NOBLE METAL ALLOYS INTRODUCTION For casting of dental restorations, it is necessary to combine various metals to produce alloys with adequate properties, for dental applications. These alloys are produced largely from gold combined with other noble metals and certain base metals to produce properties most acceptable for their intended dental applications such as inlays, onlays, bridges, removable cast restorations etc. The noble metals are those elements with a good metallic surfaces that retain their surfaces in dry air. The 6 metals of the platinum group are platinum, palladium, iridium, rhodium, osmium, ruthenium, and along with gold, are called noble metals. The history of dental casting alloys has been influenced by three major factors: 1) The technologic changes of dental prosthesis. 2) Metallurgic advancements. 3) Price changes of the noble metals since 1968. Taggart’s presentation to the New York odontological group in 1907 on the fabrication of cast inlay restorations often has been acknowledged as the first reported application of the lost wax technique in dentistry. The inlay technique described by Jaggart was an instant success.
It soon led to the casting of complex inlays such as onlays, crowns, fixed partial dentures and removable partial denture frameworks. Because pure gold did not have the physical properties required or these dental restorations, existing jewellery alloys were quickly adopted. These gold alloys were further strengthened with copper, silver, or platinum. In 1932, the dental materials group at the National Bureau of Standards surveyed, the alloys being used and roughly classified them as: Type I – Soft-VHN-50 to 90. Type II – Medium VHN of 90 to 120. Type III – Hard VHN of 120 to 150. Type IV – Extra Hard VHN ≥ 150. By 1948, the composition of dental noble metal alloys for cast metal restorations had become rather diverse. With these formulations, the tarnishing tendency of the original alloys apparently had disappeared. It is now known that in gold alloys, palladium is added to counter act the tarnish potential of silver. By 1978, the price of gold was climbing so rapidly that attention focused on the noble metal alloys – to reduce the precious metal content, yet retain the advantages of the noble metals for dental use.
Desirable properties of casting alloys: Cast metals used in dental laboratories must exhibit the following properties: 1) It should be biocompatible. 2) Easy to melt. 3) Easy to carry out casting, brazing (soldering) and polishing. 4) Little solidification shrinkage. 5) Minimal reactivity with the mold material. 6) Good wear resistance. 7) High strength and sag resistance (metal ceramic alloys). 8) Excellent tarnish and corrosion resistance.
Classification of Dental Casting Alloys According to ADA specification No.5, the castings alloys can be classified as: 1) Type I (Soft) small inlays, easily burnished and for restorations subject to very slight stress such as inlays. 2) Type II (Medium) Inlays subject to moderate stress including onlays, thick ¾ crowns, thin cast backings, abutments, pontics and full crowns. 3) Type III (Hard) Inlays subject to high stress including thin ¾ crowns, thin cast backings, abutments, pontics, full crowns denture bases and short span fixed partial dentures.
4) Type IV (Extra hard) Inlays subjected to very high stresses including denture base bars and clasps, partial dentures, frameworks and longspan fixed dentures, full crowns are often made for this type high stresses such as endodontic posts & core. The development of modern direct tooth coloured filling materials has almost eliminated the use of type I- II alloys. Types I and II alloys are often refined to as “Inlay alloys. Types III and IV are generally called “Crown & bridge” alloys. In 1984, the ADA proposed a simple classification for dental casting alloys: Alloy Type 1) High noble metal.
Total Noble Metal Content Contains≥ 40wt%Au+≥60wt% of noble metal elements (Au+IS+Os+Pd+Rh+Ru).
2) Noble metal.
Contains ≥25wt% of the noble metal elements.
3) Predominantly base metal. Contains <25wt% of the noble metal elements. According to Marzouk Type I Type II 1) Class I – Gold and platinum group based alloys Type III Type IV 2) Class II Low gold alloys (gold content <50%). 3) Class III Non-gold palladium based alloys. 4) Class IV Nickel-chromium based alloys. 5) Castable moldable ceramics.
Other Classifications: 1. Yellow golds Yellow coloured
2. Low gold / economy gold.
Gold content > 60%
Usually yellow coloured. But with gold content 60%
3. White Golds White coloured Gold content > 50% CLASSIFICATION OF ALLOYS FOR ALL-METAL RESTORATION, METAL CERAMIC RESTORATIONS, AND FRAMEWORKS FOR REMOVABLE PARTIAL DENTRUES Alloy Type
1) High Noble
Au-Ag-Cu-Pd Metal-Ceramic alloys
Ag-Pd-Cu Ag-Pd Metal-Ceramic alloys
Au-Pt-Pd Au-Pd-Ag (5-12wt%Ag) Au-Pd-Ag (>12wt%Ag) Au-Pd (Zn Ag) Pd-Au (Zo Ag) Pd-Au-Ag Pd-Au Pd-Cu Pd-Co Pd-Ga-Ag
Removable Partial Denture Au-Ag-Cu-Pd
Note : The principle reasons that alloys for all-metal restorations cannot be used for metal-ceramic restorations are that: 1) the alloys may not form thin, stable oxide layers to promote bonding to porcelain, 2) their melting range may be too low to resist sag deformation or melting at porcelain firing temperatures their thermal contraction co-efficients may not be close rough to those of commercial porcelains. Ingredients of noble metal alloys : The most important element in dental gold alloys are gold, copper, silver, platinum metals and zinc).
Gold 1) Gold is primarily responsible for Deformability (ductility) Ranks lowest in strength. Characteristic yellow colour with a strong metallic luster. Density (sp.gravity) 19.3g/cm3. Tarnish resistance. Fusion temperature – 1063°C. Not soluble in sulphuric, nitric or hydrochloric acids. Lowest density necessitates more force in centrifugal casting to compensate for lower wt/vol. However, lower density will allow more restorations per unit wt which can be economical to some extent lowest density necessitates more force in centrifugal castings to compensate for the lower wt/vol. High density enables case of castings. Alloys for dental use should be atleast 16K resistance. 2) Platinum : May be added to: 1. Strengthen the alloy. 2. Raise the fusion point (1755). 3. Import rigidity, nobility, hardness. 4. Whiten the alloy. 5. Sp gravity 21.37 6. Also malleable and ductile. Its coefficient of exp is close to that of porcelain to prevent buckling of the metal or fracture of porcelain during changes in temperature.
3) Palladium Serves the same functions but is much less expensive than platinum. Disadvantages – Palladium hydrogen gas because gold containing palladium may be more porous when cast SP.Gr 11.4 melting point – 1555°C. 4) Iridium, Ruthenium and Rhodium Trace amounts of these metals are added as “Grain Refiners” Melting point 960, 5°C below the melting point and both gold and copper as little as 0.005% is sufficient to refine the grain size. Grain refiners produce smaller grains. Fine grained alloys have smaller grains compared to coarse-grained alloys which relatively larger grains. Fine-grained alloys are generally more stronger and more ductile than coarse grained alloys. Indium – can also act as a savenger for the alloy during cast procedure. Can also serve to increase the tarnish and corrosion. 5) Silver also contributes to hardness and strength of the alloy. Although it mimics gold in its deformability effect, it adversely affect the mobility. In other words it lowers tarnish resistance. -
Food containing sulphur compounds cause severe tarnish on silver.
Silver serves to balance the red colour given by copper.
Adding small amounts of palladium to silver containing alloys prevents the rapid corrosion of such alloys in the oral environment. Also occludes appreciable quantities of O2 in the molten state. Porosities or rough casting surface can be prevented by adding 5-10% Cu.
6) Copper : Contributes strength and hardness, but decreases the mobility of the alloy, i.e. it decreases the tarnish and corrosion resistance because the maximum content should not exceed 16%. 1) Gives the alloy and reddish appearance. 2) Lowers, the fusion temperature. Zinc : (Present only in low percentages, around 0.5%) acts as a deoxidizer and reduces the oxygen content. (Because O2 released during solidification results in porosity). Karat and Fineness Karat ďƒ is the traditional unit expressing gold content in an alloy. Karat refers to the parts of pure-gold in 24 parts of an alloy. -
Pure gold is 24 karat.
22K gold is an alloy containing 22 parts pure gold in and 2 parts of other metals.
Fineness ďƒ Fineness is the percentage of gold multiplied by 10 because a 24 karat alloy would have a fineness of 100 x 10, or 1000 and a 12-karat alloy would be 500 fine (500f).
Heat Treatment of Gold Alloys Gold alloys can be significantly hardened if the alloy contains a sufficient amount of copper (at least 8 wt/copper). Heat treatment is a process of healing a metal to improve its properties. An alloy can be subjected to – Hardening heat treatment also “age hardening”, softening heat treatment also referred to as “solution heat treatment”.
“Softening Heat Treatment Casting is placed in an electric furnace for 10 minutes at a temperature of 700°C (1292°F) and then quenched in water. During this period, all intermediate phases are presumably changed to a disordered solid solution and the rapid quenching prevents ordering from occurring during cooling. Softening heat treatment reduces - Tensile strength - Proportional limit - Hardness but improves – Ductility. Softening heat treatment is indicated for, structure that are to be ground, shaped or otherwise cold worked. Hardening heat treatment – can be accomplished in many ways. One of the most practical hardening treatments is by “soaking” or aging the casting at a specific temperature for a definite time usually 15-30 minutes before it is water quenched. 9
The aging temperature on the alloy composition but is generally between 200°C (400°F) and 450°C (840°F). Age hardening improves- Strength (yield). - Proportional limit. - Hardness. Age hardening is indicated for metallic partial dentures, saddle bridges and other similar structures. For small structures such as inlays, a hardening heat treatment is not usually employed. Ideally before the alloy is given an age- hardening heat treatment, it should be subjected, 1) to a softening heat treatment for 2 reasons: to reline all strain hardening, if it is present, 2) to start the hardening treatment with the alloy as a disordered solid solution.
Casting Shrinkage: Most metals and alloys including gold and the noble metal alloys shrink when they change from the liquid to the solid state. The shrinkage occurs in 3 stages: 1) The thermal contraction of the liquid metal between the temperature to which it is heated and the liquidus temperature. 2) The contraction of the metal from liquid to solid shape. 3) The thermal contraction of the solid metal that occurs down to room temperature.
The values for casting shrinkage differ for the various alloys presumably because of difference in their composition. It has been shown that platinum, palladium and copper all are effective in reducing the casting shrinkage of an alloy. The following table shows the linear solidification shrinkage of casting alloys: Alloy Type I, gold base
Type II, gold base
Type III, gold base
Composition of Traditional I to IV alloys Type I
Sn, In, Fe, Zn, Ga Balance
Silver-Palladium Alloys These alloys are white in colour. Main component is silver, but small amounts of palladium (attract 25%) is also present (Palladium added to provide mobility and tarnish resistance). Copper may or may not be present and a small amount of gold.
Copper-free silver-palladium alloys may contain 70-72% silver and 25% palladium and may have physical properties of type III gold alloys. Other silver-based alloys might contain roughly 60% silver, 25% palladium and as much as 15% or more of copper. These alloys have properties triangular to Type IV gold alloy. Disadvantages : Greater potential for tarnish and corrosion.
High Noble Alloys for Metal-Ceramic Restoration: The original metal-ceramic alloys contained 85% gold and were much too soft for stress-bearing restorations such as fixed partial dentures. There was no evidence of a chemical bond between these alloys and dentinal porcelain. Therefore mechanical retention and undercuts were used to prevent detachment of the ceramic veneer. Using the “Stress bond test”, it was found that the bond strength of the porcelain to this type of alloy was less than the cohesive strength of the porcelain itself. So, stress was concentrated at Porcelain-metal interface. By adding less than 1% of oxide-forming elements such as iron, indium and tin to this high-gold content alloy, the porcelain metal bond strength was improved. (Iron also increases the proportional limit, and strength of the alloy). Because this 1% addition of base metals to the gold, palladium and platinum alloy produced a slight oxide film on the surface of the substructure to
achieve a porcelain-metal bond strength level that surpassed the cohesive strength of the porcelain itself. This new type of alloy, with small amounts of base metals added became the standard for the metal-ceramic restoration. In spite of vastly different composition, these alloys share at least three common features: 1) They have the potential to bond to dental porcelain. 2) They possess coefficients of thermal contraction compatible with those of dental porcelains. 3) Their solidus temperature is sufficiently high to permit the application of low-fusing porcelains. The coefficient of thermal expansion tends to have a reciprocal relationship with the melting point of alloys. The higher the melting temperature of a metal, the lower its CTE. The high noble alloys for metal-ceramic restorations are: 1) Gold-platinum-palladium alloys: Gold content ranges upto 88%. Varying amounts of palladium, platinum and small amounts of B metals. Colour – yellow Disadvantages : Susceptible to sag deformation and FPD should be restricted to 3-unit spans, amount cantilever or crowns. 2) Gold-palladium-silver alloys:
Gold content ranges between 39% - 77% gold. Palladium – 35%. Silver – less than 22%. Silver increases the thermal contraction coefficient. Disadvantages : Silver present in this alloy can discolour same porcelains. 3) Gold-Palladium alloys Gold content ranges for 44% to 55% Palladium 35% to 45%. These alloys have remained popular despite of their relatively high cost. Disadvantage : The lack of silver results in freedom from silver distry. The lack of silver results in a decreased coefficient thermal content because these alloys must be used only with porcelains that have low CT contraction to avoid the development of initial circumferential tensile stresses in deny cooling point of ---------.
Noble Alloys for Metal-Ceramic Resins According to ADA classification of 1984, noble alloys must contain at least 25wt% of the noble metals but do not necessarily contain any gold. Noble palladium-based alloys after a compromise between the high-noble gold alloys and predominantly base metal alloys. Also, the price / ounce of a palladium alloy is generally one half to one third that of a gold alloy. Density is midway between that of base metal and high noble alloys.
Palladium-based alloys have a workability angular to gold and scrap value. 1) Palladium-silver alloys: These alloys were introduced widely in the late 1970s (to overcome some disadvantages of early B.M. alloys e.g. : castability, porcelain bonding and workability problems). In the recent years the popularity of these alloys has declined because of the disadvantages with these alloys is it discolours porcelain during firing. This discolour, is usually a greenish-yellow discolor and is popularity termed as “Greening”. Greening occurs mainly because of the presence of silver Silver vapor escapes for the surface of these alloys during firing of the porcelain. Diffuses as ionic silver into the porcelain. Finally reduced to colloidal metallic silver in the surface layer of porcelain. Not all porcelains are susceptible to silver discolour because some do not contain the necessary elements to reduce the ionic silver. To eliminate the greening problem, palladium alloys with no silver were developed. These alloys contain 75% to 90% palladium. Some of these high palladium alloys develop a layer of dark oxide on their surface during cooling from the degassing cycle. This oxide layer has proven difficult to mark by the opaque porcelain.
Other high palladium alloys such as the Pd-Ga-Ag-Au type do not seem to be plaqued. This alloy type was introduced to the U.S. market in 1974 as the 1 st free noble metal available for metal ceramic restorations. Composition : Palladium – 53% to 61%. Silver – 28% to 40%. Small amounts of tin + indium or both are added to promote oxide formation for adequate bonding of porcelain. In some alloys, the formation of an internal oxide layer rather than an ext. oxide layer has been reported. Increasing the silver content tends to a lower the metal range and raises the contraction coefficient of an alloy. Because of the high silver content, silver discolouration effect is most severe for these alloys. However, gold metal conditioners or ceramic coating agents may minimize this effect. The low specific gravity of these alloys (10% to 11%) combined with their low intrinsic cost makes these alloys attractive as economical alternatives to the gold-based alloys.
Palladium-copper Alloys This alloy type is comparable in cost to the Pd-Ag alloys because alloys of this type are recent introductions to the dental market, little clinical information is available on their long-term clinical success. Disadvantages because of their low melting range of approx 1170°C to 1190°C, these alloys are susceptible to creep deformation (Sag) at elevated firing temperatures. Thus, one should exercise caution in using these alloys for long-span fixed partial dentures with relatively small connectors. Composition: Palladium – 74% and 80%. Copper – 9% to 15%. Some may contain as little as 2% gold – this small amount of gold serves no useful purpose. Porcelain discolouration due to copper is possible but does not appear to be a major problem. There has also been some concern recently over the potential cytotoxic effect of copper released intra-orally from certain Pd-Cu alloys. One should also be aware of the potential effect on aesthetics of the dark brown or black oxide formed during oxidation and subsequent firing cycles. Care should be taken by the technician to mask the oxide completely with opaque porcelain and to eliminate the unaesthetic dark bond that develops at metal-porcelain junctions.
It is also important that technician ensure that a brown rather than a black oxide is formed on the metal surface during oxidation treatment otherwise, poor adherence to porcelain may result. Some of these alloys are technique sensitive with respect to casting. In some instances the molten alloy should have a thin oxide firm appearing on its surface at the casting temperature. Some instructions specify that heating be maintained for an additional 7 seconds beyond the point at which a rolling motion of the alloys is observed. Because of the lack of a specifically defined melt appearance, there may be a tendency to overheat the alloy to eliminate this film. This error would cause significant changes in the properties of the alloy and a decrease in metal-porcelain bond strength. Underheating of the molten alloy is also possible because of the difficulty in judging the proper melt appearance this could result in incomplete castings or rounded margins. Because alloys have a poor potential burnishing, except when the marginal areas are relatively thin. Although thermal incompatibility is not considered to be a major distortion of ultrathin metal copings (0.1mm) has been occasionally reported. The exact cause of this effect is not known. It could be : -
Because of metal-porcelain incompatibility stresses.
High creep rate of these alloys at temperatures near the glass transitions temperature of porcelain.
Relaxation of elastic stresses due to solidification, grinding and sandblasting.
This distortion of single unit / multiple unit castings could be overcome by using thicker copings / connectors or changing the band of porcelain or by using a different metal-ceramic system with more acceptable overall properties.
Palladium-Cobalt Alloys This alloy group is comparable in cost to Pd-Ag and Pd-Cu alloys. They are often advertised as gold-free, nickel-free, beryllium free and silver free alloys. Like many noble metals, these alloys have a fine grain size to minimize hot tearing during the solidification process. This Pd-Cu group is the most sag resistant of all noble metal alloys. Composition : -
Palladium – 78% to 88%.
Cobalt – 4% to 10wt%.
One commercial alloy contains 8% gallium. An example of typical properties of a Pd-Cu alloy is as follows: Hardness – 250DPH. Yield strength – 586 MPa. Elongation – 20%.
Modulus of elasticity 85.2 GPa Although these alloys are silver free, discoloration of porcelain can still result because of the presence of cobalt. But this is not a significant problem. Failure of the technician to completely mask out the dark metal oxide colour with opaque porcelain is a more common cause of unacceptable aesthetic results. No metal casting agents are required to mask the oxide colour or to promote adherence to porcelain. Like the Pd-Ag and Pd-Cu alloys the Pd-Co alloys generally tends to have a relatively high thermal contraction coefficient. Hence would be more compatible with high expansion porcelains.
Palladium-Gallium-Silver and Palladium-Gallium-Silver-Gold alloys These alloys are the most recent of the noble metals. This groups of alloys was introduced because they tend to have a slightly lighter coloured oxide than the Pd-Cu or Pd-Co alloys. The silver content is relatively low (5-8wt%) and is inadequate to cause porcelain greening. Since they have a low coefficient of thermal contraction they would be more compatible with lower expansion porcelains (e.g. vita porcelains).
Conclusion Numerous types of casting alloys have been used for the restoration of teeth while gold-based alloys have played the major role for many years their dominance has been challenged now by base metal system. The main reason is that base metals are less expensive as compared to gold alloys. Unfortunately these substitutes have been less than ideal in numerous way and one of greatest questions about them is their biological compatibility. However, further investigation need to be conducted to evaluate their biocompatibility. However, the performance of any restoration is related to multiple factors for ex: the design of the appliance, the skill and accuracy with which it has been fabricated and the properties of the materials used.
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