Establishment And Performance Analysis Of Functionally Graded Structures For Dental Application

BackgroundOwing to its good chemical properties, dimensional stability, and high mechanical strength and toughness, zirconia is currently widely utilized in prosthetic devices. Since it is opaque, it is typically veneered with a more translucent, albeit weaker, porcelain layer to obtain a shade comparable to that of natural teeth. However, a higher incidence of veneer chipping and fracture has been reported in clinical research and practices on porcelain-fused-to-zirconia (PFZ) systems compared with porcelain fused with metal crowns. Fracture propagates through the weakest component in these veneer-zirconia bilayer structures resulting in delamination of the veneer or extension to the core. The location of the interface has been shown to be the origin of failure, which suggests that the bond strength between the veneering ceramic and the zirconia framework is weak in layered all-ceramic restoration under load. Therefore, in order to reduce clinical failures, much effort has been focused on how to increase the bonding strength between veneering ceramic and zirconia.A functionally structure with high mechanical strength maybe a possible choice to increase the longevity of restorations by infiltrating matching glass composition into a dense zirconia substrate.ObjectivesThe purpose of the study was to test the hypothesis that infiltrating glass into the pre-sintered zirconia-based ceramics can improve the core-veneer bond by diminishing both flexure-and contact-induced tensile stresses on the outer surface of the diffusion layer.This study was aimed to optimize the formula of glass composition, infiltration technique and design of functionally graded structure. We present a new method by preparing a new composition of glass that was infiltrated into a zirconia substrate to form a functionally graded structure to improve the bonding strength between zirconia and veneer. The microstructure and the physical and mechanical properties of the graded-layer structure was then investigated. This study will provide a new idea for design of core-veneer interface and then effectively increase the long term successful rates of zirconia based restorationsMaterials and methodsPartⅠThe sessile drop technique was used to investigate the wettability of the zirconia substrate and the seven formula of glass composition. The glass frit was placed on the middle of the sintered zirconia substrate, which was then placed in a muffle furnace. A digital camera was used to take a photo of the glass from1100℃to1200℃at a interval of10℃. The contact angles of the glass frits on the zirconia substrate were then investigated at a temperature of1200℃. Then, we selected the best glass formula which has excellent wettability with the zirconia substrate.PartⅡ3Y-TZP substrate samples5mm thick and lmm in diameter were cut from the compressed powder and then pre-sintered at1200℃for2h inside a muffle furnace, producing a somewhat porous template for glass infiltration. The glass composition was reduced to a frit and applied, in slurry form, onto the top surface of the pre-sintered Y-TZP porous substrate samples. The coated samples were then infiltrated (1350℃for2h) to produce a glass/zirconia-glass/zirconia (G/G-Z/Z) layered structure.The cross-sectional surface morphology of the3Y-TZP matrix after glass infiltration was observed by a scanning electron microscope. One of the sample cross-sections was etched with dilute hydrofluoric acid (10%solution in water) for30min and then gold coated for SEM examination. The crystalline phases present in the graded-layer structure were identified using an X-ray diffractometer(XRD).Part IIIThe radius of the zirconia samples, before and after glass infiltration, was determined by digital caliper with an accuracy of0.01mm. The percent shrinkage during glass-infiltration was then calculated.Young’s modulus and hardness of residual glass layer, graded glass-zirconia layer and the zirconia interior were measured using nano-indentations.The shear bond strength of two groups between glass-infiltrated zirconia and the veneering ceramic was then investigated.Results1. The second formula of glass frits almost fully wet the zirconia substrate at1200℃, showing a contact angle of45.7°, which was much better than the other formulas of glass.2. Within the range of1200℃-1400℃, The viscosity of glass was low, which was suitable for glass infiltration.3. SEM image shows the outer surface residual glass layer (black) and the graded layer. The glass appeared to be dark while zirconia grains appear to be white. The graded layers contained a relatively high glass content at the residual glass layer interface and gradually transformed to a dense zirconia interior. The molten glass infiltrated the porous zirconia matrix by capillary forces at high temperature. After etching for20min, the SEM image reveals a three-dimensional network morphology consisting of traces of residual glass, glass-coated zirconia grains, and intergranular voids.4. Compared with the XRD patterns of the non-infiltrated, the main crystalline phase of zirconia, tetragonal (t-ZrO2), was maintained after glass-infiltration, but a portion was found to be in a monoclinic phase, m-ZrO2-5. Compared with the control group, the radius of shrinkage of the test group was as low as (24.007±0.011)%after infiltration. There was no significant difference between the test groups and control group6. The residual glass layer possessed a elastic modulus value of E=121.8±26.5GPa(mean±S.D) and a hardness value of H=10.1±2.5GPa(mean±S.D). For the glass-zirconia graded layer, the elastic modulus and hardness varied from E=204.7GPa and H=13.4GPa near the residual glass/graded layer interface to E=256.2GPa and H=16.4GPa near the graded layer/Y-TZP layer boundary. The Y-TZP interior exhibited elastic modulus and hardness values of E=279.4±2.8GPa(mean±S.D) and H=21.3±1.1GPa(mean±S.D), respectively.7. The mean SBS values were (26.43±0.58)MPa for the test group with glass infiltration and (8.47±0.33)MPa for the control group without glass infiltration. The SBS values of graded zirconia with glass infiltration were statistically higher than the control group without glass infiltration (P<0.05).Conclusion1. A good wettability of glass composition and zirconia plays an important role in the glass-infiltration process. The prepared glass has excellent wettability with the zirconia substrate.2. The functionally graded structure can be fabricated by using a glass infiltration/densification technique. By using the prepared glass, we have successfully established infiltration conditions to fabricate G/G-Z/Z structures.3. The glass infiltration process could not lead to a significant phase transition.4. Using a glass infiltration/densification technique can only slightly reduce the shrinkage of the radius of the zirconia.5. The functionally graded structures can improve its core/veneer bond, aesthetics, endurance, and fatigue resistance by diminishing both flexure-and contact-induced tensile stresses on the outer surface.6. The shear bond strength between glass-infiltrated zirconia and the veneering ceramic in the test group was (26.43±0.58)MPa, which indicated that the core-veneer bond between the zirconia core and the veneering ceramic can be improved by glass-infiltration.