Preparation And Mechanical Properties Of Lithium Disilicate Glass-Ceramics

The lithium disilicate glass-ceramics and their composites with high relative density were prepared by melting method and hot-pressing technology, respectively. The influences of heat treatment processes, content of nucleation agent and sintering temperature on crystalline phase, microstructure and mechanical properties have been studied. Furthermore, the crystallization mechanism of glass-ceramics has been analyzed during heat treatment and hot-pressed sintering. In the glass-ceramics composite systems, strengthen and toughened mechanisms also have been investigated.With regard to lithium disilicate glass-ceramics prepared by melting method, with increasing the content of P₂O₅, the crystallization temperatures increase and followed by a decrease slightly. Meanwhile, the relative content of Li₂Si₂O₅ crystals decrease and those of Li₂SiO₃ and Li₃PO₄ crystals increase in the case of single-stage and two-stage heat treatments. With increasing the content of P₂O₅, the microstructure is transformed from plate-like polycrystalline aggregates to randomly rod-like crystals and even spherical grains. Two-stage heat treatment is favorable to obtain the higher crystallinity and more Li₂Si₂O₅ crystals than single-stage treatment. Moreover, the specimens heat-treated by two-stage treatment with 1mol% and 2mol% P₂O₅ appear the better fracture toughness and flexural strength, respectively. According to the analysis of thermodynamics and crystallography, Li₂Si₂O₅ crystals probable can epitaxial growth on some crystallographic planes of Li₂SiO₃ phase.By varyting the mole ratio of SiO₂:Li₂O and P₂O₅ content, a series of lithium disilicate glass-ceramics are prepared by hot-pressing technology. With increasing the mole ratio of SiO₂:Li₂O, the relative density of glass-ceramics decreases because the viscosity increases. The relative content of Li₂SiO₃ and SiO₂ crystalline phases decrease and followed by an increment. Furthermore, when the content of P₂O₅ increase from 1.0mol% to 4.0mol%, the relative density decreases owning to the phase separation which lead to the increment of viscosity in the Li-rich regions. In addition, the relative contents of Li₂SiO₃ and Li₂Si₂O₅ crystals decrease. Meanwhile, Li₂SiO₃ crystals disappear in the hot-pressed glass-ceramics in which P₂O₅ are 3.0mol% and 4.0mol%, respectively. During hot-pressing sintering, the growth mechanism of Li₂Si₂O₅ crystals is reaction transform which is at the cost of consuming Li₂SiO₃ crystalline phase.With increasing the mole ratio of SiO₂:Li₂O, the crystal morphology is transformed from equiaxed to rod-like grains, and then some equiaxed grains emerge again. Meanwhile, the mechanical properties of glass-ceramics incresase and followed by a decrease, and the specimen whose mole ratio is 2.0 appears better mechanical properties. Additionally, with increasing the content of P₂O₅, the microstructure is refined from rod-like crystals to needle-like structure. On the other hand, because of the phase separation, the relative density and mechanical properties decrease. By varying the sintering temperature, the specimen hot-pressed at 820℃with 1mol% P₂O₅ has the better mechanical properties. The flexural strength and fracture toughness are 290MPa and 3.3MPa·m^1/2, respectively. In addition, with increasing the sintering temperature, the grain size increases slightly. The species of crystalline phases and distribution do not change significantly.Zirconia/lithium disilicate glass-ceramics composites have been prepared according to Li₂O-SiO₂-ZnO-K2O-CaO-1mol%P₂O₅ system with different content of 3Y-TZP by hot-pressing technology at 800℃in vacuum. The crystallinity of the composites increases slightly and followed by a decrement with increasing 3Y-TZP from 5wt% to 30wt%. Furthermore, the microstructure is refined by comparing with that of zirconia-free glass-ceramics. The crystals morphology transforms from rod-like to equiaxial and then converts to short rod-like crystals. In addition, the mechanical properties tend to increase firstly and followed by a decrease. G1P15Z has the best mechanical properties whose flexural strength and fracture toughness are 333MPa and 3.5MPa·m^1/2, respectively. Strengthening and toughening mechanisms, such as strengthening by residual compressive stress, transformation toughening, crack deflection and bridge connection mechanisms are confirmed.