0. Overview – Why electrical contact materials matter

An electrical contact is the point in an electrical circuit where current is actually “made and broken.”
This small interface largely determines the overall performance of:

• System reliability

• Lifetime

• Energy efficiency

A good contact material must satisfy several properties at the same time:

• Low electrical resistivity → minimizes heat generation and energy loss

• High thermal stability → maintains structural integrity under repeated arcing

• Excellent wear resistance → reduces mechanical wear and deformation

• Arc erosion resistance → resists melting, evaporation, and oxidation under arc exposure

For example:

• Ag-based MMCs (Ag-based metal matrix composites) are highly advantageous for low-voltage switching

• Cu-based MMCs are reported to be more suitable for high-voltage sliding contacts and thermal management systems

In the following, we focus on materials widely used in industry:
silver (Ag), gold (Au), copper (Cu), tungsten (W), and palladium (Pd), and discuss:

• Basic properties

• Arc erosion mechanisms

• Recent research directions

• Application fields and selection guidelines

1. Silver (Ag) and Ag-based alloys

1-1. Fine silver: the conductivity benchmark material

Silver (Ag) serves as a benchmark material for electrical contacts.

• Electrical conductivity: close to or above 100% of the International Annealed Copper Standard (IACS)

• Thermal conductivity: very high, roughly around 400 W/m·K

• High-purity silver (99.9% or higher) offers the lowest contact resistance, minimizing heat generation

However, it has clear disadvantages:

• Sulfidation by sulfur (S) and chloridation by chlorine (Cl) form a contamination film on the surface → increases contact resistance

• The metal is relatively soft, making it vulnerable to mechanical wear and deformation

• Under high arc loads, welding and spattering can occur

Therefore, fine silver is still widely used in:

• Low- and medium-current switches and relays

• General household and industrial low-voltage switching components

But in high-current and high-arc environments, it is usually used in the form of alloys or composites rather than pure silver.

1-2. Ag-Cu, Ag-SnO₂, Ag-CdO – alloys and oxide composites

1. Ag-Cu alloy (silver–copper)

• Typical example: sterling silver (about 92.5% Ag)

• Higher hardness (HV) than pure silver → improved resistance to deformation and wear

• Reported to reduce arc welding rate by about 20–30% compared to pure silver

• Widely used in general relays and universal switches

2. Ag-CdO (silver–cadmium oxide)

• Has long served as a standard material for power switching applications

• Advantages:

  • Very high arc resistance

  • Maintains relatively stable contact resistance under repeated switching

• Disadvantages:

  • CdO (cadmium oxide) is toxic and regulated under RoHS and similar environmental regulations

• As a result, new designs are increasingly moving toward alternative materials

3. Ag-SnO₂ (silver–tin oxide)

• A representative eco-friendly alternative to Ag-CdO

• In high-current environments (50 A and above), studies report that it:

  • Shortens arc duration by about 10–20%

  • Improves wear resistance by nearly a factor of two

• With CuO addition:

  • Reduces coarse agglomeration of SnO₂ particles

  • Suppresses the increase in contact resistance

• With rare-earth doping (for example La, Y):

  • Enhances thermal stability

  • Stabilizes the molten pool during arc erosion

In summary, Ag-SnO₂ is a key material that targets both environmental compliance and performance, especially in:

• Low-voltage circuit breakers

• Contactors

• PV inverter relays

where high peak current and repeated arcing occur.

1-3. Ag-based MMCs and nano-reinforcement – CNT, graphene

Recently, MMCs with a silver matrix reinforced by ceramics or nano-carbon materials have been actively studied.

• Ag-TiN and Ag-CNT reinforced composites:

  • Reduce friction coefficient

  • Maintain stable contact characteristics even beyond 100,000 switching cycles

• Graphene nanoplatelet reinforcement:

  • Reduces excessive spattering of molten Ag under arc

  • Helps stabilize surface morphology and conduction paths

In short, Ag-based MMCs are a high-end material family that aim to achieve:

• Intrinsically excellent electrical conductivity

• Enhanced resistance to arc erosion and mechanical wear

at the same time.

2. Gold (Au) and gold alloys

Gold (Au) is a noble metal that effectively does not oxidize, and therefore offers outstanding contact stability under:

• Corrosion

• Humidity

• Contamination

Even after more than 10⁶ operating cycles, changes in contact resistance are typically very small (less than about 1%).

Although its electrical conductivity is lower than that of silver (around 70–75% IACS), gold provides essentially top-tier reliability in low-current and low-voltage regimes.

2-1. Characteristics of gold and Au-Pd alloys

• Because almost no surface oxide layer forms, gold maintains stable contact even for very small signals in the mV/mA range

• Au-Pd alloys:

  • Reduce wear and welding compared to pure gold

  • Have been reported to limit signal loss to below about 5% in RF connectors

Typical application fields:

• Precision electronics

• Sensors and measuring instruments

• High-frequency/high-speed communication equipment
  (such as 5G infrastructure connectors and relays)

2-2. Gold plating and thickness design

Because the bulk material cost of gold is extremely high:

• It is usually applied as a thin plating layer, typically around 0.5–1 μm

• In connectors subject to repeated mating and unmating:

  • Once the gold layer wears through, the underlying nickel (Ni) or copper (Cu) alloy is exposed

  • From that point, corrosion and increases in contact resistance progress rapidly

Therefore, at the design stage it is crucial to:

• Define the minimum gold plating thickness according to the required number of mating cycles
  (for example 10,000 / 50,000 / 100,000 operations)

3. Copper (Cu) and copper alloys

Copper (Cu):

• Has electrical conductivity second only to silver (almost 100% IACS)

• Is relatively inexpensive

which makes it very suitable for mass production.

However, it also has clear drawbacks:

• Highly susceptible to oxidation → once an oxide film forms, contact resistance increases significantly

• Pure copper has relatively low hardness (HV 50–80), making it vulnerable to mechanical deformation

• Under arc erosion, surface tarnishing and crack formation are likely to occur

To compensate, copper is often used in the following forms:

• Cu-based MMCs such as Cu-Cr and Cu-W

• Surface protection via Ag or Sn plating

For example:

• Cu-20Cr alloys are reported to:

  • Shorten arc duration during high-voltage switching

  • Reduce mass loss due to erosion

• CNT-reinforced Cu-Cr (Cu-Cr-CNT) composites are being studied to:

  • Increase the viscosity of the molten pool

  • Suppress metal splashing during arc erosion

Representative application fields:

• Busbars

• High-current contact parts (high-current terminals, EV power connectors)

• Mass-produced relays and terminals where cost is a critical factor

4. Tungsten (W) and tungsten alloys – a shield for high-temperature, high-arc environments

Tungsten (W):

• Has an extremely high melting point (about 3422°C)

• Possesses high strength and hardness

which give it excellent resistance to arc erosion.

However:

• Its electrical conductivity is lower than that of silver and copper

• It is difficult to machine

So in practical electrical contact applications, tungsten is usually used as part of a composite material.

Representative combinations include:

• Ag-W (silver–tungsten composites)

• W-Cu (tungsten–copper composites)

• Mo-W-Cu systems

These materials are used in environments such as:

• High-voltage circuit breakers

• Gas-insulated switchgear (GIS)

• Tap-changers

• Welding equipment

Recent studies show that:

• As the tungsten content increases:

  • Arc resistance and erosion resistance improve

  • Electrical conductivity decreases, defining a clear trade-off

• Processes such as spark plasma sintering (SPS) are employed to:

  • Improve microstructural uniformity

  • Control porosity

and thereby fine-tune performance.

5. Palladium (Pd) and Pd alloys – a practical alternative to Au

Palladium (Pd) is a noble metal that:

• Is somewhat less expensive than gold

• Offers excellent corrosion resistance and high-temperature stability

Key characteristics:

• Electrical conductivity is about 80% of that of silver

• Pd-Cu and Ag-Pd alloys have been reported to show:

  • Relatively low wear and small increases in contact resistance even after around one million switching cycles

• During arc erosion, these alloys exhibit:

  • Reduced bridge erosion

  • Lower welding tendency

Typical application fields:

• Automotive electronic components (especially in high-humidity, high-temperature environments)

• Precision relays

• Communication devices (5G relays, connectors, etc.)

Layered Au-Pd structures are attracting attention as a design that:

• Combines the low-resistance properties of Au

• With the corrosion and wear resistance of Pd

6. Comparison of key physical and electrical properties by material

Basic property comparison of representative contact materials

As seen in this table:

• In terms of electrical and thermal conductivity alone, silver and copper are advantageous

• In high-temperature, high-arc environments, tungsten-based systems are essential

• For corrosion and environmental stability, gold and palladium-based materials are dominant

7. Arc erosion and switching performance comparison

Arc erosion has a major impact on contact lifetime.
Many studies treat it as a key factor that determines more than 70% of overall contact life.

The table below summarizes representative trends, based on tests under DC 24 V / 15 A conditions and 10⁴–10⁵ switching cycles.
(Exact values depend strongly on specific test conditions.)

Comparative arc erosion and switching characteristics of representative alloys

In summary:

• Noble-metal systems such as Au-Pd plating and Pd-Cu
  are well suited for signal-level, high-reliability applications due to low mass loss and low welding risk

• Composite systems such as Ag-SnO₂-CuO and W-Cu-rGO
  are being intensively studied for power switching environments, where they effectively reduce arc erosion and welding

8. Material selection guidelines

In practical design, selecting an electrical contact material is less about finding “the best material” in absolute terms and more about finding “the best compromise” for the specific product conditions.

Key factors to consider include:

• Current level and waveform

• Presence and frequency of arcing

• Environmental stress (temperature, humidity, gas composition, corrosive atmosphere, etc.)

• Required lifetime and switching cycles

• Cost (BOM cost) and regulations (such as RoHS and other environmental rules)

Based on these, typical selection examples are:

1. Low-current, small-signal applications (sensors, 5G communication, high-speed connectors)

   • Recommended: Au plating, Au-Pd, Pd-Cu

   • Reason: Very high corrosion resistance, minimal change in contact resistance even after more than one million mating cycles

2. General switches and relays (roughly 10–50 A, inductive loads)

   • Recommended: Ag-SnO₂, Ag-Ni, Ag-Cu

   • Reason: Good balance of arc resistance and wear resistance versus cost; widely validated in EV relays and similar applications

3. High-current, high-arc environments (circuit breakers, welding machines, 1 kV and above)

   • Recommended: W-Cu, Ag-W, Mo-W-Cu composites

   • Reason: Minimize erosion and welding under high temperature and heavy arcing; can keep mass loss below about 0.1%

4. Cost-sensitive, high-volume production (busbars, automotive power terminals)

   • Recommended: Cu-based materials (with Ag or Sn plating, and optional CNT/graphene reinforcement)

   • Reason: Optimal in terms of conductivity and cost; oxidation and wear issues can be mitigated through plating and coatings

5. Designs where environmental compliance and regulations are critical

   • Recommended: Ag-SnO₂, Ag-ZnO, Cd-free MMCs

   • Reason: Provide switching performance comparable to Ag-CdO in an environmentally friendly, regulation-compliant way