Changes

To improve the physical and contact properties of fine silver melt-metallurgicalproduced silver alloys are used ''(Table 2.13)''. By adding metal components themechanical properties such as hardness and tensile strength as well as typicalcontact properties such as erosion resistance, and resistance against materialtransfer in DC circuits are increased ''(Table 2.14)''. On the other hand however,other properties such as electrical conductivity and chemical corrosionresistance can be negatively impacted by alloying ''(Figs. 2.47 and 2.48)''. Fig. 2.47: Influence of 1-10 atom% of different alloying metals on the electrical resistivity of silver[[File:Influence of 1 10 atom of different alloying metals.jpg|right|thumb|Influence of 1-10 atom% of different alloying metals on the electrical resistivity of silver]]

Fine-Grain Silver (ARGODUR-Spezial) is defined as a silver alloy with an additionof 0.15 wt% of Nickel. Silver and nickel are not soluble in each other in solidform. In liquid silver only a small amount of nickel is soluble as the phase diagram''(Fig. 2.51)'' illustrates. During solidification of the melt this nickel addition getsfinely dispersed in the silver matrix and eliminates the pronounce coarse graingrowth after prolonged influence of elevated temperatures ''(Figs. 2.49 and 2.50)''.

Fine-grain silver has almost the same chemical corrosion resistance as finesilver. Compared to pure silver it exhibits a slightly increased hardness andtensile strength ''(Table 2.14)''. The electrical conductivity is just slightly decreasedby this low nickel addition. Because of its significantly improved contactproperties fine grain silver has replaced pure silver in many applications.

Using copper as an alloying component increases the mechanical stability ofsilver significantly. The most important among the binary AgCu alloys is that ofAgCu3, known in europe also under the name of hard-silver. This material stillhas a chemical corrosion resistance close to that of fine silver. In comparison topure silver and fine-grain silver AgCu3 exhibits increased mechanical strengthas well as higher arc erosion resistance and mechanical wear resistance''(Table 2.14)''.

Increasing the Cu content further also increases the mechanical strength ofAgCu alloys and improves arc erosion resistance and resistance againstmaterial transfer while at the same time however the tendency to oxide formationbecomes detrimental. This causes during switching under arcing conditions anincrease in contact resistance with rising numbers of operation. In specialapplications where highest mechanical strength is recommended and a reducedchemical resistance can be tolerated, the eutectic AgCu alloy with 28 wt% ofcopper ''(Fig. 2.52)'' is used. AgCu10 also known as coin silver has beenreplaced in many applications by composite silver-based materials while sterlingsilver (AgCu7.5) has never extended its important usage from decorative tablewear and jewelry to industrial applications in electrical contacts.

Besides these binary alloys, ternary AgCuNi alloys are used in electrical contactapplications. From this group the material ARGODUR 27, an alloy of 98 wt% Agwith a 2 wt% Cu and nickel addition has found practical importance close to thatof AgCu3. This material is characterized by high resistance to oxidation and lowtendency to re-crystallization during exposure to high temperatures. Besideshigh mechanical stability this AgCuNi alloy also exhibits a strong resistanceagainst arc erosion. Because of its high resistance against material transfer thealloy AgCu24.5Ni0.5 has been used in the automotive industry for an extendedtime in the North American market. Caused by miniaturization and the relatedreduction in available contact forces in relays and switches this material hasbeen replaced widely because of its tendency to oxide formation.

The attachment methods used for the hard silver materials are mostly close tothose applied for fine silver and fine grain silver.

Hard-silver alloys are widely used for switching applications in the informationand energy technology for currents up to 10 A, in special cases also for highercurrent ranges ''(Table 2.16)''.

Dispersion hardened alloys of silver with 0.5 wt% MgO and NiO (ARGODUR 32)are produced by internal oxidation. While the melt-metallurgical alloy is easy tocold-work and form the material becomes very hard and brittle after dispersionhardening. Compared to fine silver and hard-silver this material has a greatlyimproved temperature stability and can be exposed to brazing temperatures upto 800°C without decreasing its hardness and tensile strength.