Amorphous Carbon Film Coating 2

[Vig16]

We're working with a piece of 17-4 PH stainless steel which is annealed. We had applied one coat of metal containing hydrogenated amorphous carbon film to the steel. The vendor then, without consenting us, coated the steel again; but this time with a hydrogenated amorphous carbon film (containing no metal).

My question is, by them doing this, will the two layers of film actually bond to each other? If so, how apt will the bond be to chipping?

 

[JTreglio]

The stress in the coating is compressive, so another factor is the curvature of the part. The coating will adhere better to convex surfaces than concave surfaces if the stress is significant.


Jim Treglio

 

[Vig16]

Thanks for your input guys!

I had another question about the electropolishing. At times, we’ve received parts of the same lot that have come back with different finishes on the 17-4. Some come back with a matte finish, while others come back with a shiny mirror-like finish. Is there any reason that you can see this happening? Could the location in the tank affect the finish of certain parts? All parts are fixtured in the same location, so that shouldn’t be a factor…

 

[kenvlach]

If the parts from the same lot are identical in alloy chemistry, heat treatment, machining or other mechanical treatment, and pre-cleaning, then the matte finish is poor electropolishing (etching) due to improper current density. A matte or snowy finish can also be from gas bubbles trapped on the surface.

But, more likely, the current density issue. Anode (part) to cathode spacing should be nearly uniform, both anode and cathode must have enough current carrying capacity (cross-section relative to surface area), plus the rectifier must be able to supply enough power to do the entire load simultaneously. If underpowered, do fewer parts at a time.

Please describe the relative geometries of shiny and matte parts vs. cathodes.

 

[Vig16]

The parts are all the same part and they are shaped like a "V" where the arms flex and crush a piece of titanium between the ends of the arm tips. The base of the V shape is a rectangle about a half inch long. The entire part is about 4" long, 1/8" thick, and the outermost tips of the top of the V shape are about 1" wide.

The parts are typically hung 3 pieces to one copper wire. Each one of the parts is about 2-3" from the closest part. All of the parts are always the same part when they are sent to be electropolished, so the part geometry is always constant.

Does this give you any more information that you didn't already know?

 

[kenvlach]

Do you know the wire size & whether the parts are positioned with the flat surfaces vertical (to avoid trapping gas bubbles)? Need 5-10 amps per part (assuming 8 sq. inches area).

IMHO, it's better to rack the parts (e.g., with copper, brass, or titanium spring clips on a copper or titanium rod) than hang from a wire. If loose or undersize wire, amperage to the part maybe below the critical current density (which varies depending on electrolyte, e.g., minimum 50 Amp/sq. ft. for 63 wt% H3PO4-15 wt% H2SO4-22 wt% H2O).

It's also essential that parts be clean (no oil!) when electropolishing. If the racker has dirty hands or gloves... See any oil sheen on the electrolyte surface?

Surfactant in the EP solution helps if parts are less than perfectly clean. Also, helps gas bubbles exit surface of part.

 

[Vig16]

After seeing the wire, I can recall it being about 1/8" diameter. As far as the orientation of the flat surfaces of the part, I don't recall them having them set in any particular way.