The influence of K2O on the crystallization and microstructure of glass-ceramics in the SiO2-Li2O-K2O-ZnO-P2O5 system

Five lithium disilicate glass-ceramics were prepared in the SiO2-Li2O-K2O-Z nO-P2O5 system, varying the K2O content from 0 to 6.0 mol %, while the mola r ratio of the other components was kept constant. Crystallization processe s were investigated using high temperature X-ray diffraction and differenti al scanning calorimetry. K2O mainly influenced the phase formations during heating in the temperature range of 500 degreesC to 1100 degreesC. The crys talline phases lithium disilicate, lithium metasilicate, lithium phosphate, cristobalite and quartz were detected in different successions. Samples wi thout K2O exhibited multiple cracking during isothermal heat treatment, as the developed internal strains could not be relieved by viscous flow. Glass -ceramics with K2O contents of 1.5 and 2.9 mol % showed a fine grained micr ostructure with randomly oriented, rod-shaped crystals, which were responsi ble for high mechanical strengths. A K2O content of 4.5 mol % caused a coar se crystallization of lithium disilicate crystals up to 20 mum in length. L ithium metasilicate was the main crystal phase in the 6.0 mol % K2O contain ing glass ceramic.