- Email: [email protected]
A study of ceramic-metal restoration process
DENTAL DANIEL
TECHNOLOGY Section editor
H. GEHL,
A study George
of ceramic-metal
K. Koseyan*
and Chandi
restoration P. Biswas,
process
Ph.D.**
Clifton, N. J.
P
orcelain fused to metal for dental restorations has been in use for almost a century. The fused dental porcelain is nonreactive to oral fluids and gum tissues and has excellent abrasion resistance and dimensional stability. However, this vitreous material has poor ductility and poor compressive, shear, and impact strength. Therefore, a substructure of cast alloy fused to the porcelain facing is used for reinforcement. In the past, alloys of various precious metals, such as gold, platinum, and palladium, have been successfully used with porcelain facings. The recent escalation in the cost of precious metals has initiated the introduction of certain nonprecious alloys. The success of the porcelain-metal composite for dental restorations depends on the strength characteristics of the metal and the ceramic, the metal-to-ceramic bond strength, and the handling and processing techniques for the restoration. A typical restoration process consists of the following steps: (a) wax-up for the coping, (b) sprueing for the casting, (c) investing, (d) burnout, (e) melting and casting of the alloy, (f) devesting and cleaning and finishing of the coping, (g) application of bonding agent, (h) opaque coating (to mask the color of the coping), (i) porcelain buildup, (j) glazing and staining, and (k) finishing and polishing. The following technique of ceramic-metal dental restoration using nonprecious alloys has been studied carefully, and an account of the possible reasons for frequent failures of these restorations along with some fundamental concepts of an alloy and its bond with ceramics are presented. ALLOY
An alloy is a multicomponent solid (or liquid) in which the primary component is a metal. Brass is an alloy of copper and zinc; steel is an alloy of iron and carbon. The properties of an alloy normally are different from those of its parent metals, and by mixing suitable metals, alloys can be made considerably stronger with physical characteristics that are more desirable than those of pure metals. *Dental
ceramist.
**Metallurgist. 694
pll;;ry
Study of ceramic-metal restoration process 695
‘u
The metals and other chemical elements are composed of atoms which are the smallest particles retaining the individual characteristics of the element in question. A cubic millimeter of dental alloys contains about 9 x 1222 atoms. The atoms have a specific dispersion pattern in a metal or alloy. Different metals have different atomic sizes and arrangements. The composition of an alloy determines its atomic arrangement, microstructure, and, therefore, its properties. Different types of furnaces are used for melting metals, such as a gas furnace, electric resistance furnace, electric arc furnace, induction furnace, and so on. The different types of casting processes1 are sand-casting, permanent-mold casting, diecasting, investment casting, and centrifugal casting. Centrifugal casting can be a true or a semicentrifugal process with the mold cavity containing the axis of rotation or centrifugation in which the entire mold cavity is spun off the axis of rotation. Castings that require a close tolerance and delicate surface features normally are made by the investment technique. In this technique, an expendable pattern made of wax or plastic is used for making the mold. The pattern is burnt out before the metal is poured in the mold. Presently, dental restorations are made by high-speed centrifuge casting using the lost-wax technique. ALLOY-PORCELAIN
BOND
The bond between two materials such as metal and ceramic is mechanical and chemical or adhesive. The mechanical bond is achieved by roughening the surface prior to coating so that the two materials are joined through physical interlocking. The chemical bond, on the other hand, is achieved through atomic bonding. The chemical bond strength or the work of adhesion between metal and ceramic is given by the formula2
w, =
ymv
+ ycv
-
yme~
where ymv is the surface free energy* of the metal, ycv is that of the ceramic, and Ymc is the free energy of the metal-ceramic interface. This equation states that the work per unit of area necessary to separate a metal and ceramic is equal to the free energy of the two new interfaces created (metal-vapor and ceramic-vapor) minus the free energy of the interface which was destroyed (metal-ceramic). RESTORATION PROCESS Wax-up for coping. Since wax is highly susceptible to permanent deformation under slight pressure, the wax patterns require very careful handling. Otherwise, a deformed coping and a disfigured restoration will result. Various substances, such as olive oil and soap, are applied on the die as releasing agents for the wax. These materials sometimes cause the formation of microscopic gas bubbles on the inside surface of the coping. The removal of these gas pockets *The molecules at the surface of a solid are attracted inward and to each side by their neighbors; but, there is no outward attraction to balance the inward pull, becausethere are relatively few molecules in the vapor phase. This unbalanced force possessed by surface molecules due to their position
is regarded
as surface free energy.
696
J. Prosthet. Dent. December, 1976
Koseyan and Biswas
requires grinding which, unless done carefully, often causes surface oxidation through excessive heating. Znuesting. Some investment materials shrink during setting which is accompanied by a rise in their temperature. This causes some distortion in the wax pattern and, therefore, dimensional inaccuracies in the coping. Melting. If the oxyacetylene flame is oxidizing, it will oxidize the metals during melting of the alloy. The oxides may subsequently react with the opaque layer, causing it to discolor and become brittle and perhaps uselessfor further processing. Therefore, a reducing or neutral flame should be used for melting nonprecious alloys to prevent any oxidation, Casting. Since the sprue and coping sections are thin, a high speed of rotation is required to force the metal into the mold in the centrifugal casting machine, and this speed must be maintained for a few seconds after it is started. Normally, however, this speed drops from the very start, and the centrifugal force becomes insufficient for completely filling the mold with lighter nonprecious alloys (the ratio of density of pure gold to pure cobalt is 2.18 to 1) . As we know, the centrifugal force is expressed bY F = m w’r or 4~’ m r n?, where m is the mass of the liquid, w is the angular velocity, r is the radius of rotation, and n is the speed of revolution. Therefore, an increase in speed will increase the force drastically. An insufficient centrifugal force causes misruns in the mold; i.e., improper filling of the small cavities, corners, and thin sections of the mold (featheredge, knife-edge, bevel-edge, chamfer, normal shoulder, or bevel shoulder). TO ensure complete filling of the mold, the gas and/or air present in the mold cavity must escape through vents. These gases sometimes become entrapped in the alloy, causing porous castings (the porosity in the marginal areas cannot be ground or polished), and react with it. Before the centrifuge is released, it is essential that the metal is melted sufficiently and superheated enough to prevent any cold shut. The broken-arm centrifuges are suitable only for noble metals. When the centrifuge is released, the molten alloy in the crucible experiences tremendous turbulence. The alloy enters the sprue in a spiral manner, dislodging and carrying with it some investment particles from the sprue walls. For casting nonprecious alloys, the existing centrifuge machine should be modified and the gates or sprues designed properly SO that the metal enters the mold directly at a very high speed. Soldering. The solder preferably should be of the same base metal as the coping alloy. Otherwise, the solder deposit will behave differently from the alloy. Application of bonding agent. A bonding agent is often used to improve the bond between the coping and the opaque, but in some instances, it is not used at all. During curing of the bonding agent, its metal particles undergo some oxidation, and small gas bubbles are formed at the coping-bonding agent interface. Due to the difference in the thermal expansions of the alloy and the bonding agent, the bonding layer cracks during heating and cooling, causing some oxidation of the exposed coping surface.
Volume 36 Number 6
Study
of ceramic-metal
restoration
process
697
Opaque coating. Firing of the opaque above 1,800’ F. is preferred since highfusing porcelain bakes and bonds better with the alloy than low-fusing porcelain. During application of the opaque, the die or model is vibrated for proper packing, and the excess water from the wet opaque is soaked up by an absorbent paper. When the bonding agent is used, the vibration and the pressure required for absorption break the small gas bubbles formed in the layer of bonding agent. The fine particles that are formed during this breaking get mixed with the opaque and, due to vibrations, sometimes move to the outer surface of the opaque. These particles sometimes aggregate inside the opaque. After the opaque is applied, it is dried by placing it near the open furnace door. During drying, the opaque faces the heat buccally, causing the distal buccal and mesial surfaces to shrink which produces cracks in the lingual surface. The finishing lines, especially the margins, also develop very fine cracks due to shrinking of the opaque. The opaque also separates from the coping surface in the marginal areas. The opaque is then baked by inserting it in three gradual steps toward the hot zone of the muffle. The furnace is closed, a vacuum of about 30 inches of Hg is drawn, and the firing temperature is set. During baking, numerous hairline cracks develop randomly on the opaque surface. Peeling of the opaque layer near the marginal areas also occurs in this stage. When a bonding agent is used, most of the gas bubbles that were left in the layer of bonding agent break, and the gas escapes through the opaque layer. All these openings produced in the opaque layer provide easy access of air, causing some oxidation of the coping surface. When the opaque is taken out of the furnace and cooled, it develops further cracks due to the difference in thermal contractions of the alloy and the opaque. The differential thermal expansion and contraction also cause interfacial stresses to develop that affect the alloy-to-opaque bond strength. Sometimes in the baked condition. the opaque layer appears extremely brittle, overbaked, and uneven, is mixed with metal particles (from bonding agent), and has cracks that chip easily under slight pressure. The repair of these cracks by adding extra opaque only causes additional cracking problems. Porcelain buildup. The porcelain is applied and packed over the opaque layer. While vibrating the model, microscopic cracks develop in the opaque layer. and the porcelain penetrates these cracks along with the existing cracks and openings in the opaque. The porcelain is then dried and baked in the same manner as the opaque, at about 1,800’ F. in a vacuum of about 30 inches of Hg. During drying, the cracking of the lingual and the marginal areas occurs due to shrinking of the porcelain as happens in opaque. During firing, cracking occurs on the surface due to differential expansion. Also, tiny gas bubbles are formed in the porcelain, especially in the marginal areas. The structure becomes weak because of the interfacial s’tresses developed due to differential thermal expansion. During cooling, the porcelain develops some additional cracks, especially near the tip of the soldering joint. Carving. The most commonly used tools for carving are diamond discs, black separating discs, soft green discs, and the like. Prolonged grinding at one spot may cause localized heating, leading to cracking and chipping of the porcelain and
698
Koseyan
and Biswas
J. Prosthet. Dent. December, 1976
opaque. The presence of gas bubbles in the porcelain promotes cracking. The extra porcelain added for repair also causes chipping and cracking. Glazing and patching. A thin layer of low-fusing porcelain mixed with tap water, distilled water, alcohol, or glycerin is applied on the porcelain surface. Vibration is important during this application so that the cracks and surface porosities on the ceramic surface are filled up. The glaze is then dried by placing the restoration on a hemostat or tray near the open furnace door. After it is dried, the restoration is inserted in three or four gradual steps toward the hot zone of the furnace (4OOO F.), and then the temperature is raised to that for glazing. During this heating, the porcelain develops cracks, especially in the marginal and finishing areas for single units and in the soldering areas for fixed partial dentures. After glazing for 5 to 10 seconds, the restoration is removed from the furnace and cooled under a cover for about 10 minutes. Further cracks develop during cooling, and also some abnormal coloration is developed, giving an unpleasant appearance to the restoration. Sandblasting. After glazing, the inside of the crown is sandblasted for cleaning, and this sometimes produces cracking in the porcelain near the edge of the crown. Final marginal polish. Final marginal polishing can cause thermal stresses and sometimes causes cracks in the porcelain. SUMMARY The conventional process of ceramic-metal dental restorations has several drawbacks, produces substandard quality restorations, and may result in frequent failures. The lack of adequate development of nonprecious alloys has limited the bond strength between the coping and opaque to only 14,000 p.s.i. The porcelain layer oftentimes contains cracks and gas bubbles, making it weak and hence susceptible to failures even during processing. The restorations, ranging from single-unit to full-arch, undergo severe stresses in a person’s mouth, and therefore, a high degree of strength is essential. Attempts are being made to improve the present technique to obtain better restorations. Several modifications have been made to the conventional process, and a much stronger alloy-to-opaque bond and a defect-free ceramic layer have been achieved. Since the work on our technique has not been completed, the detailed description and experimental results will be the subject of a future article. References 1. Heine, R. W., and Rosenthal, P. C.: Principles of Metal Casting, New York, 1955, McGraw-Hill Book Company, Inc., p. 22. 2. Harkins, W. D.: The Physical Chemistry of Surface Films, New York, 1952, Reinhold Publishers Corp., p. 414. MR. KOSEYAN DENTOID PORCELAIN STUDIO, INC. 62 HUEMMER TERRACE CLIFTON, N. J. 07013 DR. BISWAS ABEX RESEARCH CENTER MAHWAH, N. J. 07430
TECHNOLOGY Section editor
H. GEHL,
A study George
of ceramic-metal
K. Koseyan*
and Chandi
restoration P. Biswas,
process
Ph.D.**
Clifton, N. J.
P
orcelain fused to metal for dental restorations has been in use for almost a century. The fused dental porcelain is nonreactive to oral fluids and gum tissues and has excellent abrasion resistance and dimensional stability. However, this vitreous material has poor ductility and poor compressive, shear, and impact strength. Therefore, a substructure of cast alloy fused to the porcelain facing is used for reinforcement. In the past, alloys of various precious metals, such as gold, platinum, and palladium, have been successfully used with porcelain facings. The recent escalation in the cost of precious metals has initiated the introduction of certain nonprecious alloys. The success of the porcelain-metal composite for dental restorations depends on the strength characteristics of the metal and the ceramic, the metal-to-ceramic bond strength, and the handling and processing techniques for the restoration. A typical restoration process consists of the following steps: (a) wax-up for the coping, (b) sprueing for the casting, (c) investing, (d) burnout, (e) melting and casting of the alloy, (f) devesting and cleaning and finishing of the coping, (g) application of bonding agent, (h) opaque coating (to mask the color of the coping), (i) porcelain buildup, (j) glazing and staining, and (k) finishing and polishing. The following technique of ceramic-metal dental restoration using nonprecious alloys has been studied carefully, and an account of the possible reasons for frequent failures of these restorations along with some fundamental concepts of an alloy and its bond with ceramics are presented. ALLOY
An alloy is a multicomponent solid (or liquid) in which the primary component is a metal. Brass is an alloy of copper and zinc; steel is an alloy of iron and carbon. The properties of an alloy normally are different from those of its parent metals, and by mixing suitable metals, alloys can be made considerably stronger with physical characteristics that are more desirable than those of pure metals. *Dental
ceramist.
**Metallurgist. 694
pll;;ry
Study of ceramic-metal restoration process 695
‘u
The metals and other chemical elements are composed of atoms which are the smallest particles retaining the individual characteristics of the element in question. A cubic millimeter of dental alloys contains about 9 x 1222 atoms. The atoms have a specific dispersion pattern in a metal or alloy. Different metals have different atomic sizes and arrangements. The composition of an alloy determines its atomic arrangement, microstructure, and, therefore, its properties. Different types of furnaces are used for melting metals, such as a gas furnace, electric resistance furnace, electric arc furnace, induction furnace, and so on. The different types of casting processes1 are sand-casting, permanent-mold casting, diecasting, investment casting, and centrifugal casting. Centrifugal casting can be a true or a semicentrifugal process with the mold cavity containing the axis of rotation or centrifugation in which the entire mold cavity is spun off the axis of rotation. Castings that require a close tolerance and delicate surface features normally are made by the investment technique. In this technique, an expendable pattern made of wax or plastic is used for making the mold. The pattern is burnt out before the metal is poured in the mold. Presently, dental restorations are made by high-speed centrifuge casting using the lost-wax technique. ALLOY-PORCELAIN
BOND
The bond between two materials such as metal and ceramic is mechanical and chemical or adhesive. The mechanical bond is achieved by roughening the surface prior to coating so that the two materials are joined through physical interlocking. The chemical bond, on the other hand, is achieved through atomic bonding. The chemical bond strength or the work of adhesion between metal and ceramic is given by the formula2
w, =
ymv
+ ycv
-
yme~
where ymv is the surface free energy* of the metal, ycv is that of the ceramic, and Ymc is the free energy of the metal-ceramic interface. This equation states that the work per unit of area necessary to separate a metal and ceramic is equal to the free energy of the two new interfaces created (metal-vapor and ceramic-vapor) minus the free energy of the interface which was destroyed (metal-ceramic). RESTORATION PROCESS Wax-up for coping. Since wax is highly susceptible to permanent deformation under slight pressure, the wax patterns require very careful handling. Otherwise, a deformed coping and a disfigured restoration will result. Various substances, such as olive oil and soap, are applied on the die as releasing agents for the wax. These materials sometimes cause the formation of microscopic gas bubbles on the inside surface of the coping. The removal of these gas pockets *The molecules at the surface of a solid are attracted inward and to each side by their neighbors; but, there is no outward attraction to balance the inward pull, becausethere are relatively few molecules in the vapor phase. This unbalanced force possessed by surface molecules due to their position
is regarded
as surface free energy.
696
J. Prosthet. Dent. December, 1976
Koseyan and Biswas
requires grinding which, unless done carefully, often causes surface oxidation through excessive heating. Znuesting. Some investment materials shrink during setting which is accompanied by a rise in their temperature. This causes some distortion in the wax pattern and, therefore, dimensional inaccuracies in the coping. Melting. If the oxyacetylene flame is oxidizing, it will oxidize the metals during melting of the alloy. The oxides may subsequently react with the opaque layer, causing it to discolor and become brittle and perhaps uselessfor further processing. Therefore, a reducing or neutral flame should be used for melting nonprecious alloys to prevent any oxidation, Casting. Since the sprue and coping sections are thin, a high speed of rotation is required to force the metal into the mold in the centrifugal casting machine, and this speed must be maintained for a few seconds after it is started. Normally, however, this speed drops from the very start, and the centrifugal force becomes insufficient for completely filling the mold with lighter nonprecious alloys (the ratio of density of pure gold to pure cobalt is 2.18 to 1) . As we know, the centrifugal force is expressed bY F = m w’r or 4~’ m r n?, where m is the mass of the liquid, w is the angular velocity, r is the radius of rotation, and n is the speed of revolution. Therefore, an increase in speed will increase the force drastically. An insufficient centrifugal force causes misruns in the mold; i.e., improper filling of the small cavities, corners, and thin sections of the mold (featheredge, knife-edge, bevel-edge, chamfer, normal shoulder, or bevel shoulder). TO ensure complete filling of the mold, the gas and/or air present in the mold cavity must escape through vents. These gases sometimes become entrapped in the alloy, causing porous castings (the porosity in the marginal areas cannot be ground or polished), and react with it. Before the centrifuge is released, it is essential that the metal is melted sufficiently and superheated enough to prevent any cold shut. The broken-arm centrifuges are suitable only for noble metals. When the centrifuge is released, the molten alloy in the crucible experiences tremendous turbulence. The alloy enters the sprue in a spiral manner, dislodging and carrying with it some investment particles from the sprue walls. For casting nonprecious alloys, the existing centrifuge machine should be modified and the gates or sprues designed properly SO that the metal enters the mold directly at a very high speed. Soldering. The solder preferably should be of the same base metal as the coping alloy. Otherwise, the solder deposit will behave differently from the alloy. Application of bonding agent. A bonding agent is often used to improve the bond between the coping and the opaque, but in some instances, it is not used at all. During curing of the bonding agent, its metal particles undergo some oxidation, and small gas bubbles are formed at the coping-bonding agent interface. Due to the difference in the thermal expansions of the alloy and the bonding agent, the bonding layer cracks during heating and cooling, causing some oxidation of the exposed coping surface.
Volume 36 Number 6
Study
of ceramic-metal
restoration
process
697
Opaque coating. Firing of the opaque above 1,800’ F. is preferred since highfusing porcelain bakes and bonds better with the alloy than low-fusing porcelain. During application of the opaque, the die or model is vibrated for proper packing, and the excess water from the wet opaque is soaked up by an absorbent paper. When the bonding agent is used, the vibration and the pressure required for absorption break the small gas bubbles formed in the layer of bonding agent. The fine particles that are formed during this breaking get mixed with the opaque and, due to vibrations, sometimes move to the outer surface of the opaque. These particles sometimes aggregate inside the opaque. After the opaque is applied, it is dried by placing it near the open furnace door. During drying, the opaque faces the heat buccally, causing the distal buccal and mesial surfaces to shrink which produces cracks in the lingual surface. The finishing lines, especially the margins, also develop very fine cracks due to shrinking of the opaque. The opaque also separates from the coping surface in the marginal areas. The opaque is then baked by inserting it in three gradual steps toward the hot zone of the muffle. The furnace is closed, a vacuum of about 30 inches of Hg is drawn, and the firing temperature is set. During baking, numerous hairline cracks develop randomly on the opaque surface. Peeling of the opaque layer near the marginal areas also occurs in this stage. When a bonding agent is used, most of the gas bubbles that were left in the layer of bonding agent break, and the gas escapes through the opaque layer. All these openings produced in the opaque layer provide easy access of air, causing some oxidation of the coping surface. When the opaque is taken out of the furnace and cooled, it develops further cracks due to the difference in thermal contractions of the alloy and the opaque. The differential thermal expansion and contraction also cause interfacial stresses to develop that affect the alloy-to-opaque bond strength. Sometimes in the baked condition. the opaque layer appears extremely brittle, overbaked, and uneven, is mixed with metal particles (from bonding agent), and has cracks that chip easily under slight pressure. The repair of these cracks by adding extra opaque only causes additional cracking problems. Porcelain buildup. The porcelain is applied and packed over the opaque layer. While vibrating the model, microscopic cracks develop in the opaque layer. and the porcelain penetrates these cracks along with the existing cracks and openings in the opaque. The porcelain is then dried and baked in the same manner as the opaque, at about 1,800’ F. in a vacuum of about 30 inches of Hg. During drying, the cracking of the lingual and the marginal areas occurs due to shrinking of the porcelain as happens in opaque. During firing, cracking occurs on the surface due to differential expansion. Also, tiny gas bubbles are formed in the porcelain, especially in the marginal areas. The structure becomes weak because of the interfacial s’tresses developed due to differential thermal expansion. During cooling, the porcelain develops some additional cracks, especially near the tip of the soldering joint. Carving. The most commonly used tools for carving are diamond discs, black separating discs, soft green discs, and the like. Prolonged grinding at one spot may cause localized heating, leading to cracking and chipping of the porcelain and
698
Koseyan
and Biswas
J. Prosthet. Dent. December, 1976
opaque. The presence of gas bubbles in the porcelain promotes cracking. The extra porcelain added for repair also causes chipping and cracking. Glazing and patching. A thin layer of low-fusing porcelain mixed with tap water, distilled water, alcohol, or glycerin is applied on the porcelain surface. Vibration is important during this application so that the cracks and surface porosities on the ceramic surface are filled up. The glaze is then dried by placing the restoration on a hemostat or tray near the open furnace door. After it is dried, the restoration is inserted in three or four gradual steps toward the hot zone of the furnace (4OOO F.), and then the temperature is raised to that for glazing. During this heating, the porcelain develops cracks, especially in the marginal and finishing areas for single units and in the soldering areas for fixed partial dentures. After glazing for 5 to 10 seconds, the restoration is removed from the furnace and cooled under a cover for about 10 minutes. Further cracks develop during cooling, and also some abnormal coloration is developed, giving an unpleasant appearance to the restoration. Sandblasting. After glazing, the inside of the crown is sandblasted for cleaning, and this sometimes produces cracking in the porcelain near the edge of the crown. Final marginal polish. Final marginal polishing can cause thermal stresses and sometimes causes cracks in the porcelain. SUMMARY The conventional process of ceramic-metal dental restorations has several drawbacks, produces substandard quality restorations, and may result in frequent failures. The lack of adequate development of nonprecious alloys has limited the bond strength between the coping and opaque to only 14,000 p.s.i. The porcelain layer oftentimes contains cracks and gas bubbles, making it weak and hence susceptible to failures even during processing. The restorations, ranging from single-unit to full-arch, undergo severe stresses in a person’s mouth, and therefore, a high degree of strength is essential. Attempts are being made to improve the present technique to obtain better restorations. Several modifications have been made to the conventional process, and a much stronger alloy-to-opaque bond and a defect-free ceramic layer have been achieved. Since the work on our technique has not been completed, the detailed description and experimental results will be the subject of a future article. References 1. Heine, R. W., and Rosenthal, P. C.: Principles of Metal Casting, New York, 1955, McGraw-Hill Book Company, Inc., p. 22. 2. Harkins, W. D.: The Physical Chemistry of Surface Films, New York, 1952, Reinhold Publishers Corp., p. 414. MR. KOSEYAN DENTOID PORCELAIN STUDIO, INC. 62 HUEMMER TERRACE CLIFTON, N. J. 07013 DR. BISWAS ABEX RESEARCH CENTER MAHWAH, N. J. 07430