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Mirror coating on SS for 600¦C

Mirror coating on SS for 600¦C

Mirror coating on SS for 600¦C

(OP)
We are searching for a high reflectivity coating or surface treatment on SS to make mirrors which can operate at up to 600°C without irreversible degradation of the reflectivity at 800 nm wavelength. Any company that can do it for us?
Thank you in advance!

RE: Mirror coating on SS for 600¦C

Check into passivate and electropolish, depending on your reflectivity requirements...

RE: Mirror coating on SS for 600¦C

What is the atmosphere the surface will see?

RE: Mirror coating on SS for 600¦C

(OP)
Unclesyd, the mirror surface  will operate in vaccum.

RE: Mirror coating on SS for 600¦C

(OP)
Alexit, we require high relfectivity which remains stable at high temperatures. What do you mean by passivate and electropolish?

RE: Mirror coating on SS for 600¦C

You take the stainless itself, mechnically grind and polish it (being carefull to not smear any metal) and then electropolish.  You can get very fine surfaces, highly reflective.
I don't know the spectal characteristic of stainless.
You would need to be working from special mateiral if you are using these as part of an optical system.  There is very clean 304 available that would not have microscopic imperfections.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Mirror coating on SS for 600¦C

soptic,

You may want to consider using molybdenum mirrors which are quite commonly used for CO2 lasers.  It has 98% reflectivity at 10.6 microns and can certainly take your temperature when used in a vacuum.  I don't know what the exact reflectivity would be at 800 nm, but a laser mirror supplier could probably tell you.  They are available off the shelf in variuos sizes and are not terribly expensive, probably a lot cheaper than polishing your own mirror or having a custom one made.   Search "moly laser mirrors" to find suppliers.

Timelord

RE: Mirror coating on SS for 600¦C

(OP)
Timelord,
The substrate of our mirror must be SS so we cannot use molybdenum laser mirrors. Further, at 800 nm the reflectivity is under 70%. What we need is a coating that exhibits no ageing at high temperatures!

RE: Mirror coating on SS for 600¦C

Here is a company that I’ve seen referenced several times for high temperature reflective coatings.
The specific reference is to their MicroDyn process.

http://depsci.com/newsite/index.php

Alumiplate is a little under your temperature requirements but this site has some good information on the capabilities of and references to single point diamond turning.  

http://www.alumiplate.com/html/optics.html

RE: Mirror coating on SS for 600¦C

Hi,
A plated finish should be brighter than 304 SS.  For good chemical stability vis-a-vis 304,  try     
1.) bright Ni electroplating,
2.) bright electroless nickel (low phosphorus) plating,
3.) electroplated Cr, and
4.) electroplated Fe.  

I only having visual impressions of the above finishes, not reflectivity at your wavelength.

Do several test panels of each, plus the bare 304.  Test reflectivity before and after accelerated testing, e.g., 700oC in your vacuum.  

Prepare the substrate test panels as suggested above by EdStainless.  Use 304L (low carbon) or 304 ELC (extra low carbon) for best appearance. For electropolishing information, see these electropolishers' illustrated websites
http://www.electromatic.com/benefits.html  (California)
http://www.ableelectropolishing.com/electrop.htm (Chicago)

Electropolish Systems sells both chemicals and equipment (& will run samples):
http://www.ep-systems.com/

RE: Mirror coating on SS for 600¦C

(OP)
thank you Kenvlach,
do you know if your proposed technique is usual for high temperature applications? Will diffusion from SS to the surface degrade the reflectivity with time?
Axel

RE: Mirror coating on SS for 600¦C

Hi Axel,

My experience is with the plating & electropolishing. We sometimes do a "diffusion bake" to improve coating adhesion, but only up to 650oF, depending on substrate. That's why I suggested accelerated testing at 700oC. With coating thicknesess & diffusion data, can calculate via Fick's Law how long before the coating interdiffuses into matrix. But 1) actual test data is better,  2) my guess is that it will remain better for long time -- diffusion will slow down as concentration gradient decreases, and finally, 3) if you plate Fe (maybe over a Wood's Ni strike, that's usual first step for plating on SS), there won't be much gradient anyway. Anything more I say would be speculation.
Ken

RE: Mirror coating on SS for 600¦C

How's progress?
Came across some relevant NASA research; abstract below.  

Reflectivity of Silver and Silver-coated Substrates from 25°C to 800°C
A bench top facility was used to evaluate the reflectivity of several candidate coating-substrate combinations in vacuum at elevated temperatures. Silver was selected as the reflective coating of choice, while copper, nickel, electroless nickel on copper, and 304 stainless steel were selected as substrates. Pure silver, with no coating at all, was also evaluated. An optically flat silver-coated sapphire substrate was used as a standard. All metal substrates were either metallurgically polished or diamond turned to mirror finish prior to silver deposition. Silicon dioxide was used as a protective coating in most cases. Reflectivity measurements were made at room temperature in the visible range with a spectrophotometer, and at elevated temperatures up to 800°C with a helium-neon laser at 632 nm. Results from the high temperature reflectivity measurements will be presented. “
http://www.grc.nasa.gov/WWW/epbranch/thermal/therpropabs.htm

 Note that 304 SS was one substrate.  The binary phase diagrams show negligible solubility between Ag & Cr, Fe and Ni, so should be OK for long-term use.

RE: Mirror coating on SS for 600¦C

(OP)
Thank you Kenvlach for the valuable NASA article. I will try to get a copy of the work if avalable. It's exactly what we are looking for. Do you thing diamond turning is better than conventional polishing for the substrate manufacturing (less impurities?).

Axel

RE: Mirror coating on SS for 600¦C

Prefer electropolishing to to follow machining and conventional polishing. Electropolishing leaves an ~atomically clean surface w/o any machining smears, for lower outgassing and better spectral properties.

RE: Mirror coating on SS for 600¦C

Back when our company had an instrument division our lab was doing some work on front surface mirrors for IR research we tried to electropolish lapped surfaces with little success.  As we were looking for extremely flat mirrors we found that it took very little electropolishing to generate gradations in reflective surface.  We went to single point diamond turning and a mechanical lap and sometimes a flash electropolish to get an acceptable surface.  

Depending on your requirements both electropolishing and single point diamond turning have come a long way so you may be able to use either one or a combination of both.     

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