05-09-2012, 02:35 PM
The effect of WS2 nanoparticles on friction reduction in various lubrication regimes
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ABSTRACT
A study of the tribological behavior of nested inorganic fullerene-like (IF) nanoparticles of WS2, as a potential additive to
base oils is presented. Friction measurement results obtained from three different test rigs over a wide range of normal loads
and sliding velocities are shown. Stribeck curves are used to reveal the lubrication regimes where the IF are most effective. It is
found that the addition of IF-WS2 nanoparticles to the base oils results in up to 50% reduction in friction coefficient in the
mixed lubrication regime. The mechanism of improved friction and wear behavior with the IF additive is discussed.
Introduction
Solid lubricants, such as metal dichalcogenide MoS2
or graphite, have been extensively studied and used for
a long time as additives in lubricating oils to reduce
friction and wear of rubbing pairs [1]. The role of
MoS2 powder as a solid lubricant has been discussed
in great detail in Ref. [2]. Fewer studies have been
focused on other metal dichalcogenides such as WS2,
which can also display good lubricating properties [3].
Metal dichalcogenides of the form MX2 (M ¼ W, Mo;
X ¼ S, Se) have a characteristic anisotropic layered
structure. Strong covalent forces bind M and X atoms
within a lamella, whereas adjacent lamellae interact
through relatively weak van der Waals forces. The unit
cell has a hexagonal symmetry and includes two adjacent
lamellae (2H arrangement). This graphite-like
structure is considered to be responsible for the lubricating
properties of these materials, because the weak
inter-lamellar bonding facilitates the shear when the
direction of sliding is parallel to the planes of the
material [1–5].
Test devices and procedures
Tests were performed on three different test rigs
covering a wide range of operating conditions, and
three different contact geometries and sliding modes.
In addition each test rig can simulate the conditions of
boundary, mixed and hydrodynamic lubrication
regimes. The most important test conditions for the
three test rigs are listed in table 1. As can be seen from
this table most of the tests were performed with a flaton-
flat tribotester that was specially designed and built
for this purpose. The contact of two flat surfaces, an
upper quasi-static tribo-element in the form of a flat
circular disk and a lower tribo-element in the form of
a flat plate performing a linear motion characterize the
new test device. A schematic diagram of the apparatus
is illustrated in figure 1. A weight (1) drives the lower
tribo-element plate (2), which is mounted in a selfaligning
frame on a linear bearing (3).
Conclusion
The effect of IF-WS2 as a potential additive to base
oils was investigated over a wide range of operating
conditions in terms of contact pressures, sliding velocities,
lubricant viscosities and different sliding modes.
Tests were performed on three different test rigs and
with three different base oils. The different test rigs
enable studying the IF effect in three distinct lubrication
regimes namely, boundary, mixed and hydrodynamic
lubrication. Both friction and contact electrical
resistance were measured and some post-test optical
microscopy and XPS analyses were also conducted.
The main findings of the present investigation are the
following.