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Friction and wear of MoS
2
films on laser textured steel surfaces
L. Rapoport
a,
⁎, A. Moshkovich
a
, V. Perfilyev
a
, I. Lapsker
a
,
G. Halperin
b
, Y. Itovich
b
, I. Etsion
b
a
Holon Institute of Technology, Holon 58102, Israel
b
Department of Mechanical Engineering, Technion, Haifa 32000, Israel
Received 30 July 2007; accepted in revised form 6 December 2007
Available online 23 December 2007
Abstract
Incorporation of solid lubricant into micro-reservoirs produced by Laser Surface Texturing (LST) and its effect on the tribological properties of
surfaces under dry friction is studied. The density of the dimple reservoirs and the height of the bulges around them are investigated in terms of the
longevity of solid lubricant films burnished on LST steel surfaces. Friction tests were performed using a ball-on-flat device. Optimum density (40–
50%) of the dimples is revealed. It is shown that the adhesion of solid lubricant in the space between the dimples is provided by mechanical
engagement of particles in the rough surface and by smearing the solid lubricant around the dimples. Best results are obtained with the surfaces
that were lapped to half of the height of bulges. Long wear life of burnished film on LST steel surfaces is apparently provided by preservation of
thin MoS
2
film around the bulges and by supply of solid lubricant from the dimples to the surface.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Laser texturing; Burnishing; Solid lubricant; Wear life
1. Introduction
Surface texturing as a means for enhancing the tribological
properties of mechanical components is well known for many
years. Fundamental research work on various forms and
shapes of surface texturing for tribological applications is
carried out by several research groups worldwide and various
texturing techniques are employed in these studies including
machining, ion beam texturing, etching techniques and laser
texturing. Interestingly almost all these fundamental works are
experimental in nature and most of them are motivated by the
idea that the surface texturing provides micro-reservoirs to
enhance lubricant retention or micro-traps to capture wear
debris. The laser surface texturing (LST) seems to be the most
advanced of all known methods of surface texturing for
tribological applications [1].LSTisstartingtogainmoreand
more attention in the Tribology community as is evident from
the growing number of publications on this subject. Indeed,
LST provides substantial improvement of tribological perfor-
mance under friction with fluid lubricant. The geometrical
parameters of LST were optimized for fluid lubrication of flat
surfaces under different contact conditions (see, for example,
[2–5]).
In recent years, laser texturing was combined with in-
corporation of solid lubricant into micro-reservoirs. Stored in
the dimples solid lubricant can be released to the interface
and thus increase the longevity of rubbed surfaces. However,
friction and wear of solid lubricant films on LST steel surfaces
has not been practically investigated. The authors know of only
limited number of works where laser treatment was combined
with formation of self-lubricating films on ceramic surfaces
[6,7]. The micro-reservoirs were machined by a focused UV
laser beam on the surface of hard TiCN coatings [7]. It was
found that the optimum area density for dimple reservoirs is
about 10%, which corresponds to 50 µm separation of 10 µm
sized dimples. The surfaces of hard coatings were then filled
with MoS
2
and graphite films. Burnishing and sputtering were
A
vailable online at www.sciencedirect.com
Surface & Coatings Technology 202 (2008) 3332–3340
www.elsevier.com/locate/surfcoat
⁎Corresponding author. Department of Science, Holon Institute of Technology,
52 Golomb St., Holon 58102, Israel. Tel.: +972 3 5026616; fax: +972 3 5026619.
E-mail address: rapoport@hit.ac.il (L. Rapoport).
0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2007.12.009
used in order to deposit solid lubricant films on the laser
textured surfaces.
Bonded molybdenum disulfide (MoS
2
) lubricant films are
widely used in different applications, especially in space craft
and launch vehicles (e.g. [8,9]). MoS
2
coatings usually show
low friction (0.02–0.04) in dry and vacuum conditions and high
friction in humid environment [8,10]. Burnished films created
by a rubbing process transfer solid lubricant onto the contact
surfaces. Burnishing of MoS
2
or other solid lubricants is widely
applied in order to improve the tribological properties of
roughened substrates [11–13]. To improve the tribological
properties of solid lubricant films under different environmental
conditions (humidity, vacuum, high temperature, etc.), MoS
2
or
WS
2
powders are mixed with other powders before burnishing
(e. g. [14,15]).
The wear life is one of the main parameters in the analysis of
the efficacy of solid lubricant films (e. g. [11,16]). Wear life is
determined as the number of cycles (or time) of sliding to reach
a high value of the friction coefficient (failure of the film).
The objective of this work was to study the effect of area
density of dimple reservoirs on friction and wear of solid
lubricant films. Another aim of this experiment was to evaluate
the effect of bulge's height on the wear life of solid lubricant
films burnished on LST steel surfaces.
2. Experimental
The LST was applied on the test specimens with a 5 kHz
pulsating Nd:YAG laser with a power of 11 kW and pulses of
30 ns duration and 4 mJ each (courtesy of Surface Technologies
Ltd.).
Two types of tests were performed in order to evaluate the
effect of LST on friction and wear of smeared solid lubricant
films.
In the first test, the effect of the density of LST dimples was
assessed. The density of the dimple reservoirs was changed
from 10 to 60%. In order to use small amount of solid lubricant
nanoparticles under burnishing, a small depth dimples close to
2 µm with a diameter 65 ± 15 µm were applied. In order to
assess the effect of the depth of dimples on the wear life of the
solid lubricant film, a series of samples with a depth of 4 µm
was used. The samples were lapped after LST so that the height
of the bulges around the dimples was close to 0.5 µm. Five
roughness tracks in different directions in relation to the dimples
were measured for each sample. The spread of the values of
roughness parameters of the textured surfaces with different
area density of dimples are presented in Table 1. A scheme of a
LST surface is shown in Fig. 1.
Laser dimples were produced on the surface of hardened
steel disks (HR
c
= 60). The roughness of the virgin steel surface
(Ra) was close to 0.1 µm. Since the roughness of the substrate
affects the wear life of burnished solid lubricant, rough grinded
samples (Ra = 0.4–0.6 µm) were compared with the textured
samples. These values were chosen since a lot of textured
surfaces had the same values of Ra. It is known that steel
surfaces are oxidized during LST in air. It is mainly the surface
of bulges around the dimples. Since application of different
stages of lapping (full or 50%) and different density of dimples
led to different and uncontrolled amount of oxides, the textured
Table 1
The values of the roughness parameters for the LST surfaces with different
density of dimples
Density of
dimples, %
Ra, µm Rq, µm Rz, µm Sm, µm Rsk, µm
58 0.58–0.64 0.45–0.70 2.44–2.57 50–70 −0.2–0.5
42 0.64–0.67 0.73–0.77 2.82–2.84 78–85 −0.3–0.7
26 0.47–0.55 0.59–0.66 2.3–2.6 100–145 −1.2–1.4
10 0.38–0.42 0.53–0.56 2.29–2.49 120–180 −1.7–1.8
Fig. 1. A scheme of a LST surface with the geometrical parameters of dimples. h—height of the bulges, H—depth of the dimples, d—diameter of dimples,
D—external diameter of bulges, S—distance between the dimples, burnished powder layer.
3333L. Rapoport et al. / Surface & Coatings Technology 202 (2008) 3332–3340
surfaces were etched before a burnishing. The surfaces were
treated by 1% of chloride acid during 30 s. After that the
surfaces were neutralized and rinsed in ultrasonic bath. Finely,
the samples were dried at 70 °C during 1 h. Cloth burnishing
was used in order to deposit thin solid film of MoS
2
on the
surface of the steel disks. Commercially available MoS
2
powder
(b2 µm) was used in order to burnish the steel surfaces. The
thickness of the solid lubricant film was close to 1 µm (a little
more than 0.5 µm height of the bulges). Friction tests were
performed using a ball-on-flat device under sliding velocity of
0.25 m/s and 45 ± 5% RH. Bearing ball (AISI 51100) with a
diameter of 10 mm was used as a counter body. The same
material was used for rings that were then LST treated and
burnished with solid lubricant. The friction coefficient, diameter
of the contact spot on the surface of the ball and the width of the
wear track on the laser treated samples were studied. In order to
evaluate the effect of the dimples density and the height of the
bulges on the wear life of the storage films the load was
increased by steps according to the following scheme. Initially,
the load was increased by steps of 15 N every minute during
6 min up to 90 N. Then, the load was further increased by steps
of 36 N every minute during additional 16 min up to a total load
of 666 N. Under these severe contact conditions (virgin contact
pressure close to 3 GPa) the test was continued until the friction
coefficient increased to about 0.3 when the test was stopped.
The test time under the load of 666 N until stopping the test was
selected as the wear life of the solid lubricant film. The wear life
of solid lubricant depends mainly on the adhesion of the
burnished layers to the laser textured surfaces. A larger wear life
corresponds to a better adhesion of the burnished layers to the
substrate.
A second type of tests was performed in order to assess the
effect of the depth of the dimples and the height of the bulges on
the wear life of solid lubricant films. The samples with
maximum density of dimples and depth of 2 µm were compared
with the samples with dimple depth of 4 µm. Three heights of
bulges controlled by the extent of post LST lapping were
studied. These were: 0–0.2 µm (full lapping), 0.4–0.6 µm (50%
lapping), 1–1.5 µm (without any lapping). The friction and
Fig. 2. The surfaces of steel samples with dimples area density of 58% (a) and
10% (b).
Fig. 3. The surface of steel samples after LST (the density of dimples 42%). The
melted ranges around the dimples are seen on the figure.
Fig. 4. The effect of dimples area density on bearing ratio, Tp, of LST samples.
RTp is a depth of the profile.
3334 L. Rapoport et al. / Surface & Coatings Technology 202 (2008) 3332–3340
wear properties of the LST surfaces were compared with
reference grinded samples. These reference steel samples were
grinded (Ra = 0.6 µm) and burnished with MoS
2
solid lubricant
powder similar to that performed for the LST surfaces. The
structure and morphology of the thin films before and after
friction tests were studied by SEM, EDS and optical micro-
scopy. The roughness of the surfaces was measured before and
after the friction tests.
3. Results
3.1. The characterization of LST surfaces
The surfaces of steel samples treated by LST are shown in
Fig. 2.
The magnified micrograph of LST surface is shown in Fig. 3.
In order to assess the effect of LST on the roughness of the
surfaces, a bearing ratio parameter, Tp, was used, Fig. 4. It can
be seen that at a depth of 1 µm the space of the dimples of 10%
density is 10%, while it is about 60% for the dimple densities of
42–58%. Consequently, the space for solid lubricant nanopar-
ticles is essentially bigger for the samples with high density of
dimples.
Analysis of the roughness parameters of the textured surfaces
with different area density of the dimples (Table 1) showed that
the three parameters, Ra, Rq, Rz, remain practically constant
while the mean spacing between the profile peaks, Sm and the
measure of the symmetry of the amplitude distribution curve
about the mean line, Rsk, changed significantly. A strong
correlation was found between Sm, and the area density of
Fig. 5. The profiles of laser textured surfaces with different density of dimples (a) −42%, (b) −26%, (c) −10%.
3335L. Rapoport et al. / Surface & Coatings Technology 202 (2008) 3332–3340
dimples. The Sm parameter, related to the distance between
dimples, decreases with increasing area dimples density.
3.2. Solid lubricant films on LST surfaces
The profilograms of LST surfaces are shown in Fig. 5.
It can be seen that the average depth of the dimples is about
2 µm regardless of the dimples' density. In order to characterize
the amount of the solid lubricant that filled the dimples, the area
of the dimples cross section was colored (red area). The area of
the dimples, 288, 265 and 108 µm
2
corresponds to the densities
of 42%, 26% and 10%, respectively.
The SEM micrograph of MOS
2
film burnished on LST
surface is shown in Fig. 6. It may be seen that MoS
2
powder fills
both the space of dimples and the places around the dimples.
3.3. The effect of density of dimples on friction and wear life of
solid lubricant films
Fig. 7 shows the effect of area density of dimples on wear
life of burnished MoS
2
film. Increasing the density of dimples
usually led to increasing the wear life. According to the test,
the transition to seizure occurred after 6 min under load of
90 N for the sample having density of 10% while it happens
under load of 666 N for the samples with other densities of
dimples.
The wear life increases significantly with increasing the
density from 10% to 42% and then the life levels off. It is
expected that larger amount of solid lubricant powder burnished
onto the textured surface resulting from higher density of
dimples is responsible for wear life of solid lubricant film. The
solid lubricant film was usually well smeared over the entire
contact range especially on the LST surface with a high density,
Fig. 8.
The change of the friction coefficient with time for the
samples with the density of dimples of 26% and 42% is shown
in Fig. 9.
The virgin coefficient of friction under low load (15 N) was
about 0.12. With increasing load (or time), the friction
coefficient decreased down to 0.03–0.04 and under maximum
load (666 N) it remained constant. At the transition to seizure
the coefficient of friction jumped strongly. With sliding time a
transition to seizure was observed. A detachment of the solid
Fig. 7. The effect of density of dimples filled with MoS
2
particles on wear life of
solid lubricant film.
Fig. 8. The smeared film of MoS
2
particles in the steady friction state. Density of
dimples is 42%.
Fig. 9. The change of the friction coefficient with time for the samples with
density of dimples 26% (1) and 42% (2). Load, P= 666 N.
Fig. 6. SEM micrograph of MoS
2
film burnished on LST steel surface.
3336 L. Rapoport et al. / Surface & Coatings Technology 202 (2008) 3332–3340
lubricant film begun on the surface of the bulges around the
dimples, Fig. 10. It is expected that seizure inception occurs
originally on the surface of bulges. A removal of solid lubricant
film from around the dimples led to increased friction and the
cracking of solid lubricant film in the dimples. Finally, the
seizure occurred in some of places where the amount of solid
lubricant was limited, Fig. 11.
3.4. The effect of the size of bulges and the depth of dimples on
wear life of solid lubricant films
The study showed strong influence of the height of bulges on
the amount of solid lubricant, the adhesion of burnished layers
onto textured surface and the wear life of solid lubricant film.
Fig. 12 shows SEM micrographs of LST surfaces that were
burnished by MoS
2
powder after leaving different heights of
bulges. It can be seen that the diameter of filled dimples and
consequently their area on the surface is changed depending on
the height of bulges.
In the fully lapped surfaces, the MoS
2
particles are mainly
found within the micro-dimples, Fig. 12(a). A very thin MoS
2
film is present on the smooth surface between the dimples and
the grinding marks are still apparent on this surface. In the
samples without lapping, the amount of MoS
2
powder covers
the entire contact surface with a much thicker film compared to
the fully lapped surfaces case and the laser dimples become
invisible on the burnished surfaces, Fig. 12(c). The experiment
showed that the shortest wear life was obtained with the fully
Fig. 11. The microphotograph of solid lubricant film in the inception to seizure.
Fig. 12. MoS
2
film burnished on the LST samples with different height of
bulges. The density of the dimples is 35%. The height of bulges is: 0–0.2 µm (a),
0.4–0.6 µm (b), 0.8–1 µm (c).
Fig. 10. A detachment of solid lubricant from the bulges (around the dimples).
3337L. Rapoport et al. / Surface & Coatings Technology 202 (2008) 3332–3340
lapped surfaces. The samples without lapping revealed a
medium wear life. The best longevity was obtained with the
surfaces that had 50% lapping where the height of the bulges
was 0.4–0.6 µm. SEM micrographs of the rubbed surfaces
revealed that the solid lubricant film is better preserved on the
samples where half of the original height of the bulges was
removed by lapping, Fig. 13.
The friction behavior of LST surfaces with depth of dimples
of 2 µm and 4 µm was compared with a reference ground
surfaces, Fig. 14. As it can be seen the wear life of burnished
film on ground surfaces is very low in comparison to laser
textured surfaces. The wear life of MoS
2
film on ground
surfaces was less than 15 min, so the transition to seizure
occurred at relatively low load in comparison to textured
samples (the loads of 126 N and 306 N for ground surfaces with
Ra = 04–05 µm and Ra = 0.6–0.7 µm, respectively). Larger
amount of solid lubricant stored within deeper dimples tend to
increase considerably the wear life of the solid lubricant films.
As can be seen from Fig. 14, increasing the dimple depth from 2
to 4 µm increased the wear life from 50 to 80 min. Therefore,
the wear life of burnished layers depends mainly on the amount
of solid lubricant and its preservation in the dimples. Solid
lubricant is better preserved in deeper dimples and thus provides
longer wear life.
4. Discussion
The wear life of the rubbed surfaces covered by solid
lubricant is mainly determined by the adhesion of solid
lubricant aggregates to the substrate and by the supply of
lubricant from the dimples to the interface. Solid lubricant
particles fill the dimples of LST surface under low contact
pressure during cloth burnishing. The adhesion of solid
lubricant in the space between the dimples is provided by the
mechanical engagement of particles in the rough surface and by
smearing the solid lubricant around the dimples. Therefore, a
smaller space between the micro-dimples should facilitate both
the smearing and the supply of solid lubricant to the interface. In
our case the highest wear life was obtained with dimples density
of 40–50%. The bearing capacity of LST surfaces is provided
both by thin solid lubricant films in the space between the
dimples and by solid lubricant supply from the dimples. The
analysis of the contact surfaces permits to conclude that the
damage of solid lubricant film and the transition to seizure is
determined by the preserved amount of solid lubricant in the
space between the dimples where the thickness of the film is
relatively small. The time of supply of solid lubricant to the
space between dimples apparently depends on the amount of
solid lubricant in the dimples. The deeper the dimples are, the
longer is the wear life of the solid lubricant films. The main
advantage of MoS
2
film burnished on laser textured surfaces in
comparison to burnished film on ground surface is their high
critical load of transition to seizure. Burnished films on the
ground surface have usually low endurance because of poor
adhesion e.g. [8]. The transition to seizure with the grinded
surfaces occurred after less than 15 min sliding under a load of
about 300 N while with the burnished LST surfaces seizure
occurred after 80 min of sliding test under load of 666 N. Long
wear life of burnished film on LST steel surfaces is apparently
provided by preservation of thin MoS
2
film around the bulges
and by supply of solid lubricant from the dimples to the surface.
The effect of load on the friction behavior of burnished MoS
2
film was similar to that observed for sputtered films (e.g. review
Fig. 13. The surface of wear track at the end of steady friction state for solid
lubricant films burnished on LST surface with a height of the bulges of 0.4–
0.6 µm (a) and 0.0–0.2μm (b).
Fig. 14. The effect of the depth of dimples on the wear life of solid lubricant film.
(1) depth of 4 µm, load 666 N; (2) depth of 2 µm, load 666 N; (3) ground
surface, Ra = 0.6–0.7 µm, load 306 N;(4) ground surface, Ra = 0.3–0.4 µm, load
126 N.
3338 L. Rapoport et al. / Surface & Coatings Technology 202 (2008) 3332–3340
[9]). The friction coefficient of burnished film on textured
surfaces decreased with increasing load and reached the value of
0.03–0.04 at the maximum load (virgin contact pressure of
about 3 GPa). Similar low values of the friction coefficient were
previously observed for MoS
2
film in vacuum, while in humid
air the friction coefficient was much higher reaching a value of
0.2 [8–10]. It should be noticed however, that the above
mentioned environmental effects were studied under relatively
low pressure of about 0.5 GPa (see [8–10]). In our tests in
humid air (50%) and under the same low pressure of 0.5 MPa
the friction coefficient was also high about 0.15. However,
under the high contact pressure of 3 GPa the friction coefficient
decreased to values typical to these in dry air. The exact effect of
load on friction in humid environment was not yet studied. At
this point we can only speculate that under high contact pressure
a high local temperature develops that can provide conditions
close to this in dry air.
The dimple processing on steel surfaces is associated with
melting and vaporization leading to formation of brittle hard
oxidized bulges around the dimples. The average external
diameter of the bulges was about 40–45 µm, while the diameter
of the dimples was about 40 µm. It is clear that the amount of
solid lubricant particles on the LST surfaces is different for the
samples with and without bulges. For the full lapped surfaces
(without bulges), solid lubricant is mainly preserved in the
dimples. Very thin film of lubricant is observed on the smooth
space between the dimples. A confirmation of the very thin film
preservation is provided by the visible grinding marks left after
lapping and burnishing on these surfaces and by the short
longevity of the solid lubricant film under friction test. It was
expected that for the LST surfaces with full bulges, where the
amount of solid lubricant on the surface is maximum, the wear
life of the film will also be maximum. However, the longevity of
these films was less than for the samples with 50% of lapping
(half of the original full height of bulges). Since the full height
of the bulges was 1–1.5 µm, which is close or even larger than
the thickness of the lubricant film, some brittle and oxidized
summits of the bulges can plough the surface layers of the
rubbed surfaces and thus decrease the wear life. The analysis of
the effect of the height of bulges on the longevity of solid
lubricant films revealed that a lapping of half of the height of the
bulges provides maximum longevity. MoS
2
film is well
smeared on all contact surfaces.
In order to evaluate the morphology of the burnished film
before and after friction, SEM and EDS analyses were used.
Fig. 15 shows SEM microphotograph of the area around the
wear track. The numbers in the photo present the points where
EDS analysis was carried out. The straight horizontal line is the
scan line along the wear track obtained in the secondary
electrons. Line above gives the distribution of the characteristic
X-rays for sulfide (S Kα) in the MoS
2
film. The intensity of
those characteristic X-rays usually correlates with the thickness
of the film [17].
An EDS analysis confirmed the presence of solid lubricant
film both in the dimples (point 3) and in the space between
dimples (points 4, 5), Fig. 16.
The solid lubricant films are preserved both in the dimples
and in the space between the dimples. Fig. 16 shows the
composition of the film in the different points (1–3). It may be
seen that the amount of MoS
2
in the dimples near the wear track
is essentially higher (point 3) in comparison with the solid
lubricant film in the wear track before the transition to seizure
(points 1, 2).
Fig. 16. The spectrum corresponding to MoS
2
film in a dimple, point 3 (a) and in
the wear track, points 1 and 2 (b).
Fig. 15. The surfaces with half height of bulges at the end of steady friction state.
Points (1, 2) characterize the film in the wear track, points 3–5 indicate
burnished layer in the dimple (3) and around the dimples (4, 5).
3339L. Rapoport et al. / Surface & Coatings Technology 202 (2008) 3332–3340
5. Conclusions
1. The effect of the density of dimples on the longevity of MoS
2
solid lubricant film burnished on LST surface was studied.
The optimum density of the dimples was found to be
between 40 to 50%. In this case the solid lubricant films are
smeared better in the space between the dimples.
2. The effect of the height of bulges around the dimples on the
longevity of the solid lubricant films was analyzed. The
minimum longevity of the solid lubricant films corresponded
to full lapped surfaces. This result is explained by low
adhesion of the MoS
2
in the smooth space between the
dimples. The best results were obtained with the surfaces that
were lapped to half of the original full height of the bulges. In
this case the film was preserved on the contact surface during
a long time. EDS analysis confirmed the presence of MoS
2
film on the rubbed surfaces in the steady friction state.
3. The effect of the depth of the dimples on the wear life of
solid lubricant film on LST surfaces was analyzed. Two
times increase in the depth of dimples led to the same value
of increase of the wear life of the burnished solid lubricant
layers. The wear life of burnished MoS
2
film is significantly
higher than that for the same films on ground surfaces. The
low value of the friction coefficient (0.03–0.04) was
obtained for LST surface burnished MoS
2
film under friction
in humid air and high contact pressure.
Acknowledgement
This work was supported by a grant from the Israeli Ministry
of Science.
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