Nano Indentation Theory |
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Hardness Scale Conversion
Conversion between the different hardness scales is not exact since the definition of hardness varies with the type of test used. It is a system property not an intrinsic material property like modulus.
Conversion between Vickers Hardness and indentation hardness
The Vickers hardness number HV (or HV) is the test force divided by the sloping (i.e. surface) area of the indenter up to a depth where it has the same cross-sectional area as the residual indent:-
HV = constant x Test force/Surface area of indentation.
In contrast, indentation hardness, HIT, is defined as the test force (F) divided by the projected area of contact (A):-
HIT = F/A(hc)
The projected contact area A(hc) is calculated from knowledge of the geometry of the indenter and stiffness of the contact.
From ISO 14577-1.2 (Indentation standard):-
The ratio of surface area (As) to projected area (Ap) is a constant for a perfect Vickers or modified Berkovich indenter:-
As/Ap = 26.43/24.5 = 1.08
The diagonal length measured in conventional Vickers tests is related to Ap by:-
d2 = 2 Ap
which leads to:-
HV = 0.0945 x HIT (in MPa)
IMPORTANT: It is also necessary to check units. The above equation converts a Vickers hardness number to a HIT value in MPa (as kg/mm2 = 106 N/m).
(note: it is common practice for HV to be quoted in MPa and HIT in GPa – so the above equation becomes:-
HV = 94.5 x HIT (in GPa)
Example:- HV 700 is approximately equivalent to HIT of 7.5 GPa)
Note that this correlation does not allow for tip blunting at small depths and the two scales diverge as the indentation contact becomes more elastic.
