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topic 59489

Brush plated silver is flaking at the edges

December 6, 2013

Q. Dear Sir,

I am doing silver brush plating of test samples. Base material is AISI 4140 (quench and temper steel). Base coat is copper on Nickel. The thickness of silver plating is 0.35 mm. First, I plated 0.175 mm thickness. After that, the sample was heat treated at temperature of 190 °C for 3 hours. Then I plated a second layer 0.175 mm on the first layer. After finishing the brush plating, the sample was ground and polished, heat treated at 190 °C for 4 hours.

Then It was tested by bend test. The silver coating flaked at the edge of test sample during the bend test. Why? Please explain your suggestions about the root cause of silver flaking at the edge of sample.

Ye Htut
- Singapore

First of two simultaneous responses -- December 19, 2013

A. Hello Ye,
I can't say that I have heard much about heat treatment of silver. Are you sure your under plate is Cu on Ni, and not the other way around? The silver you are brush plating directly on Cu will not hold up very well. The Cu will migrate into the silver layer and you end up with a Cu - Ag inter metallic. A Ni barrier plate is normally used under the silver plate. Anyway, its a good possibility that the first coating of silver wasn't properly cleaned after "heat treatment". The oxide from the heat treatment must all be removed. Did both layers of silver peel? If only the second layer peeled, that would be your answer. Are the Ni and Cu under plate also brush plated? I was also considering if these plated layers were done in a conventional plating bath, there could be some high current density burning which can also cause peeling on the edges of a part. Please let us know what you discover.

Mark Baker
Process Engineer - Malone, New York, USA

Second of two simultaneous responses -- December 19, 2013

A. Ye,
Perhaps the nickel oxidized (dried in the air a bit). Your statement mentions the silver flaking so if the nickel remained there I would suspect oxidation. If the nickel dries or becomes oxidized the silver can flake off later, especially when tested because of a weak bond.

blake kneedler
Blake Kneedler
Feather Hollow Eng. - Stockton, California

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