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Impermeable coating for 200 micron diameter alloy spheres

I am working on a research project that requires a non-destructive method of plating small diameter spherical particles made of a steel alloy. The objective is to prevent hydrogen transfer through the coating into the particle, which creates embrittlement. Our research group has utilized electroless nickel and electroless copper plating with limited success, as the thin coatings required by the project do not seem to be sufficiently impermeable. Is there a method available for plating 200-300 micron particles with a more resistant barrier such as chrome or PVD aluminum?

Jeremy Chell
Mechanical Engineer - Madison, Wisconsin, USA
April 17, 2009

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Embrittlement is not absolutely a bad word like cancer, but it is an unwarranted bogeyman to many.
First it is not really a problem until above RC39.
Second, you can diminish it massively by acid activating the material only enough to work. Arbitrary etches such as 50% HCl for 3 minutes are not required for clean parts without heat treat scale. If so, blast the parts to remove the scale.
Third, bake the parts for 4 hours at 375F immediately after the final rinse and blow dry.
Or, is there something about the situation that I do not understand?

James Watts
- Navarre, Florida
April 27, 2009

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May 5, 2009

Hydrogen embrittlement is a VERY bad word if applied to, e.g., Aircraft undercarriage parts!
The point is that Jeremy needs to avoid it for a research project.
The first problem is that the substrate is 200-300 micron particles - difficult to see how he could blast clean these.
Any electrolytic finish is going to be a problem to apply.
Chromium is particularly difficult as it generates great quantities of hydrogen. This is usually baked out - easy because chrome tends to be either micro-cracked or porous. The problem is that it will not keep hydrogen out.
Try http://www.rebresearch.com/H2perm2.htm for quantitative permeability.
As you will inevitably have to use a thin coating, gold or silver may be the best bet with copper a useful third.
Electroplated gold is non porous at about 5 microns on a decent surface.

Geoff Smith
Hampshire, England

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Yes, it it is possible to coat the steel balls with aluminum, titanium etc by PVD. The problem could be in coating the entire surface uniformly. This would need some special arrangement to tumble the steel ball during coating.

H.R. Prabhakara - Consultant
Bangalore Plasmatek - Bangalore Karnataka India
June 25, 2009