Dental Casting Alloys and Metal Joining
Age hardening—Process of hardening certain alloys by controlled heating and cooling, which usually is associated with a phase change.
Antiflux—A substance such as graphite that prevents flow of molten solder on areas coated by the substance.
Copy milling—Process of cutting or grinding a desired shape to the same dimensions as a master pattern in a manner similar to that used for cutting a key blank from a master key.
Flux—Compound applied to metal surfaces that dissolves or prevents the formation of oxides and other undesirable substances that may reduce the quality or strength of a soldered or brazed area.
Lost wax technique—Process in which a wax pattern, prepared in the shape of missing tooth structure, is embedded in a casting investment and burned out to produce a mold cavity into which molten metal is cast.
Noble metal—Gold and platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, and osmium), which are highly resistant to oxidation and dissolution in inorganic acids. Gold and platinum do not oxidize at any temperature, rhodium has excellent oxidation resistance at all temperatures, osmium and ruthenium form volatile oxides, and palladium and iridium form oxides in the temperature ranges of 400 °C to 800 °C and 600 °C to 1000 °C, respectively.
Postsoldering—Process of brazing or soldering two or more metal components of a prosthesis after the metal substructure has been veneered with a ceramic.
Presoldering—Process of brazing or soldering two or more metal components of a prosthesis before a ceramic veneer is fired or hot-pressed on the metal structure.
Soldering—Process of building up a localized metal area with a molten filler metal or joining two or more metal components by heating them to a temperature below their solidus temperature and filling the gap between them using a molten metal with a liquidus temperature below 450 °C. In comparison with welding, fusion of the joined alloy part(s) does not usually occur during this process. Bonding of the molten solder to the metal parts results from flow by capillary action between the parts without appreciably affecting the dimensions of the joined structure. In dentistry, many metals are joined by brazing, although the term soldering is commonly used. If the process is conducted above 450 °C, it is called brazing.
The twentieth century has generated substantial changes in dental prosthetic materials. The major factors driving new development are (1) economy—the new material performs the same function as the old but at a lower cost; (2) performance—the new material performs better than the old in some desirable way, as in the processing method, handling characteristics, or delivery system; and (3) esthetics—the new material provides a more esthetic result. A brief description of the evolution of the currently marketed alloys is presented below in order to clarify the rationale for the development of the wide variety of alloy formulations.
Historical Perspective on Dental Casting Alloys
1907: the Lost Wax Process
In his 1907 US Patent #865823, Taggart described a method of making gold inlays using the lost wax technique. It led to the casting of onlays, crowns, multiple-unit fixed dental prostheses (FDPs), and frameworks for removable partial dentures.
1932: Classification of Gold-Based Casting Alloys
The dental materials group at the National Bureau of Standards (now National Institute of Standards and Technology) evaluated the properties of alloys being used and classified them by their Vickers hardness number (VHN): type I (soft, VHN, 50 to 90), type II (medium, VHN, 90 to 120), type III (hard, VHN, 120 to 150), and type IV (extra hard, VHN 150 and above).
1933: Cobalt-Chromium and Nickel-Chromium Alloys
The advantages of cobalt-chromium and nickel-chromium alloys are their lighter weight, greater stiffness (elastic modulus), greater strength, and reduced cost. For these reasons, they have largely replaced gold-based alloys for making removable partial dentures. The rapidly fluctuating escalating price of gold between 1980 and 2012 has made them logical alternatives to gold alloys for FDPs.
1971: the End of the Bretton Woods System
The Bretton Woods system, implemented after World War II, was an international monetary framework of fixed exchange rates between gold and currency. The system ended on August 15, 1971, a step that completely abandoned the gold standard. Gold then became a commodity freely traded on the open market. As a result, the price of gold rose steadily over the next 9 years. The price of gold skyrocketed again between 2006 and 2011, approaching $1900 per ounce in August 2011.
In response to the ever-increasing price of gold, new dental alloys were introduced by replacing gold partially or entirely with a less expensive noble metal or adapting removable partial denture alloys for crown and bridge casting applications.
1976: the Medical and Dental Devices Act
This law placed the U.S. dental industry under the auspices of the U.S. Food and Drug Administration (FDA). Dental alloys for prosthetics were classified as passive implants. All materials on the market prior to 1976 were automatically “grandfathered” as acceptable for market distribution. Manufacturers were required to have a quality system in place, but no product standards were established.
1996: the European Medical Devices Directive
The European Union (EU) established that any imports of dental devices required a CE (Certification-Expert) mark. To sell products in the EU, a company must be compliant with an International Organization for Standardization (ISO) standard (ISO 9000) and meet the requirements of the European Medical Device Directive. This performance standard extended beyond the FDA requirements, since no existing products were grandfathered and safety had to be demonstrated. Information and data on the development process were also required. Again, no specific product standards were established.
1998: the Clean Air Acts
The price of palladium increased to a record high of $1000 per troy ounce in 2000 because of the increased demand for palladium-containing catalytic converters. At the same time during that decade, the price of gold was trading below $300 per troy ounce and gold-based dental alloys were less expensive than palladium-containing alloys. The poor economy of 2001 decreased the demand for palladium and its price fell to a level comparable to that of gold, in the range of $300 to $350 in 2002. In 2012, the price of palladium was less than half that of gold.
Classification of Dental Casting Alloys
Traditionally, dental gold alloys have been specified by the gold content based on carat or fineness. The carat system specifies the gold content of an alloy based on parts of gold per 24 parts of the alloy. Fineness is the unit that describes the gold content in noble metal alloys by the number of parts of gold per 1000 parts of alloy. For example, pure gold is 24-carat or 1000 fine, whereas an 18-carat alloy contains 75% pure gold and is 750 fine. The terms carat and fineness are rarely used to describe the gold content of current dental alloys. However, fineness is often used to identify gold alloy solders.
Since the classification of gold alloys was established in 1932 by the National Bureau of Standards, the number of alloy compositions has increased astronomically. Dental alloys currently available for dental castings can be classified according to their composition, their intended usage, or their mechanical properties.
Alloy Classification by Noble Metal Content
In 1984, the American Dental Association (ADA) proposed a simple classification for dental casting alloys based on the content of noble metals. Three categories were described: high noble (HN), noble (N), and predominantly base metal (PB). This classification is presented in Table 16-1. Noble metals comprise a group of seven metals that are resistant to corrosion and tarnish in the mouth. In order of increasing melting temperature, they include gold, palladium, platinum, rhodium, ruthenium, iridium, and osmium. Only gold, palladium, and platinum, which have the lowest melting temperatures of the seven noble metals, are currently of major importance in dental casting alloys. The noble metals and silver are sometimes called precious metals, referring to their high economic values, but the term precious is not synonymous with noble. Silver is reactive in the oral cavity and is not considered a noble metal.
Alloy Classification by Noble Metal Content—American Dental Association (1984)
|Alloy Type||Total Noble Metal Content|
|High noble (HN)||Must contain ≥40% Au and ≥60% by weight of noble metal elements*|
|Noble (N)||Must contain ≥25% by weight of noble metal elements|
|Predominantly base metal (PB)||Contains <25% by weight of noble metal elements|
*Noble metal elements include Au, Pd, Pt, Rh, Ru, Ir, and Os.
As described in Chapter 5, the IdentAlloy program was established by manufacturers to provide documentation of certified alloys. Under the program, each alloy has a certificate that lists its manufacturer, alloy name, composition, and ADA classification. Some insurance companies use it as well to determine the cost of crown and bridge treatment. This system lacks the potential to discriminate among alloys within a given category, which may have quite different mechanical properties.
Alloy Classification by Mechanical Properties
Over the past few decades base metal alloys have been developed to the point where they are superior to high noble and noble alloys in several respects, such as low cost and low density, excellent strength, high stiffness, and stable oxide formation (which is required for bonding to porcelain). The classification described in ADA specifications and ISO standards has changed over time.
ADA Specification No. 5 formerly classified gold alloys as types 1 through 4, depending on the content of gold, palladium, and platinum. The content of noble metals by weight ranges from 83% (type 1) to 75% (type 4). Both the current ADA Specification No. 5 (1997) and ISO Standard 1562 (2004) have classified four types of casting alloys using similar minimal yield strength and percent elongation values for each type of alloy. The only difference is that the ISO standard is specific for casting gold alloys whereas ADA Specification No. 5 covers all alloys that pass the tests for toxicity and tarnish. Table 16-2 lists the classification and mechanical properties described in ISO 1562.
Mechanical Property Requirements Proposed in ISO Draft International Standard 1562 for Casting Gold Alloys (2002)
|Type||Descriptor||Yield Strength (MPa)||Elongation (%)||Examples of Applications|
|2||Medium||180||10||Inlays and onlays|
|3||Hard||270||5||Onlays, thin cast backings, pontics, full crowns, saddles|
|4||Extra hard||360||3||Saddles, bars, claps, crowns, bridges, and partial denture frameworks|
The more recent ISO 22674 standard classifies metallic materials for fixed and removable restorations and appliances into six types according to their mechanical properties without referring to the composition of the alloys (Table 16-3). Although the properties for heat-treated specimens are not given in the table, it is assumed that the specimens are bench-cooled.
Classification of Metallic Material for Dental Applications—ISO 22674 (2006)*
|Type||Yield Strength (MPa)||Elongation (%)||Examples of Applications|
|0*||–||–||Single-tooth fixed restorations—e.g., small veneered one-surface inlays, veneered crowns|
|1||80||18||Single-tooth fixed restorations, veneered or nonveneered one-surface inlays, veneered crowns|
|2||180||10||For single-tooth fixed restorations—e.g., crowns or inlays without restriction on the number of surfaces|
|3||270||5||For multiple-unit fixed restorations—e.g., bridges|
|4||360||2||For appliances with thin cross sections that are subjected to very high forces—e.g., removable partial dentures, clasps, thin veneered crowns, wide-span bridges or bridges with small cross sections, bars, attachments, implant retained superstructures|
|5||500||2||For thin removable partial dentures, parts with tin cross sections, clasps|
*Metallic materials for metal-ceramic crowns produced by electroforming or sintering belong to type 0.
Alloy Classification by Principal Elements
Alloys may be classified based on the principal or most abundant element (e.g., a palladium-based alloy), or they may be named based on the two or three most important elements (e.g., Pd-Ag, Co-Cr, or Ni-Cr-Be alloys). When an alloy is identified according to the elements it contains, the components are listed in declining order of composition, with the largest constituent first followed by the second largest constituent. An exception to this rule is the identification of certain alloys by elements that significantly affect physical properties, which represent potential biocompatibility concerns, or both. For example, Ni-Cr-Mo-Be alloys are often designated as Ni-Cr-Be alloys because of the contributions of beryllium to the control of castability and surface oxidation at high temperatures and the relative toxicity potential of beryllium compared with other metals. Alloy groups shown in Table 16-4 are identified by principal elements.
Classification of Casting Metals for Full-Metal and Metal-Ceramic Prostheses and Partial Dentures
|Metal Type||All-Metal Prostheses||Metal-Ceramic Prostheses*||Partial Denture Frameworks|
|High noble (HN)|
*Alloys for metal-ceramic prostheses can be used for all metal prostheses, but not vice versa.
Alloy Classification by Dental Applications
There are three categories of dental alloys designated by their applications, including all-metal fixed prostheses, metal-ceramic prostheses, or removable partial dentures. Alloys for all-metal prostheses are also used as substrates for resin-veneered metal prostheses. As shown in Table 16-4, each type of alloy by noble metal content (HN, N, and PB) is available in all three categories. High noble and noble alloys for metal-ceramic prostheses can be used for all-metal prostheses, whereas those for all-metal restorations are rarely used for metal-ceramic restorations. The reasons are as follows: (1) the alloys may not form thin, stable oxides required for porcelain bonding; (2) their melting range may be too low to resist deformation or melting at porcelain-firing temperatures; and (3) their thermal contraction coefficients may not be close enough to those of commercial porcelains. The introduction of ultralow-fusion high-expansion porcelains, which sinter below 850 °C, has led to the use of some yellow-colored, high gold alloys that can be veneered with porcelain. There is a group of high gold-containing alloys that are designated for both all-metal and metal-ceramic applications.
Base metal alloys, on the other hand, are often marketed for both all-metal and metal-ceramic prostheses because of their oxide formation at room temperature. In this chapter, dental alloys are discussed by their applications.
Desirable Properties of Dental Casting Alloys
Depending on the primary purpose of the prosthesis, the choice of casting alloy or metal is made by the dentist in collaboration with a qualified dental laboratory technician or technologist. From the standpoint of patient safety and the risk of medico-legal issues, it is highly important to understand the following clinically important requirements and properties of dental casting alloys.
The alloy must tolerate oral fluids and not release any harmful products into the oral environment. When components of the alloy are released in the oral environment, they can cause a toxic or allergic reaction. Toxic materials are eliminated by national or international regulations as well as sound business practices. Allergic reactions, however, are peculiar to the individual patient, and the practicing dentist has an obligation, morally and legally, to minimize this risk.
Tarnish and Corrosion Resistance
As previously discussed in Chapter 3, corrosion is the physical dissolution of a material in the oral environment and tarnish is a thin film of a surface deposit that is adherent to the metal surface. Corrosion resistance is derived from the use of noble metals that do not react in the oral environment (e.g., gold and palladium) or by the ability of one or more of the metallic elements to form an adherent passivating surface film, which inhibits any subsurface reaction (e.g., chromium and titanium).
The melting range of the casting alloys must be low enough to form smooth surfaces with the mold wall of the casting investment (Chapter 10). To achieve an accurate fit of cast prostheses, oversized dies for waxing and controlled mold expansion are needed to compensate for casting shrinkage of the alloy and provide space for the luting cement. For metal-ceramic prostheses, the alloys must have closely matching thermal expansion coefficients to be compatible with given porcelains, and they must tolerate high processing temperatures without deforming via a creep process.
The alloy must have sufficient strength for the intended application. Alloys for bridgework require higher strength than alloys for single crowns. Alloys for metal-ceramic prostheses are finished in thin sections and require sufficient stiffness to prevent excessive elastic deflection from functional forces, especially when they are used for long-span frameworks.
Fabrication of Cast Prostheses and Frameworks
The molten alloy should flow freely into the most intricate regions of the investment mold, without any appreciable interaction with the investment material, and wet the mold surface without forming porosity within the surface or subsurface regions of the alloy. This property is also termed castability, which is measured by percent completion of a cast mesh screen pattern or other castability patterns. The selection of an investment material suitable for the metal to be cast is critical.
Cutting, grinding, finishing, and polishing are necessary steps in obtaining a prosthesis with a satisfactory surface finish. The hardness of an alloy is a good primary indicator of the likely difficulty of cutting and grinding it.
For the dental laboratory owner who must guarantee the cost of prosthetic work for a certain period of time, the cost of fabricating prostheses must be adjusted periodically to reflect the fluctuating prices of casting metals, mostly those of high noble and noble metal alloys.
Functional Mechanical Properties of Casting Alloys
The strength of an alloy is an important factor in ensuring that the prosthesis for which it is used will serve its intended functions effectively, safely, and for a reasonable time (Chapter 4). In a general sense, mechanical properties are the measured responses of materials under an applied force or distribution of forces, such as elastic deformation, plastic deformation, or a combination of both. The level of strength needed depends on the intended categories of application and types of prostheses to be made. The following are important functional characteristics of casting alloys.
This property represents a proportional constant between stress and strain during the elastic deformation of a material. One characteristic of a material with high elastic modulus is its rigidity or stiffness. For a dental prosthesis, it is equivalent to its flexure resistance. For long-span FDPs, resistantce to flexure is important. When such a prosthesis flexes during loading of the pontic, the mesiodistal bending moment exerted on the abutment teeth can act as a dislodging force, lifting the mesial and distal aspects of the prosthesis. Furthermore, a flexing bridge can induce lateral forces on the abutment teeth, resulting in the loosening of teeth. For a metal-ceramic prosthesis, the overlying brittle porcelain will fail catastrophically when the metal substructure flexes beyond the flexural strength limit of the ceramic. Elastic modulus is also important for the major connectors of removable partial dentures, which must have enough rigidity to prevent flexure during placement and function of the prosthesis. Resistance to flexure also allows clasps to fit into areas of minimal undercuts and still provide adequate retention.
The elastic moduli of base metal alloys, excluding titanium alloys, are up to twice as high as those for some popular noble metal alloys. This is not considered a major disadvantage for noble metal alloys if proper geometries are employed for the framework connectors. The deflection of a cantilever beam is inversely proportional to t3E, where t is the beam thickness in the plane of bending and E is the elastic modulus. Calculations show that increasing the thickness of the gold alloy connector by 26% yields the same resistance to bending stress as a base metal alloy connector. For a circular cross section, the diameter must increase by 19% to achieve the same degree of stiffness.
The yield strength, proportional limit, and elastic limit all are essentially measures of the same property under tension (Chapter 4). It is important to note the difference between these three properties. Yield strength is defined as the amount of stress needed to cause 0.2% plastic deformation of the material, which is why it is also called 0.2% offset yield strength. Proportional limit is the elastic stress above which the linear relationship between stress and strain no longer exists on the stress-strain plot. The elastic limit is the greatest stress to which the alloy can be subjected, such that it will return to its original dimensions when the forces are removed. Both yield strength and proportional limit can easily be located on the stress-strain curve, but elastic limit cannot be identified precisely.
Yield strength reflects the capacity of a cast prosthesis to withstand mechanical stresses without permanent deformation. Ideally, the alloys should have a high yield strength, so that a great deal of stress must be applied before a permanent change in dimensions occurs. Generally, alloys with tensile yield strengths above 300 MPa function satisfactorily in the mouth.
This mechanical property represents the amount of plastic deformation under tensile stress that an alloy can undergo before it fractures. When this tensile strain is converted to a percent of the original length of the specimen, the property is called percent elongation. If the force applied is in compressive mode, the property is called malleability. A reasonable amount of ductility and malleability are essential if the clinical application requires some plastic deformation of the as-cast structure, as is needed for clasp and margin adjustment and for burnishing. Some base metal alloys have a ductility higher than that of age-hardened Au-Cu noble metal alloys. High ductility means that the amount of deformation that one can produce by adjusting a prosthesis or by burnishing a cast metal margin plastically is higher for the alloy. In order to burnish an alloy, one must exert a high enough stress that is sufficient to exceed its yield strength. Therefore, if the alloy with high ductility also has a high yield strength, the alloy may not achieve the full benefit of high ductility.
Hardness is a measure of the resistance of the surface to indentation by an object in the shape of a spherical or a diamond-shaped point. Hardness of the metal should be high enough to resist scratching and abrasion and also to maintain the smoothness of the prosthesis in the oral environment. A hard restoration surface can also cause excessive wear of the opposing dentition or restoration(s) and requires more energy in grinding and polishing of the restorations.
An important requirement of inlay and crown and bridge alloys is that they must be easily burnished by the dentist. One measure of burnishability is the percent elongation divided by the hardness or yield strength. Some researchers believe that the burnishability of alloys can be compared by dividing the elongation (ductility) by the product of yield strength and hardness. This would indicate that the gold alloy would be easier to burnish. The extremely high hardness of most base metal alloys renders them difficult to cut, grind, and polish. From a clinician’s point of view, the lower hardness and greater ductility of most noble alloys are major advantages compared with those of base metal alloys.
This phenomenon occurs when a material is subjected to repeated loading and unloading below its elastic limit. Most fractures of prostheses and restorations develop progressively over many stress cycles. When the load is above a certain threshold, it initiates cracks from surface flaws of the material. Eventually a crack propagates to a critical size, and sudden fracture occurs.
Fatigue behavior is often determined experimentally by subjecting a material to a cyclic stress between two values and determining the number of cycles required to produce failure. When a removable partial denture is inserted and removed daily, the clasps are strained elastically as they slide over the undercuts of abutment teeth. A comparison of number of constant-deflection cycles that caused fracture of cast clasps made of cobalt-chromium alloy, commercially pure CP Ti, Ti-6Al-4V alloy, and type 4 gold alloy after a number of constant-deflection cycles that caused fracture of cast clasps has shown that cobalt-chromium alloys exhibit the highest fatigue resistance, followed by the type 4 gold alloy, Ti-6Al-4V alloy, and CP Ti. Any casting procedure that produces porosity or that produces carbides in the microstructure of a clasp arm will reduce fatigue resistance because the former represents internal flaws and the latter reduces the elongation of the casting.
Other expressions of fatigue fracture resistance include fatigue strength and endurance limit. Fatigue strength (SNf) is defined as the stress at which failure occurs after Nf cycles. Endurance limit is the maximum stress that can be maintained without failure over an infinite number of cycles.
Alloys for All-Metal Prostheses
These alloys are discussed in three main categories: noble (includes high noble), predominantly base metal, and CP Ti and titanium alloys. Titanium alloys are discussed separately, as they possess properties that are different from those of other base metal alloys.
High Noble and Noble Alloys
For prosthetic dental applications, it is necessary to incorporate various elements in gold to produce alloys with suitable properties. Platinum increases the hardness and elasticity of gold and raises the melting temperature of the alloy. When palladium became expensive in the late 1990s, platinum was used in place of palladium. Copper in a sufficient quantity relative to the gold content renders the alloy heat-treatable (Chapter 5). Silver forms solid solutions with gold and palladium and is a common component in this group of alloys. In gold-copper (Au-Cu) alloys, silver is effective in neutralizing the reddish color of copper. In the case of palladium-based alloys, silver is needed to develop the white color of the alloys. Palladium has a good range of solubility with several metals, such as gold, silver, and copper, and an ability to impart good mechanical properties. It has excellent tarnish/corrosion resistance and is relatively biocompatible. It has also been incorporated in small quantities (about 1.5% by weight) in gold alloys to improve resistance to tarnish and corrosion without a significant loss of gold color. Zinc acts as an oxygen scavenger during melting and casting of noble and high noble alloys. Indium can be used in place of zinc and, when added in greater amounts (18% to 30% by weight), it can promote a gold-like color in Pd-Ag-based alloys. Iridium or ruthenium is added in small quantities as a grain refiner, since smaller grains improve yield strength.
The compositions of selected high noble and noble alloys are given in Table 16-5. Their physical and mechanical properties are shown in Table 16-6. Since some noble metals contain no gold and rely on palladium for corrosion resistance, these alloys are discussed in two categories: Au-based alloys and Ag-Pd alloys.
Typical Compositions of High Noble and Noble Alloys for All-Metal Prostheses
|ELEMENTAL COMPOSITION (PERCENT BY WEIGHT)|
|Pale yellow||20||20||36||–||–||18||Zn:6; Ir|
Physical and Mechanical Properties of Some Modern High Noble and Noble Metal Dental Alloys for Full-Metal Prostheses
|Alloy Type||ADA Classification||Density (g/cm3)||Yield Strength (soft/hard) (MPa)||Hardness (soft/hard) (VHN)||Elastic Modulus (GPa)||Percent Elongation (soft/hard)|
These alloys are generally yellow in color (see Table 16-5). Type 1 gold alloys are soft and designed for inlays supported by teeth and not subjected to significant mastication forces. Type 2 alloys are widely used for inlays because of their superior mechanical properties, but they have less ductility than type 1 alloys. Type 3 alloys are used for constructing crowns and onlays for high-stress areas. Increasing the Pt or Pd content raises the melting temperature, which is beneficial when components are to be joined by soldering (or brazing). Type 4 gold alloys are used in high-stress areas such as bridges and partial denture frameworks. The cast alloy must be rigid to resist flexure, possess high yield strength to prevent permanent distortion, and be ductile enough for adjustment if the clasp of a framework has been distorted or needs adjustment.
Changes of alloy color caused by the reduction in gold content are compensated for by an increase in copper, silver, and palladium. Higher silver and copper content reduces the corrosion resistance of these alloys. These reduced gold alloys have moderate moduli of elasticity but a higher hardness and yield strength than their high noble counterparts.
Heat Treatment of Gold-Copper Alloys
To soften the alloy, the casting is placed in a furnace for 10 minutes at 700 °C and then quenched in water. All intermediate phases in the alloy are changed to a disordered solid solution at 700 °C, and the rapid quenching prevents ordering from occurring during cooling. The tensile strength, proportional limit, and hardness are reduced by such a treatment, and the ductility is increased. To harden the alloy, the temperature of the furnace is set between 200 °C and 450 °C and the casting is heated for 15 to 30 minutes before it is quenched in water. Ideally, before the alloy is age-hardened, it should be subjected to a softening heat treatment to relieve all residual strain hardening (Chapter 17) before the alloy is hardened again by heat treatment to produce a disordered solid solution. Otherwise the amount of solid-state transformation will not be properly controlled. In metallurgical terminology, the softening heat treatment is referred to as a solution heat treatment and the hardening heat treatment is termed age hardening. Mechanical properties of both the softened and age-hardened states are shown in Table 16-6.
The softening heat treatment is indicated for structures that are ground or reshaped plastically to a different form, either in or out of the mouth. Because the proportional limit is increased during age hardening, a considerable increase in the modulus of resilience can be expected. The hardening heat treatment is indicated for metallic partial dentures, saddles, FDPs, and other similar structures where rigidity of the prosthesis is needed. For small structures, such as inlays, a hardening treatment is not usually required. Age hardening reduces the ductility of gold alloys. A reasonable amount of ductility is essential if the clinical application requires some permanent deformation of the as-cast structure, as is needed for clasp and margin adjustment and for burnishing.
These alloys are white and predominantly silver in composition, but they contain at least 25% of palladium to provide nobility and increase the tarnish resistance of the alloy. They may also contain copper and a small amount of gold. Casting temperatures are in the range of those for yellow gold alloys. The copper-free Ag-Pd alloys may have physical properties similar to those of a type 3 gold alloy. With 15% or more copper, the alloy may have properties more like those of a type 4 gold alloy. Despite reports of poor castability because of the lower density and propensity of dissolving oxygen in the molten-state, Ag-Pd alloys can produce acceptable castings when close attention is paid to precise control of the casting and mold temperatures. The major limitation of Ag-Pd alloys in general and in the Ag-Pd-Cu alloys in particular is their greater potential for tarnish and corrosion. The amount of corrosion expected during service is negligible if the palladium content is greater than 25%.
By melting palladium and indium at the composition of 50% In and 50% Pd in atomic percent (52% In and 48% Pd by weight), the alloy is copper colored but increasing the palladium content causes the alloy to lose its reddish color and acquire a gold color. A minimum of 15% by weight of Pd-In intermetallic compound is needed to maintain the yellowish color. A much higher proportion of Pd-In intermetallic compound is used in commercial dental alloys (see Table 16-5). The tarnish resistance of the alloys is especially dependent upon the composition and the integrity of the casting. The colored phase of the Pd-In binary alloy system is hard and brittle and is not a strenghener. Silver, copper, and/or gold can be added to increase the ductility and improve the castability of the alloy for dental applications.
Predominantly Base Metals
Base metal alloys generally comprise the group of cast metals that rely on chromium for corrosion resistance. Chromium on the surface of the alloy rapidly oxidizes to form a thin layer of chromium oxide, which prevents the diffusion of oxygen into the underlying metals and improves its corrosion resistance. Chromium also strengthens the alloy by solution hardening. Since the introduction of cobalt-chromium alloys as cast dental appliances in 1928 and subsequent development of nickel-chromium and cobalt-nickel-chromium alloys, base metal alloys have gained widespread acceptance as the predominant choice for the fabrication of removable partial denture frameworks. Because of the high cost of noble metals, these base metals have been adapted also for dual applications such as the production of all-metal and metal-ceramic prostheses.
Currently there are two main groups of base metal dental alloys: nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr). The Ni-Cr alloys can be further divided into those with and without beryllium, which improves castability and promotes the formation of a stable metal oxide for porcelain bonding. The majority of Ni-Cr alloys are for small castings such as crowns and FDPs, and Co-Cr alloys are primarily used for casting removable partial dentures in which high elastic modulus and yield strength are needed. Some Ni-Cr alloys, which are used for partial denture frameworks, are formulated for their relative ease of finishing and polishing compared with Co-Cr alloys, which are used for crowns and FDPs in spite of their low ductility. Table 16-7 lists compositions and properties of selected base alloys for all metal and metal–ceramic applications. Molybdenum increases corrosion resistance and strength and decreases the thermal expansion coefficient of base metal alloys. The latter is beneficial for porcelain bonding and minimizes the risk of porcelain cracking or fracture. Base metal alloys for partial frameworks are discussed later.
Typical Compositions of Base Metal Alloys for Crown, FDP, and Metal-Ceramic Applications
|ELEMENTAL COMPOSITION (PERCENT BY WEIGHT)||MECHANICAL PROPERTIES|
|Alloy (supplier)||Ni||Co||Cr||Mo||Balance*||Yield Strength (MPa)||Hardness (VHN)||Elastic Modulus (GPa)||Elongation (%)|
|IPS d.Sign 15 (Ivoclar)||58.7||1||25||12.1||Fe:1.9; Si:1.7; Ce||340||230||200||13|
|Rexalloy (Pentron Alloys)||67||–||14||8||Ga:8; Al; Fe; Si; Mn; Zr; Cu||300||177||191||27|
|Heraenium S (Heraeus Kulzer)||62.9||–||23||10||Si:2; Fe:15.5; Ce:0.5||310||220||120||29|
|NPX-III (CMP Industries)||76.5||x†||14||4.5||Al:2.5; Be:1.6; Ti||784||350||200||9|
|Argeloy Bond (Argen)||77||–||14||4.7||Mn; Fe; Si; C||630||370||207||10|
|Norex (Pentron)||–||55||25||–||W:10; Ru:5; Al, Nb, Y, Zr||621||350||204||7|
|Heraenium P (Heraeus Kulzer)||–||59||25||4||W:10; Si:1; Mn:0.8; N||650||330||200||8|
|Jelbond Supreme (Jelenko)||–||61||27||6||W:5; Mn; Si; Fe; C||475||365||223||8|
*Elements wihtout value are less than 1 percent by weight.