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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. Hardness testing comes in two ranges: macrohardness and microhardness. Macrohardness includes testing with an applied load above 1 kg or around 10 Newton (N). Microhardness testing with below 10N applied loads is generally used for tiny samples, thin specimens, thin films or plated surfaces. The two most popular microhardness testing techniques used nowadays are Vickers and Knoop hardness tests. For more exact and repeatable results, microhardness testing has to be responsible for the effects of sample preparation, size and environment. Samples should be perpendicular to the indenter tip and fit in the sample stage. 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. For samples having several phases or grain size variations, statistical data will be required. 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. 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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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 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. Besides the above, microhardness data accuracy and precision will also be influenced by this effect. Knoop Hardness Like the Vickers hardness test, the Knoop hardness test is also a microhardness technique. The process involves a Knoop indenter pressing into a surface for measuring 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 is often used for the microhardness testing of lighter loads, and requires meticulous sample preparation. 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. For a pre-set dwell time, an assigned load will be applied. 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.