Dental Casting Alloys Notes

Metal Restorations In Dentistry

Metal restorations and prostheses are an integral part of dentistry.

Metals are among the strongest materials and provide strength and durability to any structure.

• There are two ways of constructing a metal restoration—direct and indirect.
• Direct techniques have been used in modern dentistry since the introduction of direct filing gold and amalgam in the 19th century.
• Indirect dental restorations were introduced into the dental profession with the patenting of the centrifugal casting machine and the lost wax technique by William H. Taggart in 1907.

Read And Learn More: Basic Dental Materials Notes

Terminology

Alloy

An alloy is defined as a metal-containing two or more elements, at least one of which is metal and all of which are mutually soluble in the molten state.

Noble Metals

• Noble metals have been used for inlays, crowns, and FDPs because of their resistance to corrosion in the mouth.
• Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver are the eight noble metals.
• However, in the oral cavity, silver can tarnish and therefore is not considered a noble metal.

Precious Metals

• The term precious indicates the intrinsic value of the metal.
• The eight noble metals are also precious metals and are defied so by major metallurgical societies and federal government agencies.
• For example National Institute of Standards and Technology and the National Material Advisory Board.
• All noble metals are precious but all precious metals are not noble.
• Of the eight noble metals, four are very important in dental casting alloys, i.e. gold, platinum, palladium, and silver.
• All four have a face-centered cubic crystal structure and all are white colored except for gold.
  • Gold Pure gold is a soft and ductile metal with a yellow ‘gold’ hue. It has a density of 19.3 g/cm³ and a melting point of 1063 °C. Gold has a good luster and takes up a high polish. It has good chemical stability and does not tarnish and corrode under normal circumstances.
  • Silver is Sometimes described as the ‘whitest’ of all metals. It has the lowest density (10.4 g/ cm³) and melting point (961°C) among the precious casting alloys. Its CTE is 15.7 × 10^-6/°C which is comparatively high.
  • Palladium Density is 12.02 g/cm³. Palladium has a higher melting point (1552°C) and lower CTE (11.1 × 10^-6/°C) when compared to gold.
  • Platinum It has the highest density (21.65 g/cm³) highest melting point (1769°C) and the lowest CTE among the four precious metals.

Semiprecious Metals

There is no accepted composition which differentiates ‘precious’ from ‘semiprecious’. Therefore, the term semiprecious should be avoided.

Base Metals

• These are non-noble metals. They are important components of dental casting alloys because of their influence on physical properties, control of the amount and type of oxidation, and their strengthening effct.
• Such metals are reactive with their environment and are referred to as ‘base metals’.
• Some of the base metals can be used to protect an alloy from corrosion by a property known as passivation.
• Although they are frequently referred to as nonprecious, the preferred term is base metal.

Examples Chromium, cobalt, nickel, iron, copper, manganese, etc.

History And Classification Of Dental Casting Alloys

At the beginning of the twentieth century when dental casting techniques were evolving, the alloys were predominantly gold-based.

Taggart in 1907 was the first to describe the lost wax technique in dentistry. The existing jewelry alloys were quickly adopted for dental purposes.

• Initially, copper, silver, and platinum were the main alloying elements. As the alloys evolved it was felt that a classifiation was needed.
• In 1932, the National Bureau of Standards classified the alloys according to their hardness (Type 1, Type 2, etc.).
• At that time it was felt that gold alloy with less than 65% gold, tarnished too easily in the oral cavity.
• By 1948, metallurgists experimenting with various alloys were able to decrease the gold content while maintaining their resistance to tarnish.
• This breakthrough was due to palladium. It counteracted the tarnish potential of silver.

The main requirements of the original dental casting alloys were simple

• They should not tarnish in the mouth.
• They should be strong (for use as bridges).

This soon changed with the introduction of special alloys (metal-ceramic alloys) that could bond to porcelain in the late 1950s.

• The composition and requirements of these alloys became more complex.
• For example, they had to contain elements that could enhance the bond to porcelain, they had to have a higher melting temperature (because porcelain had high fusion temperatures), etc.
• Another important development were the rapid increase in gold prices in the 1970s.
• As gold became more expensive, people began to look for less expensive metals for dental castings.
• Manufacturers began experimenting with base metal alloys like nickel-chromium and cobalt-chromium.
• These alloys were already in use since the 1930s for the construction of cast partial denture frameworks.
• Prior to this, the Type 4 gold alloys were used extensively for this purpose.
• These base metals soon replaced the Type 4 gold alloys for partial denture use because of their lightweight, lower cost and tarnish resistance.
• When the gold prices shot up, these base metal partial denture alloys were quickly adapted for use in field prosthodontics.
• Subsequently, newer formulations allowed their use as metal-ceramic alloys.
• Today there is such a wide variety of alloys in the market that classifying them is not easy.
• A number of different classifications are mentioned below.

According To Use

• Alloys for all metal and resin veneer restorations (for example inlays, posts, resin, and composite veneered crowns, and FDPs).
• Alloys for metal-ceramics restorations (for example PFM crowns and FDPs).
• Alloys for removable dentures (for example RPD frames and complete denture bases).

Based On Yield Strength And Percent Elongation (Ada Sp. 5 )

• Type 1 – Soft
• Type 2 – Medium
• Type 3 – Hard
• Type 4  – Extra-hard
  • This 1934 classification was originally intended for gold alloys and was based on hardness.
  • Since 1989, it was relaxed to include any dental alloy as long as it met the new yield strength and percentage elongation criteria.
  • Types 1 and 2 are known as ‘inlay alloys’ and Types 3 and 4 are known as ‘crown and bridge alloys’. Type 4 is occasionally used for RPD frames).

According To Nobility (Ada 1984)

• High noble metal alloys (HN) – Contains > 40 wt% Au and > 60 wt% noble metals
• Noble metal alloys (N) – Contains > 25 wt% of noble metals
• Predominantly base metal alloys (PB)- Contains < 25 wt% of noble metals
• Base metal

This classification is popular among manufacturers.

Based On Mechanical Properties (Iso 22674:2006)

• The current ISO classification supersedes all previous classifications and covers all metals used in dentistry for restorations and prostheses.
• It makes no distinction between noble and base metal. ISO 22674:2006 classifies all metallic materials into six types according to their mechanical properties.
  • Type 0 – Intended for low-stress bearing single-tooth fixed restorations, for example, small veneered one-surface inlays, and veneered crowns.
  • Type 1 – Intended for low-stress bearing single-tooth fixed restorations, for example, veneered or unveneered one-surface inlays, and veneered crowns.
  • Type 2 – Intended for single tooth fixed restorations, for example, crowns or inlays without restriction on the number of surfaces.
  • Type 3 – Intended for multiple unit fixed restorations, for example, bridges.
  • Type 4 – Intended for appliances with thin sections that are subject to very high forces, for example, removable partial dentures, clasps, thin veneered crowns, wide-span bridges or bridges with small cross-sections, bars, attachments, and implant retained superstructures.
  • Type 5 – Intended for appliances in which parts require the combination of high stiffness and strength, for example, thin removable partial dentures, parts with thin cross-sections, and clasps.

According To Major Elements

• Gold alloys
• Silver alloys
• Palladium alloys
• Nickel alloys
• Cobalt alloys
• Titanium alloys

According To The Three Major Elements

• Gold-palladium-silver
• Palladium-silver-tin
• Nickel-chromium-molybdenum
• Cobalt-chromium-molybdenum
• Iron-nickel-chromium
• Titanium-aluminum-vanadium

According To The Number Of Alloys Present

• Binary—two elements
• Ternary—three elements
• Quaternary (and so forth)—four elements

Classification According To Use Of Dental Casting Alloys

The huge choice of alloys in the market makes the process of identification a difficult task.

They are similar in some aspects, but yet, each has its own distinct features.

• These alloys vary not only in the type of metal but also in the percentage of each within the alloy.
• In spite of their wide variation in composition, they must meet the requirements of their intended use.
• For example, all metal-ceramic alloys regardless of whether they are noble or base must meet the requirements of porcelain bonding.
• For this reason, the classification according to use is recommended and will be the basis of the subsequent discussion of alloys.

Alloys for all metal and resin veneer restorations

• High noble
• Noble
• Predominantly base metal
• Base metal

Alloys for metal-ceramics restorations

• High noble
• Noble
• Predominantly base metal
• Base metal

Alloys for casting large structures

• High noble
• Noble
• Predominantly base metal
• Base metal

General Requirements Of Casting Alloys

All cast metals in dentistry have some basic common requirements.

• They must not tarnish and corrode in the mouth.
• They must be sufficiently strong for the intended purpose.
• They must be biocompatible (nontoxic and nonallergenic).
• They must be easy to melt, cast, cut, and grind (easy to fabricate).
• They must flow well and duplicate file details during casting.
• They must have minimal shrinkage on cooling after casting.
• They must be easy to solder.
  • Not all of them meet all the requirements. Some have shown a potential for allergic reactions (nickel-containing alloys) and other side effects when used without proper precautions.
  • Some are quite difficult to cast. Some are so hard (base metal alloys) that they are difficult to cut, grind, and polish. All alloys shrink on cooling.
  • Some (base metal alloys) show more shrinkage than others.
  • The shrinkage cannot be eliminated, but it can be compensated for (see investments). Besides these general requirements, alloys intended for a certain specific use must meet requirements for that.
  • For example, metal-ceramic alloys must have additional requirements in order to be compatible with porcelain.
  • The requirements for metal-ceramic alloys will be described later.

Alloys For All Metal Restorations

These alloys were among the earliest alloys available to dentistry.

• The early alloys were mostly gold alloys.
• Since they were intended for all metallic and later for resin-veneered restorations, they just had to meet the basic requirements (see general requirements).
• No special requirements are needed for veneering with resin.

Currently, the use of these alloys are slowly declining because of

• Increased esthetic awareness has reduced the trend for metal displays.
• Increasing popularity of all-ceramic and metal-ceramic restorations.
• Reducing the popularity of resin and composite as veneering material. Resin facings have a number of disadvantages.
• They wear rapidly (poor wear resistance).
• They may change color (color instability and stain absorption).
• They are porous. They tend to absorb food material and bacteria. This makes it unhygienic and gives it a bad odor.

Alloys For All Metal Restorations Classification

• (As mentioned before this 1934 classification was originally intended for gold alloys and was based on hardness.
• In 1989, it was relaxed to include any dental alloy as long as it met the new yield strength and percentage elongation criteria).
  • Type 1: Soft Small inlays, Class III and Class V cavities which are not subjected to great stress. They are easily burnished.
  • Type 2: Medium Inlays subject to moderate stress, thick 3/4 crowns, abutments, pontics, full crowns, and sometimes soft saddles.
  • Type 3: Head Inlays, crowns, and bridges, situations where there may be great stresses involved. They usually can be age-hardened.
  • Type 4:Extra- Hard Inlays subjected to very high stresses, partial denture frameworks, and long-span bridges. They can be age-hardened.
• Types 1 and 2 are generally called ‘inlay alloys’ and Types 3 and 4 are known as ‘crown and bridge alloys’.
• Because of the increased use of composite and ceramic inlays, the Type 1 and 2 inlay alloys are rarely used currently.
• Most of the discussion will focus on the Types 3 and 4 alloys.

Alloys For All Metal Restorations Uses

These alloys are not intended for porcelain bonding. They may be used as an all-metal restoration or with a resin veneer.

• Inlays and Onlays
• Crowns and FDPs
• Partial denture frames (only the Type 4)
• Post-cores

Alloys For All Metal Restorations Types

These alloys will be discussed under the following categories.

• High noble — Gold alloys
• Noble—silver palladium alloys
• Base metal — Nickel-chrome alloys
  Cobalt-chrome alloy
  Titanium and its alloys
  Aluminum-bronze alloys

Gold Alloys- For All-Metal Restorations

Synonyms Traditional gold alloys, Au-Ag-Cu alloys.

Why do we alloy gold?

• Pure gold is a soft and ductile metal and so is not used for casting dental restorations and appliances in its pure state.
• It is alloyed commonly with copper, silver, platinum, nickel, and zinc.
• Alloying gold with these metals not only improves its physical and mechanical properties but also reduces its cost.
• The display of metal particularly gold was once acceptable and probably was even a symbol of social status.
• The current trend is to avoid the display of metal.
• At the same time, increases in the platinum, palladium, and silver content of modern alloys have resulted in whiter-colored gold alloys.
• Thus, there are ‘yellow gold alloys and ‘white gold alloys’.
• The rise in gold prices has also led to the availability of alloys with low gold content. These are the ‘low golds’.
• The gold alloys discussed here are high noble alloys because of their high noble metal content (see classification according to nobility).

Gold Content Traditionally, the gold content of dental casting alloys was called

• Karat
• Fineness

KARAT It refers to the parts of pure gold present in 24 parts of alloy, for example

• 24-karat gold is pure gold.
• 22 karat gold is 22 parts pure gold and 2 parts of other metal.

Note In current dental alloys, the term karat is rarely used.

Fineness

• The Fineness of a gold alloy is the parts per thousand of pure gold. Pure gold is 1000 fie. Thus, if 3/4 of the gold alloy is pure gold, it is said to be 750 files.
• Note The term fitness also is rarely used to describe gold content in current alloys (however, it is often used to describe gold alloy solders).

Percentage Composition

• The percentage composition of gold alloys is preferred over karat and fitness.
• Since 1977, ADA has required manufacturers to specify the percentage composition of gold, palladium, and platinum on all their dental alloy packaging.

⇒ \(\frac{\text { Karat } \times 100}{24}=\% \text { gold }\)

Fineness is 10 times the percentage gold composition, i.e. fitness × 10 = %gold.

Composition Of Gold Alloys

Functions Of Constituents

Gold

It provides tarnish and corrosion resistance and has a desirable appearance. It also provides ductility and malleability.

Copper

• It is the principal hardener. It reduces the melting point and density of gold.
• If present in sufficient quantity, it gives the alloy a reddish color.
• It also helps to age harden gold alloys. In greater amounts, it reduces resistance to tarnish and corrosion of the gold alloy.
• Therefore, the maximum content should not exceed 16 percent.

Silver

• It whitens the alloy, thus helping to counteract the reddish color of copper.
• It increases strength and hardness slightly. In larger amounts, however, it reduces tarnish resistance.

Platinum

• It increases strength and corrosion resistance.
• It also increases melting point and has a whitening effect on the alloy.
• It helps reduce the grain size.

Palladium

• It is similar to platinum in its effect. It hardens and whitens the alloy.
• It also raises the fusion temperature and provides tarnish resistance.
• It is less expensive than platinum, thus reducing the cost of the alloy.

The minor additions are

Zinc

• It acts as a scavenger for oxygen. Without zinc, the silver in the alloy causes the absorption of oxygen during melting.
• Later during solidification, the oxygen is rejected producing gas porosities in the casting.

Indium, tin, and iron

They help to harden ceramic gold-palladium alloys, iron being the most effective.

Calcium

It is added to compensate for the decreased CTE that results when the alloy is made silver-free (the elimination of silver is done to reduce the tendency for green stain at the metal porcelain margin).

Iridium, ruthenium, rhenium

• They help to decrease the grain size. They are added in small quantities (about 100–150 ppm).
• Note All modern noble metal alloys are fine-grained.
• The smaller the grain size of the metal, the more ductile and stronger it is. It also produces a more homogeneous casting and improves the tarnish resistance.
• A large grain size reduces the strength and increases the brittleness of the metal.
• Factors controlling the grain size are the rate of cooling, the shape of the mold, and the composition of the alloy.

Properties Of Gold Alloys

Color

• Traditionally, the gold alloys were gold colored.
• The color of modern gold alloys can vary from gold to white.
• It depends on the amount of whitening elements (silver, platinum, palladium, etc.) present in the alloy.

Melting Range

• Ranges between 920–960 °C. The melting range of an alloy is important.
• It indicates the type of investment required and the type of heating source needed to melt the alloy.

Density

It gives an indication of the number of dental castings that can be made from a unit weight of the metal. In other words, more cast restorations per unit weight can be made from an alloy having a lower density, than one having a higher density. Gold alloys are lighter than pure gold (19.3 g/cm³).

• Type 3 — 15.5 g/cm³
• Type 4 — 15.2 g/cm³

The castability of an alloy is also affected by density. Higher-density alloys cast better than lower-density alloys.

Yield Strength

The yield strength is

• Type 3 — 207 MPa
• Type 4 — 275 MPa

Hardness The hardness indicates the ease with which these alloys can be cut, ground or polished. Gold alloys are generally more user-friendly than base metal alloys which are extremely hard.

The hardness values

• Type 3 — 121 MPa
• Type 4 — 149 MPa

Elongation

• It indicates the ductility of the alloy.
• A reasonable amount is required especially if the alloy is to be deformed during clinical use, for example, clasp adjustment.
• For removable partial dentures, margin adjustment, and burnishing of crowns and inlays. Type I alloys are easily furnished.

Alloys with low elongation are very brittle. Age hardening decreases ductility.

• Type 3 —30–40%
• Type 4 —30–35%.

Modulus Of Elasticity

• This indicates the stiffness/flexibility of the metal.
• Gold alloys are more flexible than base metal alloys (Type IV—90 × 10³ MPa).

Tarnish And Corrosion Resistance

• Gold alloys are resistant to tarnish and corrosion under normal oral conditions.
• This is due to their high noble content. Noble metals are less reactive.

Casting Shrinkage

• All alloys shrink when they change from liquid to solid.
• The casting shrinkage in gold alloys is less (1.25–1.65%) when compared to base metal alloys.

The shrinkage occurs in three stages:

• Thermal contraction of the liquid metal.
• Contraction of the metal while changing from liquid to solid state.
• Thermal contraction of the solid metal as it cools to room temperature.

Shrinkage affects the fi of the restoration. Therefore, it must be controlled and compensated for in the casting technique.

Biocompatibility

Gold alloys are relatively biocompatible.

Casting Investment

Gypsum-bonded investments may be used for low-fusing gold alloys.

Heat Treatment Of Gold Alloys

• Heat treatment of alloys is done in order to alter their mechanical properties.
• Gold alloys can be heat-treated if it contains a sufficient amount of copper.
• Only Type 3 and Type 4 gold alloys can be heat treated.

There are two types of heat treatment.

• Softening heat treatment (solution heat treatment).
• Hardening heat treatment (age hardening).

Softening Heat Treatment

Softening heat treatment increases ductility, but reduces strength, proportional limit, and hardness.

Softening Heat  Indications

It is indicated for appliances that are to be ground, shaped, or otherwise cold worked in or outside the mouth.

Softening Heat  Method

• The casting is placed in an electric furnace for 10 minutes at 700 °C and then it is quenched in water.
• During this period, all intermediate phases are changed to a disordered solid solution and the rapid quenching prevents ordering from occurring during cooling.
• Each alloy has its optimum temperature. The manufacturer should specify the most favorable temperature and time.

Hardening Heat Treatment (Or Aging)

• Hardening heat treatment increases strength, proportional limit, and hardness but decreases ductility.
• It is the copper present in gold alloys that helps in the age-hardening process.

Hardening Heat Indications

• For strengthening metallic dentures, saddles, FDPs, and other similar structures before use in the mouth.
• It is not employed for smaller structures, such as inlays.

Hardening Heat Method

• It is done by ‘soaking’ or aging the casting at a specific temperature for a definite time, usually 15–30 minutes.
• It is then water quenched or cooled slowly.
• The aging temperature depends on the alloy composition but is generally between 200 and 450 °C.
• During this period, the intermediate phases are changed to an ordered solid solution (the proper time and temperature for the age hardening of an alloy is specified by its manufacturer).
• Ideally, before age hardening an alloy, it should first be subjected to a softening heat treatment in order to relieve all strain hardening (stresses that occur during finishing).
• Starting the hardening treatment when the alloy is in a disordered solid solution allows better control of the aging process.

Low Gold Alloys

• Also known as ‘economy golds’.
• They are crown and FDP alloys having gold content below 60% (generally in the 42–55% range).
• However, gold must be the major element.

Technic alloy

• This is a gold-colored base metal alloy that was frequently misused in India to make all-metal crowns and FDPs for many years.
• They are also sometimes referred to as Japanese gold or K-metal. These alloys do not contain any gold or precious metal.
• The alloy is absolutely contraindicated for any intraoral dental use because of its low strength, low wear resistance, and tendency to tarnish.
• It has a high initial gold-like luster and patients are deliberately misled by unscrupulous practitioners into believing it was gold.
• Thanks to the availability of better materials its use has declined considerably.
• Unfortunately, one does come across restorations made from this alloy even to this day.
• Some practitioners still offer this material as a lower-cost alternative, in addition to the regular alloys.
  
• The low gold alloys were developed because of the increase in gold prices.
• However, reducing gold content increased tarnish and corrosion. This problem was overcome by two discoveries.
• Palladium made the silver in gold alloy more tarnish-resistant. 1% palladium was required for every 3% of silver.
• The silver-copper ratio had to be carefully balanced to yield a low silver-rich phase in the microstructure.

Low Gold Alloys Advantages

• Because of this research, numerous low-gold alloys were introduced into the market.
• Thus, these alloys were not only less expensive but also had good tarnish and corrosion resistance.
• Their properties are comparable to Types 3 and 4 gold alloys.

Silver-Palladium Alloys

These alloys were introduced as a cheaper alternative to gold alloys.

It is predominantly silver in composition. Palladium (at least 25%) is added to provide nobility and resistance to tarnish.

They may or may not contain copper and gold. They are white in color.

• The properties of the silver-palladium alloys are similar to the Types 3 and 4 gold alloys.
• However, they have lower ductility and corrosion resistance.
• They also have a significantly lower density than gold alloys. This may affect its castability.
• A major difference between Types 3 and 4 Ag-Pd alloys is that the latter can be significantly age-hardened because of its gold and copper content.

Nickel-chrome and Cobalt-Chromium Alloys

These are known as base metal alloys and are extensively used in many of developing countries.

In India, because of their relatively low cost many of the laboratories use these alloys along with resin facings.

• These metals are very strong and hard. Because of this, they are generally difficult to work with (cutting, grinding, polishing, etc.).
• They are dealt with in more detail in subsequent sections.

Titanium And Titanium Alloys

These metals can be used for all metal and metal-ceramic restorations, as well as partial dentures. They are described later under metal-ceramic restorations.

Aluminum-Bronze Alloy

Bronze is an alloy known to man since ancient times.

Traditional bronze is copper alloyed with tin. The ADA-approved bronze does not contain tin.

The composition is as follows:

Being relatively new, the information on these alloys is relatively scanty.

Properties
Color                            –    Yellow gold
Melting range               –    1012–1068 °C
Density                         –    7.8 g/cm 3
Brinell hardness number –    104
Yield strength                –    30,000 psi
Elongation                     –     29%

Metal-Ceramic Alloys

Metal-ceramic alloys are those alloys that are compatible with porcelain and capable of bonding to it.

• A layer of porcelain is fused to the alloy to give it a natural tooth-like appearance.
• Porcelain is a brittle material that fractures easily, so these alloys are used to reinforce the porcelain.
• Several types of alloys are used to cast substructures for porcelain-fused-to-metal crowns and FDPs.
• They may be noble metal alloys or base metal alloys (see classification).
• All have coefficient of thermal expansion (CTE) values that match that of porcelain.
• Note CTE has a reciprocal relationship with a melting point, i.e. the higher the melting point of a metal, the lower its CTE.

Metal-Ceramic Alloys Synonyms

Porcelain-fused-to-metal (PFM), cream metal alloys, porcelain-bonded-to-metal (PBM). The preferred term, however, is metal ceramic or PFM.

Evolution Of Metal-Ceramic Alloys

• The metal-ceramic alloys evolved from the resin-veneered crown and bridge alloys.
• Resin faced the problem of gradual wear and had to be replaced over time.
• Besides resin could not be used on the occlusal surface.
• To retain a resin-veneered restoration undercuts had to be provided.
• The early metal-ceramic alloys were high gold alloys (88% gold).
• They were not strong enough for FDP use.
• In the early days before porcelain-metal bonding was clearly understood, porcelain had to be retained by mechanical means with the help of undercuts.
• Later, it was discovered that adding 1% of base metals like iron, tin, indium, etc.
• Induced chemical bonding by the formation of an oxide layer.
• This significantly improved the bond strength between porcelain and metal.

Requirements Of Alloys For Porcelain Bonding

In addition to the general requirements of alloys mentioned earlier, metal-ceramic alloys have certain specific requirements in order to be compatible with porcelain veneering.

• Its melting temperature should be higher than porcelain filling temperatures.
• It should be able to resist creep or sag at these temperatures.
• Its CTE should be compatible with that of porcelain.
• They should be able to bond with porcelain.
• It should have a high stiffness (modulus of elasticity). Any fixing of the metal framework may cause the porcelain to fracture or delaminate.
• It should not stain or discolor porcelain.

Uses Of Metal-Ceramic Alloys

• As the name implies these alloys are intended for porcelain veneered restorations.
• They can also be used for all-metal restorations.

Types (Classification) Of Metal-Ceramic Alloys

Alloys for metal ceramics restorations may be categorized as

• High noble (commonly referred to as gold alloys)
  • Gold-palladium-platinum alloys
  • Gold-palladium-silver alloys
  • Gold-palladium alloys
• Noble (commonly referred to as palladium alloys)
  • Palladium-silver alloys
  • Palladium-gallium-silver alloys
  • Palladium-gold alloys
  • Palladium-gold-silver alloys
  • Palladium-copper alloys
  • Palladium-cobalt alloys
• Base metal
  • Nickel-chromium alloys
  • Nickel-chromium-beryllium alloys
  • Cobalt-chromium alloys
  • Pure titanium
  • Titanium-aluminum-vanadium

The High Noble Metal-Ceramic Alloys

The high noble alloys contain more than 40 wt% gold and are therefore also referred to as gold alloys or gold-based alloys.

Common Features Of High Noble (Gold-Based) Alloys

• Cost: These are the most expensive crown and bridge alloys. However, in spite of the cost, these alloys are user-friendly and are preferred in practices where the clientele can afford the cost.
• Color: The color can range from white to gold depending on the gold content. The whitening alloys are palladium and platinum. The gold color when present can enhance the vitality of the porcelain thus improving the esthetics.
• Melting range: Porcelain is found at a temperature of 900–960 °C. Thus obviously these alloys must have melting temperatures much higher than the temperatures at which porcelain is found. Pure gold has a melting temperature of 1063 °C. The melting temperature is raised by the addition of platinum (1769 °C) and palladium (1552 °C).
• The melting temperatures of these alloys range from 1149–1304 °C.
• Density: Ranges from 13.5 to 18.3 g/cm³ (depending on the gold content). Because of the high gold and noble metal content, these alloys have a high density. The density reduces as more palladium is added.
• Castability: The high density of these alloys makes them easy to cast. If done well one can expect most of the fie features to be accurately duplicated.
• Yield strength: Ranges from 450 to 572 MPa.
• Hardness and workability: Ranges from 182 to 220 VHN. These alloys are relatively softer when compared to the base metal alloys and so are extremely easy to work with. They are easy to cut, grind, and polish.
• Percent elongation: Ranges from 5 to 20%. This gives an indication of the ductility of the alloy. The higher the value, the more ductile it is.
• Porcelain bonding: The presence of an oxide layer on the surface of metal-ceramic alloys assists in the chemical bonding of porcelain to the alloy. Pure noble metal alloys rarely form an oxide layer. To induce the formation of an oxide layer, 1% of base metals like tin, indium, iron, etc. are added to the alloy. This significantly improved the bond strength between the porcelain and the metal.
• Sag resistance: During porcelain firing, the metal frame has to withstand temperatures as high as 950 °C. At these temperatures, there is a danger of the metal substructure sagging under its own weight, thereby deforming. The longer the span, the greater the risk. The ability of a metal to resist sag is known as sag resistance. Compared to base metal alloys, gold alloys are less sag-resistant.
• Tarnish and corrosion: Because of their high noble metal content, these alloys are extremely stable in the oral environment. Noble metals have low reactivity to oxygen and therefore do not tarnish easily.
• Biocompatibility: High noble alloys have had a good and safe track record. They are not known to cause any problems in the mouth.
• Reusability: These alloys are stable so scrap from these alloys can be recast at least two or three times. However, the more volatile base metals like zinc, indium, tin, etc., may be lost. To compensate for this, equal amounts of new alloys should be mixed. The scrap should be cleaned by sandblasting and ultrasonic cleaning before use. Alloys from different manufacturers should not be mixed as it may change their composition and properties.
• Scrap value: The high noble alloys have a good scrap value. Many suppliers and manufacturers accept used alloy scrap.
• Soldering: Gold-based alloys are quite easy to solder.

Types The following three types will be briefly described:

• Gold-palladium-platinum alloys
• Gold-palladium-silver alloys
• Gold-palladium alloys

Commercial names Some of the available alloys are presented in Table.

Gold-Palladium-Platinum Alloys

Composition

Sag resistance These alloys have a slightly lower sag resistance. Therefore, long-span FDPs should be avoided with this alloy.

Gold-Palladium-Silver Alloys

Composition

Silver has a tendency to discolor some porcelains.

Gold-Palladium Alloys

Composition

The absence of silver eliminates the discoloration problem.

Noble  Metal-Ceramic Alloys

By definition, these alloys must contain at least 25% of noble metal alloy. Currently, noble metal-ceramic alloys are mostly palladium-based.

• The high cost of gold prompted the development of cheaper base metal alloys.
• Unfortunately, many soon became disillusioned because of the difficulty of working with these alloys (poor castability and high hardness).
• The palladium-based alloys were developed during this period.
• Their properties were between that of the high noble alloys and the base metal alloys. They also had good scrap value.

Common Features Of Palladium-Based (Noble) Alloys

• Cost: Their cost range between that of the gold alloys and the base metal alloys.
• Color: They are white in color. Density They are less denser than the gold alloys (10.5–11.5 g/cm³).
• Castability: These alloys have a lower density than the gold alloys and so do not cast as well. However, they are better than the base metal alloys in this regard.
• Workability: Like the gold alloys these alloys are extremely easy to work with. They are easy to cut, grind, and polish.
• Melting range: A typical melting range is 1155–1304 °C. The melting range of these alloys like the gold ceramic alloys are high. This is desirable to ensure that these alloys do not melt or sag during porcelain firing.
• Yield strength: Ranges from 462 to 685 MPa. These compare favorably with the high noble ceramic alloys which in turn compare favorably to the Type IV alloys.
• Hardness: Ranges from 189 to 270 VHN. They tend to be slightly harder than the high noble metal-ceramic alloys.
• Percent elongation: Ranges from 10 to 34%. This gives an indication of the ductility of the alloy. The higher the value the more ductile it is.
• Porcelain bonding: Like gold alloys, base metals like tin, indium, etc. are added to enhance porcelain bonding.
• Tarnish and corrosion: Because of their high noble metal content, these alloys are extremely stable in the oral environment.
• Scrap value: The palladium-based alloys have a good scrap value. Many suppliers and manufacturers accept used alloy scrap.
• Biological considerations: These alloys are very safe and biocompatible. Some concerns have been expressed over the copper content.

Palladium-Based (Noble) Alloys Types The following are the palladium-based alloys.

• Palladium-silver alloys
• Palladium-copper alloys
• Palladium-cobalt alloys
• Palladium-gallium-silver alloys
• Palladium-gold alloys
• Palladium-gold-silver alloys

Brand names The representative alloys are presented in the table.

Palladium-Silver Alloys These alloys were introduced in the 1970s as an alternative to gold and base metal alloys. Their popularity has declined a little because of the greening problem.

Composition

• Esthetics (greening) The high silver content causes the most severe greening (greenish-yellow discoloration) problem among all the metal-ceramic alloys.
• This must be kept in mind when using it for anterior teeth. Some manufacturers have provided special agents to minimize this effect (gold metal conditioners and coating agents).
• Another alternative is to use special non-greening porcelain.

Palladium-Copper Alloys

These are relatively new alloys. Little information is available regarding their properties.

Composition

• Esthetics Copper can cause a slight discoloration of the porcelain but is not a major problem.
  • During the oxidation firing the metal acquires a dark brown almost black oxide layer.
  • Care should be taken to mask this completely with opaquer. Also of concern is the dark line which develops at the margins.
• Castability These alloys are technique sensitive. Slight errors can lead to faulty castings.

Palladium-Cobalt Alloys

Composition

• Esthetics: Cobalt can cause some insignificant discoloration. However, more care should be taken for masking the dark oxide layer with opaque.
• Sag resistance: They are the most sag-resistant of all the noble alloys.

Palladium-Gallium Alloys

There are two groups of these alloys, viz. the palladium-gallium-silver and the palladium gallium-silver-gold.

Palladium-Gallium Alloys Composition

Esthetics The oxide layer though dark is still somewhat lighter than the palladium copper and palladium cobalt alloys. The silver content does not cause any greening.

Base Metal Alloys For Metal-Ceramic Restorations

Alloys that contain little or no noble metals are known as base metal alloys.

• As mentioned earlier, these alloys were introduced as a cheaper alternative to the more expensive noble metal-ceramic alloys.
• In countries like the USA, Western Europe, and the oil-rich Middle-Eastern states, there is a preference for noble and high noble metal-ceramic alloys.
• In contrast, developing countries have shown a preference for base metal-ceramic alloys.
• This is because the economic concerns far outweigh the advantages of the more user-friendly high noble alloys.
• The first base metal alloys were the cobalt-chromium alloys primarily used for removable partial denture alloys.
• The nickel-chrome alloys were introduced later. The latest in the series are titanium and its alloys.
• Like the gold alloys, the base metal alloys can be used for many purposes.
• However, one must differentiate between the ones used for all-metal and metal-ceramic restorations.
• Obviously, the metal-ceramic alloys would be formulated with specific properties since they are to be used with ceramics.

Base metal alloys used for metal ceramics include

• Nickel-chromium (nickel-based) alloys
• Cobalt-chromium (cobalt-based) alloys
• Pure titanium
• Titanium-aluminum-vanadium alloys

Commercial names Trade names of some metal-ceramic alloys are presented in Table.

Nickel-Chromium Alloys

• Although cobalt-chromium alloys are used for metal-ceramic crowns and FDPs, many laboratories prefer to use nickel-chromium alloys.
• For this reason, the discussion will focus mostly on these alloys. Cobalt-chromium will be discussed later under alloys for removable dentures.
• Representative commercial alloys are shown.

Nickel-Chromium Alloys Composition

Basic elements

(Some alloys occasionally contain one or more minor elements).

The minor additions include

(Functions of the ingredients are described under removable partial denture alloys).

General Properties Of Nickel-Based Alloys

• Cost: They are the cheapest of the casting alloys.
• Color: They are white in color.
• Melting range: A typical melting range is 1155–1304 °C. The melting range of these alloys like the gold ceramic alloys is high.
• Density: Ranges from 7.8 to 8.4 g/cm³. They have just half the density of the gold alloys making them much lighter. One can get more castings per gram compared to the gold alloys.
• Castability: They are extremely technique sensitive. One reason may be their lower density compared to the gold alloys.
• Hardness and workability: Ranges from 175 to 360 VHN. They tend to be much harder than the high noble metal ceramic alloys. Unlike gold alloys, these alloys are extremely difficult to work with in the laboratory. Their high hardness makes them very difficult to cut (sprue cutting), grind, and polish. In the mouth, more chair time may be needed to adjust the occlusion. Cutting and removing a defective crown or FDP can be quite demanding. The high hardness results in rapid wear of carbide and diamond burs.
• Yield strength: Ranges from 310 to 828 MPa. These alloys are stronger than the gold and palladium-based alloys.
• Modulus of elasticity: Ranges from 150 to 218 GPa. This property denotes the stiffness of the alloy. Base metal alloys are twice as stiff as gold ceramic alloys. Practically, this means that we can make thinner, lighter castings or use it in long-span FDPs where other metals are likely to fail because of fixing. Gold alloys require a minimum thickness of at least 0.3–0.5 mm, whereas base metal alloy copings can be reduced to 0.3 mm (some even claim 0.1 mm).
• Percent elongation: Ranges from 10 to 28%. This gives an indication of the ductility of the alloy. Though they may appear to be ductile, these alloys, however, are not easily punishable. This may be related to additional factors like the high hardness and yield strength.
• Porcelain bonding: These alloys form an adequate oxide layer which is essential for successful porcelain bonding. However, occasionally the porcelain may delaminate from the underlying metal. This has been blamed on a poorly adherent oxide layer which occurs under certain circumstances that have not been fully understood.
• Sag resistance: These materials are far more stable at porcelain filling temperatures than gold-based alloys. They have a higher sag resistance.
• Esthetics: A dark oxide layer may be seen at the porcelain metal junction.
• Scrap value: As may be expected these alloys have poor scrap value because of the low intrinsic value of the elements.
• Tarnish and corrosion resistance: These alloys are highly resistant to tarnish and corrosion. This is due to the property known as passivation. Passivation is the property by which a resistant oxide layer forms on the surface of chrome-containing alloys. This oxide layer protects the alloy from further oxidation and corrosion. These alloys can maintain their polish for years. Other self-passivating alloys are titanium and aluminum.
• Soldering: Soldering is necessary to join bridge parts. Long-span bridges are often cast in two parts to improve the fi and accuracy. The parts are assembled correctly in the mouth and an index is made. The parts are then reassembled in the laboratory and joined together using solder. Base metal alloys are much more difficult to solder than gold alloys.
• Casting shrinkage: These alloys have a higher casting shrinkage than gold alloys. Greater mold expansion is needed to compensate for this. Inadequate compensation for casting shrinkage can lead to poorly fitting casting.
• Etching: Etching is necessary for resin-bonded restorations (e.g. Maryland bridges) to improve the retention of the cement to the restoration. The etching of base metal alloys is done in an electrolytic etching bath.
• Biological considerations: Nickel may produce allergic reactions in some individuals. It is also a potential carcinogen. Beryllium which is present in many base metal alloys is a potentially toxic substance. Inhalation of beryllium-containing dust or fumes is the main route of exposure. It causes a condition known as ‘berylliosis’. It is characterized by fl-like symptoms and granulomas of the lungs.
• Precautions: Adequate precautions must be taken while working with base metal alloys. Fumes from melting and dust from grinding alloys should be avoided (wear a mask). The work area should be well-ventilated. Good exhaust systems should be installed to remove the fumes during melting.

Casting Investments For Metal Ceramic Alloys

• Due to the high melting temperature of these alloys, only phosphate-bonded or silica-bonded investments are used.
• However, in the case of gold-based metal-ceramic alloys, carbon-containing phosphate-bonded investments are preferred.

Titanium And Its Alloys For Metal-Ceramic Applications

Titanium in the form of the oxide rutile, is abundant in the earth’s crust. The ore can be referred to as metallic titanium using a method called Kroll’s process.

• Titanium and its alloys have been available to the dental profession since the 1970s.
• Historically, titanium has been used extensively in aerospace, aeronautical, and marine applications.
• Because of its high strength and rigidity, its low density and corresponding low weight, its ability to withstand high temperatures, and its resistance to corrosion.
• The use of titanium for medical and dental applications has increased dramatically in recent years.
• Over the past three decades, the development of new processing methods computer-aided machining and electric discharge machining has expanded titanium’s useful range of applications in biomedical devices. Titanium has become available for use in metal ceramics.
• It is also used for removable partial denture alloy frames and of course commercial implants. It has been adopted in dentistry, because of its excellent biocompatibility, lightweight, good strength, and ability to passivate.

Titanium Uses

• In dentistry
  • Metal-ceramic restorations.
  • Dental implants.
  • Partial denture frames.
  • Complete denture bases.
  • Bar connectors.
  • Titanium mesh membranes (Tiomesh) are used in bone augmentation.

(In dentistry, it is especially useful as an alternative alloy to those who are allergic to nickel).

• In surgery
  • Artificial hip joints.
  • Bone splints.
  • Artificial heart pumps.
  • Artificial heart valve parts.
  • Pacemaker cases.

Astm Grades Of Titanium

ASTM International (the American Society for Testing and Materials) recognizes four grades of commercially pure titanium (CpTi) and three titanium alloys (Ti-6Al-4V, Ti-6Al-4V extra-low interstitial [low components], and Ti-Al-Nb).

Supplied As

Ingots weighing 18–40 g (height of 11.8–16.8 mm) in 1 kg boxes.

Representative products

• Rematitan M (Dentaurum) — Grade 4
• Tritan (Dentaurum) — Grade 1

Properties Of Commercially Pure Titanium

Phases: In its metallic form at ambient temperature, titanium has a hexagonal, close-packed crystal lattice (α phase), which transforms into a body-centered cubic form (β-phase) at 883 °C. The phase is susceptible to oxidation.

• Color: It is a white color metal.
• Density: It is a lightweight metal (density 4.5 g/cm 3) when compared to nickel chrome (8 g/cm³) and gold alloys (15 g/cm³).
• Modulus of elasticity: Its modulus of elasticity is 110 Gpa which makes it only half as rigid as base metal alloys. However, this appears to be sufficient for most dental uses.
• Melting point: Its melting point is quite high (1668 °C). Special equipment is needed for casting titanium.
• Yield strength: Varies from 460 to 600 MPa.
• Tensile strength: Varies from 560 to 680 MPa.
• Coefficient of thermal expansion: This is an important property when it is used as a metal-ceramic alloy. When used as a metal-ceramic alloy the CTE (8.4 × 10–6/ °C) is far too low to be compatible with porcelain (12.7 to 14.2 × 10–6/ °C). For this reason, special low-fusing porcelains have been developed to get around this problem.
• Tarnish and corrosion: Titanium has the ability to self-passivate. The metal oxidizes almost instantaneously air to form a tenacious and stable oxide layer approximately 10 nanometers thick. The oxide layer protects the metal from further oxidation. In addition, the oxide layer allows for the bonding of fused porcelains, adhesive polymers or, in the case of endosseous implants, plasma-sprayed or surface-nucleated apatite coatings.
• Biocompatibility: It is non-toxic and has excellent biocompatibility with both hard and soft tissues.

Fabrication Of Titanium Restorations Titanium structures can be made by

• Casting or
• Machining.

Casting

• The casting of titanium is a challenge because of its high melting temperature, low density, and high reactivity to atmospheric air.
• Machines for casting titanium are generally more expensive than those for other dental casting alloys.
• Dental castings are made via pressure-vacuum or centrifugal casting methods.
• The metal is melted using an electric plasma arc or inductive heating in a melting chamber filled with inert gas or held in a vacuum.
• The inert gas prevents surface reaction with the molten metal.
• Investments with high-setting expansion are used to compensate for the high casting shrinkage of titanium.

Machining

• Dental implants generally are machined from billet stock of pure metal or alloy.
• Dental crowns and FDP frameworks also can be machined from metal blanks via CAD/CAM.
• Abrasive machining of titanium, however, is slow and inefficient, which greatly limits this approach.
• Another method for fabricating dental appliances is electric discharge machining.
• Which uses a graphite die (often reproduced from the working die) to erode the metal to shape via spark erosion.
  

Ceramic Veneering

• Special low-fusing porcelains with fusing temperatures below 800 °C are used with titanium.
• This is because titanium changes to the β-form (at 883 °C) which is susceptible to oxidation.

Advantages And Disadvantages Of Titanium

Advantages

• High strength.
• Lightweight.
• Binary.
• Low tarnish and corrosion because of the ability to passivate.
• Can be laser welded.
• Limited thermal conductivity.

Disadvantages

• Poor castability.
• Highly technique-sensitive.
• Requires expensive machines for casting and machining.
• Low-fusing porcelains (below 800 °C) are required to prevent β phase transformation.

Removable Denture Alloys

Larger structures like complete denture bases and partial denture frames are also made from dental alloys.

• Being larger structures they require more quantities of alloy, which can make them quite heavy and expensive (if gold were to be used).
• Thus it became necessary to develop lighter and more economical alloys.
• Most of the large castings today are made from base metal alloys, occasionally Type 4 gold alloys are used.

Additional Requirements For Partial Denture Alloys

Besides all the earlier mentioned general requirements of casting alloys, RPD alloys have a few special requirements.

• They should be light in weight. Being much larger structures, the lighter weight aids in retention in the mouth.
• They should have high stiffness. This aids in making the casting more thinner. This is important, especially in the palate region, where having a thin palatal portion makes it more comfortable for the patient. The high stiffness prevents the frame from bending under occlusal forces.
• They should have good fatigue resistance. This property is important for clasps. Clasps have to be fixed when inserted or removed from the mouth. If they do not have good fatigue resistance they may break after repeated insertion and removal.
• They should be economical. Large structures would require more metal and therefore the cost of the alloy should be low.
• They should not react to commercial denture cleansers.

Types

• The alloys for removable denture use are
• Cobalt-chromium alloys
• Nickel-chromium alloys
• Aluminum and its alloys
• Type IV noble alloys
• Titanium

Cobalt-Chromium Alloys

• Cobalt-chromium alloys have been available since the 1920s. They possess high strength.
• Their excellent corrosion resistance especially at high temperatures, makes them useful for a number of applications.
• These alloys are also known as ‘satellites’ because of their shiny, star-like appearance.
• They are bright lustrous, hard, strong, and possess non-tarnishing qualities.

Cobalt-chromium alloys Supplied As

• Small ingots (cuboidal, cylindrical shapes) in 1 kg boxes.
• Representative products Wironium plus (Bego), Sheralit imperial (Shera).
  

Cobalt-Chromium Alloys Applications

• Denture base
• Cast removable partial denture framework
• Crowns and field partial dentures
• Bar connectors.

Cobalt-Chromium Alloys Composition

According to ADA Sp. No. 14 a minimum of 85% by weight of chromium, cobalt, and nickel is required.

Functions Of Alloying Elements

Cobalt Imparts hardness, strength, and rigidity to the alloy. It has a high melting point.

Chromium

• Its passivating effect ensures corrosion resistance.
• The chromium content is directly proportional to tarnish and corrosion resistance.
• It reduces the melting point. Along with other elements, it also acts in solid solution hardening
• . 30% chromium is the upper limit for attaining maximum mechanical properties.

Nickel

• Cobalt and nickel are interchangeable.
• It decreases strength, hardness, MOE, and fusion temperature. It increases ductility.

Molybdenum or tungsten

• They are effective hardeners.
• Molybdenum is preferred as it reduces ductility to a lesser extent than tungsten.
• Molybdenum refines grain structure.

Iron, copper, and beryllium

They are hardeners. In addition, beryllium reduces fusion temperature and refines grain structure.

Manganese and silicon

Primarily oxide scavengers prevent oxidation of other elements during melting. They are also hardeners.

Boron

Deoxidizer and hardener, but reduces ductility.

Carbon

• Carbon content is most critical. Small amounts may have a pronounced effect on strength, hardness, and ductility.
• Carbon forms carbides with metallic constituents which is an important factor in strengthening the alloy.
• However, excess carbon increases brittleness. Thus, control of carbon content in the alloy is important.

Properties

The Cobalt-chromium alloys have replaced Type IV gold alloys, especially for making RPDs, because of their lower cost and good mechanical properties.

Density

The density is half that of gold alloys, they are lighter in weight (8 to 9 g/cm³).

Fusion temperature

• Thus casting temperature of this alloy is considerably higher than that of gold alloys (1250 °C to 1480 °C).
• ADA Sp. No. 14 divides it into two types, based on fusion temperature, which is defined as the liquidus temperature.
  • Type-1 (high fusing)—liquidus temperature greater than 1300 °C.
  • Type-2 (low fusing)—liquidus temperature not greater than 1300 °C.

Yield strength

It is higher than that of gold alloys (710 MPa).

Elongation

• Their ductility is lower than that of gold alloys.
• It depends on composition, rate of cooling, and
• The fusion and mold temperatures were employed.
• The elongation value is 1–12%.
• Caution These alloys work harden very easily
• so care must be taken while adjusting the clasp arms of the partial denture.
• They may break if bent too many times.

Modulus of elasticity

• They are twice as stiff as gold alloys (225 × 103 MPa).
• Thus, casting can be made thinner, thereby, decreasing the weight of the RPD.

Hardness

• These alloys are 50% harder than gold alloys (432 VHN).
• Thus, cutting, grinding, and finishing are difficult.
• It wears of the cutting instrument.
• Special hard, high-speed fishing tools are needed.

Tarnish and corrosion resistance (passivation)

• The formation of a layer of chromium oxide on the surface of these alloys prevents tarnish and corrosion in the oral cavity.
• This is called ‘passivating effects.
• Caution Hypochlorite and other chlorine-containing compounds that are present in some denture cleaning solutions will cause corrosion in base metal alloys.
• Even oxygenating denture cleansers will stain such alloys.
• Therefore, these solutions should not be used to clean chromium-based alloys.

Casting shrinkage

• The casting shrinkage is much greater (2.3%) than that of gold alloys.
• The high shrinkage is due to their high fusion temperature.

Porosity

• As in gold alloys, porosity is due to the shrinkage of the alloy and the release of dissolved gases.
• Porosity is affected by the composition of the alloys and its manipulation.

Technical Considerations For Casting Alloys

Based on the melting temperatures of the alloys, we can divide the alloys into high-fusing and low-fusing alloys.

Low-Fusing Alloys

• The gold alloys used for all-metal restorations may be considered as low fusing. Obviously, the technical requirements of these alloys would be different from the high-fusing alloys.
• Investment material Gypsum bonded investments are usually sufficient for low-fusing gold alloys.
• Melting The regular gas-air torch is usually sufficient to melt these alloys.
• High-Fusing Alloys The high-fusing alloys include noble metal-ceramic alloys (gold and palladium alloys) as well as base-metal alloys (all-metal, metal-ceramic alloys, and partial denture alloys).
• Investment material for noble metal alloys The high melting temperatures prevent the use of gypsum-bonded investments. Phosphate-bonded or silica-bonded investments are used for these alloys.
• Investment material for base-metal alloys Phosphate-bonded or silica-bonded investments are also used for these alloys. However, there is one difference. These alloys are very sensitive to a change in their carbon content. Therefore, carbon-containing investments should be avoided when casting base-metal alloys.
• Burnout A slow burnout is done at a temperature of 732–982 °C. It is done two hours after investing.
• Melting The high fusion temperature also prevents the use of gas-air torches for melting these alloys. Oxygen-acetylene torches are usually employed. Electrical sources of melting such as carbon arcs, argon arcs, high-frequency induction, or silicon-carbide resistance furnaces may also be used.

Technique For Small Castings

The wax pattern is usually constructed on a die-stone model. The wax pattern is removed and then invested (for more details see the chapter on casting techniques).

Technique For Large Castings

• The procedure for large castings like RPD frames is slightly more complex.
• Unlike the crown or FDP pattern, the RPD pattern is difficult to remove from the model without distortion and damage.
• Therefore, a modification in the technique is required.
• A duplicate of the model is made using investment material (this is called a refractory cast).
• The wax pattern is constructed on the refractory cast.
• The pattern is not separated from the refractory cast, instead, the refractory cast is invested along with the pattern.
  

Advantages And Disadvantages Of Base Metal Alloys

Advantages Of Base Metal Alloys

• Lighter in weight.
• Better mechanical properties (exceptions are present).
• As corrosion-resistant as gold alloys (due to the passivating effect).
• Less expensive than gold alloys.

Disadvantages Of Base Metal Alloys

• More techniques is sensitive.
• Complexity in the production of dental appliances.
• High fusing temperatures.
• Extremely hard, so requires special tools for finishing.
• The high hardness can cause excessive wear of restorations and natural teeth contacting the restorations.

Comparison Of A Gold Alloy And A Base Metal Alloy

A comparison of the 2 alloys is shown in Table.