RE: ATM Amateur vacuum coatings.

["Sam Michael" <samm@idcomm.com>]

> Its great to see you on the list again.  I figured you had lost
> interest in
> our
> little ramblings.  I for one would be interested in hearing about
> your mirror
> coating exploits.  I have gotten into Anodizing,(a much less expensive
> enterprise;)

Thanks David.  No, I didn't lose interest.  I have been working on a new
kind of support system and getting the Vacuum Stuff figured out.  I am
considering adding a section in the new atmsite describing how it all works.

Currently, I have two systems.  The first uses a leak detector to pull the
vacuum.

It was convenient to use this as opposed to just using a roughing pump, a
diffusion pump and a cold trap and doing my own plumbing.

As it turned out, the leak detector works just fine and they can be found
here and there if someone wants to find one.

I have a barn full or the darned things, hee hee.

For those who followed the thread the basic problem was, that I was able to
coat my first blank but after that, the coating looked worse and worse.
Eventually the coatings were almost Black!

Everything I tried seemed to make things worse.  I cleaned and cleaned and
cleaned to no avail.  At first I suspected that it may have been the vacuum
grease.  That somehow the grease was being heated or burned during the
evaporation process or during plasma cleaning.

I cleaned all the grease, cleaned the chamber with a multitude of cleaners
ranging from acetone to acids to etching solution, etc.

As it turned out, THIS was the problem.  I began to notice that I was no
longer able to sustain my low vacuums.  Eventually all I could get was a
10-4!  That was with a COLD TRAP!  Turns out that acids OUTGAS!  I had been
using stronger and stronger cleaners that would outgas more and more.

The moral of the this problem is that if you really have to clean your
chamber, SAND IT!  Then use some acetone to wipe it down and then use some
Alcohol to remove the Acetone.  Nothing else.

I was sure that it was this outgassing that was contaminating my coating.
So now that my chamber was clean again, a 10-6 was simple to get.  All of a
sudden, my high vacuum gage was pegged at the very bottom.

Once again, I tried to coat, but to my disappointment while the coating was
MUCH better, it continue to have problems.  There was always a bit gray to
it.  I tried again, this time with a brand new filament and EURIKA, a
PERFECT coating.  Excited, I put a REAL mirror in and tried a new coating
but once again the coating was contaminated!  I tried a new filament, being
extremely careful not to handle anything and once again a GREAT coating.
Obviously my filament was being contaminated.

It was than that the light bulb FINALLY went on.  It was the air that was
being introduced onto the hot filament once the process was completed.  Hot
Aluminum sitting on the filament was reacting with the air to form Aluminum
Oxide.  A not too terribly attractive compound that is gray.  I refitted my
system to allow me to fill the chamber first with a small amount of
nitrogen.

Excited, I tried again with a new filament.  The first coating applied and
this time I back filled the chamber with a some nitrogen and allowed the
filament to cool for 30 minutes before allowing air in.  Ready to test the
theory, I put another piece of glass into the chamber and tried again.  This
time the mirror was MUCH better.  Still it was not perfect.  Apparently some
nitrides form, so I needed a more inert gas.  I am now using Helium.

Perfect coatings every time.  So LONG as there is LN2 in the cold trap.  If
there is not, back streaming from the diffusion pump will corrupt the
process and once again the coating will not quite be up to that of the pros.

What I have found out, is that a cheap Diffusion Pump and an old Rotary
Roughing pump is all you really need to get it all work but it is an awful
lot of work and you have to really be careful.

I also discovered that cleaning the mirror is not all that hard in
preparation for the coating.

The most difficult part is getting the low vacuum and waiting to open the
chamber.  While all of this was going on, I landed several large Cryogenic
pumps and compressors for them.  I also bid and got a couple of really big
chambers.  The only thing that I lacked was a plate for the bottom of the
chamber.  I eventually traded a couple of pumps for a plate and I was in
business for the new chamber (a 24" beauty that came from a medical company
that was bought out - never used with a cooling jacket).

Unfortunately, all of the cryo pumps that I had were very large (10" !).
These were too large for the chamber but more than that it was difficult to
find a gate valve that large so that the system did not have to be warmed
and cooled every time I wanted to use it.  Eventually I found a really nice
Leibold Cryo pump that already had the gate valve.  This unit was only 6"
and the perfect size.

I have also just found a really nice turbo molecular pump but I haven't
offered enough for them to bite.

This chamber is being set up with ALL the amenities.  I landed some REALLY
nice mechanical passthroughs to allow me to turn the mirror while coating
and I am working on an iris to allow me to begin the evaporation process
with the iris closed so that the initial crap does not deposit on the
mirror.

When all is said and done, I expect coat mirrors for friends and maybe wow
the kids every once in a while, but I have no intention of doing this for a
living.  This is hard work.  Still a lot of fun though...  Nothing like
coating your own mirror with a variety of evaporants.  I have already
attempted Silver once.  I will again.  I just need a better source for pure
Silver.  Something not quite so expensive.

I have run an initial vacuum test on the new chamber and it went down very
quickly and maintained a solid 10-8 mm Hg.  Of course there was nothing in
it and it is a perfectly clean chamber.  That chamber is in the process of
modifications though to get the pass-throughs, the filaments and the high
voltage source fitted.  Hopefully it wont be too long though.  A month or
two at the most given how busy I am.

I am also thinking of doing a gold coating at some point.  Oh well, sky's
the limit huh?

Sam.