Influence of cast surface finishing process on metal-ceramic bond ...

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Medicinski Glasnik, Volumen 6, Number 2, August 2009
which ceramics starts to get separated from metal.
After point B, there is a short relaxation <strong>of</strong> the material,
which is reflected in the decrease <strong>of</strong> force
with simultaneous increase <strong>of</strong> deflection. After
this relaxation the force-deflection diagram corresponds
to the diagram for metal alone.
Based on the performed analysis <strong>of</strong> the results
it may be concluded that in order to make
valid conclusions on the strength <strong>of</strong> the bond between
the metal and the ceramics point B is the
most important one, i.e. force and deflection in
which the ceramics starts to get separated. Figures
4 and 5 show the diagrams <strong>of</strong> these values.
The method <strong>of</strong> separation <strong>of</strong> the ceramic layer
in the performed tests is almost equal for all the
groups <strong>of</strong> samples. The separation always starts at
the end <strong>of</strong> the sample and propagates towards the
middle, which corresponds to the guidelines <strong>of</strong>
standard HRN EN ISO 9693. In Figure 4, which
shows the deflection that has resulted in the separation
<strong>of</strong> ceramics from the metal frame, one may
notice that the mean values <strong>of</strong> deflection in all
the samples are approximate and range from 0.07
mm (sample 6) to 0.17 mm (sample 1).
Figure 5 shows that the highest mean value
<strong>of</strong> force at which ceramics separation was recorded
in sample 3, whereas the minimal mean
value <strong>of</strong> force is recorded in sample 6. The forcedeflection
diagrams make it possible to quantify
the difference in the bond strength <strong>of</strong> the tested
system based on the additional parameters.
The analysis <strong>of</strong> variance has been used to determine
characteristics <strong>of</strong> the difference between
the samples (p < 0.05) for load only, because the
separation <strong>of</strong> ceramics in all the samples occurs in
the approximate amount <strong>of</strong> deflection. The arithmetic
means <strong>of</strong> forces significantly differ among
individual groups <strong>of</strong> samples at a risk <strong>of</strong> 5%. In
sample 3 the force at which the separation <strong>of</strong> ceramics
comes is significantly greater compared
to other tested samples. In sample 6 the bond between
ceramics and metal fractures at significantly
lower forces than in all the other samples.
Sample 5 treated with bonding agent shows
on the overall fracture area the presence <strong>of</strong> a
layer, which corresponds to the fired agent. The
value <strong>of</strong> force necessary to separate the ceramics
from metal in this sample is not substantially
different.
DISCUSSION
An understanding <strong>of</strong> the bonding mechanism
is essential for successful metal-ceramic restorations.
Although number theories and concepts
have been proposed for the actual mechanism <strong>of</strong>
bonding, it still remains elusive. Different tests
have been used to determine metal-ceramic bond
strength and beam failure loads (16). Though it is
difficult to accurately quantify real bond strength,
the 3-point flexural test is frequently used. Flexural
tests were subjected to criticism because maximal
tensile stresses were created the <strong>surface</strong> <strong>of</strong> the ceramics
and resulted in predictable tensile failures.
The validity <strong>of</strong> these tests to evaluate different alloys
has been questioned because ceramic breakage
depended on the modulus <strong>of</strong> elasticity <strong>of</strong> the
metal tested. An alloy with an elevated modulus <strong>of</strong>
elasticity would resist flexural to a greater extent,
Figure 4. Deflection values (during a separation <strong>of</strong> ceramics
from the metal frame) Figure 5. Load in which ceramics were separated