Microhardness Testing Essentials – Vickers and Knoop Hardness Tests

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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. 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. 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 Los Angeles 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 must be in complete isolation from vibrations. If samples vary in grain size or have several phases, statistical data is a must.

 

Vickers Hardness

 

The Vickers hardness test utilizes a Vickers indenter that is pushed into a surface at a particular force sustained for around 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.

 

Some degree of operator bias in this procedure must be expected, specifically in the lower range of the applied load. As per ASTM E384-11, the length of indentation diagonals must be greater than 17 microns. 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. Aside from the above, this effect will also influence accuracy and precision for microhardness data at http://burbanksteel.com/services/.

 

Knoop Hardness

 

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, 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 is often used for the microhardness testing of lighter loads, and requires meticulous sample preparation. 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. Making use of a chart, the resulting indentation measurements will then be converted to a Knoop hardness number.

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