Vickers microhardness studies on solution-grown single crystals of magnesium sulphate hepta-hydrate

A Vickers microhardness study has been carried out on (1 0 0) and (0 1 0) faces of solution-grown single crystals of magnesium sulphate hepta-hydrate (MgSO4·7H2O) over a load range of 10-80 g. The Vickers hardness numbers (Hv) are found to decrease initially with increase in load and then appear to level-off. The (1 0 0) face is the softest one. The Meyer index 'n' of the two faces is less than 2 as expected theoretically if the particular crystal system belongs to the soft material category. Neither Kick's law nor Hays and Kendall's law can fully explain the nonlinear variation of microhardness with load. Instead, preference is given to Li and Bradt's proportional specimen resistance (PSR) model. The elastic stiffness coefficient, c11, has also been calculated using Wooster's empirical relation from the hardness data and it shows reasonable agreement with a reported value. A hardness anisotropy for both planes has been observed in accordance with the orientation of the crystallographic planes.