friction in metals added version
TRANSCRIPT
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NATIONAL INSTITUE OF TECHNOLOGY
HAMIRPUR
APresentation
OnFriction in Metals
By:
Ajay Goyal
Ajay Philip
Rahul Singh
Naveen kumar
Kulbhushan Phogat
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Frictional Behaviour of Metals
Metallic friction occurs due to shearing of
welded metallic junctions formed between
contacting surfaces
In the absence of contaminant films these
junctions are formed easily and the friction is
very high.
Clean metals exhibit high coefficient of friction
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Theories of Metallic Friction
From the research conducted by Amonton,
Coulomb, Hirn, Bowden and Tabor following
results were obtained:
1.Frictional force is:
directly proportional to load applied.
independent of area of surfaces in contact.
independent of velocity of sliding
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2. Effect of velocity, surface area and load
different in lubricated & unlubricated surfaces.
3. Friction force in metals is caused due toadhesion and deformation forces.
Theories of Metallic Friction
Continued..
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Basic Friction modes in Metals
Friction on virgin surfaces
Friction on clean oxidised surfaces
Boundary friction Transition zone Pure boundary friction
Boundary hydrodynamic friction
Hydrodynamic friction
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Virgin surfaces
These surfaces have very high values for
coefficient of friction
These types of surfaces are mainly encountered
with adhesion friction.
Metallic bondings are very strong
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Clean oxidized surfaces
Less coefficient of friction as compared to
virgin surfaces.
Coefficient of friction depends on type and
properties of oxide films.
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Boundary friction transition zone
It is characterized by the presence of a very thin
layer of interstitial phase between the surfaces.
Frictional magnitude is in between clean surface
friction and boundary friction
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Pure boundary Friction
In this a very thin layer of transitionphase separates the two surfaces.
This type of friction depends on followingproperties:
1. Physical nature, state and properties of hard
surfaces,2. Structure and properties of oil molecules,
3. Experimental conditions like pressure,
& temperature.
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Boundary Hydrodynamic Friction
It shows the transition between boundary
friction to hydrodynamic friction
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Hydrodynamic Friction
It is observed in presence of interposing layer
of a lubricant
It is largely dependent of the liquid viscosity
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Factors Influencing Friction In Metals
Friction in metals are affected mainly by the
following factors :
Degree of metallic nature
Oxide film formation
Alloying Physically Adsorbed gases
Atmosphere that the metal is surrounded
Structural Effects
Crystal transformation Orientation effects
Temperature
Interfacial Potential
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Degree of Metallic Nature
All metals in the periodic table in their clean
state exhibits a stick- slip behavior when they
slides over each other
This occurs due to strong metallic bonding
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Oxide film formation
Most clean metals like Cu, Fe , Al, Ni, Zn etc forms
oxides on their surfaces.
These oxides prevent the metal surfaces from being
in direct contact with each other thereby reducing
the adhesive forces .
This reduces the coefficient of friction in metals
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Oxide film formation in Copper
Dr Whitehead (1950) studied the frictional behavior
due to the oxide film formation in copper.
He found that the coefficient of friction remainedindependent at a value of 1.6 of load for all loads
above 100gms
Coefficient of friction reduces which reduction in
load and reaches a value of 0.4 for a load of about
10gms.
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Oxide formation in Aluminum
The coefficient of friction remains constant at a high
value of 1.2 for a load between 10-2 to 104 gms
This is because the oxide film breaks up and there is
characteristic metallic welding even at smallest
loads
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Alloying Effects
Small proportions of alloying elements cancause marked changes in frictional properties
of metals
Some alloying elements increases friction
while some others reduces it
Main reason for change in friction is surface
segregation
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Addition of 5 weight percentage of
chromium iron can increase thecoefficient of friction from 0.5 to 1.
When there is difference in size between soluteand solvent atoms there occurs a tendency to
squeeze out the solute atoms out of the parentlattice to reduce energy
There are situations where solute atoms cannotbe squeezed out of the parent lattice
In such situations there is a difference in atomicsize and a strain is produced in the crystal lattice
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This strain in bulk lattice influences the
frictional behaviour of metals
This is due to the fact that it influences the
shear properties and resistance to shear themetal
The greater the strain, the greater theresistance to shear the metal
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Physically Adsorbed Gases
Adsorption is explained as the adhesion of atoms,
ions, molecules into a surface.
It creates a film of adsorbate on the surface of
adsorbent. The static friction coefficient for most metals
decreases with increasing concentration of
adsorbate.
This happens because the adhesion between
surfaces gets greatly reduced even for a small layer
of adsorbate on the solid metal surface.
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Effect of Atmospheric gases
Atmospheric gases can also cause change in
frictional characteristics of metals
This occurs largely due to chemisorption
activity of gas molecules on each metal
surface
The coefficient of friction decrease with
increase in pressure
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The frictional characteristics mainly depends
upon the metal and the atmospheric gas
molecules
In the experiments of Bowden and Hughes the
friction of copper, gold and nickel were not
affected by hydrogen and nitrogen while
Buckley reported that the friction of tungstenwas decreased by hydrogen
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Structural Effects
Hexagonal closed packed metals exhibit lowcoefficient of friction than a Face Centred Cubic
Metal.
Hexagonal metals have limited number of slipplanes.
Accordingly when hexagonal metals like cobalt
deforms by slippage when pressed against each
other leaving many air gaps at each other.
In contrast cubic metals having 12 slip planes have
no such air gaps and for this reason the contact is
stronger and friction is hence higher
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Crystal Transformation
Many metals exhibit more than one crystalstructure
Change in crystal structure can cause markeddifference in frictional behaviour
Cobalt transforms from HCP structure to FCCat 411 degree Celcius,thereby showingincreased coefficient of friction
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Orientation Effects
The crystallographic orientation in a metal canalso influence the frictional properties of metals.
The frictional coefficients of metals are usuallylowest for highest atomic density, low surfaceenergy planes in the metal
In a BCC system this is {110} surface In FCC system it is the { 111} surface
For HCP system it is the {0001} surface
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When 2 dissimilar metals are brought insliding contact, adhesion & frictional forcesmeasured are less than that in case of likematerials of same crystallographic orientation
in contact
It has been experimentally proved that
adhesive forces for copper in contact withcobalt is half that for copper in contact withnickel
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Effect of Temperature
In general friction decreases when temperature is
raised, but the effect is slight even at 1000 degrees
An exceptional case is that occurs in case of goldwhere there is little change in coefficient of friction
untill 600 degrees after it raises to a high value
This occurs due to pronounced softening of gold at
this temperature, so that metal flows readily and
the sliding surface would weld together over a large
area
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Effect of Temperature
Various metals have been examined at temperature ranging
between 25K to a room temperature:Specimens were gold , silver , copper , molybdenum etc.
Observations:
1. With FCC metals at room temperature, coefficient of friction
was quite high (=2.5).When the temperature is lowered from room temperature
coefficient of friction decreases to 1.5.This will also lead in work
hardening rate.
2. With BCC metals similar change at low temperature is observed
but coefficient of friction was lower than FCC at lower
temperature i.e. =1.0
3. In hexagonal metals, behaviour of metals depends on the
ductility of the metal. At lower temperatures coefficient of
friction will reduce to a order of 0.5.
Case Study:
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Effect of interfacial potential
In 1936, Bastow studied the friction of metals
immersed in an electrolyte
He observed that friction in immersed metals
depended on concentration of ions deposited
on the metal surface
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Case study
Measurement of friction in platinum withchanging interfacial potential:
Platinum metal is immersed in a solution ofsulphuric acid
When the interfacial potential is made +1 volt the
electrode surface becomes covered withmonolayer of oxygen and if potential is mademore, we have a region of oxygen over potential
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If the potential is reduced below 1 volt the
monolayer of oxygen is reduced
At a region of near 0 volt monolayer of hydrogenis deposited
Further decrease inve potential hydrogen overpotential occurs
In the region of +1 volt which corresponds tooxygen deposition coefficient of friction wasobserved to be about 0.7
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As the potential is decreased, the
monolayer of oxygen gets removed and
coefficient of friction becomes 3.4 at 0.3 Volts
Still lowering the potential hydrogen depositionstarts and coefficient of friction value falls to 2.3
Further lowering the potential, coefficient offriction decreases, but still have high value as
compared to oxygen deposition
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Metals sliding on polymers
Reduced friction can be observed when
metals slides over polymers and vice versa
This occurs due to the deposition of a transfer
film of the polymer on the surface of the
metal
Studies shows that such a transfer film of
polyethylene oxide was observed on iron
surface after sliding
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Metal films
We know that
F= A.S
where A- area of contact
S- shear strength
To reduce friction between surfaces we must make A & Ssmall
If we choose metal with low shear strength, it is usuallysoft, so that for a given load A becomes large
If we choose a hard metal area of contact will be small butthe shear strength will be large
For this reason the coefficient of friction for most metalsremains in the same range
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We can achieve a condition of reduced friction
by depositing a very thin film of soft metal on
the surface of the hard metal
Provided the metallic film does not breakdown theshear strength S will be that of the softer metal
At the same time A remains small even for heavy loads,since the load is borne by the hard substrate
Friction decreases as thinner films are used.
But the minimum thickness of the film should be upto10-5mm below which the layer wont be effective
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Comparison between Metal films and
Lubricating films
Metal films
Reduces friction
Should be considerably thick
to the order of 10-5 mm
Does not obey Amontons law
ie; with metallic films as load is
decreased coefficient of
friction decreases
Gets worn away at faster rate
Lubricating films
Reduces friction
Less thicker than metal films
to be effective
Obeys Amontons law
Does not get worn away at
easily
V i i f ffi i f f i i i h
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Variation of coefficient of friction with
respect to temperature
V i i f ffi i f f i i i h
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Variation of coefficient of friction with
respect to sliding velocity
V i ti f ffi i t f f i ti ith
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Variation of coefficient of friction with
respect to sliding velocity
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References
Principles & Application ofTribology By
Bharat Bhushan
Surface effects of adhesion, friction, wear &
Lubrication by Donald .H.Buckley
Friction and lubrication of solids, Volume 1
By Frank Philip Bowden & David Tabor
Engineering Tribology by G. Stachowiak &
A.Batchelor
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THANK YOU