Intermittent electrical discontinuities in tin-plated automotive contacts
This study investigates rapid changes in contact resistance, termed intermittences or short duration discontinuities, on tin-plated electrical contacts that are undergoing fretting tests. Voltage and current levels are comparable to the levels found in automotive systems. The thesis begins with an overview of the theory of mechanical and electrical contact, factors that determine the performance of an electrical contact and previous work on intermittences. Building upon this background information a low frequency fretting apparatus and a high¬speed data acquisition system are developed, capable of monitoring the voltage-drop across and current through the contact with a sample rate of up to 1 gigasamples per second. During the fretting experiments the build-up of debris is observed. As expected in this type of experiment the contact resistance rises with increasing number of fretting cycles. An increase in current accelerates the contact degradation. The contact surface is analysed using SEM and SEM/EDX. Several features are found in the recorded signals. These include plateaus, slopes of several tens of kilovolts per second, voltage-drops across the contact interface of above the melting voltage of tin and intermittences that can occur, when no forced relative motion between the contact partners is present. The observations are combined to a set of requirements of a model to explain intermittency phenomena. Previously proposed models are evaluated on the basis of these findings. Finally the asperity model is extended to incorporate the results of the experiments. It is proposed that melting of asperiti.es causes short-duration discontinuities and events that occur when no forced relative motion is present. Both could not be explained within the asperity model before. The thesis concludes with a definition of the term 'intermittence' as the temporary disruption of metallic conduction in a closed electrical contact. This definition is an improvement over the previous definition, because it allows setting a specific resistance threshold to detect intermittences in fretting experiments. Future work is suggested.