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Vickers hardness test
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== Conversion to SI units == To convert the Vickers hardness number to SI units the hardness number in [[kilograms-force]] per square millimeter (kgf/mm<sup>2</sup>) has to be multiplied with the [[standard gravity]], <math>g_0</math>, to get the hardness in MPa (N/mm<sup>2</sup>) and furthermore divided by 1000 to get the hardness in GPa. <math>\text{surface area hardness (GPa)} = \frac{g_0}{1000}HV = \frac{9.80665}{1000}HV</math> Vickers hardness can also be converted to an SI hardness based on the projected area of the indent rather than the surface area. The projected area, <math>A_{\rm p}</math>, is defined as the following for a Vickers indenter geometry:<ref>{{Cite book|last=Fischer-Cripps, Anthony C.|title=Introduction to contact mechanics|date=2007|publisher=Springer|isbn=9780387681887|edition=2nd|location=New York|pages=212β213|oclc=187014877}}</ref> <math>A_{\rm p} = \frac{d_{\rm avg}^2}{2} = \frac{1.854}{2}{A_s}</math> This hardness is sometimes referred to as the mean contact area or [[Meyer hardness test|Meyer hardness]], and ideally can be directly compared with other hardness tests also defined using projected area. Care must be used when comparing other hardness tests due to various size scale factors which can impact the measured hardness. <math>\text{projected area hardness (GPa)} = \frac{g_0}{1000}\frac{2}{1.854}HV \approx \frac{HV}{94.5}</math>
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