Letter 25628

Best way to measure chrome plating hardness  

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Which is the best instrument to measure hardness of hard chrome plating? Can we use Ultrasonic hardness tester for this purpose??

FINNY VARGHESE
Petrochemical industry - BARODA, GUJARAT, INDIA

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.

James Watts
- FL

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 [link is to spec at TechStreet]-99e1 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 E3-01 Standard Practice for Preparation of Metallographic Specimens.

Use the finishing.com search engine for coating microhard Knoop vicker’ 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

Ken Vlach - Goleta, California

The above procedure is partly from ASTM B578-87(1999) Standard Test Method for Microhardness of Electroplated Coatings. This is the Knoop microhardness test required for Cr plating per ASTM B-177 [link is to spec at TechStreet] and also for electroless nickel per MIL-C-26074E and ASTM B733 [link is to spec at TechStreet], Class 2. An advantage of this method is that the samples can also be used for microscopical thickness measurement per ASTM B487 [link is to spec at TechStreet]. 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 (http://www.finishing.com/Testing/fischer.html) 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.

Ken Vlach
- Goleta, California

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