Silver Based Materials

Pure Silver

Pure silver (also called fine silver) exhibits the highest electrical and thermal conductivity of all metals. It is also resistant against oxidation. Major disadvantages are its low mechanical wear resistance, the low softening temperature, and especially its strong affinity to sulfur and sulfur compounds. In the presence of sulfur and sulfur containing compounds brownish to black silver sulfide layer are formed on its surface. These can cause increased contact resistance or even total failure of a switching device if they are not mechanically, electrically, or thermally destroyed. Other weaknesses of silver contacts are the tendency to weld under the influence of over-currents and the low resistance against material transfer when switching DC loads. In humid environments and under the influence of an electrical field silver can creep (silver migration) and cause electrical shorting between adjacent current paths.

Table 1 shows the typically available quality grades of silver. In certain economic areas, i.e. China, there are additional grades with varying amounts of impurities available on the market. In powder form silver is used for a wide variety of silver based composite contact materials. Different manufacturing processes result in different grades of Ag powder as shown in Table 2. Additional properties of silver powders and their usage are described in Precious Metal Powders und Table Different Types of Silver Powders.

Semi-finished silver materials can easily be warm or cold formed and can be clad to the usual base materials (Figure 1 and Figure 2). For attachment of silver to contact carrier materials welding of wire or profile cut-offs and brazing are most widely applied. Besides these mechanical processes such as wire insertion (wire staking) and the riveting (staking) of solid or composite contact rivets are used in the manufacture of contact components.

Contacts made from fine silver are applied in various electrical switching devices such as relays, pushbuttons, appliance and control switches for currents < 2 A (Table 6). Electroplated silver coatings are widely used to reduce the contact resistance and improve the brazing behavior of other contact materials and components.

Table 1: Overview of the Most Widely Used Silver Grades

Designation	Composition minimum Ag [wt%]	Impurities [ppm]	Notes on Usage
Spectroscopically Pure Ag	99.999	Cu < 3 Zn < 1 Si < 1 Ca < 2 Fe < 1 Mg < 1 Cd < 1	Sheets, strips, rods, wires for electronic applications
High Purity Ag, oxygen-free	99.995	Cu < 30 Zn < 2 Si < 5 Ca < 10 Fe < 3 Mg < 5 Cd < 3	Ingots, bars, granulate for alloying purposes

Table 2: Quality Criteria of Differently Manufactured Silver Powders

Impurities		Ag-Chem.*	Ag-ES**	Ag-V***
Cu	ppm	< 100	< 300	< 300
Fe	ppm	< 50	< 100	< 100
Ni	ppm	< 50	< 50	< 50
Cd	ppm			< 50
Zn	ppm			< 10
Na + K + Mg + Ca	ppm	< 80	< 50	< 50
Ag CI	ppm	< 500	< 500	< 500
NO3	ppm	< 40	< 40
Nh4CI	ppm	< 30	< 30
Particle Size Distribution (screen analysis)
> 100 μm	%	0	0	0
< 100 bis > 63 μm	%	< 5	< 5	< 15
< 36 μm	%	< 80	< 90	< 75
Apparent Density	g/cm3	1.0 - 1.6	1.0 - 1.5	3 - 4
Tap Density	ml/100g	40 - 50	40 - 50	15 - 25
Press/Sintering Behavior
Press Density	g/cm3	5.6 - 6.5	5.6 - 6.3	6.5 - 8.5
Sinter Density	g/cm3	> 9	> 9.3	> 8
Volume Shrinkage	%	> 34	> 35	> 0
Annealing Loss	%	< 2	< 0.1	< 0.1

* Manufactured by chemical precipitation
** Manufactured by electrolytic deposition
*** Manufactured by atomizing of a melt

Figure 1: Strain hardening of Ag 99.95 - cold working

Figure 2: Softening of Ag 99.95 after annealing for 1 hr after different degrees of strain hardening

Silver Alloys

To improve the physical and contact properties of fine silver, melt-metallurgical produced silver alloys are used (Table 3). By adding metal components, the mechanical properties such as hardness and tensile strength as well as typical contact properties such as erosion resistance and resistance against material transfer in DC circuits are increased (Table 4). On the other hand however, other properties such as electrical conductivity and chemical corrosion resistance can be negatively impacted by alloying (Figure 3 and Figure 4).

Table 3: Physical Properties of Silver and Silver Alloys

Material	Silver Content [wt%]	Density [g/cm3]	Melting Point or Range [°C]	Electrical Resistivity [μΩ·cm]	Electrical Conductivity [MS/m]	Thermal Conductivity [W/mK]	Temp. Coefficient of the Electr.Resistance [10-3/K]	Modulus of Elasticity [GPa]
Ag	99.95	10.5	961	1.67	60	419	4.1	80
AgNi0.15	99.85	10.5	960	1.72	58	414	4.0	82
AgCu3	97	10.4	900 - 938	1.92	52	385	3.2	85
AgCu5	95	10.4	910	1.96	51	380	3.0	85
AgCu10	90	10.3	870	2.0	50	335	2.8	85
AgCu28	72	10.0	779	2.08	48	325	2.7	92
Ag98CuNi ARGODUR 27	98	10.4	940	1.92	52	385	3.5	85
AgCu24.5Ni0.5	75	10.0	805	2.20	45	330	2.7	92
Ag99.5NiMg ARGODUR 32 Not heat treated	99.5	10.5	960	2.32	43	293	2.3	80
ARGODUR 32 Heat treated	99.5	10.5	960	2.32	43	293	2.1	80

Figure 3: Influence of 1-10 atom% of different alloying metals on the electrical resistivity of silver

Figure 4: Electrical resistivity p of AgCu alloys with 0-20 weight% Cu in the soft annealed and tempered stage a) Annealed and quenched b) Tempered at 280°C

Table 4: Mechanical Properties of Silver and Silver Alloys

Material	Hardness Condition	Tensile Strength Rm [MPa]	Elongation A [%] min.	Vickers Hardness HV 10
Ag	R 200 R 250 R 300 R 360	200 - 250 250 - 300 300 - 360 > 360	30 8 3 2	30 60 80 90
AgNi0.15	R 220 R 270 R 320 R 360	220 - 270 270 - 320 320 - 360 > 360	25 6 2 1	40 70 85 100
AgCu3	R 250 R 330 R 400 R 470	250 - 330 330 - 400 400 - 470 > 470	25 4 2 1	45 90 115 120
AgCu5	R 270 R 350 R 460 R 550	270 - 350 350 - 460 460 - 550 > 550	20 4 2 1	55 90 115 135
AgCu10	R 280 R 370 R 470 R 570	280 - 370 370 - 470 470 - 570 > 570	15 3 2 1	60 95 130 150
AgCu28	R 300 R 380 R 500 R 650	300 - 380 380 - 500 500 - 650 > 650	10 3 2 1	90 120 140 160
Ag98CuNi ARGODUR 27	R 250 R 310 R 400 R 450	250 - 310 310 - 400 400 - 450 > 450	20 5 2 1	50 85 110 120
AgCu24,5Ni0,5	R 300 R 600	300 - 380 > 600	10 1	105 180
Ag99,5NiMg ARGODUR 32 Not heat treated	R 220 R 260 R 310 R 360	220 260 310 360	25 5 2 1	40 70 85 100
ARGODUR 32 Heat treated	R 400	400	2	130-170

Fine-Grain Silver

Fine-Grain silver is defined as a silver alloy with an addition of 0.15 wt% of nickel. Silver and nickel are not soluble in each other in solid form. In liquid silver, only a small amount of nickel is soluble as the phase diagram illustrates (Figure 7). During solidification of the melt, this nickel addition gets finely dispersed in the silver matrix and eliminates the pronounce coarse grain growth after prolonged influence of elevated temperatures (Figure 5 and Figure 6).

Figure 5: Coarse grain micro structure of Ag 99.97 after 80% cold working and 1 hr annealing at 600°C

Figure 6: Fine grain microstructure of AgNi0.15 after 80% cold working and 1 hr annealing at 600°C

Figure 7: Phase diagram of silver nickel

Fine-Grain silver has almost the same chemical corrosion resistance as fine silver. Compared to pure silver, it exhibits a slightly increased hardness and tensile strength (Table 4). The electrical conductivity is just slightly decreased by this low nickel addition. Because of its significantly improved contact properties, fine grain silver has replaced pure silver in many applications.

Hard-Silver Alloys

Using copper as an alloying component increases the mechanical stability of silver significantly (Figure 9, Figure 10 and Figure 11). The most important among the binary AgCu alloys is that of AgCu3, in europe also known as hard-silver. This material still has a chemical corrosion resistance close to that of fine silver. In comparison to pure silver and fine-grain silver, AgCu3 exhibits increased mechanical strength as well as higher arc erosion resistance and mechanical wear resistance.

Increasing the Cu content further also increases the mechanical strength of AgCu alloys and improves arc erosion resistance and resistance against material transfer while simultaneously the tendency to oxide formation becomes detrimental. This causes - during switching under arcing conditions - an increase in contact resistance with rising numbers of operation. In special applications, where highest mechanical strength is recommended and a reduced chemical resistance can be tolerated, the eutectic AgCu alloy with 28 wt% of copper is used (Figure 8). AgCu10, also known as coin silver, has been replaced in many applications by composite silver-based materials while sterling silver (AgCu7.5) has never extended its important usage from decorative table wear and jewelry to industrial applications in electrical contacts.

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

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

Hard-silver alloys are widely used for switching applications in the information and energy technology for currents up to 10 A, in special cases also for higher current ranges (Table 6).

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 to cold-work and form, the material becomes very hard and brittle after dispersion hardening. Compared to fine silver and hard-silver, this material has a greatly improved temperature stability and can be exposed to brazing temperatures up to 800°C without decreasing its hardness and tensile strength. Because of these mechanical properties and its high electrical conductivity ARGODUR 32 is mainly used in the form of contact springs that are exposed to high thermal and mechanical stresses in relays and contactors for aeronautic applications.

Figure 8: Phase diagram of silver-copper

Figure 9: Strain hardening of AgCu3 by cold working

Figure 10: Softening of AgCu3 after annealing for 1 hr after 80% cold working

Figure 11: Strain hardening of AgCu5 by cold working

Figure 12: Softening of AgCu5 after annealing for 1 hr after 80% cold working

Figure 13: Strain hardening of AgCu 10 by cold working

Figure 14: Softening of AgCu10 after annealing for 1 hr after 80% cold working

Figure 15: Strain hardening of AgCu28 by cold working

Figure 16: Softening of AgCu28 after annealing for 1 hr after 80% cold working

Figure 17: Strain hardening of AgNiO15 by cold working

Figure 18: Softening of AgNiO15 after annealing

Figure 19: Strain hardening of AgCu1.8Ni0.2 (ARGODUR 27) by cold working

Figure 20: Softening of AgCu1.8Ni0.2 (ARGODUR 27) after annealing for 1 hr after 80% cold working

Table 5: Contact and Switching Properties of Silver and Silver Alloys

Material	Properties
Ag AgNi0.15	Highest electrical and thermal conductivity, high affinity to sulfur (sulfide formation), low welding resistance, low contact resistance, very good formability	Oxidation resistant at higher make currents, limited arc erosion resistance, tendency to material transfer in DC circuits, easy to braze and weld to carrier materials
Ag Alloys	Increasing contact resistance with increasing Cu content, compared to fine Ag higher arc erosion resistance and mechanical strength, lower tendency to material transfer	Good formability, good brazing and welding properties

Table 6: Application Examples and Forms of Supply for Silver and Silver Alloys

Material	Application Examples	Form of Supply
Ag AgNi0.15 AgCu3 AgNi98NiCu2 ARGODUR 27 AgCu24,5Ni0,5	Relays, Micro switches, Auxiliary current switches, Control circuit devices, Appliance switches, Wiring devices (≤ 20A), Main switches	Semi-finished Materials: Strips, wires, contact profiles, clad contact strips, toplay profiles, seam- welded strips Contact Parts: Contact tips, solid and composite rivets, weld buttons; clad, welded and riveted contact parts
AgCu5 AgCu10 AgCu28	Special applications	Semi-finished Materials: Strips, wires, contact profiles, clad contact strips, seam-welded strips Contact parts: Contact tips, solid contact rivets, weld buttons; clad, welded and riveted contact parts
Ag99.5NiOMgO ARGODUR 32	Miniature relays, aerospace relays and contactors, erosion wire for injection nozzles	Contact springs, contact carrier parts

Silver-Palladium Alloys

The addition of 30 wt% Pd increases the mechanical properties as well as the resistance of silver against the influence of sulfur and sulfur containing compounds significantly (Table 7 and Table 8). Alloys with 40-60 wt% Pd have an even higher resistance against silver sulfide formation. At these percentage ranges however, the catalytic properties of palladium can influence the contact resistance behavior negatively. The formability also decreases with increasing Pd contents.

AgPd alloys are hard, arc erosion resistant, and have a lower tendency towards material transfer under DC loads (Table 9). On the other hand, the electrical conductivity is decreased at higher Pd contents. The ternary alloy AgPd30Cu5 has an even higher hardness, which makes it suitable for use in sliding contact systems.

AgPd alloys are mostly used in relays for the switching of medium to higher loads (> 60V, > 2A) as shown in Table 10. Because of the high palladium price, these formerly solid contacts have been widely replaced by multi-layer designs such as AgNi0.15 or AgNi10 with a thin Au surface layer. A broader field of application for AgPd alloys remains in the wear resistant sliding contact systems.

Figure 21: Phase diagram of silver-palladium

Figure 22: Strain hardening of AgPd30 by cold working

Figure 23: Strain hardening of AgPd50 by cold working

Figure 24: Strain hardening of AgPd30Cu5 by cold working

Figure 25: Softening of AgPd30, AgPd50, and AgPd30Cu5 after annealing of 1 hr after 80% cold working

Table 7: Physical Properties of Silver-Palladium Alloys

Material	Palladium Content [wt%]	Density [g/cm3]	Melting Point or Range [°C]	Electrical Resistivity [μΩ·cm]	Electrical Conductivity [MS/m]	Thermal Conductivity [W/m·K]	Temp. Coefficient of the Electr. Resistance [10-3/K]
AgPd30	30	10.9	1155 - 1220	14.7	6.8	60	0.4
AgPd40	40	11.1	1225 - 1285	20.8	4.8	46	0.36
AgPd50	50	11.2	1290 - 1340	32.3	3.1	34	0.23
AgPd60	60	11.4	1330 - 1385	41.7	2.4	29	0.12
AgPd30Cu5	30	10.8	1120 - 1165	15.6	6.4	28	0.37

Table 8: Mechanical Properties of Silver-Palladium Alloys

Material	Hardness Condition	Tensile Strength Rm[MPa]	Elongation A [%]min.	Vickers Hardness HV
AgPd30	R 320 R 570	320 570	38 3	65 145
AgPd40	R 350 R 630	350 630	38 2	72 165
AgPd50	R 340 R 630	340 630	35 2	78 185
AgPd60	R 430 R 700	430 700	30 2	85 195
AgPd30Cu5	R 410 R 620	410 620	40 2	90 190

Table 9: Contact and Switching Properties of Silver-Palladium Alloys

Material	Properties
AgPd30-60	Corrosion resistant, tendency to Brown Powder formation increases with Pd content, low tendency to material transfer in DC circuits, high ductility	Resistant against Ag2S formation, low contact resistance, increasing hardness with higher Pd content, AgPd30 has highest arc erosion resistance, easy to weld and clad
AgPd30Cu5	High mechanical wear resistance	High Hardness

Table 10: Application Examples and Forms of Suppl for Silver-Palladium Alloys

Material	Application Examples	Form of Supply
AgPd 30-60	Switches, relays, push-buttons, connectors, sliding contacts	Semi-finished Materials: Wires, micro profiles (weld tapes), clad contact strips, seam-welded strips Contact Parts: Solid and composite rivets, weld buttons; clad and welded contact parts, stamped parts
AgPd30Cu5	Sliding contacts, slider tracks	Wire-formed parts, contact springs, solid and clad stamped parts

Silver Composite Materials

Silver-Nickel Materials

Since silver and nickel are not soluble in each other in solid form and also show very limited solubility in the liquid phase, silver nickel composite materials with higher Ni contents can only be produced by powder metallurgy. During extrusion of sintered Ag/Ni billets into wires, strips and rods, the Ni particles embedded in the Ag matrix are stretched and oriented in the microstructure into a pronounced fiber structure (Figure 30 and Figure 31)

The high density produced during hot extrusion, aids the arc erosion resistance of these materials (Table 11). The typical application of Ag/Ni contact materials is in devices for switching currents of up to 100A (Table 14). In this range, they are significantly more erosion resistant than silver or silver alloys. In addition, they exhibit with nickel contents < 20 wt% a low and over their operational lifetime consistent contact resistance and good arc moving properties. In DC applications Ag/Ni materials exhibit a relatively low tendency of material transfer distributed evenly over the contact surfaces (Table 13).

Typically Ag/Ni materials are usually produced with contents of 10-40 wt% Ni. The most common used materials Ag/Ni 10 and Ag/Ni 20- and also Ag/Ni 15, mostly used in north america-, are easily formable and applied by cladding (Figure 26, Figure 27, Figure 28, Figure 29). They can be, without any additional welding aids, economically welded and brazed to the commonly used contact carrier materials. The Ag/Ni materials with nickel contents of 30 and 40 wt% are used in switching devices, requiring a higher arc erosion resistance and where increases in contact resistance can be compensated through higher contact forces.

The most important applications for Ag/Ni contact materials are typically in relays, wiring devices, appliance switches, thermostatic controls, auxiliary switches and small contactors with nominal currents > 20A (Table 14).

Table 11: Physical Properties of Silver-Nickel Materials

Material	Silver Content	Density	Melting Point	ElectricalResistivityp	Electrical Resistivity (soft)
	[wt%]	[g/cm3]	[°C]	[µΩ·cm]	[% IACS]	[MS/m]
Ag/Ni 90/10	89 - 91	10.2 - 10.3	960	1.82 - 1.92	90 - 95	52 - 55
Ag/Ni 85/15	84 - 86	10.1 - 10.2	960	1.89 - 2.0	86 - 91	50 - 53
Ag/Ni 80/20	79 - 81	10.0 - 10.1	960	1.92 - 2.08	83 - 90	48 - 52
Ag/Ni 70/30	69 - 71	9.8	960	2.44	71	41
Ag/Ni 60/40	59 - 61	9.7	960	2.70	64	37

Table 12: Mechanical Properties of Silver-Nickel Materials

Material	Hardness Condition	Tensile Strength Rm [Mpa]	Elongation A (soft annealed) [%] min.	Vickers Hardness HV 10
Ag/Ni 90/10	soft R 220 R 280 R 340 R 400	< 250 220 - 280 280 - 340 340 - 400 > 400	25 20 3 2 1	< 50 50 - 70 65 - 90 85 - 105 > 100
Ag/Ni 85/15	soft R 300 R 350 R 380 R 400	< 275 250 - 300 300 - 350 350 - 400 > 400	20 4 2 2 1	< 70 70 - 90 85 - 105 100 - 120 > 115
Ag/Ni 80/20	soft R 300 R 350 R 400 R 450	< 300 300 - 350 350 - 400 400 - 450 > 450	20 4 2 2 1	< 80 80 - 95 90 - 110 100 - 125 > 120
Ag/Ni 70/30	R 330 R 420 R 470 R 530	330 - 420 420 - 470 470 - 530 > 530	8 2 1 1	80 100 115 135
Ag/Ni 60/40	R 370 R 440 R 500 R 580	370 - 440 440 - 500 500 - 580 > 580	6 2 1 1	90 110 130 150

Figure 26: Strain hardening of Ag/Ni 90/10 by cold working

Figure 27: Softening of Ag/Ni 90/10 after annealing for 1 hr after 80% cold working

Figure 28: Strain hardening of Ag/Ni 80/20 by cold working

Figure 29: Softening of Ag/Ni 80/20 after annealing for 1 hr after 80% cold working

Figure 30: Micro structure of Ag/Ni 90/10 a) perpendicular to the extrusion direction b) parallel to the extrusion direction

Figure 31: Micro structure of Ag/Ni 80/20 a) perpendicular to the extrusion direction b) parallel to the extrusion direction

Table 13: Contact and Switching Properties of Silver-Nickel Materials

Material

Properties

Ag/Ni

High arc erosion resistance at switching currents up to 100A, Resistance against welding for starting current up to 100A, low and over the electrical contact life nearly constant contact resistance for Ag/Ni 90/10 and Ag/Ni 80/20, ow and spread-out material transfer under DC load, non-conductive erosion residue on isolating components resulting in only minor change of the dielectric strength of switching devices, good arc moving properties, good arc extinguishing properties, good or sufficient ductility depending on the Ni content, easy to weld and braze

Table 14: Application Examples and Forms of Supply for Silver-Nickel Materials

Material	Application Examples	Switching or Nominal Current	Form of Supply
Ag/Ni 90/10-80/20	Relays Automotive Relays - Resistive load - Motor load	> 10A > 10A	Semi-finisched Materials: Wires, profiles, clad strips, Seam-welded strips, Toplay strips Contact Parts: Contact tips, solid and composite rivets, Weld buttons, clad, welded, brazed, and riveted contact parts
Ag/Ni 90/10, Ag/Ni 85/15-80/20	Auxiliary current switches	≤ 100A
Ag/Ni 90/10-80/20	Appliance switches	≤ 50A
Ag/Ni 90/10	Wiring devices	≤ 20A
Ag/Ni 90/10	Main switches, Automatic staircase illumination switches	≤ 100A
Ag/Ni 90/10-80/20	Control Thermostats	> 10A ≤ 50A
Ag/Ni 90/10-80/20	Load switches	≤ 20A
Ag/Ni 90/10-80/20	Contactors circuit breakers	≤ 100A
Ag/Ni 90/10-80/20 paired with Ag/C 97/3-96/4	Motor protective circuit breakers	≤ 40A
Ag/Ni 80/20-60/40 paired with Ag/C 96/4-95/5	Fault current circuit breakers	≤ 100A	Rods, Profiles, Contact tips, Formed parts, brazed and welded contact parts
Ag/Ni 80/20-60/40 paired with Ag/C 96/4-95/5	Power switches	> 100A

Silver-Metal Oxide Materials Ag/CdO, Ag/SnO₂, Ag/ZnO

The family of silver-metal oxide contact materials includes the material groups: silver-cadmium oxide, silver-tin oxide, and silverzinc oxide. Because of their very good contact and switching properties like high resistance against welding, low contact resistance, and high arc erosion resistance, silver-metal oxides have gained an outstanding position in a broad field of applications. They are mainly used in low voltage electrical switching devices like relays, installation and distribution switches, appliances, industrial controls, motor controls, and protective devices (Table 20).

• Silver-cadmium oxide materials

Silver-cadmium oxide materials with 10-15 wt% are produced by both, internal oxidation and powder metallurgical methods.

The manufacturing of strips and wires by internal oxidation starts with a molten alloy of silver and cadmium. During a heat treatment below it's melting point in an oxygen rich atmosphere of such a homogeneous alloy, the oxygen diffuses from the surface into the bulk of the material and oxidizes the Cd to CdO in a more or less fine particle precipitation inside the Ag matrix. The CdO particles are rather fine in the surface area and getting larger towards the center of the material (Figure 38).

During the manufacturing of Ag/CdO contact material by internal oxidation, the processes vary depending on the type of semi-finished material. For Ag/CdO wires, a complete oxidation of the AgCd wire is performed, followed by wire-drawing to the required diameter (Figure 32 and Figure 33). The resulting material is used for example, in the production of contact rivets. For Ag/CdO strip materials two processes are commonly used: Cladding of an AgCd alloy strip with fine silver, followed by complete oxidation, results in a strip material with a small depletion area in the center of it's thickness and an Ag backing suitable for easy attachment by brazing (sometimes called "Conventional Ag/CdO"). Using a technology that allows the partial oxidation of a dual-strip AgCd alloy material in a higher pressure pure oxygen atmosphere, yields a composite Ag/CdO strip material that has - besides a relatively fine CdO precipitation - also an easily brazable AgCd alloy backing. These materials are mainly used as the basis for contact profiles and contact tips.

During powder metallurgical production, the powder mixed made by different processes are typically converted by pressing, sintering and extrusion to wires and strips. The high degree of deformation during hot extrusion, produces a uniform and fine dispersion of CdO particles in the Ag matrix while at the same time achieving a high density which is advantageous for good contact properties (Figure 39). To obtain a backing suitable for brazing, a fine silver layer is applied by either com-pound extrusion or hot cladding prior to or right after the extrusion.

For larger contact tips, and especially those with a rounded shape, the single tip Press-Sinter-Repress process (PSR) offers economical advantages. The powder mix is pressed into a die close to the final desired shape, the "green" tips are sintered, and in most cases, the repress process forms the exact final shape while at the same time, increasing the contact density and hardness.

Using different silver powders and minor additives for the basic Ag and CdO, starting materials can help influence certain contact properties for specialized applications.

Figure 32: Strain hardening of internally oxidized Ag/CdO 90/10 by cold working

Figure 33: Softening of internally oxidized (i.o.) Ag/CdO 90/10 after annealing for 1 hr after 40% cold working

Figure 34: Strain hardening of powder metallurgical (p.m.) Ag/CdO 90/10 by cold working

Figure 35: Softening of powder metallurgical Ag/CdO 90/10 after annealing for 1 hr after 40% cold working

Figure 36: Strain hardening of powder metallurgical Ag/CdO 88/12

Figure 37: Softening of powder metallurgical Ag/CdO 88/12 after annealing for 1 hr after different degrees of cold working

Figure 38: Micro structure of Ag/CdO 90/10 i.o. a) close to surface b) in center area

Figure 39: Micro structure of Ag/CdO 90/10 p.m.: a) perpendicular to extrusion direction b) parallel to extrusion direction

• Silver–tin oxide materials

Over the past years, many Ag/CdO contact materials have been replaced by Ag/SnO₂ based materials with 2-14 wt% SnO₂ because of the toxicity of Cadmium. This changeover was further favored by the fact that Ag/SnO₂ contacts quite often show improved contact and switching properties such as lower arc erosion, higher weld resistance and a significant lower tendency towards material transfer in DC switching circuits (Table 19). Ag/SnO₂ materials have been optimized for a broad range of applications by other metal oxide additives and modification in the manufacturing processes that result in different metallurgical, physical and electrical properties (Table 17 and Table 18).

Manufacturing of Ag/SnO₂ by internal oxidation is possible in principle, but during heat treatment of alloys containing > 5 wt% of tin in oxygen, dense oxide layers formed on the surface of the material prohibit the further diffusion of oxygen into the bulk of the material. By adding Indium or Bismuth to the alloy, the internal oxidation is possible and results in materials that typically are rather hard and brittle and may show somewhat elevated contact resistance and is limited to applications in relays. Adding a brazable fine silver layer to such materials results in a semifinished material, suitable for the manufacture as smaller weld profiles (Figure 56). Because of their resistance to material transfer and low arc erosion, these materials find for example a broader application in automotive relays (Table 20).

Powder metallurgy plays a significant role in the manufacturing of Ag/SnO₂ contact materials. Besides SnO₂ a smaller amount (<1 wt%) of one or more other metal oxides such as WO₃, MoO₃, CuO and/or Bi₂O₃ are added. These additives improve the wettability of the oxide particles and increase the viscosity of the Ag melt. They also provide additional benefits to the mechanical and arcing contact properties of materials in this group (Table 15).

Table 15: Physical and Mechanical Properties as well as Manufacturing Processes and Forms of Supply of Extruded Silver-Tin Oxide Contact Materials

Material	Silver Content [wt%]	Additives	Theoretical Density [g/cm3]	Electrical Conductivity [MS/m]	Vickers Hardness [HV0,1]	Tensile Strength [MPa]	Elongation (soft annealed) A[%]min.	Manufacturing Process	Form of Supply
Ag/SnO2 98/2 SPW	97 - 99	WO3	10,4	59 ± 2	57 ± 15	215	35	Powder Metallurgy	1
Ag/SnO2 92/8 SPW	91 - 93	WO3	10,1	51 ± 2	62 ± 15	255	25	Powder Metallurgy	1
Ag/SnO2 90/10 SPW	89 - 91	WO3	10	47 ± 5		250	25	Powder Metallurgy	1
Ag/SnO2 88/12 SPW	87 - 89	WO3	9.9	46 ± 5	67 ± 15	270	20	Powder Metallurgy	1
Ag/SnO2 92/8 SPW4	91 - 93	WO3	10,1	51 ± 2	62 ± 15	255	25	Powder Metallurgy	1,2
Ag/SnO2 90/10 SPW4	89 - 91	WO3	10					Powder Metallurgy	1,2
Ag/SnO2 88/12 SPW4	87 - 89	WO3	9,8	46 ± 5	80 ± 10			Powder Metallurgy	1,2
Ag/SnO2 88/12 SPW6	87 - 89	MoO3	9.8	42 ± 5	70 ± 10			Powder Metallurgy	2
Ag/SnO2 97/3 SPW7	96 - 98	Bi2O3 and WO3						Powder Metallurgy	2
Ag/SnO2 90/10 SPW7	89 - 91	Bi2O3 and WO3	9,9					Powder Metallurgy	2
Ag/SnO2 88/12 SPW7	87 - 89	Bi2O3 and WO3	9.8	42 ± 5	70 ± 10			Powder Metallurgy	2
Ag/SnO2 98/2 PMT1	97 - 99	Bi2O3 and CuO	10,4	57 ± 2		215	35	Powder Metallurgy	1,2
Ag/SnO2 96/4 PMT1	95 - 97	Bi2O3 and CuO						Powder Metallurgy	1,2
Ag/SnO2 94/6 PMT1	93 - 95	Bi2O3 and CuO						Powder Metallurgy	1,2
Ag/SnO2 92/8 PMT1	91 - 93	Bi2O3 and CuO	10	50 ± 2	62 ± 15	240	25	Powder Metallurgy	1,2
Ag/SnO2 90/10 PMT1	89 - 91	Bi2O3 and CuO	10	48 ± 2	65 ± 15	240	25	Powder Metallurgy	1,2
Ag/SnO2 88/12 PMT1	87 - 89	Bi2O3 and CuO	9,9	46 ± 5		260	20	Powder Metallurgy	1,2
Ag/SnO2 90/10 PE	89 - 91	Bi2O3 and CuO	9,8	48 ± 2	55 - 100	230 - 330	28	Powder Metallurgy	1
Ag/SnO2 88/12 PE	87 - 89	Bi2O3 and CuO	9,7	46 ± 5	60 - 106	235 - 330	25	Powder Metallurgy	1
Ag/SnO2 88/12 PMT2	87 - 89	CuO	9,9		90 ± 10			Powder Metallurgy	1,2
Ag/SnO2 86/14 PMT3	85 - 87	Bi2O3 and CuO	9,8		95 ± 10			Powder Metallurgy	2
Ag/SnO2 94/6 LC1	93 - 95	Bi2O3 and In2O3	9,8	45 ± 5	55 ± 10			Powder Metallurgy	2
Ag/SnO2 90/10 POX1	89 - 91	In2O3	9,9	50 ± 5	85 ± 15	310	25	Internal Oxidation	1,2
Ag/SnO2 90/10 POX1	87 - 89	In2O3	9,8	48 ± 5	90 ± 15	325	25	Internal Oxidation	1,2
Ag/SnO2 90/10 POX1	85 - 87	In2O3	9,6	45 ± 5	95 ± 15	330	20	Internal Oxidation	1,2

1 = Wires, Rods, Contact rivets, 2 = Strips, Profiles, Contact tips

In the manufacture for the initial powder mixes, different processes are applied which provide specific advantages of the resulting materials in respect to their contact properties . Some of them are described here as follows:

a) Powder blending from single component powders
In this common process all components, including additives that are part of the powder mix, are blended as single powders. The blending is usually performed in the dry stage in blenders of different design.

b) Powder blending on the basis of doped powders
For incorporation of additive oxides in the SnO₂ powder, the reactive spray process has shown advantages. This process starts with a waterbased solution of the tin and other metal compounds. This solution is nebulized under high pressure and temperature in a reactor chamber. Through the rapid evaporation of the water, each small droplet is converted into a salt crystal and from there gets transformed by oxidation into a tin oxide particle in which the additive metals are distributed evenly as oxides. The so created doped AgSnO₂ powder is then mechanically mixed with silver powder.

c) Powder blending based on coated oxide powders
In this process, tin oxide powder is blended with lower melting additive oxides such as for example Ag₂ MoO₄ and then heat treated. The SnO₂ particles are coated in this step with a thin layer of the additive oxide.

d) Powder blending based on internally oxidized alloy powders
A combination of powder metallurgy and internal oxidation this process starts with atomized Ag alloy powder which is subsequently oxidized in pure oxygen. During this process the Sn and other metal components are transformed to metal oxide and precipitated inside the silver matrix of each powder particle.

e) Powder blending based on chemically precipitated compound powders
A silver salt solution is added to a suspension of for example SnO₂ together with a precipitation agent. In a chemical reaction, silver and silver oxide respectively are precipitated around the additive metal oxide particles, who act as crystallization sites. Further chemical treatment then reduces the silver oxide with the resulting precipitated powder, being a mix of Ag and SnO₂.

Further processing of these differently produced powders follows the conventional processes of pressing, sintering and hot extrusion to wires and strips. From these contact parts, contact rivets and tips are manufactured. To obtain a brazable backing, the same processes as used for Ag/CdO are applied. As for Ag/CdO, larger contact tips can also be manufactured using the press-sinter-repress (PSR) process (Table 16).

Figure 40: Strain hardening of Ag/SnO₂ 92/8 PE by cold working

Figure 41: Softening of Ag/SnO₂ 92/8 PE after annealing for 1 hr after 40% cold working

Figure 42: Strain hardening of Ag/SnO₂ 88/12 PE by cold working

Figure 43: Softening of Ag/SnO₂ 88/12 PE after annealing for 1 hr after 40% cold working

Figure 44: Strain hardening of oxidized Ag/SnO₂ 88/12 PW4 by cold working

Figure 45: Softening of Ag/SnO₂ 88/12 PW4 after annealing for 1 hr after 30% cold working

Figure 46: Strain hardening of internally oxidized Ag/SnO₂ 88/12 TOS F by cold working

Figure 47: Softening of Ag/SnO₂ 88/12 TOS F after annealing for 1 hr after 30% cold working

Figure 48: Strain hardening of internally oxidized Ag/SnO₂ 88/12P by cold working

Figure 49: Softening of Ag/SnO₂ 88/12 SP after annealing for 1 hr after 40% cold working

Figure 50: Strain hardening of Ag/SnO₂ 88/12 WPD by cold working

Figure 51: Softening of Ag/SnO₂ 88/12 WPD after annealing for 1 hr after different degrees of cold working

Figure 52: Micro structure of Ag/SnO₂ 92/8 PE: a) perpendicular to extrusion direction b) parallel to extrusion direction

Figure 53: Micro structure of Ag/SnO₂ 88/12 PE: a) perpendicular to extrusion direction b) parallel to extrusion direction

Figure 54: Micro structure of Ag/SnO₂ 88/12 SPW: a) perpendicular to extrusion direction b) parallel to extrusion direction

Figure 55: Micro structure of Ag/SnO₂ 88/12 TOS F: a) perpendicular to extrusion direction b) parallel to extrusion direction

Figure 56: Micro structure of Ag/SnO₂ 92/8 WTOS F: a) perpendicular to extrusion direction b) parallel to extrusion direction,1) AgSnO2 contact layer, 2) Ag backing layer

Figure 57: Micro structure of Ag/SnO₂ 88/12 WPD: parallel to extrusion direction 1) AgSnO2 contact layer, 2) Ag backing layer

Table 16: Physical Properties of Powder Metallurgical Silver-Metal Oxide Materials with Fine Silver Backing Produced by the Press-Sinter-Repress Process

Material	Additives	Density [ g/cm3]	Electrical Resistivity [µS ·cm]	Electrical Conductivity		Vickers Hardness HV 10.
				[%IACS]	[MS/m]
AgCdO 90/10		10.1	2.08	83	48	60
AgCdO 85/15		9.9	2.27	76	44	65
AgSnO2 90/10	CuO and Bi2 O3	9.8	2.22	78	45	55
AgSnO2 88/12	CuO and Bi2 O3	9.6	2.63	66	38	60

Form of Support: formed parts, stamped parts, contact tips

• Silver–zinc oxide materials

Silver zinc oxide contact materials with mostly 6 - 10 wt% oxide content, including other small metal oxides, are produced exclusively by powder metallurgy (Figs. 58 – 63). Adding WO₃ or Ag₂WO₄ in the process - as described in the preceding chapter on Ag/SnO₂ - has proven most effective for applications in AC relays, wiring devices, and appliance controls. Just like with the other Ag metal oxide materials, semi-finished materials in strip and wire form are used to manufacture contact tips and rivets. Because of their high resistance against welding and arc erosion Ag/ZnO materials present an economic alternative to Cd free Ag-tin oxide contact materials (Table 19 and Table 20).

Table 17: Physical and Mechanical Properties as well as Manufacturing Processes and Forms of Supply of Extruded Silver-Zinc Oxide Contact

Material	Silver Content [wt%]	Additives	Density [g/cm3]	Electrical Resistivity [μΩ·cm]	Electrical Conductivity [% IACS] [MS/m]		Vickers Hardness Hv1	Tensile Strength [MPa]	Elongation (soft annealed) A[%]min.	Manufacturing Process	Form of Supply
Ag/ZnO 92/8P	91 - 93		9.8	2.22	78	45	60 - 95	220 - 350	25	Powder Metallurgy a) indiv. powders	1
Ag/ZnO 92/8PW25	91 - 93	Ag2WO4	9.6	2.08	83	48	65 - 105	230 - 340	25	Powder Metallurgy c) coated	1
Ag/ZnO 90/10PW25	89 - 91	Ag2WO4	9.6	2.17	79	46	65 - 100	230 - 350	20	Powder Metallurgy c) coated	1
Ag/ZnO 92/8WP	91 - 93		9.8	2.0	86	50	60 - 95			Powder Metallurgy with Ag backing a) individ.	2
Ag/ZnO 92/8WPW25	91 - 93	Ag2WO4	9.6	2.08	83	48	65 - 105			Powder Metallurgy c) coated	2
Ag/ZnO 90/10WPW25	89 - 91	Ag2WO4	9.6	2.7	79	46	65 - 110			Powder Metallurgy c) coated	2

1 = Wires, Rods, Contact rivets, 2 = Strips, Profiles, Contact tips

Figure 58: Strain hardening of Ag/ZnO 92/8 PW25 by cold working

Figure 59: Softening of Ag/ZnO 92/8 PW25 after annealing for 1 hr after 30% cold working

Figure 60: Strain hardening of Ag/ZnO 92/8 WPW25 by cold working

Figure 61: Softening of Ag/ZnO 92/8 WPW25 after annealing for 1hr after different degrees of cold working

Figure 62: Micro structure of Ag/ZnO 92/8 PW25: a) perpendicular to extrusion direction b) parallel to extrusion direction

Figure 63: Micro structure of Ag/ZnO 92/8 WPW25:a) perpendicular to extrusion direction b) parallel to extrusion direction, 1) Ag/ZnO contact layer, 2) Ag backing layer

Table 18: Optimizing of Silver–Tin Oxide Materials Regarding their Switching Properties and Forming Behavior

Material/ Material Group	Special Properties
Ag/SnO2 PE	Especially suitable for automotive relays (lamp loads)	Good formability (contact rivets)
Ag/SnO2 TOS F	Especially suited for high inductive DC loads	Very good formability (contact rivets)
Ag/SnO2 WPD	Especially suited for severe loads (AC-4) and high switching currents
Ag/SnO2 W TOS F	Especially suitable for high inductive DC loads

Table 19: Contact and Switching Properties of Silver–Metal Oxide Materials

Material	Properties
Ag/SnO2	Environmentally friendly materials, Very high resistance against welding during current-on-switching, Weld resistance increases with higher oxide contents, Low and stable contact resistance over the life of the device and good temperature rise properties through use of special additives, High arc erosion resistance and contact life, Very low and flat material transfer during DC load switching, Good arc moving and very good arc extinguishing properties
Ag/ZnO	Environmentally friendly materials, High resistance against welding during current-on-switching (capacitor contactors), Low and stable contact resistance through special oxide additives, Very high arc erosion resistance at high switching currents, Less favorable than Ag/SnO2 for electrical life and material transfer, With Ag2WO4 additive especially suitable for AC relays

Table 20: Application Examples of Silver–Metal Oxide Materials

Material	Application Examples
Ag/SnO2	Micro switches, Network relays, Automotive relays, Appliance switches, Main switches, contactors, Fault current protection relays (paired against Ag/C), (Main) Power switches
Ag/ZnO	Wiring devices, AC relays, Appliance switches, Motor-protective circuit breakers (paired with Ag/Ni or Ag/C), Fault current circuit breakers paired againct Ag/C, (Main) Power switches

Silver–Graphite Materials

Ag/C contact materials are usually produced by powder metallurgy with graphite contents of 2 – 6 wt% (Table 21). The earlier typical manufacturing process of single pressed tips by pressing - sintering - repressing (PSR) has been replaced in Europe for quite some time by extrusion. In North America and some other regions however the PSR process is still used to some extend mainly for cost reasons.

The extrusion of sintered billets is now the dominant manufacturing method for semi-finished AgC materials. The hot extrusion process results in a high density material with graphite particles stretched and oriented in the extrusion direction (Figs. 68 – 71). Depending on the extrusion method in either rod or strip form, the graphite particles can be oriented in the finished contact tips perpendicular or parallel to the switching contact surface (Figure 69 and Figure 70).

Since the graphite particles in the Ag matrix of Ag/C materials prevent contact tips from directly being welded or brazed, a graphite free bottom layer is required. This is achieved by burning out (de-graphitizing) the graphite selectively on one side of the tips.

Ag/C contact materials exhibit on the one hand an extremely high resistance to contact welding but on the other have a low arc erosion resistance. This is caused by the reaction of graphite with the oxygen in the surrounding atmosphere at the high temperatures created by the arcing. The weld resistance is especially high for materials with the graphite particle orientation parallel to the arcing contact surface. Since the contact surface after arcing consists of pure silver, the contact resistance stays consistantly low during the electrical life of the contact parts.

A disadvantage of the Ag/C materials is their rather high erosion rate. In materials with parallel graphite orientation this can be improved, if a part of the graphite is incorporated into the material (Ag/C DF) in the form of fibers (Figure 71). The weld resistance is determined by the total content of graphite particles.

Ag/C tips with vertical graphite particle orientation are produced in a specific sequence: Extrusion to rods, cutting of double thickness tips, burning out of graphite to a controlled layer thickness, and a second cutting to single tips. Such contact tips are especially well suited for applications which require both, a high weld resistance and a sufficiently high arc erosion resistance (Table 22). For attachment of Ag/C tips welding and brazing techniques are applied.

Welding the actual process depends on the material's graphite orientation. For Ag/C tips with vertical graphite orientation the contacts are assembled with single tips. For parallel orientation a more economical attachment starting with contact material in strip or profile tape form is used in integrated stamping and welding operations with the tape fed into the weld station, cut off to tip form and then welded to the carrier material before forming the final contact assembly part. For special low energy welding, the Ag/C profile tapes can be pre-coated with a thin layer of high temperature brazing alloys such as CuAgP.

In a rather limited way, Ag/C with 2 – 3 wt% graphite can be produced in wire form and headed into contact rivet shape with low head deformation ratios.

The main applications for Ag/C materials are protective switching devices such as miniature molded case circuit breakers, motor-protective circuit breakers, and fault current circuit breakers, where during short circuit failures, highest resistance against welding is required (Table 23). For higher currents the low arc erosion resistance of Ag/C is compensated by asymmetrical pairing with more erosion resistant materials such as Ag/Ni, Ag/W and Ag/WC.

Figure 64: Strain hardening of Ag/C 96/4 by cold working

Figure 65: Softening of Ag/C 96/4 after annealing

Figure 66: Strain hardening of Ag/C DF by cold working

Figure 67: Softening of Ag/C DF after annealing

Figure 68: Micro structure of Ag/C 97/3: a) perpendicular to extrusion direction b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer

Figure 69: Micro structure of Ag/C 95/5: a) perpendicular to extrusion direction b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer

Figure 70: Micro structure of Ag/C 96/4: a) perpendicular to extrusion direction b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer

Figure 71: Micro structure of Ag/C DF: a) perpendicular to extrusion direction b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag/Ni 90/10 backing layer

Table 21: Physical Properties of Silver–Graphite Contact Materials

Material	Silver Content [wt%]	Density [g/cm3]	Melting Point [°C]	Electrical Resistivity [μΩ·cm]	Electrical Conductivity [% IACS] [MS/m]		Vickers-Hardnes HV10 42 - 45
Ag/C 98/2	97.5 - 98.5	9.5	960	1.85 - 1.92	90 - 93	48 - 50	42 - 44
Ag/C 97/3	96.5 - 97.5	9.1	960	1.92 - 2.0	86 - 90	45 - 48	41 - 43
Ag/C 96/4	95.5 - 96.5	8.7	960	2.04 - 2.13	81 - 84	42 - 46	40 - 42
Ag/C 95/5	94.5 - 95.5	8.5	960	2.12 - 2.22	78 - 81	40 - 44	40 - 60
AgC DF GRAPHOR DF1	95.7 - 96.7	8.7 - 8.9	960	2.27 - 2.50	69 - 76	40 - 44

₁ Graphite content 3.8 wt%, Graphite particles and fibers parallel to switching surface

Table 22: Contact and Switching properties of Silver–Graphite Contact Materials

Material

Properties

Ag/C

Highest resistance against welding during make operations at high currents, High resistance against welding of closed contacts during short circuit, Increase of weld resistance with higher graphite contents, Low contact resistance, Low arc erosion resistance, especially during break operations, Higher arc erosion with increasing graphite contents, at the same time carbon build-up on switching chamber walls increases, silver-graphite with vertical orientation has better arc erosion resistance, parallel orientation has better weld resistance, Limited arc moving properties, therefore paired with other materials, Limited formability, Can be welded and brazed with decarbonized backing, GRAPHOR DF is optimized for arc erosion resistance and weld resistance

Table 23: Application Examples and Forms of Supply of Silver– Graphite Contact Materials

Material	Application Examples	Form of Supply
Ag/C 98/2	Motor circuit breakers, paired with Ag/Ni	Contact tips, brazed and welded contact parts, some contact rivets Contact profiles (weld tapes), Contact tips, brazed and welded contact parts
Ag/C 97/3 Ag/C 96/4 Ag/C 95/5 Ag/C DF	Circuit breakers, paired with Cu, Motor-protective circuit breakers, paired with Ag/Ni, Fault current circuit breakers, paired with Ag/Ni, Ag/W, Ag/WC, Ag/SnO2, Ag/ZnO, (Main) Power switches, paired with Ag/Ni, Ag/W	Contact tips, brazed and welded contact parts, some contact rivets with Ag/C97/3

References

References