Essentials of Microhardness Testing - Vickers and Knoop Hardness Tests
Hardness test techniques utilize an indenter probe displaced into a surface under a defined load. The indentation generally has a set 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 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 at http://burbanksteel.com/services/ 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 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 should be totally separated from vibrations. If samples vary in grain size or have several phases, statistical data is a must.
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. 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.
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. Aside from the above, this effect will also influence accuracy and precision for microhardness data at http://burbanksteel.com/services/.
Similar to the Vickers hardness test is the Knoop hardness test, another microhardness technique. 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.
A particular load is applied for a pre-defined dwell time. The Knoop test method only makes use of the long axis, in contrast to the Vickers hardness method. Making use of a chart, the resulting indentation measurements will then be converted to a Knoop hardness number.