For the ceramometal restorations, the type of finish line is:
a- Chamfer
b- Beveled shoulder ***
Beveled shoulder: According to the results of this study as the shoulder bevel had better fit than shoulder and deep chamfer designs and also there was significant difference between shoulder bevel and shoulder, so it is recommended to use shoulder bevel finish line in the metal ceramic restoration.
The realization of reinforcement and screeds for CCM techniques is very close to that of only metallic constructions. A significant difference exists in the re-use of alloys. As has already been said, during melting and casting, some elements of the alloy can be lost, especially those involved in the formation of oxides. These elements are very important for the formation of the metal-ceramic bond with precious alloys. That is why, at each casting of these alloys, at least half of the alloy must be new.
The surface treatments of screeds are also important. These are sandblasting of the surface (Al2O3 between 25 and 50μm) and the formation of oxides (either in air or under partial vacuum). With some alloys containing Pd, this cooking not only allows to create this outer layer of oxides, but also of internal oxides which penetrate into the metal from its surface and significantly increase its roughness which leads to an improvement of metal-ceramic adhesion. Some non-precious alloys lead to the formation of an oxide layer too thick which weakens the bond. With these alloys the screeds undergo a heat treatment and are then sanded to remove excess oxides.
The first layer of ceramic is very important since it must hide the metal. Special opaque ceramics must be used. After application and firing of this layer, the dentin ceramic which contains less opaque oxides and fluorescent pigments and oxides is applied and sintered. Finally, translucent enamel ceramics are applied and sintered.
Enameling and metal ceramic bonding:
The quality of a ceramic metal bond is dependent on three main modes of interaction occurring between the alloy and the ceramic during cooking and during cooling:
- The first is physicochemical. It is characterized by the wettability of the molten ceramic on the surface of the metal substrate. The wetting ability of the ceramic is controlled by its surface tension at the firing temperature, the surface energy of the metal substrate, and the nature of the interactions at the interface. Good spreading of the molten ceramic is obtained by increasing the surface energy of the alloy by prior oxidation.
- The second is chemical. It determines the nature of the bond that is created at different interfaces between ceramic, surface oxides and metal. This connection, difficult to characterize experimentally, depends on the composition and the microstructure of the alloy, as well as the nature of the ceramic. It is the culmination of complex redox phenomena at interfaces, following interdiffusions or ion migrations occurring during cooking.
- The third is mechanical. It results from two phenomena.
+ the anchoring of the ceramic after cooling in the roughnesses; surface of the alloy. The observation of metal ceramic interfaces reveals two distinct forms of roughness: those resulting from surface treatment of the alloy before firing, of macroscopic order (eg sandblasting), and those of microscopic order. which are created during the firing by selective dissolution of certain phases of the alloy, generally localized in inter-dendritic spaces;
+ the compression of the ceramic at the interface due to a slight difference in contraction between the alloy and the ceramic during cooling. This prestressing of the ceramic (hooping) is mechanically favorable, since it makes it possible to oppose the initiation or propagation of cracks from interfacial defects. It requires a superior contraction of the alloy.
In practice, the establishment of a quality bond between an alloy and a ceramic successively requires the following conditions:
- Selection of an alloy of high modulus of elasticity, whose solidus is at least 100 ° C higher than the firing temperature of the first opaque ceramic layers (980 ° C). The high rigidity of nickel and cobalt-based alloys gives them excellent resistance to deformation during baking cycles; this quality is essential for the dimensional stability of long span spans;
- Creation on the surface of the roughness infrastructure by sanding, to improve the anchoring of the ceramic. This mechanical surface treatment is followed by scrupulous cleaning and degreasing;
- Formation by prior thermal treatment of a stable and adherent oxide film on the surface of the metal frame. To be mechanically resistant, this oxide layer must have a small thickness and good chemical homogeneity. This criterion seems more accessible to nickel-based alloys, whose kinetics of oxide formation is lower than that of cobalt-based alloys. However, the role of the main constituents seems less decisive than that of certain minor additions such as beryllium or certain rare earths which contribute to promote the formation of thin and adherent oxide layers. It is also possible to use a homogenization heat treatment of the metal substructure to obtain a layer of oxides of more uniform composition. For this, the first oxide film formed during this annealing must be removed by sanding, and a second heat treatment will lead to the formation of the final oxide layer;
- Rigorous application of the enamelling protocol recommended for the ceramic used, because there are indeed significant variations, especially in the dilatometric behavior, from one brand to another. Some ceramics are better suited for use on precious metal alloy substructures, while others are better suited for enamelling frames of non-precious alloys. Particular care must be taken to respect the cooling kinetics: variations in the cooling rate can induce changes in the contraction mode of the ceramic. A precise dilatometric agreement between the alloy and the opaque ceramics between 650 ° C and the ambient temperature thus appears as a necessary condition, but not sufficient.
The realization of reinforcement and screeds for CCM techniques is very close to that of only metallic constructions. A significant difference exists in the re-use of alloys. As has already been said, during melting and casting, some elements of the alloy can be lost, especially those involved in the formation of oxides. These elements are very important for the formation of the metal-ceramic bond with precious alloys. That is why, at each casting of these alloys, at least half of the alloy must be new.
The surface treatments of screeds are also important. These are sandblasting of the surface (Al2O3 between 25 and 50μm) and the formation of oxides (either in air or under partial vacuum). With some alloys containing Pd, this cooking not only allows to create this outer layer of oxides, but also of internal oxides which penetrate into the metal from its surface and significantly increase its roughness which leads to an improvement of metal-ceramic adhesion. Some non-precious alloys lead to the formation of an oxide layer too thick which weakens the bond. With these alloys the screeds undergo a heat treatment and are then sanded to remove excess oxides.
The first layer of ceramic is very important since it must hide the metal. Special opaque ceramics must be used. After application and firing of this layer, the dentin ceramic which contains less opaque oxides and fluorescent pigments and oxides is applied and sintered. Finally, translucent enamel ceramics are applied and sintered.
Enameling and metal ceramic bonding:
The quality of a ceramic metal bond is dependent on three main modes of interaction occurring between the alloy and the ceramic during cooking and during cooling:
- The first is physicochemical. It is characterized by the wettability of the molten ceramic on the surface of the metal substrate. The wetting ability of the ceramic is controlled by its surface tension at the firing temperature, the surface energy of the metal substrate, and the nature of the interactions at the interface. Good spreading of the molten ceramic is obtained by increasing the surface energy of the alloy by prior oxidation.
- The second is chemical. It determines the nature of the bond that is created at different interfaces between ceramic, surface oxides and metal. This connection, difficult to characterize experimentally, depends on the composition and the microstructure of the alloy, as well as the nature of the ceramic. It is the culmination of complex redox phenomena at interfaces, following interdiffusions or ion migrations occurring during cooking.
- The third is mechanical. It results from two phenomena.
+ the anchoring of the ceramic after cooling in the roughnesses; surface of the alloy. The observation of metal ceramic interfaces reveals two distinct forms of roughness: those resulting from surface treatment of the alloy before firing, of macroscopic order (eg sandblasting), and those of microscopic order. which are created during the firing by selective dissolution of certain phases of the alloy, generally localized in inter-dendritic spaces;
+ the compression of the ceramic at the interface due to a slight difference in contraction between the alloy and the ceramic during cooling. This prestressing of the ceramic (hooping) is mechanically favorable, since it makes it possible to oppose the initiation or propagation of cracks from interfacial defects. It requires a superior contraction of the alloy.
In practice, the establishment of a quality bond between an alloy and a ceramic successively requires the following conditions:
- Selection of an alloy of high modulus of elasticity, whose solidus is at least 100 ° C higher than the firing temperature of the first opaque ceramic layers (980 ° C). The high rigidity of nickel and cobalt-based alloys gives them excellent resistance to deformation during baking cycles; this quality is essential for the dimensional stability of long span spans;
- Creation on the surface of the roughness infrastructure by sanding, to improve the anchoring of the ceramic. This mechanical surface treatment is followed by scrupulous cleaning and degreasing;
- Formation by prior thermal treatment of a stable and adherent oxide film on the surface of the metal frame. To be mechanically resistant, this oxide layer must have a small thickness and good chemical homogeneity. This criterion seems more accessible to nickel-based alloys, whose kinetics of oxide formation is lower than that of cobalt-based alloys. However, the role of the main constituents seems less decisive than that of certain minor additions such as beryllium or certain rare earths which contribute to promote the formation of thin and adherent oxide layers. It is also possible to use a homogenization heat treatment of the metal substructure to obtain a layer of oxides of more uniform composition. For this, the first oxide film formed during this annealing must be removed by sanding, and a second heat treatment will lead to the formation of the final oxide layer;
- Rigorous application of the enamelling protocol recommended for the ceramic used, because there are indeed significant variations, especially in the dilatometric behavior, from one brand to another. Some ceramics are better suited for use on precious metal alloy substructures, while others are better suited for enamelling frames of non-precious alloys. Particular care must be taken to respect the cooling kinetics: variations in the cooling rate can induce changes in the contraction mode of the ceramic. A precise dilatometric agreement between the alloy and the opaque ceramics between 650 ° C and the ambient temperature thus appears as a necessary condition, but not sufficient.
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