| Metal-ceramic restoration is also named porcelain-fused-to-metal. Theycombine the natural esthetics of porcelain with the strength and accuracy of castmetal. Metal-ceramic restorations become ideal materials for dental crown andbridge prostheses. The key of metal-ceramic restorations depends on the goodbond between the ceramic and the metal substructure. The poor bond betweenceramic and metal accounts for 60% of all metal-ceramic restoration failures casesand results in ceramic crack or detachment. It is still a goal for the scholars,dentists and technicians to adopt new standard and scientific methods to increasethe bond strength.Mechanisms have been described to explain the bond between the ceramicveneer and the metal substructure: (1) chemical bonding;(2) mechanicalinterlocking;(3) Van der Waal's forces;(4) compressive forces. Chemical bondingacts as key role and occupies 2/3 of the whole bond forces. Chemical bonding isindicated by the formation of an oxide film on the metal. The ideal oxide filmmakes for the wetting of the metal surface with porcelain during sintering and theexerting of all forces. As a result, a continuous electronic structure is formed fromthe metal through the oxide layer to the porcelain, chemically bonding theporcelain to the metal.Anodic oxidation is a kind of inexpensive and effective electrochemistrymethods. It can control the formation of the oxide film on the metal by adjustingthe value of volts D.C, current and time. The metal samples were joined to theanode and oxidation reaction took place while deoxidize reaction occurred on thecathode. Oxidation reaction emits electron and deoxidize reaction absorbs electron.Electrode departs from balance potential and polarization forms. Selectivesolution appears on the anode with high value of electric current. The value ofcurrent reduces notably along with the potential heightening, and the metal almoststops dissolving. This conversion of the estate of metal is named passivation. Thereason of this phenomenon lies in the formation of oxide film on metal, whichhinders the alloy from dissolving.The polarization curves of SHOFU UNI METAL II Ni-Cr alloy wereobtained under different value of volts in various solutions. The Ni-Cr alloyshowed passivation in 10% (NH4)2S2O8 and constant voltage +950mV. It indicatesthat the ideal oxide film on the surface of the metal can form by the adjustment oftime. The reaction mechanism of the oxide film is : nMe+mH2O →MenOm+2mH++2me. Me stands for metal element. The theory of electronarrangement has been widely accepted that the passivation of alloy related to theelectron arrangement of the atom on the alloy surface, especially d-layer electron.The more passive the metal element is, the less electrons fill the d-layer oftransitive metal elements, such as Cr, Ni, Mo and Fe. SHOFU UNI METAL IINi-Cr alloy contains Ni (78.0%), Cr (11.8%), Mo (6.0%). It is sensitive to thepassivation.The oxide film on the surface of metal forms poorly when the value ofconstant voltage is low and the time is not sufficient. On the contrary, theinsoluble oxide film on the surface of the alloy can translate into solvable highvalence oxide. The experiment results showed that degassing and preoxidationdoesn't affect the bond strength of SHOFU UNI METAL II Ni-Cr alloy (p>0.05).The bond strength increases significantly in the group of anodizing for 950mVand 90 seconds (p<0.05). Proper surface anodizing procedure has been proven tocontribute to the formation of strong bonding of porcelain to Ni-Cr alloy.The surface shape of the anodized alloy samples was analyzed usingelectronic scanning microscope (SEM). Lightly selective solution appearedbecause the different component parts have dissimilar electrode, whose dissolvevelocity are quite unlike. The sample's metal surface expanded, so the mechanicalinterlocking of porcelain and metal increased bond strength.The anodized samples were measured by x-ray photoelectron spectroscopy(XPS) and Auger electron spectroscopy (AES). The thickness of oxide film is 1.72times of the control samples. The oxide percentage of major metal element suchas Cr, Ni and Mo were higher, especially Cr. It initially involves the formation ofan adherent thin oxide bound to the alloy and second, the ability of the formedoxide to saturate the porcelain, completing the chemical bond of porcelain tometal. The degassing-preoxidation firing produces the thick oxide layer, whichwas 10.12 times of the control samples. The relative content of oxygen was higherthan the other 2 groups. The thermal expansion coefficient of the thick oxide filmwas different from Ni-Cr ally&ceramic. Residual thermal stresses exist in metalceramic interface and results in a poor bond. The thick oxide film hindered themetallic ions from migrating into porcelain and formed some carbonate resultingin the weak chemical bonding of porcelain to metal.The metal-porcelain specimens were analyzed using SEM and electronicx-ray spectrometry. The main elements of porcelain and metal interdiffused andthe interaction zones were formed which Sn characteristically accumulated.Diffusion bonding is established not only by interdiffusion but also byreduction-oxidation reactions. As a result, a continuous electronic structure isformed from the metal through the diffusion zone to the porcelain, chemicallybonding the porcelain to the metal. The different physical and chemical propertiesof metal and porcelain obtain gradually transition through the interaction zone.The compounds of Sn all have lower solidity points, modulus of elasticity andyield strength. They can release the stresses and benefit the wet ability of themetal surface by the porcelain. New resultant was found at metal-porcelaininterface and increased the bond strength. There was no porosity at interface forthe anodizing pretreatment specimen. Anodizing pretreatment on the alloy surfacecould improve wetting of the metal surface by the porcelain and formed apreferable interface. The porosities at the metal-porcelain interface may serve asareas of stress concentration and be the focal points of crack propagation. Theymay move to the surface or inside the porcelain during further firing such astrimming and glazing. This situation may influence the porcelain-metal bond anddecrease in resisting mechanical forces encountered in clinical function.The metal-porcelain specimens were analyzed using SEM and x-raydiffraction (XRD) to investigate the interface reaction microcosmic mechanism ofporcelain fused to Ni-Cr alloy. On the porcelain side, the resultant was observed atmetal-porcelain interface. They are the product of AlNi3 and SnCrO compoundoxide possessed the qualities of metal and porcelain. On the metal side, moderatediffusion interaction zone formed realizing the good bonding between Ni-Cr alloyand porcelain.Periodontal disease is an oral frequently encountered disease and one of themajor reasons for tooth losing. Some tooth suffering from periodontal disease canbe reserved for a longer time if the disease have been treated efficiently in time. Itis good for preserving alveolar bone and disease tooth.Metal-porcelain splints involve more tooth for periodontal disease and aredifficult to be made. It must be remade when the porcelain debonds or cracks.This instance not only increases the workload for the dentists and technicians butalso become a psychological and economic burden for the patients. Anodizingpretreatment on the Ni-Cr alloy were applied to the metal-porcelain splints forperiodontal tooth after some research work had been done. The curative effectswere well.Practice has proved that the anodizing pretreatment to the Ni-Cr alloy is anew technically feasible, inexpensive and practical method. It can easily controlthe forming of oxide film and requires considerable short time. The surface shapeof the metal was uniform and doesn't lead to stress concentration.Based on the results obtained in the work, the innovatory conclusions can bedrawn: (1) Chemical bonding and mechanical interlocking are developed toincrease the bond strength between the metal and the porcelain for the anodizingpretreatment. Among the various metal surface pretreatments, single bond force isdeveloped to create the bond strength. (2) Some compound oxide possesses thequalities of metal and porcelain realizing the good bond. It's a further step on themicrocosmic mechanism of porcelain fused to alloy.
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