Correlation of the Vickers hardness of RO–Al2O3–SiO2 glasses with predicted liquidus temperatures

Vickers hardnesses of RO–Al₂O₃–SiO₂ glasses were measured over a broad range of compositions, ranging from ternary endpoints to mixed ratios of RO (CaO with MgO, SrO, or BaO), with the systematic variation of Al₂O₃ and SiO₂ levels. The hardnesses of CAS and MAS glasses are similar, ranging from 6.7 to 7.2 GPa, with the replacement of CaO with MgO producing a marginal increase in hardness. The substitution of SrO or BaO for CaO generally produced in a decrease in hardness down to 4.5 GPa with BaO. The sensitivity to alumina and silica levels, however, was much greater ranging from a minimum of 4.5 GPa to a maximum of ∼8.2 GPa. The correlation of the Vickers hardness with melting temperature was observed in the CAS system but generally not in the RO-blended glasses. Overall, a combined cation field strength of modifier cations determined the hardness above the critical RO blending ratio.