Influence of the electrical current on wear in a sliding contact copperchrome steel, and connection with the environment

Vacuum/volume 41/numbers 7-9/pages 2213 to 2216/1990

0042-207X/9053.00 + .00 © 1990 Pergamon Pressplc

Printed in Great Britain

Influence of the electrical c u r r e n t on w e a r in a sliding c o n t a c t c o p p e r - c h r o m e steel, and c o n n e c t i o n w i t h the e n v i r o n m e n t D P a u l m i e r , A B o u c h o u c h a and H Zaidi, Laboratoire ERMES, ENSEM-INPL - CNRS, 6, rue du Joli Coeur,

54000 Nancy, France

The couple studied has been chosen because it is used to bring current to railway engines and there are problems with the wear of overhead lines. We have shown the influence of environment with oxygen and of the direction of the electrical current on the wear of copper. The interpretation of results according to the theory predicts a more significant wear of copper when it is the anode. We have calculated contact conditions between the copper wire and the steel rod (area, contact pressure, temperature . . . . ) and we have deduced the speed of growth of the oxidation layer on the copper taking into account the effect of the electrical field. When the steel is the anode, we have shown that copper and its superficial oxide are strongly abraded by the chrome oxide particles formed on the steel rod and this phenomenon is preponderant.

1. Introduction

4. Discussion

During the sliding contact of overhead lines with the collector rod used to bring the electrical current to railway engines, the friction coefficient and the wear are affected by the current. Our experiments (presented here) detail this effect: we explain how the current influences the growth of oxide layers on metals by refering to the Cabrera and Mott theory 1.

We have shown that in our experimental conditions, the wear of the wire results from oxidation mechanisms2. To analyse the effect of the electrical current on the oxidation speed, it is necessary to evaluate the contact area, the temperature reached during the contact and the thickness of the oxide layer. Then, we use the Cabrera and Mott theory to pinpoint the effect of the current.

2. Experiment

A classic pin-disc tribometer has been modified (Figure l(a)). A 16-sector on one side wheel is fixed on the rotating disc, and a stretched copper wire is fixed on a U-shaped frame, itself held by the arm of the tribometer instead of the pin. The direct electrical current is brought to the wheel through a mercury contact in the axis of rotation to avoid the effect of the centrifugal force (Figure l(b)). The wire is either an anode or a cathode according to the experimental conditions. A device permits us to reach the value of the contact resistance during the experiment. Friction coefficient # is deduced from measurement of the tangential force applied on the arm by the rotating wheel through the wire and wear values W are obtained by measures of weight loss of the wire. 3. Results

It appears that the presence of an electrical current and its direction have an insignificant effect on the friction coefficient below 40 A (Figure 2). However, the wear is directly affected by the presence of electrical current and its direction (Figure 3). This effect is confirmed when the other mechanical parameters are varying: sliding speed, (Figure 4), normal load (Figure 5), tension force of the wire (Figure 6).

4.1. Valuation of the contact area. The normal load is P = 10 N in these experiments. The radius of the circle equivalent to the contact area is: a = ( P / n H ) i/2

(H is the hardness of the softer material). When the breaking occurs in the copper metal (H = 4 x l0 s Pa), a equals 89 #m. When the breaking occurs in the copper oxide (H = 13 x l0 s Pa), a is approximately 50 #m. It is reasonable to take these two values as extreme values for a: 5 0 < a < 100 gm.

4.2. Valuation of the contact temperature. Figure 7 represents a circular contact area of the radius a. According to Archard method 3, the heat released in the contact comes from two sources: one fixed source transmits the heat quantity Qd per unit time to the rotating disc, and a mobile source transmits the heat quantity Q , per unit time to the wire. The total heat quantity realised by friction and Joule effect in the interface is: Q = Qd-F Qw.

This is shared between the two surfaces so that the rise of temperature AOm is the same on each side. 2213

D Paulmier et el: Influences on wear in a sliding contact copper-chrome steel

0.7

0.6

[] fil cathode ( - ) • fil anode (+) P = 1 kgf v = 1 , 2 m s -1 T = 36 kgf tf = 60 rain

0.5

0.4

0.3

0.20

~

L 20 I[A]

10

= 30

40

Figure 2. Effect of the polarity of the electrical current on the friction coefficient vs intensity.

~

~

50

disc

[] fil cathode ( - ) • fil anode (+) P = 1 kgf v = 1 , 2 m s -1 T = 36 kgf tf = 60 min

40

~ 30

20

Rotating axis

[]

B I

o I

~ I

B

n

•

Figure 1. (a) Modified tribometer: front view; l support, 2 arm of the tribometer, 3 stop-screw, 4 counterweight, 5 clinometer, 6 stress-gauge, 7 normal load, 8 U-shaped frame, 9 16-sectors on one side wheel, l0 axis of rotation, I I copper wire, 12 adjusting plate. (b) Vertical section of the rotating disc. When the sliding speed is low, the rise of the temperature of the wire contact surface is given by the following relation:

[]

|

1(

[

10

I

20 IIA]

I

30

Figure 3. Effect of the polarity of the electrical current on the wear of the copper wire vs intensity.

Qw A0m - 4a3.,'

0.31. Q . ( z * ~1/2

the rise of the temperature on the disc contact surfaces is

A0m --

Qd A0m = 4a~.---:'

where Z, is the thermal diffusivity of copper,

2 , and ~-d are the thermal conductivities of the copper wire and of the steel disc. (2, (copper) = 385.8 J . m m -2 s - l ° C -=, 2 d (steel NS22S) = 14.9 J . m m -2 s - I ° C - l . ) It appears that the wire exhausts a 25 times greater heat quantity than the disc, that is to say, almost all the heat released at the interface. For high sliding speeds, the rise of temperature is given by the relation: 2214

~

-" a

\v • a/

'

A pc

with p: density of the material, c: specific heat. The limits between low and high speeds are given by the values of a parameter L: L = ( v . a)/(2 - z)

with v sliding speed.

D Paulmier et ah Influences on wear in a sliding contact c o p p e r - c h r o m e steel I

r

I O []

o1=0

[]

• I + = 40A P = 1 daN T = 36 daN

50

[]

50

[] o

tf = 60 min

v = 1 . 2 m s -1 tf = 60 • i n

• O

=1 =0

• I + = 40A P = 1 daN

•

40

40 121

E 30

E3(

[]

[]

20

[]

•

da

10

r•

2(

oqbo • o []

10

[] []

I

I

2

4

,

I

i

I

6

L

5

8

t

I

I

15

25 TldaN]

35

v[m s -11 Figure 4. Wear of the copper wire with and without electrical current

vs sliding speed.

Figure 6. Wear of the copper wire with and without electrical current

vs mechanical tension force of the wire.

Table 1. Temperature rise at the interface with and without electrical current for two values of area circle radius, and for two sliding speeds []1=0 I + = 40A v= 1,2ms 1 T = 36 daN h = 60 min

I [A]

•

50

a [/~m] 50

40

lO0

E 30

50

40

100

[] oO

B

20

V (m s - ']

L

1.2 7.2 1.2 7.2

0.26 1.56 0.52 3.14

1.2 7.2 1.2 7.2

0.26 1.60 0.53 3.2

~t

Qf

A0 m

[W]

[°C]

0.8 0.55 0.8 0.4

7.2 29 7.2 29

74 205 37 74

0.8 0.55 0.8 0.4

16.8 38.6 16.8 38.6

174 275 87 I00

•

[]

l o B

10

mll

I

•

•

I

0

I

I

L

0.5

1 P[daN]

1.5

Figure 5. Wear of the copper wire with and without electrical current

vs normal load.

As the total transfer coefficient o f the wire to exhaust the released heat is low, the equilibrium wire t e m p e r a t u r e varies between 30 a n d 350°C. The values o f Table 1 show t h a t the t e m p e r a t u r e at the interface d u r i n g the sliding m o v e m e n t can rise u p to 600°C. 4.3. Valuation of the thickness of the oxide layer. According to the relation,

R Low speeds are considered when L < 0.1; high speeds are considered w h e n L > 5. W h e n 0.1 < L < 5 the relation used to give A0 m is:

aw

""-"-C--,

A0m = ~ 4a • Zw with ~ varying from 0.85 to 0.35. Table I gives the results w i t h o u t a n d with current for two values of contact-circle radius a n d two values o f the sliding speed.

Pc

Po X

with R = contact resistance, Pc = electrical resistivity o f copper, Po = electrical resistivity o f a thin layer o f c o p p e r oxide, X = thickness o f the oxide layer (where the first term o f the sum represents the constriction resistance a n d the second term the resistance of the oxide layer), R values are suspected to be of the order o f 4 x 10 -3 f h n . R values m e a s u r e d in o u r experim e n t s are a r o u n d 12 x 10 - 4 f] w h e n the conditions lead to low oxidation ( u n d e r argon). In this case: X = 2 0 / ~ when a=50#m, X=90A when a = 100 # m . 2215

D Paulmier et al: Influences on wear in a sliding contact copper-chrome steel

Table 2. Ratio of growth speed values when the copper wire is the anode, or cathode, for two interface temperatures and two values of external potential difference

£(vo+, T) r

[K]

Vo

[V]

x

[A]

£(vo_, T)

20 100 100 250

2.46 1.20 1.8 1.27

100 250 250 I000

1.1 1.04 1.13 1.03

rent to pass, a' = distance between an equilibrium position and the potential barrier which stops the migration to the nearest site, q = electrical ion charge. Supposing that the thickness of the oxides on the wire and on the wheel surface are the same, the ratio between the growth speeds is:

"Y(V~+' T) =explq " a'(V~+ - V~ ) ] 300

0.15 0.5

600

0.15 0.5

In the presence of significant oxidation (under oxygen), R values rise to 12 x 10 -3 Q, and X = 240 A when a = 50/~m, 950 .A, when a = 100 #m. 4.4. Effect of the current direction on the wear. According to the Cabrera and Mott theory, the growth speed of an oxide layer on a metal submitted to an external potential is:

~'(lle, T) = N'. ~ . v exp -

[q " a" " exp[j(-. ~ - ~ ) " e x p ( ~ . k i ~ ) , with: N' = number of sites per unit surface able to accomodate a metallic ion, ~ = oxide volume per metallic ion, v = atomic vibration frequency, k = Boltzmann constant, T = temperature, Vi = internal potential difference due to the oxide presence, V¢ = electrochemical potential difference which allows the cur-

2216

.'?(Vo , T)

X. k :):

with V e + - Ve_ between 0.15 and 0.5V, k . T varying from 0.026 eV at 300 K to 0.052 at 600 K. (a' = 3 A). The values of this ratio show the effect of the polarity on the wear speed (Table 2). It reveals that, when the copper is the anode, the copper oxidation speed is higher while the steel oxidation speed is lower. In this case, 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 released as grains at the sliding interface, abrating strongly the wire. Our observations under microscope confirm this abrating action. 5. Conclusion

This paper shows that the wear mode in this sliding contact between copper and chrome steel is not the same for different current directions. When the copper wire is the anode, the wear mode is soft adhesive, and when the wire is a cathode it appears as an abrasive mode. References

I N Cabrera and N F Mott, Rep Prog Phys, 12, 163 (1949). 2 A Bouchoucha, Thesis, INPL Nancy (1988). 3j F Archard, Wear, 2, 438 (1958).