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By | May 17, 2017

Microhardness Testing Essentials – 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. The depth or size of indentation is measured to establish hardness in traditional mechanical testing, 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). Microhardness testing with below 10N applied loads is generally used for tiny samples, thin specimens, thin films or plated surfaces. There are two very common microhardness methods used today, and they are the Vickers and Knoop hardness tests. 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. For samples having several phases or grain size variations, statistical data will be required. Vickers Hardness
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The Vickers hardness test makes use of a Vickers indenter that is pressed against a surface to a pre-determined force maintained usually for 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. 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 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. Besides the above, microhardness data accuracy and precision will also be influenced by this effect. Knoop Hardness Another microhardness technique is known as the Knoop hardness test, which is similar to the Vickers hardness test. The procedure involves a Knoop indenter pushing into a surface in order 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 follows a very meticulous sample preparation process, is generally used on lighter loads set 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. The Knoop test method only makes use of the long axis, in contrast to the Vickers hardness method. Using a chart, the indentation measurements that come out of this are then converted to a Knoop hardness number.