Materials Hardness: Vickers Hardness Scale
Vickers & Brinell Hardness Test
The Vickers materials hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd., as a method of quantifying a given material's ability to resist plastic deformation when force is applied from a standardized source. The Vickers test's unit of hardness is known as the "Vickers Pyramid Number," or (HV).
The Vickers test has one of the widest scales of any hardness test, and can be used on all metals or materials. This test can also be easier to use than other hardness tests since its calculations are independent of the size of the indenter, and the indenter can be used for all materials, irrespective of their hardness.
To preform the test, a uniform amount of force (with variable magnitudes depending on the hardness of the material) is applied to an "indenter" which makes an impression in the material to be tested. The indentation's shape is a diamond pattern that forms a geometric, square-based pyramid indentation in the material, providing well defined points of measurement in the impression, irrespective of size.
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Vickers Hardness Tester (photo: public domain) |
The final hardness number is determined by the load over the surface area of the indentation, and not the area normal to the force, and is therefore not a pressure. When performing the test the distance between multiple indentations must be more than 2.5 indentation diameters apart to avoid interaction between work-hardened regions.
Vickers vs Brinell & Mohs Hardness Test
The Vickers HV number is determined by the ratio F/A where "F" is the force applied to the material, and "A" is the surface area of the resulting indentation. The hardness number can be converted into units of Pa, but should not be confused with a pressure, which also has units of Pa. The Vickers is used as an alternative to the Brinell method, and is completely different than the Mohs hardness scale which tests a material's scratch resistance.
The Brinell hardness scale was developed by a Swedish engineer named Johan August Brinell in 1900. The Brinell test utilizes a 10mm diameter steel ball as an indenter, applying a uniform 3,000 kgf (29 kN) force. A smaller amount of force is used on softer materials, and a tungsten carbide ball is used for harder materials.
With the Mohs Scale, the hardness of a material is measured against the scale by finding the hardest material that it can scratch, and/or by identifying the softest material that can scratch it. If a given material can be scratched by quartz but not by topaz, its hardness on Mohs scale is 7.5.
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