Metal finishing Q&As since 1989

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PVD process is giving inconsistent "Brass" color

August 18, 2011

The color required is that of polished brass C36000 or C38000, applied as PVD to polished 304 grade stainless steel flat material. Variations between batches are being blamed on preparation, but the differences between surface roughness are minute. The suggestion is that polishing residue can also be the problem, causing contamination within the work chamber. The difference can sometimes be seen within one batch where the majority of the color is acceptable but a selection of parts are very light.

My problem is establishing just how consistent the process can be, how much it depends on the supply condition and what variables the PVD process offers for "tweaking".

Also, what is the usual practice for setting color range limits? Should there be one target color, leaving the vendor to get it as close as possible, or is it common to select light and dark limits to stay within?

Noel Woolley
product designer - Brisbane, Queensland, Australia

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If you are seeing a lighter color in different regions of the array, it is likely due to outgassing, where the gasses coming off of the part replace or reduce the reactive gas(es) in the chamber. This can be reduced by increasing coating time, or, preferably, doing a better job cleaning the parts before coating. If the light color appears always at the top of the chamber or bottom of the chamber, you could have a gas flow problem. Check to make sure the holes in the gas feed manifold are not clogged.

I don't know of anyone allowing a range of colors, especially for the polished brass appearance. The general goal is to be consistent.

Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover - San Diego,
California
August 22, 2011