Proceedings of WTC2005
Proceedings
of WTC2005:
World
Tribology Congress
III
September 12-16,World
2005, Washington,
USA III
Tribology D.C.,
Congress
September 12-16, 2005, Washington, D.C., USA
WTC2005-63460
WTC2005-63460
INCREASING THE PERFORMANCE OF MOTOR VEHICLE CLUTCHES BY THE USE OF
CERAMICS AS FRICTION MATERIAL AND THE RESULTING EFFECTS ON SYSTEM DESIGN
Albers, A., Mitariu, M., Arslan, A
Institute of Product Development
University of Karlsruhe (TH)
Karlsruhe, Germany
ABSTRACT
The experimental and constructive results illustrate that
engineering-ceramic materials have a high potential in the field
of dry running friction systems. According to first estimations,
it is possible to build the vehicle clutch 53% smaller or to
transmit up to 180% higher torque with the same size by an
appropriate selection of the system friction pairing and an
adequate ceramic design [1, 2]. The friction coefficient
characteristic (decreasing friction coefficient above sliding
speed) is unfavourable with regard to comfort (self-induced
grab oscillations [3]) of the vehicle clutch. Furthermore, it is
important to select the test procedure of the experimental
analyses to be as close to the system as possible in order to
obtain exact information concerning the target system.
KEYWORDS
Ceramic materials, clutch, unlubricated sliding contact,
friction, wear.
INTRODUCTION
Today, vehicles get heavier and heavier despite
increasingly light-weight construction activities. In order to
avoid drawbacks in the drive dynamics in spite of the additional
weight, a rise in the performance of the drives is required, while
the construction space is partially reduced at the same time.
Due to this tendency the requirements of clutch systems also
increase.
A high power density is achieved by means of a high
friction coefficient and a high admissible surface pressure in the
working surface pairs of the tribological system. Additionally,
clutches generally transform kinetic energy into thermal energy
with the aid of friction. Therefore, they can only fulfil their
function when the conditions such as a high level of heat
dissipation, a high heat capacity and a high temperature
resistance of the friction partners can be realised. Today,
vehicle clutches are mainly based on organic facing materials.
These materials only allow a relatively low thermal stress (≤
250°C) and a maximum surface pressure of ≥ 0.4 MPa.
Although friction coefficients ranging from 0.27 - 0.3 are
reached, considerably higher friction coefficients and
admissible surface pressures are demanded. Sinter metal
facings based on copper and above all iron alloys have friction
coefficients of µ = 0.4 - 0.7 and a high admissible operating
temperature of more than 800°C. Because of an unfavourable
gear shifting discomfort they are only employed in the field of
disc and drum brakes and flywheel clutches for heavy
commercial vehicles [4-6]. Due to their high material and
manufacturing costs the carbon fibre reinforced carbon and
compound ceramics can only maintain their position in racing
sports, plane manufacturing and upper-class vehicles [7]. A
possible approach to solving this problem is provided by the
monolithic, ceramic materials. Because of their high friction
coefficient and the high admissible surface pressure a high
power density can theoretically be expected. As demonstrated
in this paper, the results of the tests with the materials Al2O3,
SSiC mated with 100Cr6 show the potential of ceramic
materials in the dry friction system. Due to the high stiffness,
hardness, the low fracture toughness and the specific
tribological properties of ceramics, the integration of technical
ceramics into a complex technical system – such as the vehicle
clutch discussed in this paper – is a great challenge. In the
following, a new design solution for clutches is presented.
MATERIALS & EXPERIMENTAL METHODS
In order to analyse the system behaviour of different
friction combinations, experimental analyses have been carried
out at a dry friction test bench. In this analysis, the friction
coefficient as well as its behaviour concerning the sliding speed
and the local working surface temperature of the ceramic have
been determined.
The tribological tests were carried out at the working
surface pairs [8] Al2O3/100Cr6, SSiC/100Cr6, Al24TN/100Cr6
(Al24TN: Laser surface modified ceramic [9]), cloth-reinforced
C/SiC/100Cr6 (SGL Brakes Ltd.; in a real application
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combined with an organic facing) and organic facing/100Cr6
(industrial clutch). The two last combinations are exclusively
used for estimating the potential of monolithic ceramic. The
steel 100Cr6 was employed with a hardness of 180 ± 5 HV.
With 1670 HV (Al2O3: F99.7; Fa. Friatec) and 2500 HV (SSiC:
EkasicF; ESK Ceramics) the hardness of the ceramic was
clearly higher than that of the metallic material. The pellets
(Fig. 1) were used with an untreated, "as sintered" surface
(Al2O3: Rz ≤ 7.5 µm; SSiC: Rz ≤ 1.9 µm), whereas the discs are
employed surfaced with a Rz value of ≤ 13.6 µm. For the
testing of the surface influence Al2O3 pellets are used in a
polished state (Rz ≤ 1.9 µm).
Figure 02: Test procedure 1
Test procedure 2 (Fig. 03): After the application of the
normal load FN (distributed on the number of pellets) the probe
reaches the final speed nF due to a predefined acceleration in
the slip time tS. Subsequently, the working surface pair is
separated (FN = 0 N). The next test starts when the probe has
come to a standstill.
Figure 03: Test procedure 2
For test procedures 1 and 2 a fully factorial test matrix with
the parameters engine speed ndr, normal load FN and a specific
thermal energy has been developed. These matrices can be seen
in tables 1 and 2 of the appendix. Assuming that there is no
load moment and no decrease in engine speed, this matrix
results in the calculated (expected) specific thermal energy in
the clutch presented in column 3. In contrast to a conventional
vehicle clutch with organic facings as partner in the friction
contact, the specific thermal energy of this system is higher by
a factor of up to 10.
The specific thermal energies qK were determined as
follows: 1. For tests with 30 pellets the mass moment of inertia
has been varied from JF = 1 kgm² (middle-class vehicle without
loading) to JF = 2 kgm² (middle-class vehicle with loading) and
for the model tests from JF = 0.1 kgm² to JF = 0.3 kgm² .
2. By means of these mass moments of inertia, the speed
and the sum of surfaces of all functional contacts in the
working surfaces (AWSP) [8] between the pellets and the
pressure plate, the specific thermal energy qK for each test is
calculated to be:
2
0.5 ⋅ J F ⋅ (2 ⋅ π ⋅ ndr )
qK =
AWSP
The influence of the surface quality was analysed during
test procedure 1. Here, 6 pellet retainers were arranged on a
radius of r = 84 mm. The test matrix is presented in the
following table 2.
The thermo graphic recordings of the ceramic working
surface were carried out in test procedure 2. In order to have a
direct view of the pellet surface, a bore was recessed into the
discs (see Fig. 04). At the working surface the bore had a
diameter of 3 mm.
Figure 01: Test Bench
Fig. 01 illustrates the test bench structure. The test head is
designed modularly. It allows a variation of the friction radius
(R75, R84 and R93 mm) and the number of pellets. In the tests
discussed in this paper the retainers are arranged on a radius of
rm = 84 mm. For the tests, the pellets are glued into one of the
retainers by means of a two-component epoxy resin adhesive.
Five pellets are linearly made resilient at the same time for each
test with 30 pellets. In the case of tests with six pellets every
pellet is made resilient individually. The carrier and the
resilience are identical for both tests.
In order to determine the system properties of the working
surface pair [8], tests with conditions that are as similar as
possible, are carried out with regard to the target system vehicle
clutch. During a test, the slip was gradually reduced to zero
starting from a high slip (see test procedure 1). This
corresponds to the target system.
Test procedure 1 (Fig. 02): The probe with the pellets is
accelerated to a predefined engine speed ndr. After reaching the
speed, the normal load FN (distributed according to the number
of pellets) is generated by means of a stepper motor-operated
carriage and the output end engine shaft is synchronised with
regard to the engine speed (npto = 0 rpm for npto = ndr). After the
successful synchronisation the working surface pair is separated
(FN = 0 N) and the output shaft is stopped (npto = 0 rpm) in
order to start the next test directly afterwards. In order to
analyse the influence of the experimental procedure, further
tests have been carried out in which the slip was gradually
increased starting at slip equal to zero (test procedure 2). The
metallic working surface is fixed during the whole test.
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Figure04: Principal of Temperature Measuring
Figure06: Screening of different friction pairings
RESULTS
In Fig. 6 the friction coefficient is entered above the sliding
speed at different system friction pairings Al2O3 / 100Cr6, SSiC
/ 100Cr6, C/SiC / 100Cr6 and organic facing / 100Cr6 (1100
rpm; 11.2 J/mm²; 1800 N) with the same test procedure 1. This
representation shows that the friction coefficient decreases in
all friction pairings with an accelerating sliding speed. Apart
from that, the facing and friction coefficient behaviour of the
system friction pairings SiC / 100Cr6 are similar to those of the
system friction pairing organic facing / 100Cr6. The value of
the friction coefficient of the system friction pairing
Al2O3 / 100Cr6 is the lowest of all four pairings. Furthermore,
the friction coefficient gradient of this pairing is less
appropriate for a clutch system.
Figure05a: Influence of different test procedures
Figure07a: Influence of pressure force
Figure05b: Influence of different test procedures
In the Fig. 5 a, b the influence of different test procedures is
illustrated with the example of the system friction pair analysis
Al2O3 / 100Cr6 and SSiC / 100Cr6 with a normal load of
FN = 2400 N, a final speed ndr and nF = 1500 rpm and a specific
energy of 20.9 J/mm². Moreover, the friction coefficients at the
sliding speed are presented. The tests with test procedure 1 start
with a high slip (sliding speed) and a high energy input into the
system. Test procedure 2, however, starts with the sliding speed
vS = 0 m/s. Here, the energy input is low at the beginning of the
test. The tests with test procedure 2 show a higher friction
coefficient at low sliding speeds than the tests carried out
according to test procedure 1. Moreover, Fig. 5 a, b
demonstrate that the friction pairing SSiC / 100Cr6 has a higher
friction coefficient and a friction coefficient gradient that is
more suitable for vehicle clutch systems [3] than the friction
pairing Al2O3 / 100Cr6.
Figure07b: Influence of pressure force
The Fig. 7 a, b illustrate the influence of the pressure force
at the same drive engine speed (700 rpm and 1100 rpm
respectively). The influence of the pressure force (1200 N,
1800 N and 2400 N) can be neglected at different energy levels
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(qK = 9.1 J/mm² and 16.5 J/mm² respectively). However, the
fact that the friction coefficient depends on the sliding speed is
a dominating factor for the analysed area of the normal load
[10].
Figure09b: Influence of different energy grades
Fig. 9 a, b show the influence of the different grades of
energy. The friction coefficient and the friction value pattern in
Fig. 9a are approximately identical except for different grades
of energy with the same normal load (FN = 1800 N) and engine
speed (ndr = 1100 rpm). There was the variation of the mass
moment of inertia (J = 1 and 2 kgm² respectively). Here the
friction value pattern in Fig. 9b shows an approximately
identical grade of energy with the same normal load (FN = 1800
N) and different drive engine speed (ndr = 1100 and 1500 rpm
respectively). Here also the friction coefficients are nearly
identical.
Figure08a: Influence of surface roughness
Figure08b: Influence of surface roughness
Fig. 8 a, b show the influence of the surface roughness at
the same pressure force (450 N) and the same engine speed
(700 and 1500 rpm respectively). In test procedure 1, 6 pellets
with different surfaces („as sintered“ Rz=7.5 µm, „polished“
Rz = 1.9 µm) were analysed at the same time. An influence of
the surface quality at low sliding speeds can be recognised
(∆µ = 0.1). However, this influence decreases with an
increasing sliding speed or higher energy levels.
Figure10a: Temperature distribution on the pellet contact
surface
In the Fig. 10 a, b the temperature distribution on the pellet
surface of the friction pairing SSiC / 100Cr6 and
C/SiC / 100Cr6 is presented in a sequence at similar speed
(nF = 1100 rpm), normal load (FN = 600 N) and specific energy
(qK = 5.6 J/mm²). The mean surface temperature of both
pairings is almost identical in the circular measuring range. The
system friction pairing C/SiC / 100Cr6 shows a maximum
temperature which is twice as high as that of SSiC / 100Cr6.
Figure11: Exemplary thermograph record of system
friction pairings
Figure09a: Influence of different energy grades
By using the possibility of adjusting the brightness it is
possible to localize the areas and the geometric size of the high
temperatures. Fig. 11 shows exemplary two picture series
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(SSiC / 100Cr6 and C/SiC / 100Cr6). These picture series show
that there is a permanent transfer of the working contact. These
working contacts are changing in their geometrical size. The
local rise in temperature and the permanent changing of the
working contact is more distinct at C/SiC / 100Cr6 than
SSiC / 100Cr6.
Figure13: Example for development process of
constructive solutions of engineering ceramic clutches
Engineering-ceramic materials excel in their high
temperature resistance, compressive strength, rigidity and
hardness while having relatively low density. These advantages
with regard to pressure and temperature are accompanied by
disadvantages such as low bending stress, shear stress and
tensile stress. Furthermore, the integration of the ceramic into a
metallic environment is a big challenge for the design.
Different thermal expansion coefficients for example can lead
to temperature-dependent pressures or constructive clearances
in the contact area of the working surface pairs. Distortion by
temperature gradients and manufacturing deviations from
parallelism would lead to bending stress in the ceramic ring
when the clutch is closed. Furthermore, the supporting surface
part during the slipping phase is small, due to low yielding and
therefore "adaptability” of the ceramic. For these reasons the
round-shaped facing which is common in most clutches can not
be substituted directly by an engineering-ceramic. This shows
the effects of material choice on the constructive shape. As a
first approach a pellet design, which is used in the study
described here, was developed. The further search for
conceptual solutions was broadened by the solution varieties of
systematic variation in the form of the working surface and
arrangement, starting from the pellet design approach (Fig. 14).
Figure12a: Influence of Laser modified ceramics
Figure12b: Influence of Laser modified ceramics
In the Fig. 12 a, b the influence of the laser surface
modification is demonstrated. The friction coefficient of the
system friction pairing Al24TN / 100Cr6 is bigger than
Al2O3 / 100Cr6 by µ = 0.2. The friction coefficient gradients of
both system friction pairings are almost identical.
DESIGNING WITH ADVANCED CERAMICS AND
EFFECTS
The pronounced iterative character, with multiple reevaluation of the individual process steps, is characteristic of
the whole product development process from profile
development to product use and, if necessary, revitalisation.
The choice of ceramics as a material for components in the
early drafting phase essentially determines type and method of
product validation prior to release and product realisation. This
research work concentrates on the phases of finding the product
concept, product development, product formulation and product
validation (see Fig. 13). These four fundamental steps
constitute the actual constructive development process. It is
carried out until the conversion of the prototype.
Figure14: Example for reducing bending stresses by design
rule “Separating of working surface pairs and parallel
connection”
Step 1: Initial situation, a ceramic ring with high bending
stresses.
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Step 2: Reduce the tangential bending stresses by separation
of the channel and support structure and parallel
connection of working surface pairs.
Step 3: Reduce the radial bending stresses by separation of the
channel and support structure and parallel
connection of working surface pairs.
Step 4: To afford a phase of cooling down for the metallic
working surface by controlled reduction of the
working surface pairs.
Step 5: Adaptation of the ceramic working surface to ensure
that the angles are never a working surface.
Step 6: Simplification of the geometry of the ceramic working
surface
With these design steps and many other design methods we
succeeded in designing and realising a clutch prototype with
ceramic facings (Fig. 15). This prototype is able to transfer
torques of M < 260 Nm.
Figure16a: Influence of different energy grades of the prototype
retainer plate with ceramic pellets
Figure16b: Influence of different energy grades of the
prototype retainer plate with ceramic pellets
CONCLUSIONS
In this paper the potential of engineering-ceramics was
illustrated with regard to the use of vehicle clutches. Since in
system friction analyses the friction coefficient above the
sliding speed decreases, it will be important to find solutions
which show a neutral (desirably increasing) friction coefficient
behaviour above the sliding speed. For this purpose, laser
barrier layer modifications are carried out in the SFB 483. For
the development of new system solutions it is important to gain
a profound knowledge of the heat balance of the clutch.
Therefore, in further research projects more detailed FEM
models are developed for some constructive solution
alternatives. From this, the thermal and mechanic stress of the
engineering-ceramics in a construction unit as well as the
operating behaviour of the clutch alternatives under the
boundary conditions of the thermo mechanical deformations of
the components are derived.
Figure15: Prototype of the retainer plate with ceramic
pellets
After analysing different, the friction behaviour affecting
parameters with the modular test head similar test procedures
have been maintained at the first prototype retainer plate with
ceramic pellets. Exemplary two possibilities of varying the
specific thermal energy are shown in figure 16 a & b. The
pressure in both figures is kept at a constant normal load (FN =
4200 N; p = 2 N/mm²) while either the mass moment of inertia
is changed (J = 1 and 1.5 kgm² respectively) or the driving
speed (ndr = 700 – 1500 rpm). Both variations exert small
influence on the friction coefficient over the sliding speed.
Significant for the prototype retainer plate is a small friction
coefficient gradient comparative to the friction coefficient
gradient of the test head. For this flattening of the friction
coefficient gradient the tribological causes are not yet solved
recently and so this behaviour has to be focused more
vehemently in the future.
ACKNOWLEDGMENTS
The authors would like to thank the Deutsche
Forschungsgemeinschaft (DFG) for financial support within
the frame of the Center of Excellence in Research CER 483
"High performance sliding und friction systems based on
advanced ceramics".
.
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