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pc
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Blisters on surface of Ag-C contacts

07/28/2008 1:44 AM

Hi,

I am facing some queer problem.

We are manufacturing contacts for MCCB. The contacts are made of 98% Ag & 2% Graphite. They are flame brazed to ETP copper fingers. Afterwards they are silver plated.

Now, when the end of the Copper finger (which at a distance of 50 to 60 mm from the above Ag-C contacts) is then again brazed to braid assy by Resistance brazing. It is observed that when this brazing is going on, the Ag-C button develops blisters on surface.

I want to know, why this is happening?
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aurizon
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#1

Re: Blisters on surface of Ag-C contacts

07/28/2008 10:49 PM

melted silver absorbs oxygen from the air. WHen it cools, the oxygen is excluded, making bubbles, blisters etc.

Solution, exclude the oxygen with an inert gas while the brazing job goes on.

less pure silver alloy might mitigate this.

http://www.google.com/search?hl=en&q=%22melted+silver%22+%2Boxygen&btnG=Google+Search

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MUKULMAHANT
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#2
In reply to #1

Re: Blisters on surface of Ag-C contacts

07/29/2008 12:03 AM

HOW will this work:

While resistance brazing to braid-- keep silver/graphite button dipped in flowing cold water.This setup can be easily rigged up.

mm
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nap
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#3

Re: Blisters on surface of Ag-C contacts

07/29/2008 1:32 AM

If the silver plating is going over the silver button as a final coating, there's a good chance that the graphite gas released by the silver button re-heated after plating is causing the problem.

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#4

Re: Blisters on surface of Ag-C contacts

07/29/2008 8:13 AM

Uh... this might be a bit simplistic but why dont you do it the other way. Braze the braid before you plate the silver...
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aurizon
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#5

Re: Blisters on surface of Ag-C contacts

07/29/2008 8:33 AM

A second look at this show that the sintered silver-graphite button,

""Contact materials consist of graphite particles dispersed in a matrix of silver. Graphite has a high melting point (high refractoriness) and is insoluble in silver, resulting in superior anti-welding characteristics in high power and cycle applications such as circuit breakers. The graphite burns ups in the arcing or switching process and maintains a clean, low-resistivity contact surface. The silver typically bonds the graphite together, forming continuous high-conductivity paths and a lower voltage drop. These characteristics allow silver graphite brushes to perform well under high current density and low voltage conditions. Silver is more oxidation-resistant than copper, which can maintain cleaner contact surface during down times.""

from the description, the button is not remelted, thus will not extract oxygen from the atmosphere, followed by release on cooling.

A sintered material usually has some internal voids, and thus the only cause I can think of that will cause a blister is expansion of trapped gas. The buton has been pressed at high pressure and temperature, which consolidates the surface against the die into a gas tight barrier, thus leaving it susceptible to this problem. I suspect this might be caused by lack of welded contact between the deeper particles of the sintered mass, and thus would be a common Quality Control problem at the button makers end. So call them, as it appears you purchase preformed buttons. Suggest you also call a few other button makers to see if the problem is endemic in low end price competitive buttons, but goes away with costly better made buttons.

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pc
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#6
In reply to #5

Re: Blisters on surface of Ag-C contacts

07/30/2008 12:06 AM

At first even I thought of the same rpoblem.

I took cross section of two contacts where the blister size is small & large. It was quite evident that the blister was slowly growing out of nowhere.

We wrap wet cloth around the button before the braid brazing. So, possibility of overheating is less. Again, why the bubbles are not formed when the button is being brazed to copper finger, as it is subjected higher temp while being brazed.

We observed cross sections of around 20 buttons through microscope and found no cracks no impurities.
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pretzel
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#7

Re: Blisters on surface of Ag-C contacts

08/08/2008 2:41 AM

graphite in its "normal state" has a small [micro] amount of moisture trapped in its crystalline structure. It is this quality that enables graphite to be a "dry lubricant". when this moisture is released at high temperature it no longer has low friction properties. I actually thought that the moisture would have been removed during the alloy process. if this is not so then what you are witnessing may be graphite releasing moisture from pre alloy process or moisture released that has been absorbed by graphite after alloy processing. [improper storage] this moisture is released at quite a high temperature however absorbed at ambient temp [ I believe].
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pc
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#8
In reply to #7

Re: Blisters on surface of Ag-C contacts

08/11/2008 2:06 AM

Do you mean to say that when the button is brazed to the copper finger the graphit in button absorbs moisture while cooling to room temp & when the finger is again brazed to braid assy the moisture is released by graphite at less heating temperature.
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