ARTICLE Copyright © 2016 by American Scientific Publishers All rights reserved. Printed in the United States of America Journal of Advanced Physics Vol. 6, pp. 1–6, 2016 (www.aspbs.com/jap) Mechanical Studies of Pure and L-Lysine Doped Ammonium Dihydrogen Phosphate Single Crystal S. Patle 1, * and K. Rewatkar 2 1 Department of Applied Physics, Priyadarshini J. L. College of Engineering, Nagpur 440009, India 2 Department of Physics, Dr. Ambedkar College of Science, Nagpur 440010, India Pure and Doped ADP crystals were subjected to Microhardness study to understand the mechanical behavior of pure and L-lysine doped ammonium dihydrogen phosphate (ADP) single crystals grown by slow evaporation technique. Indentations were made on selected 001 plane of pure and doped crystal for loads varying from 10 to 100 g. Vickers microhardness value, yield stress and elastic stiffness constant were found to be higher for doped ADP crystal as compared to pure ADP crystal. The crystals show Reverse Indentation size effect (RISE). Mayer’s law, Hays and Kendall’s law and Proportion specimen resistance (PSR) model were applied and verified for pure and doped crystal. It is observed that both the crystals come under soft material category. The values of the load independent hardness and Newtonian pressure were higher in doped ADP crystal. The results indicate that the doping of L-lysine increases mechanical strength of ADP crystal. KEYWORDS: ADP Crystal, Hays and Kendall’s Law, Kick’s Law, PSR Model, Vickers Micro-Hardness. 1. INTRODUCTION The property called as hardness in the crystals is very dif- ficult to define on the basis of the analytical approach. Physically we can define it as the capability of the crys- tal or in general body to oppose external deforming force. From the mechanical properties of the materials we can sense out to what extent the stress yield between the molecular bonds comprising the material. Vickers micro- indentation hardness test is very sensitive to surfaces and anisotropy of elastic and plastic properties of the crystals due to the small size of indenter and complex nature of the stress field around it. 1 The measurement of the hard- ness in the crystals depends on many factors viz. inden- tation condition, temperature and strain rate and depth to which indentation is done. 2 Generally, the indentation weight between 1 to 100 g is advisable in micro-hardness test. The mechanical strength of the crystals can be sensed from the micro-hardness tests ex. propagation of dislo- cation under particular load, effect of impurities on the plastic deformation, radiation hardening and identification of slip systems. 3 The micro-indentation hardness can also provide information about creep, hardness anisotropy, plas- ticity, and fracture behaviors. 4 Micro-indentation hardness studies were performed and discussed by several authors in detail. 5-9 ∗ Author to whom correspondence should be addressed. Email: shruti.patle@gmail.com Received: 18 January 2017 Accepted: 28 February 2017 Ammonium Dihydrogen phosphate (ADP) crystals can be explored to great extents for various application in advanced technology, due to their excellent electro- optical, non-linear optical properties easy to growth of sin- gle crystal, broad transparency range, high optical dam- age threshold, and relatively low cost of production. It will be interesting to know and carry out investiga- tions on their mechanical properties to reveal the durabil- ity, mechanical stability and ruggedness for long-lasting applications. Number of researchers reported Vickers micro-hardness for ADP crystals. 10–13 It has been observed that various additives like L-lysine mono-hydrochloride dehydrate, 14 L-asparagine monohydrate doped, 15 oxalic acid, 16 DL- malic acid, 17 cobalt, 18 ammonium acetate 19 increases the micro-hardness of ADP crystal. Introduction of additives into the crystals may result in enhancement of lattice sta- bility and increase in bond energy. Altogether, several reports are available on Vickers micro-hardness studies of various amino acid doped ADP crystals. 20–23 The enhancement in transparency and efficiency second harmonic generation (SHG) of L-lysine doped ADP had been reported. 24 Also they have studied Vickers micro- hardness to access their mechanical properties. In the present research module we have carried out variation of Vickers hardness with load, Meyers’s index, yield stress and elastic stiffness constant are reported along with apply- ing Hays and Kendall’s law and Proportional Specimen Resistance (PSR) model to the micro-hardness data. J. Adv. Phys. 2016, Vol. 6, No. xx 2168-1996/2016/6/001/006 doi:10.1166/jap.2016.1353 1