Proceedings of WTC2005: World Tribology Congress III

September 12-16, 2005, Washington, D.C., USA

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

Proceedings of WTC2005 World Tribology Congress III

September 12-16, 2005, Washington, D.C., USA

WTC2005-63460

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 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.

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:

( )WSP

drFK A

nJq225.0 ⋅⋅⋅⋅

=π

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.

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Figure04: Principal of Temperature Measuring

RESULTS

Figure05a: Influence of different test procedures

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.

Figure06: Screening of different friction pairings

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.

Figure07a: Influence of pressure force

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].

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.

Figure09a: Influence of different energy grades

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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.

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 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.

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 re-evaluation 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.

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).

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.

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.

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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.

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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REFERENCES [1] A. Albers; A. Arslan: Einsatz von Keramik zur

Leistungssteigerung von Kraftfahrzeugkupplungen und die Auswirkung auf die Systemkonstruktion 44. Tribologie Fachtagung; Reibung, Schmierung und Verschleiß; 22.-24.Sept.2003, Göttingen, Bd. II, GfT, Gesellschaft für Tribologie e.V., Moers, 2003, S. 70/1-70/10

[2] A. Albers; A. Arslan: Potenzialabschätzung keramischer Teilbeläge für den Einsatz in trockenlaufenden Kupplungssystemen mittels experimenteller Untersuchungen

[3] A. Albers, D. Herbst. Schwingungen in Reibkupplungen - Ursachen, Auswirkungen, Abhilfen VDI-Berichte 1568, Düsseldorf (2000) S. 273-295.

[4] M. Fischer: Potenziale und Grenzen der PKW-Trockenkupplung. VDI Berichte Nr. 1610, 2001, S. 203-222.

[5] J. Gramberger; K.-D. Löcker: Trockenlaufende Sinterreibbeläge. Antriebstechnik 28 (1989) 11, S. 68-75.

[6] A. Albers; A. Arslan; D. Herbst: Keramik für den Einsatz in Bremsen und Kupplungen. ATZ- Automobiltechnische Zeitschrift 103 (2001) H. 5, 414-419.

[7] Messier-Bugatti: Method for Making Parts, in Particular Brake Disks, in Composite Carbon-Carbon Material. PCT-Anm.: WO 98/27026 (1996).

[8] A. Albers, S. Matthiesen: Konstruktionsmethodisches Grundmodell zum Zusam¬menhang von Gestalt und Funktion technischer Systeme, Konstruktion, 7/8 2002,(2002), S. 53–59

[9] K. Poser, J. Schneider, K.-H. Zum Gahr: Keramik/Metall-Friktionspaarungen unter ungeschmierter Gleitbeanspruchung bei kurzzeitiger Belastung. Materialwissenschaften und Werkstofftechnik 2004, 35, No. 10/11, S. 794-800.

[10] A. Albers; A. Arslan: Potenzialabschätzung keramischer Teilbeläge für den Einsatz in trockenlaufenden Kupplungssystemen mittels experimenteller Unter¬suchungen; VDI-Berichte Nr. 1786, Kupplungen und Kupplungssysteme, S.265-280, ISBN 3-18-091786-5, VDI-Verlag, Düsseldorf, 2003

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