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

ORIGINAL ARTICLE
<strong>Influence</strong> <strong>of</strong> <strong>cast</strong> <strong>surface</strong> <strong>finishing</strong> <strong>process</strong> on metal-ceramic
bond strength
Ketij Mehulić 1 , Martina Lauš-Šošić 2 , Zdravko Schauperl 3 , Denis Vojvodić 1 , Sanja Štefančić 2
1 Department <strong>of</strong> Prosthodontic School <strong>of</strong> Dental Medicine, University <strong>of</strong> Zagreb, 2 Dental Polyclinic, 3 Faculty <strong>of</strong> Mechanical Engineering and
Naval Architecture, University <strong>of</strong> Zagreb, Zagreb, Croatia
Corresponding author:
Ketij Mehulić,
University <strong>of</strong> Zagreb,
School <strong>of</strong> Dental Medicine,
Department <strong>of</strong> Prosthodontic
Gundulićeva 5, 10 000 Zagreb,
Croatia
Phone: ++385 1 4802 112;
Fax: ++385 1 4802 159;
e-mail: mehulic@sfzg.hr
Original submission:
19 March 2009.;
Revised submission:
06 April 2009.;
Accepted:
10 April 2009.
Med Glas 2009; 6(2): 235-242
ABSTRACT
Aim To investigate the influence <strong>of</strong> different <strong>cast</strong> <strong>surface</strong> <strong>finishing</strong>
<strong>process</strong> on metal-ceramics bond strength.
Methods Six Co-Cr alloy sample groups were <strong>cast</strong> (Wirobond C,
BEGO, Bremen, Germany) and randomly selected for use in one
<strong>of</strong> the six different final <strong>process</strong>ing <strong>of</strong> the <strong>cast</strong>ing <strong>surface</strong> (oxidation,
sandblasting with 110 and 250 µm Al 2 O 3 , bonding agent,
hydrochloric acid solution) prior to application <strong>of</strong> feldspathic ceramic
(Duceram Kiss, DeguDent, Hanau-Wolfgang, Germany).
The testing was carried out with a tensile testing machine (LRX
with Nexygen s<strong>of</strong>tware, Lloyd Instr., Fareham, UK) (ISO 9693).
Results The highest force (66.902 N) for the separation <strong>of</strong> ceramics
measured with the sample sandblasted with 250µm Al 2 O 3 ,
oxidised and repeatedly sandblasted with 250 µm, and the lowest
force (36.260 N) with the sample treated with hydrochloric acid
solution. With all sample groups except the group with the bonding
agent (cohesive fracture), an adhesive fracture <strong>of</strong> the medium
and an adhesive-cohesive fracture <strong>of</strong> the peripheral part <strong>of</strong> the
fracture <strong>surface</strong> were observed. The oxidation, prolonged oxidation
and the bonding agent do not influence the bond strength <strong>of</strong>
the tested metal-ceramic system.
Conclusion Different <strong>cast</strong>ing <strong>surface</strong> treatments have an important
role on the bond strength <strong>of</strong> the ceramic-metal interface.
Key words: <strong>cast</strong>ing <strong>surface</strong> <strong>process</strong>ing, bond strength, metal-ceramic
restoration, metal-ceramic interface
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INTRODUCTION
Ceramics are being used increasingly as a
restorative material in a variety <strong>of</strong> dental restorations,
including metal-ceramic crowns, all-ceramic
restorations, and fixed partial dentures, mainly
as a result <strong>of</strong> their excellent aesthetic properties,
durability, biocompatibility and resistance to wear
(1). Ceramic for dental reconstructive work are
multiphase silicate ceramics, glass ceramics or
monophased glasses with varying compositions
(2,3). Structure composed <strong>of</strong> ceramic layers on
a metal frame combined the strength <strong>of</strong> a metal
substrate (dental alloy) with aesthetic <strong>of</strong> a ceramic.
Currently, these ceramic fused to metal appliances
are widespread in use in prosthodontics. Because
<strong>of</strong> their inherently brittle nature susceptibility to
their failure was identified at localized areas <strong>of</strong> high
stress concentration on the ceramic <strong>surface</strong>, metalceramic
interface or within the microstructure (4).
In any laminate composite system the strength <strong>of</strong>
the interfacial bond between the individual laminates
is a major factor in determining the overall
resistance <strong>of</strong> the system to deformation and failure
(5,6). A strong interface should provide sufficient
stress transfer between the individual laminates to
allow the applied loads to be transferred and accommodated.
Conversely, a weak interface will
frequently result in failure by a <strong>process</strong> <strong>of</strong> delaminating
under an applied load possibly arising
from crack initiation and propagation within and
along the layer (7). These bilayered composites
have attracted considerable attention from laboratory
researchers seeking to understand the failure
characteristics <strong>of</strong> ceramic fused to metal systems.
Alterations to the interfacial region between bilayered
structures are <strong>of</strong> considerable interest and
authors have reported the effects <strong>of</strong> variations in
the interfacial <strong>surface</strong> roughness on the mechanical
properties <strong>of</strong> metal-ceramics specimens (8).
By improving a final <strong>surface</strong> treatment <strong>of</strong> metal
substructure could be significantly improved functional
durability <strong>of</strong> metal-ceramic appliances.
Oxidation heat treatment <strong>of</strong> the metal is used
to remove the entrapped gas, eliminate <strong>surface</strong>
contaminants, and form the metal oxide layer.
An alloy is deliberately given an oxidation treat-
ment prior to ceramic application, or whether it
oxidizes during the portion <strong>of</strong> the firing cycle
before flow <strong>of</strong> the ceramics begins, the fusing ceramics
comes into immediate contact with oxide
rather than with metal <strong>surface</strong> (9,10). Different
opinions exist as to how this oxide interacts with
ceramic during the firing cycle. It is widely believed,
that the fusing ceramic dissolves away the
oxide originally formed and produces an interaction
zone responsible for the formation <strong>of</strong> a bond
(11). King rejected the oxide layer theories extend
at that time (Kauzt 1936.) which postulated
that a layer <strong>of</strong> oxide adherent to the metal is wetted
by the ceramics and becomes the transition
zone between the metal and glassy matrices (12).
Pask (13) otherwise suggest a direct chemical
bonding between the ceramic and metal. According
to Mackert (11) the chromium-containing alloys
all contain oxygen-active elements: beryl,
aluminium, vanadium, titanium, and/or yttrium.
Boron oxide makes these alloys self-fluxing during
melting and gives them unique melting and
<strong>cast</strong>ing behaviour. The addition <strong>of</strong> aluminium to
these alloys adversely affects this behaviour because
<strong>of</strong> its tendency to produce slag (14). Because
<strong>of</strong> the close correspondence between oxide
adherence and ceramic bonding, it can only be
concluded that the adherence <strong>of</strong> the oxide plays a
dominant role in ceramic bonding (11).
The aim <strong>of</strong> this study was to investigate the
influence <strong>of</strong> different <strong>cast</strong> <strong>surface</strong> <strong>finishing</strong> <strong>process</strong>
on metal-ceramics bond strength.
Table 1. Procedures <strong>of</strong> metal <strong>surface</strong> treatment for each
group <strong>of</strong> samples
Specimen Metals <strong>surface</strong> treatment
1 Sand blasting with 110 μm Al2O3 particles
2
3
4
Sand blasting with 110 μm Al2O3 particles
Oxidation
Sand blasting with 110 μm Al2O3 particles
Sand blasting with 250 μm Al2O3 particles
Oxidation
Sand blasting with 250 μm Al2O3 particles
Sand blasting with 110 μm Al2O3 particles
Extended oxidation
Sand blasting with 110 μm Al2O3 particles
Sand blasting with 110 μm Al2O3 particles
Oxidation
5
Sand blasting with 110 μm Al2O3 particles
Bonding agent
6 Etching in acid mixture

MATERIALS AND METHODS
Six groups <strong>of</strong> same three metal <strong>cast</strong> plates,
25×3×0.5 mm have been produced according
to the manufacturer’s instructions. The used alloy
(Wirobond C, BEGO, Bremen, Germany)
Mehulić et al Surface <strong>finishing</strong> and bond strength
(weight percentage: Cr 26%, Mo 6 %, W 5 %, Si
1 %, Fe 0.5 %, Ce 0.5 %, C 0.02 %, and the rest
Co) belongs to the group <strong>of</strong> cobalt-chrome alloys
free <strong>of</strong> the contents <strong>of</strong> beryllium and nickel. Thus
produced samples are cleaned and handled in
same direction, and the <strong>surface</strong>s to which ceram-
Sample 1 Sample 2
Sample 3 Sample 4
Sample 5 Sample 6
Figure 1. Specimens’ <strong>surface</strong> <strong>of</strong> the sample prepared and recorded by a scanning electronic microscope (SEM) with the secondary
electron detector (SE) (Faculty <strong>of</strong> Mechanical Engineering and Naval Architecture, University <strong>of</strong> Zagreb, Croatia, 2008., with permission)
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ics is applied are treated by different procedures
and combinations <strong>of</strong> procedures (Table 1).
Sandblasting was achieved with 110 and
250 μm Al2O3 particles (Shera, Lemförde, Germany).
The used bonding agent (3C-Bond, Al-
Sample 1 Sample 2
Sample 3 Sample 4
Sample 5 Sample 6
Figure 2. Results <strong>of</strong> 3-point bending test performed on six groups <strong>of</strong> specimens

phadent N.V., Antwerpen, Belgium) is applied to
the samples in group 5. The samples <strong>of</strong> group 6
are kept in the solution obtained by mixing 50 ml
<strong>of</strong> distilled water and 50 ml <strong>of</strong> 32% hydrochloric
acid for 30 minutes. After etching these samples
are first <strong>of</strong> all washed in distilled water, and then
in the compound <strong>of</strong> ethyl alcohol and acetone in
ratio 1:1. Figure 1 shows the characteristic <strong>surface</strong>
<strong>of</strong> the sample prepared in this way recorded
by a scanning electronic microscope (Tescan
Vega TS5136LS, Tescan, Brno, Czech R) with
the secondary electron detector (SE).
Along the middle <strong>of</strong> thus prepared metal
plates the ceramics (Duceram Kiss, DeguDent,
Hanau-Wolfgang, Germany) is fired (ceramic
furnace Focus 2006, Shenpaz, Tel Aviv, Israel) in
the length <strong>of</strong> 8 mm, width <strong>of</strong> 3 mm, and thickness
<strong>of</strong> 1 mm. The ceramics corresponds to the manufacturer’s
instructions and belongs to the group
<strong>of</strong> ceramics with the fired temperature <strong>of</strong> up to
980°C, suitable for coating <strong>of</strong> the mentioned alloy.
The samples are tested by bending in three points
on the tester machine (LRX Lloyd Instruments,
Fareham, Great Britain) with installed Nexygen
programme for the <strong>process</strong>ing <strong>of</strong> results. The
samples are set so that the <strong>surface</strong> with ceramics
is turned opposite to the pin, and the metal part
resting on the supports at a distance <strong>of</strong> 20 mm,
and the diameter <strong>of</strong> pin that loads the sample is 1
mm. The shift <strong>of</strong> pin is constant during testing at
a speed <strong>of</strong> 1.5 mm/min, and the testing continues
Figure 3. Typical areas during three-point bending test
Mehulić et al Surface <strong>finishing</strong> and bond strength
until the fracture, i.e. to full separation <strong>of</strong> the ceramics
from the metal.
Testing procedure has been carried out according
to the guidelines given in ISO 9693
(15).
After testing the samples type <strong>of</strong> fracture <strong>surface</strong>s
(cohesive, adhesive or cohesive-adhesive)
were examined by scanning electronic microscope
(Tescan Vega TS5136LS, Tescan, Brno, Czech R).
The same person has performed all the tests.
The multiple range tests, Fischer’s LSD test
and ANOVA have been used for statistic analysis.
RESULTS
The results <strong>of</strong> 3-point bending test performed
on 6 groups <strong>of</strong> specimens, (each group has three
specimens) are presented in Figure 2.
The diagrams obtained by testing on the tester
and presented in Figure 2 show the same trend,
i.e. the behaviour <strong>of</strong> all samples during testing is
inter-compatible. Therefore, Figure 3 can generally
explain the behaviour <strong>of</strong> all metal-ceramic
systems in a three-point flexure bond test.
According to Figure 3 it is possible to define
three characteristic areas during testing. The
beginning <strong>of</strong> testing where the force-deflection
diagram is a horizontal line, i.e. the pin is lowered
without increase <strong>of</strong> force, represents the first
area. Such behaviour is caused by preparation <strong>of</strong>
testing and represents the period from beginning
<strong>of</strong> testing to the moment <strong>of</strong> achieving the predefined
pre-load.
Point A (Figure 3), where a sudden increase
in force is noticed, represents the moment <strong>of</strong>
contact between the pin and the sample and the
actual beginning <strong>of</strong> the testing area 2. The linear
part <strong>of</strong> the diagram that follows from this point
represents common resistance to flexing <strong>of</strong> the
metal-ceramic sample, since in this area the bond
between metal and ceramics is still strong.
Point B (Figure 3) represents the start <strong>of</strong> the
third area, i.e. the moment <strong>of</strong> loosening <strong>of</strong> the bond
between metal and ceramics and the moment at
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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

creating a higher bond (17). It is difficult to quantify
the real bond strength because in vitro testing
is not usually in correlation with ceramic breakdown
in function. The shear strength <strong>of</strong> the metalceramic
bond was evaluated by the Shell-Nielsen
test described by Dent (18) method similar to that
used by Anthony (19), M<strong>of</strong>fa (20), Diaz (21),
Anusavice (22), Warpeha (23), Miller (24), Riley
(25). The authors suggested that the differences
in oxide composition and amount, influenced by
different <strong>surface</strong> <strong>finishing</strong> procedures. Sandblasting
the finished <strong>surface</strong> is though to remove furrows,
overlaps, and flakes <strong>of</strong> metal created by the
grinding <strong>process</strong>. A sandblasted <strong>surface</strong> may have
higher <strong>surface</strong> energy that alloys increased wetting
<strong>of</strong> the metal during ceramic application. Evidence
suggested that this roughened <strong>surface</strong> could also
provide mechanical interlocking and increase the
<strong>surface</strong> area for metal-ceramic bonding (26). According
to Brantley (27), oxide layer is different
before and after sandblasting. Graham (28) suggested
final <strong>finishing</strong> <strong>process</strong> in the order: sandblasting,
grinding, sandblasting and oxidation.
Smoother <strong>surface</strong> achieved the lowest values <strong>of</strong>
bond strength and bonding agent did not improve
bond strength because <strong>of</strong> hermetical sealing <strong>of</strong> <strong>cast</strong>
<strong>surface</strong> (29). It created alumina layer on <strong>cast</strong> <strong>surface</strong>
and thus change oxide ratio on it (30). The gold
rich bonding agent reduced the interfacial stress by
improving the compatibility between ceramic and
metal (31). Basic elements oxidised selective; and
created Fe 2 O 3 , In 2 O 3 i SnO 2 on <strong>cast</strong> <strong>surface</strong> (32).
The amount <strong>of</strong> oxides is not always in proportions
with elements, which were added. Rake (33) suggested
opaque in two layers on unutilised <strong>surface</strong>s.
In Ni-Cr alloy (34) and alloy with Pd (35) ceramic
fired in vacuum produced excessive amount <strong>of</strong> oxides,
and argon reduced their appearance.
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ACKNOWLEDGEMENT
Grant No. 065-0650446-0435, and No. 120-
1201767-1762 from the Ministry <strong>of</strong> Science,
Education and Sports <strong>of</strong> the Republic <strong>of</strong> Croatia
supported this work.
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