Analytical Methods to Identify Root Causes for Early Failure of an AC Contactor with Heavy Load Service Life Test

Recently developed Silver/Tin oxide contact material Ag/SnO
₂
14 WPA has been proven to fulfill AC contactor switching requirements as service life, make capacity, and overtemperature behavior in a very satisfactory way. Based on these experiences, early failures with an additional AC4 electrical endurance test at app. 60% of service life goal were unexpected and triggered in depth analytical evaluations to fix root causes. Applying microsections, microtome sectioning, ultrasonic analysis, optical and scanning electron microscopy (SEM) combined with energy dispersive x-ray spectroscopy (EDX) two different main root causes could be identified. Mixed oxide layers with high amounts of phosphorous as well as some layers with iron oxide were detected on the contact surface, probably originating from excess braze alloy rising the contact tip side and splitter plate splashing and evaporation, respectively. These contaminants on the surface react with tin oxide leading to thick mixed oxide layers causing probably severe arcing on make. This leads to high contact erosion and finally welding of the contacts.