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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 normally comes with a defined dwell time. Traditional mechanical testing calls for the measurement of size and depth as a way to determine hardness. Hardness testing comes in two ranges: macrohardness and microhardness. Macrohardness involves testing with over 1 kg or some 10 Newton (N) in applied load. Microhardness testing that has applied loads not reaching 10 N, is often reserved plated surfaces, thin films, smaller samples or thin specimens. There are two very common microhardness methods used today, and they are the Vickers and Knoop hardness tests. For greater accuracy and repeatability of results, microhardness testing should account for sample size, environment and preparation effects. Samples have to fit in the sample stage and be in a position that is perpendicular to the indenter tip. A really rough surface could reduce indentation data’s accuracy; a tested method for polishing samples is the safest. The microhardness tester should be totally separated 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. 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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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, this test will not apply for coating thicknesses not reaching 60 microns. 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 Another microhardness technique is known as the Knoop hardness test, which is similar to the Vickers hardness test. The process involves a Knoop indenter pressing into a surface for measuring 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, 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. An assigned load will be used applied for a particular dwell time. Unlike the Vickers hardness method, the Knoop test method solely uses the long axis. The indentation measurements that result from this are then converted to a Knoop hardness number with the use of a chart.