Crystallization and mechanical properties of MgO/Al2O3/SiO2/ZrO2 glass-ceramics with and without the addition of yttria

Glasses in the system of MgO/Al2O3/SiO2 were melted using yttria stabilized tetragonal zirconia or monoclinic zirconia as nucleation agent. In some of them, MgO was partially replaced by ZnO. After melting and casting the glasses, the samples were annealed in the temperature range from 950 to 1150 degrees C. The obtained glass-ceramics were colourless and transparent to opaque. This is in contrast to glass-ceramics doped with titania or a mixture of titania and zirconia which appear purple to blue. In compositions using 4 mol% tetragonal zirconia stabilized with 3 mol% yttria only beta-quartz solid solution was detected. Using monoclinic zirconia or doped zirconia with concentrations >4 mol% resulted in the formation of alpha-quartz solid solutions. Additionally the crystal phases spinel or gahnite/spinel-solid solution were formed. The formation of these crystal phases also leads to improved mechanical properties. Bending strengths up to 475 MPa, Young's moduli up to 131 GPa, Vickers hardness up to 12.5 GPa and fracture toughness up to 2.3 MPa m(1/2) were obtained. (C) 2011 Elsevier Masson SAS. All rights reserved.