Date of Completion

9-7-2017

Embargo Period

9-7-2017

Keywords

Circuit breaker contacts, electron microscopy, UL testing, aging, intermetallics, solidification, defects, microstructure

Major Advisor

Mark Aindow

Associate Advisor

S. Pamir Alpay

Associate Advisor

Rainer J. Hebert

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Contact materials composed of Ag and W have been used in circuit breakers and other electrical switching systems for decades due to their excellent properties. A series of standard acceptance tests have been established by UL (Underwriters Laboratories) to ensure that circuit breakers meet certain criteria for performance and safety. These standard tests include: overload/temperature-rise, endurance and short-circuit conditions. In this dissertation, a study on the near-surface microstructures exhibited by untested commercial Ag/W circuit breaker contacts, and by those that have been subjected to the UL standard tests, is presented. The microstructural characteristics were investigated to deduce the degradation mechanisms during each type of test. A similar approach was also adopted to investigate the effects of natural aging and accelerated aging in dry and humid environments on the structure and degradation mechanisms for this type of contacts. Alloys composed of Ag3Sn and Cu3Sn intermetallic phases were proposed as candidate alloys for use in circuit breaker contacts to reduce the cost without compromising the breaker performance. Fundamental physical metallurgy studies were conducted to provide a more comprehensive understanding of the structure-property relationships for the candidate alloys. Results from solidification microstructures revealed the phase equilibria for Ag3Sn/Cu3Sn alloys. Studies on the defect structures in Ag3Sn/Cu3Sn alloys and in Ag3Sn single crystals showed that different deformation mechanisms operate in these iso-structural phases. The consequences of these observations for the potential of using Ag3Sn/Cu3Sn alloys in circuit breaker contacts is discussed.

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