Effects of microstructures on the mechanical properties of lithium disilicate glass-ceramics for the SiO2–Li2O–P2O5–K2O–ZnO system

Abstract

The effects of microstructures on the mechanical properties of lithium disilicate (Li 2 Si 2 O 5 :LS2) glass-ceramics (GCs) were carefully investigated for the SiO 2 –Li 2 O–P 2 O 5 –K 2 O–ZnO system. The microstructures of GC samples, having three different compositions but the same total 6 mol% of additives (P 2 O 5 , ZnO, K 2 O), 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 3 PO 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 2 O 5 , 2.75 mol% K 2 O 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.