Сraig. Dental Materials

tice. The location of the solder junction on the restoration and the total amount of solder applied are clearly factors that influence color matching.

Because of their gold content, the solders of high fineness are assumed to be more resistant to tarnish, discoloration, and corrosion in the mouth than the lower-fineness solders. However, there is little evidence or data to support such claims, because no good tests are available for tarnish and discoloration of such structures. High-fineness solders are often recommended to prevent tarnish in service. However, the qualities of increased fluidity and mechanical properties associated with the lower fineness (0.650 or less) may outweigh any increased tendency of those solders to tarnish in service. Actually, the lowerfineness solders are used extensively for the assembly of dental appliances without serious tendencies to discolor.

Biocompatibility Little information exists about the biocompatibility of noble dental solders. However, the same principles govern the biocompatibility of solders as govern alloys (see previous discussion, this chapter). In vitro evidence indicates that the biological properties of solders may be completely different when the solder is tested by itself versus when it is tested in combination with a substrate alloy. Most solders appear to release less mass and have better in vitro biocompatibility when used on an appropriate substrate metal, but a few will be more cytotoxic on the substrate. The character of the substrate alloy also plays a role, as do the surface area of the solder, any pitting present, and other factors. Generally, these results indicate that the biological liabilities of any solder should be assessed with the solder-substrate combination versus the solder by itself.

Pitted SolderJoints Hard solders have a tendency to be pitted after the soldering operation. In general, pitting results from improper heating of the solder, although some compositions of solder may be more susceptible to pitting than others. When solders of typical fineness are used, a pitted solder can often be associated with

either excessive heating during the fusion of the solder or with improper fluxing during heating.

If the solder is heated to too high a temperature or for prolonged periods, the lower-melting point tin and zinc in the solder can boil or oxidize and form pits or porosities as the solder solidifies. These pits often become apparent only during finishing and polishing operations. If the solder is underheated and the flux is applied in excess or improperly melted, it may be trapped in the melted solder and form pits that are uncovered during polishing. To avoid pitting from these causes, the solder should be heated promptly to the fusion temperature, and heating should be stopped as soon as the solder has flowed into position.

MICROSTRUCTURE OF SOLDERED JOINTS

Microscopic examination of well-formed soldered joints has shown that the solder does not combine excessively with the parts being soldered. When the solder has fused properly and has not been overheated, a well-defined boundary forms between the solder and the soldered parts. When the joint is heated too high or for too long, diffusion of elements between the solder and the parts occurs in proportion to the time and temperature excess. Studies indicate that diffusion in the soldered joint reduces the strength and quality of the joint.

Fig. 15-5 shows a microscopic view of a soldered joint between a high-noble casting alloy and a section of a Pt-Au-Pd wire used as a clasp on a removable partial denture. The sample has been etched with acid to reveal its microstructure. The wire has a typical fibrous appearance of a wrought form, and the alloy has a typical granular appearance of a cast form. The solder also has a normal granular appearance, and a sharp boundary exists between the solder and wire and the solder and the cast alloy, indicating that time or temperature of the soldering procedure was not excessive.

A less satisfactory soldered joint is shown in Fig. 15-6. The boundary between the wire and the solder is less sharp and wider. This poorer adaptation may have resulted from either im-

474 Chapter 15 NOBLE DENTAL ALLOYS AND SOLDERS

proper fluxing or improper heating when the solder was applied. The solder is also not ideally adapted to the cast alloy. However, there is no evidence of recrystallization of the wire and the grain size in the alloy is unchanged, indicating that the time and temperature of heating was not excessive. Overheating would also cause diffusion between the alloys. Recrystallization of the wire, grain growth of the casting alloy, and diffusion are all undesirable because they result in a loss of physical properties necessary for the successful function of the dental appliance.

The microstructure of an overheated wire is shown in Fig. 15-7.Some evidence of the original fibrous wire remains, but recrystallization has taken place throughout most of the wire. The section of wire shown in Fig. 15-7was 2 mm from the soldered joint. More severe recrystallization occurred close to the solder, and the wire broke in service within the area adjacent to the solder. When cast structures are overheated in the presence of solder, the solder will diffuse into the alloy, resulting in a new alloy that lacks strength and ductility. Overheating can also cause warpage and distortion of the appliance from large dimensional changes.

Silver Solder Hard solders composed of silver-based alloys are used extensively in some industries, but their application in dentistry has been limited. These solders are also commonly known as silver solders. Silver-based solders are used when a low-fusing point solder is needed for soldering onto stainless steel or other basemetal alloys. Orthodontic appliances are commonly soldered using silver-based solders. In general, the resistance of silver solders to tarnish is not as good as gold-based solders, but

Fig. 15-7Evidence of reciystallization resulting from excessive heating of gold wire. Right, Original fibrous microstructure of the wire; bottom and

left, granular structure resulting from recrystallization. Heat was applied 2 mm from this point on the wire.

the strengths of the two types of solders are comparable.

The silver-based solders are composed of silver (10 to 80 wt%), copper (15 to 30%), and zinc (4 to 35%), with some products containing small percentages of cadmium, tin, or phosphorus to further modify the fusion temperature. The formation of the silver-copper eutectic is responsible for the low melting range and higher corrosion rate found in the silver-based solders. The liquidus temperatures for these solders range from 620" to 700' C, which is slightly below those of gold-based solders. This difference is important in the soldering of stainless steel.

1 SELECTED PROBLEMS

Problem 1

Table 15-2 shows that cast 24k gold has a hardness and strength that are not sufficient for dental restorations, yet gold foil, which is also 24k gold, has three times the hardness and more than twice the tensile strength.Why?

Solution

The manufacturing of gold foil starts with the cast form of the element, then imparts severe cold work until it is very thin. Thus gold foil has a wrought microstructure and exhibits superior mechanical properties because of its fibrous form. Condensation of the foil into the restoration imparts further work hardening to the metal. Note, however, that the elongation of the gold foil is less than half of the original cast 24k gold. Thus by strengthening the gold through work hardening, it has also become more brittle. This trend is typical of wrought metals relative to their cast counterparts.

Problem 2

Inspection of Table 15-5 shows that most noble casting alloys are based either on gold, palladium, or silver, with smaller amounts of copper, platinum, and zinc added. Why do dental manufacturers pick these elements for noble dental casting alloys?

Solution

Gold and palladium are selected as major elements for noble casting alloys because they impart corrosion resistance to the alloys and are miscible (freely soluble) with other the other elements (see Fig. 15-1). In the past, palladium was often used because it was considerably cheaper than gold, and maintained corrosion resistance of the alloy. However, palladium prices have approached and even surpassed those of gold in recent times. Nev-

ertheless, palladium is used to add strength and hardness without sacrificing corrosion resistance. Silver is used as a less expensive alternative to palladium, but cannot provide as much corrosion resistance because it is not a noble metal. Furthermore, silver is not miscible with copper. The other elements in Table 15-5 (copper, zinc, gallium, iridium ruthenium, and indium) are inappropriate as major elements. Copper, zinc, gallium, and indium lack corrosion resistance, and iridium and ruthenium have extremely high melting points and are very expensive. These minor elements are used because they enhance the properties of the major elements. For example, copper provides solid-solution hardening, zinc is a deoxidizer during casting, and iridium is a grain refiner. Platinum, although a noble metal, is not generally used as a major element because of its cost and because it is not miscible with Au or Pd (see Fig. 15-1).

Problem 3

An alloy was listed by a manufacturer as an I l k alloy containing 4.2% Pd, 25% Cu, and 25% Ag (all wt%). What is the atomic percentage of Au in this alloy?Is the carat based on the atomic or weight percent?

Solution

The alloy contains 25.9 atomic percentage of gold. The answer is arrived at in the following manner: an 11-carat alloy contains 11/24 x 100 = 45.8 wt% gold. Thus the carat is based on weight percentage. The weight percentage of each element is divided by the respective atomic weight, giving the following results: Au 45.8/197 = 0.232; Cu 2Y63.5 = 0.394; Ag 25/208 = 0.231; Pd 4.2/106.5 = 0.039. The atomic percentage of Au is 0.232 x 100,' (0.232 + 0.394 + 0.231 + 0.039) = 25.9 atomic percent.

Problem 4

Upon inspection of the elements in Table 15-1, which elements do you think would be candidates as grain refiners and why? (Hint: evaluate the melting range of most alloys in Table 15-6 before answering this question).

Solution

Because Table 15-6 shows that the liquidus of most noble and high-noble casting alloys is below1200•‹C, any grain refiner would have to have a melting point well above (>500•‹ to 600" C) this value. The elements that are best in this regard are ruthenium, rhodium, osmium, and iridium. All but osmium, which is much too expensive to justify its use over the others, are used in practice.

Problem 5

A dentist tells a laboratory technician that he wants a Type I11 alloy for a restoration. The laboratory then asks the dentist whether he wants a high-noble or noble alloy. The dentists believes that the laboratory's question is unnecessary, because Type I11 specifies composition of the alloy. Who is right?

Solution

In this case, the laboratory is correct in asking about the composition of the alloy. Table 15-4 shows that a Type I11 alloy is a hard alloy suitable for restorations subject to high stress, but Type I11 gives no indication of composition. Therefore, a number of compositions, noble or high-noble, might meet the requirements for a Type I11 alloy. Thus the laboratory is correct in asking. Confusion about this issue comes from the past definition of the term Type III. In the past, alloy typing for goldbased alloys specified composition and physical properties. However, when the use of alloys other than gold-based varieties became common, the compositions and physical properties were separated. This change still causes confusion among dentists.

Problem 6

In Table 15-5, the atomic percentages of elements in the alloys are sometimes different than their corresponding weight percentages and sometimes quite similar. Why?

Solution

If an alloy contains elements with vastly different atomic weights, then the atomic percentages and weight percentages will be significantly different. The heaviest elements will be lower in atomic percentage than weight percentage and the lightest elements will be higher in atomic percentages than weight percentages. If an alloy contains elements that are comparable in atomic weights, the atomic and weight percentages of the elements will not differ as much. Thus, in Table 15-5,the atomic and weight percentages of the Ag-Pd alloys are similar because the majority of the alloy is composed of silver, palladium, and indium, which are very close to the same atomic weight (Ag = 107.9, Pd = 106.4, and In = 114.8). The other element in this alloy, zinc, does not change much because it is a minor component. If zinc were more prevalent in this alloy, its atomic and weight percentages would be significantly different because its atomic weight (Zn = 65.4) is quite different than that of the other components.

Problem 7

A gold crown made of Au-Cu-Ag-Pd-I alloy was inadvertently allowed to slowly cool to room temperature after casting. Because the crown was then in the hardened condition, it was placed into an oven at 400" C for 20 minutes and then quenched in water. However, the casting was still as difficult to finish. Why?

Solution

To convert the hardened alloy to the softened condition, it must be heated above 424" C to affect the conversion (see phase diagram in Fig. 15-1, A). Alloys of this composition are

often heated to 700" C, which is above the transition temperature of the ordered phase but well below the solidus, to affect the conversion. At this temperature, the alloy converts to the softened condition, then quenching maintains it in the softened condition by cooling it so fast that it does not have enough time to form the ordered phase.

Problem 8

A casting composed of Ag-Pd alloy produced incomplete margins. Why might this have happened?

Solution

Centrifugal casting depends on the density of the molten alloy to create hydrostatic pressure. Low-carat alloys have a lower density, which requires that the casting machine be wound tighter (more turns) for compensation. Alternatively, the fluidity of low-carat alloys is difficult to judge when casting because of a heavier oxide that forms on the surface of the molten mass. If the alloy was not heated sufficiently, then the molten metal cooled before it could reach the fine margin areas of the mold.

Problem 9

Close inspection of Table 15-9 shows that the composition of noble dental solders are quite similar to many noble alloys. Why would this be true?

Solution

Because the solder will be in intimate contact with the alloys, and because corrosion is often enhanced when dissimilar metals are in contact in the mouth, the formulation of solders with compositions similar to noble alloys makes sense. However, the composition of the solders cannot be identical because the solder must have a liquidus below that of the substrate alloy. Also, other elements must be added to ensure good flowability, fluidity, and resistance to pitting.

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B ase-metal alloys are used extensively in dentistry for appliances and instruments, as shown in the outline below. Cast cobaltchromium and nickel-chromium alloys have been used for many years for fabricating partialdenture frameworks and have replaced Type IV gold alloys almost completely for this application. Cast nickel-chromium alloys are used in fabricating crowns and bridges. These alloys were developed as substitutes for Type I11 gold alloys, and in some cases may be used as a substructure for dental porcelain. Nickelchromium and cobalt-chromium alloys are used in porcelain-fused-to-metal restorations, and these alloys are discussed in greater detail in Chapter 19. Titanium and titanium alloys are used in cast and wrought forms for crowns, bridges, implants, orthodontic wires, and endodontic files. Stainless steel alloys are used principally for orthodontic wires, in fabricating endodontic instruments, and for preformed crowns. Dental applications of cast and wrought base-

metal alloys are summarized as follows:

1. Cast cobalt-chromium alloys a. Partial-denture framework

b. Porcelain-metal restorations (see Chapter 19)

2.Cast nickel-chromium alloys a. Partial-denture framework

b.Crowns and bridges

c.Porcelain-metal restorations (see Chapter 19)

3.Cast titanium and titanium alloys a. Crowns

b. Bridges

c. Partial dentures d. Implants

4. Wrought titanium and titanium alloys

a.Implants

b.Crowns

c.Bridges

5 . Wrought stainless steel alloys a. Endodontic instruments

b. Orthodontic wires and brackets

c.Preformed crowns

6.Wrought cobalt-chromium-nickel alloysorthodontic wires and endodontic files

7.Wrought nickel-titanium alloysorthodontic wires and endodontic files

8.Wrought beta-titanium alloys-orthodontic wires

GENERA1 REQUIREMENTS

OF A DENTAL ALLOY

The metals and alloys used as substitutes for gold alloys in dental appliances must possess certain minimal fundamental characteristics:

The alloy's chemical nature should not produce harmful toxicologic or allergic effects in the patient or the operator.

The chemical properties of the appliance should provide resistance to corrosion and physical changes when in the oral fluids.

The physical and mechanical properties, such as thermal conductivity, melting temperature, coefficient of thermal expansion, and strength should all be satisfactory, meeting certain minimum values and being variable for various appliances.

The technical expertise needed for fabrication and use should be feasible for the average dentist and skilled technician.

The metals, alloys, and companion materials for fabrication should be plentiful, relatively inexpensive, and readily available, even in periods of emergency.

This list of requirements for the ideal substitute for dental gold alloys calls attention to the fact that a combination of chemical, physical, mechanical, and biological qualities is involved in the evaluation of each alloy. Properties depend on material, compositional, and processing factors.

Cast and wrought base-metal alloys, including cobalt-chromium-nickel, nickel-chromium-iron, commercially pure titanium, titanium-aluminum- vanadium, stainless steel, nickel-titanium, and titanium-molybdenum (beta-titanium) alloys are discussed in this chapter. The discussion is based on the synergistic relationship between process-

ing, composition, structure, and properties of the materials.

. ,"-

COBALT-CHROMIUMAND NICKEL-CHROMIUM CASTING

Ever since cobalt-chromium casting alloys became available for cast removable partial-denture restorations, they have continued to increase in popularity. It was estimated as early as 1949 that more than 80% of all partial-denture appliances were cast from cobalt-chromium alloys. By 1969, more than 87% of all partial-denture appliances made in this country were cast from some type of base-metal alloy. Currently, almost all the metal frameworks of partial-denture appliances are made from cobalt-chromium or nickelchromium alloys.

ANSIIADA SPECIFICATION NO. 14

According to ANSI/ADA Specification No. 14 (IS0 6871), the weight of chromium should be no less than 20%, and the total weight of chromium, cobalt, and nickel should be no less than 85%. Alloys having other compositions may also be accepted by the ADA, provided the alloys comply satisfactorily with requirements on toxicity, hypersensitivity, and corrosion. Elemental composition to the nearest 0.5% must be marked on the package, along with the presence and percentage of hazardous elements and recommendations for processing the materials. The specification also requires minimum values for elongation (1.5%), yield strength (500 MPa), and elastic modulus (170 GPa).

An important feature of this specification is that it made available a standardized method of testing, which has, in turn, made it possible to compare results from one investigation with those of another.

COMPOSITION

The principal elements present in cast base metals for partial dentures are chromium, cobalt, and nickel, which together account for 82 to 92 wt%

of most alloys used for dental restorations. Representative compositions of four commercially available dental casting alloys, including two (on the right) that are used for porcelain-fused- to-metal restorations, are listed in Table 16-1. Chromium, cobalt, and nickel compose about 85% of the total weight of these alloys, yet their effect on the physical properties is rather limited. As discussed in this chapter, the physical properties of these alloys are controlled by the presence of minor alloying elements such as carbon, molybdenun~,beryllium, tungsten, and aluminum.

Function of Various Alloying Elements

Chromium is responsible for the tarnish and corrosion resistance of these alloys. When the chromium content of an alloy is higher than 30%, the alloy is more difficult to cast. With this percentage of chromium, the alloy also forms a brittle phase, known as the sigma (0) phase. Therefore cast base-metal dental alloys should not contain more than 28% or 29% chromium. In general, cobalt and nickel, up to a certain percentage, are interchangeable elements. Cobalt increases the elastic modulus, strength, and hardness of the alloy more than does nickel.

The effect of other alloying elements on the properties of these alloys is much more pronounced. One of the most effective ways of increasing the hardness of cobalt-based alloys is by increasing their carbon content. A change in the carbon content of approximately 0.2% changes the properties to such an extent that the alloy would no longer be usable in dentistry. For example, if the carbon content is increased by 0.2%over the desired amount, the alloy becomes too hard and brittle and should not be used for making any dental appliances. Conversely, a reduction of 0.2% in the carbon content would reduce the alloy's yield and ultimate tensile strengths to such low values that, once again, the alloy would not be usable in dentistry. Furthermore, almost all elements in these alloys, such as chromium, silicon, molybdenum, cobalt, and nickel, react with carbon to form carbides, which change the properties of the alloys. Note that, as