Re: [ATM] Applying Protective Coatings

[Pete <lenses@adelphia.net>]

Hi Preston,
The process for applying MgF2 to lenses is done under heat (500 degrees 
F) so aluminum fixturing is normally undesirable.
It will tend to mark up the edges of the glass with aluminum but nothing 
that couldn't be cleaned off usually with some solvents or light rubbing 
with polishing compound. Or a Q-tip dipped in "green rivers" .
We generally use stainless steel for fixturing. Its much cleaner when 
doing a lot of production work.
If your going to purchase a chamber that has a pyrex bell jar the same 
procedure for removing aluminum from mirrors works for this.
The plus side, container is ready made, negative side, real messy job. 
If you do this make sure you protect the chamber, they only weigh about 
70 lbs but are a bear to handle. An extra person is helpful. And of 
course your removing it from the hoist. You don't want to chip the edge 
or worse yet break it, cost last time I checked (lots of years ago) was 
around $1000.00 for replacement 18" X 30" pyrex bell jar. Or just leave 
it and start coating.
In answer to your question, no the aluminum will not kill the reaction, 
its evaporated onto the heated substrate (lens) and your done.
Quick procedure.
1. Clean lenses and load into racks.
2. Prepare chamber and load MgF2.
3. Evacuate chamber.
4. Turn on heater. Wait about 1 hour.
5. Coat, turn off heater and wait 1 hour (were being safe here, wait 
longer depending on glass type and size)
6. Vent and remove, repeat!
If your chamber doesn't have a heater, before purchase make sure you 
have enough feedthroughs in the baseplate to add the electrical connections.
And you can make a heater real easy with formed stainless steel and 
nichrome wire. Or you can coat MgF2 cold but it won't hold up to any 
kind of cleaning.
Another plus here, removing the coating is easy and you can recoat as 
needed.
Rambling now talking about stuff you didn't ask about.

Have fun!
Peter Nance, Jr.
lenses@adelphia.net



Preston Engebretson wrote:
> Hi Peter,
>
> A quick question regarding  applying protective coating to lenses. 
>
> I have the  option of purchasing a nice aluminum vacuum chamber and wish to
> us it for applying MgF2 to glass lenses.  However, I seem to recall seeing
> or reading somewhere that aluminum in the chamber would kill the reaction
> somehow.
>
> However in the book "Fabrication Methods for Precision Optics" by
> Karow, he mentions on page 578 that "The insert rings for coating
> racks are made of either aluminum or steel."
>
> Thus my confusion...Can anyone comment on this subject please.
>
> Regards,
> Preston Engebretson
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>
>   

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