Development Of Machinable Block For CAD/CAM And Mechanical Study Of Glass-infiltrated Composite

ObjectiveDental ceramic materials are increasingly popular for the dental field because of their outstanding esthetics, inertness and biocompatibility which have been the primary selection and will be the direction for the current prosthodon-tics. Problematic aspects are their brittleness and thereby their sensitivity to tensile stresses coupled with the difficulty of machinability. How to get through these problems is still a challenge. Following computer aided design and computer aided manufacturing being introduced in dentistry, a more efficient and convenient method occurred with the development of In - Ceram technique by Rinke in 1996.Because of the limited specimens tested, a larger standard deviation and non -uniform normal distribution,we can not have them compared with each other statistically. Also the values from the laboratory can not be indicated for clinical service. To a great extent,the strengths of brittle ceramic materials depend on the size of the inherent critical flaws and often vary with the processing and conditioning methods from environment and itself, respectively. So influenced by the distribution randomicity of inherent flaws, the strength of the brittle materials will show a statistical tendency of distribution. Only strength values should not be e-valuated on the structural reliability correctly. The alternative use of Weibull a-nalysis permits the determination of the fracture strength distribution and well -understanding of the tested values with the aid of the integral over the volume whereby volume integral accounts for internal cracks and porosity. It can be used as a design parameter of brittle materials for clinical practice which will intro-duce a long - time survival rate.Dental adhesive is an important branch in dentistry and gives us an new scope for difficult clinical problems. The wide use of all - ceramic restoration attributes to the study and development of adhesive materials. A reliable adhesion is needed in case of fracture of the all - ceramic restoration. A successful fixed restoration is a integrity of multi - layered parts. It is the same as the In - Ceram ceramics with a strengthened core which makes stress distribution more complicated. A reduction of the integral stress plays an very primary role in the brittle materials by a reliable adhesion.The purpose of this study is to explore the structural reliability of machinable infiltrated ceramics made from ultrafine alumina powder for dental CAD/ CAM by means of flexural strength together with Weibull analysis and to investigate the effect of nonuniform adhesive layer thickness on the stress distribution of axially loaded all —ceramic crowns. With the former,clinical limitations and indications regarding to this kind of brittle ceramics can be made. With the latter, a detailed information on the best stress distribution can be noticed. A better survival rate of the In - ceram all - ceramic restoration will be induced with these knowledge.Methods1. Production of ultrafine a - alumina powderThe rough material of ammonium aluminum sulfate was placed into a computer - controlled hearth. The elevated temperature rate was 10℃ per minute. Every 200℃ was coupled with a holding time of a half hour. When 700℃ arrived , a three hours were hold. After the material was cooled naturally, it was transferred to an agate jar for mixing with ethyl alcohol for 48 hours. Then it was dried at 65℃ for 24 hours. Again,a higher temperature of 1180℃ was set for a new firing cycle. After holding for 3 hours and ball - mixing for 48 hours, it was dried again for preparation. A batch of powder preparation was prepared again by mixing 500ga - Al2O3and 2.5g MgO in ethyl alcohol with Al2O3ball in an agate jar.2. Production of a machinable porous Al2O3 bulkAl2O3 bulks of pure Al2O3 powder and mixing powder were formed by the i-sostatic - pressing method at 200MPa , and sintered at 1300℃, 1350℃ and 1400℃ .respectively. The holding time was set for 2 hours.3. Production of a glass - infiltrated compositeAn uniform aqueous slurry was applied directly onto the Al2O3 bulk for the infiltration in a VITA VACUMAT 40 furnace according to the manufacture's directions.4. Analysis of scanning microscopy and X - ray diffractionThe fractured parts of specimens were sputter - coated with gold - palladium. A X - ray diffraction examination was also performed.5. Flexural stength and Weibull analysis30 bar specimens with the size of 3 × 2 × 34mm3 were made for flextural strength testing after wet - grounding with 600,800 and 1200 grit size silicone carbide paper. The fracture stress values were analyzed by Weibull analysis to determine the Weibull modulus values(m) and the 1 and 5% probabilities of fracture. Weibull plots of fracture stress were generated with computer. The data points were described by a straight line produced by a least -squares fit (maximum likelihood method) of the fracture stress data available. The description of the Weibull distribution is given by Pf = 1 - exp [ - ( σ/σ )m ] , where Pf is the fracture probability,defined by the relation Pf = κ/( N + 1) , κ is the rank in strength from least to greatest, N denotes the total number of specimens in the sample,m is the shape parameter( Weibull mudulus) ,andσ0 is the characteristic strength,and the strength at a probability of failure of 1% and 5% were obtained through the use of a computer program designed to perform the Weibull analysis from the fracture data.6. Construction of two - dimentional finite element model of premolarThe prepared abutment had a convergence angle of 10 degrees, a shoulder margin redius of 0.6mm,and an axial/occlusal line angle radius of 0.5mm. Located along the vertical axis of this abutment core was a pulp cavity that had a width of 1.0mm and tapered off in a bullet - like fashion 7. 5mm from the base. No elements were constructed in this region. The In - ceram core coping and theceramic porcelain were veneered along the abutment with a uniform thickness of 0. 6mm. Panavia adhesive and Zinc phosphoric cement with four different layer thicknesses were made.7. Stress analysis of the all - ceramic restorationEach model was made in an axisymmetrie fashion with 2 - D elements. The FE meshes of each group modeling the radial slice of the tooth have approximately the same axisymmetrie elements, equivalent to 2300 2 - D ones, most of which were quadrilateral four - node elements and the remainder , three - node triangular elements developed automatically with Ansys 6. 1 software. A normal stress of magnitude 71.3Mpa was applied uniformly through the symmetric axis on the occlusal surface. This is equivalent of applying a vertical bite force , over a cir-cular areea with a radius of 1. 54mm. The total magnitude of this load was 540N. All the materials involved were surposed to be with linear elastic properties. Sress distribution and magnitude within the all - ceramic crowns were ex-amed with the Ansys 6. 1 FEA software coupled with identifying the regions where peak tensile stresses occur.Results1. Analysis of scanning microscopy and X - ray diffractionThe apparent neck - growth and surface diffusion happened with regards to the MgO -added group when 1350℃ or higher temperature arrived. A continuous infiltrated porous skeleton was formed with the temperature 1400℃. The composite made from the pure Al2O3 bulk was porous. The intergranular fracture was shown by scanning microscopy. XRD indicated the major crystal phase was a - alumina.2. Flexural stength and Weibull analysisThe mean flexural strength and standard deviation were 375.5 ±66. 88Mpa coupled with Weibull modulus of 6. 2. Strength values at failure probability of 1% and 5% were 191. 79Mpa and 249. 83Mpa,respectively. The σ63.21% value was 404.44 MPa.3. Stress analysis of the all - ceramic restorationWith regard to panavia adhesive,the porcelain peak tensile principal stress of group 100μm decreased by 12% compared to group 25 μm and was the lowest in all the four groups. As to the adhesive layer peak tensile stress, both group 100μm and group 100 -25μm were lower than the other groups and decreased by 33. 3% and 33. 1% , respectively in comparison with group 25μm. When 25μm adhesive layer thickness occurred in occlusal region, the core ceramic peak tensile stresess lowered, but were still the highest among the all - ceramic crowns parts. As to zinc phosphoric cement, There was not significantly different among the four groups with regard to the core ceramic peak tensile stress. For all the groups, the porcelain tensile stresses were located around the inner surface of the axial/occlusal region, the core ceramic on the inner surface of the axial/ occlusal line angle region , while the adhesive layer in the chamfer region turning up toward axial region.Conclusions1. ultrafine grain - sized alumina powder made from ammonium aluminum sulfate and infiltrated porous alumina bulk can be produced with the proper controlled firing cycle set. After being infiltrated by lanthanum glass powder, a glass- infiltrated composite can be developed.2. The glass - infiltrated composite can be densified when 0. 5wt% MgO additive was introduced which will be favorable in the fracture resistance.3. Within the limitation of the study, the sintering temperature did not change the fracture pattern of the composite. Only intergranula fracture happened which indicates the intergranula energy is lower than the innergranula energy.4. Depending on the average fracture strength can not be used reliably as a design parameter for brittle all - ceramic crown materials. Compared with the average fracture strength alone, the use of flexure strength test combined with the Weibull analysis as a method of predicting survival rates from the strength data could be favorable in identifying the failure mechanisms involved in ceramic crown fracture, which would result in an improved validation of the strength data.