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 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. Macrohardness and microhardness are the two ranges of hardness testing. Macrohardness involves testing with over 1 kg or some 10 Newton (N) in applied load. Microhardness testing with below 10N applied loads is generally used for tiny samples, thin specimens, thin films or plated surfaces. Vickers and Knoop hardness tests are the two most common microhardness techniques used today. 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 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 must be in complete isolation from vibrations. For samples having several phases or grain size variations, statistical data will be required. 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. 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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A certain degree of operator bias should be in this method, particularly in the applied force’s lower range. 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. In addition, this effect also has an impact on 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, the more rectangular or elongated shape of the Knoop indenter makes it look different from a Vickers indenter, which is used in microhardness testing, or a Berkovich indenter, which is used in nanoindentation. The Knoop hardness test method is often used for the microhardness testing of lighter loads, and requires meticulous sample preparation. 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. The Knoop test method only makes use of the long axis, in contrast to the Vickers hardness method. Making use of a chart, the resulting indentation measurements will then be converted to a Knoop hardness number.

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