Hard chromium plating has a wide range of applications where wear and a low coefficient of friction are considerations.

Applications

In our Laboratory, we see hard chromium used for rebuilding worn or mis-manufactured parts, original manufacture and wear resistance of tooling and gages. It's also used for protection of molds running corrosive or abrasive plastics and to provide mold release characteristics.

Typical chromed parts would be: automotive valve stems, piston rings, shock rods, Mac Pherson struts, the bores of diesel and aircraft engine cylinders, and hydraulic shafts.

Hard chromium plating is commonly used to restore original dimensions to worn surfaces of large crank-shafts for diesel, gas engines and compressors. Hard chrome also finds use in the repair of impaired printing and paper making roll bearing journals.

Hard chromium deposits are intended primarily to increase service life of functional parts by increasing their resistance to wear, abrasion, heat and corrosion. Plus hard chrome is used to restore dimensions of undersized parts.

Hardness

Hardness testing of thinly plated parts can be a problem. Typical Rockwell regular and superficial tests cannot be employed since they read an average between the plated surface and the substrate hardness. Transverse metallography and microhardness is both destructive and expensive. The use of a witness sample of the parent material that is plated at the same time can be helpful in determining hardness.

Also, since the hardness testing results will vary, depending on the deposition rate, it will make a difference whether testing is near an inside corner, on a flat, or at an outside corner.

Although not a quantifiable test method, it often works out that a more satisfactory method is to establish acceptable and unacceptable limits for the surface appearance. Hardness correlates pretty well with appearance, so some agreed upon reference surfaces can be made up and set aside to establish standards.

Metals Handbook, Volume 5, 9th edition, pg. 84, offers an informative table. The scratch hardness scale that they use is somewhat arbitrary.

You can write in your Purchase Order a hardness requirement based strictly on appearance. If you really insist on having a hardness requirement then you will have to specify all of the testing details: exact location, thickness range where the test was taken, microhardness scale such as Vickers or Knoop, load on indentor, method of sample preparation and retest privileges.