Section I
*
Fundamentals of Dental Materials
66
Chemical Properties
Metals tend to lose electrons easily to form cat-
ions. They react with oxygen in the air to form
oxides; the time taken may vary. For example,
iron takes years to rust while potassium burns in
seconds. Transition metals such as iron, copper,
zinc, and nickel take much longer to oxidize
while noble metals do not react with atmo-
spheric oxygen at all.
Mechanical Properties
These properties are also the result of the
crystalline structure and metallic bonds of a
metal. Most metals are generally
ductile
(able to draw into a thin wire) and
malleable
(able to beat to a thin sheet). This is due to
the ability of the layers of atoms to slide against
each other into new positions within the same
crystal structure without breaking the metallic
bond.
If a small stress is put on the metal, the layers
of atoms will start to roll over each other. When
they fall back to their original positions on
release of this stress, the metal is said to be
elas-
tic
. This rolling of layers of atoms over each
other is hindered by grain boundaries, because
here the rows of atoms are not aligned properly.
Therefore, the smaller the individual grains, the
more the grain boundaries, and hence the
harder the metal becomes. Since atoms in
the grain boundaries are not in good contact
with each other, the metal tends to fracture at
these grain boundaries. Hence increasing the
number of grain boundaries makes the metal
not only harder, but also more brittle. It can be
concluded that
hardness
and
brittleness
of a
metal depend on its
grain size
.
Yield strength
is the amount of stress
required to produce a pre-established amount
of permanent strain (i.e., change in length) of
the alloy. Ideally, alloys used in the oral cavity
should have tensile yield strengths of more than
300 MPa so that a great deal of stress can be
applied before they permanently deform.
Modulus of elasticity
is the measure of
rigidity
or
stiffness
.
Since the distance between the atoms in a
crystal lattice is different in the horizontal
and vertical directions (depending on their
lattice structure;
Fig. 5.1
), the properties may
also vary if a single crystal is observed. This
directional property principle is followed in
the manufacture of microchips for computers.
However, in general practice it is not possible
to evaluate properties in different directions;
hence, the properties of a whole metal or alloy
are taken into consideration. So a fine-grained
structure is desirable for metals and their alloys
to have uniform properties in any direction.
Nonuniformity of directional properties is
termed
anisotropy
.
Dislocations
Metallic crystals are not perfect; they have flaws
called
dislocations
. Grain boundary is a type of
dislocation. Dislocations are of several types, but
generally all alloweasy deformation of themetal.
Fracture of metals occurs when the atomic centers
cannot slide over one another freely, because
impurities block the flow of dislocations. Hence,
all methods that increase the strength of the
metals act by controlling the movements
of these dislocations.
Coring
During solidification of an alloy, the last liquid to
solidify is the metal with lowest solidus tempera-
ture. Hence when an alloy is rapidly cooled, the
alloy has core structures, with themetal with high
solidus temperature forming the dendrites within
these cores. Themicrostructure of the rest of the
matrix between these cores consists of
metals with lower solidus temperatures.
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