Gold Based Materials

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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 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 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 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 3.

Table 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

Table 2: 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)


Table 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 4.

Table 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 5.

Table 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


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