friction in metals added version

8/4/2019 Friction in Metals Added Version

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NATIONAL INSTITUE OF TECHNOLOGY

HAMIRPUR

APresentation

OnFriction in Metals

By:

Ajay Goyal

Ajay Philip

Rahul Singh

Naveen kumar

Kulbhushan Phogat


2/41

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


27/41

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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respect to sliding velocity

V i ti f ffi i t f f i ti ith


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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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friction in metals added version

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