Indenter and Specimen Surfaces

Both the indenter and the specimen surfaces should be smooth and homogeneous in order to minimize friction. If the indenter is not smooth, under pressure that is sufficient to cause plastic flow. The specimen will become embossed by the indenter, tending to lock the surfaces. This will induce a large effective friction coefficient. 

Irregularities of a specimen s surface will result in local deformation with accompanying deformation hardening. This may lead to erroneous hardness numbers, although such errors may be small. 

More serious errors may result when the grain-size of a specimen is small compared with the size of an indentation. Then, since all crystals are elastically anisotropic a rigid indenter will produce differing amounts of elastic strain in the grains depending on their orientations. This will create an effective roughening of the surface and increase the friction coefficient. This may result in overestimates of hardnesses. For example, this may underlie reports of nanocrystalline materials being harder than diamond. 

Armstrong and C. Cm. Wu, Lattice Misorientation and Displaced Volume for Microhardness Indentations in MgO Crystals , Jour. Amer. Cer. Soc., 61, 102 (1978). 

Slip Circle Constructions for Inhomogeneous Rotational Flow , Maer. Sci. Forum, 550,105 (2007). 

---