| Mica based glass-ceramics have been widely applied in medicine, chemistry, electronic engineering for a long time since they were developed in the 19th century. The new fabricating technology also expanded their application in many fields. The mica based glass-ceramics were layered silicas, and the combining force was very weak, which caused the mechanical properties decreased. Therefore, it has been the focus for researchers to increase the strength and fracture toughness.As a special forming method, dental heat pressing changed the traditional lost-wax casting fashion. Nucleators are included in heat-pressing glass-ceramics, so they can be crystallized after heat pressing. The heat pressing can also improve the strength by raising the density and the percent of crystal. At present, the dental heat-pressing ceramic depends on import, and is rare in variety. The category of crystal limits in lutecium and lithium silicate. The traditional castable mica based glass-ceramics has not been fabricated with dental heat pressing until now.To solve these problems, our group developed a novel mica based glass-ceramic which was without interlayer and exhibited high fracture toughness and had excellent machinable property in previous study. A continuous band chips could be formed in machining at a high velocity for this kind of mica glass-ceramic, and it was helpful to avoid surface damage and edge crack. At present, the heat pressing is still the main processing manner in dentistry because of the high cost and the need for technician's capacity in CAD/CAM. Therefore, the research of appling this experimental glass-ceramic to heat pressing is very helpful for clinic. Previous study has shown that the coefficient of thermal expansion of mica was in 8-13×10-6K-1, and it was 9-10-6K-1 in tooth. So it was essential to adjust the crystal content and moderate coefficient of thermal expansion. Because the coefficient of thermal expansion in spodument was 9×10-6K-1, it was hopful to moderate coefficient of thermal expansion by raise the content of spodument. In this study, the SiO2- Al2O3- MgO- Na2O- K2O- F glass-ceramic was used. In the first part of this study, we prepared three kinds of ceramic ingots with different transparency. On the basement of first part, we applied this experimental glass-ceramic to heat-pressing procedure. The effects of fabricating procedure on microstructure and properities of the glass-ceramics were investigated and compared. The aim of this study was to evaluate the principle of crystal and the effects of variables to the experimental glass-ceramics. We also explored the proper variable in the procedure of heat pressing to establish foundation for the dental application of the experimental glass-ceramics. Two parts were included in this study:Part one Effects of Li2O addition on the crystallization of experimental mica based glass-ceramicsObjective: To investigated the effects of Li2O on the crystallization behavior of SiO2-Al2O3-MgO-Na2O-K2O-F system by varying the Li2O content in the glass composition. Method: Different amounts of Li2O were added into SiO2-Al2O3-MgO-Na2O-K2O-F system. Differential thermal analysis (DTA), X-ray diffractometry (XRD) and scanning electron microscopy (SEM) were used to evaluate the crystallization behavior, crystalline phases and microstructures of the glass-ceramics. Three point bending strength was also measured. Result: The glass-ceramics used in this study could be prepared with one-step heat treatment. The temperature of fusing was 1400℃, and the main crystal was mica when it was heat treated at 650℃-710℃. In the group heat treated at 680℃for 1h, the crystal developed well in the glass containing 4% Li2O, and showed higher bending strength (183.82±12.47 MPa)than the others. Addition of Li2O decreased the temperature of crystal, and promoted the crystallization. With the increasing of Li2O, the category of crystal was changed, the coefficient of thermal expansion decreased, and the softening point raised greatly compared with parent glass. Conclusion: The composition of Li2O can improve the crystallization property of SiO2-Al2O3-MgO-Na2O-K2O-F glass-ceramics. At same time, the coefficient of thermal expansion was decreased.Part two Effects of dental heat pressing on the structure and properties of experimental mica based glass-ceramicObjective: The purpose of this study was to evaluate the effect of dental heat pressing on the microstructure of a glass-ceramic located in SiO2-Al2O3-MgO-Na2O-K2O-F system. Method: glass was cut into bars and ingots. Ingots were used for heat pressing, and bars served as controls. DTA, SEM, XRD techniques and mechanical testing methods were used to investigate the effects of dental heat pressing on crystal and microstructure. The erosive liquid was prepared in accordance to ISO6701 standard, and the artificial saliva served as control. Specimens were put into erosive liquid or artificial saliva at 80℃for 16h. SEM observation and three bending test were taken after complete cleaning and drying. Results: The results confirmed that the glass-ceramic could be successfully pressed with commercially available dental equipment. The SEM observation revealed crystals not only aligned along the direction of pressing, but also parallel to the surface of specimens. The results of bending test revealed that the bending strength of pressed specimens were higher than that of control ones. Erosive pores could be observed after corrosion at the surface of control specimens, and strength decreased significantly. Compared with control ones, the pressed specimens showed higher resistance of chemical corrosive capability, and the bending strength was not significantly different from that of before corrosion. Conclusion: It was concluded dental heat pressing procedure led to an anisotropic structure, which can improve the mechanical property, and dental heat pressing procedure led to higher resistance of chemical corrosion.
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