High hardness sodium alumino-borosilicate glasses containing Ca and Mg ions

Sodium alumino-borosilicate glasses of two series: 25Na 2 O-10Al 2 O 3 -xMO-5B 2 O 3 -(60-x) SiO 2 (M = Ca and Mg, x = 10 and 15 mol%) and 25Na 2 O-xAl 2 O 3 -10CaO-5B 2 O 3 -(60-x)SiO 2 (x = 15 and 17.5 mol%) were prepared. 27 Al and 11 B Magic Angle Spinning Nuclear Magnetic Resonance and FTIR spectroscopy studies were used to determine Al-O and B-O coordination. In the first glass series containing Al 2 O 3 concentration of 10 mol %, on replacing 10 mol % of CaO with 10 mol % of MgO the glass transition temperature increases significantly from 558 to 594 ± 1 °C, the Al-O coordination number increases from 4.11 ± 0.01 to 4.15 ± 0.01, and the Vickers’ hardness increases from 5.88 ± 0.06 GPa to 7.63 ± 0.22 GPa. Similar variations in glass properties are found in samples containing 15 mol% of CaO and MgO. In the second glass series containing 10 mol % of CaO, on increasing Al 2 O 3 concentration from 10 to 17.5 mol%, the glass transition temperature increases from 558 to 581 ± 1 °C, Al-O coordination increases from 4.11 ± 0.01 to 4.17 ± 0.01, B-O coordination number decreases drastically from 3.27 ± 0.01 to 3.07 ± 0.01 and hardness increases significantly from 5.88 ± 0.06 GPa to 7.57 ± 0.08 GPa. The hardness of glasses is found to correlate with an increase in the concentration of penta- coordinated AlO 5 units and with a decrease in the fraction of non-bridging oxygens in the glass structure. The chemical strengthening of glasses was carried out in molten KNO 3 for 6 h at 460 °C, which increased the hardness up to a maximum value of 9.59 ± 0.09 GPa. It is found that the addition of CaO, MgO and Al 2 O 3 in the borosilicate glass system significantly enhances network connectivity, the glass transition temperature, and the hardness of glass.