Manufacturing Methods of Electrical Contact Materials

Rivet, Plate, Powder Metallurgy, Internal Oxidation, and Other Techniques

Introduction

Electrical contacts are essential components in switches, relays, and circuit breakers.
They perform functions such as current transmission, arc suppression, and welding resistance.

The performance of these contacts strongly depends on electrical and thermal conductivity, wear resistance, and arc erosion resistance.
To optimize these properties, various manufacturing methods have been developed.

Major materials include silver (Ag), Ag alloys (e.g., Ag-Cu, Ag-Ni), and composites (e.g., Ag/CdO, Ag/SnO₂, Ag/C).
Due to environmental regulations restricting cadmium use, alternative materials such as Ag/SnO₂ and Ag/LSCO have emerged.

Manufacturing techniques are mainly categorized into powder metallurgy, internal oxidation, rivet, and plate methods.
Supplementary processes such as melt-casting and mechanical alloying are also applied.

This document focuses on the four main methods—rivet, plate, powder metallurgy, and internal oxidation—covering their processes, materials, advantages, disadvantages, and applications.

Rivet Method

The rivet method produces contacts in pin form, fixed mechanically onto a metal base such as copper or steel.
It is cost-effective for low-current applications (below 20A).

Wires or rods made via internal oxidation or powder metallurgy are cut, headed, and staked onto the carrier.

Process:
(1) Production of internally oxidized or extruded composite wire
(2) Cutting and heading (forming the head under low deformation ratio)
(3) Riveting or staking onto the base plate

Materials: Pure Ag (99.95%), Ag-Ni(90/10), Ag/CdO(90/10), Ag/SnO₂(92/8), Ag/C(2-3 wt%).
Ag/CdO rivets offer excellent arc erosion resistance and are suitable for automotive relays.

Advantages: Easy forming, good brazing compatibility, and low cost.
Disadvantages: Mechanical weakness under high current (>20A), and cracking during heading if graphite content exceeds 3 wt%.

SEM analysis shows that internally oxidized Ag/CdO rivets have fine CdO particles (1–5 μm) distributed near the surface, improving welding resistance.

Applications: Micro switches, automotive relays, and low-voltage circuit breakers.
For example, Ag/Ni 90/10 rivets exhibit effective arc-quenching properties in 10A relays.

Plate Method

The plate method produces flat contacts (strip or sheet form) that are attached to a base through cladding or plating.
This method provides a larger contact area and is suitable for high-current applications.

Process:
(1) Production of internally oxidized or extruded composite strip
(2) Cladding with pure Ag or Ag alloy layer (hot or cold rolling)
(3) Welding or brazing onto the carrier

Materials: Ag/CdO (clad with Ag backing), Ag/SnO₂(88/12, In₂O₃ added), Ag/Ni, Ag/Pd(30–60 wt%).
Electroplated Ag coating reduces the contact resistance of non-silver contacts.

Advantages: Excellent brazing stability (up to 800°C), improved corrosion resistance, and economical profile production.
Disadvantages: Possible interlayer delamination during cladding and thermal softening of Ag/CdO strips.

Internally oxidized Ag/CdO plates apply Ag cladding on one side to avoid oxide-depleted zones.

Applications: Contactors, circuit breakers, and power switches.
Ag/SnO₂ clad plates show stable material transfer under DC switching conditions.

Powder Metallurgy

Powder metallurgy mixes Ag powder with metallic or oxide powders, compacts, and sinters them into composite materials.
It allows uniform dispersion of insoluble components such as Ni or SnO₂.

Process:
(1) Powder mixing (dry blending or doping)
(2) Compacting (forming green tips)
(3) Sintering (vacuum or inert atmosphere, 800–900°C)
(4) Hot extrusion or PSR (press-sinter-repress)

Materials: Ag/Ni(10–40 wt% Ni), Ag/CdO(85/15), Ag/SnO₂(88/12, WO₃/MoO₃ added), Ag/ZnO(90/10), Ag/C(2–6 wt%).
The particle size of Ag powder (1–10 μm) determines the sintered density.

Advantages: High density (>99% of theoretical), excellent arc-quenching properties, and fibrous microstructure (e.g., Ag/Ni fibers resist up to 100A erosion).
Disadvantages: High cost (especially when indium is added) and anisotropy in mechanical and electrical behavior.

Cr-doped Ag/SnO₂ produced via sol-gel and powder metallurgy shows a 20% improvement in arc erosion resistance.

Applications: Relays, contactors, and vacuum switches.
Ag/WC (via PSI infiltration) is used in high-voltage circuit breakers, achieving a density of 15–17 g/cm³.

Internal Oxidation

Internal oxidation forms fine oxide precipitates within Ag alloys by heating them in an oxygen-rich environment.
It is mainly used for producing Ag/CdO and Ag/SnO₂ materials.

Process:
(1) Melting Ag-Sn/Cd alloy (in vacuum or inert atmosphere)
(2) Heating in oxygen (600–800°C, following parabolic rate law: x² = kₚt)
(3) Oxide precipitation (CdO, SnO₂)
(4) Drawing and rolling

Materials: Ag/CdO(10–15 wt% CdO), Ag/SnO₂(2–14 wt% SnO₂, In₂O₃ added), ARGODUR 32(Ag-0.5 wt% MgO/NiO).

Advantages: Fine and uniform oxide distribution (1–5 μm), enhanced arc and welding resistance, and high-temperature stability (brazing up to 800°C).
Disadvantages: High indium cost, oxide depletion in the core (25% thickness unoxidized), and cadmium toxicity.

Applications: Power switches and relays.
Ag/SnO₂ POX1 (10 wt% SnO₂) is an eco-friendly alternative with 15% lower arc erosion rate compared to Ag/CdO.

Additional Techniques

Melt-Casting: Alloying Ag with Cu, Ni, or Pd, followed by casting, annealing, and rolling.
Materials: AgCu3 (3 wt% Cu), AgPd30.
Advantages: Improved mechanical strength (hardness 80–100 HV).
Disadvantages: Reduced conductivity (AgCu3 ≈ 90% IACS).
Applications: 10A switches.

Mechanical Alloying: High-energy ball milling of Ag with LSCO particles, followed by PSR sintering at 880°C.
Material: Ag/LSCO(12 wt% LSCO).
Advantages: No indium required, high uniformity (density 9.72 g/cm³, resistivity 3.10 μΩ·cm), CdO replacement.
Disadvantages: Risk of contamination.
Compared to conventional powder metallurgy, mechanical alloying provides ~25% better arc resistance.