| Glass-ceramics are composed of crystalline phases and a vitreous matrix phase. These materials combine the advantages of glass and ceramic, and play an important role in dental all-ceramic restorations. Oweing to the suitable coefficient of thermal expansion and refractive index of lithium disilicate crystals, lithium disilicate glass-ceramics could obtain high mechanical strength and translucent effect simultaneously, which show a brilliant future in dental aesthetic restorations. The purpose of the study was to investigate the effects of glass composition, content of P2O5 and heat treatment procedures on microstructure and properties of final glass-ceramic products. And the possibility of application of experimental lithium disilicate glass-ceramic in dental heat-pressing technique was also studied. The appropriate heat-pressing parameters were determined and the influence of heat-pressing on microstructure and properties was also analyzed, which could provide valuable information for developing novel dental glass-ceramic with intellectual property.The whole experiments were composed of two parts.Part one: the preparation of lithium disilicate glass-ceramic1. In this experiment, several glasses were designed based on the Li2O-SiO2 binary glass system. After analysis of crystalline composition, microstructure and mechanical properties, Li2O–SiO2–K2O–Al2O3–ZrO2 glass system was selected for following investigations. P2O5 as nucleation agent could promote the crystallization of base glass.2. The effect of P2O5 content and heat treatment on the crystallization, microstructure and properties of glass-ceramic was studied. Results demonstrated that surface crystallization occurred in the glass ceramic with P2O5 content of 0.5mol.%, and glass-ceramics with P2O5 content in the range of 1.0-2.0 mol.% showed obvious volume crystallization characteristics. With an increasing P2O5 content, the crystallization temperature of parent glasses decreased. There appeared a decreasing trend in crystal size and flexural strength as P2O5 content increased. Glass ceramic with P2O5 content of 1.0mol.% after two-stage heat treatment showed the highest flexural strength and stable microstructure.3. The effect of heat treatment temperature on microstructure and properties of glass-ceramic (Li2O–SiO2–K2O–Al2O3–ZrO2-P2O5) was studied. Results demonstrated that as temperature increased, the content of Li2Si2O5 increased and became the main crystallize phase. The crystal size also increased, even from spherical shape into bar shape (2-5μm). Flexural strength and contrast ratio showed an increasing trend followed the increased temperature. Glass-ceramic in group C (650℃(1h)+900℃(1h)) showed the highest flexural strength and suitable translucent characteristics, in which appropriate crystal size and interlocking microstructure were found.4. The crystalline composition, microstructure and mechanical properties of experimental lithium disilicate glass-ceramic (EDLC) and IPS e.max Press were compared. No obvious difference in mechanical properties and crystalline composition was found between these two materials. The high mechanical properties of dental lithium disilicate glass-ceramics were attributed to the relatively high crystalline content, residual stress and the interlocking microstructure.Part two: the application of experimental lithium disilicate glass-ceramic in dental heat-pressing technique5. The effect of different heat-pressing procedures on the porosity and flexural strength was studied. The feasibility of processing ELDC with current dental heat-pressing equipments was verified. The appropriate parameters were determined as follow: heat-pressing temperature-950℃; holding time-15min; pressing time-5min.6. The influence of dental heat-pressing on the crystalline composition, microstructure and chemical stability of ELDC was studied. Dental heat-pressing had no influence on the crystalline composition and content. Lithium disilicate (Li2Si2O5) represented the main crystalline phase for both ELDC glass-ceramics before or after heat-pressing. After pressing, crystal size increased slightly and seemed to align along the direction of pressing. Superior chemical stability was found for both glass-ceramics before or after heat-pressing.7. In this experiment, all-ceramic restoratives were made with experimental lithium disilicate glass-ceramic using dental heat-pressing technique. The cores made from ELDC were intact and showed superior marginal fitness. No gap was found at the interface between ELDC and veneers ceramic. All restoratives showed good appearance and superior translucent characteristics. |
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