Letter 6561

Hard Chrome Plating: Hardness vs. Thickness 

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I would like to know if there is a way to determine what thickness of hard chrome plating will yield a hardness of 69-71 Rockwell C on a substrate of 32-36 Rockwell C.

I am attempting to achieve this surface hardness with a thickness of 1.5 mils. Is this possible.

Also, in general, where could I find data on the correlation between coating thickness and surface hardness for various coatings and substrates.

Barry M. Burnett
- Pacolet, SC, USA

All chrome plating is very hard. Nor does it, to my knowledge, get harder as the thickness increases. Rather, the indentor simply encounters more difficulty in cracking through the "egg shell".

So your question is not really how thick the chromium has to be before it gets hard, your question is really: "How thick does chromium have to be before my thickness tester will correctly measure what it's supposed to?"

I can't answer your question, but if you describe your tester and your technique, it's possible that someone who is more skilled in the met lab than I will be able to help you.

Regarding your more general question, most platings are at their hardest right at the interface with the substrate because of the tiny grain structure, as the atoms of one metal (the plating) try unsuccessfully to match the crystal spacing of another metal (the substrate). As the plating gets thicker, all other things being equal, the grain will get larger and softer as it is allowed to adopt its own natural structure.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Ted's comments above are excellent and valuable for those who always wonder why hardness varies with thickness or why some measurements are lower than expected.

I would like add that the minimum coating thickness at which you could measure the hardness with above 90% accuracy depends on the substrate hardness, surface smoothness, as well as the type of instruments used to measure hardness. Our observations on near-mirror finished smooth surfaces using microhardness tester indicated that, for a relatively soft substrate with about 20 to 25 HRc, you need at least 1 mil coating thickness. I suggest using a polishing method and microhardness tester to get accurate data. You will probably need to develop your own data for various coatings and substrates.

Mandar Sunthankar
- Fort Collins, CO

To expand on the comments above:

The accuracy of an indentation hardness test, such as Rockwell or microhardness methods, is a function of the depth of the indentation and the thickness of the material to be tested. The test will be affected by material within a depth of about 10 times the depth of the indenter penetration.

For Knoop microhardness, the load can be varied to test very thin coatings. At typical test loads, Mandar's recommendation of a 0.001 in. minimum thickness is about right. Specialized testers with capability for very low loads can test coatings with much lower thickness, i.e. down to a few microinches. These tests, however, cannot be readily converted to Rockwell hardness values.

ASTM gives minimum thickness requirements for each of the test scales based on the material hardness. The thinnest material or coating that can be tested by the Rockwell method is 0.006 in. for a material that is at least 60 HRC. This test could be done using the Rockwell 15N superficial hardness scale. Direct Rockwell C testing will test to a depth of 0.022 in. at a hardness of 69 HRC. Any thinner than this, and you will have substrate effects.

Hope this helps.

Larry Hanke materials testing laboratory Minneapolis, Minnesota

Is it possible to have an extremely abrasion resistant and durable hard chrome coating of thickness 0.3-0.5 mil . If so what should be the minimum substrate hardness for the same with the required surface hardness of 68 Rc.

Gaurav Garg
- New Delhi, India

Hello, Gaurav. Although I cannot answer your question exactly, I can rephrase it a little and say that I believe that hard chrome plating thicknesses of 0.3 to 0.5 mils are widely used on hard substrates. I think if you look into the ubiquitous "air springs" that are used to support automobile life gates and hoods that you will find chrome thicknesses in this range.

Ted Mooney, P.E. 
finishing.com
Brick, New Jersey

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