Re: [ATM] Applying Protective Coatings

[Pete <lenses@adelphia.net>]

Hi Jason,
You can buy silicon monoxide and evaporate it directly.
The material sublimes, i.e. it doesn't melt just goes from solid to gas 
vapour.
You can evaporate it from just about anything as it doesn't take much power.
The easiest way to evaporate it is using crushed up pieces about 1/8" in 
a crucible with a tungsten wire spiral above it.
I am not sure if its even referenced in books (maybe 1950 or so ) but 
one of the easiest way to monitor the sio film as
it grows on a mirror is look across at a grazing angle at the freshly 
aluminized surface and as the sio deposits with a polarizing
filter the color change can be observed.
You will want a 1/2 wave of sio, so if you use a poliarizer as you go 
through one color phase rotate the polarizer by 90 degrees
and when you just finish the second color phase (kinda straw yellow 
color) you will be close enough to 1/2 wave for visible light.
If your chamber is pyrex this is very easy, put a flourescent light 
across from where you are observing.
Or the method of letting the aluminum film form aluminum oxide will work 
well, just be aware that if your in a humid environment
this doesn't give you much of a moisture barrier and the film (might) 
not last too long.

If you have any more questions or if you need any small quantity 
supplies I can help you out.

Happy coating!
Peter Nance, Jr.
lenses@adelphia.net

Jason Hissong wrote:

>Hi guys,
>
>Our club ATM group at Perkins Observatory is working on a vacuum chamber 
>project and I was interested in knowing ways to add the protective 
>coatings to t he mirror once aluminized.  The specific chamber we have 
>currently used the Tungsten wire method of evaporating the aluminum.  We 
>have most of the pieces now including the diffusion pump, roughing pump, 
>and some other stuff.
>
> From what I researched in the list, it appears that you can simply 
>leave the mirror alone for a few weeks and the layer of Aluminum Oxide 
>will form on the mirror all by itself.  I also saw a reference in 
>Wikipedia about some who bake the mirror in an oven to accelerate the 
>oxidation process?  I would be curious if anyone has done this.
>
>Also, there is a vague reference for applying Silicon Monoxide by 
>combining silicon and quartz in a crucible and evaporating it in the 
>chamber.
>
>Any references in some books would be great!
>
>Thanks for the assistance...
>
>Jason Hissong
>
>  
>
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