The growth of largest L-LMHCl dihydrate crystal by SR method Essay

The mechanical studies of SR method grown L-LMHCl with various pH values were carried out by Vicker's microhardness test. The hardness of the material depends on different parameters such as lattice energy, Debye temperature, heat of formation and inter-atomic spacing [22-24]. According to Gong [25] during an indentation process, the external work applied by the indenter is converted to a strain energy component, which is proportional to the volume of the resultant impression. Also the surface energy component is proportional to the area of the resultant impression. Microhardness is a general microprobe technique for assessing the bond strength, apart from being a measure of bulk strength [26-28]. The selected smooth surfaces of the crystal were subjected to indentation test. The indentation load was varied from 25 g to 100 g. For each load, the microhardness value was calculated using the equation [29].
Hv = 1.8544P/d2 kg mm-2 --------------------- (1)
Where Hv is the Vicker's hardness number, P is the applied load (kg), d is the average diagonal length of indentation mark (mm).
Figure 9 shows the variation of Vicker's hardness number with applied load. From the Figure, it is observed that, the Vicker's hardness number increases with applied load. Also it is confirmed that, the hardness number is high for the crystal grown with lower pH value compared to other crystals. Due to the application of mechanical stress by the indenter, dislocations are generated locally in the region of indentation. The higher hardness value for lower pH crystal indicates that greater stress is required to form dislocation thus confirming greater crystalline perfection. Similar results were reported in KDP crystal [30]. Thus the major contribution of hardness is attributed to the high stress required for homogeneous nucleation of dislocation in the small dislocation free region indented [31]. The Mayer index number was calculated from the Mayer's law [26], which relates the load and indentation diagonal length.
P = k1dn --------------------- (2)
log P = logk1 + n logd
Where k1 is the standard hardness and n is the Mayer's index (or work -hardening coefficient). The above relation indicates that Hv should increase with load P if n > 2 and decrease with load P when n < 2. A plot has been drawn between log p and log d and shown in Figure 10. The work hardening coefficient (n) was found to be 2.5, 2.8 and 2.67 for the crystal with pH value 4.5, 6.5 and 8.5 respectively. From the result, it is revealed that the crystal which is grown with the pH value of 6.5 having higher work hardening coefficient. The standard hardness k with be calculated from the plot between P and dn (Figure 11). In order to satisfy the kick's law, a correction factor may be added to the observed 'd' value. Correction factor is nothing but, it is the time taken by the material to revert its elastic mode after the removal of the applied load [26]. The Kick's law may be written as
P = k2 (d+x)2 --------------(3)