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Tuesday, November 10

Tribological Investigation of Electrical Contacts

Dinesh Bansal, Georgia Institute of Technology, Atlanta
Materials Science and Technology Division Seminar
10:00 AM, High Temperature Materials Laboratory (Building 4515), Conference Room 265
Contact: Peter Blau (blaupj@ornl.gov), 865.574-5377

Abstract

The interface of an electrical contact, either stationary or dynamic, is a complex environment as several different physical phenomena can occur simultaneously. The main motivation of the current work stems from the need to provide means for accurate determination or prediction of the critical contact parameters viz., contact resistance and temperature. Understanding the behavior of electrical contacts both static and dynamic under various operating conditions can provide new insights into the behavior of the interface. The current work covers three major topics: (1) evaluating temperature rise at the interface between two sliding bodies, (2) a study of static electrical contacts, and (3) a study of factors influencing behavior of sliding electrical contacts under high current densities.

A model for determining steady-state temperature distribution at the interface of two sliding bodies, with arbitrary initial temperatures and subjected to Coulomb and/or Joule heating, is developed. The model utilizes the technique of least squares regression to apply the condition of temperature continuity at every point in the domain. This model is the first of its kind, and enables the prediction of full temperature field. The analysis can be applied to a macro-scale contact, ignoring surface roughness, between two bodies and also to contact between two asperities. For the ease of design purposes, curve fit equations for determining the maximum and the average interface temperature for circular and elliptical contacts, with semi-ellipsoidal form of heat distribution, are developed. These curve fit equations are also applicable for the case when both the bodies have dissimilar initial bulk temperatures. The equations are presented in terms of non-dimensional parameters, and hence can easily be applied to any practical scenario.

The effect of current cycling through static electrical contact is presented. It is observed that, the voltage drop across the contact initially increases with current until a certain critical voltage is increased. Beyond this critical point, any increase in the current causes essentially no increase in steady-state contact voltage. This critical voltage is referred to as "saturation voltage." The effect of load and surface roughness on voltage saturation is also demonstrated experimentally. An explanation based on the softening of the interface, due to temperature rise, is proposed rather than more widely referred hypothesis of recrystallization.

A multi-scale model, which closely relates to the multi-scale nature of rough surfaces, is developed. This model overcomes the sensitivity to sampling resolution inherent in many asperity based models in the literature. The model predictions are compared with experimental measurements for different surface roughnesses and loads.

The behavior of sliding interfaces of aluminum–copper (Al–Cu) and aluminum–aluminum (Al–Al) are analyzed under high current densities. Experimental results are presented that demonstrate the influence of load, speed, current and surface roughness on coefficient of friction, contact voltage, contact resistance, interface temperature and wear rate. The experimental results reveal that thermal softening of the interface is the primary reason for accelerated wear under the test conditions.