Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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control, analysis, and testing MICROHARDNESS TESTING OF PLATED COATINGS BY JOHN D. "JACK" HORNER CONSULTANT, MILFORD, MICH. Hardness has been defined simply as the resistance to penetration. In order to mea- sure this resistance to penetration a testing machine is used to push a specific inden- ter into the test material with a specified force, and either the depth of penetration or the area of the "footprint" left by the indenter is used to derive a hardness "number." Hardness as measured by indentation is not a single fundamental property but a combination of properties, and the contribution of each to the hard- ness number varies with the material and the type of test. Variously shaped inden- ters have been developed and are in use in different industries. The hardness number varies with the size and shape of the indenter and the force used, among other variables, which results in several different hardness scales. The metalwork- ing industry uses several with one or more of the many Rockwell scales being among the more common. Unless the coatings are extremely thick Rockwell methods cannot be used for plated parts. The results of hardness testing of plated coatings have been the subject of dis- agreements for several years. Standardized methods such as ASTM E 384 (Standard Test Method for Microhardness of Materials), which had been devel- oped to measure the hardness of solid materials, were found to need certain mod- ifications to be adapted to relatively thinly plated coatings. This problem was rec- ognized with ASTM B 578 (Standard Test Method for Microhardness of Electroplated Coatings). The term microhardness is used to describe hardness as measured with small indenters and comparatively lower forces. (Some have pro- posed that microhardness would be more accurately described as microinden- tation.) Generally, the term microhardness is used for hardness testing when the forced used is 1,000 grams or less. WHICH HARDNESS SCALE TO USE There are two indenters commonly used for testing plated coatings. The Vickers indenter has been in use longer and is a small, pyramid-shaped diamond that pro- duces a square footprint or impression. The Knoop indenter was developed by the National Institute of Standards and Technology (NIST) and is a small, rhom- boid diamond that produces a longer, thinner, less-penetrating impression. This feature makes it more adaptable for thinly plated coatings. Currently, the ASTM specification specific for microhardness testing of plated coatings is B 578 and cov- ers only the Knoop indenter. It is expected that future revisions of this specification would include the Vickers indenter. It is important to know that these two inden- ters are not the same and microhardness numbers, although fairly close, are not interchangeable. There are some advantages to using each method. Generally, Knoop is preferred for thinner coatings because of its narrower indent and because it penetrates only about one-half as deep as the Vickers indenter. The Vickers indenter is reported to be more tolerant of slight surface irregularities. Modern test- ing machines permit the use of either indenter. If necessary to cite an ASTM Standard Test Method the Knoop indenter has the advantage currently. 511

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