Aus Wissenschaft und Forschung 14 Tribologie + Schmierungstechnik · 67. Jahrgang · 5-6/ 2020 DOI 10.30419/ TuS-2020-0024 The effect of lubrication and thermal storage on the fretting behavior of electrical contacts Haomiao Yuan, Jian Song* Eingereicht: 1. 9. 2020 Nach Begutachtung angenommen: 22. 11. 2020 Bei der Entwicklung von cyber-physischen Systemen werden zahlreiche Steckverbinder zur Stromleitung und Signalübertragung zwischen Modulen und Geräten verwendet. Der elektrische Kontakt ist der kritische Bereich im Steckverbinder. Das Verhalten und die Lebensdauer von elektrischen Kontakten beeinflussen die Zuverlässigkeit des gesamten cyber-physischen Systems stark. Zur Verbesserung des Verhaltens und der Zuverlässigkeit von elektrischen Kontakten wird in der Regel eine Schutzschicht verwendet. Häufig wird hierfür, aufgrund ihrer guten Lötbarkeit, hohen Korrosionsbeständigkeit und niedrigen Kosten, eine Zinnbeschichtung verwendet. Allerdings ist die Zinnbeschichtung anfällig für Reibkorrosion. Deswegen ist die Lebensdauer herkömmlicher verzinnter elektrischer Kontakte eingeschränkt, wenn Mikrobewegung unvermeidbar ist. In der Kontaktzone können Schmierstoffe verwendet werden, um die elektrischen Kontakte vom Sauerstoff zu trennen. Dadurch kann die Oxidation der Zinnschicht verhindert und somit die Lebensdauer verzinnter elektrischer Kontakte verlängert werden. Allerdings sind nicht alle Schmierstoffe für Anwendungen bei elektrischen Kontakten ausreichend thermisch stabil. In diesem Beitrag wird der lebensdauererhöhende Effekt von zwei Schmierstoffen, nämlich einem PFPE- Öl und einem Schmierfett auf PFPE-Basis, auf den verzinnten elektrischen Kontakten untersucht. Darüber hinaus wird durch Reibkorrosionsuntersuchungen nach Wärmelagerung die thermische Degradation auf den elektrischen Kontakten mit Schmierstoffen untersucht. Unsere Untersuchungen bestätigen, dass die Lebensdauer verzinnter elektrischer Kontakte durch den Einsatz von Schmierstoffen deutlich erhöht werden kann. Schlüsselwörter elektrische Kontakte, Reibkorrosion, PFPE-Öl, Schmierfett auf PFPE-Basis, thermische Degradation, Zinnbeschichtung With the development of cyber-physical systems, a large number of electrical connectors are used to transfer currents and signals between terminals. The electrical contact is the critical area in the electrical connector. The performance and the lifetime of the electrical contacts greatly influence system reliability. In order to improve the performance and reliability of electrical contacts, plating is generally used. Tin plating is widely used due to good solderability, corrosion resistance and low cost. However, tin plating is prone to fretting corrosion and consequently the lifetime of electrical contacts, with conventional tin surfaces, is relatively short, if micromotion is unavoidable. Some lubricants can be used to form a protective layer to separate the electrical contacts from oxygen, which can improve the performance of the tin plating. As a consequence, the lifetime of the tin-plated electrical contacts can be prolonged. However, not all the lubricants are sufficiently thermally stable for connector applications. In this paper, two kinds of lubricants, namely PFPE oil and perfluorinated oil-based grease, are applied on the tin plating. The development of electrical contact resistance and the lifetime of electrical contacts are measured by a fretting corrosion test device and compared to the results from unlubricated samples. The effect of the two lubricants on the lifetime is studied. Moreover, the thermal degradation of the electrical contacts with lubricants is also investigated using fretting corrosion tests, after storing these contacts at an elevated temperature. With the application of the lubricants, a large improvement in the lifetime of the tin-plated electrical contacts is achieved. Keywords electrical contacts, fretting corrosion, PFPE oil, perfluorinated oil-based grease, thermal degradation, tin plating Kurzfassung Abstract * Haomiao Yuan, M.Sc. Orcid-ID: https: / / orcid.org/ 0000-0003-2175-9588 Prof. Dr.-Ing. Jian Song (Corresponding author) Orcid-ID: https: / / orcid.org/ 0000-0002-7627-9824 Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences and Arts, 32657 Lemgo, Germany TuS_5_6_2020.qxp_TuS_Muster_2020 09.12.20 16: 08 Seite 14 1 Introduction With the development of cyber-physical systems, such as smart and networked factories and autonomous vehicles, a large number of sensors and electronic devices will be used, which in turn will require large quantities of electrical connectors to connect these terminals. For instance, in a modern middle class car there are approx. 250 electrical connectors with about 2000 electrical contacts [1]. Electrical connectors transmit signals and the current in the cyber-physical systems. Thus, the performance and the lifetime of the electrical contacts greatly influence the system reliability. As electrical contacts are employed for current transmission, electrical contact resistance is the main criterion for the assessment of their reliability [2]. In order to improve the performance and reliability, plating is generally used in the electrical contacts. Tin plating is widely used due to its good solderability, good corrosion resistance and low cost [3-5]. However, the tin plating can also be oxidized because it is a non-noble material. One of the critical failure mechanisms is fretting corrosion [6]. In order to prolong the lifetime of tin plated electrical contacts, some lubricants can be used to form a protective layer to separate the electrical contacts from oxygen, which can assist in retarding the oxidation of the tin plating. With regard to the use of lubricants for electrical contacts, one of the main problems is the thermal degradation of the lubricants. In this investigation, two kinds of lubricants, namely PFPE oil and perfluorinated oil-based grease, are applied on the tin plating. The development of electrical contact resistance and the lifetime of electrical contacts are measured by a fretting corrosion test device and compared to the results from unlubricated samples. The effect of the lubricants on prolonging lifetime is studied. Moreover, in order to test the degradation of the lubricants at a high temperature, thermal storage is applied to the samples with lubricants. 2 Lubrication of electrical contacts 2.1 Fretting corrosion of tin plated electrical contacts Fretting corrosion is an oxidation phenomenon which takes place during the cyclic relative micro motions between contact parts during operation. This micro motion can be induced by the vibration and/ or the difference in the thermal expansion coefficients of the holding materials. Figure 1 illustrates the failure mechanism of fretting corrosion schematically. Before operation a thin airtight oxide layer exists on the tin surface, preventing further oxidation into the fresh tin. After the mating of the contact parts, the air-tight oxide layer is broken by the normal load and then a metallic contact is built to transfer currents. In the meantime, the fresh tin is exposed to the oxidative ambient atmosphere and oxidized. During the operation, the oxides are aggregated. After a Aus Wissenschaft und Forschung 15 Tribologie + Schmierungstechnik · 67. Jahrgang · 5-6/ 2020 DOI 10.30419/ TuS-2020-0024 Figure 1: Schematic illustration of the progress of fretting corrosion: (a) intact oxide passivating layer on the tin plating before mating; (b) breaking of the thin oxide layer and establishment of the metallic contact; (c) new generation of the oxides; (d) aggregation of the oxides and electrical failure of the contacts ( ) (b) Initial oxides Newly formed oxides (a) (b) (c) (d) TuS_5_6_2020.qxp_TuS_Muster_2020 09.12.20 16: 08 Seite 15 3 Experimental 3.1 Materials The samples have a spherical/ flat configuration. The radius of the spherical part is 4.5 mm. The base material of the samples is pure copper. The tin plating of approx. 7~8 µm is electroplated on the base material. The coating thickness is measured by an X-ray fluorescence device (XRF, Fischerscope ® X-Ray system XDAL, from Helmut Fischer GmbH + Co.KG, Germany). Two kinds of lubricants, PFPE-oil (Krytox ® GPL 105) and perfluorinated oil-based grease (Lubrinox 2), are used in this investigation. The electrical contacts are firstly degreased with ethanol to remove any potential residual oil from the sample fabrication. After the evaporation of the ethanol, lubricants are applied using a cotton swab. To test the stability of the lubricants, the electrical contacts with lubricants are stored in an oven at 130 °C for 15 weeks. The sample catalogs used in this investigation are summarized in Table 1. Aus Wissenschaft und Forschung 16 Tribologie + Schmierungstechnik · 67. Jahrgang · 5-6/ 2020 DOI 10.30419/ TuS-2020-0024 long-term operation, the oxide layer is too thick to break and the current path is blocked. Thus, the electrical contact resistance increases sharply and the electrical contact fails electrically. If a lubricant is applied on the surface of the electrical contacts, the contact is separated from oxygen, thereby the oxidation of the tin plating can be retarded. As a consequence, the lifetime of the tin plated electrical contacts is expected to be prolonged. 2.2 Requirements for the lubricants used for electrical contacts Lubricants can be used on various platings in electrical contacts, for instance platings of Au [7, 8], Ag [9], Sn [10, 11], and Ni [12]. Since the lubricants are used for electrical contacts, some additional requirements should be fulfilled comparing to lubricants for other mechanical components, such as gears: • No electrical contact resistance increase After the use of the lubricants, the electrical contact resistance should not increase after the mating of the electrical contacts. • Non-oxidative Since the failure mechanism of fretting corrosion is closely related to oxidation, the lubricants should not be oxidative in order to prevent oxidation. One of the mechanisms which enables the use of lubricants to improve the performance of electrical contacts is the suppression of oxidation on the surface of electrical contacts [13, 14]. • Non-reactive with the materials If the lubricants can react with the materials of the electrical contacts, the products of reaction can increase the electrical contact resistance and degrade the protection effect. Since the electrical contacts work in different environments and have various finishes, an inappropriate lubricant may be harmful to the surface [15] and the electrical contact resistance can be increased due to the reaction of the lubricant with the surface material [16, 17]. • Good thermal stability Due to the electrical contact resistance, the temperature at the contact area will increase during operation. Moreover, depending on the installation position of the electrical contacts, such as near the engine of a car, the ambient temperature is elevated. In order to guarantee the protection effect of lubricants, good thermal stability is required, which means the lubricants should be resistant to an elevated temperature [18]. Sample 1 Tin plating, without lubricant, without thermal storage Sample 2 Tin plating, with PFPE-oil, without thermal storage Sample 3 Tin plating, with perfluorinated oil based grease, without thermal storage Sample 4 Tin plating, with PFPE-oil, with thermal storage Sample 5 Tin plating, with perfluorinated oil based grease, with thermal storage Figure 2: Self developed wear and fretting corrosion test device Table 1: Sample catalogs 3.2 Fretting corrosion test The lifetime of the samples is tested by a fretting corrosion test with an apparatus developed in our laboratory, Figure 2. The relative motion between contact pair with- TuS_5_6_2020.qxp_TuS_Muster_2020 09.12.20 16: 08 Seite 16 in µm-range is generated by a voice-coil motor. The test parameters are listed in Table 2. During the tests, the electrical contact resistance is recorded. The lifetime of the electrical contacts in the test is defined as the fretting cycle when the electrical contact resistance reaches 300 mΩ. The maximum test cycle is 100,000. After the fretting corrosion test, the wear scar is observed using a digital microscope (Keyence Corporation microscope VHX-2000). 4 Results and discussion 4.1 Protection effect of the lubricants The average initial electrical contact resistance is shown in Table 3. Generally, all the samples have very low initial electrical contact resistance. After thermal storage, the electrical contact resistance decreases slightly, due to the softening of the tin surface and the reduction in the viscosity of lubricants. As a result, the contact area is enlarged which leads to a low electrical contact resistance. The protection effect of the lubricants is reflected by the prolonged lifetime in the fretting corrosion test. Since the fretting corrosion is only run to 100,000 cycles and some of the tested electrical contacts did not fail during the test, the lifetime is shown as 100,000 cycles in the test results as illustrated in Figure 3. It can be seen that the lubricants assist in effectively prolonging the lifetime. Between the two tested lubricants, the perfluorinated oil-based grease shows better protection without thermal storage compared to the PFPE oil. 4.2 Effect of thermal storage The lifetime of the electrical contacts after the thermal storage of the lubricants is shown in Figure 4. As in Section 4.1, the lifetime is recorded as 100,000 cycles, if the electrical contact did not fail during the fretting corrosion test. A great difference between the two tested lubricants can be found. The lifetime improvement disappears after thermal storage of the PFPE oil, indicating that the electrical contacts with PFPE oil experience severe thermal degradation. In contrast, the lifetime of the electrical contacts with the perfluorinated oil-based grease is prolonged compared to the unlubricated samples, although the protection effect also degrades after thermal storage. As Aus Wissenschaft und Forschung 17 Tribologie + Schmierungstechnik · 67. Jahrgang · 5-6/ 2020 DOI 10.30419/ TuS-2020-0024 Parame te rs Value s Amplitude peak to peak 500 μm Initial normal force 3 N Radius of the spherica l sample 4.5 mm Temperature Ambient temperature (20 °C ~ 23 °C) Fretting frequency 1 Hz Relative humidity 35 ~45 % Termination criteria Electrical contact resistance≥300 mΩ Max. tested cycle: 100,000 Table 2: Test parameters in fretting corrosion test Table 3: Average initial electrical contact resistance Figure 3: Spread of lifetime of electrical contacts without thermal storage as e + the rmal rage mΩ Without lub 0.7 m Without lubricant Oil Oil + The rmal storage Gre as e Gre as e + the rmal storage 0.7 mΩ 0.8 mΩ 0.5 mΩ 0.6 mΩ 0.4 mΩ + the rmal TuS_5_6_2020.qxp_TuS_Muster_2020 09.12.20 16: 08 Seite 17 tendency that the electrical contacts with perfluorinated oil-based grease [10] have a better resistance to thermal degradation than with PFPE oil [11]. 4.3 Status of lubricants after thermal storage Figure 5 illustrates the status of the lubricants on the electrical contacts after thermal storage. It can be observed that little PFPE oil remains on the electrical contacts after thermal storage, while a large amount of grease still covers the surface of the electrical contacts. This can lead to a better performance of the electrical contacts in the fretting corrosion test as in Section 4.2. 4.4 Electrical contacts after fretting corrosion testing In Figure 6, the status of electrical contacts after the fretting corrosion test is shown. After the test, there is still sufficient perfluorinated oil-based grease on the surface of the electrical contacts. On the failed samples with perfluorinated oil-based grease without thermal storage, more severe wear can be observed compared to the passed samples, and there is trace of black oxide in the contact area. For the thermal stored samples, the passed samples have a larger contact area, so the oxide of the tin plating is spread over a large area and thus diluted compared to the failed samples. As a consequence, a better metal-metal contact can be established. However, when PFPE oil acts as the lubricant, for the electrical contacts with or without thermal storage, the color in the contact area turns dark, which is an indicator of fretting corrosion [6]. Consequently, a better protection effect of the perfluorinated oil-based grease is achieved compared to the PFPE oil. Aus Wissenschaft und Forschung 18 Tribologie + Schmierungstechnik · 67. Jahrgang · 5-6/ 2020 DOI 10.30419/ TuS-2020-0024 a consequence, the resistance to thermal degradation on the electrical contacts with the perfluorinated oil-based grease is higher than with the PFPE oil. In this investigation the measured lifetime of electrical contacts with PFPE oil and the perfluorinated oil-based grease is lower, compared to the studies in [10, 11]. The reasons for the differences are the low tested amplitudes (25 µm [11] or 50 µm [10]) and short thermal storage time (250 h [11] or 500 h [10, 11]). The testing parameters used in this study are much closer to real operating conditions of most electrical contacts. Nevertheless, the measured lifetime shown in Figure 4 correlates to the (a) (b) Figure 4: Spread of lifetime of the electrical contacts with lubricants after thermal storage: (a) PFPE oil; (b) the perfluorinated oil-based grease Figure 5: Lubricants after thermal storage: (a) contact with PFPE oil after thermal storage; (b) contact with perfluorinated oil based grease after thermal storage (a) (b) TuS_5_6_2020.qxp_TuS_Muster_2020 09.12.20 16: 08 Seite 18 4.5 Element content after the fretting corrosion test The element content in the contact area is tested by energydispersive X-ray spectroscopy (EDS, XFlash-Detector 4010 from Bruker). Figure 7 illustrates the element content of Cu, Sn, O and F. Compared to the samples without lubricant, all the samples with lubricants have a lower oxygen content in the contact area, which means the oxidation on the surface of the electrical contact in the contact area is retarded. As a consequence, the lifetime is prolonged by using lubricants. Less copper content and higher tin content are detect- Aus Wissenschaft und Forschung 19 Tribologie + Schmierungstechnik · 67. Jahrgang · 5-6/ 2020 DOI 10.30419/ TuS-2020-0024 (g) (h) (i) (j) (k) (l) (a) (b) (c) (d) (e) (f) Figure 6: Electrical contacts after the fretting corrosion test: (a) with PFPE oil, without thermal storage, flat part, failed sample; (b) with PFPE oil, without thermal storage, spherical part, failed sample; (c) with PFPE oil, with thermal storage, flat part, failed sample; (d) with PFPE oil, with thermal storage, spherical part, failed sample; (e) with perfluorinated oil based grease, without thermal storage, flat part, passed sample (f) with perfluorinated oil based grease, without thermal storage; spherical part, passed sample; (g) with perfluorinated oil based grease, without thermal storage, flat part, failed sample; (h) with perfluorinated oil based grease, without thermal storage; spherical part, failed sample; (i) with perfluorinated oil based grease, with thermal storage, flat part, passed sample; (j) with perfluorinated oil based grease, with thermal storage, spherical part, passed sample; (k) with perfluorinated oil based grease, with thermal storage, flat part, failed sample, (l) with perfluorinated oil based grease, with thermal storage, spherical part, failed sample TuS_5_6_2020.qxp_TuS_Muster_2020 09.12.20 16: 08 Seite 19 nectors development,” IEEE Trans. Compon. Hybrids Manuf. Technol., 1991, vol. 14, pp. 90-94. [3] Y.W. Park, T.S.N.S. Narayanan, and K.Y. Lee, “Fretting corrosion of tin-plated contacts: evaluation of surface characteristics,” Tribol. Int., vol. 40, pp. 548-559, 2007. [4] N.N., “Surface coating technologies,” in E. Vinaricky and V. Behrens (Eds.), “Data book of electrical contacts,” 3 rd ed., Muehlacker: Stieglitz Verlag, 2012, pp. 283-284. [5] T.S.N.S. Narayanan, Y.W. Park, and K. Y. Lee, “Frettingcorrosion mapping of tin-plated copper alloy contacts,” Wear, vol. 262, pp. 228-233, 2007. [6] E. M. Bock and J. H. Whitley, “Fretting Corrosion in Electric Contacts,” Proc. 20th Annu. Holm Semin. Electr. Contacts, 1974, pp. 128-138. [7] O. Graton, S. Fouvry, R. Enquebecq, and L. Petit, “Effect of Lubrication on DC and RF Electrical Endurance of Gold Plated Contacts Subjected to Fretting Wear,” Proc. IEEE Holm Conf. Electr. Contacts, 2018, pp. 426-434, doi: 10.1109/ HOLM.2018.8611662 [8] S. Noel et al., “A new mixed organic layer for enhanced corrosion protection of electric contacts,” Proc. 50 th IEEE Holm Conf. Electr. Contacts and 22nd Int. Conf. Electr. Contacts, 2004, pp. 274-280, doi: 10.1109/ HOLM.2004. 1353130. [9] E. Larsson, A.M. Andersson, Å. K. Rudolphi, “Grease lubricated fretting of silver coated copper electrical contacts,” Wear, vols. 376-377, Part A, pp. 634-642, 2017. [10] S. Noël, A. Brézard-Oudot, P. Chrétien and D. Alamarguy, “Fretting behaviour of tinned connectors under grease lubrication,” Proc. IEEE Holm Conf. Electr. Contacts, 2017, pp. 109-116, doi: 10.1109/ HOLM.2017.8088072. [11] S. Noël, N. Lécaudé, C. Bodin, L. Bayer, L. Tristani, E.M. Zindine, “Effect of heat treatment on electrical and tribological properties of hot-dipped tin separable contacts with fluorinated lubricant layers”, Proc. Int. Conf. Electr. Contacts (ICEC), 2000, pp 229-234. [12] S. Noël, D. Alamarguy, A. Brézard-Oudot, P. Gendre, “An investigation of fretting wear behavior of nickel coatings for electrical contacts application in dry and lubricated conditions,” Wear, vol. 301, pp. 551-561, 2013. [13] J. Swingler, “The automotive connector: the influence of powering and lubricating a fretting contact resistance,” Proc. Inst. Mech. Eng. Part D J. Automob. Eng., vol. 214, pp. 615-623, 2000. [14] T.S.N.S. Narayanan, Y.W. Park, and K.Y. Lee, “Fretting corrosion of lubricated tin plated copper alloy contacts: Effect of temperature,” Tribol. Int., vol. 41, pp. 87-102, 2008. [15] B. H. Chudnovsky, “Lubrication of electrical contacts,” Proc. 51st IEEE Holm Conf. Electr. Contacts, 2005, pp. 107-114, doi: 10.1109/ HOLM.2005.1518230. [16] R.S. Timsit, E.M. Bock and N.E. Corman, “Effect of Surface Reactivity of Lubricants on the Properties of Aluminum Electrical Contacts,” Proc. 43 rd Annu. Holm Conf. Electr. Contacts, 1997, pp. 57-66. [17] S.L. McCarthy, R.O. Carter, and W.H. Weber, “Lubricant - Induced Corrosion in Copper Electrical Contacts,” Proc. 43 rd Annu. Holm Conf. Electr. Contacts, 1997, pp. 115- 120. [18] R.S. Timsit and M. Antler, “Tribology of electronic connectors: Contact sliding wear, fretting, and lubrication,” in: P.G. Slade (Ed.), “Electrical contacts: principles and applications,” 2 nd ed., Boca Raton: CRC Press, 2013, pp. 413-518. Aus Wissenschaft und Forschung 20 Tribologie + Schmierungstechnik · 67. Jahrgang · 5-6/ 2020 DOI 10.30419/ TuS-2020-0024 ed on the lubricated samples, indicating that the wear on the lubricated samples is less severe than the samples without lubricant. More fluorine content is detected on the lubricated samples after thermal storage, implying a strong diffusion of the lubricants to the tin plating due to the thermal storage. No obvious difference between the failed and passed samples with perfluorinated oil based grease with regard to the oxygen content is found. 5 Conclusion and outlook The protection effect of the lubricants, namely the PFPE oil and the perfluorinated oil-based grease, for tin plated electrical contacts against fretting corrosion and the resistance to thermal degradation of electrical contacts with lubricants at a high temperature are investigated in this paper. At the peak to peak amplitude of 500 µm, the protection effect varies greatly for the various lubricants. The grease can provide better protection compared to the oil. The lifetime of the tin plated electrical contacts with the PFPE oil decreases significantly after thermal storage. The resistance to thermal degradation on the electrical contacts with the PFPE oil is very low. The reasons for the lifetime decrease of the electrical contacts with the perfluorinated oil-based grease after thermal storage will be investigated in a further study. Reference [1] M. Blauth, “Parametrisierte Modelle zur konstruktiven Auslegung optimierter elektrischer Steckverbinderkontakte,” Dissertation, TU Ilmenau, Ilmenau, Germany, 2017, pp. 1. [2] J.L. Queffelec, N. Ben Jemaa, D. Travers, G. Pethieu, “Materials and contact shape studies for auto-mobile con- Figure 7: Element contents in the contact area TuS_5_6_2020.qxp_TuS_Muster_2020 09.12.20 16: 08 Seite 20