| Because of its Aesthetics, biocompatibility and wear resistance, all-ceramic dental materials are more widely used. However, dental ceramics shows brittleness, low strength and expensive, which has a certain restrictions for prosthodontics materials. It has been proved that the composition of dispersed nano particles could greatly improve strength, toughness and high temperature performance of the ceramic matrix. Nano-zirconia ceramic is a new biological material, because of its high strength and high toughness for the best choice of ceramic toughening. In this study, we select the diatomite as substrate materials, enhanced its mechanical properties by the addition of nano-zirconia toughened ceramic.In order to obtain high density of the ceramic material, we should first study the sintering mechanism. Preparation of ceramic sintering is the most critical part of the process, which determines the performance of the final product, while the sintering process of one of the most important factor is the sintering curve. Dilatometry is the most common instrument which usually used to test the sintering behavior to predict sintering curve, by testing the relationship between the size of the sample and time/temperature. In the first part of this project, we aim to determine the best sintering ceramics through the different sintering temperature under the guidance of dilatometry.Particle size, size distribution, agglomeration of powders influence mechanical properties and microstructure of the ceramics. Small particle size, van der Waals forces between particles, static electricity and so on are easily lead to particle agglomeration. Powder surface modification is an effective way to solve agglomeration. Layer-by-layer technique, by adjusting the pH value of the solution, using electrostatic attraction between the opposite charges that produce the driving force in the formation of multi-surface layer polyelectrolyte, alternating adsorption of charged polyelectrolyte surface, to achieve the purpose of surface modification by electrostatic steric stabilization. The second part of this project aims to reduce agglomeration of diatomite through layer-by-layer technique, and to adsorbe closely with nano-zirconia powder to form a stable dispersion of ceramic powders, and finally to improve mechanical properties of the nano-composite ceramics.Most all-ceramic restorations utilize double system design, that is, support on the base of high-strength ceramic materials, then sintered better aesthetics veneering porcelain ceramic. The interface between substrate ceramic and the veneering porcelain is one of the most vulnerable areas, which prone to the phenomenon of porcelain collapse leading to restoration failure. Therefore, the key to success of high strength all-ceramic restorations is to achieve good matching between substrate ceramic and the veneering porcelain. The third part of this project aims to detect the bonding strength between diatomite-based dental ceramic and veneering porcelains and observe the microstructure and elements distribution of interface, in order to choose the best veneering porcelain for diatomite-based dental ceramic substrate. Objective: To investigate optimum densification rate of diatomite-based nano-composite dental ceramics, we tested different sintering curve and the effect on mechanical properties and microstructure in order to enhance the properties of diatomite-based dental ceramics.Methods: The weight ratio of diatomite: bentonite: quartz sand: Na2CO3= 11:4:4:1, in addition of 20wt% nano-zirconia powder.Then mechanical milled for 24h with the same qualitr of ethanol , followed with drying, screening, adding an appropriate amount of PVA solution, grinding, compression and polishing to the thickness of 1 mm. The diatomite-based nano-composite dental ceramic was sintered to 1150℃using dilatometry with the heating rate of 3℃/min in order to test the conteaction rate during sintering. Based on this result, the ceramics divided into five groups and respectively sintered to 1050℃, 1070℃, 1085℃, 1100℃, 1150℃. The mechanical properties of bending strength, Vickers hardness and fracture toughness were measured. SEM was used to observed microstructure of the ceramic cross-sections. Results: With the final sintering temperature increased, mechanical properties of the ceramics gradually increase and porosity decreased. As the sintering temperature further increased to 1150℃, lower mechanical properties appear, and the porosity increased. Sintering temperature to 1100℃, its mechanical properties showed the best and significantly better than the other groups (P <0.05). Bending strength, Vickers hardness and fracture toughness was 207.45±8.00MPa, 5.77±0.36GPa, 3.04±0.40 MPa·m1/2, respectively, and linear shrinkage rate was 13.28%±0.56, which were consistent with the measured thermal expansion curves. SEM showed the porosity of ceramic cross-section significantly smaller, the density increases.Conclusion: The sintering curve shows a great effect on the ceramic sintered bodies. The thermal expansion curve could provide a preliminary assessment of sintering densification process, and further experiments proved that their optimum sintering temperature is 1100℃, which provide a theoretical basis for the further improvement of the mechanical properties . Objective: To investigate modification effect on the diatomite-based dental ceramic powders and its adsorption capacity with nano-zirconia in order to reduce the agglomeration of them by layer-by-layer technique. A core - shell structure and well dispersed composite particles were prepared and the mechanical properties of sintered ceramics were improved.Methods: Two polyelectrolyte were respectively assembled on the surface of diatomite-based particles by layer-by-layer technique, and then adsorbed nano-zirconia powders through opposite charge. Zeta potential, particle size and size distribution changes of diatomite-based particles were compared before and after modification. Infrared spectra (IR) analysised the molecular structure of functional group changes on the surface of diatomite particles. SEM observed morphology of the nano-composite ceramic powders. The unmodified and modified diatomite-based particles, respectively, included 0wt.%, 20wt.%, 25wt.%, 30wt.%, 35wt.% nano-zirconia were sintered to 1100℃, which were recorded as U0Z, U20Z, U25Z, U30Z, U35Z, and M0Z, M20Z, M25Z, M30Z, M35Z. The mechanical properties of bending strength, Vickers hardness and fracture toughness were measured. XRD detected phase changes of M30Z before and after sintering. SEM and EDS tested microstructure and distribution of elements of U30Z and M30Z. Propagation phase contrast synchrotron X-ray microtomography (PPCSR-CT) were newly used to observe the internal structure.Results: Two polyelectrolyte successfully assembed on the diatomite-based surface, and the particle size and size distribution of the particles significantly reduced, d(0.5) reduced from 16.421μm to 0.420μm; SEM showed the improved dispersion of the modified diatomite-based particles and a good adsorption with nano-zirconia powders. Mechanical properties of the modified sintered groups were significantly better than unmodified groups, M30Z showed the best mechanical properties of three-point bending strength, Vickers hardness and fracture toughness respectively increased to 276.26±20.55MPa, 11.88±2.09GPa and 5.07±0.35MPa ? m1/2. SEM and EDS analysis displayed the formation of core-shell structure and the reduction porosity of sintered body, PPCSR-CT confirmed obvious dense of internal structure. Conclusion: The layer-by-layer technique can significantly improve the dispersion of diatomite-based particles and the mechanical properties of sintered ceramics. Objective: We detected the bonding strength between diatomite-based dental ceramic and veneering porcelains and observed the microstructure and elements distribution of interface, in order to choose the best veneering porcelain for diatomite-based dental ceramic substrate.Methods: The coefficient of thermal expansion(CTE) of diatomite-based dental ceramic between 25-500℃were detected by dilatometry with the heating rate of 5℃/min. Three veneering porcelain materials were selected with the best CTE matching which were alumina veneering porcelain (A group), titanium porcelain veneering porcelain (B group), E-max veneering porcelain (C group). The veneering porcelain materials were sintered to diatomite-based dental ceramic substrate according to the sintering curves from manufacturers. Shear bonding strength were detected. SEM and EDS observed the interface microstructure and element distribution.Results: The CTE of diatomite-based dental ceramic between 25-500℃was 8.85*10-6℃. The results showed that, shear bonding strength between A group and B group showed no significant difference(P>0.05), A group and C group showed significantly difference(P<0.05), in addition, B group and C group also showed statistical significance (P<0.05). According to the fracture interface, A group displayed the fracture of alumina, B group displayed the complete drop of titanium porcelain, and C group showed mixed failure of part of E-max damage and part of E-max drop. SEM and EDS found that the interface sintered tightly with no gap, and penetration of elements on both sides of the interface. This indicated that the diatomite-based substrate ceramic combine best with E-max porcelain veneer. Conclusion: The diatomite-based substrate ceramic combine best match with E-max porcelain veneer in the form of mixed failure destruction.
|
PDF Full Text Request