Vickers Hardness Testing
It is the standard method for measuring the hardness of metals, particularly those with extremely hard surfaces: the surface is subjected to a standard pressure for a standard length of time by means of a pyramid-shaped diamond. The diagonal of the resulting indention is measured under a microscope and the Vickers Hardness value read from a conversion table.
Vickers hardness is a measure of the hardness of a material, calculated from the size of an impression produced under load by a pyramid-shaped diamond indenter. Devised in the 1920s by engineers at Vickers, Ltd., in the United Kingdom, the diamond pyramid hardness test, as it also became known, permitted the establishment of a continuous scale of comparable numbers that accurately reflected the wide range of hardnesses found in steel.
The indenter employed in the Vickers test is a square-based pyramid whose opposite sides meet at the apex at an angle of 136. The diamond is pressed into the surface of the material at loads ranging up to approximately 120 kilograms-force, and the size of the impression (usually no more than 0.5 mm) is measured with the aid of a calibrated microscope. The Vickers number (HV) is calculated using the following formula:
HV = 1.854(F/D2),
with F being the applied load (measured in kilograms-force) and D2 the area of the indentation (measured in square millimetres). The applied load is usually specified when HV is cited.
In this test, a small pyramidal diamond is pressed into the
metal being tested. The Vickers Hardness number (HV) is the ratio of the load
applied to the surface area of the indention. This is done with the following
formula.
The indenter is made of diamond, and is in the form of a
square-based pyramid having an angle of 136 degrees between faces. The facets
are highly polished, free from surface imperfections, and the point is sharp.
The loads applied vary from 1 to 120 kg; the standard loads are 5, 10, 20, 30,
50, 100, and 120 kg. For most hardness testing, 50 kg is maximum.
A Vickers hardness tester should be calibrated to meet ASTM standard E10 specifications, acceptable for use over a loading range.
Vickers Hardness Testing
Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth of the indentation. More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material. Indentation hardness value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods. Click here to learn more about hardness testing basics.
The Vickers hardness test method, also referred to as a microhardness test method, is mostly used for small parts, thin sections, or case depth work. The Vickers method is based on an optical measurement system. The Microhardness test procedure, ASTM E-384, specifies a range of light loads using a diamond indenter to make an indentation which is measured and converted to a hardness value. It is very useful for testing on a wide type of materials as long as test samples are carefully prepared. A square base pyramid shaped diamond is used for testing in the Vickers scale. Typically loads are very light, ranging from a few grams to one or several kilograms, although "Macro" Vickers loads can range up to 30 kg or more. The Microhardness methods are used to test on metals, ceramics, composites - almost any type of material.
Since the test indentation is very small in a Vickers test, it is useful for a variety of applications: testing very thin materials like foils or measuring the surface of a part, small parts or small areas, measuring individual microstructures, or measuring the depth of case hardening by sectioning a part and making a series of indentations to describe a profile of the change in hardness. The Vickers method is more commonly used.
Sample preparation is usually necessary with a microhardness test in order to provide a small enough specimen that can fit into the tester. Additionally, the sample preparation will need to make the specimen’s surface smooth to permit a regular indentation shape and good measurement, and to ensure the sample can be held perpendicular to the indenter. Usually the prepared samples are mounted in a plastic medium to facilitate the preparation and testing. The indentations should be as large as possible to maximize the measurement resolution. (Error is magnified as indentation sizes decrease) The test procedure is subject to problems of operator influence on the test results.
|
