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[ATM] Fwd: [atm_free] Effect of pH on polishing fused silica was Re. Session 24...]
Hello!
So, I missed the start of this thread, but FYI, I
thought I would add that sometimes a surface is called
out as a fused silica Super Polish. Minimum rms
surface, plus, must stand a Hyrofluric Acid (3%) bath
for 10 minutes, and not show any defects that had
their 'flowed' glass filler removed. PH of 4, using
Hastalite PO refined ceria. Slurry is kept in
suspension using a electric mixer like you would find
in your kitchen.
Works great
Bill Marriott
http://StarlightOptical.com
--- Guy Brandenburg <gfbrandenburg@yahoo.com> wrote:
> Interesting comments on pH levels and polishing with
> CeO2
>
> Alan Bromborsky <brombo@comcast.net> wrote: Date:
> Thu, 30 Aug 2007 17:37:14 -0400
> From: Alan Bromborsky <brombo@comcast.net>
> To: Guy Brandenburg <gfbrandenburg@yahoo.com>
> Subject: [Fwd: [atm_free] Effect of pH on polishing
> fused silica was Re. Session
> 24...]
>
>
> To: atm_free@yahoogroups.com
> From: "Mark Cowan" <toolontop@yahoo.com>
> Date: Thu, 30 Aug 2007 19:47:54 -0000
> Subject: [atm_free] Effect of pH on polishing fused
> silica was Re. Session 24...
>
> I was looking through
> the paper I cited in the last post and noted the
> following about hydration layers and silicate
> redeposition:
>
> Buffering the CeO2 slurry to pH 4 increased the
> drag greatly, but
> according to the paper and the assorted
> micrographic and AFM imagery
> the resulting surface is much smoother - /because/
> (they say) the
> redeposition from the slurry is greatly reduced.
> Thus the polishing
> action is surface removal primarily. They also say
> the lap lasts much
> longer because the "ceria particles are suspended
> in the slurry rather
> than embedded in the lap." The lack of the
> redeposited layer means
> that the final surface is made from the pure
> substrate, and so might
> bond to the coating better, improving coating life.
>
> Perhaps you need to start with a fresh lap for this
> experiment. I plan
> to test some of this with the robotic figuring
> machine, which is much
> more capable than I am of maintaining constant lap
> speed against high
> resistance.
>
> Best,
> Mark
>
> > Here's an interesting experiment that
> demonstrates that.
> >
> > H. Highstone set me a copy of an Applied Optics
> paper from '92 (pp
> > 7164-7172, Vol 31, no 34) that explored the use
> of various agents
> > (CeO2, ZrO2, Al2O3, Y2O3, YF3) on fused silica at
> varying pH ratios.
> > Al2O3 has by far the highest roughness of any of
> these compounds.
> > Anyway, lowering the pH from 7 to 4 with CeO2 on
> fused silica lowers
> > the roughness by a factor of almost 4x under
> controlled conditions.
> > You can use citric acid to buffer the pH of a
> slurry down to these
> > levels. I've tried it. The lap drag immediately
> becomes /enormous/,
> > to the point that the lap can't be moved
> smoothly. Although it may
> > result in lower roughness under machine control,
> by hand the result
> > was horrible, with roughness quite visible under
> Foucault testing.
>
>
>
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> Guy Brandenburg, Washington, DC
> My home page on astronomy, mathematics, education:
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> or else
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> capable of conceiving, namely, the production of the
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