FORMAT i)
Introduction
ii)
Historical Perspective on Dental Casting Alloys.
iii)
Metallic Element used in dentistry.
iv)
Desirable Properties of casting Alloys.
v)
Classification of Dental casting Alloys.
vi)
Alloys for all Metal and Resin Veneer Restoration a. Gold Alloys. b. Silver Palladium Alloy c. Aluminium Bronze Alloy.
vii)
Heat Treatment of high noble and Nobel metal alloy.
viii) High Noble Alloy for Metal Ceramic Restoration a. Gold – Platinum- Palladium alloy b. Gold – Palladium Silver alloy c. Gold – Palladium alloy. ix)
Noble Alloys for Metalic Ceramic Restoration a. Palladium Based Alloy
i)
Palladium silver alloy
ii)
Palladium Copper alloy
iii)
Palladium Cobalt alloy
iv)
Palladium – Gallium – Silver and Palladium – Gallium – Silver – Gold alloy.
x) Base Metal Alloys for Cast Metal and Ca Metal Ceramic Restoration. a. Classification b. Handling Hazard and Patients Hazard
c. Cobalt – chromium alloys. d. Nickel – Chromium Alloys. e. Aluminum Bronze. x)
Metals for Partial Denture Alloys.
xi)
Titanium
xii)
Conclusion.
Dental Casting Alloys Metals and alloys have many uses in dentistry. Steel alloys are commonly used for the construction of instruments and of wires for orthodontics. Gold alloys and alloys containing chromium are used for making crowns, inlays and denture bases whilst dental amalgam, an alloy containing mercury, is the most widely used dental filling material. With the exception of Hg, metals are generally hard and lustrous at ambient temperatures, and have crystalline structures in which the atoms are closely packed together. Metals are opaque and are good conductors of both heat and electricity. The shaping of metals and alloys for dental use can be accomplished by one of three methods, namely, casting, cold working or amalgamation. Casting involves heating the material until it becomes molten. When it can be forced into an investment mould which has been prepared from wax pattern. Cold working involves mechanical shaping of the metal at relatively low temperatures, taking advantage of the high values of ductility and Malleability possessed by many metals. Some alloys can be mixed with mercury to form a plastic mass which gradually hardens by a chemical reaction followed by crystallization. The material is shaped by packing it into a tooth cavity whilst still in the plastic state.
HISTORICAL PERSPECTIVE ON DENTAL CASTING ALLOYS The 20th century generated substantially new changes to dental prosthetic materials. The major factors that are driving new developments are: i)Economy→ The new material performs the same function as the old material but at a lower cost. ii)Performance → The new material performs better than the old product in some desirable way, such as ease of processing, improved handlinig characteristics, or increased fracture resistance. iii) Aesthetics → The new material Provides a more aesthetic result, such as increased translucency. 1905 – The Lost – Wax Process → i)
Taggart’s presentation to the New York Odontological group in 1907 on the fabrication of cast inlay restorations developed in 1905 often has been acknowledged as the first reported application of the lost – wax technique in dentistry;. It was an instant success.
ii)
It soon led to the casting of inlays, onlays, crowns, FPDs, and frame works for RPD.
iii)
Jewelry alloys were quickly adopted. These gold alloys were further strengthened with additions of copper, silver, or platinum.
Gold
alloys
were
biocompatibility and ease of use.
used
because
of
their
1932 – Classification of Gold – Based Casting Alloys: i)
In 1932, the dental materials group at the National Bureau of standards surveyed the alloys being used and classified them as
Type I (Soft , VHN between 50 and 90) Type II (Medium VHN between 90 and 120) Type III (Hard VHN between 120 and 150) Type IV (Extra hard, VHN ≥ 150) ANSI/ADA Specification no.5 ISO standard 15592. ii)
During this period, the results of some tarnish tests suggest that alloys with a gold content lower than 65% to 75% tarnished too readily for dental use.
iii)
It is now known that, in gold alloys, palladium counteracts the tarnish potential of silver, allowing alloys with a lower gold content to be used successfully.
1933 – Cobalt – chromium Partial Denture Alloys i)
Base metal removable partial denture alloys were introduced in the 19305. Since that time, both nickel – chromium and cobalt – chromium formulations have become increasingly popular compared with conventional Type IV gold alloys.
ii)
The advantages of the base metal alloys are their lighter weight, greater stiffness, other beneficial mechanical properties, and reduced costs.
iii)
For these reasons, nickel – and cobalt – based alloys have largely replaced noble metal alloys for removable partial denture.
iv)
Likewise, by 1978 the price of gold was increasing so rapidly that attention was focused on the noble metal alloys.
1959 – Porcelain – Fused – to – Metal Process i)
In the late 1950s, there was the successful Veneering of a metal substructure with dental porcelain. Until that time, dental porcelain had a markedly lower coefficient of thermal expansion than did gold alloys. This thermal mismatch often led to impossible to attain a bond between the two structural components.
ii)
It was found that adding both platinum and palladium to gold lowered the coefficient of thermal expansion/contraction of the alloy sufficiently to ensure physical compatibility between the porcelain Veneer and the metal substructure.
iii)
The first commercially successful alloy contained gold, platinum, and palladium.
1971 – The Gold Standard i)
The United States abandoned the gold standard in 1971.
ii)
Prices of gold increased, in response to that, new dental alloys were introduced through the following charges. a. In some alloys, gold was replaced with palladium. b. In other alloys, palladium eliminated gold entirely.
c. Base metal alloys with nickel as the major element eliminated the exclusive need for noble metals.
1976 – The Medical and Dental Devices Act i)
Dental alloys for prosthetics were classifies as passive implants.
ii)
All materials on the market before 1976 were automatically grandfathered as acceptable for market distribution.
Manufacturers were required to have a quality system in place, but no product standards were established. 1996 – The European Medical Devices Directive i)
The European Union established that any imports of dental devices required a CE mark.
ii)
Information and data on the development process were also required. Again, no specific product standards were established.
1998 – The Clean Air Acts i)
To meet the requirements of reduced nitrogen and carbon monoxide emissions, automakers use palladium – containing catalytic converters.
ii)
The demand for palladium soared sevenfold from 1993 to 1999.
iii)
Supply could not meet the demand, and the price of palladium increased to new record highs.
iv)
At the same time the price of gold was trading during the decade.
The result was an increased demand for gold – based dental alloys.
Desirable Properties of Dental Casting alloys All casting alloys must first be biocompatible and then exhibit sufficient physical and mechanical properties to ensure adequate function and structural durability over long periods of time. The only nearly pure metal cast for dental applications is commercially pure titanium (often written as CPTi). From a stand point of patient safety and to minimize the risk for medico-legal situations, it is highly important to understand the following clinically important requirements and properties of dental casting alloys. Biocompatibility → The material must tolerate oral fluids and not release any harmful products into the oral environment. Corrosion Resistance → Corrosion is the physical dissolution of a material in an environment. Corrosion resistance is derived from the material components being either too noble to react in the oral environment (e.g., gold and palladium) or by the ability of one or more of the metallic elements to form an adherent passivating surface film, which inhibits any subsurface reaction (e.g., chromium in Ni – Cr and Co – Cr alloys and titanium in commercially pure titanium [CPTi] and in Ti – 6Al – 4V alloy).
Tarnish Resistance→ Tarnish is a thin film of a surface deposit or an interaction layer that is adherent to the metal surface. These films are generally found on gold alloys with relatively high silver content or on silver alloys.
Allergic Components in casting Alloys → A
restorative
material
should
not
cause
adverse
health
consequences to a patient. The patient’s “right – to – know” extends to having some knowledge of what is being placed into their bodies. Laws in some states are explicit in this respect. It is wise for the dentist to maintain a record of the material used for each restoration or prostheses, as well as an understanding of any known allergies stated by the patient. Aesthetics→ Considerable controversy exists over the optimal balance among the properties of aesthetics, fit, abrasive potential, clinical survivability, and cost of cast metal prostheses compared with directfilling restorations, ceramic- based prostheses (all-ceramic and metal – ceramic), and resin-veneered prostheses. Thermal Properties→For metal – ceramic restorations, the alloys or metals must have closely matching thermal expansion to be compatible with a porcelain, and they must tolerate high processing temperatures.
Melting Range→The melting range of the alloys and metals for cast appliances must be low enough to form smooth surfaces with the mold wall of the casting investment. Compensation for Solidification: To achieve accurately fitting cast inlays, on lays , crowns and more complex frameworks or prostheses, compensation for casting shrinkage from the solid us temperature to room temp must be achieved either through Computer – generated oversized dies or through controlled mold expansion. In addition, the fit of a cemented prosthesis must be tailored to accommodate the layers of bonding adhesive (if used) and the luting cement. Strength Requirements: i)
For the full cast alloys the strength requirements increase as the number of tooth surfaces being replaced increases.
ii)
Likewise, alloys for bridge work require higher strength than alloys for single crowns.
iii)
Copings for metal – ceramic pros these are finished in thin sections and require a sufficient elastic modulus (stiffness) to prevent excessive elastic deflection from functional forces ,especially when used for long – span frameworks.
iv)
The elastic moduli of many base metal alloys are considerably greater than gold – based alloys.
Values for the elastic modulus of dental alloys are as follows: Co - Cr→125 to 220Gpa Ni - Cr→145 to 190Gpa CPTi→117 Gpa
Pd-based alloys→ 110-135 Gpa Au-based alloys→75 to 119 Gpa Fabrication of cast Prosthese and Frame works i)
The use of cobalt – chromium alloys rather than gold alloys for partial denture applications may require different casting investment products and casting equipment in order to produce high – quality restorations consistently.
ii)
Selection of a suitable casting investment is a major problem when a dentist decides to use titanium for all metal prosthese or as a metal- ceramic restorative material.
Castability→ To achieve accurate details in a cast frame work or prosthesis, the molten metal must be able to wet the investment mold material very well and flow into the most intricate regions of the mold without any appreciable interaction with the investment and without forming porosity with in the surface or subsurface regions. i)
The castability of some base metals is extremely challenging in this regard, because these alloys tend to readily form oxides or interact chemically with the mold wall during the casting process.
Finishing of Cast Metal → Cutting, grinding, of some metals is quite demanding, and extra time is required to produce a satisfactory surface finish. i)
Hardness, ductility (percent elongation), and ultimate strength are important properties in this regard.
ii)
The hardness of the alloy is a good primary indicator of cutting and grinding difficulty, and this property varies widely among the current casting metals. For example, Co – Cr and Ni – Cr alloys are quite hard compared with other metals.
List of Vickers hardness numbers: Co - Cr→450 to 650 Ni - Cr→330 to 400 Ti – 6 Al –4 V →320 Tooth enamel → 300 to 400 Type IV Au alloy →250 Pd – based alloys→235 to 400 CPTi→210 (bulk) Ag - Pd→143 to 154 Dentin → 60 Type I Au alloy →55 Porcelain Bonding→To achieve a sound chemical bond to ceramic veneering materials, a substrate metal must be able to form a thin, adherent oxide, preferably one that is light in color so that it does not interfere with the aesthetic potential of the ceramic. i)
The metal must have a thermal expansion/contraction coefficient that is closely matched to that of the porcelain. Stresses that develop in the ceramic adjacent to the metal/ceramic interface can enhance the fracture resistance of a metal – ceramic prosthesis or they can increase the susceptibility to crack fo;rmation. (if tensile in nature)
Economic Considerations→ The cost of metals used for single – unit prostheses or as frame works for fixed or removable partial dentures is a function of the metal density and the cost per unit mass. For example, compared with a palladium alloy having a density of 11g/cm3 , a gold alloy with a density of 18g/cm3 will cost 164% (18/11x100) more for the same volume and unit cost of metal. Laboratory Costs→ The metal cost is a major concern for the dental Laboratory owner who must guarantee prices of prosthetic work for a certain period of time. Because of the fluctuating prices of noble metals over the past two decades, the cost of fabricating prostheses made from noble elements must be adjusted periodically to reflect these changes.
FUNCTIONS OF EACH INGREDIENT METAL IN CASTING ALLOY Gold→ i)
Yellow in colour
ii)
Ductility
iii)
Resistance to tarnish and corrosion.
Silver→ i)
Hardness and strength
ii)
Whitens the alloy thus over comes the reddening effect of copper. But tarnishes the alloy.
iii)
Forms solid solution with gold and partial solubility with copper.
Copper→ i)
Hardness and strength
ii)
Reddish color but lowers tarnish resistance.
iii)
Lowers fusion temperature.
iv)
Forms solid solution with gold
v)
Reduces the density of the alloy.
Palladium→ i)
Increases resistance to tarnish and corrosion.
ii)
Whitens the alloy
iii)
Cheap
iv)
Absorbs gases formed during casting, and thus reduces porosity.
v)
Increases hardness.
Zinc→ i)
Acts as a scavenger and removes the oxides.
Makes the alloy more castable
CLASSIFICATION OF DENTAL CASTING ALLOYS IMPORTANCE→ i)
The dental casting alloy classification is useful for estimating the relative cost of alloys, because the cost is dependent on the noble metal content as well as on the alloy density.
ii)
It is also useful for identification of the billing code that is used for insurance reimbursement.
iii)
It simplify the communication between dentists and dental laboratory technologists.
Dental casting alloys are classified according to: (According to Anusavice) I)
According to American Dental Association (1984)
II)
According to ANSI/ADA specification No.5 (1997)
III)
According
to
mechanical
property
Requirements
proposed In ISO Draft international standard 1562 for Casting Gold Alloys (2002) IV)
Classification of casting metals for Full – metal and Metal – ceramic Prostheses and Partial Dentures
Classification according to Anusavice I)
According to American Dental Association (1984)
Alloy Type
Total Nobel Metal content
High Noble (HN)
Must contain ≥ 40 wt% Au And ≥ 60 wt% of noble metal elements (Au, Pt, Pd, Rh,Ru,Ir, Os) Must contain ≥ 25wt% of noble
Noble (N)
metal elements (Au, Pt, Pd, Rh, Ru, Ir, Os) Predominantly Base Metal (PB)
Contain <25 wt% of noble metal elements.
II)
According to ANSI/ADA Specification No.5 (1997) Mechanical Property Requirements
Alloy
Yield strength (0.2% offset) Elongation Annealed Hardened Annealed Hardened Max. Mini. Minimum Minimum Minimum
type Type I Type II Type III Type IV
(Mpa) 80 180 240 300
(Mpa) 180 240
(Mpa) ------450
(%) 18 12 12 10
(%) ----3
(III) According to mechanical property requirements proposed in ISO Draft International standard 1562 for casting Gold alloys (2002) Minimum yield strength (0.2%) or proof stress of nonproportional elongation (Mpa) 80 180 270 360
Alloy Type Type 1 Type 2 Type 3 Type 4
Minimum elongation after fracture (%) 18 10 5 3
(IV) Classification of Casting Metals for full- metal and Metal â&#x20AC;&#x201C; ceramic Prostheses and Partial Dentures Metal Type
All-
Metal Metal
Prostheses High (HN)
Noble Au-Ag-Pd Au-Pd-Cu-Ag HN
Ceramic
frameworks
Prostheses Pure Au (99.7 Au-Ag-Cu-Pd wt%)
Metal- Au-Pt-Pd
Ceramic alloys
â&#x20AC;&#x201C; Partial denture
Au-Pd-Ag (5-12 wt% Ag) Au-Pd-Ag
(>12wt% Ag) Noble(N)
Ag-Pd-Au-Cu
Au-Pd Pd-Au
Ag-Pd Pd-Au-Ag Noble Metal – Pd-Ag Cesamic alloys Predominantly
CPTi
Pd-Cu-Ga Pd-Ga-Ag CPTi
CPTi
(Base Metal (PB) Ti – Al – V Ni-Cr-Mo-Be Ni-Cr-Mo Co-Cr-Mo Co-Cr-W Cu-Al
Ti – Al – V Ni-Cr-Mo-Be Ni-Cr-Mo Co-Cr-Mo Co-Cr-W
Ti – Al – V Ni-Cr-Mo-Be Ni-Cr-Mo Co-Cr-Mo Co-Cr-W
METALLIC ELEMENTS USED IN DENTAL ALLOYS For dental restorations, it is necessary to combine various elements to produce alloys with adequate properties for dental applications because none of the elements themselves have properties that are suitable. These alloys may be used for dental restorations as cast alloys, or may be manipulated into wire. The metallic elements that make up dental alloys can be divided into two major groups, the noble metals and the base metals.
BASE METAL ALLOYS INTRODUCTION Base metal alloys contain no gold, silver, platinum or palladium. The two most commonly used base metal alloys in dentistry are the nickel â&#x20AC;&#x201C; chromium (Ni/Cr) alloys which are commonly used for crown and bridge casting, including porcelain fused to metal (PFM) restorations, and the cobalt- chromium (Co/Cr) alloys which are commonly used for partial denture frame work castings. i)
These alloys contain less than 25% of nobel metals
ii)
They are tarnish and corrosion resistant due to the presence of chromium (passivation)
iii)
These alloys are presently widely used for their superior mechanical properties and low cost.
Base metals are invaluable components of dental casting alloys because of their low cost and their influence on weight , strength, stiffness, and oxide formation (which is required for bonding to porcelain) iv)
Compared with noble metals are still frequently referred to as non precious or no noble, the preferred designation is predominantly base metal. One reason for this designation is that some base metal alloys in the past have contained a minor amount of palladium, but because the properties of these alloys were controlled primarily by the base metals present, they should not have been classified as noble alloys of these alloys were controlled primarily by the base metals present, they should not have been classified as noble alloys.
Noble metals are not currently included in most of the base metal alloys in use. The percentage of base metal use in dentistry decreased between 1981 and 1995. Although the increased acceptance of these alloys during this period was greatly influenced by the rapidly fluctuating international cost of gold and other noble metals, the subsequent decline in the cost of noble metals has had a small effect on reversing this trend. The Ni â&#x20AC;&#x201C; Cr â&#x20AC;&#x201C; Be alloys have retained their popularity despite the potential toxicity of beryllium and the allergenic potential of nickel. There are several reasons for the use of nickel â&#x20AC;&#x201C; chromium alloys in dentistry: i)
Nickel is combined with chromium to form a highly corrosion resistant alloy.
ii)
Ni – Cr alloys became popular in the early 1980s as low cost metals ($2 to $3 per conventional avoirdupois ounce) when the price of gold rose to more than $ 500 per troy ounce. Because metal – ceramic restorations made with Ni – Cr – Be alloys have exhibited high success rates from the mid – 1980s to the present, many dentists have continued to use these alloys.
iii)
Alloys such as Ticonium 100 have been used in removable partial denture frameworks for many years with few reports of allergic reactions. However, it is believed that palatal epithelium may be more resistant to allergic reactions (contact dermatitis ) than gingival secular epitheliums .
iv)
The Ni – Cr and Ni – Cr – Be alloys are relatively inexpensive compared with high noble or noble alloys. The price of nickel – base alloys is stable, unlike the price of palladium based alloys.
v)
Although beryllium is a toxic metal, dentists and patients should not be affected because the main risk occurs primarily; in the vapor form, which is a concern for technicians who melt and cast large quantities of Ni – Cr – Be alloys without adequate ventilation or fume hoods in the melting area.
vi)
Nickel alloys have excellent mechanical properties, such as high elastic modulus (stiffness), high hardness, and a reasonably high elongation (ductility). The majority of nickel – chromium alloys for crowns and FPD prostheses contain 61 wt% to 81 wt% nickel, 11 wt% to 27 wt% chromium and 2wt% to 4wt% molybdenum. i) These alloys may also contain one or more of the following elements:
aluminum, beryllium, boron, carbon, cobalt, copper,; cerium, gallium, iron, manganese, niobium, silicon, tin, titanium, and zirconium.
The cobalt – chromium alloys typically contain 53 wt% to 67 wt% cobalt, 25 wt% to molybdenum, which could affect the metal ceramic bond strength.
Classification of Base Metal Alloys i) Nickel – cobalt – Chromium alloys i)
Cobalt – Chromium: • Co – Cr – Mo • Co – Cr - W
ii)
Nickel – Chromium: • Ni –Cr – Mo – Be. • Ni – Cr – Mo.
iii)
Cobalt – Chromium – nickel
ii)
Titanium alloys: • Pure Ti. • Ti – Al - V
iii)
Others: • Aluminum bronze.
Nickel – Cobalt – Chromium Alloys Composition: Percentage of alloying elements. i)
Nickel – Chromium: • Ni up to 80%
• Cr – 13 – 22% • Be – up to 2% ii)
Cobalt – chromium: • Co – 55 – 68% • Cr – up to 25 – 27% • Cobalt – chromium: (vitallium) Co – 60% Cr – 25 – 30% • Nickel – chromium: Ni – 67% Cr –26% • Cobalt – chromium – nickel: Co – 54% Cr – 26% Ni – 14%
Advantages And Disadvantages Of Base Metal Alloys ADVANTAGES DISADVANTAGES • Cheaper and harder than gold • Density is low alloys • High yield strength • High melting range and high
• Casting shrinkage is more.
modulus of elasticity • Exceptional strength at high temperature. • Superior sag resistance – means less deformation
•
Oxidize readily. • Not resistant to tarnish and corrosion.
than gold alloys.
APPLICATION OF BASE METAL ALLOYS i)
Inlays and onlays.
ii)
Cast post
iii)
Orthodontic appliances.
iv)
For metal ceramic restorations
Base metal alloys generally have higher hardness and elastic modulus values are more sag resistant at elevated temp. v)
For making cast removable partial dentures.
It has the
following disadvantages when used metal ceramic alloys. a) They are more difficult to cast and presolder than Au – Pd or Pd – Ag alloys. More technique sensitive than noble metal alloys. b) Ni – based or Co – based alloys have a potential for porcelain debonding due to separation of a poorly adherent oxide layers from the metal substrate. c) Small differences in composition may produce wide variations in metal ceramic bond strength.
Composition COBALT – CHROMIUM ALLOYS The chemical composition of these alloys specified in the ISO standard for Dental Base Metal Casting Alloys is as follows: Cobalt Chromium Molybdenum Cobalt + nickel +chromium
Main constituent No less than 25% No less than 4% No less than 85%
A typical material would contain 35 – 65% cobalt, 25 – 35% chromium, 0-30% nickel, a little molybdenum and trace quantities of other elements such as beryllium, silicon and carbon. i)
Cobalt and Nickel are hard, strong metals the main purpose of the chromium is to further harden the alloy by solution hardening and also to impart corrosion resistance by the passivating effect.
Chromium exposed at surface of the alloy rapidly becomes oxidized to form a thin, passive, surface layer of chromic oxide which prevents further attack on the bulk of the alloy. ii)
The minor elements are generally added to improve casting and handling characteristics and modify mechanical properties. E.g. silicon imparts good casting properties to a nickel â&#x20AC;&#x201C; containing alloy and increases its ductility.
Molybdenum and beryllium are added to refine the grain structure and improve the behavior of base metal alloys during casting. iii) Carbon affects the hardness, strength and ductility of the alloys and the exact concentration of carbon is one of the major factors controlling alloy properties. iv) The presence of too much carbon results in a brittle alloy with very low ductility and an increased danger of fracture. v)
During crystallization the carbides become precipitated in the interdendritic regions which form the grain boundaries. The grains are generally much larger than those produced on casting gold alloy. If this occurs the alloy becomes extremely hard and brittle as the carbide phase acts as a barrier to slip. A discontinuous carbide phase is preferable since it allows some slip and reduces brittleness.
vi) Whether a continuous or discontinuous carbide phase is formed depends on the amount of carbon present and on the casting technique. High melting temp during vii) In general, cobalt â&#x20AC;&#x201C; chromium alloys are resistant to pitting and crevice corrosion, even with in the body. By contrast, relatively little is known about their susceptibility to stress corrosion cracking or corrosion fatigue. viii) Co â&#x20AC;&#x201C; Cr alloys may undergo fretting corrosion quite readily. The process of fretting is a mechanical one and involves rubbing in the
form of a prolonged series of cyclic micro – movements. The result is localized damage to one or both surfaces. ix) In fretting corrosion, the process continually exposes new surfaces, and these undergo oxidation. The fretting debris that becomes trapped between the surface damage and exposure of new metal, and the whole process leads to loss of metal from the assembly. BIOCOMPATIBILITY OF COBALT – CHROMIUM ALLOYS USE OF CHROME – COBALT – BASED ALLOY i)
As a denture base to complete denture, as a denture base to partial denture.
ii)
As a part of implant denture.
iii)
For making surgical screws and plates.
iv)
In orthopedic surgery.
v)
For making dental wires. Casting favour discontinuous carbide phases but there is a limit to
which this can be used to any advantage since the use of very high casting temperatures can cause interactions between the alloy and the mould. NICKEL – CHROMIUM ALLOYS The chemical composition of these alloys specified in the ISO Standard for Dental Base Metal Casting Alloys (Part 2) is as follows. Nickel Chromium Molybdenum Beryllium Nickel + Cobalt + Chromium
Main constituent No less than 20% No less than 4% No more than 2% No less than 85%
As for the Co/Cr alloys the concentrations of minor ingredients can Have a profound effect on properties. The concentration of carbon and the nature of the grain boundaries are major factors in controlling the properties of these alloys.
MANIPULATION OF BASE METAL CASTING ALLOYS i) The fusion temperatures of the Ni/Cr and Co/Cr alloys vary with composition but are generally in the range 1200 – 1500 0c. This is considerably higher than for the casting gold alloys which rarely have fusion Temperatures above 9500c. ii)
Melting of gold alloys can readily be achieved using a gas – air mixture.
iii)
For base metal alloys, however, either acetylene – oxygen flue or an electrical induction furnance is required. The latter method is to be favored since it is carried out under more controlled conditions.
iv)
When using oxyacetylene flames the ration of oxygen to acetylene must be carefully controlled. Too much oxygen may cause oxidation of the alloy whilst an excess of acetylene produces an increase in the metal carbide content leading to embrittlement.
v)
Investment moulds for base metal alloys must be capable of maintaining their integrity at the high casting temperatures used. Silica bonded and phosphate bonded materials are favored with the latter product being most widely used.
vi)
Gypsum – bonded investments decompose above 12000c to form sulphur dioxide which may be absorbed by the casting, causing embrittlement. This effect can be reduced by the incorporation of oxalate in the investment; however the problem is generally avoided
by choosing an investment which is more stable at elevated temperatures. vii)
The density values of base metal alloys are approximately half those of the casting gold alloys. For this reason the thrust developed during casting may be somewhat lower, with the possibility that the casting may not adequately fill the mould. Casting machines used for base metal alloys must therefore be capable or producing extra thrust which overcomes this deficiency.
viii)
The problem may be aggravated if the investment is not sufficiently porous to allow escape of trapped air and other gases. Careful use of vents and sprues of adequate size is normally sufficient to overcome such problems.
ix)
The greatest expense involved in producing a Co/Cr dental casting is in the time required for trimming and polishing.
x)
In the â&#x20AC;&#x2DC;as castâ&#x20AC;&#x2122; state, the alloy surface is normally quite rough, partially due to the coarse nature of some investment powders. Finer investments can be used to give a smoother surface requiring less finishing.
xi)
One common technique involves painting the wax pattern with fine investment â&#x20AC;&#x201C;this then forms the inner surface of the investment mould. The bulk of the mould is then formed from the coarser grade material.
xii)
Base metal alloys, and particularly the Co/Cr type are very hard and consequently difficult to polish. After casting, it is usual to sandblast the metal to remove any surface roughness or adherent investment material as well as the green layer of oxide which coats the surface after casting. Electrolytic polishing may then be carried out. This procedure is essentially the opposite of electroplating.
xiii)
If a rough metal surface is connected as the anode in a bath of strongly acidic electrolyte, a current passing between it and the cathode will cause the anode to ionize and lose a surface film of metal. With a suitable electrolyte and the correct current density, the first products of electrolysis will collect in the hollows of the rough metal surface and so prevent further attack in these areas. The prominences of the metal surface will continue to be dissolved and in this way the contours of the surface are smoothed. Final polishing can be carried out using a high â&#x20AC;&#x201C; speed polishing buff.
xiv)
The process of electro polishing is not generally used for Ni/Cr alloy castings. These products are normally used for crown and bridge work and it is essential to maintain the accuracy of fit, particularly at the margins of crowns. This accuracy may be lost during polishing procedures and care is required to avoid such problems.
COMPARATIVE PROPERTIES OF Ni/Cr AND TYPE 3 CASTING
GOLD
ALLOYS
FOR
SMALL
CAST
RESTORATION PROPERTY Density (gcum
Ni/Cr -
8
Type 3
Comments
Gold alloy 15
More difficult to produce defect â&#x20AC;&#x201C;
3
) Fusion
As high Normally
temperature
as
lower
Casting
13500c 2.0
10000c 1.4
shrinkage (1%) Tensile strength 600
540
free castings for Ni/Cr alloys. Ni/Cr alloys require electrical
than induction furnance or oxyacetylene equipment. Mostly compensated for by correct choice of investment. Both adequate for the applications
(Mpa) Proportional
500
290
limit (Mpa)
being considered. Both high enough
to
prevent
distortions for applications being considered; note that values are lower than for partial denture alloys
Modulus
of 220
85
elasticity (Gpa)
Table 8-1) Higher modulus of Ni/Cr is an advantage for larger restorations, e.g. bridges and for porcelain â&#x20AC;&#x201C; bounded
Hardness (Vickers) Ductility
300 3-30
150
restorations. Ni/Cr more difficult to polish during
20
service. Relatively large values suggest that
(%elongation)
burnishing is possible; however, large
proportional
suggest
wish
limit
forces
values
would
be
required.
METALS AND ALLOYS FOR IMPLANTS Implants offer an alternative method of treatment for the replacement of missing teeth which can be used instead of dentures or fixed bridges. Biocompatibility and stability are often seen as closely related in that some materials are known to encourage bone growth which produces a very intimate interface between bone and in plant which helps to stabilize the latter. Function primarily depends upon the rigidity of the implant structure. This in turn is related to the dimensions and the modulus of elasticity of the material from which the implant is manufactured. Dental implants are normally classified according to the way in which they are stabilized. The three most common types are: - Subperiosteal - Blade â&#x20AC;&#x201C; vent end osseous
- Osseo integrated. Subperiosteal implants consist of an open framework of cast alloy which rests on top of the bony ridge but beneath the mucosa. Cost cobalt â&#x20AC;&#x201C; chromium alloys are most commonly used for these applications. The very high modulus of elasticity of these materials combined with reasonable cast ability is the main factors affecting this choice. Attempts have been made to improve the biocompatibility of the alloys by using hydroxyapatite coatings. - Blade â&#x20AC;&#x201C; vent implants are normally constructed from titanium which has excellent biocompatibility.
BIOLOGICAL HAZARDS AND PRECAUTIONS: RISKS FOR DENTAL LABORATORY TECHNICIANS Laboratory technicians may be exposed occasionally or routinely to excessively high concentrations of beryllium and nickel dust and beryllium vapor. Although the beryllium concentration in dental alloys rarely exceeds 2% by weight, the amount of beryllium vapor released into the breathing space during the melting of Ni-Cr- beryllium alloys may be significant over an extended period of time. i)
Actually, the potential hazards of beryllium should be based on its atomic concentration in an alloy.
ii)
One can demonstrate that an alloy which contains 80% Ni, 11.4% Cr, 5% Mo, 1.8% Fe, and 1.8% Be on a weight basis contains 73.3% Ni, 11.8% Cr, 2.6% Mo, 1.6% Fe, and 10.7% Be on an atomic basis. Thus toxicity considerations for beryllium should be based on the atomic concentration rather than the weight percentage.
iii)
The Vapor pressure of pure beryllium is app 0.1 torr (mmHg) at an assumed casting temp of 1370 o C. Comparable vapor pressures for chromium, nickel, and Molybdenum are 5x10 -3 torr, 8x10-4 torr, and 3x10-11 torr, respectively.
iv)
The risk for beryllium Vapor exposure is greatest for dental technicians during alloy melting, especially in the absence of an adequate exhaust and filtration system.
v)
The Occupational Health and Safety Administration (OSHA) specifies that exposure to beryllium dust in an should be limited to particulate beryllium concentration of 2µg/m3 of air (both respirable and non respirable particles) determined from an 8-h time-weighted average the allowable maximum concentration is 5µg/m3 (not to be exceeded for a 15-min period). For a minimum duration of 30 min, a maximum ceiling concentration of 25µg/m3 is allowed. The National Institute for Occupational Safety and Health (NIOSH) recommends a limit of 0.5 µg/m3 based on a 130 – min sample.
vi)
Moffa et al (1973) reported that high levels of beryllium were accumulating during finishing and polishing when a local exhaust system was not used. When an exhaust system was used, the concentration of beryllium in the breathing zone was reduced to levels considered safe by the authors. Workers exposed to moderately high conc. of beryllium dusts over a short period of time, or prolonged exposure to low conc., may experience signs and symptoms representing acute disease states.
vii)
Physiological responses vary from contact dermatitis to severe chemical pneumonitis, which can be fatal. The chronic disease state is characterized by symptoms persisting for more than 1 year, with
the onset of symptoms separated by a period of years from coughing, chest pain, and general weakness to pulmonary dysfunction. Prevention → i)
Well –ventilated work areas.
ii)
Protection against inhalation of dust particles during trimming with masks.
Nickel → Nickel is common in the general population. The source can also be due to other contacts like utensils and artificial jewelry. The most common manifestation is contact dermatitis. A patient with a base metal alloy bridge can show erythematous inflammation in the area of contact. Other manifestations due to inhalation include pulmonary irritation, pneumoconiosis, lung carcinomas, leading to decrease in lung function and death. Prevention → Patch test to confirm allergy. Use of alternative metals like palladium or titanium alloy.
Titanium alloys → Their major advantages are biocompatibility to oral tissues, significant strength and ductility. Composition: i)
Titanium alloy
ii)
Chromium – 5 – 15%
iii)
Nickel – 5 – 15%
iv)
Molybdenum – 3%
v)
Silicon, manganese, iron and carbon- small quantities.
Advantages→ i)
High modulus of elasticity
ii)
Easy cast ability.
iii)
Excellent bio compatibility
iv)
Has high tarnish and corrosion resistance and does not form corrosion products.
v)
Oxidizes upon contact with air or oral fluids.
vi)
Low thermal conductivity.
vii)
Capability of bonding t resin and porcelain.
Disadvantages → Special equipment is required.
Aluminium – Bronze i)
Aluminium Bronze → 7-11 wt%
ii)
Copper → 71-88 wt%
iii)
Nickel → 2-4 wt%
iv)
Iron → 1-4 wt% These alloys are still in experimental stage. No particular clinical
trial has been done. Poor resistance to tarnish is a major drawback.
NOBLE METALS - Elements with a good metallic surface that retain their surface in dry; air. They react easily with sulfur to form sulfides, but their resistance to oxidation, tarnish, and corrosion during heating, casting, soldering, and use in the mouth is very good. The noble metals are gold, platinum, palladium, iridium, sodium, osmium, and ruthenium. - The noble metals, together with silver, are some times called precious metals. Some metallurgists consider silver a noble metal in dentistry because it corrodes considerably in the oral cavity. Thus the terms noble and precious are not synonymous in dentistry. GOLD (Au) â&#x2020;&#x2019; Pure gold is a soft, malleable ductile metal that has a rich yellow color with a strong metallic luster. i)
It ranks much lower in strength.
ii)
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 causes gold to be extremely brittle.
iii)
Air or water at any temp doesnâ&#x20AC;&#x2122;t affect or tarnish gold.
iv)
Gold is not soluble in sulfuric, nitric or hydrochloric acids. However, it readily dissolves in combinations of nitric and HCl (aqua rugia, 18 Vol% nitric acid and 82 vol% Hclacids ) to form the trichloride of gold (Aucl3).It is also dissolved by a few other chemicals, such as potassium cyanide and solutious of bromine or chlorine.
v)
Gold must be alloyed with Cu, Ag, Pt and other metals to develop the hardness, durability, and elasticity necessary in dental alloys, coins, and jewelry.
PLATINUM (Pt) → Platinum is a bluish – white metal, and is toughs ductile, malleable, and can be produced as foil or fine – drawn wire. i)
Platinum has hardness similar to copper.
ii)
Pure pt has numerous applications in dentistry because of its high fusing point and resistance to oral conditions and elevated temp.
iii)
Pt has been used for pins and posts in crown and bridge restorations and alloys may be cast or soldered to the posts without damage.
iv)
Adds greatly to the hardness and elastic qualities of gold.
v)
Tends to lighten the color of yellow gold based alloys.
PALLADIUM (Pd) i)
White metal some what darker than Pt.
ii)
Its density is a little more than half that of Pt and gold.
iii)
It has a quality of absorbing or occluding large quantities of hydrogen gas when heated. This can be an undesirable quality when alloys combining Pd are heated with an improperly adjusted gas – air torch
iv)
Palladium can be combined with gold, silver, Cu, Co, Sn, In or Ga for dental alloys.
Iridium (Ir), Ruthenium (Ru), and Rhodium (Rh) i) Iridium and Ruthenium are used in small amounts in dental alloys as grain refiners to keep the grain size small. A small grain size is desirable because in improves the mechanical properties and uniformity of properties with in an alloy. As little as 0.005% of Ir is effective in reducing the grain effect. ii) Ru has a similar effect. The grain refining properties of these elements occurs largely because of their extremely high melting points. iii) Ir melts at 24100C and Ru at 23100C. Thus these elements donâ&#x20AC;&#x2122;t melt during the casting of the alloy and serve as nucleating centers for the melt as it cools, resulting in a fine â&#x20AC;&#x201C; grained alloy. iv) Rh also has a high melting point (199660C) and has been used in alloys with Pt to form wire for thermocouples. These thermocouples help measure the temp in porcelain furnaces used to make dental restorations. Osmium (Os) â&#x2020;&#x2019; Because of its tremendous expense and extremely high welting point Os is not used in dental casting alloys. i)
a deoxidizing agent.
ii)
Because of its low density, the resulting ZuO large behind the denser molten mass during casting, and is therefore excluded from the casting.
iii)
If too much Zinc is present, it will markedly increase the brittleness of the alloy.
Indium (In) i)
In is a soft, gray- white metal with a low melting point of 156.60c.
ii)
It is not tarnished by air or water. It is used in some gold- based alloys as a replacement for Zn, and is a common minor component of some noble ceramic dental alloys.
iii)
Recently, Indium has been used in greater amounts (up to 30% by wt) to impart a yellow color Pd – Ag alloys.
BINARY COMBINATIONS OF METALS Although most noble casting alloys have three or more elements, the properties of certain binary alloys are imp because these binary combinations constitute the majority of the mass of many – noble alloys. An understanding of the physical and manipulative properties of these binary – combinations constitute the majority of the mass of many noble alloys. Among the noble alloys, six binary combinations of elements are important: i)
Au – Cu, Pd – Cu, Au – Ag, Pd – Ag, Au – Pd, and Au – Pt Phase diagrams are powerful tools for understanding the physical and manipulative properties of binary alloys.
ALLOY COMPOSITION AND TEMPERATURE i)
In each phase diagram, the horizontal axis represents the composition of the binary alloy.
ii)
For example, in fig A, the horizontal axis represents a series of binary alloys of gold and copper ranging in composition from 0% gold (or 100% copper) to 100% gold.
iii)
The composition can be given In atomic percent (at %) or weight percent (wt%)
iv)
Weight percent compositions give the relative mass of each element in the alloy, where as atomic percentages give the relative numbers of atomies in the alloys. It is a simple calculation to convert weight percentages to atomic percentages, or vice versa.
v)
Note that for the binary alloys shown in fig, the atomic percent composition is shown along the bottom of the phase diagram whereas the weight percent composition is shown along the top.
vi)
The atomic and weight percent compositions of the binary alloys can differ considerably.
vii)
For example, for the Au – Cu system, an alloy that is 50% gold
by weight is only 25% gold by atoms. viii)
For other systems, like the Au –Pt system ‘F’, there is little
difference between atomic and weight percentages. The difference between atomic and weight percentages depends on the differences in the atomic masses of the elements involved. ix)
The bigger the difference in atomic mass, the bigger the difference between the atomic and weight percentages in the binary phase diagram.
x)
Because it more convenient to use masses in the manufacture of alloys, the most common method to report composition is by weight
percentages. However, the physical and biological percentages. However, the physical and biological properties of alloys relate best to atomic percentages. It is therefore important to keep the difference between atomic and weight percent in mind when selecting and using noble dental casting alloys. Alloys that appear high in gold by weight percentage may in reality contain for fewer gold atoms than might be thought. xi)
Other aspects of the phase diagrams that deserve attention are the liquid us and solid us lines. The y axes show temperature.
xii)
If the temp is above the liquid us line (marked L), the alloy will solid us line (marked S), the alloy will be solid. If the temp lies between the liquid us and solid us lines, the alloy will be partially molten.
xiii)
Note that the distance between the liquidus and solidus lines varies among systems in Fig. For example the temp difference between these lines is small for the Ag – Au system, much larger for the Au – Pt system (‘F’) and varies considerably with composition for the Au – Cu system. (‘A’)
xiv)
If the liquidus – solidus line is broad, the alloy will remain at least partially molten for a longer period after it is cast.
xv)
The temp. of the liquid us line is also imp, and varies considerably among alloys and with composition. For example the liquidus line of the Au- Ag system ranges from 962 0 to 10640C (‘C’), but the liquidus line of the Au – Pd system ranges from 1064 0 to 15540C [‘E’]. It is often desirable to have an alloy with a liquidus line at lower temperatures; the method of heating is easier, fewer side reactions occur, and shrinkage is generally less of a problem.
PHASE STRUCTURE OF NOBLE ALLOYS:
i)
The area below the solidus lines in fig is also imp to the behavior of the alloy.
ii)
If this area contains no boundaries, then the binary system is a series of solid solutions. This means that the two elements are completely soluble in one another at all temp and compositions.
iii)
The Ag –Pd system (‘D’) and Pd – Au system (‘F’) are examples of solid solution systems.
iv)
If the area below the solidus line contains dashed lines, then an ordered solution is present with in the dashed lines. An ordered solution occurs when the two elements in the alloy assume specific and regular positions in the crystal lattice of the alloy. This situation differs from a solid solution where the positions of the elements in the crystal lattice are random. Examples of systems containing ordered solutions are the Au – Cu system (‘A’).The Pd – Cu system ‘B’ and Au – Ag system ‘C’.
v)
Note that the ordered solutions occur over a limited range of compositions because the ratios between the elements must be correct to support the regular positions in the crystal lattices.
vi)
If the area below the solidus line contains a solid line, it indicates the existence of a second phase. A second- phase is an area with a composition distinctly different from the first phase.
vii)
In the Au – Pt system (‘F’) a second phase forms between 20 and 90 at% platinum. If the temp. is below the phase boundary live with in these compositions, two phases exist in the alloy. The presence of a second phase is imp because it significantly changes the corrosion properties of an alloy.
HARDENING OF NOBLE ALLOYS i)
The use of pure cast gold is not practical for dental restorations because cast gold lacks sufficient strength and hardness.
ii)
Solid – solution and ordered solution hardening are two common ways of strengthening noble dental alloys sufficiently for use in the mouth.
iii)
By mixing two elements in the crystal lattice randomly (forming a solid solution), the force needed to distort the lattice may be significantly increased. For example, adding just 10% by weight of copper to gold, the
tensile strength increases from 105 to 395 Mpa and the Brinell hardness increases from 28 to 85. The 90:10 Au – Cu mixture is the composition used in U.S. gold coins. iv)
If the positions of the two elements become ordered (forming an ordered solution), the properties of the alloy are improved further. For a typical gold – based casting alloy, the formation of an ordered solution may increase yield strength by 50%, tensile strength by 25% and hardness by at least 10%. It is important to note that the elongation of an alloy is reduced by formation of the ordered solution. For the typical gold – based alloy, the percentage elongation will decrease from 30% to about 12%.
v)
The formation of ordered solutions has been commonly used to strengthen cast dental restorations, particularly in gold – based alloys. As shown in fig ‘A’, the Au- Cu system supports ordered solutions between about 20 and 70 at %gold. However, the manipulation of the alloy during casting will determine if the ordered solution will form.
vi)
If Au – Cu containing about 50 at % gold is heated to the molten state and then cooled slowly, the mass will solidify at about 8800C as solid solutions. As the mass cools slowly to 424 0 C, the ordered solutions will then form and will remain present at room temp.
vii)
However, if the mass is cooled rapidly to room temp. after the initial solidification, the ordered solution will not form because there is insufficient time for the mass reorganize. Thus the alloy will be trapped in a non – equilibrium state of a solid solutions and will be softer, weaker, and have greater elongation. By heating an alloy in either condition above 424 0C, the state of
viii)
the alloy can be selected by picking the cooling rate. ix)
Rapid cooling will preserve the solid solution and the soft condition, whereas slow cooling will allow the formation of the ordered solution and the hardened condition.
FORMULATION OF NOBLE ALLOYS The desired qualities of noble dental casting alloys determine the selection of elements that will be used to formulate the alloys. The ideal noble casting alloy should have i)
A low melting range and narrow solidus – liquidus temperature range.
ii)
Adequate strength, hardness, and elongation
iii)
A low tendency to corrode in the oral environment
iv)
Low cost among other properties.
- Solid – solution systems are desirable for the formulation of alloys because for the formulation of alloys because they are generally
easier to manufacture and manipulate, have a lower tendency to corrode than multiple- phase systems, and provide increased strength through solid – solution or ordered – solution hardening.
CARAT AND FINENESS OF GOLD – BASED ALLOYS For many years the gold content of gold containing alloys has been described on the basis of the carat, or in terms of fineness, rather than by wt%. The term carat refers only to the gold content of the alloy; a carat represents a 1/24 part of the whole. Thus 24 carat indicates pure gold. The carat of an alloy is designated by a small letter R, for example, 18k or 22k gold. The use of term carat to designate the gold content of dental alloy is less common now. It is not unusal to find the weight percentage of gold listed or to have the alloy described in terms of finer ness. Fineness also refers only to the gold content, and represents the number of parts of gold in each 1000 parts of alloy. Thus 24k gold is the same as 100% gold or 1000 fineness. (i.e. 1000 fine). The fineness represents a precise measure of the gold content of the alloy and is often the preferred measurement. - An 18k gold would be designated as 750 fine, or, when the decimal system is used, it would be 0.750 fine; this indicates that 750/1000 of the total is gold. - The fineness system is somewhat less relevant to day because of the introduction of alloys that are not gold- based. It is imp to emphasize that the terms carat and fineness refer only to gold content, not noble –metal content.
ALLOYS FOR ALL – METAL AND RESIN – VENEERED RESTORATIONS In 1927, The National Bureau of standards established gold casting alloy types I through IV according to dental function, with hardness increasing from type I to type IV. a) Gold Alloys b) Silver Palladium alloy c) Aluminium Bronze alloy.
HEAT TREATMENT OF HIGH NOBLE AND NOBLE METAL ALLOYS
i)
Gold alloys can be significantly hardened if the alloy contains a sufficient amount of copper. Types I and II alloys usually donâ&#x20AC;&#x2122;t harden, or they harden to a lesser degree than do the types III and IV gold alloys.
ii)
The actual mechanism of hardening is probably the result of several different solid state transformations.
iii)
In metallurgical engineering terminology the softening heat treatment is â&#x20AC;&#x201C; referred to as a solution heat treatment. The hardening heat treatment is termed age hardening.
SOFTENING HEAT TREATMENT OF GOLD CASTING ALLOYS i)
The casting is placed in an electric furnace for 10 min at a temp of 7000c (12920F) and then it is quenched in h 20. During this period, all intermediate phases are presumably changed to a disordered solid, solution, and the rapid quenching prevents ordering from occurring during cooling.
ii)
The tensile strength, proportional limit, and hardness are reduced by such a treatment and the ductility is in creased.
iii)
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. Although 7000c is an adequate average softening temp,
each alloy has its optimum temp, and the manufacturer should specify the most favorable temp. and time.
HARDENING HEAT TREATMENT OF GOLD CASTING ALLOYS→ It can be accomplished in several ways. i)
One of the most practical hardening treatments is by soaking or aging the casting at a specific temp, for a definite time, usually 15 to 30 minutes, before it is water – quenched.
ii)
The aging temp. depends on the alloy composition but is generally between 2000C (3920F) and 4500C (8420F).
iii)
Ideally, before the alloy is given an age – hardening treatment, it should be subjected to a softening heat treatment to relieve all strain hardening and to start the hardening treatment with the alloy as a disordered solid solution.
iv)
This treatment is indicated for metallic partial dentures, saddles, FPDs, and other similar sites.
For small sites, such as inlays, a
hardening treatment is not usually employed. Age hardening substantially increases the yield strength. v)
The hardness values for noble metal alloys correlate quite well with their yield strengths.
vi)
Age hardening reduces the percent elongation in some cases. Alloys with low elongation are relatively brittle materials and fracture readily if loaded beyond the proportional limit or yield strength.
B) SILVER – PALLADIUM ALLOYS →
i)
Silver – Pd alloys are white and predominantly silver in composition but have substantial amounts of Pd (at least 25%) that provide nobility and promote tarnish resistance. They may or may not contain copper and a small amount of gold.
ii)
The Cu-free Ag – Pd alloys may contain 70% to 72% silver and 25% Pd and may have physical properties similar to those for a type III gold alloy.
iii)
Other Ag – based alloys might contain roughly 60% Ag, 25% Pd, and as much as 15% or more Cu 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. iv)
The use of metal – ceramic restorations in posterior sites has increased relative to the use of all metal crowns and onlays. The compositions of representative high noble and noble alloys
(including Ag- Pd alloys) for all meal restorations (Type 1 to Type IV). C) ALUMINUM BRONZE ALLOY Bronze is traditionally defined as a copper – rich, copper – tin (Cu- Sn) alloy with or without other elements such as Zn and phosphorus, there exist essentially two – component (binary), three component(ternary),and four component (quaternary) bronze alloys that contain aluminum bronze (Cu – Al), silicon bronze copper – silicon, and beryllium bronze (Cu – Be). i)
The Al- bronze family of alloys may contain between 81 wt% and 88 wt% Cu, 7 wt% to; 11 wt% Al, 2 wt% to 4 wt% Ni, and 1 wt to 4 wt% iron.
ii)
There is a potential for copper alloys to react with sulfur to form copper sulfide which may tarnish the surface of this alloy in the same manner that Ag sulfide darkens the surface of gold – base or Ag – base alloys that contain a significant Ag content.
HIGH NOBLE AND NOBLE ALLOYS FOR METAL CERAMIC RESTORATION (PROSTHESES) a) Gold – Platinum – Palladium alloy b) Gold – Palladium silver alloy c) Gold- Palladium alloy. i)
The chief objection to the use of dental porcelain as a restorative material is its low strength under tensile and shears stress conditions.
ii)
Although porcelain can resist compressive stresses with reasonable success, the substructure design should not include shapes in which significant tensile stresses are produced during loading. A method by which the disadvantage can be minimized is to bond the porcelain directly to cast alloy substructure made to fit the prepared tooth.
iii)
Adding less than 1% of oxide forming element such as iron, indium, and tin to this high gold content alloy the porcelain metal bond strength was improved. Iron increases proportional limit and strength of alloy.
iv)
The 1% addition of base metal to gold, Pd and Pt alloys was all that was necessary to produce a slight oxide film on surface of substructure to achieve porcelain metal bond strength.
v)
Inspite of vastly different chemical compositions, all the alloys described in the following according to their principal chemical elements share at least three common features: a) They have the potential to bond to dental porcelain b) They possess coefficients of thermal contraction compatible with those of dental porcelains. c) Their solidus temp is sufficiently high to permit the application of low fusing porcelains.
vi)
The coefficients of thermal expansion (CTE) tend to have a reciprocal relationship with the melting points of alloys (because of an inverse dependence on the relative strength of interatomic bonding), as well as the melting range of alloys; that is, the higher the melting temp of a metal, the lower its CTE. Metal ceramic alloys of a metal, the lower its CTE. Metal ceramic alloys are also often referred to as porcelain fused to metal (PFM) or ceramo metal alloys.
GOLD – PALLADIUM – SILVER ALLOYS (Low Silver Content) i)
Au – Pd – Ag alloys, which contain 5% to 11.99% Ag are economical alternatives to the Au – Pt – Pd or Au – Pd – Pt alloys. These are resistant to tarnish and corrosion.
ii)
The principal disadvantage of this alloy group is the potential for porcelain discoloration when Ag vapor is released and deposited on the porcelain surface.
GOLD – PALLADIUM – SILVER ALLOYS (High silver content)
i)
Gold alloys that contain 12% Ag or more account for approximately; 20% of the current alloy market. These include Au – Pd – Ag, Pd – Au – Ag and Pd – Ag alloys.
ii)
The Au – Pd alloys with high silver contents (12% to 22%) have been popular alternatives to the higher gold content alloys for many years despite their potential for porcelain discoloration.
iii)
These alloys are white – colored and are used primarily for their lower cost and comparable physical properties.
iv)
The commonly used alloys in this gp contain between 39% and53% Au and 25% to; 35% Pd.
v)
The potential for porcelain discoloration is greatest with alloy which has the highest silver contents.
vi)
The factors that intensify the porcelain color changes because of the release of Ag were identified previously. In general, it is advisable to avoid these types of alloys when using lighter shades and ceramic products that are sensitive to silver discoloration.
GOLD – PALLADIUM ALLOYS This alloy was designed to overcome the porcelain discoloration effect (because it is Ag – free) and also to provide an alloy with a lower thermal contraction coefficient than that of either the Au – Pd – Ag or Pd – Ag alloys. i)
The contents are gold ranging from 44% - 55% and a Pd level 35 –45%
ii)
Alloys of this type must be used with porcelains that have low coefficient of thermal contraction to avoid the development of axial and circumferential tensile stresses in porcelain during the cooling part of porcelain firing cycle.
iii)
The yield strength, modulus of elasticity;, tensile strength and hardness of Au – Pd – Ag and Au – Pd alloys are greater and density lower, then those are Au – Pt –Pd alloys, which implies that combination; will be more resistant to masticatory force and stiffer then restoration made of Au – Pt – Pd alloys. Lower densities also mean prosthesis will be lighter in weight.
PALLADIUM – GOLD ALLOYS Its popularity has been diminished by the recent price volatility of Pd. These are free of Ag. Therefore they don’t contribute to porcelain discoloration. Physical properties are similar to those of the Au – Pd alloys. Thermal compatibility with commercial porcelain products has not yet been reported in the dental literature. PALLADIUM – GOLD – SILVER ALLOYS i)
The Pd – Au – Ag alloy group is similar to the Au – Pd – Ag types of alloys in their potential for porcelain discoloration. These alloys have gold contents ranging from 5% to 32% and Ag contents varying between 6.5% and 14%
ii)
One would expect the potential for porcelain discoloration to be greater for the higher Ag – content alloys in this group.
iii)
These alloys have a range of thermal contraction coefficients that increase with an increase in Ag content.
PALLADIUM – SILVER ALLOYS i)
It was introduced to the U.S. market in 1974 as the first gold – free noble alloy available for metal – ceramic restorations.
ii)
The compositions of Pd – Ag alloys fall with in a narrow range of 53% to 61% Pd and 28% to 40% Ag.
iii)
Tin and /or indium are usually added t increase alloy hardness and to promote oxide formation and adequate bonding to porcelain.
iv)
A proper balance is needed to maintain a reasonably low casting temp and a compatible coefficient of thermal contraction.
v)
Because of their increase Ag content compared with that of gold based alloys, the Ag discoloration effect is most severe for these alloys. Gold metal conditioners or ceramic coating agents may minimize this effect.
vi)
The low specific gravity of these alloys (10.7 to 11.1), combined with their low intrinsic cost, make them attractive as economical alternatives to the gold – based alloys.
vii)
Adherence to porcelain is considered to be acceptable for most of the Pd – Ag alloys.
viii)
Instead of the formation of the desired external oxide, Pd – Ag nodules may develop on the surface, which enhance retention of porcelain by mechanical rather than chemical bonding.
PALLADIUM-COPPER-GALLIUM ALLOYS: •
No clinical reports of adverse events have been reported for Pd-Cu-Ga alloys.
•
The clinician should be aware of the potential effect on aesthetics of the dark brown or black oxide formed during oxidation and subsequent porcelain-firing cycles.
PALLADIUM-GALLIUM-SILVER ALLOYS:
•
They tend to have a slightly lighter colored oxide than the Pd-Cu alloys and they are thermally compatible with lower expansion porcelains.
•
The silver consent is generally relatively low (5 wt% to 8 wt % in most cases) and is usually inadequate to cause significant porcelain greening.
•
Pd-Ga-Ag alloys generally have relatively low thermal contraction coefficients are expected to be more compatible with lower expansion porcelains.
METALS FOR PARTIAL DENTURE ALLOYS:
•
The majority of removable partial denture frameworks are made from alloys based primarily on nickel, cobalt, or titanium as the principal metal component.
•
Ni is a malleable, ductile, silver-colored transition element with atomic numbers and a melting point of about 14500C.
•
CO is a silver-colored transition element with atomic number 27, having a melting point of about 1500 0C and little ductility at room temperature.
•
All CO-based and Ni-based alloys container to prevent corrosion and tarnish. The passivation mechanism of the alloy occurs through a thin surface layer of chromium oxide (Cr2O3). Most CO-Cr alloys contain MO (CO-Cr-MO), and some may contain Ni (CO-CrNi). Some Ni-Cr alloys contain beryllium (Be), which lowers the melting point to improve castability.
•
Frameworks may also be made from CPTi and Ti-6Al-4V. The most biocompatible metal for frameworks is CPTi. Porcelain with ↑ Na contents are believed to exhibit a more intense
discoloration because of more rapid silver diffusion in Na-containing glass.
CONCLUSION So its important to have the proper knowledge of metals and the alloys for the proper use in dentistry.
REFERENCES: 1) Dental material properties and manipulation CRAIG 2) Notes on dental materials E.C.COMBE 3) Text book of dental materials SHARMILLA HUSSAIN 3) Essential of dental materials SH SORATUR 4) Applied dental materials JOHN F.MCCABE 5) The chemistry of medical and dental materials J.W.NICHOLSON 6) Dental materials ANUSAVICE