Silver Semi-Refractory Materials
Unlike the other contact materials that form true alloys, the silver semi-refractory materials are dispersed phase mixtures of silver and other metals or metallic oxides. These materials are produced by powder metallurgy or internal oxidation techniques. Since these materials are essentially a silver matrix with dispersed particles, they exhibit unusually good electrical conductivity. This family of materials is subdivided as follows:
a. SILVER CADMIUM OXIDE Prior to World War ll, there was little work done on this group of materials. The Germans were the first to discover the usefulness of cadmium oxide in contact applications and made great use of it during the war. Since then, many refinements have been made in the production of these materials. Today, silver cadmium oxide is employed in the majority of medium and high current applications. The advantages of these materials are high conductivity, excellent ability to resist welding and a very low rate of electrical erosion. One disadvantage is limited ductility in the higher percentages. Typical applications include medium current range relays, electric lift truck contractors, A.C. motor controllers and automotive and aircraft relays. Materials listed below are all made via both the internal oxidation or powder metallurgy processes and are all available as solid rivets, composite weld buttons and composite rivets. With the exception of DMC-91P and DMC-95P, these materials tend to fracture when doubleheaded. Properties of Silver Cadmium Oxide Materials
Hardness and tensile strengths of all materials are in the "as-annealed": condition, except for DMC-91P and DMC-95P. b. SILVER GRAPHITE The primary use of silver graphite is in sliding contact applications, such as brushes and slip rings, where the graphite provides good lubrication. These materials resist welding and sticking and are used in A.C. and D.C. applications. In the lower percentages, such as DMC-60 (99.75% Ag, .25 C) and DMC-69 (99.5 Ag, .5 C), silver graphite is headable within reasonable design range.* Above this range, the materials are brittle and are normally supplied in composite form. Properties of Silver Graphite Materials
*Refer to Engineering Data, Cold Heading Design Limits and Considerations. c. SILVER NICKEL Silver nickel semi-refractory materials exhibit good conductivity, good wearing qualities and good resistance to welding and erosion. These materials are used in A.C. and D.C. applications where they replace fine silver or silver cadmium oxide in medium duty relays. Silver nickel is ductile up to the 10% level, and can be headed into contact rivets. The 15% material is normally supplied as a composite as ductility is limited. Properties of Silver Nickel
Note: Hardness values are for comparative purposes only and should not be used in design specifications. d. OTHER SEMI-REFRACTORY MATERIALS As with the silver alloys, there are dispersed phase materials that do not fall into a family series. One of these is DMC-65 (silver, magnesium oxide, nickel oxide). This is one of the more unusual silver alloys and is normally supplied in a heat treated condition. It is capable of withstanding elevated temperatures for extended periods without loss of temper. Because of this, DMC-65 will remain hard in high temperature brazing of composite contacts. This material is frequently used as a spring arm and contact tip. Applications are in the A.C. and D.C. areas. The magnesium oxide provides an antiweld characteristic which, when paired with DMC-51 in D.C. applications, minimizes material transfer. This material can be supplied in all forms, but it will not withstand doubleheading without cracking. DMC-81 (90% silver, 10% iron) is normally used in 120 volt applications replacing fine silver to reduce welding. Typical uses are household appliances and medium duty relays. The one disadvantage of DMC-81 is the probability of high surface resistance due to oxide formations. Due to this, it is not suitable for high temperature or high humidity applications. DMC-81 is available in solid rivet or composite form. Properties of Miscellaneous Semi-Refractory Materials
*Properties of DMC-65 are in the heat treated condition. |