- Email: [email protected]
Thermal and Tribological Analysis of the Dry Sliding Steel-steel Couple Traversed by an Electrical Current
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 55 (2014) 165 – 172
Eight International Conference on Material Sciences (CSM8-ISM5)
Thermal and tribological analysis of the dry sliding steelsteel couple traversed by an electrical current C. Boubechoua,b ; A. Bouchouchab; H. Zaidic ; Y.Mouadjib a
Faculté de Technologie, Département de Génie Mécanique, Université 20 Août 1955, Skikda 21000, Algérie. Laboratoire de Mécanique, Faculté des Sciences de l'Ingénieur, Département de Génie Mécanique, Université Constantine 1, 25000, Algérie. c Laboratoire LMS (UMR-6610-CNRS), SP2MI, Téléport 2, Boulevard Marie et Pierre Curie, Université de Poitiers, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France. b
Abstract This study concerns a thermal and tribological analysis of the dry sliding steel-steel couple traversed by an electrical current. The tests were carried out by using a tribometer pin-disc under ambient air environment. The dry friction and wear of this contact are studied with different parameters such as normal load, electrical current and sliding speed (maintained constant V = 0.5 m/s). The test duration is 20 mn. The experimental results obtained show that these parameters have a more or less significant effect on the tribological behavior of the couple. Indeed, the oxidation phenomenon, the particles of wear resulting from this oxidation, their composition, their morphology and their thermal-mechanical properties, under certain conditions, play an important role and determine the life service of this couple. To highlight the effect of these parameters, theoretical calculations based on the theories of Archard and Holm have been done. These calculations allowed us to evaluate the order of magnitude of the mechanical, geometrical, thermal and electrical parameters. The discussion of the results is mainly based on these calculations, optical and SEM observations as well as other phenomena resulting from the friction process. © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2013 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection under and/or peer-review under responsibility ofCSM8-ISM5 CSM8-ISM5 Peer-review responsibility of the Organizing Committee of
Key words: friction, wear, steel, oxidation, electrical contact, wear particles, temperature.
*C. Boubechou E-mail : [email protected]
1875-3892 © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of CSM8-ISM5 doi:10.1016/j.phpro.2014.07.024
166
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
Nomenclature P[N] normal load 'Tm[°C]interface temperature of the pin a[Pm] contact radius H[MPa]
hardness
Q[J]
the heat quantity loss of the pin
V[m/s]
sliding speed
D
coefficient of heat repartition
Rc[:]
electric contact resistance
][Pm]
thickness oxide layer
O[ W/m.K] thermal conductivity U0[:.m]
electrical resistivity of oxide
I[A]
electrical current
CP[J/Kg.K] thermal capacity J>P:m]
resistivity
VC[V]
electrical contact potentiel difference
A%
lengthening
1. Introduction The industrial problems are complex by the need to optimize the life service of the different elements, and involve many complex phenomena such as friction between the antagonistic surfaces, and overheating caused to the interface [1]. The wear processes in the sliding electrical contacts are affected by the contact temperature, the microstructural transformations, and the formation of tribochemical film, the fusion of surfaces in contact or the failures induced by the mechanical, thermal and electrical stresses [1]. In fact, the increase in local temperature and loss of material in the wear tracks are the result of energy dissipation in the friction contact zone [2]. The aim of this paper is to study the influence of the thermoelectromecanical parameters on the dry friction and wear behavior of steel-steel couple. 2. Experimental device The experimental device is a pin-on-disc tribometer. Tests are carried out in an ambient air (Fig.1). The pin (XC38) and disc (XC48) are made of steel. The disc is driven in a rotational movement of constant velocity with diameter of 50 mm and 12 mm of thickness. It has a lame hole in the center. The normal load is vertically applied with a weight P. The pin has a cylindrical shape of length 20 mm and a diameter
167
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
of 8mm; has a flat allows to o fix it in a holee with a screw w on an arm loaad of aluminum m. Friction coefficient P is deduced from measurement of the tangential forcce F induced on the arm by b the rotatingg wheel b and afteer each through the pin (P = F / P). The wear W is determinedd by weighing the specimen before b with a precision of ± 0.1 mg. Thee direct electriccal current I is brought to thhe disc test using a balance through a merrcury contact in n the axis of rootation to avoidd the effect of the centrifugal force. To enssure the same experim mental condition ns for each testt, the disc is poolished mechannically with a 1200 1 grain paper.
Fig.1. Tribometter pin-disc
3. Materials The chemical composition is presented herre in table 1 Tablee 1. Chemical composition c C C%
Si %
Mn %
Cr %
XC38
0 0.38
0.30
0.50
0.040
XC48
0 0.51
0.4
0.75
-
m and d physical charracteristics are presented in taable 2. Whereas the mechanical Table 2. Mechhanical and phhysical characteeristics HB
Re [N N/mm2]
Rm [N/m mm2]
A%
XC448
200
550
710
15
XC38
190
490
630
17
3 U [Kg/m [ ]
Cp[J/Kgg.K]
O[W/m.K]
J[P:m]
78775
490
53
0.21
4. Modeling 4.1. Evaluatioon of the real contact area
F
The real suurface of contacct between pinn-disc is obtaineed from the folllowing relatioon: P P H .S .a 2 , a P .P 0 ,6.H .S .a 2 (1) H .S
4.2. Evaluatioon of the contacct temperaturee
(2)
168
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
The quantitty of heat Q distributed betw ween two surfacces leads to a rise r in the meann temperature 'Tm at the interface. We suppose th hat 'Tm is the same in the tw wo solids. The heat flux throough each comp mponent nly on: the meechanical param meters, the intensity of the electrical e curreent, the of the couple depends main c and the number of Peeclet L [2]: geometry of thhe contact, the heat-storage capacity L
V.a , F 2.F
O U.Cp
(3)
Where D = - 0.102L+ 0 0.860 x
if L < 0.1, the interrface temperatuure of the pin iss expressed by: ǻTm
x
if 0.11 < L < 5, ǻTm
x
if L > 5.0,
ǻTm
D.
Qp 4.a.O p
Qp
(4) (5)
4.a.O p
0.31Qp
Fp
a.O p
V.a
(6)
The heat quuantity loss of the pin per uniit of time is exppressed by: Q p Qm Qe Qa x withh arc and electrrical current: Q p Qm Qe Qa P .P.V Rc .I 2 V C .I
(7) (8)
Where: Qm thee mechanical heat h , Qe the electrical heat andd Qa the heat generated g by eleectrical arcs 4.3. Thicknesss oxide layer The thicknness of the oxid de layer is exprressed by: ] c
S .a 2 .Rc Uo
(9)
5. Results b 5.1. Friction behavior Figure 2 shows s the evo olution of the friction coeffficient with tim me parameter.. We observe that P increases from m (μ ~ 0. 50) to o reach a maxim mum value of about a 0.65 thenn stabilizes at a value of 0.600. Figure 3 shhows the evolu ution of frictionn coefficient with normal loadd. The friction coefficient ȝ evolves e gradually withh load.
Fig. 2. Variationn of friction coefficcient versus time (V V = 0.5 m/s; P = 18.5 1 N and I = 2A)) Fig. 3. Variation of friction coefficiient versus normall load (V= 0.5 m/s; I = 2 A and t = 20 2 mn)
Figure 4 presents p the vaariation of thee friction coeff fficient with ellectric current.. In this curvee, three distinct zones can be identiffied: m valuee of 0.60. x zonee I: friction coeefficient evolvees gradually froom 0.50 to a maximum x zonee II: friction coefficient c ȝ decreases d signnificantly with the increasinng of current until u it reaches a value of about 0.43. oefficient ȝ staabilizes at a vallue of 0.43. x zonee III: friction co
169
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
Zone I
zone II
zone III
Fig. 4. Variation of friction coefficiient versus electriccal current (P = 188.5 N; V= 0.5m/s and a t = 20 mn)
5. 2. Wear behhavior The evoluttion of wear with w the normal load is shown in figure 5. 5 This curve shows that thhe wear increases withh the applied load. l The evollution of wear according to the t intensity of o electrical currrent is represented grraphically in fiigure 6. Two phhases are also noted: n x phase I: for the low w values I = 2A A, wear is stabillized in the neiighborhoods off W = 0.135 g. x phase II: wear decrreases graduallyy with the intennsity of electriical current unttil W = 0.04 g.
Phase I
Phase II
Fig. 5. Wear variaation with normall load (V= 0.5 m/ss; I = 2 A and t = 20 2 mn) Fig. 6. Wear variaation versus electrrical current (P = 18.5 1 N; V= 0.5m/s and t = 20 mn)
'T[°C]
5. 3. Calculating parameterss me, then the ratio r Op / Od = 1. The Like the thhermal conducctivity of steels XC38and XC48 is the sam coefficient off total transfer;; with outside, is reduced, thhe temperaturee of the pin risses according to t time until thermal balance b (table 3). x for a rupture in steeel with: P =18.5 N, H = 200 Mpa and P = 0.6, we obtainn a = 170 Pm. x for a rupture in ferrric oxide Fe2O3, H =103 Mpa,, we obtain a = 70 Pm. w oxide, versus This figuree shows the theeoretical variattion of the conntact temperatuure, with and without the real contacct area. We observe that 'Tm decreases signnificantly withh the increasingg of real contacct area. Figure 8 show ws that the temp perature of conntact is inversely proportional to the normall load. ȴ ȴTtwithoxide
ȴ ȴTtwithoutoxide
1,000.00 800.00 600.00 400.00 200.00 0.00 5EͲ05
0.0001
0.00 0015
0.0002
0.00025 a[Pm]
Fig.7. Variation of o the contact temp perature with and without w oxide verssus radius contact (V = 0.5m/s, I = 2A 2 and t = 20mn) Fig.8. Variation of o the contact temp perature with and without w oxide verssus normal load (V V = 0.5m/s, I = 2A A and t = 20mn)
170
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172 Table 3: Values of thermal, geometrical, electrical and mechanical parameters I [A]
0
2
76
170
76
170
L
0.84
3.11
0.84
3.11
D
0.43
0.54
0.43
0.54
Qp [J]
2.40
2.78
16.40
16.78
'7m [°C]
65.82
38.40
449.5
232
a [Pm]
Rc [:]
-
3.5
3.5
] [Pm]
-
17
84
6. Discussion 6.1. Effect of the time parameter on the friction and wear During the first period of contact between the elements of the couple, the wear particles are metallic in nature; the mutual transfer at the interface is also metal. There is very little debris in the contact and the friction coefficient is 0.65. Over time the debris formed, wear passive are pulled out and they have no effect on the tribological behavior. However, the active wear particles play an abrasive role in friction and wear behavior. In effect, they increase friction and reduce the randomness of the electrical contacts [11]. The increase of the friction coefficient, during the first moments of sliding (Fig. 2), is due to the interaction between the asperities of the opposing surfaces. Indeed, as and when the dynamic contact continues, the adaptation of surfaces occurs and the mechanism of formation and rupture of junctions stabilized by balance of operating conditions at the interface (running late). This increase is explained by the increase in real area of contact deformation and creep. This surface shape generally elliptical deforms and elongates in the direction of sliding. In the second period, the friction coefficient is more stable. This stability results from the accommodation of the interface friction. The asperities are sheared and the beginning of a significant production of fine particles from both surfaces is clearly observed [3]. This production and the rise of the mean interfacial temperature induce a stable friction coefficient. The changes in structure and composition of surfaces and the surface oxidation phenomenon and the mutual process of transfer of particles govern the friction and wear behavior of the couple [4]. 6.2. Effect of normal load on friction and wear When the charge increases from 5 to 15 N, the friction coefficient follows a nearly linear curve (Fig. 2). For loads greater or equal to 15 N, the electrical contact resistance stabilizes at 3.5 :. Most particles are metallic, the friction coefficient increases linearly with the load. This result in an increase of the real area of contact and the friction coefficient remains substantially constant. It is always in an adhesive real area of contact. Indeed, J. and R. Blouet [5] found that there is always a critical load beyond which the wear increases significantly (Fig. 5). In the first part of the curve, under low load, the wear is almost linear with the load (up to 15N). The increase in load, results in additional wear and probably an increase in the number of contact points and then by an increase in the density of junctions. At these pressures the critical yield stresses are reached, plastic deformation of asperities contributes to increase the contact area and promotes adhesion [9].
171
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
6.3. Effect of electric current on the friction and wear The passage of electric current intensity of 2 A does not contribute significantly to oxidation. However, there is a significant effect on the wear when the current exceeds 2 A. The decrease in wear is about 7 times less than low or no current (I d 2 A). This is explained by the fact that the heating caused by Joule effect softens the steel XC38 which is a soft material [6]. Under these conditions, the friction coefficient gradually increases (when I increases) and then stabilizes and reaches lower values (when I = 3A). The increase in the electrical contact resistance reflects the existence of oxide films and other materials at the interface [7]. The high steel oxidation leads to the rapid formation of hard asperities and thin layer of oxide, then the electrical contact resistance increases. The phenomenal electrical arcs appear to form craters. These craters are 10-20 μm in diameter. 6.4. Effect of temperature rise on friction and wear The contact temperature is an invaluable element for understanding the thermal and tribological behavior of materials rubbing together. Actually, the mechanical power transmitted through a sliding contact is mainly dissipated as heat at the interface between the two contacting materials steel-steel (table 1) The resulting temperature rise can strongly affect the surface properties of the materials [10] (Fig.8), favors physico-chemical and microstructural changes and modify the rheology of interfacial elements trapped in the contact zone [8]. When this contact is crossed by an electrical current, in addition to the heat generated by friction, is added the Joule effect. Metal surfaces of a sliding contact are the site of chemical reactions and oxidation. The fresh surfaces created by abrasive wear are exposed to ambient oxygen and the moisture of the air. 6.5. Analysis of worn surfaces Analyses by scanning electron microscopy and quantitative analysis are performed on samples of steel XC38 having rubbed on steel XC48 for applied loads of 5 N and 18.5 N at a constant speed V= 0.5 m/s for 30 minutes (Fig. 9). We have observed the following: - the micrographs of figures 9(a) and (b) show the abduction and cut away the surface material and the presence of more or less deep furrows plowed in the direction of movement of the rubbing surfaces. Wear debris present on the surface are the cause of this behavior. - X-ray microanalysis of some debris showed the presence of several elements (Figs.9 (c) and (d)), in particular a high peak of iron, a small peak of chrome and oxygen and very little other constituents. This is probably iron oxides, oxides of Cr and other elements that their presence is due to a mutual transfer between the opposing surfaces. The amount of elements transferred depends mainly on the speed and amperage parameters.
(a)
(b)
(c)
(d)
Fig.9. SEM images and X-ray analysis of the worn face of the pin (× 100): ((a) P = 5N, V = 0.5m/s and I = 2A; (b) P = 18.5N)
172
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
7. Conclusion The effect of the thermoelectromechanical parameters on the tribological behavior of steel-steel dry couple sliding leads to the following conclusions: - for low values of the electrical current less than 2 A, the friction coefficient increases gradually. However, there is no effect on wear. - for values of the electrical current higher than 2 A, the friction coefficient and wear have a decreasing form. - the presence of the electrical current activates the oxidation of wear debris and leads to an elevation of mean interface temperature which plays an important role in the modification of material properties and the oxidation phenomena. - the real contact zones are evaluated and the radius of the circular form is determined by taking into account the oxidation phenomenon (a = 70 to 170 μm). In these conditions, the critical thickness of oxide coatings evolutes between 17 Pm and 8 4 Pm. - wear process of sliding electrical contact is modified continuously by its mechanical parameters according to the electrical current intensity. Surfaces can be damaged by abrasive wear, by oxidation or by electrical arcs at the contact. References [1] A. Bouchoucha et Al, Wear 203-204 (1997) 434-441. [2] A. Bouchoucha et Al. Surface and Coatings Technology 76-77 (1995) 521-527. [3] N. Laraqi and al. Temperature and division of heat in a pin-on-disc frictional device-exact analytical solution, Wear (2008) 08.016. [4] W. Park Y and al. The influence of current load on fretting of electrical contacts. Tribology International (2008) 09.004L. [5] G. Bucca and A. Collina. A procedure for the wear prediction of collector strip and contact wire in pantograph–catenary system, Wear 266 (2009) 46-59. [6] D. Majcherczak and al, Tribological, thermal and mechanical coupling aspects of the dry sliding contact, Tribology International 40 (2007) 834-843. [7] Y. C. Chiou and al. Formation mechanism of electrical damage on sliding contacts for steel pair, Wear 266 (2009) 110–118. [8] M. G. Diehl. Wear of electrical contacts, Vol. 1 (1957/58) Wear, References p. 376. [9] P. Stempfle and al. Evaluation of the real contact area in three-body dry friction by microthermal analysis. Tribology International. (2010), 10.1016. [10] J. Denape and N. Laraqi, Thermal aspect of friction: Experimental evidence and theoretical approaches, December (2000), 563579. [11] Da Hai He and al, A sliding wear tester for overhead wires and current collectors in light rail systems, Wear 239 (2000), 10–20.
ScienceDirect Physics Procedia 55 (2014) 165 – 172
Eight International Conference on Material Sciences (CSM8-ISM5)
Thermal and tribological analysis of the dry sliding steelsteel couple traversed by an electrical current C. Boubechoua,b ; A. Bouchouchab; H. Zaidic ; Y.Mouadjib a
Faculté de Technologie, Département de Génie Mécanique, Université 20 Août 1955, Skikda 21000, Algérie. Laboratoire de Mécanique, Faculté des Sciences de l'Ingénieur, Département de Génie Mécanique, Université Constantine 1, 25000, Algérie. c Laboratoire LMS (UMR-6610-CNRS), SP2MI, Téléport 2, Boulevard Marie et Pierre Curie, Université de Poitiers, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France. b
Abstract This study concerns a thermal and tribological analysis of the dry sliding steel-steel couple traversed by an electrical current. The tests were carried out by using a tribometer pin-disc under ambient air environment. The dry friction and wear of this contact are studied with different parameters such as normal load, electrical current and sliding speed (maintained constant V = 0.5 m/s). The test duration is 20 mn. The experimental results obtained show that these parameters have a more or less significant effect on the tribological behavior of the couple. Indeed, the oxidation phenomenon, the particles of wear resulting from this oxidation, their composition, their morphology and their thermal-mechanical properties, under certain conditions, play an important role and determine the life service of this couple. To highlight the effect of these parameters, theoretical calculations based on the theories of Archard and Holm have been done. These calculations allowed us to evaluate the order of magnitude of the mechanical, geometrical, thermal and electrical parameters. The discussion of the results is mainly based on these calculations, optical and SEM observations as well as other phenomena resulting from the friction process. © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2013 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection under and/or peer-review under responsibility ofCSM8-ISM5 CSM8-ISM5 Peer-review responsibility of the Organizing Committee of
Key words: friction, wear, steel, oxidation, electrical contact, wear particles, temperature.
*C. Boubechou E-mail : [email protected]
1875-3892 © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of CSM8-ISM5 doi:10.1016/j.phpro.2014.07.024
166
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
Nomenclature P[N] normal load 'Tm[°C]interface temperature of the pin a[Pm] contact radius H[MPa]
hardness
Q[J]
the heat quantity loss of the pin
V[m/s]
sliding speed
D
coefficient of heat repartition
Rc[:]
electric contact resistance
][Pm]
thickness oxide layer
O[ W/m.K] thermal conductivity U0[:.m]
electrical resistivity of oxide
I[A]
electrical current
CP[J/Kg.K] thermal capacity J>P:m]
resistivity
VC[V]
electrical contact potentiel difference
A%
lengthening
1. Introduction The industrial problems are complex by the need to optimize the life service of the different elements, and involve many complex phenomena such as friction between the antagonistic surfaces, and overheating caused to the interface [1]. The wear processes in the sliding electrical contacts are affected by the contact temperature, the microstructural transformations, and the formation of tribochemical film, the fusion of surfaces in contact or the failures induced by the mechanical, thermal and electrical stresses [1]. In fact, the increase in local temperature and loss of material in the wear tracks are the result of energy dissipation in the friction contact zone [2]. The aim of this paper is to study the influence of the thermoelectromecanical parameters on the dry friction and wear behavior of steel-steel couple. 2. Experimental device The experimental device is a pin-on-disc tribometer. Tests are carried out in an ambient air (Fig.1). The pin (XC38) and disc (XC48) are made of steel. The disc is driven in a rotational movement of constant velocity with diameter of 50 mm and 12 mm of thickness. It has a lame hole in the center. The normal load is vertically applied with a weight P. The pin has a cylindrical shape of length 20 mm and a diameter
167
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
of 8mm; has a flat allows to o fix it in a holee with a screw w on an arm loaad of aluminum m. Friction coefficient P is deduced from measurement of the tangential forcce F induced on the arm by b the rotatingg wheel b and afteer each through the pin (P = F / P). The wear W is determinedd by weighing the specimen before b with a precision of ± 0.1 mg. Thee direct electriccal current I is brought to thhe disc test using a balance through a merrcury contact in n the axis of rootation to avoidd the effect of the centrifugal force. To enssure the same experim mental condition ns for each testt, the disc is poolished mechannically with a 1200 1 grain paper.
Fig.1. Tribometter pin-disc
3. Materials The chemical composition is presented herre in table 1 Tablee 1. Chemical composition c C C%
Si %
Mn %
Cr %
XC38
0 0.38
0.30
0.50
0.040
XC48
0 0.51
0.4
0.75
-
m and d physical charracteristics are presented in taable 2. Whereas the mechanical Table 2. Mechhanical and phhysical characteeristics HB
Re [N N/mm2]
Rm [N/m mm2]
A%
XC448
200
550
710
15
XC38
190
490
630
17
3 U [Kg/m [ ]
Cp[J/Kgg.K]
O[W/m.K]
J[P:m]
78775
490
53
0.21
4. Modeling 4.1. Evaluatioon of the real contact area
F
The real suurface of contacct between pinn-disc is obtaineed from the folllowing relatioon: P P H .S .a 2 , a P .P 0 ,6.H .S .a 2 (1) H .S
4.2. Evaluatioon of the contacct temperaturee
(2)
168
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
The quantitty of heat Q distributed betw ween two surfacces leads to a rise r in the meann temperature 'Tm at the interface. We suppose th hat 'Tm is the same in the tw wo solids. The heat flux throough each comp mponent nly on: the meechanical param meters, the intensity of the electrical e curreent, the of the couple depends main c and the number of Peeclet L [2]: geometry of thhe contact, the heat-storage capacity L
V.a , F 2.F
O U.Cp
(3)
Where D = - 0.102L+ 0 0.860 x
if L < 0.1, the interrface temperatuure of the pin iss expressed by: ǻTm
x
if 0.11 < L < 5, ǻTm
x
if L > 5.0,
ǻTm
D.
Qp 4.a.O p
Qp
(4) (5)
4.a.O p
0.31Qp
Fp
a.O p
V.a
(6)
The heat quuantity loss of the pin per uniit of time is exppressed by: Q p Qm Qe Qa x withh arc and electrrical current: Q p Qm Qe Qa P .P.V Rc .I 2 V C .I
(7) (8)
Where: Qm thee mechanical heat h , Qe the electrical heat andd Qa the heat generated g by eleectrical arcs 4.3. Thicknesss oxide layer The thicknness of the oxid de layer is exprressed by: ] c
S .a 2 .Rc Uo
(9)
5. Results b 5.1. Friction behavior Figure 2 shows s the evo olution of the friction coeffficient with tim me parameter.. We observe that P increases from m (μ ~ 0. 50) to o reach a maxim mum value of about a 0.65 thenn stabilizes at a value of 0.600. Figure 3 shhows the evolu ution of frictionn coefficient with normal loadd. The friction coefficient ȝ evolves e gradually withh load.
Fig. 2. Variationn of friction coefficcient versus time (V V = 0.5 m/s; P = 18.5 1 N and I = 2A)) Fig. 3. Variation of friction coefficiient versus normall load (V= 0.5 m/s; I = 2 A and t = 20 2 mn)
Figure 4 presents p the vaariation of thee friction coeff fficient with ellectric current.. In this curvee, three distinct zones can be identiffied: m valuee of 0.60. x zonee I: friction coeefficient evolvees gradually froom 0.50 to a maximum x zonee II: friction coefficient c ȝ decreases d signnificantly with the increasinng of current until u it reaches a value of about 0.43. oefficient ȝ staabilizes at a vallue of 0.43. x zonee III: friction co
169
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
Zone I
zone II
zone III
Fig. 4. Variation of friction coefficiient versus electriccal current (P = 188.5 N; V= 0.5m/s and a t = 20 mn)
5. 2. Wear behhavior The evoluttion of wear with w the normal load is shown in figure 5. 5 This curve shows that thhe wear increases withh the applied load. l The evollution of wear according to the t intensity of o electrical currrent is represented grraphically in fiigure 6. Two phhases are also noted: n x phase I: for the low w values I = 2A A, wear is stabillized in the neiighborhoods off W = 0.135 g. x phase II: wear decrreases graduallyy with the intennsity of electriical current unttil W = 0.04 g.
Phase I
Phase II
Fig. 5. Wear variaation with normall load (V= 0.5 m/ss; I = 2 A and t = 20 2 mn) Fig. 6. Wear variaation versus electrrical current (P = 18.5 1 N; V= 0.5m/s and t = 20 mn)
'T[°C]
5. 3. Calculating parameterss me, then the ratio r Op / Od = 1. The Like the thhermal conducctivity of steels XC38and XC48 is the sam coefficient off total transfer;; with outside, is reduced, thhe temperaturee of the pin risses according to t time until thermal balance b (table 3). x for a rupture in steeel with: P =18.5 N, H = 200 Mpa and P = 0.6, we obtainn a = 170 Pm. x for a rupture in ferrric oxide Fe2O3, H =103 Mpa,, we obtain a = 70 Pm. w oxide, versus This figuree shows the theeoretical variattion of the conntact temperatuure, with and without the real contacct area. We observe that 'Tm decreases signnificantly withh the increasingg of real contacct area. Figure 8 show ws that the temp perature of conntact is inversely proportional to the normall load. ȴ ȴTtwithoxide
ȴ ȴTtwithoutoxide
1,000.00 800.00 600.00 400.00 200.00 0.00 5EͲ05
0.0001
0.00 0015
0.0002
0.00025 a[Pm]
Fig.7. Variation of o the contact temp perature with and without w oxide verssus radius contact (V = 0.5m/s, I = 2A 2 and t = 20mn) Fig.8. Variation of o the contact temp perature with and without w oxide verssus normal load (V V = 0.5m/s, I = 2A A and t = 20mn)
170
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172 Table 3: Values of thermal, geometrical, electrical and mechanical parameters I [A]
0
2
76
170
76
170
L
0.84
3.11
0.84
3.11
D
0.43
0.54
0.43
0.54
Qp [J]
2.40
2.78
16.40
16.78
'7m [°C]
65.82
38.40
449.5
232
a [Pm]
Rc [:]
-
3.5
3.5
] [Pm]
-
17
84
6. Discussion 6.1. Effect of the time parameter on the friction and wear During the first period of contact between the elements of the couple, the wear particles are metallic in nature; the mutual transfer at the interface is also metal. There is very little debris in the contact and the friction coefficient is 0.65. Over time the debris formed, wear passive are pulled out and they have no effect on the tribological behavior. However, the active wear particles play an abrasive role in friction and wear behavior. In effect, they increase friction and reduce the randomness of the electrical contacts [11]. The increase of the friction coefficient, during the first moments of sliding (Fig. 2), is due to the interaction between the asperities of the opposing surfaces. Indeed, as and when the dynamic contact continues, the adaptation of surfaces occurs and the mechanism of formation and rupture of junctions stabilized by balance of operating conditions at the interface (running late). This increase is explained by the increase in real area of contact deformation and creep. This surface shape generally elliptical deforms and elongates in the direction of sliding. In the second period, the friction coefficient is more stable. This stability results from the accommodation of the interface friction. The asperities are sheared and the beginning of a significant production of fine particles from both surfaces is clearly observed [3]. This production and the rise of the mean interfacial temperature induce a stable friction coefficient. The changes in structure and composition of surfaces and the surface oxidation phenomenon and the mutual process of transfer of particles govern the friction and wear behavior of the couple [4]. 6.2. Effect of normal load on friction and wear When the charge increases from 5 to 15 N, the friction coefficient follows a nearly linear curve (Fig. 2). For loads greater or equal to 15 N, the electrical contact resistance stabilizes at 3.5 :. Most particles are metallic, the friction coefficient increases linearly with the load. This result in an increase of the real area of contact and the friction coefficient remains substantially constant. It is always in an adhesive real area of contact. Indeed, J. and R. Blouet [5] found that there is always a critical load beyond which the wear increases significantly (Fig. 5). In the first part of the curve, under low load, the wear is almost linear with the load (up to 15N). The increase in load, results in additional wear and probably an increase in the number of contact points and then by an increase in the density of junctions. At these pressures the critical yield stresses are reached, plastic deformation of asperities contributes to increase the contact area and promotes adhesion [9].
171
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
6.3. Effect of electric current on the friction and wear The passage of electric current intensity of 2 A does not contribute significantly to oxidation. However, there is a significant effect on the wear when the current exceeds 2 A. The decrease in wear is about 7 times less than low or no current (I d 2 A). This is explained by the fact that the heating caused by Joule effect softens the steel XC38 which is a soft material [6]. Under these conditions, the friction coefficient gradually increases (when I increases) and then stabilizes and reaches lower values (when I = 3A). The increase in the electrical contact resistance reflects the existence of oxide films and other materials at the interface [7]. The high steel oxidation leads to the rapid formation of hard asperities and thin layer of oxide, then the electrical contact resistance increases. The phenomenal electrical arcs appear to form craters. These craters are 10-20 μm in diameter. 6.4. Effect of temperature rise on friction and wear The contact temperature is an invaluable element for understanding the thermal and tribological behavior of materials rubbing together. Actually, the mechanical power transmitted through a sliding contact is mainly dissipated as heat at the interface between the two contacting materials steel-steel (table 1) The resulting temperature rise can strongly affect the surface properties of the materials [10] (Fig.8), favors physico-chemical and microstructural changes and modify the rheology of interfacial elements trapped in the contact zone [8]. When this contact is crossed by an electrical current, in addition to the heat generated by friction, is added the Joule effect. Metal surfaces of a sliding contact are the site of chemical reactions and oxidation. The fresh surfaces created by abrasive wear are exposed to ambient oxygen and the moisture of the air. 6.5. Analysis of worn surfaces Analyses by scanning electron microscopy and quantitative analysis are performed on samples of steel XC38 having rubbed on steel XC48 for applied loads of 5 N and 18.5 N at a constant speed V= 0.5 m/s for 30 minutes (Fig. 9). We have observed the following: - the micrographs of figures 9(a) and (b) show the abduction and cut away the surface material and the presence of more or less deep furrows plowed in the direction of movement of the rubbing surfaces. Wear debris present on the surface are the cause of this behavior. - X-ray microanalysis of some debris showed the presence of several elements (Figs.9 (c) and (d)), in particular a high peak of iron, a small peak of chrome and oxygen and very little other constituents. This is probably iron oxides, oxides of Cr and other elements that their presence is due to a mutual transfer between the opposing surfaces. The amount of elements transferred depends mainly on the speed and amperage parameters.
(a)
(b)
(c)
(d)
Fig.9. SEM images and X-ray analysis of the worn face of the pin (× 100): ((a) P = 5N, V = 0.5m/s and I = 2A; (b) P = 18.5N)
172
C. Boubechou et al. / Physics Procedia 55 (2014) 165 – 172
7. Conclusion The effect of the thermoelectromechanical parameters on the tribological behavior of steel-steel dry couple sliding leads to the following conclusions: - for low values of the electrical current less than 2 A, the friction coefficient increases gradually. However, there is no effect on wear. - for values of the electrical current higher than 2 A, the friction coefficient and wear have a decreasing form. - the presence of the electrical current activates the oxidation of wear debris and leads to an elevation of mean interface temperature which plays an important role in the modification of material properties and the oxidation phenomena. - the real contact zones are evaluated and the radius of the circular form is determined by taking into account the oxidation phenomenon (a = 70 to 170 μm). In these conditions, the critical thickness of oxide coatings evolutes between 17 Pm and 8 4 Pm. - wear process of sliding electrical contact is modified continuously by its mechanical parameters according to the electrical current intensity. Surfaces can be damaged by abrasive wear, by oxidation or by electrical arcs at the contact. References [1] A. Bouchoucha et Al, Wear 203-204 (1997) 434-441. [2] A. Bouchoucha et Al. Surface and Coatings Technology 76-77 (1995) 521-527. [3] N. Laraqi and al. Temperature and division of heat in a pin-on-disc frictional device-exact analytical solution, Wear (2008) 08.016. [4] W. Park Y and al. The influence of current load on fretting of electrical contacts. Tribology International (2008) 09.004L. [5] G. Bucca and A. Collina. A procedure for the wear prediction of collector strip and contact wire in pantograph–catenary system, Wear 266 (2009) 46-59. [6] D. Majcherczak and al, Tribological, thermal and mechanical coupling aspects of the dry sliding contact, Tribology International 40 (2007) 834-843. [7] Y. C. Chiou and al. Formation mechanism of electrical damage on sliding contacts for steel pair, Wear 266 (2009) 110–118. [8] M. G. Diehl. Wear of electrical contacts, Vol. 1 (1957/58) Wear, References p. 376. [9] P. Stempfle and al. Evaluation of the real contact area in three-body dry friction by microthermal analysis. Tribology International. (2010), 10.1016. [10] J. Denape and N. Laraqi, Thermal aspect of friction: Experimental evidence and theoretical approaches, December (2000), 563579. [11] Da Hai He and al, A sliding wear tester for overhead wires and current collectors in light rail systems, Wear 239 (2000), 10–20.