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Pegasus Project – DLC Coating and Low Viscosity Oil
Reduce Energy Losses Significantly
By Dave Doerwald and Ruud Jacobs, Product Manager and Process Engineer Tribological Coatings
at Hauzer Techno Coating, The Netherlands
Pegasus, the flying horse from Greek mythology,
is a suitable name for the research project initiated
by a German automotive OEM with participation
of Hauzer Techno Coating and several automotive
suppliers. It will enable future automotive vehicles
to reduce fuel consumption without losing power.
The project described in this article focuses on the
rear differential, because reducing friction here can
contribute considerably to efficiency improvement of
the whole vehicle. Surfaces, coating and oil viscosity
have been investigated and interesting conclusions
have been reached.
Compensation Premium
The motivation for the research has its roots in the EU
regulations for carbon dioxid emission and increasing oil prices. From 2012 on, due to EU regulations,
car manufacturers have to pay an excess premium to
compensate for every g/km deviation from the norm
of 130 g/km. From 2012 to 2018 there is a gradient scale of calculation, but the amount of money
involved can be really significant. Furthermore, in
the last ten years the prices for gasoline and diesel
increased more than forty percent. Energy losses
occur in many parts of the vehicle. The main contributors to the energy losses in an automobile are shown
in the schematic diagram of figure 1. In the Pegasus
project concentration is on the rear axle, or more specifically on the hypoid pinion and the hypoid gear
(fig. 2). All driving power has to pass through these
components and as such they also have the highest
influence on energy loss in the rear axle. The hypoid
pinion and hypoid gear are confronted with complicated friction aspects, because the power follows different directions at this point.
Efficiency Improvement
By introducing DLC coating, an improvement of
1.5 percent in efficiency during boundary lubrication
is achievable in the rear axle. A metal-free hydrogenated PVD/PECVD-DLC coating allows to reduce
the friction, but also reduces wear. Oil splashing
Fig. 1: Energy Analysis
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Fig. 2: The rear axle with hypoid pinion and hypoid gear
losses can be reduced by low-viscosity oil and conduction sheets, but leads to increased wear and friction at increasing loads and temperatures. However,
since exactly here the coating has great potential, the
combination of low viscosity oil and coating creates
an excellent combination in friction reduction combined with low wear. In order to improve the surfaces, the existing method of grinding and lapping
are optimized, and alternative methods of polishing
the edges are researched. A Diamond Like Carbon
(DLC) coating is used to reduce friction and wear. In
order to reach an optimum for the lubricant viscosity,
test batches with coated and uncoated parts are made.
Effect of Reduced Roughness
Different surface treatments are analyzed for their
applicability to the components. Surfaces of most
hypoid pinions nowadays have a roughness of > 6 μm.
Optimized lapping procedures are investigated,
grinding and micro blasting as well as vibratory
finishing. It turns out that roughness can be successfully reduced to Rz < 3 μm. Vibratory finishing gives
the best results, but this method is not applicable for
mass production. The achieved roughness is resulting
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in a reduced friction, as well as being very suitable
for PVD coating.
DLC Coatings
For this research project, mainly hydrogenated a-C:H
coatings produced in a Hauzer Flexicoat® 1200 are
used. In order to achieve a good adhesion on the
DLC coatings, they are built-up by a combination of
a PVD-adhesion layer (sputtering) with a functional
PECVD-layer. The standard design of this DLC coating can be seen in figure 3. Total thickness of the
coating is 2 μm to 3 μm, in which the thickness of
the coating on the teeth gradually decreases closer to
the gear base. Process temperatures with this hybrid
deposition technology are below 200 °C and hence
very suitable for components made from low tempered steels. Because the coating is so thin, it is logical that the roughness of the component before coating has a relevant influence on the friction and wear
behaviour later on. As optimum we defined surfaces
with a Rz < 2 μm and a support ratio (Rmr 0.500 μm)
of more than 50 %, indicating that more than 50 %
of the surface is smooth within less than 0.5 µm of
the roughness peaks. When the values deviate signifi-
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the influence of oil temperatures. The lubricants in
these tests were identical additive oils with different
viscosity rates of 15 cSt, 12 cSt and 9 cSt (VP100).
Results of DLC Coating on Friction Coefficient
Polished, DLC coated test plates (16MnCr5) were
tested against uncoated 100Cr6 bearing balls. Oil
temperatures were 40 °C, 80 °C and 100 °C. The
results of the test (fig. 4) were as expected. At a low
sliding speed (metal to metal contact and boundary
lubrication), the improved friction properties of the
DLC coated surface show a significant advantage
over uncoated surfaces. The advantage decreases
with increasing sliding speed and increasing lubricant film thickness.
Fig. 3: Build-up of DLC coating
Influence of Surface Roughness
on Transmission Efficiency
cantly, rapid coating failure will occur, especially on
the base of the teeth.
For testing the influence of surface roughness on
transmission efficiency, grinded and vibratory finished
hypoid pinions were investigated and compared to
standard production gears. The hypoid gears were
standard production types. By decrease of the roughness on the tooth flanks, the energy losses at low
velocity could be significantly reduced. The improvements were larger at higher torques and higher
temperatures, which can be attributed to a thinner
lubricant film in these cases combined with the positive effect of the decreased roughness. The effect is
stronger from grinded (Rz = 3 μm) to vibratory finished (Rz = 1 μm) components. The graph (fig. 5)
shows that for lower lubricant temperature and higher
viscosity, a lower roughness increases friction.
Test Methods
The effects of roughness, coatings and lubricant have
been investigated using simplified test devices in
order to get conclusive results. A tribometer, where
a ball rotates under a defined and fixed load against a
plate, allows for testing the different operating points
of the Stribeck curve by adjusting the rotation speed
of the ball. Coating lifetime is tested in a gear test
bench, where the load torque and the rotation speed
are adjustable. The efficiency improvement on the
hypoid pinion and the hypoid gear was measured on a
T-testbench. This set-up allows amongst others to test
Fig. 4: Friction coefficient as function of sliding speed in test plates; DLC coated (grey)
against uncoated (black) with 15 cSt (VP100) axle oil at 80 °C
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Influence of DLC Coating
on Efficiency of Transmission
Low Viscosity Axle Oil
The tests for roughness influence have been repeated
to show the influence of the DLC coating. In this test,
DLC coated hypoid pinions are applied. The hypoid
gears are uncoated. DLC coatings have the largest
potential for friction reduction especially on (quasi)
stationary, mixed contact and boundary lubrication.
Looking at the change in power loss of figure 6, it
can be concluded that at 6000 U/min - 25 Nm - 50 °C
surfaces are separated, lubrication is hydrodynamic
and the coating has no influence on further reduction of friction. At higher temperatures and higher
torque however, the DLC coating shows its significant advantages.
Other investigations have shown, that by using a
low viscosity axle oil, the splashing losses can be
reduced. This is shown e.g. at low forces and high
turning speed. The efficiency of this lower viscous
oil deteriorates at points of high torque impact. In
the investigations only the 6000 U/min - 25 Nm and
4000 U/min - 50 Nm showed improvements for all
temperatures. Here the energy loss was reduced significantly with 7 to 18 %. The influence of oil viscosity is easy to explain. When the layer of lubricant
between the components thickens, there will be less
contact, which makes the improved friction coefficient of the DLC coating less influential.
Fig. 5: Change of energy loss with grinded hypoid pinions and 15 cSt lubricant against
serial produced with 15 cSt lubricant
Fig. 6: Change of power loss by DLC coating with 15 cSt lubricant against uncoated hypoid
pinions with 15 cSt lubricant
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Fig. 7: Change of power loss with DLC and 12 cSt lubricant against uncoated hypoid pinions and 15 cSt lubricant
Combination of Methods
From the separate tests can be concluded that the
optimum combination would be a highly polished,
coated surface with thinner oil. In this way the
reduced splashing losses at high speed and low force
can be combined with the potential of coating in a
high load range. Until now, only measurements were
done with a combination of thin oil and DLC coating. This combination gave a reduction of energy loss
between 12 % and 32 % in all working points (fig. 7).
Concluding
By using the methods explained in this article, energy
loss in the rear axle can be significantly reduced.
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These methods are especially effective for cars with
higher emission of carbon dioxid. Large scale mass
production equipment is required for coating these
large components, to keep the coating costs down.
The extensive experience Hauzer has in up-scaling
these DLC coating processes to mass production, is
their contribution to the Pegasus project. Although
investments are needed to coat the hypoid pinions,
the corresponding cost savings are impressive and
they will increase within the next years.
Reference
M. Köhn, Dr. R. Annast, Dr. J. Schnagl: Maßnahmen zur Effizienzsteigerung des Antriebsstrangs am Beispiel Achsgetriebe; München 2011
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