Chapter 7 Metal-Ceramic Alloys I. Porcelain Supported by Metal These alloys are frequently called porcelain-fuse-to-metal alloys or, simply, PFM Metal-ceramic alloys is the preferred term that will be used in this chapter. The porcelains that will be applied to these alloys will be described in Chapter 8. Porcelain has good and bad qualities as a restorative material: good: 1. Matches the color and translucence of enamel 2. Well-tolerated by soft tissue bad: 1. Difficult to form acceptable margins 2. Very brittle subject to fracture in tension The two bad characteristics can be overcome by fusing the porcelain to a metal coping. The metal supports the porcelain, increasing its resistance to fracture. Secondly, the coping is cast using lost-wax casting techniques, which can produce a crown whose margins closely fit the prepared tooth. Metal-ceramic crowns and bridges make up an increasing percentage of all crowns and bridges placed by dentists. One reason for this is that high speed handpieces make preparation of teeth for full crowns easier than in the past. (Note: this is not conservative dentistry.) Consequently, fewer 3/4 crowns are being placed. Also, the esthetics of metalceramic crowns and bridges are highly acceptable to patients. II. Desirable Properties of Metal-ceramic Alloys High Melting Range Alloys which will be covered by porcelain must melt above the fusion temperature of porcelain. Otherwise, the casting will distort during the firing of the porcelain. It is recommended that the fusing range of the alloy be 65-150 o C above the fusion temperature of the porcelain. Recall that alloys usually have a melting range, rather than a melting point (a single temperature at which melting occurs). To increase the melting range of gold-based alloys, high melting point metals such as platinum and palladium are added to the alloy. Nickel-based, cobalt-based and palladiumbased alloys have high melting ranges by virtue of the melting temperatures of nickel, cobalt and palladium, respectively. Porcelains for veneering metal-ceramic alloys typically fuse between 1000 and 1050 o C. All metal-ceramic alloys are designed to have higher fusing temperature ranges than the ADA gold casting This is so the alloy will not flow at the fusion temperature of porcelain. Because of this, investments that will not break down at high temperature (e.g., phosphate-bonded investments) and heating techniques that can achieve high temperatures (e.g., gas-oxygen flames and induction melting) are required. Bonding to Porcelain Dental porcelains are glass ceramics that consist of crystalline filler particles (quartz, alumina, or leucite) which are embedded in a feldspathic glass matrix (see Chapter 8 for more on this). This matrix extends throughout the porcelain; it is present at the porcelain's surface, but is not confined to the surface. The process of bonding porcelain to the metal coping. The dental technician applies a slurry of porcelain particles and water to the At high temperature, porcelain particles (the frit) sinter together because this glassy matrix partially melts. Particles that 1
are in contact with one another will be welded together by the glassy silica matrix. Oxides at the alloy surface. As a general rule, silica glass will not bond to alloys unless the alloy is covered with a metal oxide. Oxide-free alloys will not bond to porcelain, probably because the covalent bonding of the glass is incompatible with the metallic bonding of the alloy. Successful porcelain-metal bonds can be formed if the alloy is covered with a naturally occurring surface oxide (e.g., this occurs in Ni-Cr alloys) or if a surface metal oxide is produced by adding a small amount (1 %) of oxide former (e.g., Sn, In, or Fe) to the alloy. At high temperature, the oxide former diffuses to the alloy's surface, where it reacts with oxygen in the atmosphere to produce the surface metal oxide. Bonding mechanisms. At the former alloy surface, the feldspathic glass of the porcelain matrix will be in nearly continuous contact with the base metal oxide. Since the feldspathic glass is primarily an oxide of silicon, it is chemically similar to the base metal oxide. The oxides can diffuse (either by interdiffusion of two viscous liquid oxides or by interdiffusion of silicon and base metal atoms between two solid oxides) into one another. The diffusion is possible because the two oxides are chemically similar and, therefore, are able to mutually dissolve in one another. Note that normally there will be little diffusion between an oxidefree alloy surface and the silicate glass in dental porcelain. Because the alloy exhibits metallic bonding and the silicate glass covalent bonding, metals and glasses exhibit little mutual solubility. For successful bonding, the bond must be strong at both the metal/metal oxide interface and the metal oxide/porcelain interface. Selection of the proper oxide former ensures that the metal oxide adheres to the underlying alloy (not all will). The bonding discussed in the preceding is chemical, but it is believed that mechanical interlocking also plays a role in the bonding between some alloys and porcelains. At high magnification, many porcelain-alloy interfaces are found to be very irregular, perhaps, because of selective dissolution of some alloy phases by the glassy porcelain. The irregular interface may interlock the porcelain and the metal. Compatible Thermal Expansion With Porcelain The thermal expansion of the metal and porcelain must be compatible. Bonds between porcelain and metal formed at high temperature will break if the metal and porcelain shrink at greatly different rates as they cool. Ideally the rates of contraction of the metal and porcelain should be equal or, also acceptably, the metal should contract at a slightly higher rate than the porcelain. The latter condition places the metal side of the joint in tension and the porcelain side in compression. Since the porcelain is much stronger in compression than in tension, this strengthens the porcelain/metal joint. Reversing the thermal contractions, so that the metal is placed in tension and the porcelain in tension, is likely to produce porcelain fracture or delamination. No Discoloration A metal-ceramic alloy should not discolor the porcelain. Types III and IV ADA casting alloys cannot be veneered with porcelain. These gold casting alloys all contain copper, which will react with porcelain to form blue or blue-green reaction products at the porcelainmetal interface. Gold-based alloys that containing silver also discolor many porcelains. Porcelain on these silver-containing alloys takes on a very light green tint. Recently, porcelains that resist such "greening" have become available. These porcelains contain 2
less sodium than the porcelains traditionally used on metal-ceramic Sag Resistance A metal-ceramic alloy should resist sag at high temperatures. Sag is distortion of the alloy under the Low Noble stress produced by its own weight, which occurs at high temperature during the fusing of the porcelain onto the metal. If the sag resistance (e.g., the metal's high temperature rigidity) is inadequate, long span bridges cannot be placed on the casts from which they were prepared. III. Classification of Metal-ceramic Alloys The first metal-ceramic alloys introduced during the 1960s were highly noble gold-platinum-palladium The cost of gold and other precious metals has lead to increased use of base metals in crown and bridge The American Dental Association classifies metal-ceramic alloys as shown in Table 1. Here "Pt group" means either platinum or palladium. As was the case for the ADA's classification of bare-metal alloys (see Handout No. 8), the classification in the Table is not very useful for understanding the properties of metal-ceramic Its primary use is to classify the alloys according to cost. In Table 2 and in Section IV and V to follow, crown and bridge alloys for porcelain application are classified according to the principal elements found in each alloy. Section IV deals with noble metalceramic alloys and Section V with base metal metal-ceramic Metal-Ceramic Alloys Table 1 ADA Classification of Metal-Ceramic Alloys Classification Weight % High Noble Au + Pt group > 90 Medium Noble 90 > Au + Pt group > 90 Au + Pt group Base Metal Au + Pt group = 0 Type High Noble Table 2 Compositions of Typical Metal-ceramic Alloys of Type Composition (wt. %) Oxide Former Au-Pd-Pt 75-98% Au Fe, Sn, In Au-Pd Medium Noble Au-Pd-Ag Pd-Cu Low Noble Pd-Ag Base Metal Ni-Cr Co-Cr 50-60% Au 35-40% Pd 50-60% Au 25-30% Pd 10-15% Ag 70-80% Pd 5-15% Cu Ga, Au 50-65% Pd 30-40% Ag 70-85% Ni 9-20% Cr 65-70% Co 25-27% Cr Sn, In Sn, In Cu? Sn or In Cr, Be Cr 3
IV. Noble Metal-ceramic Alloys High Gold (Gold-Platinum-Palladium) Alloys These contain between 75 and 98% gold. Because of their high density (18 to 19 g/cc) and high precious metal content, these are the most expensive metal-ceramic alloys per dental unit (e.g., per coping or pontic). Most of these alloys are yellow-gold. This color is particularly pleasing under porcelain. The high noble metal content of these alloys makes them very resistant to tarnish and corrosion. The platinum and palladium in these alloys push their melting temperatures well above the fusion temperature of porcelain. These alloys contain iron (in the older alloys), tin, or indium to produce the oxide film which is apparently prerequisite to bonding between the metal and porcelain. Most of these alloys are more ductile, softer, and weaker than other metal-ceramic Consequently, these alloys are easier to grind, polish and burnish than other metal-ceramic Negative factors associated with these alloys include their cost, lack of rigidity (low elastic modulus) and lack of sag resistance. The cost for the alloys alone (not including the dental technician's labor) is about $50 per pontic or $28 per coping. If a long-span bridge is not rigid, flexing of the bridge can cause the bond between the metal and the porcelain to fail or the porcelain itself to fracture. As mentioned above, sag of a long-span bridge can cause it not to fit over the prepared teeth. Gold-Palladium-Silver Alloys These alloys contain between 50 and 60% gold and between 20 and 30% palladium. Again, tin and indium are added to promote bonding. The lower gold content and lower density (13 to 14 g/cc, because there is less of the dense gold) of these alloys decreases the cost per dental unit. They are corrosion resistant and can be ground, polished and burnished. These alloys are slightly more rigid and sag less than High Gold metal-ceramic The main advantage of these alloys is that they are gold-like alloys that cost only 60% as much as the High Gold Their main disadvantage is that, under some conditions, the silver in these alloys can cause the porcelain to discolor. Gold-Palladium Alloys These are similar to Au-Pd-Ag metal-ceramic alloys, except that the silver has been removed to prevent greening of the porcelain. They contain 40-59-0% gold and 3-40% palladium. The elimination of silver reduces the thermal expansion of these The reduction leads to a thermal expansion incompatibility with some high thermal expansion porcelains. Alloy manufacturers readily admit this incompatibility and list incompatible porcelains for their alloy. These silver-free alloys have largely supplanted the Au-Pd-Ag alloys in the U.S. market. Like the Au-Pd-Ag alloys, they are 60% of the cost of the High Gold Palladium-Silver Alloys These alloys contain no gold. Typical formulations would include 50 to 65% palladium, 30 to 40% silver, and 4 to 10% base metals (tin or indium). The low density of these alloys (10 to 11 g/cm) and the absence of gold makes these the least expensive of the precious metal They are only 14% of the cost of the High Gold These alloys have the highest sag resistance of the precious metalceramic Unless special procedures are followed, these alloys will tint most porcelains green. Recent studies show that silver from the alloy can diffuse through the porcelain to the metal surface. This explains why the greenish tint is 4
usually most striking where the porcelain is thin. Here diffusing silver atoms rapidly reaches the surface. More research is needed on mechanism of greening. As was mentioned earlier, there are now porcelains on the market that resist greening. It is also possible to prevent greening by painting a layer of colloidal gold on the alloy. After firing, a thin layer of gold forms on the surface of the casting. This gold layer reduces the concentration of silver available to diffuse into the porcelain. Other manufacturers recommend a ceramic conditioner. Such conditioners are also fired onto the alloy beneath the porcelain. They are thought to act as a barrier to silver diffusion. The colloidal gold and ceramic conditioners share the disadvantage that both require an additional firing cycle. Porcelain furnaces that have been used to fire porcelain on Ag-Pd alloys become contaminated with silver. Subsequently, even silverfree alloys fired in these furnaces will have green-tinted porcelain. Fortunately, this can be prevented by placing a graphite block near the alloy being fired. The graphite produces a local reducing atmosphere near the alloy. Palladium-Copper Alloys In the early 1980s, alloys with 70 to 80% palladium and up to 15% copper were introduced. Some of these alloys also contain tiny amounts of gold and up to 8% gallium. Recall that copper in gold-containing alloys turns porcelains blue or blue-green. Copper in Pd- Cu does not discolor dental porcelains. Their principal advantage is that unlike the Pd-Ag alloys, they do not turn porcelain green. V. Base Metal Alloys (Nickel-Chromium Alloys) Over 95% of the base metal alloys are nickel-chromium Most contain between 70 and 85% nickel and between 9 and 20% chromium. The remaining 5% of the available base metal alloys are iron-chromium and cobalt-chromium, both of which contain 12 to 15% chromium. Beryllium is added to many of these alloys to improve their castability and to control surface oxide thickness (see below). Advantages Because these alloys do not contain precious elements, they are very much cheaper than other metal-ceramic Other advantages of nickel-chromium alloys include their high resistance to sag and their high rigidity (high elastic modulus). Disadvantages The nickel-chromium alloys are very hard and strong. As a result, grinding and polishing are difficult. In addition, casting procedures for these alloys (see section IV) are often not as refined as for noble metal Consequently, some experimentation may be required in order to consistently obtain successful castings. Bonding to Porcelain Indium, tin, or iron must be added to noble alloys to produce a chemical bond between metal and porcelain. Without these additional elements, alloys of gold, platinum and palladium would not form the transitional oxides that are necessary for bonding. Role of chromium. There is no need to add oxide-forming elements to nickel-chromium Chromium in these alloys oxidizes very readily (much more readily than nickel), forming chromium oxide (Cr 2 O 3 ). In the mouth, this chromium oxide protects the alloy from further chemical attack. The alloy surface resists tarnish because of this protection. Unfortunately, at the high temperatures where porcelain is fired, the chromium oxide can grow too thick. Such a thick oxide may be brittle and, as a result, porcelain may not adhere to such surfaces. In addition, thick 5
chromium oxide layers are dark green or gray. This color may show through the porcelain, producing a poor color match with natural teeth. To reduce the thickness of the chromium oxide, small amounts of elements that oxidize more readily than chromium are added to the alloy. These form thin surface oxides, which act as a barrier between oxygen and chromium, thereby, decreasing the amount of chromium oxide and chromium, thereby, decreasing the amount of chromium oxide that forms. Therefore, the elements aluminum, molybdenum and beryllium are added to some nickelchromium alloys to reduce the thickness of the chromium oxides. Beryllium is the most effective of these elements for this purpose. Other manufacturers suggest that the thickness of the oxide be reduced by sandblasting the oxide. Aside from minimizing the thickness of chromium oxides, there is no general rule for achieving bonding between nickel-chromium alloys and porcelain. In some alloys, intermediary oxides, which affect the transition from the alloy (metallic structure) to the porcelain (glassy structure), are apparently formed. For a particular alloy, these transition oxides may include one or more of the following: chromium oxide (Cr 2 O 3 ), nickel oxide (NiO), beryllium oxide (BeO), and complex nickel-chromium oxides. Mechanical retention of porcelain. Other nickel-chromium alloys appear to relay wholly or, at least partially, on mechanical bonding to achieve adhesion between porcelain and metal. Many alloy manufacturers achieve bonding through trial and error experimentation. It is important that dental laboratories follow the alloy manufacturer's procedures exactly. Procedures that may or may not be used include preoxidation treatments, which produce an oxide before porcelain is applied, and sandblasting, which may increase mechanical bonding by roughening the surface of the alloy. Biocompatibility Nickel dust. The nickel and beryllium in nickel-chromium alloys are potentially hazardous. Long-term exposure to nickel dust has lead to cancers of the lung and nasal passages. However, such long-term exposure has been observed only in the nickel industry where the quantity and persistence of the exposure would be expected to be much greater than that encountered in dental laboratories. Nickel allergies. A substantial percentage of patients are allergic to nickel. Such allergies are sometimes missed by dentists. In some patients, nickel allergy manifests itself as swollen and reddened soft tissues in the mouth. In other patients, there is no evidence of intraoral reaction. Instead, nickel alloys can produce dermatitis at sites distant from the prosthesis (e.g., inside the elbow or in the arm pit). In the U.S., up to 12% of women tested and a growing percentage of men are allergic to nickel. Before nickel-chromium crowns and bridges are placed, patients should be asked if they have experienced rashes when wearing nickel plated jewelry. Beryllium. Beryllium, which is added in small quantities to some nickel-chromium alloys, is a hazard to laboratory personnel who can breathe in the dust during grinding or the fumes during casting. Exposure to very small quantities of beryllium can produce contact dermatitis, ulcers, corneal burns, and respiratory diseases. Symptoms of acute berylliosis may include pulmonary dysfunction, congestive heart failure, and spleen and liver enlargement. Fortunately, berylliosis has yet to be documented in a dental setting. Nevertheless, avoidance of grinding dust (proper masks) and 6
casting fumes (adequate venting) seems prudent. There is no evidence that beryllium in crowns and bridges is harmful to the patient. VI. Techniques for Metal-Ceramic Alloys Investing Because of their higher fusion temperatures, metal-ceramic alloys undergo more thermal contraction on cooling than do gold and silver-palladium casting Consequently, investments for metal-ceramic alloys must be expanded more than investments for gold and silver-palladium casting Also because of the higher melting range of metal-ceramic alloys, gypsum-bonded investments cannot be used. Instead, phosphate-bonded investments are used. These investments are heated to temperatures between 700 o C and 950 o C. (A specific temperature will be recommended by the alloy gold manufacturer). There are two reasons for using these high copper temperatures: (1) the high temperatures increase the thermal expansion of the investment, and (2) the high temperatures are necessary so that alloy will remain fluid when in contact with the walls of the mold. Fluidity is a problem because of the high fusion temperatures of metal- ceramic alloys; they quickly become viscous at lower temperatures. disorderedviscous liquid ordered alloy may have difficulty flowing into fine chambers of the mold cavity. Casting Metal-ceramic alloys are usually cast with a gas-oxygen torch or with an induction casting machine. The melting characteristics of all the metal-ceramic alloys except the nickelchromium alloys are fairly similar to those of gold and silver-palladium casting That is, the alloys will pool into single circular globules. Most nickel-chromium alloys, on the other hand, do not coalesce into circular globules. The relatively thick oxide scale on the alloys tends to restrict the alloy ingots to their original shapes. The correct casting temperature is recognized by noting when the ingots begin to "slump". σ c K σ =, (4) If heating is continued σ o in an effort to produce spherical globules, an overheated liquid alloy will be cast. Such a liquid alloy will attack the surfaces of the mold and the resulting casting will have a rough surface. In addition, overheating the alloy can cause low melting point elements in the alloy to evaporate. Loss of such elements can affect bonding of porcelain to the alloy. The oxide scale can also cause one to under heat the melt. The oxide scale may be lowing white-hot, but the alloy below may be considerably cooler. Because under heated alloy is relatively viscous, castings are likely to be incomplete. Another property specific to palladium-silver alloys and nickel-chromium alloys is their low density. To achieve sufficient casting pressures, arms of centrifugal casting machines may need to be wound one or two more extra turns. the extra spring tension will produce higher rotational velocities and, consequently, will increase the casting pressure. Cleaning Castings Metal-ceramic alloys should not be pickled. Acids may selectively dissolve base metals which are important in achieving bonding to porcelain. Generally, it is recommended that the alloys be cleaned by sandblasting. Sandblasting may also be necessary for freeing the castings from phosphate-bonded investments. This is particularly true for nickel-chromium alloys, most of which form surface oxides which interact with σ σ the investment. a max = 1+ 2, (5) VII. Behavioral Objectives b From a list of choices, you will be able to select: 7
1. the effect of the relative thermal contraction of metal and porcelain on the bonding of metal to porcelain. 2. the purpose of adding iron, tin and indium to noble metal-ceramic 3. the effect of copper in metal-ceramic alloys on the color of porcelain. 4. the effect of silver in metal-ceramic alloys on the color of many dental porcelains; ways of preventing porcelain greening due to silver in the alloy. 5. the recommended fusion temperatures of metal-ceramic 6. two reasons for adding beryllium to Ni-Cr 7. potential hazards to laboratory personnel and to patients associated with Ni and Be in Ni-Cr 8. two reasons why investments for metalceramic alloys are heated to higher temperatures than investments for gold and silver-palladium casting 13. the reason why metal-ceramic alloys should not begin to melt at too low a temperature. 14. elements that are typically included in gold-based metal-ceramic alloys in order to raise the alloy's melting range. 15. correct statements about the relative ease of grinding Ni-Cr and Co-Cr metal- ceramic alloys as compared to other metalceramic 16. correct statements about why the high elastic modulus of Ni-Cr is an advantage when making crowns or fixed partial dentures. You will be able to select the correct definition or description of the following terms, or, given the definition or description, select the correct term. metal-ceramic alloy PFM alloy sag resistance 9. the difference between the appearance of liquid gold alloys that are ready to cast and of liquid nickel-chromium alloys that are ready to cast. 10. the recommendations for pickling and cleaning metal-ceramic alloys and the rationale for these recommendations. 11. correct statements about the influence of beryllium, aluminum, and molybdenum on the formation of chromium oxide on nickel-chromium 12. a correct definition of sag resistance and the relative resistances of High Gold, Pd- Ag, and Ni-Cr metal-ceramic 8