1 Introduction The most common case for breakdowns of electrical connectors is fretting corrosion. The building of an isolating oxide layer between the contact metals is caused by micro movements (oscillation, vibration, or thermal expansion).[1] A well-functioning of the connectors depends on its constructional composition as well as the friction management. Lubricants have to fulfil high-quality requirements for this complex tribosystem to ensure a long and safe lifetime of electrical contacts. The challenge for lubricants is to offer the required protection against wear, fretting corrosion and friction and thus reducing forces without hindering the electrical current to flow and ensuring a low ohmic resistance. 2 Electrical contacts 2.1 Type of electrical contacts Typically, the lubricant is responsible for ensuring a long lifetime of electrical contacts. Thinking of safety switches this is especially important and the lubricant becomes a safety relevant construction component. Special lubricants wtih characteristics against wear, corrosion, and high lubricity allow a low contact resistance, constant insertion/ operating forces, and no short circuits. This functions by a coat of lubricant on the metallic connectors and its displacement upon operating force. Electrical contacts are divided in two groups: stationary and moving. Moved electrical connections are further divided in sliding and commutating ones. The way of movement in commutating connectors is much more challenging due to the harsh conditions of the tribosystem than the one in stationary contacts. Hence, the requirement of the lubricant is much higher to protect electrical contacts e.g., brush, slider, trolley and separable contacts. All tribosystems in electrical contacts depend on the: [1] • construction  surface pressure  geometry  friction mechanism • contact materials  surface roughness  hardness  electrical conductivity  thickness of the conductive layer • installation location  ambient temperature  atmosphere  electric current and voltage Research 39 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0032 Electrical contacts - the challenge for lubricants Sarah Hüttner, Verena Leumann, Rachel Kling* Electrical contacts can be found in all industries today. In these applications lubricants are essential because they protect the metal connectors from friction and fretting corrosion, which generate wear and may lead to an isolating oxide layer and finally the destruction of the electrical connection. Measurements on customary lubricants for electrical connectors were conducted with high regard to oxidation stability, wear and friction characteristics. The selected samples also differ in the thickener systems. Besides typically non-conductive lubricants for electrical contacts, there are lubricants with high electrical conductivity required mainly for EDM bearings (electrical discharge machining) to not promote a system failure. Currently, action is taken in the industry to define standards to measure the electrical conductivity of lubricants. Keywords friction, corrosion, wear, oxide layer, oxidation stability, EDM bearings, standards Abstract * Sarah Hüttner Verena Leumann Setral Chemie GmbH, Seeshaupt, Germany Rachel Kling Sétral S.à.r.l, Romanswiller, France contacts.[2] Special antioxidant additives or base oils are necessary that lubricants withstand the development of these high temperatures. New peaks on the metal surface provide more friction or even destroy the conductive layer. Both leads to higher friction and isolating oxide layers that destroy the connection. Often, for a higher conductivity the metallic surface is coated with a very conductive metal e.g., gold, silver or copper. Since most of these metals are expensive, the layer thickness is just as thick as necessary. Typically, the interior components of electrical contacts are made out of plastics - very common are ABS and PC that are sensitive to cracking. Suited lubricants for these applications need to be compatible with the listed contact materials. Further the active protection of metallic surfaces is an essential requirement to prevent oxidation. Another important role of lubricants in an electrical environment are bearings for EDM (electrical discharge machining). Thereby, the lubricant needs to be electrically conductive to not promote a system failure. These applications are common in railway, vehicle, e-mobility technology and many more, wherever variable-speed, converter-fed electric motors and generators are used.[3] 3 Electrical conductivity 3.1 Method to measure the electrical conductivity Currently, there is no standard method to measure the electrical conductivity of lubricants nor a universally recognized definition of the value above which lubricants are considered electrically conductive. For an easy pretest for the determination of the electrical conductivity, self-constructed electric measuring cells (Figure 3.1.1) are available at setral ® - one for fluids like oils, the other for consistent lubricants like pastes or greases. The grease is spread out on the contact surface with a defined layer thickness. The electric resistance is Research 40 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0032 2.2 Functioning of the tribosystem in electrical contacts Upon closure of the electrical contact the metal peaks of the connector surfaces come into contact and allow the current to flow. The lubricant remains in the troughs together with air (see cross section in Figure 2.2.1). This application is common in stationary electrical contacts. Figure 2.2.1: orange: lubricant, white: air, arrows indicate the current flow Figure 3.1.1: electric measuring cells For the best support of the lubricant and thus the wellfunctioning of electrical contacts all parameters have to be taken into account like the general type of electrical contact, the temperature, the voltage, the amperage and the material roughness, thickness and composition. 2.2 Typical applications and requirements of the lubricant In almost all industry sectors the need of electrical contacts rises for the use in switches, controllers, printed circuit boards and many more. In the automotive industry electric contacts are ubiquitous. Here, actuartors are good example. They function by electrical contacts installed in seat height adjustments, brake boosters, control tank caps, tailgates, locking mechanism of cars etc. Not every application with electrical contact needs a lubricant with “good” electrical conductivity. The usual lubricant in moved and stationary electrical contacts is non-conductive. Other properties are much more important, because its main goal is to reduce operating and friction forces as well as wear and oxidation. Special EP/ AW additives have to be added to the lubricant to prevent the formation of abrasion particles and fretting corrosion due to micro movements.[1] As a result of insufficient lubrication an oxide layer forms on the metallic connector and the metal becomes isolated in this area and thus prevents the current from flowing. Unlubricated stationary contacts often produce arcing during opening and closing. Ionisation of the air and the associated rise in temperature causes metal transfer between the contacts, accompanying the formation of new “peaks and troughs” - a common problem in high power being measured with a high resistance measuring device by taking into account the value of the conductor area and the gap. The electrical conductivity is calculated with the reciprocal value of the electrical resistance. (Table 3.1.1) metal powders e.g., copper, graphite, graphene or carbon nano tubes (CNT). The usage of metal powders might be accompanied by other undesired effects. Therefore, lubricants filled with metal powder have to be tested thoroughly and over a long period of time in the application e.g., circuit breaker, switchgear and switches. Possible results include accelerated corrosion and remaining conductive paths, which lead to corrosion and create conductive paths, leading eventually to failures.[4] Due to their tunnel structure, CNT behave like conductor and conduct the current in one direction.[5] This technology enhances the electrical conductivity in small amounts by up to pS/ m. Using graphite increases the electrical conductivity by the factor of 1000 compared to MoS 2 . Hence, graphite is often used in electrically conductive lubricants. Ionic liquids are also a possibility for increasing the electrical conductivity. The disadvantages are the bad solubility in PAO oils of most types and that other properties of the iconic liquids have to be considered since some of them promote corrosion.[6] Additionally, the REACh restriction and the consequent limitation has to be considered. Other commonly used solid lubricants, e.g., PTFE, MoS 2 and graphite should be avoided because they are considered to increase the electrical resistance after a long period of use.[4] 4 Experiments In focus of the experimental part are electrical switches which operate at temperatures of 80 °C. Standard thickened metal soap greases are commonly used for this application. Even within this type of application the lubricant faces various challenges regarding the manifold operating conditions. This is why a lot of different formulation can be found on the market. 4.1 Data evaluation A selection of them were regarded in the following experimental part with different base oils, thickeners and some even with copper content. The samples in Table 4.1.1 were generated according to the following measurements: • Oil separation acc. to ASTM D 6184 • Worked penetration acc. to DIN ISO 2137 Research 41 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0032 Table 3.1.1 Table 3.1.2 As the first preliminary tests show, all lubricants used in electric contacts are non-conductive. Recently, there are some devices for measuring the electrical conductivity of lubricants. These devices are able to measure the conductivity at a wide temperature range from minus degrees Celsius up to high temperatures. Yet, due to its construction and the close range of the measurability of electrical conductivity it is not suited for greases but rather for electric conductive fluids. Other new methods to measure the electrical conductivity use a special adapter for the rheometer. This technology has a lot of functions and options. The strength and weakness analysis should be done carefully since a lot of knowledge and experience is required to understand the measured value and, even more important, to cover them with the actual application. 3.2 Method to improve the electrical conductivity It is important to consider that every electrical contact requires a specially designed lubricant. The commonly used lubricants in electrical contacts are non-conductive and work by other characteristics (see 2.2). Yet, sometimes there is a requirement for lubricants to be used as additional electrical contact provider to boost the electrical conductivity. This property can be enhanced by The electrical conductivity of all samples is in the range of non-conductive greases. Copper powder is added to samples B, D and G to positively influence the electrical conductivity. This leads to an increase of wear compared to identical samples without copper and for sample D increases the oxidation stability, thus copper seems to act as a catalyst.[7] Many samples reach the maximum load value of 2000 N in the SRV EP test (Chart 4.1.3). Only sample D with 1900 N, sample B with 1700 N and sample H with 1500 N Research 42 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0032 • Kinematic viscosity acc. to DIN 51562 • Copper corrosion acc. to DIN 51811 • SRV (EP) acc. to ASTM D 5706 • SRV (AW) acc. to ASTM D 5707 • RapidOxy (oxidation stability) acc. to ASTM 942 • Electrical conductivity acc. to a house method (see 3.1) (Table 4.1.1, Chart 4.1.2, Chart 4.1.3) Table 4.1.1 Chart 4.1.2 Chart 4.1.3 fall short of the values. For B this is unexpected since, the only difference to A is the copper content and copper may be used as solid lubricant. H needs more measurements for an interpretation because of the variety in differences to other samples. The lowest wear values show samples E, C and A according to the SRV AW test (Chart 4.1.2). All of the three samples reach a max. load of 2000 N in the EP test. These samples have a penetration in the range of NLGI grade 2 and an oil separation between 2-3 %. Regarding the samples E and F in combination with tests of the RapidOxy oxidation stability and the electrical conductivity, the only difference between sample E and F is, that the formulation of F contains EP additives of different types and it is one NLGI class softer. Thus, the variation in additives leads to a slightly higher value in the oxidation test. Further, this difference in additives and NLGI might lead to the increase in electrical conductivity. Sample pack four with inorganic thickened pastes shows the highest wear in the SRV AW & EP (Chart 4.1.2 and Chart 4.1.3) with sample I having exhibiting the lowest oil separation but the highest electrical conductivity. 4.2 Data analysis of the COF in the SRV EP test To evaluate the influence of the thickener in electrical contact lubricants, sample A, F and I were compared. With PAO, they all have the same base oil, but different thickeners. The SRV EP focuses on the maximum load and the coefficient of friction (COF). The running-in phase for all samples was almost 16 hours. The load was then increased in steps of 200 N up to 2000 N. Frequency (50 Hz), stroke (1 mm) and the temperature at 80 °C were constant during the measurements. Sample A The structure of calcium sulfonate complex greases is based on overbased calcium sulfonate. This is a colloidal inverse micelle disperson formed from amorphous CaCO 3 . Its micell structure is stabilized by a surfactant in the oil. The tickener with calcium carbonate initally forms a viscous caoting on the surface component. Next, calcium sulfonate creates sulfonate chains that aline vertically to the surface. At higher pressures, the sulfonates are ejected from the contact and a tribochemical film forms.[8] (Chart 4.2.1) Due to its typical thickener property, the calcium sulfonate contributes to a good extreme pressure. Thus, sample A reaches 2000 N without seizing. Further it shows a low and constant running-in phase of the COF in Chart 4.2.1. Upon increased pressure the COF decreases. Another special feature of the thickener is its micelle structur, which leads to a good oil-binding property. Calcium sulfonate greases have further special features e.g., good shear stability, a barrier to water, good anticorrosion protection, etc.. Sample F The fiber structure of lithium single soaps compared to calcium sulfonate thickeners shows a similar decreasing trend in the coefficient of friction. The running-in phase is a bit more restless compared to Chart 4.2.1. This effect might be caused by the larger particle size of single soaps. Due to the hydrogen-bonding of the hydroxyl groups, the lithium single thickener has multi-purpose properties, e.g. good shear stability, good water resistance, providing a good boundary film which causes a low coefficient of friction, etc.[9, 10] Compared to sample E, the coefficients of friction are lower. These two samples are similar but with the difference in base oil viscosity, NLGI and additional additivities. (Chart 4.2.2) Research 43 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0032 Chart 4.2.1: sample pack 1, A Chart 4.2.2: sample pack 3, F struction of the conductive layer also by preventing the formation of a non-conductive oxidation layer. All these effects work together perfectly for a long and safe lifetime of the electrical connection. References: [1] V.V. Konchits, N.K. Myshkin, Tribology of Electrical Contacts, Tribologie und Schmierungstechnik. 2017, 2, 5-12 [2] Electrolube, Contact lubricants, Switch to a superior performance, Switches and Contacts [3] B. Sauer, D. Bechev, T. Kiebach, B. Radnai, Untersuchung der Auswirkungen von leitenden und nichtleitenden Schmierfetten auf die Oberflächeneigenschaften bei spannungsbeaufschlagten Wälzlager, 2018, 3, 5-11 [4] Bella H. Chudnosky, Lubrication of Electrical Contacts, Square D, West Chester, OH 45069, USA [5] Abdullah Abdulhameed, Nur Zuraihan Abd Wahab, Mohd Nazim Mohtar, Mohd Nizar Hamidon, Suhaidi Safie, Izhal Adul Hain, Methods and Applications of Electrical Conductivity Enhancement of Materials Using Carbon Nanotubes, Springer Link, 2021 [6] C. Enekes, A.Kailer, C. Dold, T. Schubert, M. Ahrens, P. Altmann, S. Grundei, Elektrisch leitfähige Schmierstoffe für adaptive Tribosysteme - Synthese und tribologisches Verhalten unter dem Einfluss von elektrischen Potenzialen, 2016, 3, 38-44 [7] C.A. Migdal, Oxidation and the testing of turbo oils, 2008 [8] Rob Bosman and Piet M. Lugt, The microstructure of calcium sulfonate complex lubricating greases and its change in the presence of water, 2018 [9] Edit by Theo Mang, Wilfried Dresel, Lubricants and lubrication, 2007, 653 [10] Atsushi Yokouchi, Michita Hokao, Joichi Sugimura, Effects of soap fiber structure on boundary lubrication of lithium soap greases, 2011 Research 44 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0032 Sample I In comparison to sample A and F, sample I with an inorganic thickened paste shows the highest electrically conductive potential. The swelling behavior and consequently the thickening of the inorganic thickener in the base oil is well pronounced and results in the observed low oil separation. Depending on the type of inorganic thickener (e.g. silica) particles themselves have a hard and rough surface finishing. This characteristic is observed in the restless running-in phase and with the start of the applied load it comes to some high peaks in the COF. This is typically for inorganic thickeners as the particles tend to form agglomerates without well lubricity. The agglomerate disintegrates into the individual particles under continuous mechanical shear and pressure. (Chart 4.2.3) 5 Conclusion In general, soaps as calcium sulfonate or lithium soaps which form a micelle or fiber thickener structure achieve constant and low coefficients of friction. Addition of metal powder (e.g., copper) enhances the electrical conductivity with the disadvantage of higher coefficients of friction, lower oil separation and a degradation of the oxidation resistance. RapidOxy tests were run for the samples B, D, E and F and show the expected catalytic effect of copper as stated in literature.[7] A low oxidation rate is important for electrical connector applications for a long lubricant lifetime and for operating under high voltages where electrical arcing may occur. The lubricant must be able to withstand the developed high temperatures without significant degradation to further protect the metal contacts and ensure a long lifetime of the electrical switch. Depending on the application the NLGI grade and the thickener system with characteristic oil separation need to be considered. Suited lubricants reduce the friction, preventing the formation of new peaks on the metal surface and the de- Chart 4.2.3: sample pack 4, I