Indentation Size Effect (ISE) of Transparent AION and MgAl2O4

Hardness is a widely reported mechanical property for materials. Aluminum oxynitride A1ON and magnesium aluminate spinel MgAl2O4 are two important materials for some U.S. Army applications since they can be transparent in their polycrystalline form. In many of these military applications, harder materials tend to perform better, hence it is necessary to properly measure and compare hardness values of competing materials. Measuring the hardness of most ceramics is straightforward, but comparing the hardness data for different ceramics can be complicated due to the well-known indentation size effect ISE. This report describes the determination of the Vickers hardness-load curves for transparent A1ON and MgA12O4 in a load range between 0.98 and 19 N. Both materials exhibited a significant decrease in hardness with increasing load. The critical hardness the point at which fracture, rather than plastic deformation, is dominant around the indentation of spinel and A1ON was found to be 13.5 and 16.8 GPa, respectively, which differs from other investigations that did not take into account the ISE.