30-07-2012, 12:43 PM
Experimental analysis of tribological properties of lubricating oils with nanoparticle additives
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Abstract
This study examined the tribological properties of two lubricating oils, an API-SF engine oil and a Base oil, with CuO, TiO2, and Nano-Diamond
nanoparticles used as additives. The friction and wear experiments were performed using a reciprocating sliding tribotester. The experimental
results show that nanoparticles, especially CuO, added to standard oils exhibit good friction-reduction and anti-wear properties. The addition of
CuO nanoparticles in the API-SF engine oil and the Base oil decreased the friction coefficient by 18.4 and 5.8%, respectively, and reduced the
worn scar depth by 16.7 and 78.8%, respectively, as compared to the standard oils without CuO nanoparticles. In addition, investigations were
performed using TEM, OM, SEM, and EDX to interpret the possible mechanisms of anti-friction and anti-wear with nanoparticles.
Introduction
Nanotechnology is regarded as the most revolutionary technology
of the 21st century. It can be used in many fields and
ushers material science into a new era. There have been many
investigations on the tribological properties of lubricants with
different nanoparticles added. A large number of papers have
reported that the addition of nanoparticles to lubricant is effective
in reducing wear and friction [1–18]. The friction-reduction
and anti-wear behaviors are dependent on the characteristics
of nanoparticles, such as size, shape, and concentration. The
size of nanoparticles is mostly in the range of 2–120 nm [1–13].
Chinas-Castillo and Spikes [1] investigated the effect of particle
size using gold particles of 5 and 20 nm. The results showed that
gold particles of 20 nm were more effective in reducing friction
and wear than gold particles of 5 nm. This difference might be
because the tiny 5 nm particles allow more asperities interaction
than do the 20 nmparticles. Liu and co-workers [2] employed the
dialkyldithiophosphate (DDP) modified copper nanoparticles as
additives in liquid paraffin.
Analysis Instruments
Several instruments were used to examine the lubricants and
the contact surfaces of specimens for more detailed analysis of
the tribological properties. The Weissenberg Rheogoniometer,
which is a cone-and-plate viscometer, was utilized to measure
the viscosity of the lubricants. The JEOLJSM-1200EX transmission
electron microscope (TEM) was employed to obtain
the morphology and size of the nanoparticles.
The optical microscope (OM) was employed to inspect the
trace on the worn surfaces of the fixed specimens. The Alphastep
500 surface profiler was utilized to measure the arithmetic
mean of surface roughness of the fixed specimens. The worn
scar depth of the fixed specimens was measured using the Tylor-
Honson 112/220 and Hommelwerke T4000 & PM2000 surface
profilers. The topography of worn surface was examined with
the HITACHI S-4700 scanning electron microscope (SEM). The
Horiba Emax-Energy energy dispersion of X-rays (EDX) was
employed to analyze the composition of chemical elements on
the rubbing surfaces.
Conclusion
Nanoparticles including CuO, TiO2, and Nano-Diamond
used as additives in lubricating oils exhibit good frictionreduction
and anti-wear behavior, especially for CuO. For the
friction-reduction test, when CuO was added to the SF oil and
the Base oil, the friction coefficients were reduced by 18.4 and
5.8%, respectively, as compared to the oils without nanoparticles.
This might be attributed to the viscosity effect at low
temperature and the rolling effect at high temperature. The
sphere-like nanoparticles may result in rolling effect between
the rubbing surfaces, and the situation of friction is changed
from sliding to rolling. Therefore, the friction coefficient can
be reduced. For the anti-wear test, when CuO was added to the
SF oil and the Base oil, the worn scar depths were decreased
by 16.7 and 78.8%, respectively, as compared to the oils without
nanoparticles. The anti-wear mechanism is attributed to the
deposition of CuO nanoparticles on the worn surface, which
may decrease the shearing stress, thus improving the tribological
properties.