Mechanical properties of soda–lime–silica glasses with varying alkaline earth contents

Abstract

The effects of varying the alkaline earth oxide content of three series of soda–lime–silica glasses with the general formulae 13.5Na2O·xMgO·10CaO·1.5Al2O3·(75−x)SiO2 (mol.%) where x=0, 1, 2, 3, 4, 5, 6, and 7; 13.5Na2O·3MgO·(7+y)CaO·1.5Al2O3·(75−y)SiO2 (mol.%) where y=0, 1, 2, 3, 4, 5, 6, and 7 and 13.5Na2O·zMgO·(13−z)CaO·1.5Al2O3·72SiO2 (mol.%) where z=1, 3, 5, 7, 9, and 11 have been examined. Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy have been used to assess network connectivity. In the first two glass series network connectivity decreases with increasing alkaline earth addition whereas in the third series connectivity tends to be greater for the more magnesia rich glasses suggesting that magnesia does have a different effect to lime on network connectivity, but only when magnesia is the dominant alkaline earth species. Fracture toughness has been measured using bend testing, which avoids many of the questions raised by the widely used indentation technique. Moduli have been assessed using acoustic means. It was found that the mechanical properties tend to decrease with increasing network connectivity for all three glass series. For the MgO–SiO2 series and CaO–SiO2 glasses increasing the alkaline earth content at the expense of the silica content resulted in increased network depolymerization, whereas for the MgO–CaO series when MgO became the dominant alkaline earth species, network depolymerization was reduced. Thus whilst MgO and CaO both act as network modifiers when more CaO than MgO is present in soda–lime–silica glasses, a difference in behaviour is seen with magnesia rich soda–magnesia–silica glasses. In contradiction to previous data no significant advantage of replacing CaO by MgO is observed. In addition it appears that the glasses with the lowest fracture toughness values may be more resilient to contact damage than those with the higher fracture toughness values.