Microhardness Anisotropy and the Indentation Size Effect in Single Crystal Magnesium Fluoride, MgF2

The Knoop microhardness anisotropy profile was determined on the (001) of MgF2 which has microhardness maxima in the <110> and minima in the <100>. This anisotropy is the same as TiO2 and SnO2 which also have the rutile crystal structure. This indicates that the slip systems are the same for MgF2 as the other two rutile structures. The (001) microhardness of MgF2 is the most anisotropic of these three rutile structures. The three rutile structures are compared with regard to their absolute hardness values. MgF2 is much softer than the oxides, only about half the hardness of SnO2 and a third that of TiO2. It reflects the bond strengths as related to the single crystal elastic constants. The hardness of MgF2 is similar to, but slightly harder than the cubic alkaline earth fluorides, all of which have the fluorite structure. The indentation size effect of MgF2 on the (001) for the Knoop indenter over the range of test loads from 10g to 300g was determined. The ISE of MgF2 is less pronounced than those of TiO2 and SnO2.