Asperities: Mass-produced switch contacts have an rms roughness ranging from 0.5 to 1 106 in. These "smooth" surfaces, in reality, comprise microscopic peaks and valleys. As a result, two mating contacts touch only at the surface peaks or asperities. Contact force between the two surfaces plastically flattens these asperities until the sum of the reaction forces equals the applied contact force. Thus, initial contact area increases and resistance decreases. Still, final contact area is much smaller than apparent contact area.
Surface film: Contact area is affected by other factors. For example, all metal surfaces have some kind of surface film. Contaminants such as sulfides condense on metal surfaces and cause chemical reactions. Film thickness depends on the properties of the metal. If the film is softer than the contact material, contact pressure squeezes the film aside at the asperities for metal-to-metal contact. Films harder than the contact material reduce contact area and increase resistance. When contacts with brittle films close, the films are fractured away to expose clean metal.
Oxide film: Contact materials including silver, platinum, palladium, tungsten, molybdenum, and nickel react with oxygen to form tough and resistive surface films. Oxide-forming contacts need relatively large contact forces to fracture these films. Gold-plated contacts, however, are oxide-free and are recommended for high-reliability applications.
Foreign particles: Soft particles may be flattened upon contact closure to allow increased metallic contact. Hard particles may be fractured and swept away by switching action. At the same time, hard particles can cause surface abrasion and wear. And if the particulate matter absorbs moisture, the contacts may corrode or develop local galvanic action.
Since the contact interface acts as a resistor, the heat produced by the closed contacts is proportional to the voltage across them and the current through them. Temperature rise of clean contacts is negligible, but when the contacts start to separate, the number of contacting asperities decreases. As the last contacting asperity decreases its contact area, contact resistance increases until contact voltage reaches melting potential (about 0.5 V for most metals). Molten metal bridges the separating surfaces and finally breaks. During this process some material may transfer between contacts, especially if direct current is interrupted.
Rupture of the molten bridge opens the circuit at low voltage. When source voltage exceeds 14 V, an arc may form as the molten bridge ruptures, delaying circuit opening and pitting the switch contacts. In addition to shortening switch life, contact arcing radiates electromagnetic noise.
The higher the switched current, the hotter the arc and the greater the erosion. Contact erosion also increases with longer arc duration because of high switched voltages. Also, contacts that switch inductive currents erode quicker than those with resistive loads. Accelerated erosion is caused by contact arcing that results when the deenergized inductor returns its stored energy to the circuit.
As switch contacts separate, the arc extinguishes when the voltage passes through zero. Therefore, up to a point, the higher the frequency the quicker the arc is likely to extinguish. At 400 Hz, however, arc interruption may be as difficult as with dc loads. High-speed contact separation also decreases arc duration, unless ionized air is present.
The dielectric strength of air decreases as barometric pressure decreases. Therefore, at high altitudes contact arcing occurs at lower voltages and lasts longer. As a result, high-altitude applications require switches with derated load and life specifications.