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"Scratch resistance of various polished metals"
January 26, 2019
Q. Hello everyone,
I am currently designing some belt buckles made of zinc and already received some of my designs from a prototyping company. However, since I am completely clueless about surface finishing, I simply said that the surface should be polished. I received polished belt buckles made of zinc but they quickly show scratches.
I don't want to make this mistake again, so here are several beginner questions and I highly appreciate every answer:
1. What is usually the least expensive finish to make zinc scratch resistant?
2. When applying gold/silver/chrome/etc. plating, does this make the material automatically scratch resistant?
3. What kind of finish is typically used for people with a nickel allergy?
4. Do you have some kind of list or resource where possible decorative finishes are listed and shown with pictures?
Thank you very much,
A. Hi Martin. Zinc is fairly soft, about 2.5 on the Mohs scale. Nickel is a bit better, about 4.0; chrome is the hardest decorative metal finish, about 8.5-9.0. Some PVD decorative finishes are even harder because they are ceramics rather than metal.
1. The least expensive finish might be a clearcoat (automotive clearcoat paint, UV-hardened nail lacquer). Good ones are probably harder than zinc but not as hard as chrome, but because the coating is clear, scratches may be less obvious.
2. Chrome is significantly harder than zinc; gold and silver are not.
3. White bronze (an alloy of three metals) is sometimes used as a nickel replacement. Cobalt is also sometimes used, but the jury is not in on whether cobalt is really any different than nickel in allergic reactions.
There are so many variations in finishes that I'm not aware of any useful primer with pictures. If you are going into significant production rather than just custom craftwork there are finishing consultants who you can retain to guide you.
Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey