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Essentials of Microhardness Testing – Vickers and Knoop Hardness Tests Hardness test systems use an indenter probe which is displaced into a surface under a precise load. The indentation generally has a set dwell time. The depth or size of indentation is measured to establish hardness in traditional mechanical testing, 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 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 greater accuracy and repeatability of results, microhardness testing should account for sample size, environment and preparation effects. Samples should be perpendicular to the indenter tip and fit in the sample stage. A really rough surface could reduce indentation data’s accuracy; a tested method for polishing samples is the safest. It is important that the microhardness tester be isolated 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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There is, in the lower range of the applied load, some degree of operator bias that must be expected using this method. The length of indentation diagonals, as per ASTM E384-11, must be more than 17 microns in length. For coated samples with coating thicknesses less than 60 microns, this test does not apply. For several kinds of samples, the contact depth is different from the displacement depth since the surrounding material becomes elastically deflected during the indentation process. 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, 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. A particular load is applied for a pre-defined 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.