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Microhardness Testing – The Difference between 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 usually has a pre-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. There are two very common microhardness methods used today, and they are the Vickers and Knoop hardness tests. For greater accuracy and repeatability of results, microhardness testing should account for sample size, environment and preparation effects. Samples should fit in the sample stage and lay perpendicular to the tip of the indenter. A very rough surface may decrease the accuracy of indentation data; an established method for polishing samples is best to use. It is important that the microhardness tester be isolated from vibrations. Samples having many phases or variations in grain sizes require statistical data. Vickers Hardness
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The Vickers hardness test utilizes a Vickers indenter that is pushed into a surface at a particular force sustained for around 10 seconds. Once the indentation is completed, the resulting indent is examined optically to determine the lengths of the diagonals, which is important in determining the size of the impression.
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There is, in the lower range of the applied load, some degree of operator bias that must be expected using this method. As per ASTM E384-11, the length of indentation diagonals must be greater than 17 microns. For coated samples with coating thicknesses less than 60 microns, this test does not apply. For various kinds of samples, the contact depth is not the same as the displacement depth because of the surrounding material that gets elastically deflected during indentation. Aside from the above, this effect will also influence accuracy and precision for microhardness data. Knoop Hardness Similar to the Vickers hardness test is the Knoop hardness test, another microhardness technique. The method requires a Knoop indenter pushing against a surface as a way 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 done on samples requiring indentations to be close together, or on the tip of a sample, both being benefitted by the different probe shape. For a pre-set dwell time, an assigned load will be applied. Unlike the Vickers hardness method, the Knoop test method solely uses the long axis. Making use of a chart, the resulting indentation measurements will then be converted to a Knoop hardness number.