This document discusses dental casting alloys and noble metal alloys. It begins by defining key terms like metals, alloys, and crystal lattices. It then describes the atomic structure and physical properties of metals, including their densities, conductivity, and ability to be cast or machined. Specific crystal lattice structures are shown for different metals. The document discusses the solidification and crystallization of metals, including how alloys solidify over a temperature range rather than at a single point. It focuses on the properties and uses of noble metal alloys for dental restorations, noting that noble metals like gold and platinum are resistant to corrosion in the oral cavity.

1. DENTAL CASTING
ALLOYS - I
D.V.BHUVANESH KUMAR
I YR MDS

2.  What is Metal ? Metalloid ? Alloy ?
 Atomic structure
 Crystal lattice
 Physical properties of Metals
 Solidification & Crystallization of Metals
 Noble metals & Base metals
 Alloys
 Types & composition of Alloys
 EQUILIBRIUM PHASE DIAGRAM FOR NOBLE ALLOYS
 Ordered & Solid solution
 Physical properties of Noble alloys
 Noble-Metal Alloys for Ceramic-Metal Restorations
 Noble-Metal alloy system

3.  Dental biomaterials are generally categorized into
four classes:
Metals,
Polymers,
Ceramics ,
Composites.
The four classes are distinctly different from
each other in terms of density, stiffness, translucency,
processing method, application, and cost

4. Metals and alloys are used in almost all aspects
of
dental practice, including the
 Dental laboratory,
 Direct and indirect dental restorations,
 Implants, and
 Instruments used to prepare teeth

5. The Metals Handbook (1992) defined a
metal as
“an opaque lustrous chemical
substance that is a good conductor of
heat and electricity and, when polished,
is a good reflector of light.”

6. As a class, metals are ductile and malleable
and therefore exhibit elastic and plastic
behavior; they are good electrical and
thermal conductors, higher in density than
other classes, exhibit good toughness, are
opaque, and can be polished to a luster.
Metals may be cast, drawn, or machined to
create dental restorations and instruments

7.  A metal is any element that ionizes positively in
solution. As a group, metals constitute nearly two
thirds of the periodic table .
.
This ability to exist as free, positively charged, stable
ions is a key factor in the behavior of metals and is
responsible for many metallic properties that are
important in dentistry

9. Another important group of elements
shown are the metalloids, including
carbon, silicon, and boron.
Although metalloids do not always
form free positive ions, their
conductive and electronic properties
make them important components of
many dental alloys

10. An alloy is a material with metallic properties
consisting of two or more chemical elements at
least one of which is a metal, the choice of
elements depends on which properties are
required for specific clinical conditions.
 Characteristics such as castability, ability to be
soldered (or brazed), solidification shrinkage,
expansion coefficient, corrosion resistance,
biocompatibility, and color are important for a
variety of clinical applications.

11. Atomic Structure
 At the atomic level, pure metals
exist as crystalline arrays that are
continuous in three dimensions.
In these arrays, the nuclei and
core electrons occupy the atomic
center with the ionizable electrons floating freely
among the atomic positions.

12. The mobility of the valence electrons is
responsible for many properties of metals, such
as electrical conductivity.
It is important to note that the positively charged
atomic centers are held together by the electrons
and their positive charge is simultaneously
neutralized by the negative electrons. Thus pure
metals have no net charge.

13.  In array, the smallest repeating unit that captures all
the relationships among atomic centers is called a
unit cell.
 The three most common crystal lattice unit cells in dental
metals and alloys. A, Body-centered cubic cell; B, face-centered
cubic cell; and C, hexagonal close-packed cell.

14. BODY
CENTERED
CUBE is noted
in
#CHROMIUM ,
#IRON ,
#MOLYBDENUM
#TUNGSTEN

15. Most pure metals –
aluminum , copper,
platinum, silver and
alloys of gold,
palladium, cobalt,
and nickel exhibit the
Face Centered Cubic
array.

16. Titanium exhibits the more
complex hexagonal
close-pack array. In this array,
the atoms are equidistant
from each other in the
horizontal plane,
but not in the vertical
direction.

17. All metals occur in one
of the lattice structures shown.
There are six families of
lattices, four of which can be
subdivided.
Each family is defined by the
distances between vertices
and the angles at the vertices.

18. All properties of metals result from the metallic
crystal structure and metallic bonds.
Metals have high densities that result from the
efficient packing of atomic centers in the crystal
lattice.
Metals are electrically and thermally conductive
because of the mobility of the valence electrons
in the crystal lattice.
Physical Properties of Metals

19. The opacity and reflective nature of metals
result from the ability of the valence electrons
to absorb and emit light.
The corrosion properties of metals depend on the
ability of atomic centers and electrons to be
released in exchange for energy. The amount of
energy required depends on the strength of the
metallic force, which is related to the freedom of
the valence electron and the energy that the released
ion can gain by solvating in solution.

20. Metals such as sodium and potassium, the
metallic bond is weaker because the valence
electrons are loosely held, and the energy of
solvation is high.
Thus these metals corrode into water with
explosive energy release. For metals such as
gold and platinum, the metallic bond is
stronger; valence electrons are more tightly
held, and solvation energies are relatively low.
Thus gold and platinum are far less likely to
corrode.

21. Melting occurs as the metallic bond energy is
overcome by the applied heat. Interestingly,
the number of valence electrons per atomic
center influences the melting point somewhat.
As the number of valence electrons increases,
the metallic bond develops some covalent
character that contributes to higher melting
points. This phenomenon occurs for iron
(Fe3+) and nickel (Ni2+)

22. Metals generally have good ductility (ability
to be drawn into a wire) and malleability
(ability to be hammered into a thin sheet)
relative to polymers and ceramics. To a large
extent, these properties result from the
ability of the atomic centers to slide against
each other into new positions within the
same crystal lattice. Because the metallic
bonds are essentially non directional, such
sliding is possible.

23. If the metallic crystals were perfect,
calculations have shown that the force required
to slide the atoms in the lattice would be
hundreds of times greater than experiments
indicate.
Less force is necessary because the crystals
are not perfect; they have flaws called
dislocations. Dislocations allow the atomic
centers to slide past each other one plane at a
time.

24. Metals fracture when the atomic centers
cannot slide past one another freely. For
example, this failure can happen when
impurities block the flow of dislocations.

27.  Consider a plate of steel 15 cm wide and 6 mm thick.
Suppose it has a 5-cm crack running into one side.
The force required to make the crack run the
remaining 10 cm would be about 1800 Newtons (N).
Without the aid of the crack, 2.2 million Newtons (MN)
would be required if the steel were the best
commercial grade available.
 If the steel were a single, flawless crystal, 44 MN
would be necessary! The fracture of metals depends
heavily on dislocations and the local rupture of the
crystal lattice.

28. SOLIDIFICATION
&
CRYSTALLIZATION OF
METALS

29. SOLIDIFICATION OF METALS
temperature decreases steadily
decreases steadily to room
temperature
decreases steadily to room
temperature

30. A pure metal solidifies at a constant temperature
equal to its freezing point (same as melting point)
Cooling curve for a pure metal during casting

31. • The temperature Tf, as indicated by the straight or
“plateau” portion of the curve at BC, is the freezing
point, or solidification temperature of the pure metal.
This is also the melting point, or fusion temperature.
• During melting, the temperature remains constant.
• During freezing or solidification, heat is released as the
metal changes from the higher-energy liquid state to the
lower-energy solid state.

32.  The initial cooling of the liquid metal from Tf
to point B' is termed super cooling.
 During the super cooling process,
crystallization begins for the pure metal.
 Once the crystals begin to form, release of the
latent heat of fusion causes the temperature to
rise to Tf where it remains until crystallization
is completed at point C.

33. Most alloys freeze over a temperature range rather than at
a single temperature

34. Liquidus temperature – Temperature at which an
alloy begins to freeze on cooling or at which the
metal is completely molten on heating.
Solidus temperature – Temperature at which an
alloy becomes solid on cooling or at which the
metal begins to melt on heating.

35. Characteristically, a
pure metal crystallizes
from nuclei in a pattern
that often resembles the
branches of a tree,
yielding elongated
crystals that are called
Dendrites.
CRYSTALLIZATION OF METALS

36. Extensions or elevated areas (termed protuberances) form
spontaneously on the advancing front of the solidifying metal
and grow into regions of negative temperature gradient.

37. Difference b/w dental base metal
casting alloys & most Nobel metal
casting alloys is solidify with an
Equiaxed polycrystalline microstructure
The micro structural features in this
figure are called grains.
Equiaxed means that the three
dimensions of each grain are similar, in
contrast to the elongated morphology
of the dendrites.

38. All modern noble metal alloys are fine grained.
Smaller the grain size of the metal, the more ductile
and stronger it is.
It also produces a more homogenous 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, shape of the mold, and composition of the
alloy.

39. NOBLE METALS
The noble metals have been the basis of inlays, crowns and
bridges because of their resistance to corrosion in the oral
cavity.
Gold, platinum, palladium, rhodium, ruthenium, iridium,
osmium, and silver are the eight noble metals. However, in
the oral cavity, silver is more reactive and not considered as
a noble metal.

40. GOLD
 Pure gold is a soft and ductile.
Yellow “Gold” hue.
 Density of 19.3 gms/cm3. Melting
point of 1063oC. Boiling point of
2970 oC.
 CTE of 14.2×10-6/°C.
 Good luster and high polish.
 Good chemical stability.
 Does not tarnish and corrode.
 Dissolves in aqua regia, pot
cyanide, br, cl

41. Gold content:
Traditionally the gold content of dental casting alloys have been
referred to in terms of:
 Karat
 Fineness
Karat:
Parts of pure gold in 24 parts of alloys.
For Eg: a) 24 Karat gold is pure gold
b) 22 Karat gold is 22 parts of pure gold
Fineness:
parts per thousand of pure gold.
For Eg: Pure gold is 1000 fine.
Thus, if ¾ of the gold alloy is pure gold, it is said to be 750 fine.

42. Material Density
(g/cm3)
Hardness
(VHN/BHN)
(kg/mm2)
Tensile
Strength
(MPa)
Elongation
(%)
Cast 24k gold 19.3 28(VHN) 105 30
Cast 22k gold -- 60(VHN) 240 22
Coin gold -- 85 (BHN) 395 30
Typical Au-based
casting alloy
(70 wt% Au)*
15.6 135/195(VHN) 425/525 30/12
Condensed gold
foil
19.1 60 (VHN) 250 12.8
Physical and mechanical properties of cast pure gold, gold
alloys, and condensed gold foil

43. SILVER
The “Whitest” of all metals.
Helps to counteract the reddish colour of
copper.
To a slight extent it increases
strength and hardness.
In large amounts however, it
reduces tarnish resistance.
lowest density 10.4gms/cm3.
melting point of 961oC.
boiling point of 2216 oC.
 CTE19.710-6/oC. (comparatively high)

44. PLATINUM
Increases the strength and corrosion
resistance.
Increases the melting point and has a
whitening effect on the alloy.
Helps to reduce the grain size.
Highest density of 21.45 gms/cm3 ,
 Highest melting point of 1769oC,
Boiling point of 4530 oC and
the lowest CTE 8.910-6/oC

45. PALLADIUM
It is similar to platinum in its effect. It hardens
as well as whitens the alloy.
Quality of absorbing or occluding large
quantities of H2 gas when heated – so
undesirable when heated with improperly
adjusted gas torch.
Raises the fusion temperature and provides
tarnish resistance.
Less expensive than platinum.
Density of 12.02gms/cm3.
Higher melting point of1552oC.
Boiling point of 3980 oC and lower CTE which
is 11.810-/oC, when compared to gold.

46. IRIDIUM, RUTHENIUM
They decrease the grain size.
They are added in very small
quantities (about 100 to 150 ppm).
IRIDIUM
high melting point of 2454°C ,
boiling point of 5300 °C ,
density of 22.5gm/cm3
CTE 6.810-6/oC.
RUTHENIUM
melting point of 1966°C ,
boiling point of 4500 °C ,
density of 12.44 gm/cm3
CTE 8.310-6/oC

47.  How they act as GRAIN REFINER ?
Extremely High melting points
so they don’t melt during casting
they serve as a nucleating centers for the metal as it
cools , resulting in fine grained alloy

48.  High melting point –
1966o c.
 Alloyed with Pt to form
wire for Thermocouples.
 These thermocouples
help to measure the
temperature in
PORCELIAN FURNACES
used to make dental
restorations.

49. BASE METALS
• These are non-noble metals.
They mainly
• Influences on physical properties,
• control of the amount added ,
• type of oxidation, or their strengthening effect.
• Although they are frequently referred as non
precious, the preferred term is base metal.
• Examples of base metals are chromium, cobalt,
nickel, iron, copper, manganese etc.

50. COBALT
 hardness,
 strength
 rigidity to the alloy.
 high melting point of
1495°C
 boiling point of 2900 °C
 density of 8.85 gm/cm3
 CTE 13.810-6/oC

51. NICKEL
Strength, hardness, modulus of
elasticity and fusion temp.
Increases ductility.
Melting point of 1453°C ,
Boiling point of 2730 °C ,
Density of 8.9 gm/cm3
CTE 13.310-6/oC
Nickel, which is the most
common metal to cause
Contact Dermatitis.

52. CHROMIUM
Passivating effect ensures
corrosion resistance.
Higher proportion greater
tarnish and corrosion resistance.
Reduces the melting point.
30% chromium is the upper
limit to get maximum
mechanical properties.
Melting point of 1875°C ,
Boiling point of 2665 °C ,
Density of 7.19 gm/cm3
CTE 6.210-6/ oC

53. COPPER
It is the principal hardener.
Reduces the melting point and density
of gold.
Reddish colour.
In greater amounts it reduces
resistance to tarnish and corrosion of the
gold alloy.
Therefore, the maximum content
should NOT exceed 16%.
Melting point of 1083°C ,
Boiling point of 2595 °C ,
Density of 8.96 gm/cm³
CTE 16.5 10-6/°C .

54. ZINC
Scavenger for oxygen.
Without zinc the silver in
the alloy causes absorption of
oxygen during melting.
Later during solidification, the
oxygen is rejected producing
gas porosities in the casting.
Melting point of 420°C ,
Boiling point of 906 °C ,
Density of 7.133gm/cm3
CTE 39.710-6/oC

55. MOLYBDENUM OR TUNGSTEN
They are effective hardeners.
Molybdenum is preferred as it
reduces ductility to a lesser
extent than tungsten.
Molybdenum refines grain
structure.
Melting point of 2610°C ,
Boiling point of 5560 °C ,
Density of 10.22 gm/cm3
CTE 4.9 10-6/oC

56. IRON,BERYLLIUM
They help to harden the metal ceramic gold - palladium alloys, iron being the
most effective. In addition, beryllium reduces fusion temperature and refines grain
structure . IRON has melting point of 1527°C , boiling point of 3000 °C , density of
7.87 gm/cm3 and CTE 12.3 10-6/oC .

57. GALLIUM
It is added to compensate for the
decreased coefficient of thermal
expansion that results when the
alloy is made silver free.
The elimination of silver reduces
the tendency for green stain at the
margin of the metal-porcelain
interface.
Their oxides are important in
bonding of the ceramic to metal

58. MANGANESE AND SILICON
Primarily oxide scavengers to prevent oxidation of other elements during
melting. They are hardeners. MANGANESE has melting point of 650°C , boiling
point of 1107 °C , density of 1.74 gm/cm3 and CTE 25.2 10-6/oC , where as
SILICON has melting point of 1410°C , boiling point of 2480 °C , density of 2.33
gm/cm3 and CTE 7.3 10-6/oC .

59. CARBON:
Small amounts may have a
pronounced effect on strength,
hardness and ductility.
Carbon forms carbides important
factor in strengthening the alloy.
However when in excess it increases
brittleness.
Melting point of 3700°C ,
Boiling point of 4830 °C ,
Density of 2.22 gm/cm3
CTE 6 10-6/oC .

60. BORON
It is a deoxidizer and
hardener, but
reduces ductility.

62. ALLOYS
• They are generally prepared by fusion of the elements
above their melting points.
For example,
• A certain amount of chromium is added to iron, carbon,
and other elements to form stainless steel, an alloy that is
highly resistant to corrosion.
• Also to nickel or cobalt alloys, which comprise two of the
major groups of base metal alloys used in dentistry.

63. At least four factors determine the extent of solid solubility
of metals; atom size, valence, chemical affinity and crystal
structure.
ATOM SIZE:
If the sizes of two metallic atoms differ by less than
approximately 15% (first noted by Hume-Rothery), they
possess a favorable size factor for solid solubility.

64. VALENCE:
Metals of the same valence and size are more likely to
form extensive solid solutions than are metals of different
valences.
CHEMICAL AFFINITY:
When two metals exhibit a high degree of chemical
affinity, they tend to form an intermetallic compound upon
solidification rather than a solid solution.

65. Types & Composition
 The ADA specification for dental
casting alloys classifies alloys by
composition, dividing alloys into
three groups:
 (1) high-noble, with a noble metal
content of at least 60 wt% and a gold
content of at least 40%;
 (2) noble, with a noble metal content
at least 25% (no stipulation for gold);
and
 (3) predominately base metal, with a
noble metal content less than 25%
(Table 10-3).

66. ANSI/ADA specification No. 5 (ISO 1562) usesa type I through IV
classification system with each alloy type recommended for
specific applications, in addition to the compositional
classification previously described

69. Equilibrium phase, since the phases that are present in
an alloy system are of different compositions and
temperatures.
Phase diagrams can provide microstructure predictions
when some cast dental alloys are subjected to heat
treatment.
This concept equilibrium phase diagram was introduced
by using the table salt-water system

70. In each phase diagram ,
the horizontal axis represents the
composition of the binary
alloy.
The composition can
be given in atomic percent (at%)
or weight percent
(wt%).
Weight percent compositions
give the relative mass of each
element in the alloy, whereas
atomic percentages give the
relative numbers of atoms in
the alloys.

71.  the physical and biological properties of
alloys relate best to atomic percentages
 Alloys that appear high in gold by weight
percentage may in reality contain far fewer
gold atoms than might be thought.
 The y-axes show temperature. If the
temperature
is above the liquidus line (marked L), the
alloy will be completely molten.
If the temperature is below the solidus line
(marked S), the alloy will be solid. If the
the liquidus and
solidus lines, the alloy will be

72.  From a manipulative standpoint, it is
desirable to have a narrow liquidus-
solidus range, because one would
like to keep the alloy in the liquid
state for as short a time as possible
before casting.
 While in the liquid state, the alloy is
susceptible to significant oxidation
and contamination.
 If the liquidus-solidus range is
broad, the alloy will remain at least
partially molten for a longer period
after it is cast.

73.  The temperature of the liquidus line is
also important, and varies
considerably among alloys and with
composition.
 For example the liquidus line of the
Au-Ag system ranges from 962° to
1064° C but the liquidus line of the
Au-Pd system ranges from 1064° to
1554° C .
 It is often desirable to have an alloy
with a liquidus line at lower
temperatures; the
, occur,
and of a
problem.

74.  The area below the solidus lines is also
important to the behavior of the alloy.
 If this area contains no boundaries,
then the binary system is a series of
solid solutions.
 This means that the two elements are
completely soluble in one another at all
temperatures and compositions.
 The Ag-Pd system and Pd-Au system
are examples of solid-solution
systems.

75.  An ordered solution occurs when
the two elements in the alloy
assume specific and regular
positions in the crystal lattice of
the alloy.
 This situation differs from a solid
solution in which the positions of
the elements in the crystal lattice
are random.
 Examples of systems containing
ordered solutions are the Au-Cu
system the Pd-Cu system and the
Au-Ag system.

76.  If the area below the solidus line contains a
solid line, it indicates the existence of a second
phase.
 A second phase is an area with a composition
distinctly different from the first phase.
 In the Au-Pt system a second phase forms
between 20 and 90 at% platinum. If the
temperature is below the phase boundary line
within these compositions, two phases exist in
the alloy.
 The presence of a second phase is important
because it significantly changes the corrosion
properties of an alloy
 Because the different phases may interact
electrochemically, the
.

77.  The use of pure cast gold is not practical for
dental restorations because cast gold lacks
sufficient strength and hardness.
 Solid-solution and Ordered-solution hardening
are two common ways of strengthening noble
dental alloys
 The formation of ordered solutions has been
commonly used to strengthen cast
dental restorations, particularly in gold-based
alloys

78.  If Au-Cu containing about 50 at% gold
is heated to the molten state and then
cooled slowly, the mass will solidify at
about 880° C as a solid solution. As
the mass cools slowly to 424° C, the
ordered solution will then form and
will remain present at room
temperature.
 However, if the mass is cooled rapidly
to room temperature after the initial
solidification, the ordered solution will
not form because there is
for the mass to reorganize.
 Thus the alloy will be trapped in a
non-equilibrium state of a solid
solution and will be softer, weaker, and
have greater elongation.

79.  The conversion between the ordered solution and solid solution is
.
 Rapid cooling will preserve the solid solution and the soft condition,
whereas slow cooling will allow the formation of the ordered solution
and the hardened condition.
 The ideal noble casting alloy should have

80. properties
:
 Do not have melting points,
but rather melting ranges,
because they are
combinations of elements
rather than pure elements.
The solidus-liquidus range
should be narrow to avoid
having the alloy in a molten
state for extended times
during casting.

81. If the alloy spends a long
time in the partially molten
state during casting, there
is increased opportunity
for the
.
Most of the alloys in have
solidus-liquidus ranges of
70° C or less. The Au-Ag-Pt,
Pd-Cu-Ga, and Ag-Pd
alloys have wider ranges,
which makes them more
difficult to cast without
problems.

82. The liquidus temperature of the alloys
determines
that must be used during casting

83. In general, the burnout temperature must
be
temperature.
For the Au-Cu-Ag-Pd-I alloys, therefore,
a burnout temperature of about 450° to
475° C should be used.
If the burnout temperature approaches
700° C, a gypsum-bonded investment
cannot be used because the calcium
sulfate will decompose and brittle the
alloys.
At temperatures near 700° C or greater, a
phosphate-bonded investment is used.

84.  The torch will
adequately heat alloys with
liquidus temperatures below
.
 Above this temperature, a gas-
oxygen torch or electrical
induction method must be
used.
 A gas-air torch would be
acceptable only for the Au-Cu-
Ag-Pd-I, II, and III and the Au-
Ag-Pd-In alloys

85.  The of the alloys
determines the
 If the alloy contains a significant
amount of an element that has a
high melting point, it is likely to
have a high liquidus.
 Thus alloys that contain
significant amounts of palladium
or platinum, both of which have
high melting points will have high
liquidus temperatures.
 These alloys include the Pd-Cu-
Ga, Ag-Pd, and Au-Ag-Pt alloys.

86.  The solidus temperature is
important
,
because during both of
these operations, the
shape of the alloys is to be
retained.
 Therefore during soldering
or hardening- softening,
the alloy may be heated
only to the solidus before
melting occurs.

87.  Density is important during the
acceleration of the molten alloy into
the mold during casting.
 Alloys with high densities will
generally accelerate faster and tend to
form complete castings more easily.
 Alloys with high densities generally
contain higher amounts of denser
elements such as gold or platinum.


88.  Strength of alloys can be
measured by either the yield
strength or tensile strength.
 For several alloys, such as Au-
Cu-Ag-Pd-I, II, and III, the
formation of the ordered
phase increases the yield
strength significantly.
 For example, the yield
strength of the Au-Cu-Ag-Pd-II
alloys increases from 350 to
600 MPa with the formation of
an ordered phase.

89.  The effect of solid-solution
hardening by the addition of copper
and silver to the gold or palladium
base is significant for these alloys.
 Pure cast gold has a tensile
strength of 105 MPa . With the
addition of 10 wt% copper (coin
gold), solid-solution hardening
increases the tensile strength to 395
MPa.
 With the further addition of 10 wt%
silver and 3 wt% palladium (Au-Cu-
Ag-Pd-I), the tensile strength
increases to about 450 MPa and 550
MPa in the hard condition.

90.  Hardness is a good indicator of the
ability of an alloy to resist local
permanent deformation under
occlusal load.
 Alloys with high hardness will usually
have high yield strengths and are
more difficult to polish.
 The Ag-Pd alloys are particularly soft
because of the high concentration of
silver, which is a soft metal.
 The Pd-Cu-Ga alloys are particularly
hard because of the high
concentration of Pd, which is a hard
metal

91.  The hardness of most
noble casting alloys is less
than that of enamel (343
kg/mm2), and typically less
than that of base-metal
alloys.
 If the hardness of an alloy
is greater than enamel, it
may wear the enamel of
the teeth opposing the
restoration.

92.  Elongation is a measure of the ductility
of the alloy.
 Alloys with high elongation can be
burnished without fracture. Elongation
is sensitive to the presence or absence
of an ordered phase.
 In the hardened condition, the
elongation will drop significantly.
 In the Au-Cu-Ag-Pd-II alloys, the
elongation is 30% in the soft condition
versus only 10% in the hard condition.
 In the soft condition, the elongation of
noble dental casting alloys ranges from
8% to 30%

93. Composition and Properties of Noble-Metal
Alloys for Ceramic-Metal Restorations
 Ceramic-metal restorations consist of a
cast metallic framework (or core) on which
at least two layers of ceramic are baked.
 It is essential that the coefficient of
thermal expansion of the alloy be slightly
higher than that of the veneering ceramic
to ensure that the ceramic is in slight
compression after cooling.
 This will establish a better resistance to
crack propagation of the ceramic-metal
restoration.

94.  1. The alloy must have
. The melting
range must be substantially
higher (greater than 100° C) than
the firing temperature of the
ceramic and solders used to join
segments of a bridge.
 2. A
and is achieved by the
interactions of the ceramic with
metal oxides on the surface of
metal and by the roughness of the
metal coping.

95.  3. of the
ceramic and metal must be so
that the ceramic does not crack during
fabrication
of the
alloy core are especially important for fixed
bridges and posterior crowns. High
stiffness in the alloy reduces stresses in the
ceramic by reducing deflection and strain.
High strength is essential in the
interproximal regions in fixed bridges.
 5. . The
alloy copings are relatively thin; no
distortion should occur during firing of the
ceramic, or the fit of the restoration will be
compromised.

96.  6.An of the metal
coping is required even with the
higher melting range of the alloy.
 7. Adequate design of the
restoration is critical. The
preparation should provide for
of alloy as well
as enough space for an adequate
thickness of ceramic to yield an
esthetic restoration.

98. The Gold-Platinum-Palladium (Au-Pt-Pd) System
This is one of the oldest metal ceramic alloy system. But these alloys are
not used widely today because they are very expensive.
Advantages Disadvantages
1. Excellent castability 1. High cost
2. Excellent porcelain bonding 2. Poor sag resistance so not suited for
3. Easy to adjust and finish long span fixed partial dentures.
4. High nobility level 3. Low hardness (Greater wear)
5. Excellent corrosion and tarnish 4. High density (fewer casting per
resistance. ounce)
6. Biocompatible
7. Some are yellow in color
8. Not “Technique Sensitive”
9. Burnishable

99. These alloys were developed in an attempt to overcome the major limitations in the gold-platinum-
palladium system (mainly poor sag resistance, low hardness & high cost)
Two variations on the basic combination of gold, palladium and silver were created and are
identified as either high-silver or low-silver group.
(High Silver Group):
Gold – 39% to 53%
Silver – 12% to 22%
Palladium – 25% to 35%
trace amount of oxidizable elements are added for porcelain bonding.

100.  Composition (Low Silver Group):
 Gold – 52% to 77%
 Silver- 5% to 12%
 Palladium – 10% to 33%
 Trace amounts of oxidizable elements for porcelain bonding.

101. Gold-Palladium (Au-Pd) System:
This particular system was developed in an attempt to overcome the major limitations in the Au-
Pt-Pd system and Au-Pd-Ag system. Mainly-
-Porcelain discoloration.
-Too high coefficient of thermal expansion & contraction.
Composition:
Gold – 44% to 55%
Gallium – 5%
Palladium – 35% to 45%
Indium & Tin – 8% to 12%
Indium, Gallium and Tin are the oxidizable elements responsible for porcelain bonding.
Advantages Disadvantages
1. Excellent castability 1. Not thermally compatible with high
expansion dental porcelain.
2. Good bond strength 2. High cost
3. Corrosion and tarnish resistance
4. Improved hardness
5. Improved strength ( sag resistance)
6. Lower density

102. Palladium-Silver (Pd-Ag) System
This was the first gold free system to be introduced in the
United States (1974) that still contained a noble metal (palladium).
It was offered as an economical alternative to the more expensive
gold-platinum-silver and gold-palladium-silver (gold based) alloy
systems.
Composition: (available in two compo.)
1. Palladium – 55% to 60% Silver – 25% to 30%
Indium and Tin
2. Palladium – 50% to 55% Silver – 35% to 40%
Tin (Little or no Indium)
Trace elements of other oxidizable base elements are also
present.

103. Advantages Disadvantages
1. Low Cost 1. Discoloration (yellow, brown or green) may
occur with some dental porcelains.
2. Low density 2. Some castibility problems reported (with
induction casting)
3. Good castibility (when torch 3. Pd and Ag prone to absorb gases.
casting) 4. Require regular purging of the porcelain
4. Good porcelain bonding, furnace.
5. Burnishability 5. May form internal oxides (yet porcelain
6. Low hardness bonding does not appear to be a problem)
7. Excellent sag resistance 6. Should not be cast in a carbon crucible.
8. Moderate nobility level 7. Non-carbon phosphate bonded investments
9. Good tarnish and corrosion recommended.
resistance. 8. High coefficient of thermal expansion.
10. Suitable for long-span fixed
partial dentures.

104. HIGH PALLADIUM SYSTEM
Several types of high palladium systems were originally introduced (Tuccillo, 1987).
More popular composition groups contained cobalt and copper.
Composition (PALLADIUM-COBALT ALLOY):
Palladium – 78% to 88% Cobalt – 4% to 10%
(Some high palladium-cobalt alloys may contain 2% gold)
Trace amounts of oxidizable elements (such as gallium and indium) are added for porcelain
bonding.
Advantages Disadvantages
1. Low cost 1. More compatible with higher expansion
2. Reportedly good sag resistance porcelains.
3. Low density means more casting 2. Are more prone to over-heating than
per ounce then gold based alloys. high Pd-Cu.
4.They Melt and cast easily 3. Produces a thick, dark oxide
5. Good polishability (Supposed 4. Colored oxide layer may cause bluing of the
to be similar to Au-Pd alloys) porcelain.
6. Reportedly easier to presolder 5. Prone to gas absorption
than Pd-Cu alloys. 6. Little information on long-term clinical
success.

105. COMPOSITION (PALLADIUM-COPPER ALLOYS
Palladium – 70% to 80% Copper – 9% to 15%
Gold – 1% to 2% Platinum – 1%
Some, but not all, high palladium-copper alloys contain small quantities ( 1% to 2%) of gold and/or
platinum. Trace amounts of the oxidizable elements gallium, indium and tin are added for porcelain
bonding.
Advantages Disadvantages
1. Good castability 1. Produces dark, thick oxides
2. Lower cost (than gold based alloys) 2. May discolor (gray) some dental
3. Low density means more castings porcelains.
Per ounce 3. Must visually evaluate oxide color to
4. Tarnish and corrosion resistance determine if proper adherent oxide was
5. Compatible with many dental formed.
Porcelains. 4. Should not be cast in carbon crucibles
6. Some are available in one unit ingots. (electric casting machines)
5. Prone to gaseous absorption.
6. Subject to thermal creep.
7. May not be suitable for long span fixed
partial denture prosthesis.
8. Little information on long term clinical
success.
9. Difficult to polish

106. ….. To be continued…..