Evaluation of the Hardness of Alkaline Earth Aluminosilicate Glasses

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New York State College of Ceramics at Alfred University. Inamori School of Engineering.
The hardness of alkaline earth aluminosilicate glasses was evaluated focusing on the alumina level and atomic packing density of the system. To identify the compositional matrices, a comprehensive investigation was conducted to determine the glass formation region in ternary aluminosilicates (R2O∙RO∙Al2O3∙SiO2). Vickers hardness of the glasses were evaluated in steps: the calcium aluminosilicate system; a range of RO or R2O modifiers, including blended flux systems; and finally, the introduction of a high valency cation into the high hardness glasses to locally increase the density of the glass structure. The alumina edge of the glass formation boundary was similarly defined by a fixed molar ratio of 1.2 (±0.1) Al2O3 to R2O (Na2O or K2O), RO (MgO, CaO, etc.), or a mixture of fluxes (R2O+RO) over a broad range of silica levels and was shown to be independent of cooling rate. The alumina saturation level determined the experimental matrix to prepare samples for hardness measurement. The hardness results for CaO-Al2O3-SiO2 glasses demonstrated that hardness is strongly related to the alumina level and that the hardness of CAS glasses correlates with the melting behavior as predicted by the phase diagram. The hardness of CAS and MAS glasses were similar, ranging from 6.7 GPa to 7.2 GPa, and the replacement of CaO with MgO produced only a marginal increase in hardness. Conversely, the blending of CaO with SrO and BaO generally resulted in a decrease in hardness. The sensitivity to alumina and silica levels, however, was much greater ranging from ~4.5 to a maximum of ~8.2 GPa. Finally, high valency cations (Mo6+) were added to high hardness CAS glasses. High valency cations should have locally increased the compactness of the glass structure and thus enhanced the indentation hardness, but there was no increase in hardness. Mo serves as a heterogeneous nucleation site for crystallization resulting in anorthite precipitation. While there was a clear trend of increasing hardness with increasing Cation Field Strength (CFS), this trend did not extend to the introduction of high valency cations did not show further improvement in hardness. Overall, it was concluded that, in general, the hardness of aluminosilicate glasses correlates to melting behavior, and that within specific compositions, with the composite CFS of the modifier cations.
Thesis completed in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Ceramic Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
Aluminum silicates, Alkaline earth oxides, Glass--Mechanical properties