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Chapter VIII Micro-hardness studies…<br />

which result in the decrease of hardness. Recently, it has been found that in<br />

NLO material L–arginine dihydrogen phosphate crystals, the metal doping<br />

showed harder crystals than the pure and amino acid substituted crystals [23].<br />

The variation in the Vickers micro-hardness number with load has been<br />

studied widely on large number of materials [24-26]. However, some of the<br />

reported results indicate that the micro-hardness of a given material is<br />

constant above a critical size of penetration of the indenter impression, but<br />

below this size a steady increase in micro-hardness occurs as the load<br />

decreases. [6, 27-30]. Gane and Cox [31] have put forward an alternative<br />

explanation suggesting that the increase in micro-hardness at small<br />

indentation size is due to an increase in the stress necessary to operate<br />

dislocation sources in a perfect crystal region. The size of the dislocation of<br />

the rosette produced around micro-indentation is useful and convenient test<br />

for determination of mechanical strength of single crystals. Palanisamy [32]<br />

has studied the indenter impression at different loads on Li2CO3 crystals and<br />

subsequently etched them. Micro-indentation hardness along with chemical<br />

etching has been reported on many organic molecular crystals [33-36].<br />

Moreover, the variation of Vickers micro-hardness with load has been studied<br />

at different temperatures in antimony single crystals [37]. The positions of the<br />

peaks in the micro-hardness versus load curves were identified by slip<br />

systems operative. Earlier, the nature of Vickers micro-hardness variation with<br />

load was studied by Vaghari [35] and Joshi [36] for bismuth single crystal<br />

cleavages. The variation of micro-hardness with applied load was also studied<br />

on many organic molecular crystals like anthracence and benzoic acid [39-<br />

42]. Altogether, the variation of Vickers micro-hardness with load was studied<br />

for KDP crystals grown with organic additives; it was found that the hardness<br />

295

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