Hardness testing of plated/coated parts

I would think that the answer would depend on your particular requirements and whether you are looking for the hardness of the part itself, seperate from the coating. Or if you need to know the hardness with the coating intact. The hardness tester will measure the depth of the diamond indentation with a precise preload and full load pressure. If the coating is significantly softer than the underlying part, the measurement may be the same or close either way. When I used a Rockwell Tester, I was required to remove all paint, coatings, etc. down to bare metal with scotchbrite prior to applying the tester. Perhaps if you provide a little more information concerning your requirements, we can give you a better answer.

Richard

Thank you for the response. The requirement is not clearly identified. I tested both coated and uncoated material and the results were the same. Only one hardness spec. is called out and it doesn't state core or outter surface. The coating on these parts is strictly a preservative application only. Not attributting to the hardness. Sorry if this is vague.

Found a link on your process

http://www.burlingtoneng.com/wear_resistance.html

But still you have quite a bit of case depth.

Nice answer sb.

milo

DPH...diamond, Penetrant, hardness. It is a destructive test used widely in the Aircraft industry. It will allow you to set up production parameters and random sample your products. It is performed using a cross-section of the part and measured optically using a microscope and viewing the diamond impression left behind after an applied force is added the area to be measured.

Keeping it simple, nitrocarburize is not a coating process it is a case hardening heat treatment.You did not state what the base metal (substrate is). The hadness achieved by nitrocarburizing will be higher than the range you gave; as SB has correctly pointed out, it is logical to deduce that the 27-34 HRC is likely to be a core hardness.

The proper way to take such a hardness is through a transverse section of the party, NOT THE SURFACE.

AND NOT IN THE NITROCARBURIZED LAYER< unless the print is indeed calling for hardness test on the surface.

Typical practice is to do core tests atr the mid radius of transverse section.

If you have come up with a way to take off the nitrrocarburized coating without affecting the hardness of the base material, please contact me at once and I will put a consortium together to purchase the rights to your process!!! (The nitrocarburise is a diffusion of chemicals in to the substrate and their subsequent transformation- either you remove this layer by machining, or else ???)

milo

I beleive this is a discussion that you should be having with your customer. With that said, the Rockwell C scale measures through hardness, the hardness of the entire part. The hardness of the surface coating would be measured using one of the superficial hardness scales.

I'm not familiar with the specifics of Melonite "QPQ"Nitrocarburizing surface process, but it appears to be a wear resistance coating with only superficial penetration and not measurable on the Rc scale.

I hope this helps.