Silver-Coated Composite Movable Contact for Wear Resistance
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Solution Overview
Problem
Silver-coated composite materials for movable contact parts face issues with adhesiveness under repeated shear stress and increased contact resistance due to wear, leading to a shortened service life, especially in small-sized switches with dome-shaped contacts.
Innovation Solution
A silver-coated composite material with a nickel or cobalt underlying layer, a copper intermediate layer, and a silver outermost layer, where the silver grain size is controlled between 0.5 to 5.0 μm and the copper layer thickness is 0.05 to 0.3 μm, is used, along with a heat treatment process to enhance adhesiveness and maintain low contact resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a stainless steel substrate is coated with silver or silver alloy to improve electrical characteristics and solderability, then the contact resistance increases over time due to wear of the silver coating, but the service life is shortened
Solution Approach 1:
The patent applies a multi-layer composite plating structure consisting of a nickel or cobalt underlying layer, a copper intermediate layer, and a silver outermost layer. This composite structure combines the advantages of each material: nickel/cobalt provides corrosion resistance and adhesion, copper provides ductility and electrical conductivity, and silver provides excellent electrical characteristics and solderability. The specific thickness ratios (copper layer 0.05-0.3 μm, silver layer 0.5-5.0 μm) are optimized to prevent wear-through while maintaining electrical performance, thereby resolving the contradiction between contact resistance stability and service life.
Solution Approach 2:
The patent applies different plating layers with specific properties at different locations and depths: the underlying layer (nickel or cobalt) provides corrosion resistance and strong adhesion to the stainless steel substrate, the intermediate copper layer provides ductility and electrical conductivity, and the outermost silver layer provides excellent electrical characteristics and solderability. This localized functional distribution ensures that each layer performs its specific function optimally, preventing wear-induced contact resistance increase while extending service life.
2Reliability
If palladium plating is applied to improve adhesion and reduce wear, then the coating becomes hard and rigid, causing cracks to occur when the number of actions increases
Solution Approach 1:
The patent replaces the hard and rigid palladium plating with a copper intermediate layer that has excellent ductility and flexibility. The copper layer maintains strong adhesion between the nickel/cobalt underlying layer and the silver outermost layer, while its ductile nature allows it to accommodate stress and deformation without cracking, even after numerous switching actions. This resolves the contradiction between adhesiveness and crack resistance.
3Reliability
If the silver grain size is not controlled, then the adhesiveness deteriorates under repeated shear stress, but controlling grain size requires additional processing steps
Solution Approach 1:
The patent specifies a controlled silver grain size range of 0.5-5.0 μm in the outermost layer to optimize adhesiveness under repeated shear stress. This parameter control is achieved through standardized plating process parameters (current density, plating time, bath temperature) that can be implemented in existing manufacturing lines without major modifications. The grain size control enhances the silver layer's resistance to delamination while maintaining electrical performance, resolving the contradiction between adhesiveness and manufacturing simplicity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides improved adhesiveness and extended service life by maintaining low and stable contact resistance over time, even after thermal hysteresis, and reduces the occurrence of cracks in the layers, resulting in a longer contact service life.
Implementation Method 1
the copper component, which has diffused through the grain boundary of silver, captures oxygen in the outermost layer, to suppress oxidation of the underlying layer
Implementation Method 2
by conducting a heat treatment at a temperature within the range of 50 to 190° C. under an atmosphere of the air
Implementation Method 3
when the average grain size of the silver or silver alloy formed in the outermost layer is set within the range of 0.5 to 5.0 μm
Data Source
AI summary
A silver-coated composite material for movable contact parts, which has: an underlying layer composed of any one of nickel, cobalt, a nickel alloy, and a cobalt alloy at least provided on a part of the surface of a stainless steel substrate; an intermediate layer composed of copper or a copper alloy provided thereon; and a silver or silver alloy layer provided thereon as an outermost layer, wherein a thickness of the intermediate layer is 0.05 to 0.3 μm, and wherein an average grain size of the silver or silver alloy provided as the outermost layer is 0.5 to 5.0 μm.