True hardness

This consists of loading a pointed diamond or a hardened steel ball and pressing it into the surface of the material to be examined. The further into the material the indenter (as it is called) sinks, the softer is the material and the lower its yield strength. The true hardness is defined as the load (F) divided by the projected area of the indent, A. (The Vickers hardness, H , unfortunately was, and still is, defined as F divided by the total surface area of the indent. Tables are available to relate H to Ff .)... 

It is widely accepted that to measure the true hardness of the Elms, the indentation depth should not exceed 10 % of the film thickness. Based on a finite element analysis, Bhat-tacharya [70] and Bhushan [71] conclude that the true hardness of the films can be obtained if the indentation depth does not exceed about 3 0 % of the film thickness. 

Anistropy may cause differences in the magnitude of indentations or result in deformed indentations (Fig. 7.1), from which the true hardness in a given crystallographic direction should be calculated (Fig. 7.2). 

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It is evident from the graph that the hardness becomes independent of load for loads more than 400 mN. Though hardness is a surface phenomenon at lower load at higher loads, beyond 400 mN, the hardness value represents the true hardness value of bulk and it is consequently independent of the load. The reason is attributed to the fact that the plasma generated excited species interacts with the surface of the polymeric films and leads to cross-linking which improve the hardness of the samples which is also corroborated with TGA thermograms (Figure 14.25). 

In this study, the nanoindentation on monoclinic-, tetragonal- and cubic-zirconia polycrystals (referred as MZP, TZP and CZP, respectively) was carried out with a home-made nanoindenter. Relationship between indentation load P and penetration depth /z of a diamond stylus was analyzed to derive the true hardness Hr as a function of the plastic depth hp, which is a measure of plasticity. The plastic deformation of each zirconia at room temperature was discussed in detail through the examination and the mutual comparison of indentation size dependence of Hr-... 

The parameter a is considered by some to be the true hardness or the hardness due to volumetric deformation processes in the absence of surface effects. 

Bemhardt related the term to plastic deformation. Others have described it as die "work of permanent deformation" or the "volume energy of deformation, or die term as the "true hardness," in the absence of surface effects. 

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