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Base metal alloys Base metal alloys Document Transcript

  • DENTAL CASTING ALLOYS INTRODUCTION For casting dental restorations and for the fabrication of wire and other structures, 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. The alloys are compounded to produce properties most acceptable for their intended dental applications, such as simple inlays, bridges, removable cast restorations, solders, (or) wrought wire forms. The noble metals are those elements with a good metallic surface that retain their surfaces in dry air. The six metals of the platinum group they are, platinum, iridium, radium, osmium and ruthemium; along with gold they are called noble metals. HISTORICAL PERSPECTIVE ON DENTAL CASTING ALLOYS The history of dental casting alloys has been influenced by 3 major factors: 1
  • 1. The technologic changes of dental prosthesis. 2. Metallurgic advancements; and 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 Taggart was an instant success. It soon led to the casting of complex inlays such as onlays, crowns, FPD, and RPD frame works. Because pure gold did not have the physical properties required of these dental restorations, existing jewellery alloys were quickly adopted. These gold alloys were further strengthened with copper, silver (or) palladium. In 1932, the dental materials group at the National Bureau of Standards surveyed the alloys being used and roughly classified them as Type 1, 2, 3 and 4. In the following years, several patents were issued for alloys containing palladium as a substitute for platinum. 2
  • 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. The base metal removable partial denture alloys were introduced in the 1930s. Since that time, both Nickel-chromium and cobalt-chromium formulations have become increasingly popular compared with conventional Type IV gold alloys, which previously were the predominant metals used for such prosthesis. The obvious advantages of base metal alloys are their lighter weight, increased mechanical properties and reduced costs. Likewise, by 1978, the price of gold was climbing so rapidly the attention focused on the noble metal alloys. To reduce the precious metal content yet retain the advantages of the noble metals for dental use. PROPERTIES OF NOBLE METAL ALLOYS Since these metals have a wide range of properties and are widely used in dentistry, it is worth while to describe some of their properties. 3
  • a. GOLD 1. Pure gold is a soft, malleable, ductile metal that does not oxidize under atmospheric conditions and is attacked by only a few of the most powerful oxidizing agents. 2. It has a rich yellow colour with a strong metallic luster. 3. Although it is the most ductile and malleable of all metals, it ranks much lower in strength. The pure metal fuses at 106°C, which is only 20° below the melting point of copper (1083°C). 4. Small amounts of impurities have a pronounced effect on the mechanical properties of gold and its alloys. The presence of less than 0.2% lead will cause gold to be extremely brittle. Mercury in small quantities also has a harmful effect on its properties. 5. Gold is nearly as soft as lead, with the result that in dental alloys, coins, and articles of jewellery it must 4
  • be alloyed with copper, silver, platinum and other metals to develop the necessary hardness, durability and elasticity. 6. The specific gravity of pure gold is between 19.30 and 19.33, making it one of the heavy metal. 7. Air (or) water at any temperature does not affect (or) tarnish gold. 8. Gold is not soluble in sulfuric, nitric (or) hydrochloric acids. PALLADIUM 1. Palladium is not used in the pure state in dentistry, but it is used in many dental alloys, combined with either gold (or) silver. It is cheaper than platinum and since it imparts many of the properties of platinum to dental alloys, it is often used as a replacement for platinum. 2. Palladium is a white metal some what darker than platinum. 5
  • 3. Its specific gravity is 11.4 (or) about half that of platinum and a little more than half that of gold. 4. It is a malleable and ductile metal with a melting point of 1555°C, which is the lowest of the platinum group of metals. 5. The metals has the quality of absorbing (or) occluding large quantities of hydrogen gar when heated. This can be an undesirable quality when alloys containing palladium are heated with an improperly adjusted gas air torch. IRIDIUM, RUTHENIUM, AND RHODIUM 1. Small amounts of iridium are some times present in dental alloys, either as impurities combined with platinum (or) as additions to modify the properties. 2. As little as 0.005% (50 ppm) is effective in refining the grain size of cast gold alloys. Ruthenium produces a similar effect. 3. Iridium is a hard metal that is quite brittle, white with a high sp. Gravity of 22.42 and an exceptionally high melting point estimated to be about 2440'C. 6
  • SILVER 1. Silver is malleable and ductile, white, the best-known conductor of heat and electricity, and stronger and harder than gold but softer than copper. 2. It melts at 960.5'C, which is below the melting point of both gold and copper. 3. It is unaltered in clean, dry air at any temperature but combines with sulfur, chlorine and phosphorous, (or) vapors containing these elements (or) their compounds. Foods containing sulfur compounds cause severe tarnish on silver. 4. Pure silver is seldom employed in dental restorations because of the black sulfide formation on the metal in the mouth, although it is used extensively for small additions to many gold alloys. Addition of small amounts of palladium to silver containing alloys prevents the rapid corrosion of such alloys in the oral environment. 7
  • KARAT AND FINENESS OF GOLD For many years the gold content of gold alloys has been described on the basis of karat, (or) in terms of fineness, rather than by weight %. The term karat refers to the parts of pure gold in 24 parts of an alloy. For example, 24-karat gold is pure gold, where as 22-karat gold is an alloy containing 22 parts pure gold and 2 parts of other metals. Fineness describes gold alloys by the number of parts per 1000 of gold. For example, pure gold has a fineness of 1000, and 650 fine alloy has a gold content of 65%. Thus the fineness rating is to timer the gold % in an alloy. Fineness is considered as more practical term than karat. The terms karat and fineness are rarely used to describe the gold content of current alloys. 8
  • CLASSIFICATION OF DENTAL CASTING ALLOYS (OR) DENTAL GOLD ALLOYS According to ADA specification No.5 these casting alloys are described simply as: Type I Type II Type III and Type IV Type I (Soft): These alloys are limited to use in inlays that are subject only to slight stress during mastication. This would include inlays for the gingival and interproximal areas of a 91 tooth and for certain occlusal inlays of such design (or) location that they are not subjected to severe stress applications. Alloys of this type often are useful for inlays prepared by the direct technique, which requires the finishing operation to be completed on the tooth with relatively simple hand instruments. Type II (Medium): These medium alloys can be used for all types of cast inlays and onlays. 9
  • Type III (Hard): These alloys are most acceptable for crowns, thin 3/4th crowns, and anterior and posterior bridge abutments, which should not be cast from the softer and weaker Type I and Type II alloys. Type IV (Extra hard): These alloys are designed to have sufficient strength and adequate properties for cast removable partial dentures with clasps, precision cast fixed bridges and ¾ crowns, which are not subjected to hard working (or) burnishing operations. Composition: The composition of the gold casting alloys that meet the requirements of ADA Sp. No. 5 are given in the Table below: TYPE GOLD PLATINUM PALLADIUM I 81-83% - 0.2-4.5% II 76-78% - 1-3% III 73-77% - 2-4% IV 71-74% 0-1% 2-5% It is apparent from the Table that there is some reduction in gold content when a comparison is made between Type I and IV alloys. 10
  • An increase in copper content is observed as the gold content is decreased. An increase in the zinc content also occurs in Type IV alloys. Platinum is rarely added to Type 1 gold alloys, but a small amount of palladium is always added to all 4 types. PROPERTIES OF GOLD CASTING ALLOYS Type Vickers hardness number (Kg/mm2 ) Softened Yield strength, (0.1% Offset) MPa Elongation Minimum Maximum Softened minimum Hardened minimum Softened minimum Hardened minimum (%) I 50 90 None None 18 None II 90 120 140 None 12 None III 120 150 200 None 12 None IV 150 None 340 500 10 2 From the above table it may be seen that as the hardness increases from Type I to Type IV, the yield strength and tensile strength values are also increased, and the elongation generally decreased. 11
  • Since the yield strength represents in general the resistance to permanent deformation under stress, it can be seen that alloys with increased hardness values offer an increased resistance to permanent bending (or) deformation. Soft alloys have a higher degree of elongation and a relatively greater quality of ductility than the alloys of higher hardness values. FUSION TEMPERATURES OF DENTAL GOLD CASTING ALLOYS METAL (or) ALLOY TYPICAL FUSION TEMPERATURE (°C) Type I 1005-1070 Type II 900-970 Type III 875-1000 Type IV 875-1000 From the above table it is observed that the fusion temperature of the 4 types of alloys decreases from Type I to Type IV. The fusion temperatures are important factors in choosing the type of investment to be used. 12
  • Alloys having fusion temperatures higher than about 1100°C should not be cast into calcium sulfate bonded investment. Numerous classification systems have been proposed to categorize the wide variety of commercial gold-based and palladium based alloys. In 1984 the ADA proposed a simple classification of dental casting alloys. ALLOY TYPE TOTAL NOBLE METAL CONTENT High noble metal Contains ≥ 40 wt% AV and ≥ 60% wt of the noble metal elements (Au+Ir+OS+Pt+Rh+Ru). Noble metal Contains ≥ 25% of the noble metal elements. Predominantly base metal Contains < 25 wt% of noble metal elements. Many manufacturers have adopted this classification to simplify the communication between dentists and dental laboratory technologists. 13
  • Some insurance companies use it as well to determine the cost of crown and bridge treatment. CLASSIFICATION OF ALLOYS FOR ALL METAL RESTORATIONS METAL, CERAMIC RESTORATIONS AND FRAME WORKS FOR REMOVABLE PARTIAL DENTURES Alloy Type All-Metal Metal- Ceramic Removable Partial Dentures High noble Au-Ag-Cu-Pd metal ceramic alloys Au-Pt-Pd Au-Pd-Ag (5- 12 wt% Ag) Au-Pd-Ag (>wt 12% Ag) Au-Pd (no Ag) Au-Ag-Cu-Pd Noble Ag-Pd-Au-Cu Ag-Pd Metal ceramic alloys Pd-Au (no Ag) Pd-Au-Ag Pd-Ag Pd-Cu Pd-Co Pd-Ga-Ag Ag-Pd-Au-Cu Ag-Pd In this we have all the 4 types of alloys, described earlier with both high noble and noble metal alloys. Heat treatment of high noble and noble metal alloys: Gold alloys can be significantly hardened if the alloy contains a sufficient amount of copper. Types I and E alloys 14
  • usually do not harden, (or) harden to a lesser degree than do the types HI and IV alloys. The actual mechanism of hardening is probably the result of several different solid - solid transformations. Alloys that can be hardened can of course, also be softened. In metallurgical terminology the softening heat treatment is referred to as solution heat treatment. The hardening heat treatment is termed age hardening. Softening heat treatment: The casting is placed in an electric furnace for 10 minutes at a temperature of 700°C (129°F) and then it is quenched in water. The tensile strength, hardness and proportional limit are reduced by such a treatment but the ductility is increased. The softening heat treatment is indicated for structures that are to be ground, shaped (or) otherwise cold worked, either in or out of the mouth. 15
  • Hardening heat treatment: The age hardening of the dental alloys can be accomplished 'in several 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 to 30 minutes, before it is water quenched. The aging temperature depends on the alloy composition but is generally between 200°C and 450°C. Ideally, before the alloy is given an age-hardening treatment, it should be subjected to a softening heat treatment to relieve all strain hardening, if it is present and to start the hardening treatment with the alloy as a disordered solid solution. The hardening heat treatment is indicated for metallic partial dentures, bridges, and other similar structures. Casting Shrinkage: Most metals and alloys, including gold and noble metal alloys, shrink when they change from the liquid to the solid state. The values for the casting shrinkage differ for the various alloys presumably because of differences in their composition. It has been shown, for example, that platinum, palladium and copper 16
  • all are effective in reducing the casting shrinkage of the alloy. It is of interest that the value for the casting shrinkage of pure gold closely approaches that of its maximal linear thermal contraction. Alloy Casting shrinkage (%) Type I, Gold base 1.56% Type II, Gold base 1.37% Type III, Gold base 1.42% Ni-Cr-Mo 2.3% Silver Palladium alloys: These alloys are white and predominantly silver 'm composition but have substantial amounts of palladium (at least 25/o) that provide nobility and promote the silver tarnish resistance. They may (or) may not contain copper and a small amount of gold. The copper (Cu) free Ag-Pd alloys may contain 70% to 72% silver and 25% palladium and may have physical properties of a Type III gold alloy. Other silver-based might contain roughly 60% silver 25% palladium and as much as 15% or more of copper and may have properties more like a Type IV gold alloy. Despite early reports of poor castability, the Ag-Pd alloys can produce acceptable castings. 17
  • Because of the increasing interest in aesthetics by dental patients, a decreased use of all metal restorations has occurred during the past decade. The use of metal ceramic restorations in posterior sites has increased relative to the use of all metal crowns and onlays. HIGH NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS The original metal ceramic alloys contained 88% gold and were much to soft for stress bearing restorations such as FPD. Because there was no evidence of a chemical bond between these alloys and dental porcelain, 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. This mean that if the failure occurred in the metal-ceramic restoration, it would most probably arise at the porcelain metal interface. By adding less than 1% of oxide forming elements such as iron, idium, and tin to this high gold content alloy, the porcelain 18
  • metal bond strength was improved by a factor of 3. Iron also increase the proportional limit and strength of the alloy. This 1% addition of base metals to the gold, palladium and platinum alloy was all that was necessary to produce a slight oxide filum on the surface of the substructure to achieve a porcelain-metal bond strength level that surpassed the cohesive strength of porcelain itself. The high noble alloys for metal ceramic restorations are: A. Gold-Platinum - Palladium Alloys: These alloys have a gold content ranging up to 88% with varying amounts of palladium, platinum and small amounts of base metals. Some of these alloys are yellow in colour. Alloys of this type are susceptible to sag deformation and FPD's should be restricted to 3-unit spans, anterior cantilevers (or) crowns. B. Gold-Palladium-Silver Alloys: These gold based alloys contain between 39% and 77% upto 35% palladium and silver levels as high as 22%. 19
  • The silver increases thermal contraction coefficient but it also has a tendency to discolor some porcelains. C. Gold-Palladium Alloys: A gold content ranging from 44% to 55% and a palladium level of 35% to 45% is present in these metal ceramic alloys, which have remained popular despite their relatively high cost. The lack of silver results in a decreased thermal contraction coefficient and the freedom from silver discolouration of porcelain. Alloys of this type must be used with porcelains that have low coefficients on to avoid the development of axial and circumferential of thermal contraction to avoid the development of axial and circumferential tensile stresses in Porcelain during the cooling part of the porcelain firing cycle. NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS A. Palladium Based Alloys 1. Palladium - Silver alloys. 2. Palladium Copper alloys: 3. Palladium - Cobalt alloys: 4. Palladium - Gallium - Silver and Palladium - Gallium – Silver – Gold alloys: 20
  • a. Palladium based alloys Noble palladium based alloys offer a compromise between the high noble gold alloys and the predominantly base metal alloys. The price per ounce of a palladium alloy is generally one half to one third that of a gold alloys. The density of a palladium based alloy is midway between that of base metal and of high noble alloys. 2) Palladium - Silver Alloys: Pd-Ag alloys were introduced widely in the late 1970s. This alloy type was introduced to the U.S. market in 1974 as the first gold free noble metal available for metal ceramic restorations. Pd-Ag alloys enjoyed wide spread popularity for a few years after they were introduced, but their popularity has declined some what in recent years because of their tendency to discolor porcelain during firing. One theory that has been proposed for this greenish yellow discoloration, popularly termed "Selling" is that the silver vapor escapes from the surface of these alloys during firing of the porcelain, diffuses as ionic silver into the porcelain and is reduced to form colloidal metallic silver in the surface layer of porcelain. 21
  • The compositions of Pd-Ag alloys fall within a narrow range: 53% to 61% palladium and 28% to 40% silver. Tin (or) Indium or both are usually added to increase alloy hardness and to promote oxide formation for adequate bonding of porcelain. In some of these alloys, the formation of an internal oxide rather than an external oxide has been reported. Other palladium alloys contain 75% to 90% palladium and no silver and were developed to eliminate the greening problem some of the high palladium alloys develop a layer of dark oxide on their surface during cooling from the degassing cycle, and this oxide layer has proven difficult to mask by the opaque porcelain. Other high palladium alloys such as the Pd-Ga-Ag-Au type seem not to be plagued by this problem. The replacement of gold by palladium raises the melting range but lower the contraction coefficient of an alloy. Increases the silver content tends to lower the melting range and raises the contraction coefficient. Because of their high silver contents compared with the gold based alloys, the silver discoloration effect is most severe for 22
  • these alloys. Gold metal conditioners or ceramic coating agents may minimize this effect. The low specific gravity of these alloys (10.7-11.1) combined with their low intrinsic cost makes these alloys attractive as economical alternatives to the gold based alloys. 3) PALLADIUM-COPPER ALLOYS: This alloy is comparable in cost to the Pd-Ag alloys. Because of their low melting range of approximately 1170°C to 1190°C, these alloys are expected to be susceptible to creep deformation (Sag) at elevated firing temperatures. Thus, one should exercise caution in using these alloys for long-span FPDs with relatively small connectors. As is true for some Pd-Ag alloys, several of these products contain 2% gold. These alloys contain between 74-80% palladium and 9-15% copper. Porcelain discolouration due to copper is possible but does not appear to be a major problem. 23
  • One should be aware of the potential effect on aesthetics of the dark brown (or) black oxide formed during oxidation and subsequent porcelain firing cycles. Care should be taken, to mask this oxide completely with opaque porcelain and to eliminate the unaesthetic dark band that develops at metals porcelain junctions. The Pd-Cu alloy have yield strengths upto 1145MPa. Elongation values of 5% to 11% and hardness values as high as some base metal alloys. Thus, these alloys would appear to have a poor potential for burnishing except when the marginal areas are relatively thin. Although thermal incompatibility is not considered to be a major concern, distortion of ultra thin metal copings (0.1mm) has been occasionally reported. 4) PALLADIUM-COBALT ALLOYS: This alloy group is comparable in cost to the Pd-Ag and Pd- Cu alloys. They are often advertised as gold free, nickel-free, beryllium-free, and silver-free alloys. The reference to nickel and beryllium indicates that these alloys, as is true with the other noble metals, are generally considered biocompatible. 24
  • Like many noble metals, these alloys have a fine grain size to minimize hot tearing during the solidification process. This Pd-Co group is the most sag resistant of all noble metal alloys. The noble metal content (based on palladium) ranges from 78% to 88%. The cobalt content ranges between 4 and 10wt% over commercial alloy contains 8% gallium. An example of typical properties of a Pd-Co alloy is as follows: Hardness 250DPH Yield strength 586MPa Elongation 20% and Modulus of elasticity 85.2Gpa Although these alloys are silver-free, discolouration of porcelain can still result because of the presence of cobalt. Any way this is not considered a significant problem. Failure of the technician to completely mask out the dark metal oxide color with opaque porcelain is a more common cause of unacceptable aesthetic results. 25
  • 5) PALLADIUM-GALLIUM-SILVER AND PALLADIUM-GALLIUM-SILVER-GOLD ALLOYS These alloys are the most recent of the noble metals. This group of alloys was introduced because they tend to have a slightly lighter coloured oxide than Pd-Cu (or) Pd-Co alloys and they are thermally compatible with lower expansion porcelains. The oxide that is required for bonding to porcelain is relatively dark, but it is somewhat lighter than those of the Pd-Cu and Pd-Co alloys. The silver content is relatively low (508 wt%) and is usually inadequate to cause porcelain “greening”. 26
  • TECHNICAL DATA CHART FROM DEGUSSA HIGH GOLD CONTAINING CROWN AND BRIDGE ALLOYS Alloy Type Colour Au Vicker’s hardness Degulor A 1, soft Deep yellow 87.5 55 Degulor B 2, medium Yellow 75.7 95 Degulor C 3, hard Yellow 74.0 145 Degulor Mo 4, extra hard Yellow 65.5 195 SILVER PALLADIUM CROWN AND BRIDGE ALLOYS Alloy Type Colour Au Pd Vickers hardness Palliag MJ 4, extra hard White 55.0 20.9 150 HIGH GOLD CONTAINING ALLOYS FOR CERAMICS Alloy Type Colour Au Vicker’s hardness Degulent G Extra hard Yellow 86 150 Biobond III Extra hard Bright yellow 82.6 160 27
  • PALLADIUM-BASE ALLOYS FOR CERAMICS Alloy Type Colour Au Vicker’s hardness Bond-on 4 Extra hard White 79.7 260 These Pd-Ga-Ag alloys generally tend to have a relatively low thermal contraction coefficient and would be expected to be more compatible with lower expansion porcelains such as vita porcelains. BASE METAL ALLOYS FOR DENTAL CASTINGS The pressures of economics, as well as a search for improved mechanical properties, have led to the development of base metal alloys for the construction of dental prosthesis devices. Composition: The principal elements present in cast base metals for partial dentures are chromium, cobalt and nickel, which together make up approximately 90% of the most alloys used for dental restorations. Representation compositions for typical dental casting alloys are listed in the table. 28
  • Composition of cast base metal alloys used in dentistry: Ingredients Alloys (% of weight) Vitallium Toconium Jelenko LG Nobilium Chromium Cobalt Nickel Molybdenum Aluminium Iron Carbon Beryllium Silicon Manganese Gallium 30.0 Balance - 5.0 - 1.0 0.5 - 0.6 0.5 - 17.0 - Balance 5.0 5.0 0.5 0.1 1.0 0.5 5.0 - 27.0 Balance 13.0 4.0 - 1.0 0.2 - 0.6 0.7 - 30.0 Balance - 5.0 - - 0.35 - 0.35 - 0.05 On close examination of the table, one can observe the following points: 1. Chromium is the only major metal that exists in all alloys of the type. Cobalt is present in all alloys except Ticonium, whereas nickel is absent in vitallium and nobelium. 2. The total wt of chromium, cobalt and nickel in these alloys is over 90% yet, their effect on the physical properties of these alloys are controlled by the presence of minor 29
  • alloying elements such as carbon, molybdenum, beryllium, tungsten and aluminium. Effect of each alloy constituents Chromium: Chromium content is responsible for the tarnish resistance and stainless properties of these alloys. When the chromium content of an alloy is over 30% of the alloy is more difficult to cast. It also forms a brittle phase, known as the zigma phase. Therefore dental alloys of these types should not contain more than 28% or 29% chromium. Cobalt and nickel: are somewhat interchangeable to a certain percentage cobalt increases the elastic modulus, strength and hardness of the alloy more than nickel does. Nickel may increase ductility. Carbon content: The hardness of cobalt base alloys is increased by the increased content of carbon. A change in the carbon content in this order of 0.2% in these alloys changes the properties to such an extent that the alloy would no longer be usable in dentistry. Molybdenum: The presence of 3% to 6% molybdenum contributes to the strength of the alloy. 30
  • Aluminium: Aluminium in nickel-containing alloys forms a compound of nickel and aluminium (Ni3-Al). This compound increases the ultimate tensile and yield strength. Berylium: Addition of 1% beryllium to nickel-base alloy reduces the fusion range of the alloy by about 100°C. It also aids in solid solution hardening. It improves the casting characteristic are possibly participate in porcelain bonding. Silicon and Manganese: are added to increase and castability of these alloys. They are present primarily on oxide to prevent oxidation of other element during melting. The presence of nitrogen which cannot be controlled unless the castings are made in a controlled atmosphere as in vacuum or argon, also contributes to the brittle qualities of these cast alloys. When the nitrogen content of the final alloy is more than 0.1%, the castings lose some of their ductility. Since the minor ingredients of carbon, nitrogen and oxygen effectively influence the properties of the final formulated and designed in such a way as to maximize the rigidity of the prosthesis. The obvious approach would be to increase the thickness of the metal substructure, since doubling the thickness increases the rigidity in bending by a factor of 8. However, the maximum 31
  • thickness of the overall restoration is limited externally by occlusion and proper anatomical contour internally by the desire to retain as much tooth structure as possible. (Esthetics requires a minimal thickness of overlying porcelain that results in severe limits as to the maximum thickness of the metal). An examination of the mechanical properties of base metal alloys and a gold alloy shows that in general the base metal alloys have a modulus of elasticity approximately twice that of previously used gold alloys. Since elastic modulus is a measure of the stiffness of rigidity of materials, this property would enhance the application of base metal alloys for long-span bridges where flexure, is a major cause of failure. Given an equal thickness of precious metal alloy and base metal alloy, the base metal alloy bridge would flex only half as much as the precious alloy material under the same occlusal forces. In a similar manner, the higher modulus of elasticity may be utilized to permit thinner castings. The Vicker’s hardness of base metal alloys may range from approximately 175 to 360DPH. Although certain of the base metal alloys may approach the hardness of noble metal alloy (approximately 160DPH), the majority of these alloys are considerably harder. Clinically, it is improbable that significant 32
  • occlusal wear of the alloy will occur. Therefore, particular attention must be directed toward perfecting occlusal equilibration. The removal of defective clinical units is also more difficult than with noble metal alloys, since the high hardness results in rapid wear of carbide burs and diamond points. The durability, as measured by the percentage elongation, of base metal alloys ranges between approximately to 10 and 28 percent. Noble metal alloys have an elongation of approximately 5 to 10 percent. The density of base metal alloys is approximately 8.0gm/cm3 , as compared with 18.39gm/cm3 for comparable noble metal alloys. Since casting alloys are purchased on a weight basis, a lower density is indirectly reflected to the purchaser, who receives more than twice the volume of material for each unit weight acquired. Also, the intrinsic value of the component elements in base metal alloys is significantly les than that of comparable noble metal alloys. Thus, on the basis of their lower density and the low intrinsic value of the component metals, the cost differential between base metal and noble metal alloys can be substantial. 33
  • When porcelain is first fired to a metal substructure, the alloy is subjected to considerable temperature variations and stresses induced by the shrinkage of the overlying porcelain. Sag resistance is the property that has been used to describe the ability of an alloy to resist the permanent deformation of creep induced by thermal stresses. It is particularly important in long-span bridges where the porcelain firing temperature may cause the unsupported structure to deform permanently. Under controlled conditions, it has been found that a base metal alloy will deform less than 0.001 inch, while a noble metal alloy will deform 0.009 inch. It is likely that the higher fusion temperature common to base metal alloys is a factor that contributes to the superior sag resistance properties of these alloys. The question of metal ceramic compatibility is basic to the selection of an alloy system for this type of restoration. Two requirements are implicit. The metal must not interact with the ceramic in such a way to discolour the porcelain at the interface or marginal regions. Moreover, the metal ceramic system must form a stable bond at the interface that can withstand normal stresses in the mouth. 34
  • Alloys for complete metal restorations are cast into calcium surface bonded investment molars then the alloys have been melted with gas-air blow porhes. The cast base metal alloys cannot be melted with the conventional blowtorch uses for gold alloys, and so it has been necessary to develop special electric melting facilities or less commonly to melt the alloys which or oxygen-oxetylene torch. Electrical somers of melting are often used to advantage, such as carbon areas, argon arcs, high frequency induction, or silicon-confide resistance furnaces. In some insurances, sophisticated electronic equipment is used to control the temperature, casting time, and similar variables in order to regulate the gain formation and confide precipitation. Less commonly oxygen-nityfine torch is used to melt the alloys. The confurizing section of the oxygen-acetylene flame caudd carbon to the alloy. The extra carbon changes not only the microstructure but also the mechanical properties. (In general, hardness and yield strength increases whereas ductility decreases). Therefore, when melting the alloy with an oxygen-acetylen torch, the proportion of the two gases, the length of the flame and the distance of the torch tip from the alloy should be standardized. 35
  • Carbon crucibles and carbon-containing investments should be avoided. Casting shrinkage compensation Because of the high fusion temperature, the casting shrinkage of the base metal alloys is greater than that of the fold casting alloys. It is in the order of 2.3%, which requires that the mold be expanded more than when the dental gold alloys are cast. (Approximately 2% for Ni-Cr alloys). Thermal expansion represents the principal method of mold expansion for compensation of the alloy shrinkage. The use of special phosphate of silicote-bonded investments permit adequate thermal expansion of the molds when they are probably located and one can produce castings that display the proper fit and adequate compensation have yield strengths of at least 450MPa (60,000 lb/inch2 ) to withstand permanent deformation when used as partial denture clasps. Tensile strength studies have indicated that the ultimate tensile strength of the cast base metal alloys is less influenced by variations in test conditions than some other properties such as elongation are. 36
  • Elongation: The percentage elongation of the an alloy is important as an indication of the relative brittleness or ductility that the restoration will exhibit. (There are many occasions therefore when it can be considered to be an important property for comparison of alloys for removable partial denture appliances). The combined effect of elongation and ultimate tensile strength is an indication of toughness of any material. Partial denture claps cast of alloys with a high elongation and tensile strength do not fracture in service as often as those with low elongation, because of their toughness. The percentage elongation is one of the properties that is critical to test accurately and to control properly during test preparatio. A very small amount of microporosity that may exist in the test specimen will alter the elongation considerably, whereas its effect on yield strength, elastic modulus and tensile strength is rather limited. One can therefore assume that practical castings may exhibit similar variations in elongation from one. Casting to another. To some degree this is borne out in practice, with some castings from the same product showing a greater tendency toward brittleness than others. This observation 37
  • indicates that the control of the melting and casting variables is of extreme importance if reproducible results are to be obtained. Although nickel and cobalt are interchangeable in cobalt- nickel-chromium alloys, in general increasing the nickel content with a corresponding reduction in cobalt will increase the ductility and elongation. Jelenko LG, a cobalt-chromium alloy with some nickel and with rigid control of molybdenum and carbon, has a high elongation without much decrease in strength. Elastic modulus: the higher the value, the more rigid the structure can be expected to be, provided the dimensions of the casting are the same in both instances. (There are those within the profession who recommend the use of a well-designed, rigid appliance on the basis that it gives the proper distribution of forces on the supporting tissues when in service. With a greater elastic moduls it is possible to design the restorations with slightly reduced dimensions. (It is a well established fact that the elastic modulus of the cast metal alloys is at least twice that of the dental gold alloys). The cast cobalt-chromium dental alloys show comparable values for elastic modulus of about 228GN/m2 (33x106lb/inch2 ), whereas nickel-chromium alloys possess an elastic modulus of 38
  • about 198MPa (27x106 lb/inch2 ), which is approximately double the value of 90MPa (13x106 lb/inch2 ) for type IV cast metal gold alloys. MICROSTRUCTURE OF CAST BASE METAL ALLOYS The microstructure of any substance is the basic parameter that controls the properties. Cobalt-chromium or nickel-chromium alloys microstructure changes by a slight alteration of manipulative conditions. The microstructure of cobalt-chromium alloys in these condition consists of an elastomeric matrix composed of a solid solution of cobalt and chromium in a cored dendritic structure. Many elements present in cast base metal alloys, such as chromium, cobalt and molybdenum, are carbide forming elements. Depending on the composition of a cast base metal alloy and its manipulative condition, it may form more or less of any given type of carbide. Further more, the arrangements of these carbides may also vary depending on the manipulative condition. The effect of the microstructure on physical properties a commercial cobalt- chromium alloy is illustrated in Figure. A one cane say that the carbides are continuous along the grain boundaries. Such a structure is obtained when the metal is cast as soon as it is 39
  • completely melted. In this condition the cast alloy possesses low elongation values with a good and clean surface. Carbides that are spherical and discontinuous like islands are shown in figure B. Such a structure can be obtained if the alloy is heated about 100°C about its normal melting temperature, and this results in casting with good elongation values but with a very poor surface. The surface is so poor that the casting cannot be used in dentistry. Dark eutectoid areas which are lamellar in nature, are shown in figure C. Such a structure is responsible for very low elongation values but a good and clean casting. Investment materials and casting operations: The manner of casting relatively 1 stage partial denture appliances differs in some details from the casting of simple restorations such as single inlays or crowns, though in principle the two operations are similar. In cast partial denture construction a suitable cast of refractory material or investment serves as the structure on which the wax pattern is formed, when the wax pattern is completed on the refractory cast, both the wax pattern and the cast are then invested in a appropriate investment material. If gold alloys are to be used, the conventional gypsum- bonded silica investment is acceptable, but only one cast nickel 40
  • chromium alloy (ticonium) has a sufficiently low melting temperature to be cast into a specially formulated gypsum-silica type of investment. For the higher melting base metals it is necessary to use-------. When casting any of the base metals in to molar designed to accommodate the higher melting temperature of these onlays, certain problems may be encountered that are less common when casting lower melting alloys. One problem is that of trapping gases in the mold during the casting process. To have sufficient strength and resistance to thermal shock, some investments for the cast base metal alloys lack sufficient porosity for the rapid escape of gases from the mold cavity when the hot metal intern. As a result, the gases may be trapped in the mold cavity and produce voids and casting defects. Numerous methods have been prepared to overcome such defects, such no venting is the surface of the mold to permit rapid elimination of gases. The skillful spruing at and venting of the mold, combined with complete elimination of the wax residue and adequate heating of the metal, tend to reduce this type of defective casting. The melting of base metal alloys must be carefully controlled to avoid inverse damage to the alloy during the melting 41
  • and casting process. Oxidation of the ingredient metals and carbide or nitrite formation at the high temperature required to melt these alloys demand procine control of the melting and casting operation. Regardless of the method employed to melt the alloy, it is possible to cause severe damage to the properties of the casting if proper melting practices are not observed. Other applications of cast base metal alloys: It has been demonstrated that cast cobalt-chromium alloys serve a useful purpose in appliances other than removable partial denture restorations. In the surgical repair of bone fractures, alloys of this type are used for bone plates, crown, various fracture appliances and splints. Metallic obturation and oral implants for a variety of purposes are formed from cast base metal alloys. The use of the cobalt-chromium alloys for surgical purposes in wall established, and these -------------------------------------- periods of time without harmful reactions. This favourable response of the tissues probably is attributable is the low solubility and electrogalvanic action of the alloy used, with the result that the metal is inert and produce no inflammatory responses. The product known as surgical vitallium is used extensively for this purposes. Cast titanium and its alloys 42
  • have recently been introduced as surgical implant materials because of their excellent tissue compatibility. Potential health hazards of nickel and beryllium It is widely recognized beryllium is potentially toxic under uncontrolled conditions. Lab-technicians may be exposed occasionally or routinely to excessively high concentration of beryllium and nickel dust and beryllium vapours. The exposure to beryllium may result in acute and chronic forms of beryllium disease (Physiologic). The responses may vary from contact dermatitis to severe chemical pneumanitic which can be fetal. However, the diagnosis of chronic beryllium disease is difficult, since it often exhibits symptoms range from coughing, chest pain and general weakness to pulmonary drifection and requires the establishment of beryllium exposure. The occupational health and safety administration specifies that exposure to beryllium dust in an air should be limited to a particular beryllium concentration of 2mg/m3 of air for a time weighed, 8 hours day. In laboratory and clinical situations in a grinding of beryllium containing alloys is of performed, adequate 43
  • local exhaust ventilation safeguards should be employed, since all forms of beryllium are toxic and the body cannot ------- beryllium. It is also believed that beryllium localization (i.e. movement of Be ions to the surface) enriches surface is the point that beryllium makes upto 30% of the composition of the surface layer. Beryllium release from the surface is enhanced by presence. POTENTIAL HAZARDS OF NICKEL TO PATIENTS In certain non-dental industraial applications and subexperimental conditions, nickel and its compounds have been implicated as potential carcinogens and as sensitizing agents. Of great concern in dental patients is the intra oral exposure to nickel, especially for patients with known allergy to this elements, Dermatitis resulting from contact with nickel solutions was reported as early as 1989. the incidence of allergic sensitivity to nickel has been found to be 5 to 10 times higher for females than for males with 5% to 8% of females showing sensitivity. Many cases of respiratory organ concerns have been documented in studies of workers involved in the plating, refining, grinding and polishing of nickel and nickel alloys. Because of the concerns over the carcinogenic potential of nickel, 44
  • the National institutes for occupational safety and health has recommended a standard to limit employee exposure to inorganic nickel in the work place to 15mg/m3 . It appears that the potential carcinogenic risks of nickel are less likely to affect dental patients. To minimize exposing of metallic dust to patients and dentists during intra oral metal grinding operations, a high-speed evacuation system should be used. Patients should be informed of the potential allergic effects of nickel exposures an a thorough medical history should be taken to try to determine the patient may be allergic to nickel. Intra oral tissues are more resistant to symptoms of sensitivity. However intra oral exposure to allergies can be manifested in locations remote from dental restorations. The systems of the sensitivity range from urticania, pruritis, xerostomia, eczema or vesicular eruptions. Etching of base metal alloys: When it was first introduced micromechanical retentions of etched metal resin-bonded retainers (Maryland bridges) was obtained by electronically etching the base metal alloys. More 45
  • recently, chemical etchants have been marketed also less expensive and more convenient for etching the metal substraction. The intagets surface of the resin are to be binded to etched enamel are treated with acid gels or liquids for a short period of time. However reports on the comparative bond strength between electrolytically etched and chemically etched surface on conflicting. More study is needed to determine the relative value of chemical etchants substitutes for conventional electrolytic etching. METAL CERAMIC ALLOYS General Features: The chief objection of the use of dental porcelain as a restorative material is its low tensile and shear strength. Although porcelain can resist compressive stresses with reasonable success, substructure design does not permit shapes in which compressive stress is the principal force. A method by which this disadvantage can be minimized is to fuse the porcelain directly to a cast alloy substructure made to fit the prepared tooth. If a strong bond is attained between the 46
  • porcelain veneer and the metal, the porcelain veneer is reinforced. Thus, brittle fracture can be avoided, or at least minimized. The original metal ceramic alloy contained 88 percent gold and were much too soft for stress-bearing restorations such as fixed partial dentures. Since there was no evidence of a chemical bond between these alloys and dental porcelain, mechanical retention and undercuts were used to prevent detachments of the ceramic veneer. By adding less than 1 percent oxide forming elements such as iron, indium and tin to this high gold content alloy, the porcelain-metal bond strength was improved by a factor of 3 iron also increases the proportional limit and strength of the alloy. This 1 percent addition of base metals to the gold, palladium, and platinum alloy was all that was necessary to produce a slight oxide film on the surface of the substructure to achieve a porcelain metal bond strength. This new type of alloy with small amounts of base metals added, became the standard for the metal ceramic restoration. In response to economic pressures, other gold-and palladium-base metal ceramic alloys emerged. In time, base metals were also developed for this same purpose. 47
  • Properties: The clinical success of a metal ceramic restoration is dependent in large measures on the ability of the underlying alloy substructure to resist the potentially destructure masticatory stresses. (Therefore it is imperative that the metal ceramic restoration be use of variable casting conditions. Therefore, these alloys are generally considered to be technique sensitive. One reason for this sensitivity is that almost all elements in these alloys such as chromium, silicon, molybdenum, cobalt and nickel react with carbon to form carbides depending on the mold and alloy-casting temperature, cooling rate, and other technical variables, carbides of any one of these elements may form. The formation of different carbides naturally changes the properties of the alloys. As a result, careful control of manipulative variables in the casting operations is necessary. ANSI/ADA Specificaiton No. 14 According to this specification, the total weight of chromium, cobalt and nickel should be not less than 85% or no less than 20% chromium. Alloys having other compositions may also be accepted by the ADA provides that the alloys comply satisfactory with requirements for toxicity hypersensitivity and 48
  • corrosion. Composition to the nearest 0.5% shall be marked on the package plus the presence and percentage of hazardous elements and precautioning recommendations for processing the materials. The specification also recommend minimum values for elongation, yield strength and elastic modulus. An important feature of this specification is that it has more a standardized method of testing available, which has in turn, made possible comparisons of results from one investigation to another. 49
  • CONTENTS  Introduction  Historical Perspective on Dental Casting Alloys  Properties of Noble Metal Alloys  Classification of Dental Casting Alloys  Alloys for All Metal & Resin Veneer Restorations  High Noble Alloys for Metal Ceramic Restorations  Base metal Alloys for Dental Castings  Composition of Base metal Alloys for Small castings  Effect of Alloy Constituents  Handling Hazards and Precautions  Summary & Conclusion 50