Best way to measure chrome plating hardness

A. I am from the old and broke school of chrome plating. The only "hardness" test that I am aware of is a microhardness test which requires a test piece or destructive testing of a part.

A. Agree with Mr. Watts. Hardness testers used for bulk materials are unsuitable for thin coatings. A sizable investment is required for proper sample preparation and measurement, so unless you already have metallography facilities, first check for a nearby metallurgical testing laboratory.

I suggest a Knoop microhardness tester, as it can measure thinner coatings than Vickers testers. Instruments with switchable Knoop and Vickers indenters are available if you also wish to measure hardened steel. The measurement method is given in ASTM E384 Standard Test Method for Microindentation Hardness of Materials. Also ISO 14577. For sample preparation (slice and dice with a diamond wafering saw, mount in a hard plastic, polish to a near-mirror finish), see ASTM E3 Standard Practice for Preparation of Metallographic Specimens.

Use the finishing.com search engine for coating microhard Knoop vickers for additional info, e.g., letter #6561

Some technical features of Microindentation Hardness Testing are given at http://www.metallography.com/amp/micro.htm Both bulk and microhardness methods are described at http://www.leco.com/customersupport/met-tips/met_tip9.pdf

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Ed. note Sept. 2019: That leco.com link is now broken.

A. The above procedure is partly from ASTM B578-21 Standard Test Method for Microhardness of Electroplated Coatings. This is the Knoop microhardness test required for Cr plating per ASTM B177 and also for electroless nickel per MIL-C-26074 [on DLA] and ASTM B733, Class 2. An advantage of this method is that the samples can also be used for microscopical thickness measurement per ASTM B487. A disadvantage is that the coating thickness must be at least 25 microns (1 mil) for hard metals Cr, Ni, etc. and 38 microns (1.5 mil) for soft metals Ag, Au and Cu.

I thank Toby Padfield for pointing out elsewhere that Fischer Technology makes a highly sensitive microhardness tester capable of measuring coatings only 1 micron thick. As such, it can measure a very flat plated surface (polished) directly, without the cross-sectioning procedure required by the ASTM procedure.

A. For chrome, a micro hardness test is required! This is a destructive test.
If you can guarantee that the shaft underneath is always within a tight hardness range, you can do destructive microhardness tests on a series of dummy shafts and compare them to rockwell C readings. This will give you an approximation of the real hardness. The chrome has to be of identical thickness as well as the shafts being of identical hardness.
In short, the only true answer is microhardness which is not quick, cheap or easy for poorly qualified help.

A. George,

Microhardness should really be done on the cross-section of the plating to get a true hardness. If you were to plate a test piece along with the job it could then be tested rather than the part.

It is possible to test the part, but it will leave a small dent where the diamond indenter lands on the surface. This just means that you need to test in a non-critical area of the part.

Finally ASTM B578-21 describes microhardness testing of electroplated deposits.