Chemical strengthening of sodium alumino‐borosilicate glasses: Effects on thermal and mechanical properties

Abstract Sodium alumino‐borosilicate glasses of two series: 20Na 2 O‐xAl 2 O 3 ‐20B 2 O 3 ‐(60‐x)SiO 2 and 30Na 2 O‐xAl 2 O 3 ‐20B 2 O 3 ‐(50‐x)SiO 2 (where x = 0, 5, 7.5, 10 and 12.5 mol %) were prepared by melt quenching in a temperature range of 1400°C–1600°C in platinum crucibles. Glass samples were characterized by density, differential scanning calorimetry, Fourier transform infrared spectroscopy, 11 B and 27 Al magic angle spinning nuclear magnetic resonance studies. The hardness of the samples was measured by Vickers’ indentation tests. Glass samples were chemically strengthened by an ion exchange process using molten KNO 3 at 460 ° C for 6 h. In the glass series containing 20 mol% of Na 2 O, density decreases from 2.525 to 2.369 ± 0.005 g·cm −3 , the glass transition temperature decreases from 607 ± 1°C to 576 ± 1°C, Al‐O coordination increases from 4.39 ± 0.01 to 4.46 ± 0.01, B–O coordination decreases from 3.74 ± 0.01 to 3.24 ± 0.01 and hardness decreases from 6.11 ± 0.03 to 4.62 ± 0.11 GPa with an increase in Al 2 O 3 concentration. In the second glass series containing 30 mol% of Na 2 O on adding Al 2 O 3 upto 12.5 mol%, density decreases from 2.502 to 2.421 ± 0.005 g·cm −3 , glass transition temperature increases from 548 ± 1°C to 575 ± 1°C, Al–O coordination increases from 4.40 ± 0.01 to 4.51 ± 0.01, B–O coordination number decreases from 3.69 ± 0.01 to 3.29 ± 0.01 while the hardness decreases from 4.87 ± 0.01 to 4.35 ± 0.13 GPa. Chemical strengthening of glasses by ion exchange process enhanced the hardness values by a maximum of 24%.