3-D Crosslinking)
– Phenol-Formaldehyde (Phenolic)
answer
THERMOSET NETWORKED Polymer (3-D Crosslinking)
– Phenol-Formaldehyde (Phenolic)
question
Describe Isomerism
answer
ISOMERISM:
– Two compounds with same chemical formula can have quite different structures

ex. C8H18 can be normal octane or 2,4-dimethylhexane
question
Thermoplastics vs. Thermosets
answer
THERMOPLASTICS
• Linear polymers with 2 links/mer
• They can be softened (and melted) repeatedly by raising the temperature.
• Weak secondary bonds between chains.
• Strong bonds within chains.

THERMOSETS
• Crosslinked with 3 links/mer
• Rigid 3-D molecules
• After a thermoset is formed, it CANNOT be reshaped or remelted.
question
Not all chains in a polymer are of the same ____
— i.e., there is a distribution of molecular weights
answer
Not all chains in a polymer are of the same LENGTH
— i.e., there is a distribution of molecular weights
question
• MW can be defined in several ways:
• Number averaged,
answer
DP = average number of repeat units per chain
question
Molecular Shape (or _______):
– Chain bending and twisting are possible by ____ of carbon atoms around their chain single bonds
– note: not necessary to ____ chain bonds to alter molecular shape
answer
Molecular Shape (or CONFORMATION):
– Chain bending and twisting are possible by ROTATION of carbon atoms around their chain single bonds
– note: not necessary to BREAK chain bonds to alter molecular shape
question
Chain ________ = r

r << total chain length

Large # of chains:
• Each may bend, coil or kink
• Extensive chain entanglement
• Governs some mechanical properties,
e.g. in elastomers
answer
Chain END-TO-END DISTANCE = r

r << total chain length

Large # of chains:
• Each may bend, coil or kink
• Extensive chain entanglement
• Governs some mechanical properties,
e.g. in elastomers
question
Physical characteristics of polymers depend on:
1.
2.
3.
answer
Physical characteristics of polymers depend on:
1. MOLECULAR WEIGHT
2. SHAPE
3. STRUCTURE
question
List the 4 covalent chain configurations in order of increased strength
answer
[image]
question
• Linear polymer:

? Repeat units joined end-to-end
? Long flexible chains (“spaghetti”)
? Extensive _____ and ____ bonds betweenchains
? PE, PVC, PS, MPPA, Nylon, fluorocarbons
answer
• Linear polymer:

? Repeat units joined end-to-end
? Long flexible chains (“spaghetti”)
? Extensive VAN DER WAALS and HYDROGEN bonds between chains
? PE, PVC, PS, MPPA, Nylon, fluorocarbons
question
Branched polymer:

• Side-branch chains connected to main chain
• Chain packing efficiency ____
• ____ density
• LDPE
answer
Branched:

• Side-branch chains connected to main chain
• Chain packing efficiency REDUCED
• LOWER density
• LDPE
question
• Crosslinked polymer:

? Adjacent chains joined by ____ bonds
? Occurs during ____
? Non-reversible
? Rubbers (via vulcanization using S atoms)
answer
• Crosslinked polymer:

? Adjacent chains joined by COVALENT bonds
? Occurs during SYNTHESIS
? Non-reversible
? Rubbers (via vulcanization using S atoms)
question
Network polymer:

• Monomers forming __ or more covalent bonds
• Form 3-D networks
• Epoxies, polyurethanes, phenol-formaldehyde
answer
Network polymer:

• Monomers forming 3 or more covalent bonds
• Form 3-D networks
• Epoxies, polyurethanes, phenol-formaldehyde
question
______ are mirror images – can’t superimpose without breaking a bond
answer
STEREOISOMERS are mirror images – can’t superimpose without breaking a bond
question
Configurations – to change must ____ ____
answer
Configurations – to change must BRAEAK BONDS
question
______ – stereoregularity or spatial arrangement of R units along chain
_____ – all R groups on same side of chain
_____ – R groups alternate sides
_____ – R groups randomly positioned
answer
TACTICITY – stereoregularity or spatial arrangement of R units along chain
ISOTACTIC – all R groups on same side of chain
SYNDIOTACTIC – R groups alternate sides
ATACTIC – R groups randomly positioned
question
Geometrical isomerism within repeat units with ____
bonds between chain C atoms:

- cis: H atom and CH3 group on the same side of the chain
- trans: H atom and CH3 group on opposite sides of the chain

Isomers have the same or different properties?
Can you convert from cis to trans?
answer
Geometrical isomerism within repeat units with DOUBLE
bonds between chain C atoms:

- cis: H atom and CH3 group on the same side of the chain
- trans: H atom and CH3 group on opposite sides of the chain

Isomers have the same or different properties? DIFFERENT
Can you convert from cis to trans? NO
question
What are 4 possible arrangements for copolymers (two or more monomers polymerized together)
answer
question
Crystallinity in Polymers:

• Involves _____ as opposed to atoms in metals & ceramics
• More complex atomic arrangements
• Crystallinity due to packing of polymer chains to generate an
ordered atomic array.
• Complex ____ ____
• Chain twisting, coiling & kinking all promote disorder:
? Disorder = amorphous regions
? Most polymers thus only partially crystalline
? Combination of ____ + ____ regions
• Range from 100% amorphous to ~95% crystalline
• Density = fn.(% crystallinity)
answer
Crystallinity in Polymers:

• Involves MOLECULES as opposed to atoms in metals & ceramics
• More complex atomic arrangements
• Crystallinity due to packing of polymer chains to generate an
ordered atomic array.
• Complex UNIT CELLS
• Chain twisting, coiling & kinking all promote disorder:
? Disorder = amorphous regions
? Most polymers thus only partially crystalline
? Combination of CRYSTALLINE + AMORPHOUS regions
• Range from 100% amorphous to ~95% crystalline
• Density = fn.(% crystallinity)
question
How does an increase in % crystallinity of a polymer affect:
TS and E

How does heat treating affect % crystallinity?
answer
How does an increase in % crystallinity of a polymer affect:
TS (INCREASE)
and E (INCREASE)

How does heat treating affect % crystallinity? It causes crystalline regions to grow and % crystallinity to INCREASE
question
Single polymer crystals – only for ____ and carefully ____ growth rates
answer
Single polymer crystals – only for SLOW and carefully CONTROLLED growth rates
question
Some semicrystalline polymers form spherulite structures
• Alternating chain-folded crystallites and amorphous regions
• Spherulite structure for relatively ___ growth rates
- has a ______ site at the center
answer
Some semicrystalline polymers form SPHERULITE structures
• Alternating chain-folded crystallites and amorphous regions
• Spherulite structure for relatively RAPID growth rates
- has a NUCLEATION site at the center
question
Strengths of polymers ~__% of those for metals
answer
Strengths of polymers ~10% of those for metals
question
Picture the deformation of BRITTLE crosslinked and network polymers
answer
[image]
question
Picture the deformation of SEMICRYSTALLINE (PLASTIC) polymers
answer
[image]
question
Picture the deformation of ELASTOMERS
answer
[image]
question
What does the stress strain curve look like for BRITTLE polymers, PLASTIC polymers, and ELASTOMERS
answer
[image]
question
Influence of T on Thermoplastics:

Decreasing T...

-- ____ E
-- ____ TS
-- ____ %EL
answer
Influence of T on Thermoplastics:

Decreasing T...

-- INCREASES E
-- INCREASES TS
-- DECREASES %EL
question
Influence of strain rate on Thermoplastics:

Increases strain rate...

-- ____ E
-- ____ TS
-- ____ %EL
answer
Influence of strain rate on Thermoplastics:

Increases strain rate...

-- INCREASES E
-- INCREASES TS
-- DECREASES %EL
question
What is Tm and Tg?
answer
Tm = MELTING TEMP
Tg = GLASS TRANSITION TEMP
question
What factors affect Tm and Tg?
answer
What factors affect Tm and Tg?

Both Tm and Tg increase with increasing chain stiffness

• Chain stiffness increased by presence of
1. Bulky sidegroups
2. Polar groups or sidegroups
3. Chain double bonds and aromatic chain groups

• Regularity of repeat unit arrangements – affects Tm only
question
How can we increase chain stiffness, in turn increasing Tm and Tg?
answer
Chain stiffness increased by presence of

1. Bulky sidegroups
2. Polar groups or sidegroups
3. Chain double bonds and aromatic chain groups
question
Regularity of repeat unit arrangements – affects [Tm/Tg] only
answer
Regularity of repeat unit arrangements – affects Tm only
question
Craze formation prior to cracking:
– during crazing, plastic deformation of spherulites
– and formation of ____ and ____ bridges
answer
Craze formation prior to cracking:
– during crazing, plastic deformation of spherulites
– and formation of MICROVOIDS and FIBRILLAR bridges
question
Processing of Polymers:

_______
– Can be reversibly cooled & reheated, i.e. recycled
– Heat until soft, shape as desired, then cool
– Examples: polyethylene, polypropylene, polystyrene.


________
– When heated forms a molecular network
(chemical reaction)
– Degrades (doesn’t melt) when heated
– Prepolymer molded into desired shape, then
chemical reaction occurs
– Examples: urethane, epoxy
answer
Processing of Polymers:

THERMOPLASTIC
– Can be reversibly cooled & reheated, i.e. recycled
– Heat until soft, shape as desired, then cool
– Examples: polyethylene, polypropylene, polystyrene.


THERMOSET
– When heated forms a molecular network
(chemical reaction)
– Degrades (doesn’t melt) when heated
– Prepolymer molded into desired shape, then
chemical reaction occurs
– Examples: urethane, epoxy
question
List 4 methods of processing polymers and which type (thermoplastic or thermoset) it applies to
answer
List 4 methods of processing polymers and which type (thermoplastic or thermoset) it applies to:

1. COMPRESSION MOLDING
thermoplastics and thermosets

2. INJECTION MOLDING
thermoplastics and some thermosets

3. EXTRUSION
thermoplastics
BLOWN-FILM EXTRUSION for plastic bags*** we watched a video on this, might actually be important
question
polymer fibers are formed by _____.

- extrude polymer through a spinneret (a die containing many small orifices)
– the spun fibers are drawn under tension
– leads to highly aligned chains
- fibrillar structure

primary use in textiles
answer
polymer fibers are formed by SPINNING.

- extrude polymer through a spinneret (a die containing many small orifices)
– the spun fibers are drawn under tension
– leads to highly aligned chains
- fibrillar structure

primary use in textiles
question
polymer fiber characteristics
answer
polymer fiber characteristics
1. high tensile strengths
2. high degrees of crystallinity
3. structures containing polar groups
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