Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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doing this unless using conversation data actually obtained from the specific material being tested. Hardness numbers should be expressed in the scale that they were tested. Similarly, conversion from one test load to another is to be dis- couraged without actual data obtained with the same test material. Measuring hardness of nonuniform deposits: Porous deposits, deposits that contain codeposited hard or soft particles (often termed composites) will produce unre- liable readings and should not be tested for hardness in the composite form using microindentation methods. Deposit too thin: Indentation made too close to the edge of the deposit. Very brittle deposits: These often crack, which is not always visible, and given unre- liable results. The temptation is to use less force, which produces much higher numbers and is not comparable to data obtained at the recommended load of 100- gram force. This is the minimum force required for harder materials by the per- tinent hardness specifications. Including "unusual indentations" in the averages: An indentation that is noticeably different, either in shape or size, from other indentations is probably the result of an unknown test variable and not a true reflection of the hardness and should be rejected. CONCLUSION Microhardness testing of plated coatings, or more properly, microindenta- tion test methods for hardness of these materials, can be a good tool in the study of materials, development of improvements, process and quality control, and other applications. The standard test methods, when followed closely and with due attention to the procedures and variables contained therein and in this article can produce dependable and repeatable results within practical limits. In the real world, however, these procedures have not always been followed. The reader is cautioned that there are data in the literature, both old and recent, that are incompletely presented and difficult to duplicate or to compare with data from other sources. The reliability of the test method has been found to improve with the number of indentations and the number of measurements if all the necessary precautions are taken and if the operator is knowledgeable on the procedure and has adequate equipment. It appears inherent to the method that wider ranges are found with the harder materials. Of the two common indenters Knoop has the smaller range when testing the harder materials. Those writing specifications and setting limits on hardness should be well aware of these factors. 515

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