Exploring The Optimum Design, Fabrication And Mechanical Behavior Of Functionally Graded Material (FGM) Layer In All-ceramic Dental Restoration

Purpose: Firstly the aim of this study was to explore the optimum design of the bio-inspired functionally graded material (FGM) layer in all-ceramic dental restoration and cambered all-ceramic restoration to achieve the excellent stress reduction and distribution. Three-dimensional finite element models (both layer and cambered one) of the ceramic restoration with multi-layer structure was established, including bilayered ceramic/ bio--inspired FGM layer/cement/dentine structure. The numerical model was analyzed by using the finite element method and first order optimization technique. Secondly research on change of glass infiltrated zirconia ceramic technology formed specimens' elastic modulus, compressive strength and damage resistance at different pre-sintering temperature.Method: 1. Three-dimensional finite element model of the bilayered ceramic/ bio-inspired functionally material graded layer/cement/dentine was established as the experimental group using the ANSYS package. The bilayered ceramic/cement/dentine model without FGM layer was constructed as the control group. Both experimental and control group used zirconia as the core. A numerical optimization procedure was applied with the above model using the first order method in ANSYS program. After the design variables were appointed, the computer program was developed to calculate the modulus of elasticity (En) of each layer of the FGM. A 600N static vertical load was applied to the model, a fully bonding interface under the dentin layer was assumed,2. Three-dirnensional data of the extracted human maxillary first molar were obtained via Mioro-CT, which were transterrcd into Mimies10.0 software generating a 3-D model of maxillary first molar. Three-dirnensional cambered finite element model of the bibvered ceramic/ bio-inspired functionally material graded layer/cement/dentine was established as the experimenial group using the ANSYS package. The cambered bilayered ceramic/cement/dentine model without FGM layer was constructed as the control group. Both experimental and control group used zirconia as the core. After the design variables were appointed, the computer program was developed to calculate the modulus of elasticity (En) of each layer of the FGM. In order to mimic the maximum occlusal force, the 3-D model was loaded 600N at the central fossa, 75N load was applied at the up one third of the bucco-inclined plane of the mesiobuccal cusp, distalbuccal cusp, and distal cusp respectively, giving a total of 225N, to mimic the masticatory force, a fully bonding interface under the dentin layer was assumed.3.The functionally graded zirconia specimens (radius r=6mm, 0.5mm thick) were divided into three groups in according to the pre- sintering temperature 900 °C, 1000 °C and 1100 °C. Specimens were fabricated using a glass-ceramic infiltration method. In this method, an in-house developed glass composition was employed to infiltrate the top surfaces of specimens. Then observe the microstructure of the gradient layer structure of the zirconia specimens. And by means of the nano indentation method and maximum fracture stress tests to determine the elastic modulus and the compressive strength of zirconia specimens. Fatigue tests and cracks analysis were also performed on the specimens to exam the damage resistance of multilayers.Results: I. Three dimensional finite element model of the bilayered ceramic/bio-inspired functionally graded material layer/cement/dentime was developed. As for the experimental group, the maximum principal stress was found in FGM with a value of 23.01 86 MPa. The stress distributed uniformly in FGM. For the control group. stress concentration was found at the interface between core ceramic and cement The stress value rose from 20.837 to l00.23MPa within the bottom of the core ceramic.2 Three dimensional cambered finite element model of the bilayered ceramie/bio-inspired functionally graded material layer/cernenl/denfine was developed. For experimental one, the stress change little In bioinspired functionally guided layer with maximum pcincipal stress value of 76.70MPa under vertical loads and 3 l.8/0MPa under inclined loads. As for the control group, under vertical and inclined loads, stress concentration was found at the sub-surface of the core with the maxim urn principal stress value of 121.90MPa and 63.82MPa respectively. 3. There was about 150-200?rn functionally graded layer formed on the surface of zirconia specimens with the electron microscopy. In the functionally graded layer, the elastic modulus was increased gradually. In the group two which pre-sintering temperature was 1000 °C , the compressive strength is the highest, and the other groups were significantly different, P<0.05. The experimental results obtained from fatigue tests showed that functionally graded zirconia specimens could bear more times of loading than monolithic zirconia ones. By the observation of specimens' surface with an in-situ Questar telescope, the cracks of experimental groups are fewer and more regular than the control group.Conclusions: 1. the existence of the bioinspired functionally graded layer can significantly reduce the stress concentration at the sun-surface of the core. The optimization of the elastic modulus graded change in the bioinspired functionally graded layer was existed. 2. The glass infiltration technology can form elastic modulus gradient structure on the surface of the zirconia core porcelain. The toughening effect on the zirconia ceramic specimens was better by 1000°C pre-sintering temperature. The results of maximum fracture stress tests., fatigue tests and cracks analysis demonstrated that functionally graded zirconia specimens had better toughening effect and damage resistance than monolithic zirconia ones. What also important was that pre-sintering temperature could effect the improvement of material properties.