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9 Metals OUTLINE Desirable Properties of Dental Alloys Alternative Technologies for Fabricating Prostheses Functional Mechanical Properties of Alloys Joining of Dental Alloys Classification of Dental Casting Alloys...

9 Metals OUTLINE Desirable Properties of Dental Alloys Alternative Technologies for Fabricating Prostheses Functional Mechanical Properties of Alloys Joining of Dental Alloys Classification of Dental Casting Alloys Wrought Alloys and Plastic Deformation of Metals Alloys for All-Metal Prostheses Effects of Annealing Wrought Alloys Alloys for Metal-Ceramic Prostheses Types of Wrought Alloys Alloys for Removable Partial Dentures KEY TERMS Age hardening Process of hardening certain alloys by controlled heat- are highly resistant to oxidation and dissolution in inorganic ing and cooling, which usually is associated with a phase change. acids. Annealing The process of controlled heating and cooling that is Precipitation hardening The process of strengthening and designed to produce desired properties in a metal. Typically, the hardening a metal by precipitating a phase or constituent from annealing process is intended to soften metals, increase their a saturated solid solution. ductility, stabilize shape, and increase machinability. Recovery A stage of heat treatment that results in the partial Antiflux A substance such as graphite that prevents the flow of restoration of properties of a work-hardened metal without a molten solder on areas coated by the substance. change in the grain structure. Base metal A metal that readily oxidizes or corrodes. Important Recrystallization The process of forming new stress-free crystals base metals for dental alloys are nickel, cobalt, iron, titanium, in a work-hardened metal through a controlled heat-treatment and chromium. process. Cold welding The metal-joining process by metallic bonding that Soldering Process of building up a localized area of a metal prosthe- does not rely on heating to achieve fusion but the pressure sis with a molten filler metal or joining two or more metal compo- applied to the interface between parts to be joined; no liquid nents by heating them to a temperature below their solidus tem- phase is produced within the interface joint. perature and filling the gap between them using a molten metal CP Ti Commercially pure titanium, which consists of 99 wt% or with a lower liquidus temperature. If the melting temperature of higher-purity titanium, with oxygen, carbon, nitrogen, and the solder is greater than 450 °C, the process is called brazing. hydrogen dissolved interstitially. Springback The amount of elastic strain that a metal can recover Dislocation An imperfection in the crystalline arrangement of at- when loaded to and unloaded from its yield strength; an impor- oms consisting of an extra partial plane of atoms (edge disloca- tant property of orthodontic wires. tion), a spiral distortion of normally parallel atom planes (screw Strain hardening The increase in strength and hardness and dislocation), or a combination of the two types. decrease in ductility of a metal that is caused by permanent Flux Compound applied to metal surfaces that dissolves or prevents deformation below its recrystallization temperature; also called the formation of oxides and other undesirable substances that work hardening or cold working. may reduce the quality or strength of a soldered or brazed area. Superelasticity The ability of certain nickel-titanium alloys to Grain growth The increase in the mean crystal size of a polycrys- undergo extensive elastic deformation resulting from a stress- talline metal produced by a heat-treatment process. assisted phase transformation, with the reverse transformation Grain refinement The process of reducing the crystal (grain) size occurring on unloading; sometimes called pseudoelasticity. in a solid metal through the action of specific alloying elements Welding Process of fusing two or more metal parts through the or compounds; the process increases the yield strength of met- application of heat, pressure, or both, with and without a filler als. metal, to produce a localized union across an interface between Lost wax technique Process in which a wax pattern, prepared in the workpieces. the shape of missing tooth structure, is embedded in a casting Working range The maximum amount of elastic strain that an investment and burned out to produce a mold cavity into which orthodontic wire can sustain before it permanently deforms. molten metal is cast. Wrought alloy A metal that has been permanently deformed to Noble metal Gold and platinum group metals (platinum, pal- alter the shape of the structure and certain mechanical proper- ladium, rhodium, ruthenium, iridium, and osmium), which ties, such as strength, hardness, and ductility. 171 172 PA RT I I I Indirect Restorative Materials In his 1907 U.S. Patent 865823, Taggart described a method of standpoint of patient safety and the risk of medico-legal issues, it making gold inlays using the lost wax technique. The lost wax is highly important to understand the following clinically impor- method of making metal objects involves pouring a molten metal tant requirements and properties of dental casting alloys: into a mold that has been created with a wax pattern (Chapter 14, 1. Biocompatibility—The alloy must tolerate oral fluids and not Wax Pattern and Sprue Design). The process led to making custom release any harmful products into the oral environment that precision casts for the restoration of missing tooth structure, such might cause a toxic or allergic reaction. Biocompatibility will as onlays, crowns, multiple-unit fixed dental prostheses (FDPs), be covered in Chapter 17. and frameworks for removable partial dentures (RPDs). Since the 2. Tarnish and Corrosion Resistance—Corrosion is the physi- early 1980s, the numbers of dental alloys, along with new alloy cal deterioration of a material in the oral environment, and systems, have increased dramatically as a result of the market-price tarnish is a thin film that is adherent to the metal surface increase of noble metals, the performance of the same function (Chapter 3, Tarnish and Corrosion). Both phenomena affect at a lower cost, the need for increasingly specialized physical and the durability and appearance of the prostheses. mechanical properties, and the awareness of the importance of 3. Thermal Properties—The melting range of the casting alloys biological properties. must be low enough to form smooth surfaces with the mold Objects of any design, intricate or simple, can be produced by wall of the casting investment (Chapter 14, Investment Materi- using the lost wax technique as long as a wax pattern can be made als). For metal-ceramic prostheses, the alloys must have closely and invested, and alloys can be melted, or by using alternative matching thermal expansion coefficients to be compatible with processing techniques such as computer-aided design/computer- the given porcelains, and they must tolerate high processing aided manufacturing (CAD-CAM) milling and three-dimensional temperatures without deforming via a creep process. (3-D) printing. On the other hand, many ancillary dental materi- 4. Strength Requirements—The alloy must have sufficient strength als and instruments are fabricated from initially cast alloys that for the intended application. For example, alloys for bridgework have been subsequently rolled to form sheets or rods, drawn into require higher strength than alloys for single crowns. wire or tubing, or forged into a finished shape. Rolling, drawing, 5. Fabrication—The molten alloy should flow freely into the and forging are major processes of permanently deforming metals. investment mold, without any appreciable interaction with the Whenever a cast pure metal or alloy is permanently deformed to investment material, and wet the mold to form a surface free of an intended shape in any manner, it is considered a wrought alloy. porosity. It should be possible to cut, grind, finish, and polish This permanent deformation alters the microstructure, and the alloy the alloy to obtain a prosthesis with a satisfactory surface finish. exhibits properties that are different from those it had in the as-cast 6. Porcelain Bonding—The alloy must be able to form a thin state. The most significant change is the increase of yield strength adherent oxide that enables chemical bonding to ceramic with a reduction in ductility. The applications of wrought alloys in veneering materials. dentistry include orthodontic wires, clasps for RPDs, root canal files 7. Economic Considerations—The cost of metals used for sin- and reamers, preformed crowns in pediatric dentistry, and surgical gle-unit prostheses or as frameworks for FDPs or RPDs is a instruments. Most metal-based restorations and prostheses are cast function of the metal density, fluctuations in metal prices, and not wrought. and the cost per unit mass. In the construction of dental appliances, there is often a need to join metal parts, cast or wrought, together using high heat. When Functional Mechanical Properties of Alloys wrought alloys are involved in such joining, the strength and frac- ture resistance of the wrought alloy will be compromised if the Mechanical properties are the measured responses of materials in metal is exposed to the temperature range at which the wrought the form of stress and strain under an applied force or distribu- structure is diminished or lost. Such weakening can occur during tion of forces. The relevant functional characteristics of casting the metal-joining process for stainless-steel orthodontic appliances. and wrought alloys are described next. The ability to work with dental alloys and associated wrought alloys is dependent on a knowledge of these materials. The goal CRITICAL QUESTIONS of this chapter is for the reader to become familiar with vari- ous types of casting alloys and associated wrought alloys. We What are two clinical disadvantages of cast metals that have lower elastic will discuss the desirable properties and the relevant mechanical moduli? Why does a long-span bridge require alloys of high elastic modulus? properties of dental alloys first. For casting alloys, the focus is the alloy classification and clinical applications of the alloys. For Elastic Modulus wrought alloys, the focus is the process of permanent deforma- tion and its effect on the properties for their specific applications One characteristic of a material with a high elastic modulus is its in dentistry. Several types of wrought alloys used in dentistry are rigidity or stiffness. For a dental prosthesis, rigidity is equivalent to described. The description of joining metals is like that for bond- the resistance to flexure (bending). When a long-span FDP flexes ing presented in Chapter 6; the emphasis is on the principles and during occluding of the pontic, the mesiodistal bending moment procedures of the process. exerted on the abutment teeth can act as a dislodging force, lift- As previously done in Chapter 2, Metals, the terms metal and ing the mesial and distal aspects of the prosthesis. Furthermore, alloy are frequently used interchangeably. a flexing bridge can induce lateral forces on the abutment teeth, resulting in the loosening of these teeth. For a metal-ceramic Desirable Properties of Dental Alloys prosthesis, the overlying brittle porcelain will fail catastrophically when the metal substructure flexes beyond the flexural limit of Depending on the primary purpose of the prosthesis, the choice the ceramic. The elastic modulus is also important for the major of casting alloy is made by the dentist in collaboration with a connectors of an RPD, which must have enough rigidity to pre- qualified dental laboratory technician or technologist. From the vent flexure during the placement and function of the prosthesis.  CHAPTER 9 Metals 173 Resistance to flexure also allows the clasps of an RPD to fit into the prosthesis. Eventually, a crack propagates to a critical size, areas of minimal undercuts and still provide adequate retention. and sudden fracture occurs. Common engineering expressions of fatigue fracture resistance are fatigue strength and endurance limit CRITICAL QUESTION (Figure 4.15). Fatigue strength (SNf) is defined as the stress at which failure occurs after a specific number of fatigue cycles. Endurance Why is it that a cast prosthesis that is subjected to tensile stress above the limit is the maximum stress that can be maintained without failure alloy yield strength will not necessarily fracture? over an infinite number of cycles. Some alloys do not have a well- defined endurance limit. Yield Strength (Proof Stress) Classification of Dental Casting Alloys Recall the discussion in Chapter 4, Stress-Strain Properties that yield strength, elastic limit, and proportional limit, by definition, In 1932, the dental materials group at the National Bureau of Stan- are different properties, but all three terms have been used to reflect dards (now National Institute of Standards and Technology) classi- the capacity of a cast prosthesis to withstand mechanical stresses fied dental gold alloys being used then by Vickers hardness number without permanent deformation. Ideally, dental alloys should have (VHN): type I (soft, VHN 50 to 90), type II (medium, VHN 90 a high yield strength so that a large amount of stress must be applied to 120), type III (hard, VHN 120 to 150), and type IV (extra hard, before a permanent change in dimensions occurs. For an orthodon- VHN 150 and above). Since then, the number of alloy compositions tic appliance, it relates to the maximum force the wire can deliver. and applications has increased vastly. They are now classified accord- Generally, alloys with tensile yield strengths above 300 MPa func- ing to composition, intended usage, or mechanical properties. tion satisfactorily in the mouth. Alloy Classification by Noble Metal Content CRITICAL QUESTION In 1984, the American Dental Association (ADA) proposed a simple How does the alloy ductility increase the fracture resistance of a margin of a classification for dental casting alloys based on the content of noble cast metal crown or a clasp arm on an RPD? metals. Three categories were described: high noble (HN), noble (N), and predominantly base metal (PB). Subsequently, a fourth Ductility (Percent Elongation) group of titanium and titanium alloys was added because of the unique characteristics of titanium. This classification is presented in Ductility represents the amount of permanent deformation under Table 9-1. Noble metals comprise a group of seven metals that are tensile stress that an alloy can undergo before it fractures. If the resistant to corrosion and tarnish in the mouth. In order of increas- force applied is in a compressive mode, the property is called ing melting temperature, they include gold, palladium, platinum, malleability. A reasonable amount of ductility and malleability is rhodium, ruthenium, iridium, and osmium. These noble metals and essential if a prosthesis requires adjustment to be functional, such silver are sometimes called precious metals, referring to their high eco- as the bending of RPD clasps and the burnishing of crown mar- nomic value, but the term precious is not synonymous with noble. gins. As discussed earlier, one needs to apply stress greater than Silver is reactive in the oral cavity and is not a noble metal. the yield strength of the material to cause permanent deformation Based on this classification, the IdentAlloy program was on a metal surface. Therefore, high ductility allows one to achieve established by manufacturers to provide documentation of cer- more permanent deformation without fracture but does not indi- tified alloys. Under this program, each alloy has a certificate cate if burnishing or adjusting the prosthesis would be easier or (Figure 9-1) that lists its manufacturer, alloy name, composition, more difficult in terms of the stress required. and ADA classification. Some insurance companies use it as well to determine the cost of crown and bridge treatment. Keep in CRITICAL QUESTION mind that this system certifies only the composition of alloys. Why is a harder metal more resistant to wear than a softer metal? TABLE 9-1 Alloy Classification by Noble-Metal Hardness Content—American Dental Association Clinically, hardness reflects the resistance of the restoration to (2003) scratching and abrasion by the opposing tooth or restoration and the ability to maintain the smoothness of the prosthesis in Alloy Type Total Noble-Metal Content the oral environment. However, a harder restoration surface can High noble (HN) Must contain ≥40% Au and ≥60% by weight cause excessive wear of softer opposing dentition or restorations. of noble-metal elements* In addition, harder surfaces are more difficult to cut, grind, finish, Noble (N) Must contain ≥25% by weight of noble-metal and polish because higher stress is required for each procedure. elements Predominantly base Contains