International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01
45
The Effect of Sliding Speed and Normal Load on
Friction and Wear Property of Aluminum
M. A. Chowdhury , M. K. Khalil, D. M. Nuruzzaman, M. L. Rahaman
Department of Mechanical Engineering
Dhaka University of Engineering & Technology, Gazipur, Gazipur - 1700, Bangladesh

E-mail: asadzmn@yahoo.com, Phone:+8801715178693

Abstract — The present paper investigates experimentally the
effect of sliding speed and normal load on friction and wear
property of an aluminum disc sliding against stainless steel pin.
To do so, a pin-on-disc apparatus was designed and fabricated.
Experiments were carried out under normal load 10-20 N, speed
500-2500 rpm and relative humidity 70%. Results show that the
friction coefficient decreases with the increase of sliding speed
and normal load for aluminum. It is also found that the wear
rates increase with the increase of sliding speed and normal
load.
Index Term— Friction Coefficient, Normal Load, S liding
S peed, Wear Rate.
I.
INT RODUCT ION
Study of mechanics of friction and the relationship between
friction and wear dates back to the sixteenth century, almost
immediately after the invention of Newton’s law of motion. It
was observed by several authors [1-13] that the variation of
friction and wear rate depends on interfacial conditions such
as normal load, geometry, relative surface motion, sliding
speed, surface roughness of the rubbing surfaces, type of
material, system rigidity, temperature, stick slip, relative
humidity, lubrication and vibration. Among these factors
sliding speed and normal load are the two major factors whose
play significant role for the variation of friction and wear rate.
The third law of friction, which states that friction is
independent of velocity, is not generally valid. The coefficient
of kinetic friction as a function of sliding velocity generally has
a negative slope. Changes in the sliding velocity result in a
change in the shear rate which can influence the mechanical
properties of the mating materials. The strength of many metals
and nonmetals is greater at higher shear strain rates as stated
by Bhushan and Jahsman [14, 15] which results in a lower real
area of contact and a lower coefficient of friction in a dry
contact. On the other hand, Bhushan reported that high normal
pressures and high sliding speeds can result in high interface
(flash) temperatures that can significantly reduce the strength
of most materials [16]. Yet in some cases, localized surface
melting reduces shear strength and friction drops to a low
value determined by viscous forces in the liquid layer. Fridmen
and Levesque [17] suggest that part of the observed friction
reduction is due to negative slope of the dependence of the
friction force upon velocity. The friction force is a function of
velocity and time of contact. For most materials when the
velocity increases, friction decreases and when duration of
contact increases, friction increases. The dependence of
friction on velocity may be explained in the following way.
When velocity increases, momentum transfer in the normal
direction increases producing an upward force on the upper
surface. This results in an increased separation between the
two surfaces which will decrease the real area of contact.
Contributing to the increased separation is the fact that at
higher speeds, the time during which opposite asperities
compress each other is reduced increasing the level on which
the top surfaces moves.
In the case of materials with surface films which are either
deliberately applied or produced by reaction with environment,
the coefficient of friction may not remain constant as a
function of load. In many metal pairs in the high-load regime,
the coefficient of friction decreases with load. Bhushan [18]
and Blau [19] reported that increased surface roughening and a
large quantity of wear debris are believed to be responsible for
decrease in friction. It was observed that the coefficient of
friction may be very low for very smooth surfaces and/or at
loads down to micro-to nanonewton range [20, 21]. In spite of
these investigations the effects of sliding speed and normal
load are yet to be clearly understood. Therefore in this study
an attempt is made to investigate the effect of sliding speed
and normal load on friction and wear behavior of aluminum
sliding against stainless steel. It is expected that the
applications of these results will contribute to the different
concerned mechanical processes.
II.
EXPERIMENT AL DET AILS
Fig. 1 shows a pin-on-disc machine which contains a pin that
can slide on a rotating horizontal surface (disc). A circular
aluminum test sample (disc) is to be fixed on a rotating plate
(table) having a long vertical shaft welded from the bottom
surface of the rotating plate. The shaft passes through three
close-fit bush-bearings which are rigidly fixed with threesquare plates such that the shaft can move only axially and
any
111701-6868 IJMME-IJENS © February 2011 IJENS
I J ENS
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01
2
1
6
3
46
7
4
5
14
8
10
11
9
1. Load arm holder
2. Load arm
3. Normal load (dead weight)
4. Horizontal load
5. Pin sample
6. T est disc with rotating table
7. Computer
8. Belt and pulley
9. Main shaft
10. Motor
11. Speed control unit
12. Base plate
13. Rubber block
14. Rotating plate
15. Vertical square bar
15
12
13
Fig. 1. Block diagram of the experimental set -up
radial movement of the rotating shaft is restrained by the bush.
To provide the rigidity to the main structure of this set-up all
these three supporting square plates along with a base plate
are rigidly fixed with four vertical square bars. The base plate
was bolted with the foundation. A 50 mm thick neoprene
rubber bearing pad was used between the base plate and the
foundation. Foundation bolts were passed through this
bearing pad to fix base plate with this concrete foundation.
Sliding velocity can be varied by two ways (i) by changing
the rotation of the shaft and (ii) by changing the radius of the
point of contact of the sliding pin.
A half-horsepower motor is mounted vertically to rotate the
shaft with the table on a separate base having rubber damper.
This separate base was used to reduce the effect of vibration
of the motor, which may transmit to the main structure. The
speed of the motor is varied as required by using an electronic
speed control unit. Contacting foot of a 6mm diameter
cylindrical pin is flat made of SS-304, fitted on a holder is
subsequently fitted with an arm. The arm is pivoted with a
separate base in such a way that the arm with the pin holder
can rotate vertically and horizontally about the pivot point with
very low friction. Pin holder is designed including the facility
of putting dead weight on it so that required normal force will
act on the test sample through the pin. To avoid the loss of
surface material of the pin the contacting surface will remain
almost constant and for this the shapes of pin were maintained
cylindrical. A load cell (TML, Tokyo Sokki Kenkyujo Co. Ltd,
CLS-100NA, Serial no. MR2947) was used to measure the
vertical force acting on the pin. A data acquisition system was
used to measure the force continuously when the system is on
and these data are sent directly to the computer. The load cell
along with its digital indicator (TML, Tokyo Sokki
Kenkyujo Co. Ltd, Model no. TD-93A), calibrated against a
standard proving ring was used for measuring loads. Losses of
frictional forces at pivot points of the pin holder were
determined and incorporated in the results. The total set-up
was placed inside a chamber whose relative humidity can be
adjusted by supplying requisite amount of moisture. A
hygrometer (Wet and Dry Bulb Hygrometer, ZEAL, England)
was used to measure the relative humidity of the chamber. A
tachometer was used to measure the rpm of the rotating shaft.
The surface roughnesses of the test sample were also
measured by surface roughness tester (Taylor Hobson
Precision Roughness Checker). The average roughnesses of
the aluminum disc before test were found to be 0.20 m (RMS).
All experiments were conducted at about 70% relative
humidity. Wear rates were calculated from the measured weight
loss of the disc after rubbing for definite time. Initial and final
weights of the disc before and after rubbing were measured on
a high-resolution weighing scale. During tests each experiment
was repeated several times with new sample of pin and disc.
III.
RESULT AND DISCUSSION
Fig. 2 shows the variation of friction coefficient with the
duration of rubbing at different sliding speed for aluminum.
The curve in Fig. 2 drawn for speed 500 rpm shows the
variation of friction coefficient of aluminum with duration of
rubbing. During the starting, value of friction coefficient is 0.53
which remains constant for few seconds then increases almost
linearly up to 0.58 over a duration of 15 seconds of rubbing
and after that it remains constant for the rest of the
experimental time. Other curves of this figure show the values
of friction coefficient at 1000, 1500, 2000 and 2500 rpm
111701-6868 IJMME-IJENS © February 2011 IJENS
I J ENS
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01
0.6
0.5
500 rpm
1000 rpm
1500 rpm
2000 rpm
2500 rpm
0.4
0
Friction coefficient
Friction coefficient
0.6
0.3
47
0.5
0.4
0.3
50 100 150 200 250 300 350
0
500 1000 1500 2000 2500 3000
Sliding Speed (rpm)
Duration of rubbing (sec)
0.7
Friction coefficient
sliding speed. All these curves show similar trend as before.
Other parameters such as normal load (10 N), surface
roughness (0.60 m) and relative humidity (70%) are identical
for these five curves. These findings are in agreement with the
findings of Chowdhury and Helali [22, 23] for mild steel and
composite materials. The friction at the time of starting is low
and remains at its initial value for some time and the factors
responsible for this low friction are due to the presence of a
layer of foreign material. This surface in general comprises of
(i) moisture, (ii) oxide of metals, (iii) deposited lubricating
material, etc.. Aluminum readily oxidizes in air, so that, at initial
duration of rubbing, the oxide film easily separates the two
material surfaces and there is little or no true metallic contact
and also the oxide film has a low shear strength. During initial
rubbing, the film (deposited layer) breaks up and clean
surfaces come in contact which increase the bonding force
between the contacting surfaces. At the same time due to the
inclusion of trapped wear particles and roughening the
substrate, the friction force increases due to the increase of
ploughing effect. Increase of surface temperature, viscous
damping of the friction surface, increased adhesion due to
microwelding or deformation or hardening of the material might
have some role on this increment of friction coefficient as well.
After a certain duration of rubbing, the increase of roughness
and other parameters may reach to a certain steady state value
and hence the values of friction co-efficient remain constant
for the rest of the time. In the curves of Fig. 2, it is also seen
that the values of friction co-efficient decreases with the
increase of sliding speed. These results are presented in Fig. 3.
The decrease of friction coefficient of aluminum with the
increase of sliding speed may be due to the change in the
shear rate which can influence the mechanical properties of the
mating materials. The strength of these materials is greater at
higher shear strain rates [14, 15] which results in a lower real
area of contact and a lower coefficient of friction in dry contact
condition. These findings are in agreement with the findings of
Chowdhury and Helali [24] for mild steel, ebonite and GFRP
sliding against mild steel. Similar trends of results
Fig. 3. Variation of friction coefficient with the variation of sliding
speed (Relative humidity =70%, Normal load =10N).
10 N
15 N
20 N
0.6
0.5
0.4
0.3
0.2
0
50
100 150 200 250 300
Duration of rubbing (sec)
Fig. 4. Variation of friction coefficient with the variation of duration
of rubbing at different normal load (Relative humidity =70%, Sliding
Speed =1500 rpm).
0.6
Friction coefficient
Fig. 2. Variation of friction coefficient with the variation of duration
of rubbing at different sliding speed (Relative humidity =70%,
Normal load =10N).
0.5
0.4
0.3
0.2
5
10
15
20
25
Normal load (N)
Fig. 5. Variation of friction coefficient with the variation of normal
load (Relative humidity =70%, Sliding Speed =1500 rpm).
111701-6868 IJMME-IJENS © February 2011 IJENS
I J ENS
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01
are obtained for friction coefficient with the variation of normal
load and duration of rubbing and these results are presented in
Fig. 4 and 5. Fig. 5 indicates coefficient of friction decreases
with the increase of normal load within the observed range.
Increased surface roughing and a large quantity of wear debris
are believed to be responsible for the decrease of friction [18,
19] with the increase of normal load.
Wear rate (mg/sec)
3.5
3.0
2.5
2.0
[2]
1.0
[3]
0.5
0
500
1000 1500 2000 2500 3000
Sliding Speed (rpm)
[4]
Fig. 6. Variation of wear rate with the variation of sliding speed
(Relative humidity =70%, Normal load =10N).
[5]
[6]
3.5
Wear rate (mg/sec)
IV.
CONCLUSION
The presence of sliding speed and normal load indeed affects
the friction force and wear rate considerably. The values of
friction coefficient decrease with the increase of sliding speed
and normal load. The wear rates, on the other hand, increase
with the increase of sliding speed and normal load. As the (i)
the friction coefficient decreases and (ii) wear rate increases
with the increase of normal load and sliding speed, therefore
maintaining appropriate level of sliding speed and normal load
friction and wear may be kept to some lower value to improve
mechanical processes.
[1]
1.5
3.0
[7]
2.5
2.0
1.5
[8]
1.0
0.5
5
10
15
20
25
Normal load (N)
Fig. 7. Variation of wear rate with the variation of normal load
(Relative humidity =70%, Sliding Speed =1500 rpm).
Several experiments are carried out to observe the effect of
normal load and sliding speed on wear rate of aluminum. Curve
of Fig. 6 shows the variation of wear rate with the variation of
speed. From this figure, it is observed that wear rate increases
with the increase of sliding speed. This is due to the fact that
duration of rubbing is the same for all sliding speed, while the
length of rubbing is more in case of higher speed [25]. The
materials mild steel-mild steel couples [25] also show similar
behavior i.e. wear rate increases with the increase of sliding
speed. Figure 7 indicates the variation of wear rate with normal
load. The curve of the figure shows that wear rate increases
with the increase of normal load. This is due to the fact as the
normal load increases frictional heat generates at the contact
surface and hence strength of materials decreases.
48
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
REFERENCES
J. F. Archard, Wear Theory and Mechanisms, Wear Control
Handbook, M. B. Peterson and W.O. Winer, eds., ASME, New
York, NY, pp. 35-80, (1980).
D. T abor, Friction and Wear – Developments Over the Last 50
Years, Keynote Address, Proc. International Conf. Tribology –
Friction, Lubrication and Wear, 50 Years On , London, Inst.
Mech. Eng., pp. 157-172, (1987).
S. T . Oktay, N. P. Suh, Wear Debris Formation and
Agglomeration, ASME Journal of Tribology, Vol. 114, pp. 379393, (1992).
N. Saka, M. J. Liou, N. P. Suh, T he role of T ribology in Electrical
Cotact Phenomena, Wear, Vol. 100, pp. 77-105, (1984).
N. P. Suh, H. C. Sin, On the Genesis of Friction and Its Effect on
Wear, Solid Contact and Lubrication, H. S. Cheng and L. M. Keer,
ed., ASME, New York, NY, AMD-Vol. 39pp. 167-183, (1980).
V. Aronov, A. F. D'souza, S. Kalpakjian, I. Shareef, Experimental
Investigation of the effect of System Rigidity on Wear and
Friction-Induced Vibrations, ASME Journal of
Lubrication
Technology, Vol. 105, pp. 206-211, (1983).
V. Aronov, A. F. D'souza, S. Kalpakjian, I. Shareef, Interactions
Among Friction, Wear, and System Stiffness-Part 1: Effect of
Normal Load and System Stiffness, ASME Journal of Tribology,
Vol. 106, pp. 54-58, (1984).
V. Aronov, A. F. D'souza, S. Kalpakjian, I. Shareef, Interactions
Among Friction, Wear, and System Stiffness-Part 2: Vibrations
Induced by Dry Friction, ASME Journal of Tribology, Vol. 106,
pp. 59-64, (1984).
V. Aronov, A. F. D'souza, S. Kalpakjian, I. Shareef, Interactions
Among Friction, Wear, and System Stiffness-Part 3: Wear Model,
ASME Journal of Tribology, Vol. 106, pp. 65-69 (1984).
J. W. Lin, M. D. Bryant, Reduction in Wear rate of Carbon
Samples Sliding Against Wavy Copper Surfaces, ASME Journal of
Tribology, Vol. 118, pp. 116-124 (1996).
D. T abor, Friction and Wear – Developments Over the Last 50
Years,”Keynote Address, Proc. International Conf. Tribology –
Friction, Lubrication and Wear, 50 Years On, London, Inst.
Mech. Eng., pp. 157-172, (1987).
E. J. Berger, C. M. Krousgrill, F. Sadeghi, October Stability of
Sliding in a System Excited by a Rough Moving Surface, ASME,
Vol. 119, pp. 672- 680, (1997).
B. Bhushan, Principle and Applications of T ribology, John Wiley
& Sons, Inc., New York, pp. 344-430, (1999).
B. Bhushan, W. E. Jahsman, Propagation of Weak Waves in
Elastic- Plastic and Elastic-viscoplastic Solids With interfaces, Int.
J. Solids and Struc.,Vol. 14, 39-51, (1978).
B. Bhushan, W. E. Jahsman, Measurement of Dynamic Material
Behavior under Nearly Uniaxial Strain Condition, Int. J. Solids
and Struc., Vol. 14, 739-753,(1978).
B. Bhushan, Effect of Shear Strain Rate and Interface
T emperature on Predictive Friction Models, Proc. Seventh LeedsLyon Symposium on Tribology (D. Dowson, C. M. T aylor, M.
111701-6868 IJMME-IJENS © February 2011 IJENS
I J ENS
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
Godet and D. Berthe, eds.), pp. 39-44, (1981) IPC Business Press,
Guildford, UK.
H. D. Fridman, P. Levesque, Reduction of static frict ion by sonic
vibrations, J. Appl. Phys., Vol. 30, pp. 1572-1575, (1959).
B. Bhushan, T ribology and Mechanics of Magnetic Storage
Devices, 2 nd edition, Springer-Verleg, New York, (1996).
P. J. Blau, Friction Science and T echnology, Marcel Dekker, New
York, (1996).
B. Bhushan, Handbook of Micro/ Nanotribology, 2 nd edition, CRC
Press, Boca Raton, Florida, (1999).
B. Bhushan, A. V. Kulkarni, Effect of Normal Load on Microscale
Friction Measurements, Thin Solid Films, Vol. 278, 49-56; 293,
333, (1996).
M. A. Chowdhury, M. M. Helali, T he frictional behavior of mild
steel under horizontal vibration. Tribology International, Vol. 42,
pp. (946-950) 2009.
M. A. Chowdhury, M. M. Helali, T he frictional behavior of
composite materials under horizontal vibration. Industrial
Lubrication and Tribology, Vol. 61, Issue 5, pp. 246-253, (2009).
M. A. Chowdhury, M. M. Helali, T he Effect of Amplitude of
Vibration on the Coefficient of Friction, Tribology International,
Vol. 41, Issue 4, pp. 307-314, (2008).
M. A. Chowdhury, M. M. Helali, , T he Effect of Frequency of
Vibration and Humidity on the Wear rate, Wear, Vol. 262, pp.
198-203, (2007).
111701-6868 IJMME-IJENS © February 2011 IJENS
I J ENS
49