Mae 493n 593t Lec7
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Transcript of Mae 493n 593t Lec7
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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
Coefficient of ploughing friction
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
Coefficient of ploughing friction
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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Friction of ceramics
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
Friction of ceramics
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Friction of ceramics
Friction of ceramics
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Friction of ceramics
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