Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 5 Aus Wissenschaft und Forschung 1 Introduction Electrical contacts are necessary components of all technical systems [1]. Figure 1 represents the most general classification of electrical contacts considering the contact kinematics, functionality and design features. Tribology is a key factor in operation of many types of electrical contacts. In fact, all the sliding contacts and most of the commutating ones are specific tribosystems. Contacts of electrical machines, current pick-offs of transport and lifting machines, and of radio-electronic devices, control and automatic systems are the most important and widely used types of sliding contacts. As a rule, sliding contacts for electrical and transportation machines transfer currents of a moderate and high density while those for radio-electronic devices, control and automatic systems are usually low-current ones. 2 Contact resistance There are many parameters that can be used to asses the operating efficiency of electrical contacts. Among these parameters, the most important are electrical (resistance and its stability) and tribological ones (the wear resistance and friction coefficient). The real surfaces are not flat; hence, in metal-to-metal contact the surface asperities will penetrate the surface contaminant films, establishing metallic contacts. As the force increases, the number and the area of these small * Professor Dr. Sc. Nikolai K. Myshkin, Director and Head of Tribology Dept., MPRI, Valery V. Konchits, Ph.D., Leading Researcher, MPRI Metal-Polymer Research Institite of Belarus National Academy of Science, Gomel, 246050, Belarus ELE C TR IC C O N TA C TS STA TIO N A R Y M O V IN G S LID IN G C O M M U TA TIN G B IN D IN G B R U SH S LID ER TR O LLEY S EPA R AB LE R ELA Y B R EA K IN G S O L D E R ED W E LD ED B O N D ED C U R R E N T- C A R R Y IN G B U SS ES C U R R EN T P IC K O FF S O F E LEC TR IC A L A N D W E LD IN G M A C H IN E S R H EO ST A T S, P O T EN TIO - M ET ER S C O D E SEN D ER S C U R R EN T P IC K O FFS O F C R A N ES A N D TR A N S PO R T PLU G C O N N EC TO R S A N D C IR C U IT B R EA K E R S O PE R A TE U N D ER C O N D ITIO N S O F FR IC TIO N A N D W E AR Figure 1: Classification of electrical contacts Tribology of Electrical Contacts N.K. Myshkin, V.V. Konchits* Eingereicht: 20. 10. 2015 Nach Begutachtung angenommen: 15. 2. 2016 The role of electrical contacts in modern engineering is very important due to the fact that all the electric energy is passing them at least once. Much more important is the fact that the reliability of control and communication systems strongly depends on electrical contacts. Tribology is a key factor in operation of many types of electrical contacts. In fact, all the sliding contacts and most of the commutating ones are specific tribosystems. In the era of high-tech we meet new problems relating to miniaturization in electronics, automation, and robotics. It means that advances in microand nanotribology should be applied to solve these problems. The practical applications of tribology to various types of electrical contacts and trends in research and development of the efficient contacts are considered. Keywords contact mechanics, electric current in tribosystem, nanoscale effects, friction and wear performance. Abstract T+S_2_17 30.01.17 11: 58 Seite 5 F = A c H R c = ( 2 H/ 4F) 1/ 2 6 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 contact spots will increase.The actual (real) contact area A r is a fraction of the expected (apparent) contact area A a , as shown in Figure 2. The current passes through the conducting spots which are much smaller than the apparent, real and load-bearing contact spots, so the current lines are constricted [2]. As a result, the electrical resistance increases. This increase is defined as the constriction resistance. Contaminant films on the mating surfaces make the contact quasi-metallic and increase the resistance of conducting spots. The total resistance due to constriction and contaminant films is termed the contact resistance. The contact load-bearing spots consist of a set of smaller spots formed by subroughness. Figure 3 is an illustration of this concept and depicts the AFM data processing. The positions of the clusters are determined by the largescale waviness of the contact surfaces and the positions of spots formed by the small-scale surface roughness. Contact resistance is then determined by the number and size of the spots and by the pattern and size of their clusters. It is usually accepted that the real contact area is determined by plastic deformation of asperities. Bowden and Tabor [3] proposed that contact pressure on contacting asperities is equal to the yield limit of the softer of contact materials and the normal load is supported by softer asperities. Under this assumption, the area of mechanical contact A c is related to normal load F and to hardness H of the softer material as This expression states that the real area of mechanical contact between two surfaces is independent of the area of nominal contact of the surfaces i. e., A c = F/ H depends only the contact force and the hardness of the contacting bodies, and is independent of the dimensions of the contacting objects. If the electrical interface does not carry electrically insulating films and is characterized by a sufficiently large number of conducting spots the contact resistance may be expressed as Holm has analyzed the basic consequences of the effect of the size and the number of contact spots on their conductivity [2]. Electric resistance of metal contact in the absence of surface films due to constriction of current lines is described by the formula derived with the assumption that contact materials have similar conductivity, all the spots are equal in size and are small comparing to the size of contact bodies: Aus Wissenschaft und Forschung Figure 2: Schematic of current constriction and real contact area Figure 3: Visualization of contact spots at various scales: a - AFM-image of an analyzed area (scan 15 ×15 μm); b - clusters of real contact spots at microlevel; c -real contact spots at submicrolevel (physical contact area) (a) (b) (c) T+S_2_17 30.01.17 11: 58 Seite 6 + = + = c a na R R R 1 1 2 2 1 ρ . + = α ρ 21 21na R , where ≠ ≠ − = j i j i ij s n 1 16 3 2 1 π α c R α π 2 3 3 16 = Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 where a is the radius of an elementary spot and α c is the size of circle enveloping the cluster of spots. Greenwood has proposed further development of calculation techniques taking into account the mutual influence of spots [4]: Here s ij are the distances between the spots in the cluster. It is reasonable to add the third term taking into account the mutual influence of clusters [5]: Value of α c can be found by the same technique as α and in first approximation it is equal to the radius of nominal contact area. The test of the relations was carried out using the setup given in Figure. 4 [5]. Plastic cage containing a certain number of steel balls having 6.3 mm diameter is compressed between two steel circular plates with diameter equal to 130 mm. Plastic cage was fabricated of glass-cloth laminate having shape of disk with diameter equal to 120 mm and thickness 2.5 mm. A total number of 109 holes having diameter slightly larger than 6.3 mm were drilled in the cage providing the possibility to arrange the balls in the regular groups with different spacing. Contact resistance was measured by four-wire technique. The analysis of the data shows that the test and calculation results describing the changes in the number of balls and their position within the nominal area are in good qualitative agreement. More uniform and less dense array of spots under approximately the same contact area results in a lower contact resistance. This trend is more apparent when the number of balls in contact is increased which can be expected from theoretical predictions and confirmed by calculation and testing the real metal specimens with different surface treatment [1]. The theoretical prediction gives a ground for design of more efficient electrical contacts and most simple solutions are used in design of the stationary contacts with surfaces patterned by regular asperities distributing uniformly the mechanical load and spots of conductivity over the nominal contact area [1, 6, 7]. Figure 5 shows the application of such contacts in the busbar joints [6]. 7 Aus Wissenschaft und Forschung Figure 4: Plastic cage retaining the balls between disc plates (left) and scheme of the test (right) Figure 5: Typical application of the multilam contacts in busbar connection [6] 3 Solution of Tribological Problems in Electrical Contacts 3.1 Interrelation of friction and electrical processes When analyzing the operation of different types of contacts they can be considered as current-passing tribosystems. A specific feature of moving electrical contacts is their wear under operation conditions. Two basic wear modes are typical, i.e. mechanical wear caused by friction and electrical wear resulting from the effect of electric current on the contact materials. The mechanical wear appears in sliding electrical contacts in a similar manner as in common friction pairs. Depending on the current passage mechanism and the combination of contact ma- T+S_2_17 30.01.17 11: 58 Seite 7 8 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 terials the electrical wear may be caused mainly by the transfer of ions of one material to another. This factor can increase molecular attraction and induce seizure with tearingoff, sparking, and arcing. The processes of friction transfer and the formation of intermediate films on friction surfaces are of primary importance for sliding electrical contacts of all types. For example, the formation of the film is characteristic for brush contacts and it is strongly controlled by the direction of current [8]. In low-current contacts the films appear due to friction even on such noble metals as platinum (the effect of frictional polymerization) [9]. Inevitable interrelation of friction and current in operation is an important feature of the moving electrical contacts. This specific feature is taken into account in requirements to contact materials, coatings, lubricants, and contact design. The interrelation of friction and electrical processes is governed also by the state of the interface and by behavior of boundary films. The analysis shows that the improvement of sliding contacts includes the following three basic directions: a) development of new contact materials, coatings, and lubricants; b) special techniques affecting the state of the interface; c) improvement in contact design (Figure 6) 3.2 Design aspects The data relating the contact conductivity to topography are essential for making the recommendations for contact design. For example, it was shown [7] that the use of regular texture of contact surfaces can greatly improve the efficiency of moving electrical contact due to homogeneity in load distribution and conduction spots, as well as trapping of debris (Figure 7). Another significant design solution in electrical contacts is the development of fiber brushes for high-current pickoffs [10, 11]. Solving the brush wear problem by increasing the number of elastic contact spots is efficient from tribological point of view and simultaneously from the point of electrical conductivity. In order to meet the strict requirements for the current density and sliding velocities with low friction and wear, brush design similar to those used in the 19 th century (brush made of copper wires) was proposed for highcurrent pick-offs. Modern brushes of this type are made of small diameter metal or metallized fibers (Figure 8). Aus Wissenschaft und Forschung MEANS OF IMPROVING RELIABILITY OF ELECTRICAL CONTACTS DESIGN CONTACT MATERIALS INTERFACE CONTACT LOAD NUMBER OF INDEPENDENT ELEMENTS GEOMETRY OF ELEMENTS REDUCING ADHESION HARDENING SURFACE TOPOGRAPHY LUBRICATION Figure 6: Means of improving reliability of sliding contacts Figure 7: Scanning electron micrograph of the modulated surface [7] Figure 8: Micrograph of the cross section of almost finished “brush-stock”, comprising bundles of 20-μm gold fiber in a copper matrix [10]. White color shows the sections of gold microfibers, grey relates to copper. At final stage of the material processing the copper is etched out exposing the gold microfibers T+S_2_17 30.01.17 11: 58 Seite 8 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 Potentially superior properties of metal fiber brushes, in particular, low resistance, low noise, and high speed and current density capabilities have been demonstrated in practical applications [10]. To ensure the long-lasting, slow-wearing sliding metalmetal contacts it is essential to make the contact operating in the elastic deformation mode. As it is shown by Kuhlmann-Wilsdorf wear rate decreases to much lower levels when contact spots are elastic. Unlubricated metal-fiber brushes can operate with mild wear due to very large number of contact spots and hence low contact pressure. The presence of moisture film (~ 5 A) at contact spots is extremely valuable for metal-fiber brush operation since it is the thinnest conceivable surface film shield which prevents cold-welding and permits almost wearless sliding at modest friction. Fiber brushes were initially invented in order to provide current collection for homopolar motors. They need to provide current density of j = 310 A cm -2 at velocity v = 40 ms -1 with a maximum total loss of L T = 0.25 W A -1 and to be operated in a protective atmosphere of moist CO 2 . Due to low heat loss and voltage drops of about 0.1 V and less, extremely low noise, and very high current density metal fiber brushes should spread to all types of brush motors as costs come down and reliability is established. They will be irreplaceable whenever very high current density is needed, even in short pulses. For example, brushes currently being developed for magnetically levitated trains, planned to run at up to 300 m.p.h. ≈ 150 ms -1 . 3.3 Materials Formation of thin transfer films on contact surfaces, which do not affect strongly the current passage but reduce essentially the probability of seizure, welding, and severe wear is a generally used method of improving the performance of sliding contacts. For these purposes the composite materials containing conductive solid lubricants are commonly used. In most electrical sliding systems graphite brushes run on copper or copper-plated surfaces. However, in a few instances steel, nickel, or noble metals have been used. Therefore compatibility effects were studied by Rabinowicz and Ross [12]. It was found that when carbon and the metal of the counterbody are incompatible (low solid solubility) the wear tended to be low, and conversely (Figure 9). When the silver-graphite brushes were used, it was found that the lowest wear was obtained when metal of the counterbody had poor compatibility against both carbon and silver. Gold and copper were the metals giving the minimum wear against graphite, while rhodium and iridium were the best against silver-graphite. Electrical wear in the composite - metal contacts results from many factors which often influence each other and whose contributions are difficult to separate. However, if we can determine the principal factors and explain the mechanism of wear under the action of current, it becomes possible to select the optimal combination of contact elements and formulate the requirements to mechanical, electrical, and other properties of the contact materials. It follows from the experimental data that the action of several electrical wear factors is facilitated by the presence of oxygen in the ambient medium [1]. Oxygen accelerates the formation of non-conductive films followed by fritting, chemical erosion, selective oxidation of the binder, etc. The search for ways reducing the harmful influence of oxygen has shown that inert gaseous atmosphere (carbon dioxide, nitrogen, helium, argon, etc.) with controlled moisture can significantly increase the wear resistance and improve electrical characteristics of carbon and metal-graphite contact materials [13]. It has been shown [14] that new types of composite contact materials produced by traditional technology on the base of common materials can operate with much higher current density and sliding velocity than it was previously assumed. An example is a new type of sintered coppergraphite composite materials with improved self-lubrication for railway current collectors. These materials use the concept of a network structure of copper matrix for the electrical conduction and self-lubrication which is provided by graphite and MoS 2 powders stored in the pores. The unique microstructure of the material, which is composed of a graphite islands in a copper matrix achieves the solid lubrication function with very little effect on electrical conduction during sliding. A special electrical conductive mechanism - network conduction plays a major role in maintaining the low resistivity of the material. Friction and wear of the composite-metal contacts are basically determined by the 9 Aus Wissenschaft und Forschung Figure 9: A plot of the average wear coefficient versus the compatibility of the flat materials against graphite for pure graphite brush tests [13] T+S_2_17 30.01.17 11: 58 Seite 9 10 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 action of electric current on the brush surface layer [8]. The reversible reduction in strength of the surface layer of carbon brushes at the Joule heating reduces microcutting and causes “lubrication” by the electric current. Irreversible changes in the surface layer of brushes with high content of non-carbonized polymer binder at certain critical magnitude of contact current lead to sharp reduction in friction and increase in wear intensity of these materials. Development of plastic deformation leading to increase in the contact area, explains the frictional behavior of brush materials with high content of metal. The changes in the collector surface layer and intermediate films during current passage influence the frictional characteristics to a lesser degree than electrical characteristics of the contact. In the absence of sparking, the basic cause of brush wear intensification under the action of electrical current is the Joule heating in the friction contact. Depending on the operation conditions and type of contact materials the additional heat release resulting from current passage may lead to mechanical weakening of the brush surface layer, roughening of the collector surface, intensification of adhesion on contact spots, reversible reduction in the brush material strength near the contact spots, mechanical stresses because of thermal expansion, etc. The electric field in the contact area can be additional factor affecting wear rate due to oxidation, mass transfer, and debris formation. 3.4 Lubricants in electrical contacts The applications of lubricants in electrical contacts are steadily increasing. Nowadays almost all types of lightduty sliding contacts operating at low velocities without arcing and electrical erosion are lubricated. Lubricants were also used in heavy-duty sliding contacts. The application of a proper contact lubricant can reduce sliding wear by several orders of magnitude [15]. In order to examine the role of lubrication it is necessary to evaluate its influence on both frictional and electrical characteristics of the contact, and this influence may evidently be ambiguous. In a general case, the effects of lubricants can be positive and negative. Along with the positive effects, leading to reductions of mechanical and electrical losses and improvement of the service life and operating reliability of the contact, negative effects are also possible. The latter are characterized by the necessity of complicated design and prevention of irreversible changes in lubricant composition, and also prevention of the hydrodynamic lubrication conditions, leading to sparking and electrical erosion of the contact. Ideal properties of a good sliding-contact lubricant were formulated by Glossbrenner [16] as follows: • Operates best if retained on the contact surface by chemical adsorption or strong physical adsorption • Must not chemically degrade metals of the contact surfaces • Must not chemically react with insulation of the contact assembly • Must not chemically react with gases of the operating environment • Must have low volatility at the operating temperature and pressure • Must have proper viscosity at given velocity, temperature, and contact force • Must not form friction polymer of sufficient viscosity to separate contacts • Should have low surface energy to wet the contact surfaces Both liquid lubricants and greases are used in sliding contacts. Some organic fluids such as esters and polyglycols provide better antifriction and antiwear properties for sliding contacts than the organic greases [17]. Among the positive effects we note one which is achieved only by the application of liquid lubricants. If a liquid lubricant wets well the contact surface, the lubricant removes other fluids, e. g. water, from the surface and forms a thin protective film. The halogen-containing liquid lubricants are effective for application in sliding electrical contacts whereas the silicon fluids are the poorest ones [18]. Oxygen-containing and hydrocarbon fluids take an intermediate position. Mass-produced perfluorinated polyalkylether (PFPE) fluids are widely used. They also serve as a base for developing filled lubricants for application in certain contact pairs, for example made of gold [19]. Greases are quite often used in sliding contacts [15]. Graphite, dichalcogenides of metals (Mo,W, Nb, Ta), and metal powders are used as fillers to greases. The bulk electrical characteristics of lubricants with the conductive filler vary slightly, yet the conductivity of thin films (from a few to hundreds microns) increases significantly. High filler concentration can deteriorate the grease lubricity, so the optimal content of the conductive filler has to be determined experimentally for each application. In high-current sliding contacts lubricants are used more rarely and they can be divided into two groups. The first group includes the compositions whose properties are similar to those of mass-produced lubricants for light-current contacts, i. e. they do not contain conducting particles. The second group comprises greases with fine conductive fillers. For example, graphite suspensions in oil are used to lubricate contacts of Aus Wissenschaft und Forschung T+S_2_17 30.01.17 11: 58 Seite 10 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 welding machines, urban transport pantographs and current pick-offs. Several base oil chemistries have been successfully applied in contact lubricants. Selecting the proper lubricant starts with the choice between the synthetic and petroleum-based products. Petroleum products begin to degrade at or before 100 °C, and become virtually intractable at sub-zero temperatures; some synthetic lubricants still function well at -70 °C to beyond 200 °C. Synthetic lubricants may cost more than petroleum-based lubricants, but they generally can be used in smaller quantities (Table 1). Material compatibility should be considered as well. While lubricants do not affect most thermoplastics, esters (diesters and polyolesters) are noted for their incompatibility with polycarbonate, polyvinyl chloride, polystyrene and acrylonitrile-buta-diene-styrene copolymer resins. Only the fluoroethers (PFPEs) are inert enough to be safe with most polymers. Compatibility charts are available from many manufacturers, but testing is the only way to guarantee a successful match between lubricant and contact material. 4 Summary. • Electrical contacts are in most cases specific tribosystems which can be considered in complexity of current conduction, friction, and changes in structure and composition of materials and surface films. • Progress in engineering, more severe operation conditions, and necessity of optimization in mechanical and 11 Aus Wissenschaft und Forschung Table 1: Lubricant application in electrical connectors and sliding electrical contacts [1] Application Lubricant alternatives Special characteristics Electrical connectors (automotive, telecommunication, computer or PC board connectors; backplanes; instrumentation accessories) • Synthetic hydrocarbon and ester based lubricants • Fluorinated ether based lubricants • Polyphenyl ether based lubricants • Provides thin protective film on tin-lead connectors, reducing corrosion and contact forces while making a weather resistant seal. • Long life without generating lubricant decomposition products. • Traditional for gold on gold. Electrical sliding contacts (automotive instrumentation; rotary and sliding switches; appliance controls; distribution switch gear) • Synthetic hydrocarbon greases • Synthetic hydrocarbon gels • Synthetic polyether greases • Polyol ester greases • Water-resistant; good plastic compatibility. • Non-melting; water-resistant; oxidation stability; low volatility. • Suitable for high temperature, arcing conditions. • High lubricity; salt water-resistant; wide temperature fluidity. Potentiometers (wire wound and conductive plastic potentiometers; trimmers; automotive sensors; aerospace controls) • Halogenated silicone oils and gels • Polyol ester greases • Fluorinated ether greases • Temperatures from -70 °C to 200 °C; good noise reduction. • High temperatures stability; good noise reduction; low thin-film volatility. • Resists aggressive chemicals and all except fluorinated solvents; temps from -65 °C to 225 °C. T+S_2_17 30.01.17 11: 58 Seite 11 12 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 energy losses require new solutions in design and materials of electrical contacts. • The research basis and achievements in surface engineering make realistic further increase in contacts efficiency and reliability. References [1] Braunovic, M., Konchits V. V., Myshkin N. K.: Electrical Contacts: Fundamentals, Applications and Technology, New York: CRC Press 2007 [2] Holm, R.: Electrical Contacts. New York: Springer, 1979. [3] Bowden F.P., Tabor D.: Friction and Lubrication of Solids, Oxford: Clarendon Press, 1986 [4] Greenwood, J.A.: Constriction resistance and the real area of contact. Brit. J. Appl. Physics. 17 (1966), 1621-1631 [5] Myshkin, N. K., Kim, C. K.: Influence of relative position and size of metal contact spots on conductivity. Journal of Friction and Wear. 15(1994), 54-58 [6] The multi-lam principles, Multi-contact Tech., Catalog Multi-Contact USA, Santa Rosa, California [7] Saka, N., Llou, M. G., Suh, N. P.: The role of tribology in electrical contact phenomena. Wear. 100 (1984), 77-105 [8] Myshkin, N. K., Konchits, V. V.: Friction and wear of metal-composite electrical contacts. Wear. 158 (1992), 119- 140 [9] Konchits, V. V.: Polymerization in Friction. Ch. 824 in “Encyclopedia of tribology” (Q.J.Wang & Yip Wah Chung eds), Springer Science+Business Media New York, 2013, 2644-2648 [10] McNab, J. R., Wikin G. A.: Carbon fiber brushes for superconducting machines. Electronics and power. (1) (1972), 8-12. [11] Kuhlmann-Wilsdorf, D.: Metal fiber brushes. In: Slade,P.G., ed. Electrical contacts. Principles and Applications. New York: Marcel Dekker, Inc., 1999, 943-1017 [12] Rabinowicz, E., Ross, A. Z.: Compatibility effects in the sliding of graphite and silver-graphite brushes against various ring materials. In: Proceedings of 30 th IEEE Holm Conference on Electrical Contacts. Chicago, USA, Sept. 17-21, 1984, 499-506 [13] Shobert, E.: Sliding electrical contacts (graphite type lubrication). In: Slade, P.G., ed. Electrical Contacts. Principles and Applications. New York: Marcel Dekker, Inc. 1999, 839-872 [14] He, D. H., Manory, R.: A novel electrical material with improved self-lubrication for railway current collectors. Wear. 249 (2001), 626-636 [15] Antler, M.: Sliding studies of new connector contact lubricants. IEEE Trans, CHMT. CHMT-10 (1987), 24-31 [16] Glossbrenner, E. W.: Sliding contact for instrumentation and control. In: Slade,P.G., ed. Electrical contacts. Principles and Applications. Marcel Dekker, Inc. New York, 1999, 885-941 [17] Myshkin, N. K., Konchits, V. V., Kirpichenko, Yu. E., Markova L. V.: Wear of contact elements in electromechanical switches. Wear 181-183 (1995), 691-699 [18] Antler, M.: Tribology of electronic connectors: contact sliding wear, fretting, and lubrication. In: Slade, P.G., ed. Electrical contacts. Principles and Applications. New York: Marcel Dekker, Inc., 1999, 309-402 [19] Smith, E. F., Klein, A. Ney, J.M., Lysonski, R., Agopovich, J. W., Dentob, R.: Screening contact materials for low speed slip ring assemblies. In: Proceedings of 39 th IEEE Holm Conference on Electrical Contacts. Pittsburg, USA, Sept.27-29, 1993, 157-170. Aus Wissenschaft und Forschung Anzeige Nutzen Sie auch unseren Internet-Novitäten-Service: www.expertverlag.de mit unserem kompletten Verlagsprogramm, über 800 lieferbare Titel aus Wirtschaft und Technik T+S_2_17 30.01.17 11: 58 Seite 12