The family of silver-metal oxide contact materials includes the material groups:silver-cadmium oxide (DODURIT CdO), silver-tin oxide (SISTADOX), and silverzincoxide (DODURIT ZnO). Because of their very good contact and switchingproperties like high resistance against welding, low contact resistance, and higharc erosion resistance, silver-metal oxides have gained an outstanding positionin a broad field of applications. They mainly are used in low voltage electricalswitching devices like relays, installation and distribution switches, appliances,industrial controls, motor controls, and protective devices ''(Table 2.13)''.

Silver-cadmium oxide (DODURIT CdO) materials with 10-15 wt% are producedby both, internal oxidation and powder metallurgical methods ''(Table 2.25)''.

The manufacturing of strips and wires by internal oxidation starts with a moltenalloy of silver and cadmium. During a heat treatment below it's melting point in aoxygen rich atmosphere in such a homogeneous alloy the oxygen diffuses fromthe surface into the bulk of the material and oxidizes the Cd to CdO in a more orless fine particle precipitation inside the Ag matrix. The CdO particles are ratherfine in the surface area and are becoming larger further away towards the centerof the material ''(Fig. 2.83)''.

During the manufacturing of Ag/CdO contact material by internal oxidation theprocesses vary depending on the type of semi-finished material.For Ag/CdO wires a complete oxidation of the AgCd wire is performed, followedby wire-drawing to the required diameter ''(Figs. 2.77 and 2.78)''. The resultingmaterial is used for example in the production of contact rivets. For Ag/CdO stripmaterials two processes are commonly used: Cladding of an AgCd alloy stripwith fine silver followed by complete oxidation results in a strip material with asmall depletion area in the center of it's thickness and a Ag backing suitable foreasy attachment by brazing (sometimes called “Conventional Ag/CdO”). Usinga technology that allows the partial oxidation of a dual-strip AgCd alloy materialin a higher pressure pure oxygen atmosphere yields a composite Ag/CdO stripmaterial that has besides a relatively fine CdO precipitation also a easily brazableAgCd alloy backing ''(Fig. 2.85)''. These materials (DODURIT CdO ZH) are mainlyused as the basis for contact profiles and contact tips.

During powder metallurgical production the powder mixed made by differentprocesses are typically converted by pressing, sintering and extrusion to wiresand strips. The high degree of deformation during hot extrusion produces auniform and fine dispersion of CdO particles in the Ag matrix while at the sametime achieving a high density which is advantageous for good contact properties''(Fig. 2.84)''. To obtain a backing suitable for brazing, a fine silver layer is appliedby either com-pound extrusion or hot cladding prior to or right after the extrusion''(Fig. 2.86)''.

For larger contact tips, and especially those with a rounded shape, the single tipPress-Sinter-Repress process (PSR) offers economical advantages. Thepowder mix is pressed in a die close to the final desired shape, the “green” tipsare sintered, and in most cases the repress process forms the final exact shapewhile at the same time increasing the contact density and hardness.

Using different silver powders and minor additives for the basic Ag and CdOstarting materials can help influence certain contact properties for specializedapplications.

Fig. 2.77:Strain hardening of internally oxidizedAg/CdO 90/10 by cold working

Fig. 2.78:Softening of internally oxidizedAg/CdO 90/10 after annealingfor 1 hr after 40% cold working

'''Table 2.25: Physical and Mechanical Properties as well as Manufacturing Processes andForms of Supply of Extruded Silver Cadmium Oxide(DODURIT CdO) Contact Materials'''

Fig. 2.79:Strain hardening ofAg/CdO 90/10 P by cold working

Fig. 2.80: Softeningof Ag/CdO 90/10 P after annealingfor 1 hr after 40% cold working

Fig. 2.81:Strain hardeningof Ag/CdO 88/12 WP

Fig. 2.82:Softening of Ag/CdO 88/12WP after annealingfor 1 hr after different degrees ofcold working

Fig. 2.83: Micro structure of Ag/CdO 90/10 i.o. a) close to surfaceb) in center area

Fig. 2.84: Micro structure of Ag/CdO 90/10 P:a) perpendicular to extrusion directionb) parallel to extrusion direction

Fig. 2.85:Micro structure of Ag/CdO 90/10 ZH:1) Ag/CdO layer2) AgCd backing layer

Fig. 2.86: Micro structure of AgCdO 88/12 WP: a) perpendicular to extrusion directionb) parallel to extrusion direction

Over the past years, many Ag/CdO contact materials have been replaced byAg/SnO₂ based materials with 2-14 wt% SnO₂ because of the toxicity ofCadmium. This changeover was further favored by the fact that Ag/SnO₂contacts quite often show improved contact and switching properties such aslower arc erosion, higher weld resistance, and a significant lower tendencytowards material transfer in DC switching circuits ''(Table 2.30)''. Ag/SnO₂materials have been optimized for a broad range of applications by other metaloxide additives and modification in the manufacturing processes that result indifferent metallurgical, physical and electrical properties ''(Table 2.29)''.

Manufacturing of Ag/SnO₂ by ''internal oxidation'' is possible in principle, butduring heat treatment of alloys containing > 5 wt% of tin in oxygen, dense oxidelayers formed on the surface of the material prohibit the further diffusion ofoxygen into the bulk of the material. By adding Indium or Bismuth to the alloy theinternal oxidation is possible and results in materials that typically are rather hardand brittle and may show somewhat elevated contact resistance and is limitedto applications in relays. To make a ductile material with fine oxide dispersion(SISTADOX TOS F) ''(Fig. 2.114)'' it is necessary to use special process variationsin oxidation and extrusion which lead to materials with improved properties inrelays. Adding a brazable fine silver layer to such materials results in a semifinishedmaterial suitable for the manufacture as smaller weld profiles(SISTADOX WTOS F) ''(Fig. 2.116)''. Because of their resistance to materialtransfer and low arc erosion these materials find for example a broader

''Powder metallurgy'' plays a significant role in the manufacturing of Ag/SnO₂contact materials. Besides SnO₂ a smaller amount (<1 wt%) of one or moreother metal oxides such as WO₃, MoO₃, CuO and/or Bi₂O₃ are added. Theseadditives improve the wettability of the oxide particles and increase the viscosityof the Ag melt. They also provide additional benefits to the mechanical andarcing contact properties of materials in this group ''(Table 2.26)''.

In the manufacture the initial powder mixes different processes are appliedwhich provide specific advantages of the resulting materials in respect to theircontact properties ''(Figs. 2.87 – 2.119)''. Some of them are described here asfollows:

Further processing of these differently produced powders follows theconventional processes of pressing, sintering and hot extrusion to wires andstrips. From these contact parts such as contact rivets and tips aremanufactured. To obtain a brazable backing the same processes as used forAg/CdO are applied. As for Ag/CdO, larger contact tips can also bemanufactured more economically using the press-sinter-repress (PSR) process''(Table 2.27).''

Fig. 2.87:Strain hardening ofAg/SnO₂ 92/8 PE by cold working

Fig. 2.88:Softening ofAg/SnO₂ 92/8 PE after annealingfor 1 hr after 40% cold working

'''Table 2.26: Physical and Mechanical Properties as well as Manufacturing Processes and

Fig. 2.8990:Strain hardening Softening ofAg/SnO₂ 88/12 PE by after annealing for 1 hr after 40% cold working

Fig. 2.9091:Softening Strain hardening of oxidized Ag/SnO₂ 88/12 PEafter annealing for1 hr after 40% PW4 by cold working

Fig. 2.9192:Strain hardening Softening of oxidizedAg/SnO₂ 88/12 PW4 by after annealing for 1 hr after 30% cold working

Fig. 2.9293:Softening Strain hardening of Ag/SnO₂ 8898/12 PW4 afterannealing for 1 hrafter 30% 2 PX by cold working

Fig. 2.9394:Strain hardening Softening ofAg/SnO₂ 98/2 PXby after annealing for 1 hr after 80% cold working

Fig. 2.9495:Softening Strain hardening ofAg/SnO₂ 9892/2 8 PXafter annealingfor 1 hr after 80%by cold working

Fig . 2.9596:Strain hardeningSoftening of Ag/SnO₂ 92/8 PXby after annealing for 1 hr after 40% cold working

Fig. 2.9697:Softening Strain hardening ofinternally oxidized Ag/SnO₂ 9288/8 PXafter annealing for 1 hrafter 40% 12 TOS F by cold working

Fig. 2.9798:Strain hardening Softening of internallyoxidizedAg/SnO₂ 88/12 TOS Fby after annealing for 1 hr after 30% cold working

Fig. 2.98:Softening ofAg/SnO₂ 88/12 TOS F afterannealing for 1 hr after 30%cold working Fig. 2.99:Strain hardening ofinternally oxidizedAg/SnO₂ 88/12Pby cold working