Mixed CaO/MgO effect on microstructure, mechanical properties and crystallization behaviour of Li2O-Al2O3-SiO2-ZrO2-P2O5 glass

The improvement of glass and glass ceramics mechanical performance, which is determined by the components and structures, has become a critical issue for various applications. In current research, alkaline earth oxides, MgO and CaO, were introduced into Li2O-Al2O3-SiO2 glass components for glass better mechanical properties. The dependency of CaO/MgO ratios on the glass structures were investigated by Raman and nuclear magnetic resonance spectroscopy. The conversion of Qn groups in glass network confirmed decreasing bridging oxygen and connectivity with the increased CaO/MgO ratios. The glass densities, molar volumes showed linear increase with the replacement small Mg2+ion with large Ca2+ ion in glass structures. The glass transition and first crystallization temperatures slightly dropped with higher CaO content, until MgO was completely replaced. The non-linear glass hardness with maximum deviation from end numbers indicated the mixed alkaline effect in Li2O-Al2O3-SiO2 glass, probably originated from the chemical bonds and topological constraints due to the coexistence of Mg and Ca in glass structure. After heat-treatment, MgAl2Si4O12 and SiO2 crystals were precipitated in Li2O-MgO-Al2O3-SiO2 glass matrix, and the crystals phases changed to the combination of MgAl2Si4O12, LiAlSi3O8, and SiO2, to LiAlSi3O8, SiO2, to LiAlSi3O8, SiO2, ZrSiO4, and finally to Li2O·Al2O3·7.5SiO2, SiO2, ZrSiO4, due to the changed ion radius and contents, with the increased CaO/(MgO+CaO) ratios. Meanwhile, the diameters of crystals grew larger. The hardness and Reduced elastic modulus of glass ceramics increased at first, from 8.30 to maximum 8.67 GPa, and from 74.00 to 84.20 GPa, with 0.2 CaO/(MgO+CaO) ratio and then dropped. The glass structures, crystals phases and sizes change were responsible for the different mechanical properties.