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Junction growth

22

21

2 haa p =+

W

F

The value of

may approach h

so that p

can be very

modest in order to maintain plastic flow

Under these high frictional conditions, there is significant

junction growth and

can increase to very large values

212 / 1 aam

m

p ==

R e a r r a n g

i n g

m = / h

(called friction factor and

represents the relative strength of the interface)

Therefore, as m increases (i.e. adhesion increases) the coefficient of friction

increases

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Junction growth

Effect of m on

when 2

=32 and for

1

/ 2

=1 and 0.9

For m close to 1 (strong adhesion),

is reaching very high values

As m decreases, coefficient of friction dramatically decreases

Remember m represents the

relative strength of the interface

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Junction growth

Experimental evidence

Nickel asperity sliding on a tungsten surface

~0.4 for normal atmospheric condition

De contaminating surfaces by heating them

Surfaces become clean strong adhesion very high

Re contaminating the junction

oxide patches form in the junction reduce

lower

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Junction growth Friction pairs

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Deformation

Two surfaces

slide

against

each

other

under

dry

conditions Scoring and surface damage is observed on at least one of them

A force that initiates or maintains tangential motion and therefore contributes to

the frictional force and to the coefficient of friction (plastic

work gone into

ploughing deformation)

The above force is added to the adhesional effects in order to describe the

surface interactions

C. V. Dharmadhikari et al 1999 Europhys. Lett. 45 215

Polished surface of polycrystalline silver

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Deformation

Modeling a surface asperity as a cone or as a sphere A groove will be formed on the surface (ploughing effort of the

asperity)

p AF =

Tangential force to create the groove Cross sectional

area of groove

Pressure needed to displace material in the surface

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Deformation

For a cone

p Rw

Rw

Rw

RF

=2

22

41

22arcsinFor a sphere

pw

F = cot

2

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Deformation

For a cone

For a sphere

H w

W = 22

cot2

W is normal loadH is hardness of surface

H w

W =4

2

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Deformation

Coefficient of ploughing friction

But we know that; W F

=

For a cone

For a sphere

H

p=

tan2

H

p

w

R

R

w

w

R

= 12

2arcsin

2222

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Deformation


For a conical asperity and assuming p= H, the coefficient of friction depends only on

For a spherical asperity, coefficient of friction depends on w / 2R (i.e. it changes as

the asperity goes deeper in the surface)

For quite rough surfaces the angle is less than 10 o and the contribution of the ploughing frictional component is less than 0.1

For w/2R>0.2, the ploughing friction component makes a significant contribution to overall friction

f ff f l h f

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Deformation


H p

w R

Rw

w R

= 12

2arcsin22

22

For a sphere

If R>>w (track width)

H W

R 34

Therefore, for a spherical asperity of a given size R

carrying a fixed normal load

W, the contribution to the overall

due to ploughing is proportional to 1/ H The above is important for softer materials

Friction of metals

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Friction of metals

I.M. Hutchings Tribology book

The friction of pure metals sliding against themselves in air is determined by the

presence of surface oxides

If the surface oxide remains intact during sliding, surface damage is slight and

is determined by the oxide surface

Coefficient of friction increases when oxide layer is removed at higher loads In general,

for an alloy is less than that of its pure components

Sliding friction of steels:

varies with composition, microstructure and often depends on load

0.4%C At low loads the uppermost layer Fe2O3 remains intact.As load increases, layer is removed and a transition of

is observed

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Effect of temperature of friction of metals When temperature increases during sliding of metals Their mechanical properties change Their rate of oxidation increases Phase transformations may take place

Frictional behaviour is influenced

Cubic close packed

Body centered cubic

Hexagonal close packed

S l i d i n g i n u

l t r a h i g h v a c u u m

Transitions are observed for ccpand bcc metals

No transitions for hcp metals but

more ductile metals exhibit higher

friction (Ti, Zr)

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Effect of temperature of friction of metals At high temperatures one of the surfaces may become molten

Its shear strength decreases and the friction force drops to a low value This occurs in the sliding of metals at very high speeds (>100m/s) This same phenomenon is observed in the sliding of a ski over ice/snow

In both cases the dissipation of frictional work generates local heat and raise the

temperature at the interface to the melting point Therefore, conditions of effective hydrodynamic lubrication are

taking place

f

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Friction of rubbers and elastomers

When a rigid counterface with a smooth

surface and a large radius of curvature slides against a rubber surface

Adhesion becomes

important

Relative motion at the interface is due to

waves of detachment

which flow across the contact patch from the leading edge These waves are called Schallamach

waves

can be as high as 2

F i i f bb d l

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Friction of rubbers and elastomers

When the radius of curvature of the

slider becomes small (needle like), the asperity penetrate deeply into the rubber, no cutting occurs because failure is prevented by adhesional effects

The rubber tears at right angles to the

direction of maximum stress (doted line)

ASTM D2228 (fig 4.12b)

Rubber

Abrasion Resistance Test

P l f i i

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Polymer friction

Unlike metals, polymers exhibit low bulk moduli and increase their density

significantly under the action of hydrostatic pressure

Change in intermolecular spacing

Material shear stress h becomes function of local

normal pressure p

Contact area A

is a non linear function of load W

aphh o +=

Experimental evidence

Empirical

observations

32, / 2

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Polymer friction

P l f i ti ff t f t t

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Polymer friction effect of temperature

At low temperatures polymers behave in a brittle manner (they posses some degree

of crystallinity)

At higher temperatures they soften, they lose any crystallinity

and become

amorphous and glassy ( g

is reached glass transition temperature)

Friction of ceramics

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Ceramics are in general more stable (thermally, chemically) and

harder than metals Ceramics of tribological interest include: Al2

O3 Si3

N4 SiC ZrO2

Made from powders


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Because of their brittle nature, various types of cracks are generated by friction in

the vicinity of the friction track Radial cracks and lateral cracks are formed during loading and unloading phases

S

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Summary

Adhesion component of friction Deformation component of friction Metals Elastomers

and rubbers

Polymers Ceramics

General discussion of Frictional behaviour

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