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1. Seminar on
Dental casting alloys
 INTRODUCTION because pure metals are apt to be soft and many tends to
corrode rapidly and also because high cost their use is quite limited in dentistry.
To optimize properties, most metals commonly used in dentistry are mixtures of
two or more metallic elements or one or more metal and/or non metals (THE
ALLOY).
 CASTING is one of the most widely used methods for fabrication of metal
restorations out of the mouth. A pattern of lost tooth structure or the dental
prosthesis to be reproduced in metal is constructed in wax. The wax is surrounded
by an investment. After the investment has hardened the wax is removed and the
molten metal is forced into the mold space.
 My seminar deals with these dental casting alloys.
“What we will be tomorrow is because of what we are today, and what we are today is
because of what we were yesterday”.
Somebody haws rightly said that History is the best teacher and a brief description of the
evolution of the currently marketed alloys is appropriate to understand the rationale for
the development of the wide variety of alloy formulations.
TABLE 19-1 ANUSAVICE PG NO 565
1. The technological changes of dental prostheses.
2. Metallurgic advancements.
3. Price changes of noble metals since 1968
Taggart’s presentation to the New York Odontological Group in 1907 on the fabrication
of cast inlay restorations often has been acknowledged as the first reported application of
lost wax technique in dentistry.
The inlay technique described by Taggart was an instant success and it soon lead
to the casting of complex restorations such as inlays, onlays, crowns, fixed partial
dentures and removable partial denture frameworks.
Because pure gold did not have physical properties required for those dental
restorations existing jewelry alloys were quickly adopted. These gold alloys were further
strengthened with copper, silver or platinum.
In 1932 the dental materials Group at the National Bureau of Standards surveyed
the alloys being used and roughly classified them as a Type I, II, III and IV.
In the following years several patents were issued for alloys containing Palladium,
as a substitute for Platinum.

2. By 1948 the composition of dental noble metal alloys for lost metal restorations had
become rather diverse, wirth these formulations, the tarnishing tendency of the original
alloys apparently had disappeared. It is now known that in gold alloys Palladium is added
to counteract the tarnishing potential of silver.
The base metal removable partial denture alloys were introduced in the 1930s. Since
that time, both Ni-Cr and Co-Cr formulati0ons have become increasingly popular
compared with conventional type IV gold alloys, which previously were the predominant
metals used for such prostheses. The obvious advantages of base metal alloys are their
lighter weight, increased mechanical properties and their reduced cost.
Likewise by 1978, the price of gold was climbing so rapidly attention focused on the
noble, metal alloys. To reduce the the precious metal content and yet retain the
advantages of noble metals for dental use.
DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYS:
All casting alloys must first be biocompatible and then exhibit sufficient physical and
mechanical properties to ensure adeqwuate function and structural durability overlong
periods of time.
1. BIOCOMPASTIBILITY: Their material must tolerate oral fluids and must not
release any harmful products into the oral environment.
2. CORROSION RESISTANCE: Corrosion is defined as a chemical or a
electrochemical process in which a solid usually a metal, is attacked by an
environmental agent, resulting in partial or complete dissolution. Metals are
generally more susceptible to such attacks because of electrochemical reactions.
Corrosion resistance is derived either by the component being too noble to react
in the oral environment(E.g.: gold, palladium) or by its ability to form an adherent
passivating surface film which inhibits any subsurface reactions( E.g.: Co-Cr, Ni-
Cr and Co-Cr alloys and Ti alloys in CP Ti and in Ti-6Al-4V alloys).
3. TARNISH RESISTANCE “process by which a metal surface is dulled or
discolored when a reaction with sulfide, oxide, chloride or chemical causes a the
film to form. These films are generally found on gold alloys with relatively high
silver content or on silver alloys.
4. ALLERGENIC COMPONENTS INC ASTING ALLOYS: A restorative material
should not cause adverse health consequences to a patient. Beryllium potentially
toxic under uncontrolled conditions.
5. ESTHETICS : Considerable controversy exists over the optimal balance among
the properties of esthetics, fit, abrasive potential, clinical survivability, and cost
of cast metal prosthesis compared with direct filling restorations, ceramic based
prosthesis(all ceramic and metal ceramic) and resin veneer prosthesis.
6. THERMAL PROPERTIES: For metal ceramic restorations the alloys or metals
must have closely, matching thermal Expansion to be compatible with as given
porcelain, and they must tolerate high processing temperature.
7. MELTING RANGE: Must be low enough to gform smooth surfaces with the
mold wall of the casting investment.

3. 8. COMPENSATION FOR SOLIDIFICATION: Compensation for casting
shrinkage must be achieved either through computer generated oversized dies or
through controlled mold expansion. The fit of the cemented prosthesis must be
tailored to accommodate the alloys of bonding adhesive and the luting cement.
9. STRENGTH REQUIREMWNTS: Alloys for bridgework require higher strength
than alloys for single crowns.
Copings for metal ceramic restorations require sufficient elastic modulus
(stiffness) to prevent elastic deflection from functional forces.
10. CASTABILITY: The molten metal must be able to wet the investment mold
material very well (decreased contact angle) and flow into the most intricate
regions of the mold without any appreciable interaction with the investment and
without forming porosity within the subsur5face ands surface regions.
11. FINISHING OF CAST METALS: Hardness, ductility (percentage elongation)
and ultimate tensile strength are important properties in this regard.
12. PORCELAIN BONDING: A substrate metal must be able to form a thin, adherent
oxide, preferably one that is light in color and also it must have thermal
expansion/contraction coefficients that are closely matched to that of
porcelain.
13. ECONOMIC CONSIDERATIONS: The cost of metals used in restorative
dentistry is a function of the metal density and the cost per unit mass.
CLASSIFICATION OF DENTAL CASTING ALLOYS
“Even the proverbial needle in the haystack can be found if there is a system and
method to the search”
In 1984, the ADA proposed a simple classification for dental casting alloys.
BOX NO 19-1 ANUSAVICE PG 570.
Cast dental alloys can be classified according to the following five categories:
1. USE (All metal inlays, crowns and bridges, metal and ceramic prostheses, posts
and cores, removable partial dentures, implants).
2. MAJOR ELEMENTS :( Gold based, palladium based, silver based, nickel based,
cobalt based and titanium based)
3. NOBILITY: High noble, noble, predominantly base metal.
4. PRINCIPAL THREE ELEMENTS: E.g.: Au-Pd-Ag
Pd-Ag- Sn
Ni-Cr-Be
Co-Cr-Mo etc
4. DOMINANT PHASE SYSTEM: Single phase, Eutectic, Peritectic, and
Intermetallic
Mechanical properties (ANSI/ ADA Specifications, 1997)- Table 19-2 pg 571 anusavice.
MICROSTRUCTURE OF ALLOYS

4. METAL: An element whose atomic structure rapidly loses electron to form positively
charged ions, and which exhibits metallic bonding, opacity good light reflectance from a
poloished surface, and high electrical ands thermal conductivity.
Of the 103 elemnts listed oinm the periodic table 80 are classed as metals and they
exhibit the following properties:
1. Metallic luster
2. Metallic ring.
3. Harder, stronger and denser than other elements
4. Solids at room temperature( Exception, mercury and gallium which are liquids at
room temperature and hydrogen which is a gas at room temperature).
5. Good conductors of heat and electricity.
6. Opaque
7. Ductile and malleable
8. Electropositive
ALLOY: A crystalline substance with metallic properties that is composed of two or
more chemical elements, at least one of which is a metal.
ALLOY SYSTEM is an aggregate of two or more metals in all combinations.
In order to specify a particular alloy it is necessary to list the metals or elements
present in the alloy and the amount of each element present. Two methods are
available.:
1. The weight percentageof each element.
2. The atomic fraction or percentage.
The properties of an alloy relate more to the atomic percentage.
PHASE DIAGRAMS(CONSTITUTION DIAGRAM).
A graph of the phase field limits as a function of temperature and composition. Phase
diagrams usually represent equilibrium conditions.
USES: They show the phases that are present in an alloy system for different
compositions and temperatures.
DIAGRAM:Fig 6-3 pg 126 and fig 6-4 pg 127 anusavice.
CLASSIFICATION OF ALLOYS BASED ON THEIR MISCIBILITY:
1. SOLID SOLUTION SUBSTITUTIONAL AND INTERSTITIAL TYPE.
2. CORING
3. EUTECTIC ALLOYS
4. PERITEC SYSTEM
5. INTERMETALLIC COMPOUNDS
PROPERTIES OF DIFFERENT ALLOYING ELEMENTS:
GOLD:
1. Pure gold is soft, malleable, ductile metal that does not oxidize under atmospheric conditions
and is attacked by only a few of the most powerful oxidizing agents.
2. It has a rich yellow color with a strong metallic luster.
3. Although it is the most malleable and ductile metal, it ranks much lower in strength.

5. 4. The pure metal fuses at 1060 degree Celsius
5. Small amounts of impurities have a pronounced effect on mechanical properties of gold and
its alloys
6. The presence of les than 2% lead will cause the metal to become extremely brittle.
7. Mercury in small quantities also has a harmful effect on its properties.
8. Gold is nearly as soft as lead with the result that in dental alloys it must be alloyed with
copper, silver, platinum and other metals to develop the necessary hardness, durability And
elasticity
9. The specific gravity of pure gold is between19.30 and 19.33 making it one of the heavy
metals
10. Air or water at any temperature doesn’t tarnish gold.
11. It is not soluble in sulfuric, nitric or hydrochloric acids.
13.BHN of 25
14. boiling point of 2970 degree Celsius
15.Linear coefficient of thermal expansion 0.142
The puritiy of gold is expressed in karat or fineness
Karat refers to parts of pure gold in 24 parts of gold alloy
Fineness refers to parts of pure gold in 1000 parts of gold alloy.
PALLADIUM:
1. Palladium is not used in pure state in dentistry but it is used in many dental alloys
combined with either gold or silver.
2. It is cheaper than platinum and since it imparts many of the properties of platinum to
dental alloys it is often used as a replacement for platinum.
3. Platinum is a white metal some what darker than platinum.
4. Its specific gravity is 11.4 i.e., about half that of platinum and a little more than half of
gold.
5. It is a malleable and ductile metal with a melting point of1555 degree Celsius which is
the lowest of the platinum group of metals.
6. It hardens the alloy, imparts it whiter color and compensates the reddening effect of
copper. Increase the melting point of the alloy and renders silver tarnish resistant.
SILVER:
1. Silver is malleable, ductile, white in color and best known for its conduction of
heat and electricity. It is stronger and harder than gold but softer than copper.
2. Melting point of 960.5 degree Celsius
3. It combines with sulfur, chlorine and phosphorus or their vapors
4. Pure silver is seldom employed in dental restorations because of the black sulfide
formation on themetal in the mouth although it is used as small additions to many
gold alloys.
5. Addition of palladium to silver containing alloys prevents the rapid corrosiojn of
such alloys in the oral environment.
6.Silver increases the hardness slightly,whitens the alloy to over come the reddening
effect of copper. Molten silver can dissolve oxygen and cause porosity in the casting and
silver can encourage corrosion.

6. COPPER
1.Hardens the alloy.
2.Reduces the melting point of alloy.
3. Reduces the density of the alloy
4. Excessive copper renders the alloy more susceptible to tarnish and corrosion and
reddens the alloy.
ZINC:
1. It is an oxide scavenger during melting of the alloy for casting procedure.
2. in the absence of zinc silver absorbs oxygen at high temperature from the
atmosphere. This oxygen is rejected during solidification tending to produce
porosity in the casting.
IRIDIUM, RUTHENIUM AND RHODIUM:
Iridium is a hard metal that is quite brittle white with a high specific gravity of 22.42 and
an exceptionally high melting point of 2440 degree Celsius.
1.As little as 0.005% of Iridium is effective in refining the grain size of cast gold alloys.
2. Ruthenium produces a similar effect.
GALLIUM:
Used mainly in silver free alloys to compensate for the the decreased thermal expansion
seen in silver free alloys. (Silver is avoided in metal ceramics as it has as greening effect)
IRON,TIN:
Increases the hardness. Also provides an oxide coat which improves bonding of porcelain
to alloy.
HEAT TREATMENT OF HIGH NOBLE AND NOBLE METAL ALLOYS:
HOMOGENIZATION:
The cast alloy is held at a temperature near its solidus to achieve the maximum amount of
diffusion without melting (up to a period of 6 hours in some instances)
This treatment allows atomic diffusion to occur which eliminates as-cast compositional
nonuniformity. This treatment results in:
1. Increase in tarnish and corrosion resistance.
2. Increase in the ductility of the alloy.
SOLUTION HEAT TREATMENT:
It involves heating the casting to a temperature below the solidus(usually 700 degree
Celsius) , holding for a short period of time(typically 10 min) so that the alloy returns to
random substitutional solid solution , and then quenching to retain this atomic
arrangement at room temperature.
The tensile strength, hardness and proportional limit are reduced by such a treatment
but the ductility is increased. This treatment is indicated for structures that are to be
ground, shaped or otherwise cold worked, either in or out of the mouth.

7. HARDENING HEAT TREATMENT:
The age hardening of the dental alloys can be accomplished in several ways. One of the
most practical hardening treatments is by SOAKING or AGEING the casting at a specific
temperature for a definite time,usually 15 to 30 minutes, before it is water quenched. The
ageing temperature depends on the alloy composition but is generally between 200 and
450 degree Celsius.
This treatment is indicated for metallic partial dentures, bridges and other similar
structures.
CASTING SHRINKAGE:
All metals and alloys of practical dental interest shrink when they change from liquid to
solid state. This occurred in three stages:
1. The thermal contraction of the liquid metal between the temperature to which it is
heated and the liquidus.
2. The contraction of the metal inherent in its change from the liquid to the solid
state.
3. The thermal contraction of the solid metal that occurs on further cooling to room
temperature.
TABLE FOR CASTING SHRINKAGE:Pg 577, ANUSAVICE.
This casting shrinkage must be compensated for by adequate casting technique(
selection of proper investment material which compensates the shrinkage and yet will be
able to withstand the fusion temperature of the alloy, for example)
SILVER PALLADIUM ALLOYS:
These alloys are white and predominantly silver in composition but have substantial
amounts of palladium(at least 25%)
that provides nobility and promote tarnish resistance. They may or may not have copper
and a small amount of gold.
ADVANTAGES:
1. Adequate strength properties.
2. Acceptable castability
3. Low cost
LIMITATION:
1. Great potential for tarnish and corrosion.
HIGH NOBLE AND NOBLE ALLOYS FOR METAL-CERAMIC PROSTHESIS
The chief objections to the use of dental porcelain as a restorative material are its low
strength under tensile and shear conditions. A method by which this disadvantage can be
minimized is to bond the porcelain directly to a cast alloy substructure made to fit the
prepared tooth.
In spite of vastly different chemical compositions all such alloys must share at least
three common features:
1. They must have the potential to bond to dental porcelain.
2. They posses coefficients of thermal contraction compatible with those of dental
porcelains.

8. 3. Their solidus temperature is sufficiently high to permit application of low fusing
porcelains.
4. the coefficients ofd thermal expansion tend to be reciprocal to melting point of the
alloys.
5. high sag resistance.
SAG DEFORMATION Fig 19.1
GOLD PALLADIUM SILVER ALLOYS (LOW SILVER CONTENT)
ADVANTAGES:
1. Economical
2. Excellent resistance to tarnish and corrosion
3. Relative freedom from technique sensitivity
DISADVANTAGE:
The potential for porcelain discoloration when silver vapor is released.
GOLD- PALLADIUM-SILVER ALLOYS (HIGH SILVER CONTENT)
Gold alloys that contain 12% Ag or more account for approximately 20% of the current
alloy market.
ADVANTAGES:
1. Lower cost
2. Favorable physical properties
DISADVANTAGE:
Potential for porcelain discoloration.
GOLD PALLADIUM ALLOYS:
The first alloy of this type was introduced in 1977 by J.F.Jelenko and Co. This alloy was
designed to overcome the porcelain discoloration effect (because it is silver free) and also
to provide an alloy with a lower thermal contraction coefficient than that of either Au-Pd-
Ag or Pd-Ag alloys. Their gold content varies from 45 to 52% and palladium content
varies between 37-45%.
A slight thermal contraction mismatch is recommended to develop compressive hoop
and axial stresses in porcelain which are protective in nature. However, significantly
higher mismatches may lead to porcelain cracking or metal-ceramic bond failure because
of the development of tensile stresses which exceed the tensile strength of porcelain or
the strength of the metal ceramic bond. Another cause is the development of radial tensile
stresses that exceed the tensile strength of porcelain.
ADVANTAGES: These alloys are considered nearly ideal because:
1. Contain no silver
2. Their surface oxide layer is virtually indiscernible
3. Their sag resistance is better than that of Au-Pt-Pd alloys
4. Their castability, corrosion resistance and adherence to porcelain are excellent
5. Cost effective
PROPERTIES:
1. HV of about 200
2. Yield strength of 570 MPa

9. 3. Elongation of 20%
PALLADIUM GOLD ALLOYS:
Relatively few alloys are available in the market due to the price volatility of palladium
and their laboratory and clinical performance has not been adequately documented.
PALLADIUM-GOLD-SILVER ALLOYS:
These have a potential for porcelain discoloration. Gold content is from 5-32% and silver
content from 6.5-14%.
These alloys have a range of thermal contraction coefficients that increase with an
increase in silver content.
PALLADIUM-SILVER ALLOYS:
This alloy type was introduced in the market in 1974 as the first gold free noble alloy
available for metal ceramic restorations
The Pd content is 53-61% and 28-40% Ag in addition to tin and/or indium.
ADVANTAGES:
1. The low specific gravity and their low cost make them attractive economic
alternatives to gold based alloys.
2. Adequate physical properties
3. Alloys of this type are easy to polish and burnish
4. Adherence to porcelain is acceptable although a predominantly mechanical type
of bonding has been suggested for this alloy.
DISADVANTAGES:
1. Silver discoloration effect is most severe for these alloys. Gold metal conditioners
or ceramic coating agents may minimize this effect. In addition many of today’s
porcelains are formulated to minimize this problem.
PALLADIUM-COPPER-GALLIUM ALLOYS:
First introduced in 1983, these alloys were very popular in 1990s. However the price
volatility of palladium required dentists to use other alloys. A brown or black oxide layer
is formed during oxidation and subsequent porcelain firing cycles. Because of all these
factors these alloys have not been well accepted in the dental practice.
PALLADIUM-GALLIUM-SILVER ALLOYS:
These alloys have a slightly lighter oxide layer than Pd-Cu alloys and they are thermally
compatible with lower expansion porcelains. In addition they have a coparatively low
hardness which allows the alloy to be adjusted in the dental lab or the chair side.
DISCOLORATION OF PORCELAIN BY SILVER:
1. The colloidal dispersion of silver atoms entering the body and incisal porcelain or
the glazed surface from vapour transport or surface diffusion may cause color
changes including green, yellow-green, yellow-orange, orange and brown hues.
This phenomenon is termed GREENING.

10. 2. Porcelains with higher sodium content are believed to exhibit more intense
discoloration because of mmore rapid silver diffusion in sodium containing glass.
3. The intensity of discoloration increases for higher silver content alloys, is more in
the cervical region, lighter shades, multiple firing procedures and certain brands
of porcelain and also in silver free alloys due to vaporization of silver from the
walls of contaminated furnaces.
PREVENTION OF DISCOLORATION:
1. Use of ultra low fusing porcelain or non greening porcelain
2. A pure gold film can be fired on a metal substrate to reduce the surface silver
concentration.
3. A ceramic conditioner can be fired as a barrier between the alloy and the
porcelain.
4. Use of a graphite block routinely to maintain a reducing atmosphere.
THERMAL COMPATIBILITY AND INCOMPATIBILITY OF METAL-CERAMIC
SYSTEMS
Thermal compatibility refers to the ability of a metal and its veneering porcelain to
contract at similar rates during cooling from the ceramic sintering temperature (>871
degree Celsius for low fusing porcelains and <871 degree Celsius for ultra low fusing
porcelains). FIG 19-4 DESCRIBE IN DETAIL
Stresses develop because of the difference in the thermal coefficients between metal
and porcelain as a prosthesis is cooled below the glass transition temperature of
porcelain. For today’s porcelain this temperature lies within the range of 500 and 650
degree Celsius.
It is also possible to develop failure level stresses in near the metal porcelain junction
when the contraction coefficient of the porcelain is much lower than that of the metal.
Incompatibility failures may result with compatible systems when atypical cooling
rates, excessive porcelain metal thickness ratios, when improper framework or coping
geometries are used, or when the number of cycles exceeds the number recommended by
the manufacturer.
Leucite(K2O.Al2O3.4SWiO2) is the principal high expansion microstructural
component of dentalporcelains and its presence may cause large increases in the
contraction coefficients of porcelain when more than five firing cycles are necessary.

12. PHYSICAL PROPERTIES OF HIGH NOBLE AND NOBLE ALLOYS
Table 19-7 pg 594 anusavice
BASE METAL ALLOYS FOR CAST METAL AND METAL CERAMIC
PROSTHESIS
The no of dental laboratories using base metal alloys steadily increased through 70s and
80s. Although the increased acceptance of these alloys during this period was greatly
influenced by the price fluctuation of the noble metals, the trend continued through 90s
even when the prices of noble metals had come down. The Ni-Cr-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 chromium and/or cobalt chromium alloys
in dentistry
1. Nickel is combined with Chromium to form a highly corrosion resistant alloy
2. Cost effectiveness
3. Alloys such as Ticonium 100 have been used in re4movable partial denture
frameworks for many years with few reports of allergic reactions.
4. Although Beryllium is a toxic metal, dentists and patients should not be affected
because the main risk occurs mainly in the vapor form which is a concern for the
technician.
5. Nickel alloys have excellent mechanical properties such as high elastic modulus,
high hardness, high sag resistance and a reasonably high elongation (ductility)
6. Lower density
On the other hand it is also important the realize the limitations of these alloys,
particularly Vis-a –Vis metal ceramic restorations:
1. These alloys are more difficult to cast and presolder
2. The ability to obtain acceptable fitting castings may require special procedures to
adequately compensate for the higher solidification shrinkage
3. Potential for porcelain delamination as a result of separation of poorly adherent
oxide layer from the metal substrate.
4. Finishing and polishing require special procedures and is not easy either in the lab
or at chairside.
5. Removal of defective restorations may take time.
6. Repair of crowns with fractured porcelain veneers which may be simply
performed on noble metal substrates using pin-retained facings or metal ceramic
onlays, is more difficult to accomplish in base metal frameworks.
COMPOSITION AND PROPERTIES OF BASE METAL ALLOYS: TABLE 19-8,19-9
Pg 599 Anusavice.
BIOLOGICAL HAZARDS AND PRECAUTIONS
Lab technicians may be exposed occasionally or routinely to excessively high
concentrations of beryllium and nickel dust and beryllium vapour. Although the amount

13. of beryllium rarely exceeds 2% by weight, the atomic concentration of beryllium is
around 10.7%. The risk for beryllium vapour exposure is grteatest to dental technicians
during alloy melting , especially in the absencedof an adequate exhaust and filtration
system. The Occupational Health and Saftey Administration (OSHA) specifies that the
exposure to beryllium dust in air should be limited to a particulate beryllium
concentration of 2micrograms/m3 of air ( both respirable and nonrespirable particles)
determined from an 8 hr time weighted average. The allowable maximum concentration
is 5microgram/m3(not to be exceeded for a 15 min period). The National Institute for
Occupation Safety and Health (NIOSH) recommends a limit of 0.5 micrograms /m3
based on a 130 min sample. Moffa et al reported that when a local exhaust system was
used the cocn of beryllium was reduced to safe levels.
Physiologic responses to beryllium vary from contact dermatitis to severe chemical
pneumonitis which can be fatal. Symptoms may range from coughing, chest pain and
general weakness to pulmonary dysfunction.
ALLERGY POTENTIAL OF NICKEL:
Of greater concern to dental patient is intra oral exposure to nickel, especially for patient
with a known allergy to this element. Inhalation, ingestion and dermal contact on nickel
or nickel containing alloys are common because nickel is found in environment sources
such as air , soil and food as well as synthetic objects such as coins, kitchen utensils and
jewelry.
Nickel allergy is determined by patch test using 5% Nickel sulfate. The effects of nickel
exposure to humans have included dermatitis, casncer of nasal sinus and larynx, irritation
and perforation of nasal septum, loss of smell, asthmatic lung disease, pulmonary
pneumoconiosis, lung dysfunction and death.
OSHA standard: 8 hr time weighted average concentration limit of 1000 microgram/m3
of nickel and nickel compounds.
PARTIAL DENTURE ALLOYS AND GUIDELINES FOR SELECTION
EFFECT OF EACH ALLOY CONSTITUENT:
CHROMIUM:
Chromium content is responsible for the tarnish resistance and stainless properties of
these alloys. When the chromium of an alloy is over 30% the alloy is more difficult to
cast. It also forms a brittle phase known as the zigma phase. Therefore dental alloys of
these types should not contain more than 28-29% chromium.
COBALT AND NICKEL:
They are some2what interchangeable to a certain extent. Cobalt increases the elastic
modulus, strength and harness of the alloy more than nickel does. Nickel may increase
ductility.
CARBON CONTENT;

14. The hardness of cobalt based alloys is increased by the increased content of carbon. A
change in the carbon content in the order of 0.2 % in these alloys changes their properties
to csuch an extent that the alloy would no longer be usable in dentistry.
MOLYBDENUM:
The presence of 3-6% molybdenum contributes to the strength of the alloy.
ALUMINIUM:
Al in Ni containing alloys forms a compound of Nickel and Aluminium (Ni3-Al).This
compound increases the ultimate tenmsile and yield strength.
BERYLLIUM:
1 % of this element to Nickel based alloys reduces the fusion range of the alloy by about
100 degree Celsius. It also aids in solid solution hardening. It improves the casting
characteristics which possibly aid in porcelasin bonding.
SILICON AND MANGANESE:
These are added to increase the castability of these alloys. They are present primarily on
oxide to prevent oxidation of other elements during melting. The presence of nitrogen
which cannot be controlled unless the castings are made in a controlled atmosphere as in
vacuum or argon also contributes to the brittle qualities of these cast alloys. When the
nitrogen content of the final alloy ois more thamn 0.1 % the ncasatings loose some of
their ductility since the minor ingredients of carbon, nitroigen and ygen effectively
increase the properties of the final formulated and designerd in such aweay as rto
maximize the rigidity of the prosthesis.
The obvious approach would be to increase the thickness of metal substructure
since doubling the tyhi9ckness increases the rigidity in bendingby a factor of 8. however,
the maximum thickness of the overall restorastion is limited externally by occlusion and
proper anatomical contour internally by the desire to retain as mucxh tooth structure as
possible(esthtics requir44es a minimal thickness of overlaying porvcelain that results in
severe ;limits asstothew maximum thickneres s ofdthe metal)
TITANIUM FOR CASTING APPLICATIONS:
Titanium was first isolated thern named 200 years ago but the metal we know is less than
40 years old. It is thew fourth most abundant metalin the easrth’s crust. It is a reactive
metal and hence difficult ti extract pure titanium. Dr. Wilhelm kroll invented useful
metallurgical processes for the commercial production of titanium metal and hence heis
called the father of titanium industry.
PROPERTIES OF TITANIUM
The physical and mechanical properties of titanium and its alloys vary greatly with the
addition of traces of other elements such as oxygen, iron and nitrogen. Commercially
pure Ti is available in four grades (Grade I to Grade IV) based on the incorporation of
small amounts of oxygen, nitrogen, hydrogen, iron and carbon during purification
procedures.
The most commonly used and important Ti alloy is Ti-6Al-4V alloy because of its
desirable proportion and predictable producibility.

15. PROPERTIES OF TITANIUM AND ITS ALLOY (Ti-6Al-4V)
Table 2 –Wang/ Fenton
1. Ti is the most biocompatible metal used for dental prostheses.
2. High melting point of 1668 degree Celsius.
3. It is highly resistant to tarnish and corrosion due to the formation of a coat of
titanium oxide on the surface.
4. But as the oxidation rate of Ti increases rapidly above a temperature of 850
degree Celsius, it is desirable to use ultra low fusing porcelains for Ti-ceramic
prostheses.
5. A special casting machine with arc melting capability and argon atmosphere is
used along with a compatible investment are used to ensure acceptable castability.
6. The most widely used Ti alloy used in dentistry is Ti-6Al-4V which is a alpha-
beta alloy. Although it is stronger than CP Ti, it is not as attractive from a
biocompatibility point of view due to slow release of Al and V atoms in vivo.
ALTERNATIVES TO CAST METAL TECHNOLOGY
To avoid the challenges and cost of associated with metal casting process, four
technologies are available;
SINTERING OF BURNISHED FOIL:
The Captek system consists of three pairs of materials:
1. CaptekP layer which is adapted first to the die and fired at a temperature of 1075
degree Celsius
2. Captek G which is applied over the Captek P coping and the former is drawen by
capillary action into the network structure of the Captek P coping vacated by the
adhesive binder.
3. Captek Repair paste and Capfil which are used to add material to Captek
structures.
The main ADVANTAGE of Captek structures is the very low thichness of metal tjhat can
be achievedehich ensures minimal tooth preparation and hence improved esthetics.
CAD-CAM PROCESSING
A CAD-CAM System electronically or digitally records surface co-ordinates of the
prepared tooth and stores these retrived data in the memory of a computer. The image
data can then be retrieved immediately to mill or grind a metal, ceramic or composite
prosthesis by computer control from a solid block of the chosen material. Within minutes
the prosthesis can be fabricated and placed in a prepared tooth and bonded or cemented in
the mouth of a patient.
The optical scanning procedure eliminates the need for an impression. An
advantage of ceramics is that homogeneous, high quality materials with minimal porosity
and other typical defects are designed for CAD-CAM applications.
COPY MILLING
This process is based on the principle of tracing the surface of a pattern that is then
replicated from a blank of ceramic, composite, or metal that is ground, cut or milled by a

16. rotating wheel whose motionis controlled by a link through the tracing device. Eg : The
Celay : Mikrona Technologies, Spreintenbach, Switzerland)
ELECTROFORMING
A master cast of the prepared tooth is prepared and coated with a special die spacer to
facilitate separation of the duplicating material. After applying aconductive silver layer to
the duplicated surface (Gypsum product) , the die is connected to a plating head and
connected to a power source and then placed in a plating solution. After a sufficentlyu
thick layer of gold or other material is deposited, the gypsum is removed and the coping
is sandblasted. Subsequent ceramic layers are condensed and sintered in a conventional
way.
REVIEW OF LITERATURE
P.J Brockhurst and R.W.S Canon in 1981 examined the requirements of alloys for metal-
ceramic crowns and bridgework and discussed the functional requirements and
manipulative behavior of as well as cost of alternatives to high gold alloys.
They concluded that base metal alloys functioned satisfactorily as compared to high
noble alloys provided proper dental lab procedures were employed. Nickel and beryllium
did not appear to be health hazards for them.
J. Robert Kelly and Thomas C.Rose in 1983 discussed the various physical properties,
biocompatibility, porcelain bonding and corrosion resistance of various non precious
alloys and concluded that though the manipulation of non precious alloys is technique
sensitive and exacting, their better physical properties and clinical performance merited
consideration. They were of the opinion that beryllium was not a health hazard provided
proper exhaust and ventilation was used in the dental lab and that the allergenic potential
of nickel needed further research.
Russel R. Wang and Aaron Fenton in 1996 reviewed the literature on Titanium for
prosthodontic applications. They described the development and properties of titanium
for the purpose of evaluating the present status and future trends in its use.
M. Bagby, S.J Marshall and G.W Marshall in 1990 reviewed the literature on metal
ceramic bio compatibility. They discuseed the various tests to predict thermomechanical
compatibility and also for measuring compatibility at the metal ceramic interface.
Selcuk Oruc and Ybrahim Tulunoglu in 2000 evaluated the marginal and inner fit of
metal –cramic restorations and frameworks made with a Nickel-Chromium alloy
(Remanium CS) and a commercially pure Titanium (Rematitan). They concluded that the
fit of base metal alloy metal ceramic crowns was better than the commercially pure
Titanium metal ceramic crowns. However both the artificial crowns were clinically
acceptable.
John C. Wataha in 2002 discussed the properties that are relevant tot eh proper selection
of an alloy for a given clinical problem. He summarized the various alloys available till

17. date and their classification and also provided simple guidelines to help dentists choose
appropriate alloys for their practices.