Simultaneous Studies Of Electrical Contact Resistance And Thermal Contact Conductance Across Metallic Contacts

Abstract

Contact resistance is the most important and universal characteristic of all types of electrical and thermal contacts. Accurate measurement of contact resistance is important, because it serves as a measure for judging the performance and operational life span of contacts. Rise in contact temperature is one of the major factors that pose a big threat to the stability of electrical contacts. Dissipation of heat by solid conduction through a contact interface is governed by its thermal contact conductance (TCC). This emphasizes the need to study the TCC of an electrical contact along with its electrical contact resistance (ECR). Simultaneous measurement of ECR and TCC is important for understanding the interconnection between these two quantities and the possible influence of one over another. Real time experimental data and analytical correlations can be extremely helpful in developing electrical contacts with improved thermal management capabilities. As a part of the experimental investigation, a test facility has been developed for making simultaneous measurement of ECR and TCC across flat contacts. The facility has the capability of measuring ECR and TCC over a wide range of operating parameters, such as contact pressure, contact temperature, interstitial gaseous media, ambient pressure, etc. It is also capable of determining the electrical resistivity and thermal conductivity of materials as a function of temperature, which is very helpful in analyzing the generated contact resistance data. Using this facility, simultaneous ECR and TCC measurements are made across bare and gold plated contacts of OFHC Cu (oxygen free high conductivity copper) and brass. Simultaneous ECR and TCC measurements are made on nominally flat contacts in the contact pressure range of 0 – 1 MPa and the interface temperature range of 20 – 120 °C. Effect of contact pressure and interface temperature on ECR and TCC is studied on bare and gold coated contacts in vacuum, N2, Ar, and SF6 environments. TCC strongly depends on the thermophysical properties of the interstitial media and shows a significant enhancement in gaseous media, because of the increased interfacial gap conductance compared to vacuum. The gas pressure is varied in the range of 1 – 2.6 bar to study its effect on the gap conductance at different contact pressures and interface temperatures. Minor increase in the ECR observed in gaseous media is found to be independent of the properties of the media. Experimental results indicated that ECR depends on the gas pressure as well as on the applied contact load. Effect of gold coating and its thickness on the ECR and TCC across OFHC Cu and brass contacts is studied. Measurements on electroplated gold specimens having different gold layer thicknesses (0.1, 0.3, and 0.5 µm) indicated that ECR decreases and TCC increases with increasing gold coating thickness. Effect of gold coating on the substrate properties, contact surface tomography, and microhardness is analyzed and correlated to the observed behavior of ECR and thermal gap conductance. An attempt is made to understand and quantify the changes in the contact surface characteristics due to contact loading and heating, by measuring various surface topography parameters before and after the experimentation. Effect of thermal stresses (generated due to temperature variations) on ECR and TCC is studied and inclusion of an experimentally measured temperature dependent load correction factor is suggested in the theoretical models to take into account the effect of thermal stresses in contact assemblies.