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When heating tungsten filaments to evaporate aluminum the first thing that is vaporized is the tungsten oxide. This is why you either need to strip the oxide off (electropolising or by chemical cleaning using an oxidizing agent) before heating or you should use a shutter (closed while premelting the aluminum on the tungsten).? The aluminum will melt and wet the tungsten at temperatures way below what is needed to evaporate the aluminum. When the temperature is increased the aluminum will evaporate Then open the shutter).? Don

Precisely!

?

Guy Brandenburg, Washington, DC?
http://gfbrandenburg.wordpress.com/
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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From: Andrew Aurigema
To: VacuumX@...
Sent: Monday, September 26, 2011 4:44 PM
Subject: Re: [VacuumX] Al sticking, Cr sticking, Cr stripping

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thanks for the tip.? So slowly on the current so the coils have a chance to heat up the aluminum and never got to bright glow till they are aluminum coated.?

On Mon, Sep 26, 2011 at 4:03 PM, Guy Brandenburg <gfbrandenburg@...> wrote:

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Andrew, Try ramping up the current and voltage very slowly on your tungsten filament in the future. Take nearly a minute to reach full current.

It worked for me; it might possibly work for you.

Guy

On Sep 26, 2011, at 3:08 PM, Andrew Aurigema <eosraptor@...> wrote:

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That is very clever actually.? Reverse electro plating.?

I too ran into overly stuck on coatings this weekend.? I almost have my chamber back up and working but the pressure is still a little to high.? When I strip the coating off there is a thin film of dark looking soot on the glass.? This will not "green river" strip off.? In fact nothing took it off.? I was forced to set back up the polishing lap and do another 20 minutes of automated figuring to fully remove the film.?

I am thinking that the problem is the first few seconds of heat to the first coils.? The tungsten is bright red / near white hot for a few seconds while the aluminum is melting and wicking up onto it.? In that time, the tungsten "burns" if the pressure is a little high.? That black film is now under the aluminum.? It is great for adhesion but taking it off is not possible without polishing.?

It may be possible to use this mistake somehow.? Maybe set up one tungsten in the center of the chamber that you burn for 10 seconds at a pressure not quite as low as for coatings.? If there is no aluminum on the tungsten then it will burn and make that tungsten soot.? Aluminizing over that soot stuff is a sure fire way to get the aluminum to stick to the glass.?

I dont know if any of this is any good but it may be worth a look if somebody is bored out there.????

Just remember, to get that film off you will be using CeO and pitch.

Drew in soggy Florida

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On Mon, Sep 26, 2011 at 1:54 PM, Attila <schneyolo@...> wrote:

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Hi All,

I just would like to share some experience about these.

Thank's for Vladimir I made the triangle glow discharge cleaning cathode
as he described in his pdf file. And it seems the Al passed mostly the tape test.

But just in case I tried to evapaorate 10nm Cr at 1,5nm/sec under the Al tn a piece of glass. And sticked very well. Later I was able to strip them by green river.

So that is why Sunday I started to make some good coating for mirrors. Four of them was loaded and coated first 20nm Cr and than 120nm Al, and before I could coat the two oxide layer I had to terminate the process because of the shutter failure.

No problem , let's strip off the Cr and Al by green river I guessed. But this time the Cr sticked so well the green river was not able to strip it.
I have no HF acid. So I tried everything without succes.
Suddenly I found an idea and tried it. And this one worked and I was able to strip off the Cr easily without eching the glass.

The methode is the reverse electroplating. I used sulphur acid(battery acid), a lead plate for the negative and a sponge on the positive electrode. The power supply was my inverting welder.

As I rubbed the surface of the immersed mirror the Cr solved and I was able to whipe it off.

So at least for 20nm Cr it works.

Have a nice day!

Br.

Attila

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