Study of the Fracture Toughness of K2NixCo(1 – x)(SO4)2 · 6H2O Crystals in Dependence of the Growth Direction and Rate

Resumo

K₂Ni(SO₄)₂ · 6H₂O (KNSH), K₂Co(SO₄)₂ · 6H₂O (KCSH), and K₂Ni_xCo_(1 – x)(SO₄)₂ · 6H₂O (KCNSH) crystals have been grown by traditional and accelerated techniques. The microhardness and crack lengths were measured by indentation with a Vickers pyramid in dependence of the crystal growth conditions in order to improve the crystal structure and properties. It was shown that, with an increase in the growth rate, the stress intensity factor of the crystals (which characterizes the fracture toughness) changes in dependence of the composition: it decreases for KCSH (due to the rise in inhomogeneity) but increases for KCNSH, which is explained by lowering the level of both the mosaic and radial inhomogeneity. In the case of KNSH crystals, the fracture toughness of the {110} sector decreases, whereas the changes in the sector {001} of “slow” crystal are differently directed, which is explained by the high inhomogeneity of its composition.