Metal ceramics /certified fixed orthodontic courses by Indian dental academy

4/12/2014 1
Metal ceramic
INDIANDENTALACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

4/12/2014 2
MECHANISM OF PORCELAIN
–METAL ATTACHMENT
Four theories have been proposed to
explain the processes that lead to
porcelain-to-metal bonding:
1. Van der waals forces.
2. Mechanical retention.
3. Compression bonding.
4. Direct chemical bonding.

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VAN DER WAALS FORCES.
1. The attraction between charged atoms that
are in intimate contact yet do not actually
exchange electrons is derived from van der
waals forces.
2. These secondary forces are generated more
by a physical attraction between charged
particles than by an actual sharing or exchange
of electrons in primary(chemical) bonding.
3. Van der waals forces are generally weak,
because nearly all the positive and negative
charges present in these atoms are satisfied in
a single molecule.

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4. It is also believed that bonding entails some
measure of true adhesion based on the extent
to which the metal substructure is wetted by
the softened dental porcelain.
5. The better the wetting of the metal surface,
greater the vanderwaal’s forces.
6. Furthermore, porcelain’s adhesion to metal
can be diminished or enhanced by alterations
in the surface characters(texture) of the
porcelain-bearing surface on the substructure.

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7. A rough, contaminated metal surface will
inhibit wetting and reduce the vanderwaals
bond strength. On the other hand, a slightly
textured surface, created by finishing with
uncontaminated aluminum oxide abrasives
and followed by air abrasion(blasting) with 50
microns aluminium oxide, reportedly will
promote wetting by the liquid porcelain.
8. Improved wetting is then accompanied by an
increase in adhesion through vanderwaals
forces.

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MECHANICAL RETENTION:
1. The porcelain-bearing area of a metal
casting contains many microscopic
irregularities into which opaque porcelain may
flow when fired.
2. Air abrading the metal with aluminum
oxide is believed to enhance mechanical
retention further by eliminating surface
irregularities ( stress concentrations) while
increasing the overall surface area available
for bonding.

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3. Despite it’s presence, mechanical retention’s
contribution to bonding may be relatively
limited.
4. Dental porcelain does not require a
roughened area to bond to metal. In fact
porcelain will fuse to a well polished
surface, but some surface roughness is
effective in increasing bonding forces.

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COMPRESSION BONDING
Dental porcelain is strongest under
compression and weakest under tension.
Hence , if the coefficient of thermal
expansion of the metal substructure is greater
than that of the porcelain placed over it, the
porcelain should be placed under
compression on cooling.
1. When cooling a restoration with a full-
porcelain veneer, the metal contracts faster
than the porcelain but is resisted by the
porcelain’s lower coefficient of thermal
expansion.

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2. This difference in contraction rates
creates tensile forces on the metal and
corresponding compressive forces on the
porcelain. Without the wraparound effect
created in a full porcelain restoration, there
is less likelihood this compression bonding
will develop fully.

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THERMAL EXPANSION
Generally substances increase in the length
and volume when they are heated. This
phenomenon is called as thermal expansion.
The specific rate of change in length of a
particular substance per unit change in
temperature is called coefficient of linear
expansion.
The rate of change in volume is called
coefficient of cubical expansion.
These may generally be called coefficient of
thermal expansion or simply thermal
expansion.

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RELATION BETWEEN METAL
AND PORCELAIN
When porcelain is fused to metal, three
possible relations can exist in thermal
expansion:
1. Thermal expansion (or contraction) is
greater in porcelain than in metal.
equal between metal and porcelain.
greater in metal than in porcelain.

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THERMAL EXPANSION IS GREATER
IN PORCELAIN THAN IN METAL.
Greater thermal expansion in porcelain means
that during the time after porcelain has lost
thermoplastic fluidity in the course of cooling,
but after melting of porcelain at high
temperature, porcelain is apt to contract to be
smaller and shorter than metal until it reaches
room temperature.
Therefore, assuming that they are separated,
there will be a difference in length between
them.

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Hence porcelain becomes shorter after cooling
although they had the same length before
heating.
In the ceramo-metallic system, porcelain side
is subjected to tensile stress while the metal
side is subjected to compressive stress as they
are fused together. As a result, the porcelain,
which is very weak against tensile stress, will
crack immediately.

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THERMAL EXPANSION IS EQUAL
BETWEEN METAL AND PORCELAIN
As metal and porcelain expand or contract at
the same rate, there will be no difference in
dimensions between them at all.
As a result, porcelain receives no stress from
metal and thus cracking does not occur in the
stable porcelain unless undue external force is
applied.
It is very difficult, however, to obtain the
identical curves for coefficient of thermal
expansion between porcelain and metal, and
under ordinary conditions there is a
discrepancy to some extent.

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THERMAL EXPANSION IS GREATER
IN METAL THAN IN PORCELAIN.
In general, this thermal expansion relationship
exists between metal and porcelain in the
dental metal-ceramic system.
The objective of such a relationship is to
obtain the most stable assembly after firing.
Fractures do not usually occur since porcelain
has very high compression strength, although
the porcelain side is subjected to compressive
stress as the metal contracts more than
porcelain during cooling to ambient
temperature after firing.

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However, this does not mean that cracking
will never occur.
If there is a significant difference in thermal
expansion between metal and porcelain, a
shearing force acts on their interface, and if
stress is sufficiently great, cracking, or
fracture may occur.

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CHEMICAL BONDING
The single most significant mechanism of
porcelain-metal attachment is a chemical bond
between dental porcelain and the oxides on the
surface of the metal substructure.
There are those who believe that two
mechanisms might exist within the chemical
(or molecular) bonding theory.
According to one hypothesis,the oxide layer
is permanently bonded to the metal
substructure on one side while the dental
porcelain remains on the other.

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The oxide layer itself is sandwiched in
between the metal substructure and the
opaque porcelain.
This sandwich theory is undesirable in that a
thick oxide layer might exist that would
weaken the attachment of metal to porcelain.
The second, and more likely, theory suggests
that the surface oxides dissolve, or are
dissolved by the opaque porcelain layer.
The porcelain is then brought into atomic
contact with the metal surface for enhanced
wetting and direct chemical bonding so metal
and porcelain share electrons.

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From a chemical standpoint, both covalent
and ionic bonds are thought to form but
only a monomolecular( single) layer of
oxides is believed to be required for
chemical bonding to occur.

Thank you
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Metal ceramics /certified fixed orthodontic courses by Indian dental academy

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