Effect of thermal history and chemical composition on hardness of silicate glasses

The prediction of hardness is possible for crystalline materials, but not possible for glasses so far. In the present paper, we describe and discuss several important factors that should be used for predicting the hardness of glasses. To do so, we have studied the influences of thermal history and chemical composition on hardness of silicate glasses. By subjecting E-glasses of different compositions to various degrees of annealing, it is found that hardness decreases with the fictive temperature. Addition of Na2O to a SiO2–Al2O3–Na2O glass system causes a decrease in hardness. However, the number of non-bridging oxygen per tetrahedron (NBO/T) is not the only parameter determining hardness. In addition, it is found that the effect of ionic radius on hardness is opposite for alkali and alkaline earth ions. Hence, changes of the structural network occurring at the atomic scale must be taken into account when predicting the effect of composition on hardness. The principles used in the calculation of hardness of crystalline materials are only partly valid for glasses.