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Mirror coating on SS for 600¦C
- Thread starter soptic
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EdStainless
Materials
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.
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.
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
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
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Here is a company that I’ve seen referenced several times for high temperature reflective coatings.
The specific reference is to their MicroDyn process.
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.
The specific reference is to their MicroDyn process.
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.
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
(California)
(Chicago)
Electropolish Systems sells both chemicals and equipment (& will run samples):
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
(California)
(Chicago)
Electropolish Systems sells both chemicals and equipment (& will run samples):
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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
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
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. “
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.
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. “
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.
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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
Axel
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.
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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