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*Good arc extinguishing capability
Materials suited for use as electrical contacts can be divided into the following groups based on their composition and metallurgical structure:
*Alloys
*Composite materials
Besides these few pure metals , a larger number of alloy materials made by melt technology are available for the use as contacts. An alloy is characterized by the fact , that its components are completely or partially soluble in each other in the solid state. Phase diagrams for multiple metal compositions show the number and type of the crystal structure as a function of the temperature and composition of the alloying components.
They indicate the boundaries of liquid and solid phases and define the parameters of solidification.
Alloying allows to improve the properties of one material at the cost of changing them for the second material. As an example, the hardness of a base metal may be increased while at the same time the electrical conductivity decreases with even small additions of the second alloying component.
Composite materials are a material group whose properties are of great importance for electrical contacts that are used in switching devices for higher
electrical currents.
Those used in electrical contacts are heterogeneous materials , composed of two or more uniformly dispersed components , in which the largest volume portion consists of a metal.
The properties of composite materials are determined mainly independent from each other by the properties of their individual components. Therefore it is , for example , possible to combine the high melting point and arc erosion resistance of tungsten with the low melting and good electrical conductivity of copper, or the high conductivity of silver with the weld resistant metalloid graphite. <xr id="fig:fig2.1"/> Powder metallurgical manufacturing of composite materials (schematic)"/> shows the schematic manufacturing processes from powder blending to contact material. Three basic process variations are typicallyapplied:
*Sintering without liquid phase (Press-Sinter-Repress, PSR)
*Infiltration (Press-Sinter-Infiltrate, PSI)
During ''sintering without a liquid phase'' (left side of schematic), the powder mix is first densified by pressing, then undergoes a heat treatment (sintering) and eventually is re-pressed again to further increase the density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the low gas content material Cu/Cr. This process is used for individual contact parts and also termed press-sinter-repress (PSR). For materials with high silver content, the starting point before pressing is mostly a large block (or billet) which is then, after sintering, hot extruded into wire, rod or strip form. The extrusion further increases the density of these composite materials and contributes to higher arc erosion resistance. Materials such as Ag/Ni, Ag/MeO and Ag/C are typically produced by this process. ''Sintering with liquid phase'' has the advantage of shorter process times due to the accelerated diffusion and also results in near-theoretical densities of the composite material. To ensure the shape stability during the sintering process , it
is however necessary to limit the volume content of the liquid phase material.
==Gold Based Materials==
Pure Gold is besides Platinum the chemically most stable of all precious metals. In its pure form , it is not very suitable for use as a contact material in electromechanical devices because of its tendency to stick and cold-weld at even low contact forces. In addition , it is not hard or strong enough to resist mechanical wear and exhibits high materials material losses under electrical arcing loads. This limits its use in form of thin electroplated or vacuum deposited layers.
Main ArticelArticle: [[Gold Based Materials| Gold Based Materials]]
==Platinum Metal Based Materials==
The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir, and Os ([[Platinum_Metal_Based_Materials|Table 1]]<!--(Table 2.6)-->). For electrical contacts , platinum and palladium have practical significance as base alloy materials and ruthenium and iridium are used as alloying components. Pt and Pd have similar corrosion resistance as gold but because of due to their catalytical properties , they tend to polymerize adsorbed organic vapors on contact surfaces. During frictional movement between contact surfaces , the polymerized compounds known as “brown powder” are formed , which can lead to significantly a significant increase in contact resistance. Therefore Pt and Pd are typically used as alloys and are rather not used in their pure form for electrical contact applications.
Main ArticelArticle: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]
==Silver Based Materials==
Main Article: [[Silver Based Materials| Silver Based Materials]]
==Tungsten and Molybdenum Based Materials==
Main Article: [[Tungsten and Molybdenum (Pure Metals), Silver–Tungsten (SIWODUR) Based Materials, Silver–Tungsten Carbide (SIWODUR C) Materials, Silver–Molybdenum (SILMODUR) Materials, Copper–Tungsten (CUWODUR) | Tungsten and Molybdenum Based Materials]]
==References==
Manufacturing Equipment for Semi-Finished Materials
(Bild)
[[de:Kontaktwerkstoffe_für_die_Elektrotechnik]]