Effects of microstructures on the mechanical properties of lithium disilicate glass-ceramics for the SiO_2-Li_2O-P_2O_5-K_2O-ZnO system

摘要

The effects of microstructures on the mechanical properties of lithium disilicate (Li_2Si_2O_5:LS2) glass-ceramics (GCs) were carefully investigated for the SiO_2-Li_2O-P_2O_5-K_2O-ZnO system. The microstructures of GC samples, having three different compositions but the same total 6 mol% of additives (P_2O_5, ZnO, K_2O), could be controlled by varying heat treatment conditions of glass samples. In addition to major LS2 crystalline phase, two other minor crystalline phases of Li_3PO_4 (LP) and cristobalite were detectable. Average crystal sizes of GC samples were in the region of 0.3-10 μm, and the volume fractions of crystalline phases were in the region of 0.70-0.73. The biaxial flexural strength was improved with decreasing the average crystal size from ~10 to ~1 μm by following the Hall-Petch relation, and the maximum flexural strength of 445.8 ± 7.2 MPa was obtainable from the GC samples with the additive compositions of 1.5 mol% P_2O_5, 2.75 mol°/o K_2O and 2 mol% of ZnO which were nucleated at 540 °C for 1 h and grown at 850 °C for 4 h. When 8.4 vol% cristobalite was incorporated into the sample, however, there was a severe scattering in flexural strength data due to the formation of microcracks induced by the beta-to-alpha transition of cristobalite and also due to a micro tensile stress in the residual glassy phase induced by a large difference in thermal expansion coefficients between crystal and residual glassy phases. Meanwhile, the Vickers hardness was increased with decreasing the average crystal size down to ~0.3 μm, indicating that the factors for the deterioration of flexural strength have a negligible effect on hardness.