Some aspects of the friction and wear of electrical sliding contacts under conditions of boundary lubrication

Some aspects of the friction and wear of electrical sliding contacts under conditions of boundary lubrication

131 Wear, 77 (1982) 131 - 137 SOME ASPECTS OF THE FRICTION AND WEAR OF ELECTRICAL SLIDING CONTACTS UNDER CONDITIONS OF BOUNDARY LUBRICATION V. A. B...

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131

Wear, 77 (1982) 131 - 137

SOME ASPECTS OF THE FRICTION AND WEAR OF ELECTRICAL SLIDING CONTACTS UNDER CONDITIONS OF BOUNDARY LUBRICATION

V. A. BELYI, N. K. MYSHKIN and V. V. KONCHITS Institute of Mechanics of Metal-Polymer Gomel, B.S.S.R. (U.S.S.R.)

Systems, Byelorussian Academy

of Sciences,

(Received July 3, 1981; in revised form September 4, 1981)

Summary The friction characteristics of lubricated electrical sliding contacts are considered. Data are presented concerning the effects of the current, the lubricant properties, the velocity and the load on the friction and wear behaviour of the contacts. In light-current electrical contacts the effectiveness of the lubricant does not depend on its conductivity but on its ability to prevent the formation of a nonconductive film by lubricating action. Under semifluid lubricating conditions the heavy current acts by discharge through the lubricant film, and the lubricant conductivity generally determines the friction and wear characteristics of the contact. Colloidal metal particles produce additional conductivity in the clearance between the contacting surfaces, prevent electrical erosion and, in some cases, form plastic films which decrease the coefficient of friction and the wear rate of the contact.

1. Introduction Electrical sliding contacts (SCs) are widely used but their reliability is inadequate for a number of applications. The problem of improving the reliability of light-current low speed SCs and of heavy-current high speed SCs is very complex. Improvements in the wear resistance and in the durability, e.g. by the application of hard coatings, usually have a deleterious effect on the electromechanical characteristics of the contacts or increase the cost of the unit, e.g. when noble metals are used. It has been shown [l -31 that lubricants can be used successfully in SCs. Light-current SCs have a low wear rate and a high reliability under boundary lubrication [ 4, 51. However, it is difficult to develop suitable lubricants for electrical contacts because their effect on the current flow and hence on the wear behaviour is poorly understood. 0043-1648/82/0000-0000/$02.75

0 Elsevier Sequoia/Printed

in The Netherlands

132

The purpose of this paper is to consider the interrelation between the lubricant properties, the current effect and the friction and wear characteristics of SCs for both light and heavy currents.

2. The wear and electromechanical electrical sliding contacts

characteristics

of light-current

low speed

Test data on wear and contact resistance are presented in Table 1 for cone contacts rubbed against different coatings under dry and lubricated conditions. The data show the effectiveness of the lubricants, which is particularly significant for the oil containing a surface-active additive. Such an oil decreases the contact voltage drop as well as the linear wear of the contact. Similar data on the effect of lubricants have been obtained elsewhere [ 1 - 51, and some workers have assumed that the decrease in the contact resistance can be explained by the high conductivity of the lubricant boundary layers. However, our earlier results [6, 71 have shown that the conductivity of a thin film of oil is equal to its value in the bulk. The contribution of the lubricant conductivity to the total conductivity of the rough metal contact is significant if the bulk conductivity of the lubricant exceeds lop2 - 10-l a-’ m-l. The value for oils in common use is about lo-l3 - 10m5 fi2-l m-l so that the contribution of lubricant conductivity is negligible. The effect of the lubricant on the conductivity of a light-current SC was investigated by rubbing a hemispherical gold indenter with a top radius TABLE 1 Wear and electromechanical Contact unit materials (cone brush-plane disc)

cu-cu

(Pt-4.5% Ni)-Cr Bronze (5% Sn, 5% Zn, 5% Pb)-Cr Bronze (3% Si, 1% MN)-Cr Bronze (2% Be)-Cr Brass (40% Zn, 1% Pb)-Cr Bronze (10% Sn, 1% P)-Cr

characteristics of light-current sliding contacts Linear wear of the cone contacta (mm) and the contact resistance (a) for the following lubrication conditions Without lubricant

Vaseline oil

AMG-10 oil (acid-refined oil fraction)

AMG-10 oil with 0.5 mass% oxypropylated glycerine

1.5; 0.07 1.0; 0.36 1.2 ; 0.24

0.6; 0.07 0.25; 0.36 0.2; 0.24

0.4; 0.07 0.1; 0.3 0.25; 0.18

0.28; 0.07 0.10; 0.3 0.15; 0.18

1.3; 1.2; 1.3; 1.2;

0.35; 0.28 0.3; 0.28 0.35; 0.29 0.35; 0.25

0.28; 0.22; 0.30; 0.14;

0.17; 0.14; 0.22; 0.08;

0.28 0.28 0.29 0.24

0.22 0.23 0.22 0.22

0.22 0.23 0.22 0.22

Test conditions: load, 0.1 N; sliding velocity, 0.1 m s-l ; J = 50 mA; U = 12 V; test duration, 36 h. aThe top curvature of the cone is 50 pm.

133

of about 50 pm against a metal plane electrode (platinum, silver, copper or aluminium) under a load of 10m2 - 10e4 N at a velocity of 10m6 - 10m3 m s-l in both dry and lubricated conditions. The contact voltage drop during sliding and the voltage-current characteristics under static conditions were obtained. The lubricants used were Vaseline and castor oil, glycerine, oleic acid, MS-20 (selective solvent-refined oil), MGE-10 (acid-refined oil fraction) and MVP (Vaseline acid-refined oil). The data obtained show that the contact conductivity does not depend on the voltage or the type of lubricant used (Fig. l(a)). The voltage-current characteristics are linear for both dry and lubricated contacts. Only in the case of the Cu-glycerine combination did the contact voltage drop decrease and stabilize (Fig. l(b)). Additional tests were carried out using oxidized copper specimens (2’ = 453 K at 7 = 18 ks). The conductivity of the dry contact was negligible (Fig. l(c)) up to high values of the external e.m.f. (U = 5 V) but it decreased in the presence of the lubricant as a result of the decomposition of the oxide film. The chemical nature of this decomposition was confirmed by tests at a low external e.m.f. (U < 0.5 V) in which no decrease in the conductivity occurred. It is clear that there was no mechanical fracture of the oxide film. The results show that the lubricant has a negligible effect on the electromechanical properties of the contact at low velocities and relatively high pressure (boundary lubrication conditions) if there is no surface chemical interaction. Thus a lubricant can be selected from data on antiwear testing by considering its ability to prevent the formation of nonconductive films.

3. Friction

and wear behaviour

of heavy-current

contacts

Friction tests were carried out by rubbing copper brushes against stainless steel rings at a sliding velocity of 2 m s-l and a load of 0.1 MPa. The ring roughness R, was 0.25 pm. The effectiveness of the lubricant antiwear action was evaluated by measuring the time that elapsed before scoring produced an increase in the coefficient of friction. When a current is passed, the coefficient of friction increases and becomes non-stable (Fig. 2). The extent of stable operation decreases with lubrication (Table 2) and sparking takes place at the beginning of the operation. For grease lubrication, sparking ceased after a short run but the frictional force increased. For oil lubrication, sparking and instability of F and U, did not increase up to the initiation of scoring. The polarity of the current affects the friction and wear characteristics. With negative brush polarity the friction track on the ring rapidly loses lubricant and seizure occurs. With positive polarity the lubricant film is preserved and the time to seizure increases. If the current is cut off during the test the frictional force does not decrease, but if the cut-off is accompanied by fresh lubricant feeding the frictional force decreases to the value obtained during operation in the absence of a current. The passage of a heavy

134

mV

Uk

0,8 094 0 t L?.L-_ platinum

platinG

oil

(a)

V

Uk

6 094 O- I 50 copper

copper+g1ycerine

(b)

V 4 2

Uk

0, 1

0, I=JA

I=01

Fig. 1. Variation contact.

in the contact

voltage

drop U, when changing

from dry to lubricated

Fig. 2. Typical dependence of the frictional force F and the contact voltage drop uk on time in the presence of an electric current for stainless steel (ShH15) rubbing on copper (MI) lubricated with grease (TSIATIM-201).

TABLE Effect

2 of the electric

current

Lubricant

Grease (TSIATIM 201lithium-soap grease) MS-20 oil MGE-10 oil Vaseline oil

on the friction

behaviour

Time before seizure (min) at the following currents

of lubricated Contact

contacts voltage drop (mV)

J=IA

J=3A

At an external e.m.f. of 15 mV

J=3A

J=O 300-350

50-60

30

10 * 13

200 - 500

200-250 180-220 120-160

15-20 10-15 lo-15

10 8 8

8 - 12 8 -12 8-12

250 - 800 300 - 1000 300 - 1000

through a lubricated metal contact produces changes in the lubricant properties, its decomposition and seizure occurrence. When the sliding velocity is increased, the probability of hydrodynamic film formation is increased and the film thickness exceeds the critical thickness for tunnel conductivity. According to Furey [8], the frequency of formation of metal contact spots can be characterized by the average contact resistance observed when a low voltage (of the order of millivolts) is applied to the contact. In the current,

135

present tests this value is relatively high before the current is switched on (Table 2, fifth column) and hence current flow occurs in the presence of contact breaking. The data on contact breaking at 1 V [ 71 have shown that electrical discharges take place up to a critical value of the clearance between the breaking electrodes. These data also confirmed the existence of “contact bridges” formed by metal particles produced during erosion. The friction and wear behaviour of heavy-current SCs depends strongly on the conductivity of the lubricant because the breaking of the contact spots results in sparking and the consequent discharge decomposition of lubricants in common use. An increase in the current density intensifies this process. Lubricants possess low conductivity which increases when colloidal metal particles are present between the contact surfaces, e.g. as the result of wear in the presence of surface-active substances [ 91. This phenomenon has been investigated using an St 45 steel ring-copper brush unit running at 2 m s-l under a pressure of 1 MPa for 30 h in glycerine. The roughness R, of the steel ring was 0.3 pm. Under the test conditions about 10’ - lOlo particles cme3 in the size range 0.01 - 0.3 I.trn were present in the lubricant, and transferred film formation took place on the steel surface. The friction and wear characteristics depend on the direction and density of the current (Figs. 3 and 4). Under optimum conditions, the current flow intensifies the film formation, increases the metallic and quasimetallic contact area and decreases the erosion and wear rates. The effect of the current on material transfer in the friction zone can be evaluated by the general transfer equation based on diffusion-convection kinetics [4] :

Fig. 3. The effect of an electric current on the coefficient of friction of a bronze (5% Sn, 5% Zn, 5% Pb)-steel (St 45) system: curve 1, positive brush; curve 2, negative brush; curve 3, external e.m.f. cut-off. Fig. 4. The effect of an electric current on the total mass loss of a bronze-steel unit: -, negative brush ; +, positive brush.

friction

136

Analysis of eqn. (1) shows that the material transfer can be controlled by an external force (e.g. the electric field intensity). The condition for the dominant external force effect can be formulated in a similar manner to the Peclet number: V&O

(2)

-21

D

where

D=hT

(3)

6vro and

(4) for the effect of an electric field on the motion of particles possessing an electrokinetic potential in a viscous liquid. The critical value of the electric potential difference according to the dominant effect is E>

-

2kT

(5)

W-0x0

and the corresponding density between rough metals is .

Exe

J=

A,R

of the current

flowing through

a contact

(6)

The.parameter x0 is the effective clearance between rough surfaces, e.g. for the plastic contact of a rough rigid solid with a smooth solid x0 = H,, - 2.8H,,2’30R,,1’3qa2’3 + R,,,

I( $)““-

($--‘“/

(7)

The substitution of typical values for the parameters in eqns. (l), (2) and (6) shows that electric current densities in the range 1 - 10 A cm-2 can determine the rate of formation of the transferred film and hence the wear rate of a friction unit. In general, the electric erosion and the wear rate of heavy-current SCs are decreased when highly dispersed metal particles are present in the contact zone. Therefore friction conditions which produce colloidal metal debris (e.g. boundary friction in certain surface-active lubricants) or lubricants with dispersed metal additives can be used.

4. Conclusions The selection of lubricants for electrical sliding contacts depends on the working conditions and on the nature of the current flow. The friction and

137 wear of light-current low speed sliding contacts (the current passes through metal contact spots and thin films by tunnel conductivity) depends mainly on the lubricating action. The conductivity of the lubricant is important in heavy-current sliding contacts and at high speeds when hydrodynamic effects and electrical discharges occur.

Nomenclature b D $ F G Hb HE i J k N Qa 4c ‘0 & Rb

R max T u u, v, x0 ; : V

bearing-curve parameter diffusion coefficient potential difference coefficient of friction frictional force total weight loss height of surface waves Brine11 hardness current density electric current Boltzmann constant number of particles per unit volume nominal pressure contour pressure radius of the particle arithmetic mean deviation of the asperity radius of surface waves maximum height of the asperities temperature (K) external e.m.f. contact voltage drop convective velocity effective clearance dielectric constant electrokinetic potential viscosity elastic constant bearing-curve parameter

height

References 1 2 3 4 5

6 7 8 9

M. Antler, Wear, 6 (1963) 44 - 65. H. E. R. Kingsbury, Electr. Times, 159 (17) (1971) 14 - 15. P. V. Chiarenzelli and B. C. Henry, Lubr. Eng., 22 (5) (1966) 174 - 180. V. N. Litvinov, N. M. Mikhin and N. K. Myshkin, Physical-Chemical Mechanics of Selective Transfer in Friction, Nauka, Moscow, 1979. N. M. Mikhin, N. K. Myshkin, N. A. Valueva and V. B. Pevzner, in Proc. Znt. Conf. on Wear of Materials, Dearborn, MI, 1979, American Society of Mechanical Engineers, New York, 1979, pp. 175 - 180. N. K. Myshkin and V. V. Konchits, Dokl. Akad. Nauk B.S.S.R., 3 (1980) 234 - 237. N. K. Myshkin and V. V. Konchits, Friction Wear, 1 (3) (1980) 483 - 494. M. J. Furey, ASLE Trans., 4 (1961) 1 - 11. N. M. Mikhin, N. K. Myshkin, V. N. Litvinov and M. N. Dobytchin, in Proc. Znt. Conf. on Wear of Materials, St. Louis, MO, 1977, American Society of Mechanical Engineers, New York, 1977, pp. 60 - 63.