24th International Colloquium Tribology - January 2024 133 Enhancing Reliability and Service Life Predictions through Friction Monitoring and Sensor-Embedded Smart Contacts A Comparative Study of Weld, Bolt, and Plug-in Connections Michael Gless 1* , Anette Schwarz 1 ContactEngineering.de, Stuttgart, Germany * Corresponding author: E-mail Contact@ContactEngineering.de 1. Introduction Electrification leads to a high interest in electrical contacts. Battery or Hydrogen Electric Drivetrains, E-Scooters and E-Bikes become increasingly popular. In December 2022, the share of battery-electric and plug-in hybrids in newly registered cars in Germany rose to more than 55%. Every electric vehicle relies on numerous electrical high-current contacts. Therefore, appropriate design with energy efficiency, performance, and the highest reliability, especially in high-current contacts, is essential. However, there remain areas for the use of green synthetic fuels. Hydrogen is considered one of the most promising energy sources of the future. Green Hydrogen generation and Fuel-Cells are based on electrochemical processes and on many electrical contacts. Furthermore, trends toward automation not only aim to enhance energy efficiency but also demand enhanced reliability in contacts. In autonomous systems, these contacts must perform their functions seamlessly throughout their lifecycle or autonomously detect and communicate any deviations. This manuscript delves into systematic decision-making by highlighting and rating different solutions. The focus is on tribology in electric vehicle connections and ensuring robust performance and reliability. Based on these and other requirements, the best type of contact must be chosen. 2. Requirements and Design-Possibilities In engineering, decisions for best design solution have been made in a very early design process. Important electric contacts are shown in Figure 1. Figure 1: Connections and Contacts grouped into Material-Fit, Force-Fit, and Form-Fit. In electrical contacts, Form- Fit Connections need Force as well. Contact Resistance and Reliability of contacts are a common concern, prompting inquiries and extensive discussions. Contact resistance affects temperature, aging and energy efficiency. Power loss P and resulting heating during operation depend on current flow I and resistance R (P = I² · R). Results are shown in Figure 2. Figure 2: Measurements of Contact Resistance and Reliability Rating of Weld, Clinch, and Bolt Connection. Reliability according to MIL-HDBK 2017 and IEC 61709. 3. Focus on tribological failures/ failure mechanisms The main goal of contacts is to fulfill required functions. In an appropriate and optimized design, most failure mechanisms can be avoided. There are electrical, mechanical, and chemical failure and aging mechanisms. Influences which are in this document deeper addressed are frictions and wear. 3.1 Friction monitoring in electrical bolted joints The friction coefficients have a significant influence on the assembly force/ clamping force in the connection, which in turn has a significant influence on the resistance of the connection. Clamping force and friction between contact partners also influence transverse force and sliding, partial 134 24th International Colloquium Tribology - January 2024 Enhancing Reliability and Service Life Predictions through Friction Monitoring and Sensor-Embedded Smart Contacts sliding, or fretting in the contact area. A general rule is to avoid sliding or micro-movements in an electrical contact. Anti-friction coatings on the connecting elements are used to reduce the distribution of coefficients of friction, to set a defined coefficient of friction window, for example from μ-= 0.09 to 0.14. This reduces the spread of the contact force achieved and thus also the spread of the contact resistance/ electrical values of the connection. Determination of the coefficients of friction from the gradients is shown in Figure 3. The big advantage is that this can also be done as part of series production. However, the settlement/ embedding of joints and also the torsional rigidity of the tightening tools are challenging. Therefore, as an alternative, the ratio of tightening and loosening torque was determined, and the coefficient of friction was calculated from these. This approach delivers very satisfactory results. Deviations in the friction values can be identified during series production based on measured torque. Figure 3: Torque vs. Torsion angle for a Bolt Connection with slide coating (black line) and try/ without (blue line). The corresponding clamping force differs from 8 kN to 14-kN 3.2 Relaxation, reduction of spring or contact force Challenging relaxation/ loss of preload force, especially with highly conductive materials. For this purpose, relaxation tests and hot aging tests are usually carried out. For plug-in connections, we explored fretting wear optimization and wear prediction of coatings in electrical contacts. Optical measurement techniques enable the measurement of wear rates after a short test duration, while service lifetime models allow for estimating the service lifetime of the contacts. 3.3 Fretting/ Friction Corrosion and Friction Wear Frictional wear describes the aging of current-carrying connections due to relative movements. Spring forces/ mating forces limit contact pressure and therefore friction force. Micro motions in contact can be caused by thermal expansion, by shocks or vibrational loads. Sliding goes with Oxidation, Corrosion, Wear, and an increasing Contact Resistance. 4. Testing and Monitoring Besides mentioned force, resistance/ voltage drop, electromagnetic field measurements or electronic sensing, temperature is an easily accessible factor in electrical contacts. Monitoring supports: a. Verification of results e.g. at other/ system levels b. Actual operating limits, e.g., to avoid overloading/ overheating of components during operation c. Operating Condition and Anomaly detection. Detection of early signs of failures, early warning, Mitigate potential risks associated with high-current contacts, avoid subsequent errors. d. Derating, Service Lifetime Prediction, Predictive maintenance, planned maintenance Temperature Measurements are implemented in serial production accompanying testing/ prototypes, to further enhance reliability and ensure long-term performance. Thermal expansion/ bimetal-based mechanisms. Positive Temperature Coefficient materials acts as a reversible thermal shutdown mechanism in case of extensive current load e.g. PTCs in consumer cells. Measurements use again temperature-dependent resistors like PT100 or thermoelectric effects are e.g. thermocouples. Thermochromic labels/ materials are a simple and effective way to see if a certain temperature/ maximum temperature that was reached. Additional contactless diagnosis is possible. The microbolometer membrane absorbs emitted infrared radiation, heats up and chances resistance. Infrared sensors or cameras can be used. Infrared is not influenced by electromagnetic/ EM noises. A very elegant and cost-effective solution is to integrate a miniature infrared thermometer on a circuit board/ PCB, shown in Figure 4. Figure 4: Miniature Infrared Thermometer (e.g. MLX90632) stacked on top of Test-Surface. Infrared Thermometer measures surface temperature of an electrical contacts, of a cooling media or lubricant. 5. Conclusion The document gives an overview of important high-current contacts. Furthermore, consideration and results of important influences and reliability rating help in making decisions and designing a robust design. Important influences, which are deeper investigated, are friction and wear. To support testing, automation and increase reliability, monitoring solutions are discussed. By combining friction monitoring, assembly process recording, and sensor-embedded smart contacts, we can significantly enhance the reliability, service life predictions, proactive/ predictive maintenance and mitigate potential risks of tribological contacts.