Microhardness and brittle fracture of garnet single crystals

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 beingK _c=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 andK _c values for CaGeGG. The constraint factor,, and yield stress,Y, were deduced from the measured hardness data, giving=2.2 andY about 7 GN m^–2.