23rd International Colloquium Tribology - January 2022 313 Wear of electrical contacts of equal motion amplitude and equal force in different directions Dirk Hilmert Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences and Arts, Campusallee 12, 32657 Lemgo, Germany Kevin Krüger Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences and Arts, Campusallee 12, 32657 Lemgo, Germany Haomiao Yuan Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences and Arts, Campusallee 12, 32657 Lemgo, Germany Jian Song Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences and Arts, Campusallee 12, 32657 Lemgo, Germany Corresponding author: jian.song@th-owl.de 1. Introduction With regard to the growing market of electric cars and connected production systems, reliable electrical connectors are becoming increasingly important. In these automotive applications and industrial production systems, electrical contacts experience various types of loads. Among these, thermal and vibrational stresses are critical loads which subsequently cause micro motions and result in the fretting wear of coatings. Also, since the connectors are mounted in various positions, these motions occur in different directions. This leads to translational relative motion and rocking motions in the mating direction and the directions orthogonal to mating direction respectively. The aim of this study is to analyse the fretting wear of silver coated electrical contacts in three different orthogonal directions. 2. Theory Electrical connectors that need to maintain low contact resistance in long term applications are often used with silver coatings. This protects the base material from atmospheric gas diffusion and thus prevents oxidation of the connector [1]. However, vibrations and different thermal expansion rates can lead to fretting wear of the coating which can result in a wear through of the protective layer. This eventually leads to the corrosion of the base material and the electrical failure of the contact. To ensure the safe design and validation of electrical connectors, test specifications and standards are available that define fretting wear tests [2]. Although failures can be observed due to rocking motions, former investigations mainly consider the mating direction [3, 4]. Therefore, initial studies of the motion in orthogonal directions to the mating direction have been conducted and a definition of directions for fretting wear tests has been proposed, as shown in Fig. 1 [5]. Fig. 1: Fretting directions according to [5] 3. Experimental Setup For this study silver coated electrical connectors with 5.5 µm silver coating and 2 µm nickel underlayer are used. In preparation for the tests, the receptacle and blade are mated which leads to an overlapping of the contact partners by 2 mm. The contact pairs are then mounted in the test rig with different mounting directions for each test. While the receptacle is clamped on a linear actuator, the blade of the connector is fixed in the test rig. In direction 1, the receptacle is clamped directly at the crimp. For directions 5 and 6 the electrical conductor is clamped with a distance of 2 mm from the crimp to take into effect the flexibility of the attached 314 23rd International Colloquium Tribology - January 2022 Wear of electrical contacts of equal motion amplitude and equal force in different directions cable. In the tests a displacement of 50 µm is given to the specimen at 1 Hz in these setups. To analyse the change of the fretting wear with respect to time, a series of 1000 cycles is conducted and the frictional energies for the 1st, 50th and 1000th cycle are calculated. These energies are then compared to determine the critical directions and displacements. 4. Results and Discussion The force-displacement curves of direction 1 for a total displacement of 50 µm are shown in Fig. 2. It can be observed that the curve changes over the course of 1000 cycles from a flat curve with a maximum force of 1.2 N to a thinner curve at about 6.7 N. Hence the curve is increasing in force while the hysteresis decreases. Fig. 2: Force-displacement curves of direction 1 for equal amplitudes of 50 µm The curves for the directions 5 and 6 are shown in Fig. 3. In comparison to direction 1 not only the maximum forces in the first cycle are lower, but also the hysteresis of the curves are lower. However, when conducting the tests with 1000 cycles it is also shown that the overall force increases for direction 5 as well as for direction 6 and that the general shape of the curve is not changing as in the case of direction 1. Fig. 3: Force-displacement curves of direction 5 and 6 for equal amplitudes of 50 µm As shown at a displacement of 50 µm in each direction for one cycle, the motion force in direction 1 is the highest with a maximum of about 1.2 N. In the orthogonal directions 5 and 6, smaller forces of 0.4 and 0.8 N are measured. Also, the maximum displacements in the orthogonal directions for a force of 1.2 N are examined in preliminary tests. Displacements of 250 µm for direction 6 and 600 µm for direction 5 are observed. Therefore, the wear behaviour corresponding to equal forces is also investigated by applying a total displacement of 1200 µm and 500 µm for directions 5 and 6 respectively. The results are shown in Fig. 4. In comparison to the displacement curves at 50 µm, the maximum forces are increased. Also, the hysteresis of the curves is increased and the overall shape is more comparable to direction 1. Comparing the 1 st , 50 th and 1000 th cycle of directions 5 and 6 at high displacements shows that the force increases while the hysteresis decreases. Fig. 4: Force-displacement curves of direction 5 and 6 for equal forces The calculation of the frictional energies for these diagrams is shown in Tab. 1. Comparing the 50 µm displacements, it can be observed that direction 1 shows the highest amount of frictional energy while direction 5 shows the lowest energies. It can also be noticed, that at equal amplitudes the frictional energies decrease over the course of 1000 cycles. In comparison to the equal amplitudes, the frictional energies of equal forces show, the highest amount of energy for direction 5. Due to the higher amplitude of displacement, these forces exceed the forces in direction 1. This comparison shows that the attached cable has a significant influence on the fretting wear of the connector. Tab. 1: Frictional energies of different directions (D1, D5, D6) and displacements Frictional Energy [µJ] 1st Cycle 50th Cycle 1000th Cycle 23rd International Colloquium Tribology - January 2022 315 Wear of electrical contacts of equal motion amplitude and equal force in different directions equal amplitude D1 50 µm 76.9 75.6 41.1 D5 50 µm 0.3 0.2 0.3 D6 50 µm 3.1 2.4 1.5 equal force D1 50 µm 76.9 75.6 41.1 D5 1200 µm 1320.5 1049.0 822.7 D6 500 µm 608.3 629.5 630.4 5. Conclusion In this study, the force-displacement curves of silver coated contacts at different displacement amplitudes are compared. In general, an increase of the overall force over the course of 1000 cycles and the corresponding change in curve shapes is observed. For an amplitude of 50 µm the mating direction (D1) is most severe and shows the highest frictional energies and forces. Therefore, this is the critical direction of motion. However, when the frictional forces are equal to direction 1, the frictional energy and the hysteresis of D5 and D6 increase compared to the equal amplitude tests. This shows a major influence of the attached cable to the electrical connector when subjected to vibrational or thermal loads. 6. Acknowledgement This study is financed by the German Federal Ministry for Economic Affairs and Energy (BMWi, IGF, 20139 N) References [1] Song, J., Wang, L., Zibart, A. et al.: Corrosion Protection of Electrically Conductive Surfaces, 2012. [2] N.N.: TechnischerLeitfaden-TLF0214: Validierung von Automotive-Niedervolt-Steckverbindern. [3] Perrinet, O., Laporte, J., Fouvry, S. et al.: The electrical contact resistance endurance of heterogeneous Ag/ Sn interfaces subjected to fretting wear, 2014. [4] Queffelec J.L., Ben Jemaa N., Travers D.: Materials and contact shape studies for automobile connector development, 1990. [5] Hilmert, D., Krüger, K., Song, J.: Vergleichende Untersuchung der Verschleißbilder von Steckverbindern aus Reibverschleiß- und Vibrationsprüfungen mit unterschiedlichen Prüfrichtungen, 2021.