Vacuum Metallizing (VM) Layer Turn Black After Solor Radiation Test

More info needed:

which metal or alloy is the vacuum coating?

which plastic?

Which type of over-coating?

Which type of undercoating (barrier to prevent the base materials to diffuse into the metallic layer).

If silver is used (not very probable) then this black is very likely due to sulphur reacting with silver.

If aluminum is used (most likely) then black color may be arising from nano-metric cracking (too brittle by deposition impurities).

If chromium is used then small amounts of nitrogen will introduce total brittleness and the chromium will convert to fine-grained powder on thermal cycling.

Many more possibilities from coating defects existing!

RHABE

Thanks RHABE,

the metal used is Tin ( Sn)

the raw substrate: PC

Base coat : UV coating

overcoating: UV coating

the process flow as following:

raw material ( PC) --> base coating (UV) --> UV curing --> VM ( Sn) --> middle coat ( coloring propose --PU paint) --> hot air oven cure ( 80 degree C)--> Topcoating (UV) --> UV curing --> finised product.

If the metallized coating is thin and the super coat is permeable to oxygen it is possible that black tin oxide is being formed.

You may be forming an allotrope:

http://en.wikipedia.org/wiki/Tin

Hi,

as the metalized tin layer deposited by sputtering process and over-coated with a layer of UV paint, how does allotrope form through this process?

Is Tin/Sn incompatiblitity with the present of Ultraviolet light ? & Chloride (Cl)?

Hi,

tin is not compatible with chlorine, nor with sulfur nor oxygen.

SnCl4 is used in a vast variety of chemical synthetics, SnS2 is used in lubricants and braking liners (substitution of Pb and Sb), SnO2 as polishing agent for gemstones.

So very likely the Cl but may be also S and O2 is responsible for your problem.

Reaction will not be complete so the color will be different from the pure material.

If you try a coating on a ceramic or glass or a plastic that is containing only C- and H- atoms then this will be a little bit clearer.

The first aluminum layers I made on glass and ceramics were black, the next grey, then silvery but non-conducting - all because reacting with residual water in the vacuum-chamber.

Did you check for tightness and gas-purity? Did you degas any rubberO-rings at elevated temperature?

Worst thing I ever met was ball-bearing-lubricant from a turbo-pump that contaminated the high-vacuum side of the turbo-pump but was not visible in pressure and pump-down time as suited for vacuum operation. But as soon as the plasma was switched on the ions that leaked into the turbo-pump cracked the oil and the low molecular-weight crack-products leaked back into the sputtering chamber.

Have success

RHABE

Hi,

black color: either chemical reaction or fine recrystallisation,

chemical reaction may be oxidation or reaction with organic compounds from under-over-coating.

Look at the black surface with a high resolution microscope and with a SEM.

Make an analysis if tin or a tin-compound.

Try without the different coating layers if the same blackening exists. Start with pure tin on your base material.

If the result is the same then it is the tin that is the problem.

Try to remelt with a short pulse of high intensity light (or heat) and look if shiny again: if yes then thy surface roughness caused the black.

Make an analysis of the tin used in the coating process: it may be too pure or too impure or both at the same time but different impurities.

And please report the results.

Switch to aluminum if allowed: fewer problems.

RHABE

Hi RHABE

i am preforming SEM-EDX analysis on the coating layer, result will be report later.

regarding to " remelt with a short pluse of high intensity light" i had trial but the color was not change and remain black, please describe in more detail why the surface roughness can cause the black,

thank you,

Hi,

if you have plate-like crystals that are oriented near vertical to the surface then the light gets trapped and may exit only after many reflections.

The steeper the flanks the more reflections. And some considerable amount of stray-light and diffracted light - into any direction - as the edges and corners will act as stray centers. And the roughness of individual crystals too.

As a considerable percentage of light intensity is lost at any reflection (usually around 20 ... 30% for non-optimised reflecting surfaces) it takes only 7 to 10 reflections to have a backscattered intensity near 1% of incident light.

This is known in some special galvanic coatings (chromium black) but the shape of the absorbing structures is more like mushrooms there.

All this is true only if the size of the crystals is above the wavelength of the light. If near the wavelength then calculation is much more difficult (solving electromagnetic theory at a conducting irregular surface).

At structure dimensions much below the wavelength of light the roughness is generating stray light and the geometry is generating reflection.

As your remelting attempt did not give a reflecting surface I assume there was no remelting but your metal is considerably mixed with oxides.

We had this effect at our early experiments with a commercial coating equipment - this had only one Viton O-ring for sealing (400mm diameter) and temperature was around 80°C there. This resulted in heavy leakage (pressure still ok) and oxidation of aluminum surface to any color between black and white and any reflectivity but nothing mirror-like.

What about resistivity and soldering possibility?

RHABE

Thank you RHABE,

The SEM analysis shown not different in terms of composite scanning between the black layer with the silver color layer, Am I choosing a wrong analysis? or what will be the best analysis method?

the resistivity is high, as the part is plastics substrate, i am not able to preform the soldering test,

regards,

surface

Hi,

what happens if you put a short piece of new soldering wire (PbSn or SnIn) on top of the metalised areas and heat this up from above with light or hot air or a soldering iron and cool from below to prevent melting of the plastic substrate?

If not wetting shortly after melting then the metalisation is contaminated.

Is your SEM anylysisi sensitive to Carbon, Oxygen and Nitrogen? You need these to judge if clean.

RHABE

Hi.

the SEm result shown the present of this element; Carbon, oxygen, silicon & sulfur for the standard specimen, while the turn back specimen contained; Carbon, oxygen, silicon, sulfur & Chlorine. Is tin react with Chlorine during solar test to form the black layer?

regards,

Surface