Effect of ZnO on the crystallization, microstructure, and properties of glass-ceramics in the SiO2-Li2O-ZnO-K2O-P2O5 system

Abstract

Six glasses in the SiO2-Li2O-ZnO-K2O-P2O5 system were prepared. Starting from a dental glass-ceramic model composition the ZnO content was varied from 0.0 to 8.3 mol% with the molar ratios of the other components being invariant. Crystallization processes of these glasses were investigated using high-temperature X-ray diffraction and differential scanning calorimetry. Crystal phases occurred in the temperature range of 500 to 1050 °C, while the main precipitated phase was lithium disilicate (Li2Si2O5). Lithium metasilicate, lithium orthophosphate, and cristobalite were secondary phases. Morphology and microstructure of the glass-ceramics were studied using scanning electron microscopy. Li2Si2O5 crystals in glass-ceramics had elongated shapes along the c-axis, crystal lengths ranging from 1 to 5 µm and aspect rados between 2 and 5. The lath-shaped Li2Si2O5 crystals were randomly oriented in the volume of the glass-ceramics and were responsible for the good mechanical properties. With increasing ZnO content a decrease of flexural strengths from 520 to 450 MPa was observed. It was concluded that the ZnO variation mainly influenced the translucency of the glass-ceramics. Relating to dental standard contrast ratios between 0.23 and 0.74, method limits of 0 and 1 provided, were determined.