Proceedings of WTC2005
World Tribology Congress III
September 12-16, 2005, Washington, D.C., USA
WTC2005-63174
STATIC AND KINETIC FRICTION OF SMOOTH GLASS SURFACES
RUBBED WITH SILICONE OILS
Rapoport, L., Holon Institute of Technology, Holon
58102,Israel
Moshkovich, A., Holon Institute of
Technology, Holon 58102,Israel
Shmukler, V., Holon Institute of Technology, Holon
58102,Israel
Verdyan, A., Holon Institute of
Technology, Holon 58102,Israel
2. EXPERIMENTAL PROCEDURE
Glass flat samples were moved with the reciprocal speed of
20 μm/s and minimal contact pressure of 65 KPa. The load
was changed from 1.5 N to 5 N The length of the friction track
was 600 μm. Three types of silicone fluid with the viscosity of
100, 450 and 5000 cSt at 250 C were used. The surfaces were
lubricated with 4-5 drops of the silicone liquid prior to the test.
ABSTRACT
The study of the effect of waiting time, loading and
unloading on static and kinetic friction for real contact microsystems was carried out. Three types of silicone fluid with the
viscosity of 100, 450 and 5000 cSt at 250 C were used. Stopstart experiments allowed us to estimate the relaxation time
after the static friction overshoot. For silicone liquid with
viscosity of 100 cSt, relaxation of friction force (F) after the
static friction overshoot occurred over one cycle of testing.
Relaxation of F for fluids with the viscosity of 450 and 5000
cSt occurred during a long time and this effect was opposite
for these silicone liquids. The analysis of loading-unloading
cycles showed only a partial reversibility of F. The results
were compared with static friction and stick-slip data obtained
in other works, using SFA and FFM techniques. In order to
explain the effect of viscosity and structure of the lubricant
layers on stick-slip phenomenon, interior and wall slip of the
lubricant film is discussed.
3. RESULTS AND DISCUSSION
The study of the effect of testing time on friction force (F)
showed that F increases with time reaching the steady friction
state. It has been revealed that the friction force is roughly
inverse to the bulk viscosity as observed for confined fluids
[4]. The time to steady friction state depended on the viscosity
of the silicone fluid. The higher the viscosity of the liquid, the
more the time for obtaining the steady friction state is needed,
Fig. 1. It is expected that original increasing of the friction
force with time is associated with squeezing out lubricant
layers from the interface. A steady friction state is apparently
associated with a preservation of thin molecular film trapped
between the contact surfaces. No damage was observed for
contact surfaces rubbed with silicone fluids under load of 65
KPa. In order to explain the effect of viscosity and structure of
the lubricant layers on stick-slip phenomenon, interior and
wall slip of the lubricant film is discussed.
Stop-start experiments allowed us to estimate the relaxation
time after the static friction overshoot. Two series of the
experiments were carried out in order to evaluate the effect of
waiting time on the static force. In the first series, the stopstart test was carried out immediately after dropping the
silicone liquid into the interface, i.e. when a bulk film was
present. The second series was performed after reaching the
steady friction state. Static friction overshoot was not observed
in the first series of experiments.
1. INTRODUCTION
Some micro-electro-mechanical systems have to provide
very precise translation during motion after pauses. Increasing
the time of the pauses (waiting time) usually raises the static
friction overshoot, limiting thus the precision of these systems.
The problems of static friction have been carefully studied in a
number of works [e.g. 1, 2] The goal of this work was to study
the effect of testing and waiting time; loading and unloading
on static and kinetic friction for real contact micro-systems.
The contact surfaces were analyzed by AFM. The results were
compared with static friction and stick-slip data obtained in
other works, using SFA and FFM techniques.
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30
30
-2
25
Static friction force, N, 10
25
Friction force, gr
100 cSt
20
15
5000
cSt
10
100 cSt
20
5000 cSt
15
10
5
450 cSt
5
0
0
450 cSt
10
20
30
40
50
60
70
Waiting time, sec
0
0
50
100
150
200
Fig. 2 The change of the static friction force with a change
of the waiting time
Number of cycles
Fig. 1 The effect of number of the cycles on the friction
force for the silicone oils
A next stage of this work was the analysis of load on stick-slip
motion. A linear dependence of friction force on load, passing
through the origin, was observed for silicone fluids with the
viscosity of 100 cSt. The plateau in the friction force that was
exhibited with load increasing up to the value of 4.5 N,
suggests a transition to “wearless” friction. An interesting
effect was found to be for silicone oil with the viscosity of 450
cSt. The friction force remained constant at a definite load
range. In this case, a remarkable decrease of the friction
coefficient was obtained. This effect may be attributed to the
formation of the totally frozen "solid-crystalline" state with
load increasing. The analysis of loading-unloading cycles
showed only a partial reversibility of F. Stick-slip motion of
silicone fluids is explained by degree of disordering and
branching of chained molecules.
Stopping in the second series, at the steady state, usually led
to static friction overshoot for all liquids. The value of static
force (Fs) increased over the waiting time, Fig. 2. Although
the same values of static friction were obtained over the
waiting time for silicone fluids with viscosity of 100 and 5000
sCt, different relaxation processes apparently occur for these
two fluids. It may be concluded that stick-slip motion occurs
only after stopping in the steady friction state, when thin
molecular layers remain in the interface. A minimal value of
static friction was obtained for the silicone fluid with the
viscosity of 450 cSt. It was found that for silicone fluid with
the relatively low viscosity of 100 cSt, relaxation of F after the
static friction overshoot occurred during the first cycle of
testing. For more viscous silicone fluids relaxation occurred
during long time and the decreasing and increasing of F were
observed for the silicone liquids with the viscosity of 450 and
5000 cSt, respectively. Furthermore, a definite connection was
observed between the rate of F increasing with testing time
and the rate of F relaxation after static friction overshoots. The
higher the value of friction force obtained in the steady friction
state, the smaller the time of relaxation is observed. It is
expected that the same mechanisms of "interdigitation" are
responsible for the increase of F with testing time at the
beginning of friction experiment and the decrease of Fs during
relaxation. In order to explain the effect of "interdigitation" on
friction of silicone fluids, “friction phase diagram” [3] has
been considered.
REFERENCES
1. Persson B.N.J., 1999, "Sliding Friction", Surface Science
Reports, 33, 83-119.
2. Gao J., Luedtke W.D., Gourdon D., Ruths M., Israelachvili
J.N., Landman U., 2004, "Frictional Forces and Amontons'
Law: From the Molecular to the Macroscopic Scale", J. Phys.
Chem. B., 108, 3410-3425.
3. Yoshizawa H., Chen Y-L., Israelachvili, 1993,
"Fundamental Mechanisms of Interfacial Friction. 1. Relation
between Adhesion and Friction", J. Phys. Chem., 97, 41284140.
4. Luengo G., Israelachvili J., Granick S., 1996, "Generalized
effects in confined fluids: new friction map for boundary
lubrication", Wear, 2000, 328-335.
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