Changes

Besides these few pure metals a larger number of alloy materials made by melttechnology are available for the use as contacts. An alloy is characterized by the factthat 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 thecrystal structure as a function of the temperature and composition of the alloying components.

They indicate the boundaries of liquid and solid phases and define theparameters of solidification.Alloying allows to improve the properties of one material at the cost of changingthem for the second material. As an example, the hardness of a base metal maybe increased while at the same time the electrical conductivity decreases witheven small additions of the second alloying component.

Composite materials are a material group whose properties are of greatimportance for electrical contacts that are used in switching devices for higherelectrical currents.Those used in electrical contacts are heterogeneous materials composed of twoor more uniformly dispersed components in which the largest volume portionconsists of a metal.The properties of composite materials are determined mainly independent fromeach other by the properties of their individual components. Therefore it is forexample possible to combine the high melting point and arc erosion resistanceof tungsten with the low melting and good electrical conductivity of copper, orthe high conductivity of silver with the weld resistant metalloid graphite.

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. Figure 2.1 Powder metallurgical manufacturing of composite materials shows the schematic manufacturing processes from powderblending to contact material. Three basic process variations are typically

During ''sintering without a liquid phase'' (left side of schematic) the powder mix isfirst densified by pressing, then undergoes a heat treatment (sintering), andeventually is re-pressed again to further increase the density. The sinteringatmosphere depends on the material components and later application; avacuum is used for example for the low gas content material Cu/Cr. Thisprocess is used for individual contact parts and also termed press-sinterrepress(PSR). For materials with high silver content the starting point atpressing is most a larger block (or billet) which is then after sintering hotextruded into wire, rod, or strip form. The extrusion further increases the densityof these composite materials and contributes to higher arc erosion resistance.Materials such as Ag/Ni, Ag/MeO, and Ag/C are typically produced by thisprocess.

''Sintering with liquid phase'' has the advantage of shorter process times due tothe accelerated diffusion and also results in near-theoretical densities of thecomposite material. To ensure the shape stability during the sintering process itis however necessary to limit the volume content of the liquid phase material.

composite material. To ensure the shape stability during the sintering process itis however necessary to limit the volume content of the liquid phase material. As opposed to the liquid phase sintering which has limited use for electricalcontact manufacturing, the ''Infiltration process'' as shown on the right side of theschematic has a broad practical range of applications. In this process thepowder of the higher melting component sometimes also as a powder mix witha small amount of the second material is pressed into parts and after sinteringthe porous skeleton is infiltrated with liquid metal of the second material. Thefilling up of the pores happens through capillary forces. This process reachesafter the infiltration near-theoretical density without subsequent pressing and iswidely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts,controlling the amount or excess on the bottom side of the contact of theinfiltration metal Ag results in contact tips that can be easily attached to theircarriers by resistance welding. For larger Cu/W contacts additional machining isoften used to obtain the final shape of the contact component.