Opinion for Hardness Testing

I would be inclined to ask: "what tester and procedure has stood the test of time with your client(s)...?"

Hardness values are, for all intents and purposes, arbitrary numbers, for which there are no absolutes standards. Hardness is not a fundamental property of a material, but rather, a response to a particular test method, following a given procedure.

Uncertainties in hardness testing have been recognized within many segments of industry since shortly after such testing first came into being. A CoP (Code of Practice) was prepared (c 2000) within UNCERT, a project funded by the Standards, Measurement and Testing Programme (under SMT4-CT97-2195) to simplify the way in which uncertainties are evaluated.

The work performed (for example) by S.N. Petrenko, in the "20's (attempting to wipe the slate clean, of the disrepute surrounding Brinell hardness testing) was only half the battle, in THAT arena (of hardness testing). Google "ACTA IMEKO Sept 2014 Proposed definition for the Brinell hardness indentation edge".

Any hardness tester "worthy of its name" (for the type of test required) could give satisfactory results, if it is used according to an agreed-upon procedure, using agreed-upon calibration references (and, of course, used by personnel who are qualified to use same!)

Find the GUM document ("Guide to the expression of uncertainty in measurement": 5+ separate pdfs) and peruse the section on hardness testing. I guarantee that it will "open your eyes" to some potential pitfalls in your process, and keep you from appearing as a "ninnyhammer".

"Cheers", and best wishes.