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topic 60496

Non porous and non conductive coating for stainless steel

August 21, 2017

Q. I have a 302SS part that we need to apply an electrically insulating coating to, or some other way render the surface non-conducting. A search of the site left hints of ideas but none that jump out as a clear solution for our needs.

More specifically we're trying to apply a non-conductive surface that we can then copper plate (either electroless or electrolytic plating are options). There can be NO electrical connection between the copper layer and the underlying stainless steel, so the non-conductive surface needs to be free of porosity. The copper surface will then be etched to create individual conductors. We'd like the insulating layer to be less than 5 microns in thickness if possible. The entire device will need to be able to withstand temperatures of 350 °C for 24 hours without degradation.

We've extensively researched the use of parylene and numerous polyimide formulations to create a thin coating over the surface. We've made significant progress with a polyimide coating but micropores that lead to shorting between the copper and SS layers remains a challenge.

If there's an surface treatment of the SS that would get us close then I'm fine with coating this with a thin polyimide to give us the robust electrical isolation that we need.

Phil Houle
Engineer - Sunnyvale, California, USA
  ^- Privately contact this inquirer -^


August 2017

A. Hi Phil. No answers, just food for thought: Electrocoating works by putting parts into an aqueous solution and applying electricity which causes the ions to reduce into a non-conductive paint, which then causes the electricity to flow to any area that is not covered. The deposit thickness is very even, and any pinholes should get covered because of its intrinsic nature. But 5 microns is pretty thin, and 350 °C sounds quite high. If the part could be made of aluminum it could be anodized to non-conductivity, but again, 5 microns isn't much. The people who make exquisite chandeliers do not apply one heavy coat of lacquer but about 5 very highly thinned coats, and that may be guidance towards a pinhole-free surface. Good luck.

Regards,

pic of Ted Mooney
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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


simultaneous August 22, 2017

A. Phil,
More food for thought, given the coating you want to apply, there's no particular reason this part needs to be made out of stainless steel.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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August 24, 2017

A. This is a tricky one! Surely there must be a different way to design your actual component which would avoid this issue?

Just to take you in completely a different direction have you looked into HVOF (High Velocity Oxygen Fuel) this could be used to apply cermet materials with a high level of ceramic which would certainly do the job of electrical insulation and these coatings are dense with an extremely low porosity. You may need to do two layers. A layer of primarily ceramic and then perhaps a layer with more metal in order for the copper to be bonded better.

This process isn't really suited for small or intricate components and is certainly not cheap but I think would fulfill your criteria.

Along with what Ted was saying about Anodising, and thinking along the lines of ceramic I have seen that some companies that offer ceramic anodising I can't claim to know much about it, but again something to have a look at?

Oliver Gwynne
Bronte - Bradford, West Yorkshire


August 24, 2017

Q. Thanks for the thoughts. Unfortunately the SS core is extremely critical to the function of the device. It provides very specific mechanical characteristics as well as serving as one of the electrical paths for the system, which is why we need total electrical isolation from the plated on copper layer.

I found reference in the literature to "Zirconia Coating on Stainless Steel Sheets from Organozirconium Compounds", which seems to hold promise.

Phil Houle [returning]
- Sunnyvale, California


August 24, 2017

A. How about PVD? In the same vacuum chamber you can coat an insulating film like TiO2 or alumina and then coat copper without breaking vacuum. Porosity may not be an issue. But one may have to consider issues like diffusion of Cu at 350 °C and breakdown voltage of the insulating layer.

H.R. Prabhkara
Bangalore Plasmatek - Bangalore, Karnataka, India



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