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Chapter VIII Micro-hardness studies…<br />

Table: 8.2 Yield stress, first order elastic stiffness constant at 100g load, work<br />

hardening coefficient and standard hardness values for pure and amino acid doped<br />

KDP crystals.<br />

Samples<br />

Pure KDP<br />

Crystal<br />

KDP + 0.3% Lhistidine<br />

KDP + 0.4% Lhistidine<br />

KDP + 0.5% Lhistidine<br />

KDP + 0.3% Lthreonine<br />

KDP + 0.4% Lthreonine<br />

KDP + 0.5% Lthreonine<br />

KDP + 0.3%<br />

DL-methionine<br />

KDP + 0.4%<br />

DL-methionine<br />

KDP + 0.5%<br />

DL-methionine<br />

Yield Stress<br />

σy (MPa)<br />

8.6 Hays and Kendall law<br />

First Order Elastic<br />

Stiffness Constant<br />

C11 (MPa)<br />

In the present study, the micro-hardness has a relatively larger value at<br />

low loads. The micro-hardness shows a steep decrease with increasing loads<br />

up to about 0.294 N and, thereafter, the micro-hardness becomes almost<br />

constant. Mott [6] quotes several reports where such behaviors have been<br />

observed. Pratap and Hari Babu [84] in their study of ammonium halide<br />

crystals and Kotru [85] in his study of rare earth compound crystals observed<br />

similar load dependence. Suthar [62] applied Hays and Kendall law to Mn +2<br />

and Cu +2 doped calcium levo-tartrate tetrahydrate crystals.<br />

According to the approach of Hays and Kendall [14] this type of load<br />

variation can be due to the sample exerting a Newtonian pressure on the<br />

loaded indenter. This resultant pressure (W) is a function of the material being<br />

306<br />

Work Hardening<br />

Coefficient<br />

n<br />

Standard<br />

Hardness a<br />

(MPa)<br />

432.1 280049.4 1.461 12887<br />

431.5 279391.9 1.468 12643<br />

430.7 278497.4 1.472 12601<br />

429.6 277269.0 1.474 12552<br />

431.5 279391.9<br />

429.8 277487.4<br />

429.4 276952.9<br />

430.8 278580.4<br />

429.3 276922.8<br />

428.0 275509.6<br />

1.463<br />

1.484<br />

1.497<br />

1.469<br />

1.472<br />

1.499<br />

12604<br />

12585<br />

12501<br />

12504<br />

12452<br />

12360

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