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Influence of the electrical sliding speed on friction and wear processes in an electrical contact copper–stainless steel
Applied Surface Science 223 (2004) 330–342
Influence of the electrical sliding speed on friction and wear processes in an electrical contact copper–stainless steel A. Bouchouchaa,*, S. Chekroudb, D. Paulmierc a
Laboratoire de Me´canique, De´partement de Ge´nie Me´canique, Faculte´ des Sciences de l’Inge´nieur, Universite´ Mentouri Constantine, Constantine 25000, Algeria b De´partement de Physique, Universite´ Mentouri Constantine, Constantine25000, Algeria c Laboratoire de Physique et Me´canique des Mate´riaux, CNRS-UMR 7554, Equipe ERMES, ENSEM-INPL, 2 Avenue de la Foreˆt de Haye, 54516 Vandoeuvre-les-Nancy Cedex, France Received 8 June 2003; received in revised form 13 September 2003; accepted 14 September 2003
Abstract Among the various parameters that influence the friction and wear behaviour of a copper–stainless steel couple crossed by an electrical current and in a dry contact is the sliding speed. The tests were carried out under ambient environment and the sliding speed was in the range of 0.2–8 ms1. The electrical current intensity was varied from 0 to 40 A and held constant during each experiment. The normal load was maintained constant corresponding to an average Hertzian stress of 107 Pa. It appears that the friction coefficient and the wear rate increase at first with the speed, reach their maximums, then slowly decrease and tend to constant values. Over the entire range of sliding speeds two types of wear are observed. These latters are essentially mild wear as long as hard debris do not appear at the interface and severe wear when debris consisting of oxides or oxide metal mixture become big enough, they are removed from the surface and have abrasive effect. The results are discussed in terms of observations of wear debris size and composition, wear track study, metallographic study of worn surfaces and friction and electrical contact resistance records. # 2003 Elsevier B.V. All rights reserved. Keywords: Friction; Wear; Sliding speed; Electric contacts; Abrasion; Oxide films
1. Introduction The copper–steel couple was used to transfer the electrical current from the catenary (copper) to the collector (steel) but, previously the noticed high wear rate has led to the present use of the copper–graphite couple. However, it is interesting to highlight the friction and wear processes in the case of copper–steel couple. These depend on the using conditions [1], especially
*
Corresponding author. Tel.: þ213-31632434; fax: þ213-31632434. E-mail address: [email protected] (A. Bouchoucha).
on the sliding speed which appears as an important operating parameter determining the useful service life of the couple. The sliding speed has an effect to increase the interface temperature by heating. Indeed, the local temperatures can attain values leading to the thermal softening of the copper. This affects the metallic structure [2] and the mechanical properties of the surface layers [3]. High and moderate sliding speeds, in presence of an electrical current, favour the formation of hard oxides and in addition to the adhesive wear, the abrasive one occurs at the interface. The results are discussed essentially taking into account the theoretical analysis and experimental phenomenon due to the friction.
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.09.018
A. Bouchoucha et al. / Applied Surface Science 223 (2004) 330–342
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2. Experimental details 2.1. Apparatus and procedure The experiments were carried out with a pin on disc system tribometer, which was modified in tensioned wire-on-disc system, as described elsewhere [4]. The disc was replaced by a side wheel with 16 sectors (Fig. 1), and the pin by a U-shaped frame over which a copper wire was stretched [4]. The tension force T of the wire was measured by strain gauges that were glued on the arm. The normal load was applied vertically with a weight P. The horizontal wheel was driven by an electric motor with variable but controlled speed. The friction coefficient m ¼ F=P is deduced from measurements of the tangential force F induced on the arm by rotating wheel through the wire. The direct electric current I is brought to the disc through a mercury contact in the axis of rotation (Fig. 2) to avoid the effect of centrifugal force. The electric contact resistance Rc was recorded using an appropriate device. The test duration tf was 60 min and the wear W was determined by weighing the wire before and after the experiment.
Fig. 2. Turning in contact mercury, 1: electrical wire conducting I, 2: mercury, 3: head of screw, 4: disc, 5: rotating axis.
2.2. The specimens The disc was made of stainless steel (Cr18, Ni10 and balance Fe), with 120 mm in diameter and 30 mm thick. The wire was made of copper of high purity (99.98%), 2.2 mm in diameter. Before use, the copper wires were annealed under vacuum for 2 h at 600 8C. Metallographic sections were prepared from specimens in order to compare their hardness before and after the test.
3. Results 3.1. Influence of the sliding speed on friction 3.1.1. Evolution of the friction coefficient versus time The general evolution of the friction coefficient with time is given in Fig. 3. Two distinct zones can easily be identified on these curves: An initial unsteady phase: during this period, the friction coefficient m is stable (no fluctuations) and evolves between two extremes mmin and mmax. A steady state: during this phase, which follows the preceding state, the equilibrium stage depends on the operating conditions at the interface.
Fig. 1. Schematic representation of disc with 16 sectors.
However, in the absence of an electric current, at low sliding speeds, small oscillations are noticed in the friction coefficient (Fig. 3a). At moderate sliding speeds they disappeared almost totally (Fig. 3b) and
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Fig. 3. Evolution of friction coefficient with time, (a) v ¼ 0:3 m s1, (b) v ¼ 1:2 m s1 and (c) v ¼ 7:2 m s1.
at higher sliding speeds (Fig. 3c) the friction coefficient fluctuates 10–15 min and then stabilises. 3.1.2. Influence of the sliding speed on the friction coefficient Fig. 4 shows the variation of the average friction coefficient m with sliding speed v in stationary regime. On this curve, we can observe three zones: 0:25 < v < 1:2 m s1: in this interval, the friction coefficient m slightly increases from the value 0.54 to reach a maximum of the order of 0.60 at around v ¼ 1:2 m s1.
5 > v 1:2 m s1: in the second zone, with increasing speed, the friction coefficient m notably diminishes to a limit value ml about m ¼ 0:40. 7:2 v 5 m s1: this region is characterised by a relative stability of friction coefficient m at the limit value ml. 3.2. Influence of the sliding speed on wear and wear rate The effect of sliding speed on the wear rate k (Fig. 5) is nearly analogous to that on friction coefficient.
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as shown in Figs. 4 and 5 when I þ ¼ 40 A (anodic wire).
4. Discussion 4.1. Influence of the sliding speed on friction and wear
Fig. 4. Variation of the average friction coefficient with speed for I ¼ 0 and I þ ¼ 40 A.
In particular, the wear rate (wear per unit sliding distance) is maximum for speed values around v ¼ 12 m s1 and then, decreases and becomes constant. The same figure shows the variation in wear with sliding speed, we notice that as the sliding speed increases, the wear varies approximately linearly. 3.3. Influence of the heat generated by mechanical friction and Joule effect on friction and wear An increase in sliding speed, as well as in the normal load and/or the electric current intensity generates a raise in the temperature at the contact. Consequently, the oxides formation is facilitated. However, whatever the value of sliding speed v, the passage of electrical current across the contact leads to fluctuations in m ¼ f (t) (Fig. 6) and RC ¼ f (t) (Fig. 7) in the stabilised regime. The amplitude of these fluctuations increases with increasing electric current intensity. The electrical current has no significant influence, on the average friction coefficient and the wear rate, except a slight decrease beyond 20 A approximately
In the domain of low speeds: 1:2 > v 0:25 m s1, the friction coefficient and the wear reach high values because the contact time is long enough, so that metallic bondings raise and the actual contact area widens out. This leads to oscillations in the friction coefficient (Fig. 3a) due to the decohesion and roughness of the worn surfaces. When the sliding speed is between 1.2 and 5 m s1, first the bonding growth time decreases; second the temperature increases with their effect on the oxidation phenomenon. The number of metallic bondings is reduced by the formation of an oxide layer on the sliding surface of the wire. Our observations (optical microscopy) showed that this layer is dense, regular and fits the irregularities of the surface on which it is firmly bounded (Fig. 8). As a result, this layer protects the surface from wear. The electric contact resistance increases (when I 6¼ 0 A) however (for I ¼ 0 A), the friction coefficient, the wear rate, the fluctuations in friction and the surface roughness decrease. When the temperature increases, the layer becomes ductile and the oxide behaves like a lubricant. At speeds of 5–8 m s1, as sliding speed increases, the surface temperature increases and then the oxidation rate increases too, this reduces the number of metallic contacts and the amount of metal transfer. The oxide layer disintegrates, the strength of the substrate is lower and the contacts break and the copper softens [5]. Plastic deformations of the subsurface extend to lesser depth surface. The total wear increases almost linearly, while the friction coefficient remains constant around 0.41. 4.2. Influence of the sliding speed on wear rate Under air (when I ¼ 0 A), the wear is of the soft adhesive mode and the oxide layer must reach a minimal depth before breaking [6]. This depends on
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Fig. 5. Wear of the copper wire vs. sliding speed with and without electrical current (a) W ¼ f ðvÞ and (b) k ¼ f ðvÞ.
time and on surface temperature. If the temperature was independent of the sliding speed (case of low speeds), the quantity of removed oxide would be independent of the speed, and the wear rate, as an inverse ratio of the speed, would decrease when the sliding speed increases. For high sliding speeds, the surface temperature increases and the oxide formation speeds up [7]. In this case, the wear rate keeps the same value and appears independent of the sliding speed. This phenomenon is observed for speeds over 5 m s1. Moreover, when the copper is anode, the growth of the copper oxidation layer is higher while the one of steel becomes lower. In this situation, we observe experimentally that the copper wear is low. When the copper is the cathode, the opposite occurs. This is due to the fact that a high steel oxidation leads to the rapid formation of hard iron and chrome oxides which are released as grains at the sliding interface, thus abrading strongly the wire. Our observations under microscope confirm this abrading action. Consequently, the variations in the wear and the wear rate with speed have tendency to decrease when I þ ¼ 40 A (Fig. 5).
4.3. Influence of the heat released by mechanical friction and Joule effect on friction coefficient and wear 4.3.1. Calculation of the contact area In these experiments, the value of the normal load is 10 N. The radius of the circle equivalent to the total contact area (Fig. 9) is: 1=2 P a¼ pH where H is the hardness of the softer material. 4.3.2. Calculation of the contact temperature According to the Archard method [8], the total heat quantity released by friction and Joule effect in the interface is: Q ¼ Qd þ Qw where Qd is the heat quantity per unit of time to the rotating disc, and Qw is the heat quantity transmitted by a mobile source per unit time to the wire.
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Fig. 6. Influence of the current intensity on the evolution of m when v is fixed (a) I ¼ 10 A, (b) I ¼ 20 A and (c) I ¼ 40 A.
Q is shared between the two surfaces so that the rise in temperature Dym is the same on each side. When the sliding speed is low, the temperature variation Dym of the wire contact surface is given by the following formula: Qw Dym ¼ 4alw
However, on the disc contact surface the rise of temperature is given by: Dym ¼
Qd 4ald
where lW and ld are the thermal conductivities of the copper wire and of the steel disc, respectively:
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Fig. 7. Evolution of the electrical contact resistance Rc with time (a) I ¼ 10 A and (b) I ¼ 40 A.
lw ¼ 385:8 J m1 s1 8C1 and ld ¼ 14:9 J m1 s1 8C1. It appears that the wire exhausts a heat quantity 25 times greater than the disc.
Fig. 8. Optical micrograph (100 ) of the worn copper showing the adaptation of oxide layer to the substate (P ¼ 10 N, T ¼ 360 N, v ¼ 1:2 m/s, tf ¼ 60 min, I ¼ 0 A).
Fig. 9. Schematic diagram of sliding contact wire–disc.
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For high sliding speeds, Dym is given by the relation: 1 0:31Qw w 2 Dym ¼ 4alw va where the thermal diffusivity of copper is w ¼ l=rc, r is the density of the material and c is the specific heat. For moderate speeds, the expression used for Dym is: Qw Dym ¼ a 4alw with a varying from 0.85 to 0.35 and Qd ¼ mPv þ RC I 2 . The limits between low and high speeds are given by the values of a parameter L: va L¼ 2w Low speeds are considered when L < 0:1, for moderate speeds 0:1 < L < 5 and high speeds are considered when L > 5. As the total transfer coefficient of the wire to exhaust the generated heat is low, the equilibrium wire temperature varies between 30 and 350 8C. The calculated values show that the temperature at the interface during the sliding can rise up to 600 8C [9]. 4.3.3. Determination of the thickness of the oxide layer Using equation [5]: rc r0 X þ 2a pa2 where Rc is the contact resistance, rc is the electrical resistivity of copper, r0 is the electrical resistivity of a thin layer of copper oxide and X is the thickness of the oxide layer. The first term of the sum represents the constriction resistance and the second term the resistance of oxide layer. The values of r0 are suspected to be of the order of 4.103 O m, Rc value measured in our experiments is about 12.103 O when the conditions lead to a significant oxidation. ˚ for a ¼ 50 mm, when In these cases, X ¼ 240 A breakage occurs in the copper oxide (H ¼ 13 108 ˚ for a ¼ 100 mm, when breakN m2) and X ¼ 950 A age occurs in the copper metal (H ¼ 4 108 N m2). Rc ¼
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Table 1 Influence of the speed on the rise in temperature at the interface with and without electrical current I (A) 0
40
˚) X (A
v (m/s)
Qf (W)
Dy (8C)
50
240
100
950
50
240
100
950
1.2 7.2 1.2 7.2 1.2 7.2 1.2 7.2
7.2 29 7.2 29 16.8 38.6 16.8 38.6
74 205 37 74 174 275 87 100
a (mm)
Table 1 gives the results with and without electrical current for two values of contact circle radius and two values of sliding speed. Under dry sliding conditions, the temperature can rise up to 600 8C. Tylecotte [10] measured the mechanical properties of wire copper oxides and the ductility of film copper oxide on copper both as a function of temperature. These results indicated that the strength in the oxide decreased rapidly as the temperature was progressively raised above 500 8C. The variation of the friction coefficient is essentially controlled by the mechanical properties of the oxide films formed; a rapid decrease in the shear strength, due to the breaking of this oxide on the wire would bring about a decrease in friction. This behaviour is due to a change in the nature of the oxide or to a local surface melting [11] when electric arcs occur at the contact. Moreover, during severe wear, our observations by microscopy revealed some structural changes induced on the surface layer of copper and it seems that these changes (Fig. 10) depend on the combined action of frictional and normal forces, the antagonistic material, the sliding speed and the intensity of the electrical current. The microhardness, measured on metallic worn wire cross sections obtained at various depths below the surface, is shown in Fig. 11. A large hardness value (400–500 Hv) is observed at the surface; our results reveal that, in the presence of high electrical current intensities, the hardness decreases with increasing depth h and increases with sliding speed v. Subsurface deformation increases and extends about e ¼ 40 mm beneath the surface (Fig. 10). The slight reduction in the wire wear (with respect to I ¼ 0 A) when
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essentially rubbing on copper. An analogous constatation has been mentioned by Montgomery [14] for the sliding of copper alloys against steel. The transfer process can lead to friction fluctuations and modify the wear behaviour in several ways, mainly at higher speeds [15], including the following:
Fig. 10. Schematic cross-sectional view of the wire.
I þ ¼ 40 A (Fig. 5a) is probably due to a change in copper hardness, and also to the oxide film which acts like a lubricant [12]. 4.4. Influence of the sliding speed on metallic transfer During sliding, a transfer of copper occurs from the wire to the track on the disc. Our observations showed that this transfer increases with increasing sliding speed v, applied load P and/or electric current intensity I. Our examinations by optical microscopy showed a mixture film of copper and oxides. This film is in general, fairly thin and uniform over the entire surface [13]. In absence of oxidation, copper is
variation of real contact area; variation at the interface of hardness and ductility; variation of the chemical composition of thin surface films; covering of the sharp hard asperities on disc by a more ductile deposit from the wire. Often the nature of the transferred deposit may be the factor, which determines whether friction variations observed are instantaneous fluctuations (Fig. 3) or longer evolutions in time [16]. Our examinations (I ¼ 0 A) have permitted to verify that as soon as, the transfer of copper on the track of the disc begins, the friction coefficient starts to fluctuate; the fluctuations disappear (Fig. 3c), when the transferred film becomes a stable mixture of copper and oxide of copper. This one plays a lubricant role as a third body. In addition, the transferred metal on the friction track of the disc oxidizes when the right temperature is reached at the interface wire–disc. This occurs at high sliding speeds, important normal loads and strong electrical current intensities.
Fig. 11. Variation of the microhardness (Hv) of the surface layer with depth h below the wire surface (P ¼ 10 N, T ¼ 360 N, v ¼ 1:2 ms1, tf ¼ 60 min, I ¼ 0 A and I ¼ 40 A).
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The traces of oxides observed on the friction track of the disc have two origins: The film oxidized on the wire breaks by mechanical and/or electrical action mainly, because this layer is harder than copper. The removed oxide is mixed with the transferred copper in the contact which leads to reduction of friction and wear. The transferred metallic layer becomes oxidized [17], the degree of oxidation increases with sliding speed. It appears that the wear process is not caused by the copper transfer, but by the oxidation of the transferred copper. Indeed, a decrease in, or an absence of this oxidation decreases the wire wear. These two processes occur simultaneously. Some observations have shown the existence of oxides on the wire and on the disc, but we don’t know if the oxidation of the transferred copper is complete or partial. Under argon, we have shown [4] that unoxidized copper is transferred, but in absence of oxidation, the wear is very low. 4.5. Influence of the sliding speed on wear types developed at the interface 4.5.1. In the absence of electrical current During the first revolutions of the disc (in the transient period) and for a constant sliding speed v, copper is transferred. This transfer continued with time and is exhausted as wear debris. Growing by successive depositions of metal, the detachment of a particle of copper occurs, due to repeated encounters, when it reaches its critical size. The wear debris are large due to the normal load, which is relatively supported by few contact areas. As sliding advances, metallic junctions develop and transfer takes place. The wear developed is the adhesive mild type. At the steady state, which follows the preceding state, the friction coefficient is stable (Fig. 3b). The wear particles are predominantly oxides (black powder) due to the oxygen of the environment, with very little metallic debris. Their average size (diameter) determined by observations in SEM was of the order of 1.5 mm. As the speed increases, the average particles size decreases. It has been shown [18] that the greater part of wear can be attributed to
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the larger particles. Other microscopic examinations of the worn face of the wire are made in evidence the presence of a black layer. The latter can readily adapt itself to substrate irregularities, but it is easily sheared. It seems that the plastic properties of copper oxide CuO [10] could play an important role. Thus, the oxidative wear is the predominant mechanism [19]. 4.5.2. In the presence of electrical current 4.5.2.1. During the transition regime. The transition period is characterised by a relatively low electric contact resistance (Fig. 7) and by a friction coefficient which depends on mechanical and/or electrical parameters. In this stage, the wire–disc contact is metallic and the transfer too, so the initial electric contact resistance and the friction coefficient are low. Then wear debris are formed, in particular, annealed copper particles are detached and they play an abrading role. These increase the friction coefficient and decrease the random character of the electric contacts, leading to low fluctuations of the electric contact resistance. The worn surfaces are bright and metallic in appearance. The result is severe wear. 4.5.2.2. During the stationary stage. When 0 < I < 20 A whatever the value of the sliding speed v, m ¼ f ðtÞ (Fig. 6a) and RC ¼ f ðtÞ (Fig. 7a) present fluctuations. The amplitude of these fluctuations increases with the electrical current intensity. The worn surface of wire is dark and dull in appearance, exhibits oxide layers and microcut marks and shows little surface damages (Fig. 8). The wear debris appear as a mixture of oxides with little particles of copper. The damages of the friction track on the disc increase with sliding speed v and the current intensity I. The oxide layer on the copper face disintegrates and subsequently the severe wear predominates. When 40 I 20 A whatever the polarity of the wire and the sliding speed v, the fluctuations become more important (Figs. 6b, c and 7b). The disc friction track is more deteriorated, contrarily to the preceding case (I < 20 A). The rubber face of the wire is almost totally covered by a very black
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Fig. 12. Optical micrograph (100 ), we note the presence of bared sites (zones) produced by a breaking of the oxide wire (P ¼ 10 N, T ¼ 360 N, v ¼ 1:2 ms1, tf ¼ 60 min, I ¼ 20 A).
layer (Figs. 12 and 13). This oxide film readily adapts itself to the surface of copper wire and adheres strongly to the substrate (Fig. 12). It also acts as a lubricant, reducing the average friction
coefficient m, the wear and the wear rate as mentioned earlier and as shown in Figs. 4 and 5. This oxide layer is therefore sheared by mechanical and/ or electrical action and frequent metal to metal
Fig. 13. Microscopic aspects (100 ) of the worn surface of the wire, we distinguish the scratches due to the action of active particles (P ¼ 10 N, T ¼ 360 N, v ¼ 1:2 m s1, tf ¼ 60 min, I ¼ 20 A).
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contacts are established. These result in higher fluctuations of the friction coefficient and of the electrical contact resistance. As mentioned above, the oxidation of the steel, principally when the disc is anode [20] leads to the rapid formation of hard iron Fe2O3 and chromium Cr2O3, which are released as grains at the sliding surface and strongly abrade the wire and the disk. There are, therefore a mixture of oxide with some other interfacial constituents. Some of the wear particles entrapped between the surface cause ploughing although, some of the debris are pushed out of the interface by the motion. The interfacial mechanisms change, the contact area decreases and the detached particles form a black powder with very little metallic debris. The Fe2O3 forms coarse-grained crystals, which have a preferred orientation on the surface [18]. These crystals cannot readily adapt themselves to the surface irregularities. An important plastic deformation occurs, the roughness of the worn surfaces is high and the deterioration process continues resulting in severe wear. The accumulation of the oxide debris at the contact level results in an increase in the electrical contact resistance. In Fig. 14, one can distinguish the stable and unstable periods corresponding to the cracking and periodic generation of the oxide layer, due to the sticking of the wear grains at the interface. This phenomenon is generally accompanied by strong acoustic emissions implying the passage of the direct contact to the contact across the oxide grains. In this situation, when the friction coefficient drops, the contact potential Vc increases simultaneously [4]. The real contact area of the surface elements
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diminishes, when the thickness of the oxide layers rises and the contact zones are more fragile, mainly when they occur at one grain. The rupture intervens then through the interface grain–metal. The torn grains oxidize more or less quickly as a function of the sliding speed v and the intensity I. They wear out equally and their size diminishes. At this moment starts the phase of stable abrasive wear [21] which governs the tribological behaviour of the couple.
5. Conclusion It stands out from this study that the sliding speed is an important parameter, which has a major effect on the tribological behaviour of the couple: At low speeds, the friction coefficient, the wear and the wear rate increase. At moderate speeds, the friction and the rate of wear diminish whereas the wear rises. At high speeds, the wear increases while the friction coefficient and the wear rate stabilise at limit values. The increase in the temperature at the interface favours the oxidation phenomenon and decreases the mechanical characteristics of copper. The composition, the size and the nature of oxide debris formed at the interface govern the friction and the wear behaviour of the couple. The presence of electrical current through the contact, for a given sliding speed v, activates the oxidation process and is at the origin of fluctuations in friction coefficient (m ¼ f ðtÞ) and electrical contact resistance (Rc ¼ f ðtÞ). It is also responsible for the passage from mild wear mode to severe wear type. The nature of the oxide layer on the wire contact, the structure of underlying copper and/or the oxidation of the transferred metal on the friction track of the disc diminish the friction, the wear and the rate of wear with increasing sliding speed and/or electrical current intensity.
Acknowledgements Fig. 14. The evolution of the potential Vc as a function of time, characterising the noisy behaviour under representative conditions.
The authors would like to thank Prof. H. Khireddine for his help in the preparation of figure captions.
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References [1] D. Paulmier, A. Bouchoucha, H. Zaidi, Vacuum 41 (1990) 2230–2232. [2] V.Ya. Berent, Eurotrib. I/A (1981) 37–44. [3] A. Bouchoucha, et al., Wear 203–204 (1997) 434–441. [4] A. Bouchoucha Thesis, INPL, Nancy, France, 1997. [5] R. Holm, Electric Contacts, fourth ed., Springer, Berlin, 1967. [6] T.F.J. Quinn, Part VII of Fundamentals of Tribology, MIT Press, 1980. [7] A. Bouchoucha, et al., Surf. Coat. Technol. 76–77 (1995) 521–527. [8] J.F. Archard, Wear 2 (1958–1959) 438–455. [9] D. Paulmier, A. Bouchoucha, H. Zaidi, Vacuum 41 (1990) 2213–2216. [10] R.F. Tylecotte, J. Inst. Met. 78 (1950–51) 301.
[11] A. Bouchoucha, D. Paulmier, Rev. Sciences et technologie, Universite´ Constantine, Alge´ rie, vol. 3, 1992, pp. 37–48. [12] A. Senouci, M. Schmitt, A. Bouchoucha, H. Zaidi, D. Paulmier, 14e`me Congre`s Francais de Me´ canique, Toulouse, France, 1999, p. 956. [13] A. Senouci, H. Zaidi, J. Frene, A. Bouchoucha, D. Paulmier, Appl. Surf. Sci. 144–145 (1999) 287–291. [14] R.S. Montgomery, Tech. Rep. ARLCB-TR 82039 (1982) 1–23. [15] A. Begelinger, A.W.J. De Gee, A.S.L.E. Trans. 10 (1967) 124–133. [16] P.J. Blau, Wear 72 (1981) 55–66. [17] A. Bouchoucha, et al., Proc. Nortrib’94 1 (1994) 163–172. [18] K. Gopinath, Wear 71 (1981) 161–178. [19] R.F. Tylecotte, J. Iron Steel Inst., London 196 (1960) 135. [20] N. Cabrera, N.F. Mott, Rep. Prog. Phys. 12 (1949) 163. [21] M. Cocks, J. Appl. Phys. 28 (Part II) (1957) 835–843.
Influence of the electrical sliding speed on friction and wear processes in an electrical contact copper–stainless steel A. Bouchouchaa,*, S. Chekroudb, D. Paulmierc a
Laboratoire de Me´canique, De´partement de Ge´nie Me´canique, Faculte´ des Sciences de l’Inge´nieur, Universite´ Mentouri Constantine, Constantine 25000, Algeria b De´partement de Physique, Universite´ Mentouri Constantine, Constantine25000, Algeria c Laboratoire de Physique et Me´canique des Mate´riaux, CNRS-UMR 7554, Equipe ERMES, ENSEM-INPL, 2 Avenue de la Foreˆt de Haye, 54516 Vandoeuvre-les-Nancy Cedex, France Received 8 June 2003; received in revised form 13 September 2003; accepted 14 September 2003
Abstract Among the various parameters that influence the friction and wear behaviour of a copper–stainless steel couple crossed by an electrical current and in a dry contact is the sliding speed. The tests were carried out under ambient environment and the sliding speed was in the range of 0.2–8 ms1. The electrical current intensity was varied from 0 to 40 A and held constant during each experiment. The normal load was maintained constant corresponding to an average Hertzian stress of 107 Pa. It appears that the friction coefficient and the wear rate increase at first with the speed, reach their maximums, then slowly decrease and tend to constant values. Over the entire range of sliding speeds two types of wear are observed. These latters are essentially mild wear as long as hard debris do not appear at the interface and severe wear when debris consisting of oxides or oxide metal mixture become big enough, they are removed from the surface and have abrasive effect. The results are discussed in terms of observations of wear debris size and composition, wear track study, metallographic study of worn surfaces and friction and electrical contact resistance records. # 2003 Elsevier B.V. All rights reserved. Keywords: Friction; Wear; Sliding speed; Electric contacts; Abrasion; Oxide films
1. Introduction The copper–steel couple was used to transfer the electrical current from the catenary (copper) to the collector (steel) but, previously the noticed high wear rate has led to the present use of the copper–graphite couple. However, it is interesting to highlight the friction and wear processes in the case of copper–steel couple. These depend on the using conditions [1], especially
*
Corresponding author. Tel.: þ213-31632434; fax: þ213-31632434. E-mail address: [email protected] (A. Bouchoucha).
on the sliding speed which appears as an important operating parameter determining the useful service life of the couple. The sliding speed has an effect to increase the interface temperature by heating. Indeed, the local temperatures can attain values leading to the thermal softening of the copper. This affects the metallic structure [2] and the mechanical properties of the surface layers [3]. High and moderate sliding speeds, in presence of an electrical current, favour the formation of hard oxides and in addition to the adhesive wear, the abrasive one occurs at the interface. The results are discussed essentially taking into account the theoretical analysis and experimental phenomenon due to the friction.
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.09.018
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2. Experimental details 2.1. Apparatus and procedure The experiments were carried out with a pin on disc system tribometer, which was modified in tensioned wire-on-disc system, as described elsewhere [4]. The disc was replaced by a side wheel with 16 sectors (Fig. 1), and the pin by a U-shaped frame over which a copper wire was stretched [4]. The tension force T of the wire was measured by strain gauges that were glued on the arm. The normal load was applied vertically with a weight P. The horizontal wheel was driven by an electric motor with variable but controlled speed. The friction coefficient m ¼ F=P is deduced from measurements of the tangential force F induced on the arm by rotating wheel through the wire. The direct electric current I is brought to the disc through a mercury contact in the axis of rotation (Fig. 2) to avoid the effect of centrifugal force. The electric contact resistance Rc was recorded using an appropriate device. The test duration tf was 60 min and the wear W was determined by weighing the wire before and after the experiment.
Fig. 2. Turning in contact mercury, 1: electrical wire conducting I, 2: mercury, 3: head of screw, 4: disc, 5: rotating axis.
2.2. The specimens The disc was made of stainless steel (Cr18, Ni10 and balance Fe), with 120 mm in diameter and 30 mm thick. The wire was made of copper of high purity (99.98%), 2.2 mm in diameter. Before use, the copper wires were annealed under vacuum for 2 h at 600 8C. Metallographic sections were prepared from specimens in order to compare their hardness before and after the test.
3. Results 3.1. Influence of the sliding speed on friction 3.1.1. Evolution of the friction coefficient versus time The general evolution of the friction coefficient with time is given in Fig. 3. Two distinct zones can easily be identified on these curves: An initial unsteady phase: during this period, the friction coefficient m is stable (no fluctuations) and evolves between two extremes mmin and mmax. A steady state: during this phase, which follows the preceding state, the equilibrium stage depends on the operating conditions at the interface.
Fig. 1. Schematic representation of disc with 16 sectors.
However, in the absence of an electric current, at low sliding speeds, small oscillations are noticed in the friction coefficient (Fig. 3a). At moderate sliding speeds they disappeared almost totally (Fig. 3b) and
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Fig. 3. Evolution of friction coefficient with time, (a) v ¼ 0:3 m s1, (b) v ¼ 1:2 m s1 and (c) v ¼ 7:2 m s1.
at higher sliding speeds (Fig. 3c) the friction coefficient fluctuates 10–15 min and then stabilises. 3.1.2. Influence of the sliding speed on the friction coefficient Fig. 4 shows the variation of the average friction coefficient m with sliding speed v in stationary regime. On this curve, we can observe three zones: 0:25 < v < 1:2 m s1: in this interval, the friction coefficient m slightly increases from the value 0.54 to reach a maximum of the order of 0.60 at around v ¼ 1:2 m s1.
5 > v 1:2 m s1: in the second zone, with increasing speed, the friction coefficient m notably diminishes to a limit value ml about m ¼ 0:40. 7:2 v 5 m s1: this region is characterised by a relative stability of friction coefficient m at the limit value ml. 3.2. Influence of the sliding speed on wear and wear rate The effect of sliding speed on the wear rate k (Fig. 5) is nearly analogous to that on friction coefficient.
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as shown in Figs. 4 and 5 when I þ ¼ 40 A (anodic wire).
4. Discussion 4.1. Influence of the sliding speed on friction and wear
Fig. 4. Variation of the average friction coefficient with speed for I ¼ 0 and I þ ¼ 40 A.
In particular, the wear rate (wear per unit sliding distance) is maximum for speed values around v ¼ 12 m s1 and then, decreases and becomes constant. The same figure shows the variation in wear with sliding speed, we notice that as the sliding speed increases, the wear varies approximately linearly. 3.3. Influence of the heat generated by mechanical friction and Joule effect on friction and wear An increase in sliding speed, as well as in the normal load and/or the electric current intensity generates a raise in the temperature at the contact. Consequently, the oxides formation is facilitated. However, whatever the value of sliding speed v, the passage of electrical current across the contact leads to fluctuations in m ¼ f (t) (Fig. 6) and RC ¼ f (t) (Fig. 7) in the stabilised regime. The amplitude of these fluctuations increases with increasing electric current intensity. The electrical current has no significant influence, on the average friction coefficient and the wear rate, except a slight decrease beyond 20 A approximately
In the domain of low speeds: 1:2 > v 0:25 m s1, the friction coefficient and the wear reach high values because the contact time is long enough, so that metallic bondings raise and the actual contact area widens out. This leads to oscillations in the friction coefficient (Fig. 3a) due to the decohesion and roughness of the worn surfaces. When the sliding speed is between 1.2 and 5 m s1, first the bonding growth time decreases; second the temperature increases with their effect on the oxidation phenomenon. The number of metallic bondings is reduced by the formation of an oxide layer on the sliding surface of the wire. Our observations (optical microscopy) showed that this layer is dense, regular and fits the irregularities of the surface on which it is firmly bounded (Fig. 8). As a result, this layer protects the surface from wear. The electric contact resistance increases (when I 6¼ 0 A) however (for I ¼ 0 A), the friction coefficient, the wear rate, the fluctuations in friction and the surface roughness decrease. When the temperature increases, the layer becomes ductile and the oxide behaves like a lubricant. At speeds of 5–8 m s1, as sliding speed increases, the surface temperature increases and then the oxidation rate increases too, this reduces the number of metallic contacts and the amount of metal transfer. The oxide layer disintegrates, the strength of the substrate is lower and the contacts break and the copper softens [5]. Plastic deformations of the subsurface extend to lesser depth surface. The total wear increases almost linearly, while the friction coefficient remains constant around 0.41. 4.2. Influence of the sliding speed on wear rate Under air (when I ¼ 0 A), the wear is of the soft adhesive mode and the oxide layer must reach a minimal depth before breaking [6]. This depends on
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Fig. 5. Wear of the copper wire vs. sliding speed with and without electrical current (a) W ¼ f ðvÞ and (b) k ¼ f ðvÞ.
time and on surface temperature. If the temperature was independent of the sliding speed (case of low speeds), the quantity of removed oxide would be independent of the speed, and the wear rate, as an inverse ratio of the speed, would decrease when the sliding speed increases. For high sliding speeds, the surface temperature increases and the oxide formation speeds up [7]. In this case, the wear rate keeps the same value and appears independent of the sliding speed. This phenomenon is observed for speeds over 5 m s1. Moreover, when the copper is anode, the growth of the copper oxidation layer is higher while the one of steel becomes lower. In this situation, we observe experimentally that the copper wear is low. When the copper is the cathode, the opposite occurs. This is due to the fact that a high steel oxidation leads to the rapid formation of hard iron and chrome oxides which are released as grains at the sliding interface, thus abrading strongly the wire. Our observations under microscope confirm this abrading action. Consequently, the variations in the wear and the wear rate with speed have tendency to decrease when I þ ¼ 40 A (Fig. 5).
4.3. Influence of the heat released by mechanical friction and Joule effect on friction coefficient and wear 4.3.1. Calculation of the contact area In these experiments, the value of the normal load is 10 N. The radius of the circle equivalent to the total contact area (Fig. 9) is: 1=2 P a¼ pH where H is the hardness of the softer material. 4.3.2. Calculation of the contact temperature According to the Archard method [8], the total heat quantity released by friction and Joule effect in the interface is: Q ¼ Qd þ Qw where Qd is the heat quantity per unit of time to the rotating disc, and Qw is the heat quantity transmitted by a mobile source per unit time to the wire.
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Fig. 6. Influence of the current intensity on the evolution of m when v is fixed (a) I ¼ 10 A, (b) I ¼ 20 A and (c) I ¼ 40 A.
Q is shared between the two surfaces so that the rise in temperature Dym is the same on each side. When the sliding speed is low, the temperature variation Dym of the wire contact surface is given by the following formula: Qw Dym ¼ 4alw
However, on the disc contact surface the rise of temperature is given by: Dym ¼
Qd 4ald
where lW and ld are the thermal conductivities of the copper wire and of the steel disc, respectively:
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Fig. 7. Evolution of the electrical contact resistance Rc with time (a) I ¼ 10 A and (b) I ¼ 40 A.
lw ¼ 385:8 J m1 s1 8C1 and ld ¼ 14:9 J m1 s1 8C1. It appears that the wire exhausts a heat quantity 25 times greater than the disc.
Fig. 8. Optical micrograph (100 ) of the worn copper showing the adaptation of oxide layer to the substate (P ¼ 10 N, T ¼ 360 N, v ¼ 1:2 m/s, tf ¼ 60 min, I ¼ 0 A).
Fig. 9. Schematic diagram of sliding contact wire–disc.
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For high sliding speeds, Dym is given by the relation: 1 0:31Qw w 2 Dym ¼ 4alw va where the thermal diffusivity of copper is w ¼ l=rc, r is the density of the material and c is the specific heat. For moderate speeds, the expression used for Dym is: Qw Dym ¼ a 4alw with a varying from 0.85 to 0.35 and Qd ¼ mPv þ RC I 2 . The limits between low and high speeds are given by the values of a parameter L: va L¼ 2w Low speeds are considered when L < 0:1, for moderate speeds 0:1 < L < 5 and high speeds are considered when L > 5. As the total transfer coefficient of the wire to exhaust the generated heat is low, the equilibrium wire temperature varies between 30 and 350 8C. The calculated values show that the temperature at the interface during the sliding can rise up to 600 8C [9]. 4.3.3. Determination of the thickness of the oxide layer Using equation [5]: rc r0 X þ 2a pa2 where Rc is the contact resistance, rc is the electrical resistivity of copper, r0 is the electrical resistivity of a thin layer of copper oxide and X is the thickness of the oxide layer. The first term of the sum represents the constriction resistance and the second term the resistance of oxide layer. The values of r0 are suspected to be of the order of 4.103 O m, Rc value measured in our experiments is about 12.103 O when the conditions lead to a significant oxidation. ˚ for a ¼ 50 mm, when In these cases, X ¼ 240 A breakage occurs in the copper oxide (H ¼ 13 108 ˚ for a ¼ 100 mm, when breakN m2) and X ¼ 950 A age occurs in the copper metal (H ¼ 4 108 N m2). Rc ¼
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Table 1 Influence of the speed on the rise in temperature at the interface with and without electrical current I (A) 0
40
˚) X (A
v (m/s)
Qf (W)
Dy (8C)
50
240
100
950
50
240
100
950
1.2 7.2 1.2 7.2 1.2 7.2 1.2 7.2
7.2 29 7.2 29 16.8 38.6 16.8 38.6
74 205 37 74 174 275 87 100
a (mm)
Table 1 gives the results with and without electrical current for two values of contact circle radius and two values of sliding speed. Under dry sliding conditions, the temperature can rise up to 600 8C. Tylecotte [10] measured the mechanical properties of wire copper oxides and the ductility of film copper oxide on copper both as a function of temperature. These results indicated that the strength in the oxide decreased rapidly as the temperature was progressively raised above 500 8C. The variation of the friction coefficient is essentially controlled by the mechanical properties of the oxide films formed; a rapid decrease in the shear strength, due to the breaking of this oxide on the wire would bring about a decrease in friction. This behaviour is due to a change in the nature of the oxide or to a local surface melting [11] when electric arcs occur at the contact. Moreover, during severe wear, our observations by microscopy revealed some structural changes induced on the surface layer of copper and it seems that these changes (Fig. 10) depend on the combined action of frictional and normal forces, the antagonistic material, the sliding speed and the intensity of the electrical current. The microhardness, measured on metallic worn wire cross sections obtained at various depths below the surface, is shown in Fig. 11. A large hardness value (400–500 Hv) is observed at the surface; our results reveal that, in the presence of high electrical current intensities, the hardness decreases with increasing depth h and increases with sliding speed v. Subsurface deformation increases and extends about e ¼ 40 mm beneath the surface (Fig. 10). The slight reduction in the wire wear (with respect to I ¼ 0 A) when
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essentially rubbing on copper. An analogous constatation has been mentioned by Montgomery [14] for the sliding of copper alloys against steel. The transfer process can lead to friction fluctuations and modify the wear behaviour in several ways, mainly at higher speeds [15], including the following:
Fig. 10. Schematic cross-sectional view of the wire.
I þ ¼ 40 A (Fig. 5a) is probably due to a change in copper hardness, and also to the oxide film which acts like a lubricant [12]. 4.4. Influence of the sliding speed on metallic transfer During sliding, a transfer of copper occurs from the wire to the track on the disc. Our observations showed that this transfer increases with increasing sliding speed v, applied load P and/or electric current intensity I. Our examinations by optical microscopy showed a mixture film of copper and oxides. This film is in general, fairly thin and uniform over the entire surface [13]. In absence of oxidation, copper is
variation of real contact area; variation at the interface of hardness and ductility; variation of the chemical composition of thin surface films; covering of the sharp hard asperities on disc by a more ductile deposit from the wire. Often the nature of the transferred deposit may be the factor, which determines whether friction variations observed are instantaneous fluctuations (Fig. 3) or longer evolutions in time [16]. Our examinations (I ¼ 0 A) have permitted to verify that as soon as, the transfer of copper on the track of the disc begins, the friction coefficient starts to fluctuate; the fluctuations disappear (Fig. 3c), when the transferred film becomes a stable mixture of copper and oxide of copper. This one plays a lubricant role as a third body. In addition, the transferred metal on the friction track of the disc oxidizes when the right temperature is reached at the interface wire–disc. This occurs at high sliding speeds, important normal loads and strong electrical current intensities.
Fig. 11. Variation of the microhardness (Hv) of the surface layer with depth h below the wire surface (P ¼ 10 N, T ¼ 360 N, v ¼ 1:2 ms1, tf ¼ 60 min, I ¼ 0 A and I ¼ 40 A).
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The traces of oxides observed on the friction track of the disc have two origins: The film oxidized on the wire breaks by mechanical and/or electrical action mainly, because this layer is harder than copper. The removed oxide is mixed with the transferred copper in the contact which leads to reduction of friction and wear. The transferred metallic layer becomes oxidized [17], the degree of oxidation increases with sliding speed. It appears that the wear process is not caused by the copper transfer, but by the oxidation of the transferred copper. Indeed, a decrease in, or an absence of this oxidation decreases the wire wear. These two processes occur simultaneously. Some observations have shown the existence of oxides on the wire and on the disc, but we don’t know if the oxidation of the transferred copper is complete or partial. Under argon, we have shown [4] that unoxidized copper is transferred, but in absence of oxidation, the wear is very low. 4.5. Influence of the sliding speed on wear types developed at the interface 4.5.1. In the absence of electrical current During the first revolutions of the disc (in the transient period) and for a constant sliding speed v, copper is transferred. This transfer continued with time and is exhausted as wear debris. Growing by successive depositions of metal, the detachment of a particle of copper occurs, due to repeated encounters, when it reaches its critical size. The wear debris are large due to the normal load, which is relatively supported by few contact areas. As sliding advances, metallic junctions develop and transfer takes place. The wear developed is the adhesive mild type. At the steady state, which follows the preceding state, the friction coefficient is stable (Fig. 3b). The wear particles are predominantly oxides (black powder) due to the oxygen of the environment, with very little metallic debris. Their average size (diameter) determined by observations in SEM was of the order of 1.5 mm. As the speed increases, the average particles size decreases. It has been shown [18] that the greater part of wear can be attributed to
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the larger particles. Other microscopic examinations of the worn face of the wire are made in evidence the presence of a black layer. The latter can readily adapt itself to substrate irregularities, but it is easily sheared. It seems that the plastic properties of copper oxide CuO [10] could play an important role. Thus, the oxidative wear is the predominant mechanism [19]. 4.5.2. In the presence of electrical current 4.5.2.1. During the transition regime. The transition period is characterised by a relatively low electric contact resistance (Fig. 7) and by a friction coefficient which depends on mechanical and/or electrical parameters. In this stage, the wire–disc contact is metallic and the transfer too, so the initial electric contact resistance and the friction coefficient are low. Then wear debris are formed, in particular, annealed copper particles are detached and they play an abrading role. These increase the friction coefficient and decrease the random character of the electric contacts, leading to low fluctuations of the electric contact resistance. The worn surfaces are bright and metallic in appearance. The result is severe wear. 4.5.2.2. During the stationary stage. When 0 < I < 20 A whatever the value of the sliding speed v, m ¼ f ðtÞ (Fig. 6a) and RC ¼ f ðtÞ (Fig. 7a) present fluctuations. The amplitude of these fluctuations increases with the electrical current intensity. The worn surface of wire is dark and dull in appearance, exhibits oxide layers and microcut marks and shows little surface damages (Fig. 8). The wear debris appear as a mixture of oxides with little particles of copper. The damages of the friction track on the disc increase with sliding speed v and the current intensity I. The oxide layer on the copper face disintegrates and subsequently the severe wear predominates. When 40 I 20 A whatever the polarity of the wire and the sliding speed v, the fluctuations become more important (Figs. 6b, c and 7b). The disc friction track is more deteriorated, contrarily to the preceding case (I < 20 A). The rubber face of the wire is almost totally covered by a very black
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Fig. 12. Optical micrograph (100 ), we note the presence of bared sites (zones) produced by a breaking of the oxide wire (P ¼ 10 N, T ¼ 360 N, v ¼ 1:2 ms1, tf ¼ 60 min, I ¼ 20 A).
layer (Figs. 12 and 13). This oxide film readily adapts itself to the surface of copper wire and adheres strongly to the substrate (Fig. 12). It also acts as a lubricant, reducing the average friction
coefficient m, the wear and the wear rate as mentioned earlier and as shown in Figs. 4 and 5. This oxide layer is therefore sheared by mechanical and/ or electrical action and frequent metal to metal
Fig. 13. Microscopic aspects (100 ) of the worn surface of the wire, we distinguish the scratches due to the action of active particles (P ¼ 10 N, T ¼ 360 N, v ¼ 1:2 m s1, tf ¼ 60 min, I ¼ 20 A).
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contacts are established. These result in higher fluctuations of the friction coefficient and of the electrical contact resistance. As mentioned above, the oxidation of the steel, principally when the disc is anode [20] leads to the rapid formation of hard iron Fe2O3 and chromium Cr2O3, which are released as grains at the sliding surface and strongly abrade the wire and the disk. There are, therefore a mixture of oxide with some other interfacial constituents. Some of the wear particles entrapped between the surface cause ploughing although, some of the debris are pushed out of the interface by the motion. The interfacial mechanisms change, the contact area decreases and the detached particles form a black powder with very little metallic debris. The Fe2O3 forms coarse-grained crystals, which have a preferred orientation on the surface [18]. These crystals cannot readily adapt themselves to the surface irregularities. An important plastic deformation occurs, the roughness of the worn surfaces is high and the deterioration process continues resulting in severe wear. The accumulation of the oxide debris at the contact level results in an increase in the electrical contact resistance. In Fig. 14, one can distinguish the stable and unstable periods corresponding to the cracking and periodic generation of the oxide layer, due to the sticking of the wear grains at the interface. This phenomenon is generally accompanied by strong acoustic emissions implying the passage of the direct contact to the contact across the oxide grains. In this situation, when the friction coefficient drops, the contact potential Vc increases simultaneously [4]. The real contact area of the surface elements
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diminishes, when the thickness of the oxide layers rises and the contact zones are more fragile, mainly when they occur at one grain. The rupture intervens then through the interface grain–metal. The torn grains oxidize more or less quickly as a function of the sliding speed v and the intensity I. They wear out equally and their size diminishes. At this moment starts the phase of stable abrasive wear [21] which governs the tribological behaviour of the couple.
5. Conclusion It stands out from this study that the sliding speed is an important parameter, which has a major effect on the tribological behaviour of the couple: At low speeds, the friction coefficient, the wear and the wear rate increase. At moderate speeds, the friction and the rate of wear diminish whereas the wear rises. At high speeds, the wear increases while the friction coefficient and the wear rate stabilise at limit values. The increase in the temperature at the interface favours the oxidation phenomenon and decreases the mechanical characteristics of copper. The composition, the size and the nature of oxide debris formed at the interface govern the friction and the wear behaviour of the couple. The presence of electrical current through the contact, for a given sliding speed v, activates the oxidation process and is at the origin of fluctuations in friction coefficient (m ¼ f ðtÞ) and electrical contact resistance (Rc ¼ f ðtÞ). It is also responsible for the passage from mild wear mode to severe wear type. The nature of the oxide layer on the wire contact, the structure of underlying copper and/or the oxidation of the transferred metal on the friction track of the disc diminish the friction, the wear and the rate of wear with increasing sliding speed and/or electrical current intensity.
Acknowledgements Fig. 14. The evolution of the potential Vc as a function of time, characterising the noisy behaviour under representative conditions.
The authors would like to thank Prof. H. Khireddine for his help in the preparation of figure captions.
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