I am using a nanoindenter to determine the hardness of my coated sample. It gives a reading in HIT=799 MPa. Can I convert this reading to Vickers hardness?
Conversion of nano indentation hardness to micro indentation or macro indentation hardness is not possible. Nano indentation represents the localized harness of a grain or grain boundary or an inclusion while micro/macro indentation represents the aggregate hardness of grains, grain boundaries, inclusions etc (more than at least 5 grains). Nano hardness has no direct relationship with the macro tensile properties of a material while micro/macro hardness has.
In some total nano to micro/macro hardness conversion and vice versa are not possible.
Vickers hardness and nanoindentation hardness use the same unit (load/area). However, as Rajendran have already mentioned, their measuring effective volume is different by order of magnitudes. Hence, I don't think the two values are convertible, but they should be comparable to give a relative comparison of the two measurements.
If you just want to convert MPa to HV, just divide MPa with 9.81 m/s2.
As mentioned before the conversion is not straight forward but it needs to be analysed on a case by case and several considerations have to be taken into account:
1) The nanohardness suffers of what it is called Indentation Size effect, the smaller the indentation mark the harder the material appears to be (this is related to geometrically necessry dislocation and strain gradients that is higher for smaller indents)
2) on the nano/micro scale the surface could be highly heterogenous with great variations of local hardness compared to the macro scale
3) it depends on the type of indenter that you are using, normally in nanoindentation you can use different types of indenters (spherical, flat punch, berkovic...) that have a different geometry from a vickers indenter sometimes used in macro hardness: for instance in the case of berkovic to vickers comparison HVit approx= Hit/10.80
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