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strip the Beilby layer

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#1 duck

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Posted 17 April 2021 - 09:07 AM

Have run into a couple of occasions when the pitch lap was ineffective in adjusting the figure.  What I found effective was covering the pitch with felt.  Afterward, take the felt off and smooth out the surface.

 

 



#2 ccaissie

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Posted 19 April 2021 - 02:24 PM

Huh.  Supposedly the "Beilby layer" was an idea that became popular when polishing was thought to be a fluidized glass layer.  Was claimed to be what the polished surface was, and able to fill pits and surface details, and could be removed, revealing the pits.

 

Whaddya you think?  Why would felt be able to remove a layer of "Stuff" that a non-glazed, charged pitch lap can't? 

 

I know felt is used, but since it can't be formed against the glass, it can create a wild undulating surface....used in low quality optics i heard.  I'd think it would turn an edge pronto, making a lot of extra work...



#3 duck

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Posted 19 April 2021 - 03:57 PM

The sub-diameter pitch lap is covered with felt.  Tape the felt to the sides of the lap.  Press the lap as usual.

 

I have more than once encountered an up/down edge.  On the last 12 1/2 paraboloid, I used a 1" diameter, felt covered pitch lap working on the up slope with small, circular strokes.  The zone was about 1/2" wide.  I tried the usual techniques with overhanging tangential strokes using full and subdiameter laps.  These were ineffective.  The edge was brought into shape monitoring with the (nulled) Ceravolo Fizeau.  Then switched to Ross nulled knife edge and a 4" subdiameter pitch (no felt) lap and removed the swirl marks.

 

I've seen plenty of evidence that polishing redeposits glass atoms and Ce atoms.  I think the Edmund Optics article on "super" polishing says something about repeatedly removing the Beilby layer in order to get at the subsurface damage left over from abrasive grinding.  I read a paper on laser damage threshold which advocated AL2O3 rather than CeO2 for fused silica optics because of the Ce atoms left in the Beilby layer.

 

If the concept of the Beilby layer is abandoned, I'm not aware of it.


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#4 mark cowan

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Posted 19 April 2021 - 04:53 PM

Polishing slurry carries silicate molecules and depending on pH and concentration and other conditions will indeed redeposit silicate compounds on the hydrated surface layer of the glass, which is only a few nm deep.  This will gradually fill in small defects, seen that many times - small bubbles in quartz will appear at the surface, disappear under normal polishing, then if the load is increased often reappear.  

 

https://www.laserfoc...he-beilby-layer



#5 MitchAlsup

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Posted 19 April 2021 - 06:21 PM

Polishing slurry carries silicate molecules and depending on pH and concentration and other conditions will indeed redeposit silicate compounds on the hydrated surface layer of the glass, which is only a few nm deep.  This will gradually fill in small defects, seen that many times - small bubbles in quartz will appear at the surface, disappear under normal polishing, then if the load is increased often reappear.  

 

https://www.laserfoc...he-beilby-layer

This explanation says that the Beilby layer is on the glass not on the pitch lap.

That corresponds to my knowledge base.



#6 mark cowan

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Posted 19 April 2021 - 08:54 PM

Yes of course it's on the glass.  It's an interfacial layer a few nms deep extending into the bulk glass that experiences some penetration of  the forces from water molecules and thus has attraction for the suspended silicates in the slurry compound.  The slurry is on the pitch and on the glass and inbetween the two as well.  Once a silicate molecule falls out of solution it remains as part of the glass unless removed again.  Or so I understand. shrug.gif

 

FWIW reflective optics aren't really affected by having small defects polished over and then aluminized, but extreme UV applications for lenses are affected (trapped silicate compounds) and laser damage occurs at such places.  

 

If you have a lot of pits and polish a long time energetically the pits eventually go away.  I once hand polished two scratches off of a small Pyrex mirror that were deep enough to catch a fingernail in when they happened.  But they didn't disappear from the top down, more like from the bottom up, because the time spent polishing wasn't nearly enough to remove the entire surface layer of the glass down to the bottom of the scratches.  Therefore they filled in somehow.


Edited by mark cowan, 19 April 2021 - 09:08 PM.


#7 duck

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Posted 20 April 2021 - 06:31 PM

Fascinating and in agreement Mr. Cowan 




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