Difference between revisions of "Contact Materials for Electrical Engineering"

From Electrical Contacts
Jump to: navigation, search
 
(51 intermediate revisions by 5 users not shown)
Line 1: Line 1:
−
The contact parts are important components in switching devices. They have to
+
The contact parts are important components in switching devices. They have to maintain their function from the new state until the end of the functional life of the devices.
−
maintain their function from the new state until the end of the functional life of the
 
−
devices.
 
  
−
The requirements on contacts are rather broad. Besides typical contact properties
+
The requirements on contacts are rather broad. Besides typical contact properties such as
−
such as
 
  
 
*High arc erosion resistance
 
*High arc erosion resistance
Line 12: Line 9:
 
*Good arc extinguishing capability
 
*Good arc extinguishing capability
  
−
they have to exhibit physical, mechanical, and chemical properties like high electrical
+
They have to exhibit physical, mechanical and chemical properties like high electrical and thermal conductivity, high hardness, high corrosion resistance etc. and besides this, should have good mechanical workability and also be suitable for good weld and brazing attachment to contact carriers. In addition they must be made from environmentally friendly materials.
−
and thermal conductivity, high hardness, high corrosion resistance, etc and besides
 
−
this should have good mechanical workability, and also be suitable for good weld and
 
−
brazing attachment to contact carriers. In addition they must be made from
 
−
environmentally friendly materials.
 
  
−
Materials suited for use as electrical contacts can be divided into the following groups
+
Materials suited for use as electrical contacts can be divided into the following groups based on their composition and metallurgical structure:
−
based on their composition and metallurgical structure:
 
  
 
*Pure metals
 
*Pure metals
 
*Alloys
 
*Alloys
 
*Composite materials
 
*Composite materials
−
*Pure metals
 
  
−
From this group silver has the greatest importance for switching devices in the higher
 
−
energy technology. Other precious metals such as gold and platinum are only used in
 
−
applications for the information technology in the form of thin surface layers. As a nonprecious
 
−
metal tungsten is used for some special applications such as for example as
 
−
automotive horn contacts. In some rarer cases pure copper is used but mainly paired
 
−
to a silver-based contact material.
 
  
−
*Alloys
+
'''Pure metals'''
 +
 
 +
Within this group, silver has the greatest importance for switching devices in the higher energy technology. Other precious metals such as gold and platinum are only used in applications for the information technology in the form of thin surface layers. As a nonprecious metal, tungsten is used for some special applications such as, for example, automotive horn contacts. In some rarer cases, pure copper is used, but mainly paired to a silver-based contact material.
 +
 
 +
'''Alloys'''
 +
 
 +
Besides these few pure metals, a larger number of alloy materials made by melt technology are available for the use as contacts. An alloy is characterized by the fact, that its components are completely or partially soluble in each other in the solid state. Phase diagrams for multiple metal compositions show the number and type of the crystal structure as a function of the temperature and composition of the alloying components.
  
−
Besides these few pure metals a larger number of alloy materials made by melt
+
They indicate the boundaries of liquid and solid phases and define the parameters of solidification.
−
technology are available for the use as contacts. An alloy is characterized by the fact
+
Alloying allows to improve the properties of one material at the cost of changing them for the second material. As an example, the hardness of a base metal may be increased while at the same time the electrical conductivity decreases with even small additions of the second alloying component.
−
that its components are completely or partially soluble in each other in the solid state.
 
−
Phase diagrams for multiple metal compositions show the number and type of the
 
−
crystal structure as a function of the temperature and composition of the alloying components.
 
  
−
They indicate the boundaries of liquid and solid phases and define the
+
'''Composite Materials'''
−
parameters of solidification.
 
−
Alloying allows to improve the properties of one material at the cost of changing
 
−
them for the second material. As an example, the hardness of a base metal may
 
−
be increased while at the same time the electrical conductivity decreases with
 
−
even small additions of the second alloying component.
 
  
−
*Composite Materials
+
Composite materials are a material group whose properties are of great importance for electrical contacts that are used in switching devices for higher
 +
electrical currents.
  
−
Composite materials are a material group whose properties are of great
+
Those used in electrical contacts are heterogeneous materials, composed of two or more uniformly dispersed components, in which the largest volume portion consists of a metal.
−
importance for electrical contacts that are used in switching devices for higher
 
−
electrical currents.
 
−
Those used in electrical contacts are heterogeneous materials composed of two
 
−
or more uniformly dispersed components in which the largest volume portion
 
−
consists of a metal.
 
−
The properties of composite materials are determined mainly independent from
 
−
each other by the properties of their individual components. Therefore it is for
 
−
example possible to combine the high melting point and arc erosion resistance
 
−
of tungsten with the low melting and good electrical conductivity of copper, or
 
−
the high conductivity of silver with the weld resistant metalloid graphite.
 
  
−
Figure 2.1 shows the schematic manufacturing processes from powder
+
The properties of composite materials are determined mainly independent from each other by the properties of their individual components. Therefore it is, for example, possible to combine the high melting point and arc erosion resistance of tungsten with the low melting and good electrical conductivity of copper or the high conductivity of silver with the weld resistant metalloid graphite. <xr id="fig:Powder metallurgical manufacturing of composite materials (schematic)"/> shows the schematic manufacturing processes from powder blending to contact material. Three basic process variations are typically applied:
−
blending to contact material. Three basic process variations are typically
 
−
applied:
 
  
 
*Sintering without liquid phase (Press-Sinter-Repress, PSR)
 
*Sintering without liquid phase (Press-Sinter-Repress, PSR)
Line 70: Line 42:
 
*Infiltration (Press-Sinter-Infiltrate, PSI)
 
*Infiltration (Press-Sinter-Infiltrate, PSI)
  
−
During ''sintering without a liquid phase'' (left side of schematic) the powder mix is
+
<figure id="fig:Powder metallurgical manufacturing of composite materials (schematic)">
−
first densified by pressing, then undergoes a heat treatment (sintering), and
+
[[File:Powder metallurgical manufacturing of composite materials (schematic).jpg|thumb|<caption>Powder-metallurgical manufacturing of composite materials (schematic) T<sub>s</sub> = Melting point of the lower melting component)</caption>]]
−
eventually is re-pressed again to further increase the density. The sintering
+
</figure>
−
atmosphere depends on the material components and later application; a
 
−
vacuum is used for example for the low gas content material Cu/Cr. This
 
−
process is used for individual contact parts and also termed press-sinterrepress
 
−
(PSR). For materials with high silver content the starting point at
 
−
pressing is most a larger block (or billet) which is then after sintering hot
 
−
extruded into wire, rod, or strip form. The extrusion further increases the density
 
−
of these composite materials and contributes to higher arc erosion resistance.
 
−
Materials such as Ag/Ni, Ag/MeO, and Ag/C are typically produced by this
 
−
process.
 
−
 
 
−
''Sintering with liquid phase'' has the advantage of shorter process times due to
 
−
the accelerated diffusion and also results in near-theoretical densities of the
 
  
−
Fig. 2.1: Powder-metallurgical manufacturing of composite materials (schematic)
+
During ''sintering without a liquid phase'' (left side of schematic), the powder mix is first densified by pressing, then undergoes a heat treatment (sintering) and eventually is re-pressed again to further increase the density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the low gas content material Cu/Cr. This process is used for individual contact parts and also termed press-sinter-repress (PSR). For materials with high silver content, the starting point before pressing is mostly a large block (or billet) which is then, after sintering, hot extruded into wire, rod or strip form. The extrusion further increases the density of these composite materials and contributes to higher arc erosion resistance. Materials such as Ag/Ni, Ag/MeO and Ag/C are typically produced by this process.
−
T = Melting point of the lower melting component
 
  
−
composite material. To ensure the shape stability during the sintering process it
+
''Sintering with liquid phase'' has the advantage of shorter process times due to the accelerated diffusion and also results in near-theoretical densities of the composite material. To ensure the shape stability during the sintering process, it
 
is however necessary to limit the volume content of the liquid phase material.
 
is however necessary to limit the volume content of the liquid phase material.
  
−
As opposed to the liquid phase sintering which has limited use for electrical
+
As opposed to the liquid phase sintering, which has limited use for electrical contact manufacturing, the ''Infiltration process'' as shown on the right side of the schematic, has a broad practical range of applications. In this process the powder of the higher melting component, sometimes also as a powder mix with a small amount of the second material, is pressed into parts. Then, right after sintering, the porous skeleton is infiltrated with liquid metal of the second material. The fill-up process of the pores happens through capillary forces. This process reaches, after the infiltration, near-theoretical density without subsequent pressing and is widely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts, controlling the amount or excess on the bottom side of the contact of the infiltration metal Ag, results in contact tips that can be easily attached to their carriers by resistance welding. For larger Cu/W contacts, additional machining is often used to obtain the final shape of the contact component.
−
contact manufacturing, the ''Infiltration process'' as shown on the right side of the
 
−
schematic has a broad practical range of applications. In this process the
 
−
powder of the higher melting component sometimes also as a powder mix with
 
−
a small amount of the second material is pressed into parts and after sintering
 
−
the porous skeleton is infiltrated with liquid metal of the second material. The
 
−
filling up of the pores happens through capillary forces. This process reaches
 
−
after the infiltration near-theoretical density without subsequent pressing and is
 
−
widely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts,
 
−
controlling the amount or excess on the bottom side of the contact of the
 
−
infiltration metal Ag results in contact tips that can be easily attached to their
 
−
carriers by resistance welding. For larger Cu/W contacts additional machining is
 
−
often used to obtain the final shape of the contact component.
 
  
 
==Gold Based Materials==
 
==Gold Based Materials==
  
−
Pure Gold is besides Platinum the chemically most stable of all precious metals.
+
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 material losses under electrical arcing loads. This limits its use in form of thin electroplated or vacuum deposited layers.
−
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. This limits its use in form of thin electroplated or vacuum deposited layers.
 
  
−
Main Articel: [[Gold Based Materials| Gold Based Materials]]
+
Main Article: [[Gold Based Materials| Gold Based Materials]]
  
 
==Platinum Metal Based Materials==
 
==Platinum Metal Based Materials==
  
−
The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir, and Os ''(Table
+
The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir and Os ([[Platinum_Metal_Based_Materials|Table 1]]<!--(Table 2.6)-->). For electrical contacts, platinum and palladium have practical significance as base alloy materials and ruthenium and iridium are used as alloying components. Pt and Pd have similar corrosion resistance as gold but due to their catalytical properties, they tend to polymerize adsorbed organic vapors on contact surfaces. During frictional movement between contact surfaces, the polymerized compounds known as “brown powder” are formed, which can lead to a significant increase in contact resistance. Therefore Pt and Pd are typically used as alloys and are rather not used in their pure form for electrical contact applications.
−
2.6)''. For electrical contacts platinum and palladium have practical significance
 
−
as base alloy materials and ruthenium and iridium are used as alloying components.
 
−
Pt and Pd have similar corrosion resistance as gold but because of their
 
−
catalytical properties they tend to polymerize adsorbed organic vapors on contact
 
−
surfaces. During frictional movement between contact surfaces the polymerized
 
−
compounds known as “brown powder” are formed which can lead to significantly
 
−
increase in contact resistance. Therefore Pt and Pd are typically used as alloys and
 
−
not in their pure form for electrical contact applications.
 
  
−
Main Articel: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]
+
Main Article: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]
  
 
==Silver Based Materials==
 
==Silver Based Materials==
−
Pure Silver, Silver Alloys, Silver Composite Materials
 
−
 
−
Main Articel: [[Silver Based Materials| Silver Based Materials]]
 
  
 +
Main Article: [[Silver Based Materials| Silver Based Materials]]
  
 
==Tungsten and Molybdenum Based Materials==
 
==Tungsten and Molybdenum Based Materials==
  
−
===Tungsten and Molybdenum (Pure Metals)===
+
Main Article: [[Tungsten and Molybdenum Based Materials| Tungsten and Molybdenum Based Materials]]
−
Tungsten is characterized by its advantageous properties of high melting and
 
−
boiling points, sufficient electrical and thermal conductivity and high hardness
 
−
and density ''(Table 2.35)''. It is mainly used in the form of brazed contact tips for
 
−
switching duties that require a rapid switching sequence such as horn contacts
 
−
for cars and trucks.
 
−
 
 
−
Molybdenum has a much lesser importance as a contact material since it is less
 
−
resistant against oxidation than tungsten.
 
−
Both metals are however used in large amounts as components in composite
 
−
materials with silver and copper.
 
−
 
 
−
Table 2.35: Mechanical Properties of Tungsten and Molybdenum
 
−
 
 
−
=== Silver–Tungsten (SIWODUR) Materials===
 
−
Ag/W (SIWODUR) contact materials combine the high electrical and thermal
 
−
conductivity of silver with the high arc erosion resistance of the high melting
 
−
tungsten metal ''(Table 2.36)''. The manufacturing of materials with typically
 
−
50-80 wt% tungsten is performed by the powder metallurgical processes of
 
−
liquid phase sintering or by infiltration. Particle size and shape of the starting
 
−
powders are determining the micro structure and the contact specific properties
 
−
of this material group ''(Figs. 2.134 and 2.135) (Table 2.37)''.
 
−
 
 
−
During repeated switching under arcing loads tungsten oxides and mixed
 
−
oxides (silver tungstates – Ag<sub>2</sub> WO<sub>4</sub> ) are formed on the Ag/W surface creating 2 4
 
−
poorly conducting layers which increase the contact resistance and by this the
 
−
temperature rise during current carrying. Because of this fact the Ag/W is paired
 
−
in many applications with Ag/C contact parts.
 
−
 
 
−
Silver–tungsten contact tips are used in a variety of shapes and are produced for
 
−
the ease of attachment with a fine silver backing layer and quite often an
 
−
additional thin layer of a brazing alloy. The attachment to contact carriers is
 
−
usually done by brazing, but also by direct resistance welding for smaller tips.
 
−
 
 
−
Ag/W materials are mostly used as the arcing contacts in disconnect switches
 
−
for higher loads and as the main contacts in small and medium duty power
 
−
switches and industrial circuit breakers ''(Table 2.38)''. In north and south america
 
−
they are also used in large volumes in miniature circuit breakers of small to
 
−
medium current ratings in domestic wiring as well as for commercial power
 
−
distribution.
 
−
 
 
−
=== Silver–Tungsten Carbide (SIWODUR C) Materials===
 
−
This group of contact materials contains the typically 40-65 wt-% of the very
 
−
hard and erosion wear resistant tungsten carbide and the high conductivity silver
 
−
''(Fig. 2.135) (Table 2.36)''. Compared to Ag/W the Ag/WC (SIWODUR C)
 
−
materials exhibit a higher resistance against contact welding ''(Table 2.37)''. The
 
−
rise in contact resistance experienced with Ag/W is less pronounced in Ag/WC
 
−
because during arcing a protective gas layer of CO is formed which limits the
 
−
reaction of oxygen on the contact surface and therefore the formation of metal
 
−
oxides.
 
−
 
 
−
Higher requirements on low temperature rise can be fulfilled by adding a small
 
−
amount of graphite which however increases the arc erosion. Silver–tungsten
 
−
carbide–graphite materials with for example 27 wt% WC and
 
−
3 wt% graphite or 16 wt% WC and 2 wt% graphite are manufactured using the
 
−
single tip press-sinter-repress (PSR) process ''(Fig. 2.136)''.
 
−
 
 
−
The applications of Ag/WC contacts are similar to those for Ag/W ''(Table 2.38)''.
 
−
 
 
−
=== Silver–Molybdenum (SILMODUR) Materials===
 
−
Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced by
 
−
the powder metallurgical infiltration process ''(Fig. 2.137) (Table 2.36)''. Their
 
−
contact properties are similar to those of Ag/W materials ''(Table 2.37)''. Since the
 
−
molybdenum oxide is thermally less stable than tungsten oxide the self-cleaning
 
−
effect of Ag/Mo contact surface during arcing is more pronounced and the
 
−
contact resistance remains lower than that of Ag/W. The arc erosion resistance
 
−
of Ag/Mo however is lower than the one for Ag/W materials. The main
 
−
applications for Ag/Mo contacts are in equipment protecting switching devices
 
−
''(Table 2.38)''.
 
−
 
 
−
Fig. 2.134: Micro structure of Ag/W 25/75
 
−
 
 
−
Fig. 2.135: Micro structure of Ag/WC 50/50
 
−
 
 
−
Fig. 2.136: Micro structure of Ag/WC27/C3
 
−
 
 
−
Fig. 2.137: Micro structure of Ag/Mo 35/65
 
−
 
 
−
Table 2.36: Physical Properties of Contact Materials Based on Silver–Tungsten (SIWODUR),
 
−
Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)
 
  
−
Table 2.37: Contact and Switching Properties of Contact Materials Based on Silver – Tungsten
+
==Contact Materials for Vacuum Switches==
−
(SIWODUR), Silver–Tungsten Carbide (SIWODUR C)
 
−
and Silver Molybdenum (SILMODUR)
 
  
−
Table 2.38: Application Examples and Forms of Supply for Contact Materials Based
+
The low gas content contact materials are developed for the use in vacuum switching devices.  
−
on Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C)
 
−
and Silver Molybdenum (SILMODUR)
 
  
−
==== Copper–Tungsten (CUWODUR) Materials====
+
Main Article: [[Contact Materials for Vacuum Switches| Contact Materials for Vacuum Switches]]
−
Copper–tungsten (CUWODUR) materials with typically 50-85 wt% tungsten are
 
−
produced by the infiltration process with the tungsten particle size selected
 
−
according to the end application ''(Figs. 2.138 – 2.141) (Table 2.39)''. To increase
 
−
the wettability of the tungsten skeleton by copper a small amount of nickel
 
−
< 1 wt% is added to the starting powder mix.
 
−
 
 
−
W/Cu materials exhibit a very high arc erosion resistance ''(Table 2.40)''.
 
−
Compared to silver–tungsten materials they are however less suitable to carry
 
−
permanent current.
 
−
 
 
−
With a solid tungsten skeleton as it is the case for W/C infiltrated materials with
 
−
70-85 wt% tungsten the lower melting component copper melts and vaporizes
 
−
in the intense electrical arc. At the boiling point of copper (2567°C) the still solid
 
−
tungsten is efficiently “cooled” and remains pretty much unchanged.
 
−
 
 
−
During very high thermal stress on the W/Cu contacts, for example during short
 
−
circuit currents > 40 kA the tungsten skeleton requires special high mechanical
 
−
strength. For such applications a high temperature sintering of tungsten from
 
−
selected particle size powder is applied before the usual infiltration with copper
 
−
(example: CUWODUR H).
 
−
 
 
−
For high voltage load switches the most advantageous contact system consists
 
−
of a contact tulip and a contact rod. Both contact assemblies are made usually
 
−
from the mechanically strong and high conductive CuCrZr material and W/Cu as
 
−
the arcing tips. The thermally and mechanically highly stressed attachment
 
−
between the two components is often achieved by utilizing electron beam
 
−
welding or capacitor discharge percussion welding. Other attachment methods
 
−
include brazing and cast-on of copper followed by cold forming steps to
 
−
increase hardness and strength.
 
−
 
 
−
The main application areas for CUWODUR materials are as arcing contacts in
 
−
load and high power switching in medium and high voltage switchgear as well
 
−
as electrodes for spark gaps and over voltage arresters ''(Table 2.41)''.
 
−
 
 
−
Table 2.39: Physical Properties of Copper–Tungsten (CUWODUR) Contact Materials
 
−
 
 
−
Fig. 2.139: Micro structure of W/Cu 70/30 G Fig. 2.140: Micro structure of W/Cu 70/30 H
 
−
 
 
−
Fig. 2.138: Micro structure of W/Cu 70/30 F Fig. 2.141: Micro structure of W/Cu 80/20 H
 
−
 
 
−
Manufacturing of Contact Parts for
 
−
Medium and High Voltage Switchgear
 
−
 
 
−
Table 2.40: Contact and Switching Properties of Copper–Tungsten
 
−
(CUWODUR) Contact Materials
 
−
 
 
−
Table 2.41: Application Examples and Forms of Supply for Tungsten–
 
−
Copper (CUWODUR) Contact Materials
 
−
 
 
−
==Special Contact Materials (VAKURIT) for Vacuum Switches==
 
−
The trade name VAKURIT is assigned to a family of low gas content contact
 
−
materials developed for the use in vacuum switching devices ''(Table 2.42)''.
 
−
 
 
−
===Low Gas Content Materials Based on Refractory Metals===
 
−
Contact materials of W/Cu, W/Ag, WC/Ag, or Mo/Cu can be used in vacuum
 
−
switches if their total gas content does not exceed approximately 150 ppm. In
 
−
the low gas content W/Cu (VAKURIT) material mostly used in vacuum contactors
 
−
the high melting W skeleton is responsible for the high erosion resistance when
 
−
combined with the high conductivity copper component which evaporates
 
−
already in noticeable amounts at temperatures around 2000 °C.
 
−
 
 
−
Since there is almost no solubility of tungsten, tungsten carbide, or molybdenum
 
−
in copper or silver the manufacturing of these material is performed powdermetallurgically.
 
−
The W, WC, or Mo powders are pressed and sintered and then
 
−
infiltrated with low gas content Cu or Ag. The content of the refractory metals is
 
−
typically between 60 and 85 wt% ''(Figs. 2.142 and 2.143)''.
 
−
 
 
−
By adding approximately 1 wt% antimony the chopping current, i.e. the abrupt
 
−
current decline shortly before the natural current-zero, can be improved for
 
−
W/Cu (VAKURIT) materials ''(Table 2.43)''.
 
−
The contact components mostly used in vacuum contactors are usually shaped
 
−
as round discs. These are then attached by brazing in a vacuum environment to
 
−
their contact carriers ''(Table 2.44)''.
 
−
 
 
−
===Low Gas Content Materials Based on Copper-Chromium===
 
−
As contact materials in vacuum interupters in medium voltage devices low gas
 
−
materials based on Cu/Cr have gained broad acceptance. The typical chromium
 
−
contents are between 25 and 55 wt% ''(Figs. 2.144 and 2.145)''. During the
 
−
powder metallurgical manufacturing a mix of chromium and copper powders is
 
−
pressed into discs and subsequently sintering in a reducing atmosphere or
 
−
vacuum below the melting point of copper. This step is followed by cold or hot
 
−
re-pressing. Depending on the composition the Cu/Cr (VAKURIT) materials
 
−
combine a relatively high electrical and thermal conductivity with high dielectric
 
−
stability. They exhibit a low arc erosion rate and good resistance against welding
 
−
as well as favorable values of the chopping current in medium voltage load
 
−
switches, caused by the combined effects of the two components, copper and
 
−
chromium ''(Table 2.43)''.
 
−
 
 
−
The switching properties of Cu/Cr (VAKURIT) materials are dependent on the
 
−
purity of the Cr metal powders and especially the type and quantity of impurities
 
−
contained in the chromium powder used. Besides this the particle size and
 
−
distribution of the Cr powder are of high importance. Because of the getter
 
−
activity of chromium a higher total gas content of up to about 650 ppm
 
−
compared to the limits in refractory based materials can be tolerated in these
 
−
Cu/Cr contact materials. Besides the more economical sinter technology also
 
−
infiltration and vacuum arc melting are used to manufacture these materials.
 
−
Cu/Cr contacts are supplied in the shape of discs or rings which often also
 
−
contain slots especially for vacuum load switches in medium voltage devices
 
−
''(Table 2.44)''. Increased applications of round discs can also be observed for low
 
−
voltage vacuum contactors.
 
−
 
 
−
Table 2.42: Physical Properties of the Low Gas Materials (VAKURIT) for Vacuum Switches
 
−
 
 
−
Fig. 2.142: Micro structure of W/Cu 30Sb1
 
−
– low gas
 
−
 
 
−
Fig. 2.143: Micro structure of WC/Ag 50/50
 
−
– low gas
 
−
 
 
−
Fig. 2.144: Micro structure of Cu/Cr 75/25
 
−
– low gas
 
−
 
 
−
Fig. 2.145: Micro structure of Cu/Cr 50/50
 
−
– low gas
 
−
 
 
−
Table 2.43: Contact and Switching Properties of VAKURIT Materials
 
−
 
 
−
Table 2.44: Application Examples and Form of Supply for VAKURIT Materials
 
  
 
==References==
 
==References==
Line 598: Line 329:
 
Manufacturing Equipment for Semi-Finished Materials
 
Manufacturing Equipment for Semi-Finished Materials
 
(Bild)
 
(Bild)
 +
 +
[[de:Kontaktwerkstoffe_für_die_Elektrotechnik]]

Latest revision as of 11:54, 26 January 2023

The contact parts are important components in switching devices. They have to maintain their function from the new state until the end of the functional life of the devices.

The requirements on contacts are rather broad. Besides typical contact properties such as

  • High arc erosion resistance
  • High resistance against welding
  • Low contact resistance
  • Good arc moving properties
  • Good arc extinguishing capability

They have to exhibit physical, mechanical and chemical properties like high electrical and thermal conductivity, high hardness, high corrosion resistance etc. and besides this, should have good mechanical workability and also be suitable for good weld and brazing attachment to contact carriers. In addition they must be made from environmentally friendly materials.

Materials suited for use as electrical contacts can be divided into the following groups based on their composition and metallurgical structure:

  • Pure metals
  • Alloys
  • Composite materials


Pure metals

Within this group, silver has the greatest importance for switching devices in the higher energy technology. Other precious metals such as gold and platinum are only used in applications for the information technology in the form of thin surface layers. As a nonprecious metal, tungsten is used for some special applications such as, for example, automotive horn contacts. In some rarer cases, pure copper is used, but mainly paired to a silver-based contact material.

Alloys

Besides these few pure metals, a larger number of alloy materials made by melt technology are available for the use as contacts. An alloy is characterized by the fact, that its components are completely or partially soluble in each other in the solid state. Phase diagrams for multiple metal compositions show the number and type of the crystal structure as a function of the temperature and composition of the alloying components.

They indicate the boundaries of liquid and solid phases and define the parameters of solidification. Alloying allows to improve the properties of one material at the cost of changing them for the second material. As an example, the hardness of a base metal may be increased while at the same time the electrical conductivity decreases with even small additions of the second alloying component.

Composite Materials

Composite materials are a material group whose properties are of great importance for electrical contacts that are used in switching devices for higher electrical currents.

Those used in electrical contacts are heterogeneous materials, composed of two or more uniformly dispersed components, in which the largest volume portion consists of a metal.

The properties of composite materials are determined mainly independent from each other by the properties of their individual components. Therefore it is, for example, possible to combine the high melting point and arc erosion resistance of tungsten with the low melting and good electrical conductivity of copper or the high conductivity of silver with the weld resistant metalloid graphite. Figure 1 shows the schematic manufacturing processes from powder blending to contact material. Three basic process variations are typically applied:

  • Sintering without liquid phase (Press-Sinter-Repress, PSR)
  • Sintering with liquid phase
  • Infiltration (Press-Sinter-Infiltrate, PSI)
Figure 1: Powder-metallurgical manufacturing of composite materials (schematic) Ts = Melting point of the lower melting component)

During sintering without a liquid phase (left side of schematic), the powder mix is first densified by pressing, then undergoes a heat treatment (sintering) and eventually is re-pressed again to further increase the density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the low gas content material Cu/Cr. This process is used for individual contact parts and also termed press-sinter-repress (PSR). For materials with high silver content, the starting point before pressing is mostly a large block (or billet) which is then, after sintering, hot extruded into wire, rod or strip form. The extrusion further increases the density of these composite materials and contributes to higher arc erosion resistance. Materials such as Ag/Ni, Ag/MeO and Ag/C are typically produced by this process.

Sintering with liquid phase has the advantage of shorter process times due to the accelerated diffusion and also results in near-theoretical densities of the composite material. To ensure the shape stability during the sintering process, it is however necessary to limit the volume content of the liquid phase material.

As opposed to the liquid phase sintering, which has limited use for electrical contact manufacturing, the Infiltration process as shown on the right side of the schematic, has a broad practical range of applications. In this process the powder of the higher melting component, sometimes also as a powder mix with a small amount of the second material, is pressed into parts. Then, right after sintering, the porous skeleton is infiltrated with liquid metal of the second material. The fill-up process of the pores happens through capillary forces. This process reaches, after the infiltration, near-theoretical density without subsequent pressing and is widely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts, controlling the amount or excess on the bottom side of the contact of the infiltration metal Ag, results in contact tips that can be easily attached to their carriers by resistance welding. For larger Cu/W contacts, additional machining is often used to obtain the final shape of the contact component.

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 material losses under electrical arcing loads. This limits its use in form of thin electroplated or vacuum deposited layers.

Main Article: Gold Based Materials

Platinum Metal Based Materials

The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir and Os (Table 1). For electrical contacts, platinum and palladium have practical significance as base alloy materials and ruthenium and iridium are used as alloying components. Pt and Pd have similar corrosion resistance as gold but due to their catalytical properties, they tend to polymerize adsorbed organic vapors on contact surfaces. During frictional movement between contact surfaces, the polymerized compounds known as “brown powder” are formed, which can lead to a significant increase in contact resistance. Therefore Pt and Pd are typically used as alloys and are rather not used in their pure form for electrical contact applications.

Main Article: Platinum Metal Based Materials

Silver Based Materials

Main Article: Silver Based Materials

Tungsten and Molybdenum Based Materials

Main Article: Tungsten and Molybdenum Based Materials

Contact Materials for Vacuum Switches

The low gas content contact materials are developed for the use in vacuum switching devices.

Main Article: Contact Materials for Vacuum Switches

References

Vinaricky, E.(Hrsg.): Elektrische Kontakte, Werkstoffe und Anwendungen. Springer-Verlag, Berlin, Heidelberg etc. 2002

Lindmayer, M.: Schaltgeräte-Grundlagen, Aufbau, Wirkungsweise. Springer-Verlag, Berlin, Heidelberg, New York, Tokio, 1987

Rau, G.: Metallische Verbundwerkstoffe. Werkstofftechnische Verlagsgesellschaft, Karlsruhe 1977

Schreiner, H.: Pulvermetallurgie elektrischer Kontakte. Springer-Verlag Berlin, Göttingen, Heidelberg, 1964

Hansen. M.; Anderko, K.: Constitution of Binary Alloys. New York: Mc Graw-Hill, 1958

Shunk, F.A.: Constitution of Binary Alloy. 2 Suppl. New York; Mc Graw-Hill, 1969

Edelmetall-Taschenbuch. ( Herausgeber Degussa AG, Frankfurt a. M.), Heidelberg, Hüthig-Verlag, 1995

Rau, G.: Elektrische Kontakte-Werkstoffe und Technologie. Eigenverlag G. Rau GmbH & Co., Pforzheim, 1984

Heraeus, W. C.: Werkstoffdaten. Eigenverlag W.C. Heraeus, Hanau, 1978

Linde, J.O.: Elektrische Widerstandseigenschaften der verdünnten Legierungen des Kupfers, Silbers und Goldes. Lund: Hakan Ohlsson, 1938

Engineers Relay Handbook, RSIA, 2006

Großmann, H. Saeger, K. E.; Vinaricky, E.: Gold and Gold Alloys in Electrical Engineering. in: Gold, Progress in Chemistry, Biochemistry and Technology. John Wiley & Sons, Chichester etc, (1999) 199-236

Gehlert, B.: Edelmetall-Legierungen für elektrische Kontakte. Metall 61 (2007) H. 6, 374-379

Aldinger, F.; Schnabl, R.: Edelmetallarme Kontakte für kleine Ströme. Metall 37 (1983) 23-29

Bischoff, A.; Aldinger, F.: Einfluss geringer Zusätze auf die mechanischen Eigenschaften von Au-Ag-Pd-Legierungen. Metall 36 (1982) 752-765

Wise, E.M.: Palladium, Recovery, Properties and Uses. New York, London: Academic Press 1968

Savitskii, E.M.; Polyakova, V.P.; Tylina, M.A.: Palladium Alloys, Primary Sources. New York: Publishers 1969

Gehlert, B.: Lebensdaueruntersuchungen von Edelmetall Kontaktwerkstoff- Kombinationen für Schleifringübertrager. VDE-Fachbericht 61, (2005) 95-100

Holzapfel,C.: Verschweiß und elektrische Eigenschaften von Schleifringübertragern. VDE-Fachbericht 67 (2011) 111-120

Schnabl, R.; Gehlert, B.: Lebensdauerprüfungen von Edelmetall- Schleifkontaktwerkstoffen für Gleichstrom Kleinmotoren. Feinwerktechnik & Messtechnik (1984) 8, 389-393

Kobayashi, T.; Koibuchi, K.; Sawa, K.; Endo, K.; Hagino, H.: A Study of Lifetime of Au-plated Slip-Ring and AgPd Brush System for Power Supply. th Proc. 24 Int. Conf. on Electr. Contacts, Saint Malo, France 2008, 537-542

Harmsen, U.; Saeger K.E.: Über das Entfestigungsverhalten von Silber verschiedener Reinheiten. Metall 28 (1974) 683-686

Behrens, V.; Michal, R.; Minkenberg, J.N.; Saeger, K.E.: Abbrand und Kontaktwiderstandsverhalten von Kontaktwerkstoffen auf Basis von Silber- Nickel. e.& i. 107. Jg. (1990), 2, 72-77

Behrens, V.: Silber/Nickel und Silber/Grafit- zwei Spezialisten auf dem Gebiet der Kontaktwerkstoffe. Metall 61 (2007) H.6, 380-384

Rieder, W.: Silber / Metalloxyd-Werkstoffe für elektrische Kontakte, VDE - Fachbericht 42 (1991) 65-81

Harmsen,U.: Die innere Oxidation von AgCd-Legierungen unter Sauerstoffdruck. Metall 25 (1991), H.2, 133-137

Muravjeva, E.M.; Povoloskaja, M.D.: Verbundwerkstoffe Silber-Zinkoxid und Silber-Zinnoxid, hergestellt durch Oxidationsglühen. Elektrotechnika 3 (1965) 37-39

Behrens, V.; Honig Th.; Kraus, A.; Michal, R.; Saeger, K.-E.; Schmidberger, R.; Staneff, Th.: Eine neue Generation von AgSnO2 -Kontaktwerkstoffen. VDE-Fachbericht 44, (1993) 99-114

Braumann, P.; Lang, J.: Kontaktverhalten von Ag-Metalloxiden für den Bereich hoher Ströme. VDE-Fachbericht 42, (1991) 89-94

Hauner, F.; Jeannot, D.; Mc Neilly, U.; Pinard, J.: Advanced AgSnO Contact 2 th Materials for High Current Contactors. Proc. 20 Int. Conf. on Electr. Contact Phenom., Stockholm 2000, 193-198

Wintz, J.-L.; Hardy, S.; Bourda, C.: Influence on the Electrical Performances of Assembly Process, Supports Materials and Production Means for AgSnO2 . Proc.24th Int. Conf. on Electr. Contacts, Saint Malo, France 2008, 75-81

Behrens, V.; Honig, Th.; Kraus, A.; Michal, R.: Schalteigenschaften von verschiedenen Silber-Zinnoxidwerkstoffen in Kfz-Relais. VDE-Fachbericht 51 (1997) 51-57

Schöpf, Th.: Silber/Zinnoxid und andere Silber-Metalloxidwerkstoffe in Netzrelais. VDE-Fachbericht 51 (1997) 41-50

Schöpf, Th.; Behrens, V.; Honig, Th.; Kraus, A.: Development of Silver Zinc th Oxide for General-Purpose Relays. Proc. 20 Int. Conf. on Electr. Contacts, Stockholm 2000, 187-192

Braumann, P.; Koffler, A.: Einfluss von Herstellverfahren, Metalloxidgehalt und Wirkzusätzen auf das Schaltverhalten von Ag/SnO in Relais. 2 VDE-Fachbericht 59, (2003) 133-142

Kempf, B.; Braumann, P.; Böhm, C.; Fischer-Bühner, J.: Silber-Zinnoxid- Werkstoffe: Herstellverfahren und Eigenschaften. Metall 61(2007) H. 6, 404-408

Lutz, O.; Behrens, V.; Finkbeiner, M.; Honig, T.; Späth, D.: Ag/CdO-Ersatz in Lichtschaltern. VDE-Fachbericht 61, (2005) 165-173

Lutz, O.; Behrens, V.; Wasserbäch, W.; Franz, S.; Honig, Th.; Späth, D.; Heinrich, J.: Improved Silver/Tin Oxide Contact Materials for Automotive th Applications. Proc.24 Int. Conf. on Electr. Contacts, Saint Malo, France 2008, 88-93

Leung, C.; Behrens, V.: A Review of Ag/SnO Contact Materials and Arc Erosion. 2 th Proc.24 Int. Conf. on Electr. Contacts, Saint Malo, France 2008, 82-87

Chen, Z.K.; Witter, G.J.: Comparison in Performance for Silver–Tin–Indium Oxide Materials Made by Internal Oxidation and Powder Metallurgy. th Proc. 55 IEEE Holm Conf. on Electrical Contacts, Vancouver, BC, Canada, (2009) 167 – 176

Roehberg, J.; Honig, Th.; Witulski, N.; Finkbeiner, M.; Behrens, V.: Performance of Different Silver/Tin Oxide Contact Materials for Applications in Low Voltage th Circuit Breakers. Proc. 55 IEEE Holm Conf. on Electrical Contacts, Vancouver, BC, Canada, (2009) 187 – 194

Muetzel, T.; Braumann, P.; Niederreuther, R.: Temperature Rise Behavior of th Ag/SnO Contact Materials for Contactor Applications. Proc. 55 IEEE Holm 2 Conf. on Electrical Contacts, Vancouver, BC, Canada, (2009) 200 – 205

Lutz, O. et al.: Silber/Zinnoxid – Kontaktwerkstoffe auf Basis der Inneren Oxidation fuer AC – und DC – Anwendungen. VDE Fachbericht 65 (2009) 167 – 176

Harmsen, U.; Meyer, C.L.: Mechanische Eigenschaften stranggepresster Silber- Graphit-Verbundwerkstoffe. Metall 21 (1967), 731-733

Behrens, V.: Mahle, E.; Michal, R.; Saeger, K.E.: An Advanced Silver/Graphite th Contact Material Based on Graphite Fibre. Proc. 16 Int. Conf. on Electr. Contacts, Loghborough 1992, 185-189

Schröder, K.-H.; Schulz, E.-D.: Über den Einfluss des Herstellungsverfahrens th auf das Schaltverhalten von Kontaktwerkstoffen der Energietechnik. Proc. 7 Int. Conf. on Electr. Contacts, Paris 1974, 38-45

Mützel, T.: Niederreuther, R.: Kontaktwerkstoffe für Hochleistungsanwendungen. VDE-Bericht 67 (2011) 103-110

Lambert, C.; Cambon, G.: The Influence of Manufacturing Conditions and Metalurgical Characteristics on the Electrical Behaviour of Silver-Graphite th Contact Materials. Proc. 9 Int. Conf.on Electr. Contacts, Chicago 1978, 401-406

Vinaricky, E.: Grundsätzliche Untersuchungen zum Abbrand- und Schweißverhalten von Ag/C-Kontaktwerkstoffen. VDE-Fachbericht 47 (1995) 159-169

Agte, C.; Vacek, J.: Wolfram und Molybdän. Berlin: Akademie-Verlag 1959

Keil, A.; Meyer, C.-L.: Der Einfluß des Faserverlaufes auf die elektrische Verschleißfestigkeit von Wolfram-Kontakten. ETZ 72, (1951) 343-346

Slade, P. G.: Electric Contacts for Power Interruption. A Review. Proc. 19 Int. Conf. on Electric Contact Phenom. Nuremberg (Germany) 1998, 239-245

Slade, P. G.: Variations in Contact Resistance Resulting from Oxide Formation and Decomposition in AgW and Ag-WC-C Contacts Passing Steady Currents for Long Time Periods. IEEE Trans. Components, Hybrids and Manuf. Technol. CHMT-9,1 (1986) 3-16

Slade, P. G.: Effect of the Electric Arc and the Ambient Air on the Contact Resistance of Silver, Tungsten and Silver-Tungsten Contacts. J.Appl.Phys. 47, 8 (1976) 3438-3443

Lindmayer, M.; Roth, M.: Contact Resistance and Arc-Erosion of W-Ag and WC-Ag. IEEE Trans components, Hybrids and Manuf. Technol. CHMT-2, 1 (1979) 70-75

Leung, C.-H.; Kim, H.J.: A Comparison of Ag/W, Ag/WC and Ag/Mo Electrical Contacts. IEEE Trans. Components, Hybrids, Manuf. Technol., Vol. CHMT-7, 1 (1984) 69-75

Allen, S.E.; Streicher, E.: The Effect of Microstructure on the Electrical th Performance of Ag-WC-C Contact Materials. Proc. 44 IEEE Holm Conf. on Electr. Contacts, Arlington, VA, USA (1998), 276-285

Haufe, W.; Reichel, W.; Schreiner H.: Abbrand verschiedener W/Cu-Sinter- Tränkwerkstoffe an Luft bei hohen Strömen. Z. Metallkd. 63 (1972) 651-654

Althaus, B.; Vinaricky, E.: Das Abbrandverhalten verschieden hergestellter Wolfram-Kupfer-Verbundwerkstoffe im Hochstromlichtbogen. Metall 22 (1968) 697-701

Gessinger, G.H.; Melton, K.N.: Burn-off Behaviour of WCu Contact Materials in an Electric Arc. Powder Metall. Int. 9 (1977) 67-72

Magnusson, M.: Abbrandverhalten und Rißbildung bei WCu-Tränkwerkstoffen unterschiedlicher Wolframteilchengröße. ETZ-A 98 (1977) 681-683

Heitzinger, F.; Kippenberg, H.; Saeger, K.E.; Schröder, K.H.: Contact Materials for Vacuum Switching Devices. Proc. XVth ISDEIV, Darmstadt 1992, 273-278

Grill, R.; Müller, F.: Verbundwerkstoffe auf Wolframbasis für Hochspannungsschaltgeräte. Metall 61 (2007) H. 6, 390-393

Slade, P.: G.: The Vacuum Interrupter- Theory; Design; and Application. CRC Press, Boca Raton, FL (USA), 2008

Frey, P.; Klink, N.; Saeger, K.E.: Untersuchungen zum Abreißstromverhalten von Kontaktwerkstoffen für Vakuumschütze. Metall 38 (1984) 647-651

Frey, P.; Klink, N.; Michal, R.; Saeger, K.E.: Metallurgical Aspects of Contact Materials for Vacuum Switching Devices. IEEE Trans. Plasma Sc. 17, (1989) 743- 740

Slade, P.: Advances in Material Development for High Power Vacuum Interrupter th Contacts. Proc.16 Int. Conf. on Electr. Contact Phenom., Loughborough 1992,1-10

Behrens, V.; Honig, Th.; Kraus, A.; Allen, S.: Comparison of Different Contact th Materials for Low Voltage Vacuum Applications. Proc.19 Int. Conf. on Electr. Contact Phenom., Nuremberg 1998, 247-251

Rolle, S.; Lietz, A.; Amft, D.; Hauner, F.: CuCr Contact Material for Low Voltage th Vacuum Contactors. Proc. 20 int. Conf. on Electr. Contact. Phenom. Stockholm 2000, 179-186

Kippenberg, H.: CrCu as a Contact Material for Vacuum Interrupters. th Proc.13 Int. Conf. on Electr. Contact Phenom. Lausanne 1986, 140-144

Hauner, F.; Müller, R.; Tiefel, R.: CuCr für Vakuumschaltgeräte- Herstellungsverfahren, Eigenschaften und Anwendung. Metall 61 (2007) H. 6, 385-389

Manufacturing Equipment for Semi-Finished Materials (Bild)

Retrieved from "https://www.electrical-contacts-wiki.com/index.php?title=Contact_Materials_for_Electrical_Engineering&oldid=5513"