Comparative study of the thermal behavior of the tribological couples copper-graphite and graphite-
graphite
A.BENFOUGHAL1, 2, A.BOUCHOUCHA1, R.ABOUD1, Y.MOUADJI1, H.SARRAR2, N.SASSANE2
1Faculty of Technology, Mechanics Laboratory Campus Chaâbat- Erssas Constantine1 University
Constantine 25000, ALGERIA [email protected]
2 Welding and Research Center (CSC) B.P 64 Cheraga-ALGERIA
Abstract— The dry tribological behavior copper-graphite and graphite-graphite pairs in sliding contact is studied. The tests were carried out with a pin-on-disc tribometer in ambient air.
The experimental results show the evolution of the temperature as a function of the normal load and the sliding speed. These results show that these two parameters have a significant influence on the variation of the average contact temperature.
The discussion of results is based on observations by optical microscope and interfacial phenomena resulting from friction.
Index Terms—Temperature, contact, friction, copper, graphite. normal load, sliding speed.
Nomenclature
ρi : masse volumique [Kg.m]
Cp : specific heat [J.Kg-1 .K- 1] λi : thermal conductivity [W.m-1.K-1]
ai : thermal diffusivity [m2 .s-1 ] t : time [mn]
I. INTRODUCTION
The contact is a multidisciplinary field. Indeed, it uses the fields of mechanics, friction, material behavior and thermal.
This is also a problem from multi-scale microscopic effects (third body, transformations tribological surfaces, etc.) to the macroscopic phenomena of heat dissipation or structural deformation, etc.
In electric motors and braking systems, the dry contacts Copper-Graphite and graphite-graphite are used.
The local temperature increases that results may significantly affect the surface properties of materials in dynamic contact. In the case of two metal surfaces and by the combined actions of the normal load and tangential force, the
peaks of the asperities are the effective surface which is subject to high contact stresses and large variations in temperature.
During recent decades, the calculations of the temperature of such systems and the development of appropriate experimental systems have been scientific interest increasingly growing [1].
The objective of this work is to study the evolution of superficial temperature in a dry dynamic contact copper- graphite and graphite-graphite and make a comparison of the thermal behavior between the two couples.
II. EXPERIMENTALDEVICE A. tribometer
The used tribometer (fig.1) is based on the principle of the machine sliding wear; it is constituted by two pieces in dry friction (pin on disk).
Fig.1. Tribometer pin-disk.
B. Contact pion-disk
Fig.2. Scheme of the contact pin-disk.
The pin used has a cylindrical shape (fig.2) comprises a flat part, which allows to fix it in a hole by means of a locking screw on a load arm aluminum. It is charged against a disc by masses of variable weight. The pin is easily exchanged by another sample, or can be removed to measure the mass loss.
The wear disk is a circular plate fixed to a support which rotates at variable rotational speeds. The power of the electric motor transmits for disk by means of a reduction gear ratio 1:20. The frequency converter which allows for a range of rotational speeds from 10 tr.mn-1
to 240 tr.mn-1
. The radius of the wear track is set to 0.02 m; the linear velocity varies between 0.020 m / s and 0.5 m / s. The normal force is transmitted to the sample holder with the loads resting on the end of a load arm. The load range on the pin does not exceed 40 N. The disc is 50 mm in diameter and 5 mm thick, the pin is 8 mm in diameter and 20 mm in length.
The force sensor holds the load arm in the horizontal plane and this records the friction force produced in the contact between the two samples.
C. Materials C. 1. Disk
The disc is made of graphite (MY3D). This type of graphite is used in mechanical seals [2].
C. 2. Pin
It is 99.9% pure copper, a good conductor of heat and electricity. CFC structure allowed elastic and ductile, easily deformable cold, its recrystallization begins at 220 ° C [3].
C. 3. Physical proprieties
TABLE I. PHYSICAL CHARACTERISTICS OF COPPER AND GRAPHITE [4].
III. EXPERIMENTALMETHODSFOREVALUATING SURFACETEMPERATURES
Measure surface temperature is a delicate operation that requires great care. Knowledge of this surface temperature is nevertheless essential to the study of heat transfer between two solids in dynamic contact.
When we want to measure the surface temperature between two elements of a sliding contact, the problem of the accessibility of the surface thermometer is often critical.
Furthermore, this latter should in no way interfere with the thermal behavior of surfaces [4].
The techniques for measuring surface temperature by electrical thermocouple were often used and are of two types:
thermocouples embedded beneath the surface friction and dynamic thermocouples.
The first technique consists in implanting measuring thermocouples embedded in blind holes, in the friction surface of the samples. The joints are adhered with a ceramic cement or adhesive polymer according to the temperature measured.
The temperature recorded depends on the distance of the thermocouple location. This type of assembly is very simple and gives good guidance for measuring average temperatures in the friction surface but is limited in the evaluation of the maximum temperatures reached the surface.
The second technique uses a measurement dynamic thermocouple junction where the acting hot weld is formed by the engaging elements. Initially developed to study the surface temperatures between tool and piece during machining operations, this technique was used to assess different surface temperatures of metal couples in sliding contacts.
The test stand is equipped with sensors to make track the evolution of the temperature. Pins are instrumented with K- type thermocouples placed at 2 mm behind the friction surface (Fig. 3). The temperature reading is not exactly that of the contact surface, but it is a good indicator.
ρ CP λ a
Graphite 2800 712 21 1,05.10-5
Cuivre 8900 93,1 385,8 0,46. 10-5
Fig.3. Experimental method for measuring the temperature [2].
IV. RESULTSANDDISCUSSION
In general, increasing the load leads to a significant reduction in the coefficient of friction [5], the pin compresses the material and forces it to flow to the sides, in the form of pads (Fig.4), thereby facilitating its movement. On the contrary to a low force, material passes below the edge pin. This phenomenon can also be explained by the effect of the hardening of the piece which prevents the adhesion phenomenon to the high values of effort [6]. Indeed, more the contact pressure is great more the contact will be perfect.
Therefore, instead of the friction between the asperities in the case of low pressure, it becomes a perfect friction in the case of high pressures. Furthermore, the phenomena related to plastic deformation increase with the contact pressure [7].
(a)
(b)
Fig.4. Image obtained by optical microscope of the worn surface in contact copper-graphite (P = 20 N): (a) pin, (b) disk A. Evolution of temperature for two couples as function of
time
In the study of the test temperature, the pins used were instrumented with thermocouples. These measurements were used to obtain an estimate of the heat flow in the pin. The measured temperatures stabilize after 1200 seconds (Fig. 5).
Figure 3 show that the temperature increases rapidly for the couple copper-graphite while for graphite-graphite, it is lower.
Moreover, we also note that the difference in temperature for the same conditions between the two pairs of materials varies up to t = 25 min, then it stabilizes at a value of T = 20 ° C.
0 5 10 15 20 25 30
20 25 30 35 40 45 50 55
temperature [0 C]
time [mn]
copper-graphite graphite-graphite
ΔT
Fig.5. Evolution of the temperature at 2 mm of the interface in the pin versus time for the two couples, P = 20 N and v = 0.5
m / s.
B. Evolution temperature for two couples according to the normal load and the sliding speed
The two curves show a significant temperature rise when the applied load and the sliding speed increase (Figs. 6 and 7).
5 10 15 20 25 30
28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58
temperature [0 C]
load [N]
copper-graphite graphite-graphite
Fig.6. Evolution of the contact temperature of the interface to 2 mm in the pin according to the load applied to v = 0.5 m / s.
0,1 0,2 0,3 0,4 0,5
30 35 40 45 50 55 60
temperature [°c]
sliding speed [m/s]
graphite-graphite copper-graphite
Fig.7. Evolution of the contact temperature of the interface to 2 mm in the pin according to the slip speed for P = 20 N.
V. CONCLUSION
We have developed in this article a study to determine interfacial temperatures in a dynamic contact.
The results obtained show that the most influential parameters of the variation of the temperature are: the normal load, the sliding speed and the nature of the materials used.
It can be seen that the couple graphite-graphite better withstand the temperature that the couple copper-graphite, and because of the thermal properties of carbon, which support very well the thermal changes.
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