Abstract
Hardness and fracture toughness were measured using the Vickers microhardness test in the low load range from 25 to 100 g near to the fracture threshold for near-perfect single crystals of garnets. The influence of crystal growth parameters, calcium impurity content and crystallographic orientation of Gd3Ga5O12 (GGG) and Ca3Ga2Ge3O12 (CaGeGG) samples was investigated. Fracture starts with radial cracking from indent corners followed by lateral fracture of two distinct modes. The mean hardness of [111] oriented GGG isH=13 GN m−2, for [111] oriented CaGeGG it is 12 GN m−2, the average fracture toughness beingKc=1.2 and 0.8 MN m−3/2, respectively for the two crystals. Impurity doping slightly increases the strength of the material. Among the investigated crystals (111) faces are the least strong, the (100) face has maximumH andKc values for CaGeGG. The constraint factor,φ, and yield stress,Y, were deduced from the measured hardness data, givingφ=2.2 andY about 7 GN m−2.
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