The hardness for monoclinic zirconia is approximately 9.2 GPa [31] for samples with a density > 98% and 4.1-5.2 GPa [32] for samples with a density > 95% of theoretical, whereas hardness values for amorphous zirconia vary between 5 and 25 GPa [33]. The hardness increases slightly to values approaching 11 GPa for yttria-stabilized zirconia of 1.5 mol% yttria, which is stabilized in the tetragonal form [31]. Addition of larger amounts of yttria dopant results in hardness values approaching 15 GPa [34].