Mechanical and structural properties of soda lime silica glasses as a function of composition.

Significant changes in mechanical and structural properties can be obtained by modifying commercial soda-lime-silica glass composition within a narrow range; and this can potentially enable the glass scientists and technologists to produce commercially viable, stronger and lighter soda-lime-silica glass products. In this research, four different series of soda-lime-silica glasses have been produced; MgO and CaO glass series are fabricated by varying the magnesia/silica and calcia/silica ratios respectively; and CaO-MgO and Al2O3 glass series were produced by altering the calcia/magnesia and (alumina + soda)/silica ratios, respectively. Mechanical properties such as Vicker’s hardness and fracture toughness were measured by indentation method; and bending fracture toughness was also obtained by the surface crack in flexure method. Differential thermal analysis was used to determine the glass transition temperatures of these glass series. The variation of mechanical properties of glass series have been interpreted in terms of acquired structural information from 29Si NMR, Raman and FTIR absorption spectroscopies. It is found that magnesia and calcia act as network modifiers when they are substituted for silica in MgO and CaO glass series, and therefore they reduce connectivity of glass series. However, at fixed silica and soda contents, addition of magnesia at the expense of calcia increases network polymerisation. Indentation experiments showed that magnesia rich soda-lime-silica glasses are more susceptible to stress-corrosion than calcia rich glasses, and that they exhibit large discrepancies between direct and 24 hour indentation toughness values. Raman spectra of MgO and CaO-MgO glass series show that the intensity reduction in the long tail of the low frequency band is less for magnesia rich soda-lime-silica glasses compared to the observed reduction in calcia rich ones, and presumably this is potentially linked to presence of relatively larger membered rings in magnesia rich glasses. And therefore, the potential higher abundance of large membered rings might reduce stress-corrosion resistance of high magnesia containing glasses. No significant trend between bending fracture toughness and indentation fracture toughness could be identified. Moreover, large discrepancies are observed between direct and 24 hours indentation toughness values of MgO glass series. And all these inconsistencies raise the doubts over the accuracy of indentation method which has also been discussed in the literature. Elastic moduli have been measured by acoustic means, and it was found that Young’s moduli of MgO, CaO, CaO-MgO and Al2O3 glass series increase with network depolymerisation; and the significant role of packing density on Young’s modulus and Poisson’s ratio is obtained. Bending (surface crack in flexure) experiment has been used to minimise the uncertainties associated with indentation method. Contrary to the reports of previous works, the addition of magnesia in place of calcia does not increase fracture toughness. However, substitution of calcia in place of silica or magnesia gives rise to higher fracture toughness values in CaO and CaO-MgO glass series. It was also found that the replacement of alumina by silica can increase fracture toughness of soda-lime-silica glasses, and this increment in fracture toughness can be attributed to reduced stiffness and easier plastic deformation of silicate backbone as a result of removal of alumina that have significantly larger bond strength than that of other conventional oxides used in soda-lime-silica glasses. Furthermore, glasses that are more resilient to sharp contact loading exhibit lower fracture toughness values; whereas, glasses that possess larger packing densities and Poisson’s ratios favour easier shear flow and show larger fracture toughness values. Therefore, increasing alkaline earth oxide content preferably using a less covalent one in place of silica; or removing structural units (i.e. AlO4) that have very high dissociation energy per unit volume from silicate network can reduce stiffness of backbone of silicate glass and hence can increase plastic deformation capacity and bending fracture toughness of soda-lime-silica glasses. Calcium oxide-rich glasses (i.e. 14CaO glass) exhibit one of the highest fracture toughness values (~0.95 MN m-3/2) whilst the lower fracture toughness values (~0.78 MN m-3/2) are observed for low calcium oxide containing silicate glasses; and the total increment of fracture toughness is ~ 22% due to the replacement of silicon dioxide by calcium oxide. This significant improvement in the fracture toughness with composition can enable to formulate new glass compositions to produce thin-walled and tougher soda-lime-silica glass products such as container glass (i.e. bottles and jars) in glass manufacturing industry. Additionally, addition of calcium oxide in place of silicon dioxide can also reduce melting temperature of the glass batch. Consequently, higher calcium oxide/silicon dioxide ratio in soda-lime-silica glass can be more beneficial for glass industry to manufacture lighter and energy-efficient glass products. Higher fracture toughness values are generally observed for calcium oxide-rich soda-lime-silica glasses that are more packed than silicon dioxide-rich glasses, and this shows that denser soda-lime-silica glasses exhibit higher fracture toughness values. However, it can be possible to produce tougher soda-lime-silica glasses that have larger network openness and relatively lower density as is obtained in Al2O3-free glass; but energy consumption will be significantly higher for these high silica containing soda-lime-silica glasses, although these glasses exhibit good chemical durability. Overall, market competitiveness and high energy costs in glass industry can dictate the use of cost-effective glass compositions such as calcium rich soda-lime-silica glasses