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Essentials of Microhardness Testing – Vickers and Knoop Hardness Tests Hardness test procedures make use of an indenter probe that is displaced into a surface under a particular load. The indentation generally has a set dwell time. Traditional mechanical testing requires the measurement of the indentation’s size or depth in order to determine hardness. There are two ranges of hardness testing – macrohardness and microhardness. Macrohardness involves testing with over 1 kg or some 10 Newton (N) in applied load. Microhardness testing that has applied loads not reaching 10 N, is often reserved plated surfaces, thin films, smaller samples or thin specimens. The two most popular microhardness testing techniques used nowadays are Vickers and Knoop hardness tests. For more accuracy and duplicability of results, microhardness testing must account for the effects of preparation, environment and sample. Samples have to fit in the sample stage and be in a position that is perpendicular to the indenter tip. A particularly rough surface can diminish the accuracy of indentation data; it is best to use a tested and proven procedure for polishing samples. The microhardness tester should be totally separated from vibrations. Samples having many phases or variations in grain sizes require statistical data. Vickers Hardness
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The Vickers hardness test makes use of a Vickers indenter that is pressed against a surface to a pre-determined force maintained usually for 10 seconds. As soon as the indentation is done, the indent will be checked optically to measure the lengths of the diagonals, which is key to determining the impression’s size.
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Some degree of operator bias in this procedure must be expected, specifically in the lower range of the applied load. As per ASTM E384-11, the length of indentation diagonals must be greater than 17 microns. This test is not valid for coated samples with coating thicknesses below 60 microns. For a lot of sample types, the contact depth is not similar to the displacement depth, because surrounding material gets elastically deflected during the process of indentation. In addition, this effect also has an impact on accuracy and precision for microhardness data. Knoop Hardness Another microhardness technique is known as the Knoop hardness test, which is similar to the Vickers hardness test. The procedure involves a Knoop indenter pushing into a surface in order to measure hardness. However, with its more rectangular or elongated form, the Knoop indenter looks different from a Vickers indenter for microhardness testing or a Berkovich indenter, which is used for nanoindentation. The Knoop hardness test method, which demands a painstaking sample preparation process, is normally used on lighter loads for microhardness testing. Knoop hardness testing is used on samples where indentations have to be close together, or should be on a sample’s edge, both gaining benefits from the different probe shape. An assigned load will be used applied for a particular dwell time. As opposed to the Vickers hardness method, the Knoop test method only utilizes the long axis. Making use of a chart, the resulting indentation measurements will then be converted to a Knoop hardness number.