Changes

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.

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-sinterrepress (PSR). For materials with high silver content , the starting point at before pressing is most mostly a larger 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

As opposed to the liquid phase sintering , which has limited use for electrical contact manufacturing, the ''Infiltration process'' as shown on the right side of the schematic , has a broad practical range of applications. In this process the powder of the higher melting component , sometimes also as a powder mix with a small amount of the second material , is pressed into parts and after sintering the porous skeleton is infiltrated with liquid metal of the second material. The filling fill-up process of the pores happens through capillary forces. This process reaches , after the infiltration , near-theoretical density without subsequent pressing and is widely 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 the infiltration metal Ag , results in contact tips that can be easily attached to their carriers by resistance welding. For larger Cu/W contacts , additional machining is often used to obtain the final shape of the contact component.

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.

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 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 not in their pure form for electrical contact applications.