Smart Ideas: Tests Revisited

Microhardness Testing Essentials – Vickers and Knoop Hardness Tests Hardness test systems use an indenter probe which is displaced into a surface under a precise load. The indentation normally comes with a defined 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 more accuracy and duplicability of results, microhardness testing must account for the effects of preparation, environment and sample. 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 must be in complete isolation from vibrations. If samples vary in grain size or have several phases, statistical data is a must. 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. With the indentation complete, the resulting indent will be scrutinized optically to measure the lengths of the diagonals, which will be used for 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. The length of indentation diagonals, as per ASTM E384-11, must be more than 17 microns in length. This test is not valid for coated samples with coating thicknesses below 60 microns. 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. In addition, this effect also has an impact on accuracy and precision for microhardness data. Knoop Hardness Like the Vickers hardness test, the Knoop hardness test is also a microhardness technique. The procedure involves a Knoop indenter pushing into a surface in order to measure 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, which follows a very meticulous sample preparation process, is generally used on lighter loads set for microhardness testing. 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. Using a chart, the indentation measurements that come out of this are then converted to a Knoop hardness number.