The effects of nickel on the switching performance of silver-tungsten-based contacts

Silver-tungsten-based contacts are commonly used in air circuit breakers. The most common contact-related problems for devices using this type of material are high contact resistance, i.e. high temperature rise in a device, and catastrophic material erosion (cracking). This paper describes how the addition of nickel affects the erosion characteristics of the base silver-tungsten material. It is shown that a multiphase tungsten/nickel structure can form on the contact surface resulting in high contact resistance. The silver remains more cohesively adherent to the tungsten phase when the nickel is present. The multiphase structure which forms appears to be more resistant to thermal stress cracking. Negative effects of the nickel addition are the reduced high temperature stability of the contact material and the potential for high contact resistance if contact is made directly with the multiphase tungsten-nickel material. Two surface structure conditions which can cause high contact resistance without the presence of oxides are described.