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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 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 covers testing that involves an applied load of more than 1 kg or roughly 10 Newton (N). With applied loads less than 10 N, microhardness testing is usually used for thin specimens, plated surfaces, thin films or smaller samples. Vickers and Knoop hardness tests are the two most common microhardness techniques used today. For more accuracy and duplicability of results, microhardness testing must account for the effects of preparation, environment and sample. 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. The microhardness tester must be in complete isolation from vibrations. Samples having many phases or variations in grain sizes require statistical data. Vickers Hardness
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In the Vickers hardness test, a Vickers indenter will be pressed against a surface at a pre-defined force held for about 10 seconds. With the indentation complete, the resulting indent will be scrutinized optically to measure the lengths of the diagonals, which will be used for determining the impression’s size.
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A certain degree of operator bias should be in this method, particularly in the applied force’s lower range. The length of indentation diagonals, as per ASTM E384-11, must be more than 17 microns in length. This test is not valid for coated samples with coating thicknesses below 60 microns. 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 Like the Vickers hardness test, the Knoop hardness test is also a microhardness technique. The procedure involves a Knoop indenter pushing into a surface in order to measure hardness. However, being more elongated or rectangular, the Knoop indenter is shaped uniquely from a Vickers indenter for microhardness, or a Berkovich indenter for nanoindentation. The Knoop hardness test method, which follows a very meticulous sample preparation process, is generally used on lighter loads set for microhardness testing. Knoop hardness testing is used on samples that need indentations to be close together or on the sample’s edge, with both benefitting from the unique 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. The indentation measurements that result from this are then converted to a Knoop hardness number with the use of a chart.