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Chapter VIII Micro-hardness studies…
[76] found that a micro-hardness decreased with decrease in load and
concluded that this dependence was due to the relative contributions of plastic
and elastic deformations in the indentation process. In view of these different
observations, it has become rather difficult to establish any definite
relationship of general validity between P and d or H and P. The Kick’s law
was applied to variation of Vickers micro-hardness with load in several
organic molecular crystals and it was found that n=2 for anthracene and
benzoic acid crystals but n=1.8 for phenanthrene crystals [36]. In case of
several non linear optical (NLO) crystals, such as L-histyidineium perchlorate
(LHPC), L-histyidineium bromide monohydrate (LHB), L-histyidineium
dihydrogen phosphate orthophosphoric acid (LHDP) and L-arginium bis
(Dihydrogen phosphate) (LADP), the application of Kick’s law to the variation
of hardness with load gave the work hardening coefficient less than 1, but for
LHPC it gave more than 2 [77]. Also, Vaidya [78] applied Kick’s law to the
Vickers hardness profile with applied load for bismuth and bismuth-zinc
systems. Notwithstanding, some studies have suggested more than one value
of coefficient n. Saraf [79] has obtained two different values of n for higher
and lower load regions in the case of baryte crystals. Ambujam [80] has
reported the profile of Vickers micro-hardness variation with load for γ-glycine
crystals. The RISE was predicted from the Kick’s law plots of log P verses log
d. The work hardening coefficient or the Meyer index n was found to be 2.58.
The RISE suggests that the apparent micro-hardness increases with
increasing applied test loads, which has been critically examined by Sangwal
[81-82] for a number of crystals. This RISE has been examined in terms of
the existence of a distorted zone near the crystal- medium interface, effect of
vibrations and indenter bluntness at low loads, the applied energy loss as a
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