[Solved] Which metal coating is used in the mirror?

Author: Helen

Apr. 29, 2024

[Solved] Which metal coating is used in the mirror?

Explanation:

The following metal are used for coating to the mirror

The commonly known process of depositing vapor-thin layers of metallic coatings, such as aluminum or silver, on glass and other kinds of substrates to make mirror reflective surfaces is known as silvering.

The actual metal coating used determines the reflectance characteristics of the mirror's surface. Aluminum (Al) is the cheapest metallic coating and it is the most commonly used metal coating, especially when it comes to telescope mirrors. For example, the aluminum coating thickness for a mirror with a 15-in. radius would have an average coating thickness of 1.53 waves.

Aluminum has a reflective yield of around 88-92% over the entire range of the visible spectrum. Silver (Ag) as a precious metal is more expensive and has a nominal reflectance value that can range from 95 to 99% and even extend far into the infrared regions of the visible spectrum, but there is a noticeable large drop in its reflective ability (<90%) in the blue and ultraviolet parts of the spectrum.

The more (most) expensive of the precious metals is gold (Au) which has the highest (98-99%) reflectance range throughout the infrared spectrum. It's reflectivity is somewhat limited at spectral wavelengths that are lower than 550 nm, and the typical gold color is the result that one would see in a gold metallic coated mirror.

Best Stainless Steel for Mirror like Surface.

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Best Stainless Steel for Mirror like Surface.

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Best Stainless Steel for Mirror like Surface.

zj4041

(Aerospace)

(OP)

13 Sep 10 13:04

What stainless steel is best for mirror like finish/polish?
I am considering 321 316 or 321. Are they all about the same.. is does one polish better than the other?

FYI: this is to reduce the emissivity/radiation HT of the surface.

Thanks.

RE: Best Stainless Steel for Mirror like Surface.

btrueblood

(Mechanical)

13 Sep 10 13:41

The hardest alloys will polish best. Hard-drawn 301 or 302 sheet, or 440...

Alloys with more chrome will hold that polish longer (not oxidize as rapidly). 309, 310...

RE: Best Stainless Steel for Mirror like Surface.

zj4041

(Aerospace)

(OP)

13 Sep 10 13:49

so in comparison between 321 and 304... 321 is harder so it will polish better? and both have about the same amount of chrome so they will both hold the polish the same way.. correct ? ... is it correct to say 321 will polish better then 304

RE: Best Stainless Steel for Mirror like Surface.

Metalguy

(Materials)

13 Sep 10 14:22

No, 321 is not harder than 304.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

RE: Best Stainless Steel for Mirror like Surface.

Compositepro

(Chemical)

13 Sep 10 14:31

Polishing will have more effect on the type of reflection (specular versus scattering) that on the absorbtivity of the surface.

RE: Best Stainless Steel for Mirror like Surface.

mcguire

(Materials)

13 Sep 10 16:07
Don't spend too much on a pretty finish. That only affects the specularity of the reflection, not the percentage of light reflected.
304 bright anneal will be very hard to improve upon. This can be purchased from service center stock. Anything more will be diminishing returns.

I just measured a group of stainless steels for reflectivity and emittance for this purpose. Austenitics are distinctly better than ferritics. Just choose one based on the environment it will face. 304 is good except near sea spray. In that case use 316.Don't spend too much on a pretty finish. That only affects the specularity of the reflection, not the percentage of light reflected.304 bright anneal will be very hard to improve upon. This can be purchased from service center stock. Anything more will be diminishing returns.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

You will get efficient and thoughtful service from optec.

RE: Best Stainless Steel for Mirror like Surface.

btrueblood

(Mechanical)

13 Sep 10 16:11

Hard-drawn or hard-rolled products are harder (have higher y.s.) than others. They will deform less, and tend to embed less abrasives during the act of polishing. 302 and 301 are more typically supplied in sheet with 1/4-hard or higher ratings.

CPro, there are published values for s.s. in most heat transfer textbooks that are much lower for "polished" than "mill" finish. Yes, specularity plays a role in lowering emissivity, but I think the ability to clean, and keep clean (i.e. free of heavy oxidation) is also part of it. It's easier to see the dullness of oxidation build-up on a previously polished surface.

RE: Best Stainless Steel for Mirror like Surface.

zj4041

(Aerospace)

(OP)

13 Sep 10 16:22

I am begining an experiment to see if nickel plated chromolly has lower emissivity than polished ss 321. Based on my application; I hope to find out that they are the same or nickel plated chromolly is better.

btrueblood

(Mechanical)

17 Sep 10 11:34

Thought I said that? ;)

We used to gold plate a lot of heat shielding for s/c use, the issue was not oxidation build up in the vacuum environment of space, but the oxidation of the surface during the months leading up to launch. Radiant heating being the only heat transfer method available, surface finishes become critical to maintaining temperature environments for spacecraft. Gold being very noble, the oxide issue is avoided. Similarly, at higher temperatures (where gold may diffuse into substrates) rhodium or other platinum group "noble" metals were sometimes used.

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