Gold Based Materials

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 losses under electrical arcing loads Table 3 (Tab. 2.4). This limits its use in form of thin electroplated or vacuum deposited layers.

For most electrical contact applications gold alloys are used. Depending on the alloying metal the melting is performed either under in a reducing atmosphere or in a vacuum. The choice of alloying metals depends on the intended use of the resulting contact material. The binary Au alloys with typically <10 wt% of other precious metals such as Pt, Pd, or Ag or non-precious metals like Ni, Co, and Cu are the more commonly used ones Table 1 (Tab. 2.2). On one hand these alloy additions improve the mechanical strength and electrical switching properties but on the other hand reduce the electrical conductivity and chemical corrosion resistance Figure 1 (Fig. 2.2) to varying degrees.

Under the aspect of reducing the gold content ternary alloys with a gold content of approximately 70 wt% and additions of Ag and Cu or Ag and Ni resp., for example AuAg25Cu5 or AuAg20Cu10 are used which exhibit for many applications good mechanical stability while at the same time have sufficient resistance against the formation of corrosion layers Table 2(Table 2.3).

Table 2.1: Commonly Used Grades of Gold

Designation	Composition Au (min. content)	Impurites ppm	Remarks on forms and application
Electronic Glod Gold	99.999	Cu < 3 Ag < 3 Ca < 1 Mg <1 Fe < 1	Wires, strips, alloying metal for semiconductors, electronic components
Pure Gold	99.995	Cu < 10 Ag < 15 Ca < 20 Mg < 10 Fe < 3 Si < 10 Pb < 20	Granulate for high purity alloys, strips, tubing, profiles
Ingot Grade-Gold	99.95	Cu < 100 Ag < 150 Ca < 50 Mg < 50 Fe < 30 Si < 10	Alloys, commonly used grade

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Table 1: Tab 2.2 Physical Properties of Gold and Gold-Alloys

Figure 1: Fig 2.2 Influence of 1-10 atomic% of different alloying metals on the electrical resistivity of gold (according to J. O. Linde)

Tab.2.3: Mechanical Properties of Gold and Gold-Alloys

Material	Hardness Condition	Tensile Strength Rm [MPa] min.	Elongation A10 [%] min.	Vickers Hardness HV
Au	R 140 R 170 R 200 R 240	140 170 200 240	30 3 2 1	20 50 60 70
AuAg20	R 190 R 250 R 320 R 390	190 250 320 390	25 2 1 1	38 70 95 115
AuAg30	R 220 R 260 R 320 R 380	220 260 320 380	25 3 1 1	45 75 95 110
AuAg25Cu5	R 400 R 470 R 570 R 700	400 470 570 700	25 4 2 2	90 120 160 185
AuAg20Cu10	R 480 R 560 R 720 R 820	480 560 720 820	20 3 1 1	125 145 190 230
AuAg26Ni3	R 350 R 420 R 500 R 570	350 420 500 570	20 2 1 1	85 110 135 155
AuAg25Pt6	R 280 R 330 R 410 R 480	280 330 410 480	18 2 1 1	60 90 105 125
AuCo5	R 340 R 390 R 450 R 530	340 390 450 530	10 2 1 1	95 105 120 150
AuCo5 prec.hardened	heterogeneous	360	3	110-130
AuNi5	R 380 R 450 R 560 R 640	380 450 560 640	25 3 2 1	115 135 160 190
AuPt10	R 260 R 310 R 370 R 410	260 310 370 410	20 2 1 1	80 90 100 105
AuCu14Pt9Ag4	R 620 R 700 R 850 R 950 prec.hardened	620 700 850 950 900	20 3 2 1 3	190 225 260 270 280

Other ternary alloys based on the AuAg system are AuAg26Ni3 and AuAg25Pt6. These alloys are mechanically similar to the AuAgCu alloys but have significantly higher oxidation resistance at elevated temperatures Table 3(Table 2.4).

Table 2.4: Contact and Switching Properties of Gold and Gold Alloys

Material	Properties
Au	Highest corrosion resistance, low hardness	High electr. conductivity, strong tendency to cold welding
AuAg8	High corrosion resistance, low thermo e.m.f.	Low contact resistance
AuPt10 AuPd5	Very high corrosion resistance	High hardness
AuAg10 - 30	Mostly corrosion resistant	Higher hardness
AuNi5 AuCo5	High corrosion resistance, low tendency to material transfer	High hardness
AuAg25Pt6	High corrosion resistance, low contact resistance	High hardness
AuAg26Ni3 AuAg25Cu5 AuAg20Cu10	Limited corrosion resistance	High hardness
AuPd40 AuPd35Ag10 AuCu14Pt9Ag4	High corrosion resistance	High hardness and mechanical wear resistance

Caused by higher gold prices over the past years the development of alloys with further reduced gold content had a high priority. The starting point has been the AuPd system which has continuous solubility of the two components. Besides the binary alloy of AuPd40 and the ternary one AuPd35Ag9 other multiple component alloys were developed. These alloys typically have < 50 wt% Au and often can be solution hardened in order to obtain even higher hardness and tensile strength. They are mostly used in sliding contact applications.

Gold alloys are used in the form of welded wire or profile (also called weldtapes), segments, contact rivets, and stampings produced from clad strip materials. The selection of the bonding process is based on the cost for the joining process, and most importantly on the economical aspect of using the least possible amount of the expensive precious metal component.

Besides being used as switching contacts in relays and pushbuttons, gold alloys are also applied in the design of connectors as well as sliding contacts for potentiometers, sensors, slip rings, and brushes in miniature DC motors Table 4(Table 2.5).

Table 2.5: Application Examples and Forms of Gold and Gold Alloys

Material	Application Examples	Form of Application
Pure Gold (electroplated)	Corrosion protection layer for contact parts, stationary contacts, bonding surfaces	Electroplated coatings, bond surface layers
Hard Gold (sputtered)	Contact parts for connectors and switches, sliding contact tracks, bonding surfaces	Electroplated coatings on contact rivets and stamped parts
Hard Gold (sputtered)	Contacts in switches and relays for low loads, electronic signal relays	Contact surface layer on miniature profiles (weld tapes)
AuAg8	Dry circuit switching contacts, electronic signal relays	Contact rivets, welded contact parts
AuAg20	Switching contacts for low loads, electronic signal relays	Contact rivets, welded contact parts
AuAg25Cu5 AuAg25Cu10 AuAg26Ni3	Contact parts for connectors, switches and relays	Claddings on Cu alloys, contact rivets, contact layer on micro profiles (weld tapes)
AuNi5 AuCo5 (heterogen)	Contacts in switches and relays for low and medium loads, material transfer resistant contacts	Contact rivets, welded contact parts, contact layer on miniature profiles (weld tapes)
AuPt10 AuAg25Pt6	Contacts for highest chemical corrosion resistance in switches and relays	Contact rivets, contact layer on micro profiles (weld tapes)
AuCu14Pt9Ag4	Sliding contacts for measurement data transfer	Wire-formed parts

Picture

Figure 2 Fig. 2.3: Phase diagram of goldplatinum

Figure 3 Fig. 2.4: Phase diagram of gold-silver

Figure 4 Fig. 2.5: Phase diagram of gold-copper

Figure 5 Fig. 2.6: Phase diagram of gold-nickel

Figure 6 Fig. 2.7: Phase diagram of gold-cobalt

Figure 7 Fig. 2.8: Strain hardening of Au by cold working

Figure 8 Fig. 2.9: Softening of Au after annealing for 0.5 hrs after 80% cold working

Figure 9 Fig. 2.10: Strain hardening of AuPt10 by cold working

Figure 10 Fig. 2.11: Strain hardening of AuAg20 by cold working

Figure 11 Fig. 2.12: Strain hardening of AuAg30 by cold working

Figure 12 Fig. 2.13: Strain hardening of AuNi5 by cold working

Figure 13 Fig. 2.14: Softening of AuNi5 after annealing for 0.5 hrs after 80% cold working

Figure 14 Fig. 2.15: Strain hardening of AuCo5 by cold working

Figure 15 Fig. 2.16: Precipitation hardening of AuCo5 at 400°C hardening temperature

Figure 16 Fig. 2.17: Strain hardening of AuAg25Pt6 by cold working

Figure 17 Fig. 2.18: Strain hardening of AuAg26Ni3 by cold working

Figure 18 Fig. 2.19: Softening of AuAg26Ni3 after annealing for 0.5 hrs after 80% cold working

Figure 19 Fig. 2.20: Strain hardening of AuAg25Cu5 by cold working

Figure 20 Fig. 2.21: Strain hardening of AuAg20Cu10 by cold working

Figure 21 Fig. 2.22: Softening of AuAg20Cu10 after annealing for 0.5 hrs after 80% cold working

Figure 22 Fig. 2.23: Strain hardening of AuCu14Pt9Ag4 by cold working

Figure 23 Fig. 2.24: Precipitation hardening of AuCu14Pt9Ag4 at different hardening temperatures after 50% cold working

Figure 2: Phase diagram of goldplatinum

Figure 3: Phase diagram of gold-silver

Figure 4: Phase diagram of gold-copper

Figure 5: Phase diagram of gold-nickel

Figure 6: Phase diagram of gold-cobalt

Figure 7: Strain hardening of Au by cold working

Figure 8: Softening of Au after annealing for 0.5 hrs after 80% cold working

Figure 9: Strain hardening of AuPt10 by cold working

Figure 10: Strain hardening of AuAg20 by cold working

Figure 11: Strain hardening of AuAg30 by cold working

Figure 12: Strain hardening of AuNi5 by cold working

Figure 13: Softening of AuNi5 after annealing for 0.5 hrs after 80% cold working

Figure 14: Strain hardening of AuCo5 by cold working

Figure 15: Precipitation hardening of AuCo5 at 400°C hardening temperature

Figure 16: Strain hardening of AuAg25Pt6 by cold working

Figure 17: Strain hardening of AuAg26Ni3 by cold working

Figure 18: Softening of AuAg26Ni3 after annealing for 0.5 hrs after 80% cold working

Figure 19: Strain hardening of AuAg25Cu5 by cold working

Figure 20: Strain hardening of AuAg20Cu10 by cold working

Figure 21: Softening of AuAg20Cu10 after annealing for 0.5 hrs after 80% cold working

Figure 22: Strain hardening of AuCu14Pt9Ag4 by cold working

Figure 23: Precipitation hardening of AuCu14Pt9Ag4 at different hardening temperatures after 50% cold working

References

References