The Effect of Oxide Layer Thickness on
Bond Strength of Porcelain to Ni-Cr Alloy
Shahin Rezaee Roknia, Hadi Baradaranb
Introduction: This study evaluated the effect of four oxidation techniques and porcelain firing on the thickness of
oxide layer on base-metal alloy (Ni-Cr) and the relationship of oxide layer to porcelain-metal bond strength.
Materials & Methods: This study had an interventional parallel design. Forty plates of base-metal alloy (Ni-Cr)
were prepared and divided into four groups to be treated with four different oxidation techniques (A- no oxidation; Boxidation in air; C- oxidation under vacuum; and D- oxidation under vacuum followed by sandblasting). Vita opaque and
body porcelain were applied on one side of the plates and baked in a furnace (Vacumat 200) following the recommended
directions. The thickness of the oxide layer was measured by an electron microscope before and after porcelain
application. The three-point bend test was used to measure the bond strength between porcelain and metal. The force
which caused breakage was recorded by computer. Data were analyzed with a one-way analysis of variance (ANOVA)
and the Duncan’s test with a 95% significance level.
Results: The mean thickness of oxide layer was statistically different between groups before and after porcelain
application (p=0.001<0.05). The Duncan multiple range test demonstrated significant differences in porcelain- metal
bond strength between different groups of oxidation. Group D showed the best bond strength and was statistically
different from the other groups.
Conclusion: Oxidation under vacuum followed by sandblasting seems to control the oxide layer and improve the
bond strength of porcelain to Ni-Cr alloy.
Key words: Oxide layer, bond strength, porcelain, Ni-Cr alloy.
Journal of Mashhad Dental School, Mashhad University of Medical Sciences, 2007; 31(Special Issue): 17-21.
eramic-metal restorations combine the beauty of
porcelain and the strength of a metal
substructure. The success and predictability of
bonding porcelain to gold base alloys are well
documented.1-4 Unfortunately, the cost of gold base
alloys for bonding porcelain has increased to such an
extent that the future use of the porcelain-gold system is
severely jeopardized. Accordingly, interest has been
renewed in the non-precious metal alloys as a substitute
for gold. However, during the past few years, the
technologic parameters of dental ceramic alloys have
incurred changes. Precious metal/ceramic alloys are
being challenged by manufacturers of numerous nonprecious alloys who claim superior physical properties
for their products. Even though non-precious alloys that
have superior mechanical properties include higher
modulus of elasticity, sag resistance, longevity, and
higher melting temperature, there appears to be an
investigative controversy about the bond strength of
non-precious alloy systems. Sced and Mclean found that
all base metal alloy systems break at the interface but do
not break in the porcelain, as is typical for gold alloy
systems.5 Moffa and associates determined that the bond
C
----------------------------------------------------------------------a
Professor, Dept of Prosthodontics, School of Dentistry and Dental
Research Center of Mashhad University of Medical Sciences.
Mashhad, Iran.
E-mail: rokni0rahmani@yahoo.com
b
Assistant Professor, Dept of Prosthodontics, Dental School,
Rafsanjan University of Medical Sciences, Rafsanjan, Iran
shear strength of two non-precious alloys fused to
ceramco porcelain was between 13,500 and 13,900 PSI,
which was comparable to the bond of a gold–ceramic
system.6 McClean demonstrated that nickel and
chromium oxide decreased the thermal coefficient of
expansion of vita porcelain that may induce stresses that
could cause failure of non-precious ceramic-metal
restorations.7
An oxide layer forms on the surface of most dental
porcelain fused to metal alloys (PFM) when they are
exposed to oxygen at high temperatures. Whether an
alloy is deliberately given an oxidation treatment prior
to porcelain application, or whether it oxidizes during
the portion of the firing cycle before the flow of the
porcelain begins, the fusing porcelain comes into
immediate contact with oxide rather than with the metal
surface. Various opinions exist as to how this oxide
interacts with porcelain during the firing cycle. It is
widely believed that the fusing porcelain dissolves away
the oxide originally formed and produces an interaction
zone responsible for the formation of a bond.8 Pask and
Fulrath reported a direct chemical bonding between the
porcelain and metal, although they did not reject the
possibility of a discrete oxide layer at the interface.9
Certain elements have been shown to accumulate at the
porcelain–metal interface. This concentration of
elements does not necessarily indicate a discrete oxide
layer, and may merely represent a solution of their ions
in the interface porcelain. Epitaxy effects or lack of
crystallinity would hinder detection of an oxide layer by
18
Journal of Mashhad Dental School, Mashhad University of Medical Sciences, 2007; 31(Special Issue): 17-21.
X-ray diffraction, as would the presence of only a very
thin layer of the oxide.10-12 However, only oxides
present in large quantities, such as Cr2o3, have been
positively identified.13 Compounds formed on Ni-Cr
alloys during conditions similar to those encountered
during dental laboratory practice was identified with Xray diffraction. Oxides of nearly all elements contained
by the alloys were found after low temperature (650oC)
oxidation, while Ni2o and particularly Cr2o3 were
predominant after oxidation at high temperatures
(1000oc).14,15
Deftary et al used different methods of pretreatment
on noble and base metal alloys to evaluate the effect of
these treatments on porcelain-metal bond strength. They
found significant differences between different metals
and suggested that each alloy should be treated
differently.16 Mackert et al reported that the strength of
oxide adherence to metal varies in different porcelain
fused to metal alloys.17 The fracture strength of
repeatedly fired porcelain veneered to high noble and
base metal alloy crowns were measured by Barghi et al.
The result showed as the number of firings increased,
the fracture strength decreased in high noble alloys but
did not significantly affect base metal alloys.18 The
shear bond strength of porcelain fused to metal was
compared in four gold and five non-precious alloys. The
bond in non-precious alloys was fairly lower and the
fracture type was adhesive in non-precious and cohesive
in gold alloys.19
The purpose of this study was to evaluate the
influence of the thickness of surface oxide layer in base
metal alloys on porcelain–metal bond strength.
Materials & Methods
Forty plates of Ni-Cr alloy (silver cast* JTC.Fulldent SA,
Switzerland) were made from green wax sheet (Krupp Dental
Co) with the dimensions of 30×5×0.5 mm. Metal plates
were sandblasted with 80 µm aluminum oxide grains
and ultrasonically cleaned.
Samples were divided into four groups to be treated
with different oxidation methods suggested by
manufacturers: Group A- no oxidation; Group Boxidation in air; Group C- oxidation under vacuum;
Group D- oxidation under vacuum followed by
sandblasting with 80 µm aluminum oxide grains at a
distant of 20 mm for 10 seconds with a 4 bar force.
Samples were oxidized in the furnace (Vacumat 200,
Vita Zahnfabrik H. Rauter GmbH & Co, Germany) under the
recommended directions for each technique. The
thickness of the oxide layer was measured by an
electron microscope (SEM) at three definitive points for
each specimen (Fig 1). The accuracy of the electron
microscope was 0.1 µm. Opaque and body porcelain
(Vita VMK 68N, Zahnfabrik H, Rauter, GmbH & Co, Germany)
were applied on one side of the plates and fired in the
*Ni : Principal content
Cr : 12.14%
Br : 1.6-1.9%
same furnace with the recommended directions for each
stage.
The thickness of porcelain was adjusted to 1 mm for
all specimens, and then they were auto-glazed. At this
stage, the thickness of oxide layer was measured at the
same definitive points by the electron microscope.
Bond strength of porcelain to metal was measured by
the three-point bend test in an Instron universal testing
machine (Instron Corp, Canton, Mass). For this purpose, test
specimens were placed into a special jig that was
tightened by two screws at its sides. The machine’s
cross head was moved at 0.5 mm speed per minute until
the bond between porcelain and metal broke. The force
which caused breakage was recorded by computer.
Mean values and standard deviations (SD) were
calculated for each group before and after porcelain
application, and compared by a one-way analysis of
variance (ANOVA), with the value of statistical
significance set at P=0.05. Multiple comparisons of
group means were made using the Duncan’s multiple
range test. All data were analyzed with a statistical
software package (SPSS/PC, SPSS Inc, Chicago). The
same statistical analysis was used to verify the relation
between bond strength and the thickness of oxide layer
for different oxidation methods.
Results
First stage result: The one-factor ANOVA indicated
a significant difference in the thickness of oxide layer
between groups before porcelain application with the
confidence of 95% (P =0.001<0.05). The thickness of
the oxide layer increased after porcelain application, and
there was also a significant difference between the 4 test
groups (P=0.001<0.05) (Fig 2). The Duncan’s multiple
range test revealed significant difference between
groups before and after porcelain application (Table 1).
Second stage result: In order to verify and compare
the porcelain-metal shear bond strength of the 4 test
groups, the one-way ANOVA test was used. This
analysis showed statistically significant difference
among the groups (P =0.00<0.05) (Fig 3).
The Effect of Oxide … (Shahin Rezaee Rokni, et al)
19
a
c
o
o
b
d
o
o
Fig 1. SEM images of oxide layer A, before oxidation. B, oxidation in air. C, oxidation under vacuum. D, oxidation under
vacuum followed by sandblasting.
NP
Groups
Mean oxide layer thickness
(0.1µm)
(SD)
After
oxidation
A
D
C
B
17.50 (2.85)
21.67 (2.28)
25.34 (3.60)
30.01 (4.01)
After
porcelain
application
56.05 (12.44)
73.21 (18.89)
85.49 (12.39)
100.40 (12.95)
Duncan test
A
*
*
*
D
*
*
C
B
Oxide layer thickness (0.1)
Table 1. Mean values and standard deviations of oxide
layer thickness after oxidation and porcelain application
and the Duncan multiple range test
P
120
100
80
60
40
20
0
*
A
B
C
D
Groups
Fig 2. Comparison of mean oxide layer before and after
porcelain application (NP=before) (P=after).
20
Journal of Mashhad Dental School, Mashhad University of Medical Sciences, 2007; 31(Special Issue): 17-21.
Bond strengths (MPa)
1800
1600
1400
1200
1000
A
B
C
D
Groups
Fig 3. Shear bond strength values of the 4 test groups.
The Duncan’s multiple range comparison test was
used to identify statistical differences between oxidation
techniques and porcelain-metal bond strength. The
result indicated that group D had the highest bond
strength and significant difference with the other
groups. There was no significant difference between
groups B and C, but these groups were different
statistically from group A. The least bond strength was
found in group A (Table 2,3).
Table 2. Statistical analysis of bond strength
Groups
A
B
C
D
N
10
10
10
10
Duncan’s test
Mean
Bond strength
(Mpa)
SD
1153.50
1407.71
1442.94
1655.92
53.3920
59.5603
66.4416
47.2953
A
B
C
*
*
*
*
*
D
Table 3. Comparison of mean oxide layer and bond strength
Groups
A
B
C
D
Mean Oxide layer (0.1µm)
After porcelain application
56.05
100.49
85.49
73.21
strength Mean bond
(Mpa)
1153.50
1407.71
1442.94
1655.92
Discussion
Different factors influence the bond between
porcelain and metal; therefore, finding a definitive
method to evaluate bond strength is difficult. Surface
roughness and contamination affect the mechanical
bond. Chemical bond is determined by base elements
existing in dental alloys and the wetting ability of alloy
by porcelain. Compression bond depends on the
geometry of the metal frame and thermal expansion
coefficient of metal and porcelain.
The most important bond mechanism is a chemical
type; and an oxide layer is responsible to adhere
porcelain to metal. The metal oxides have been studied
extensively and are thought to play an important role in
ceramo-metal bonding.20-22 Since all elements in basemetal alloys are able to be oxidized, achieving adequate
porcelain-metal bonding is fundamentally a question of
controlling the oxide layer formed on the surfaces of the
metal. Different methods are suggested for metal
oxidation and pretreatment before porcelain firing. In
this study, four pretreatment methods, which were
recommended by manufacturers, were tested on a basemetal alloy (Ni-Cr); and the thickness of the oxide layer
was measured before and after porcelain application by
the electron microscope. The results indicated a
statistically significant difference in oxide layer
thickness between oxidation methods. Oxidation in air
showed the thickest oxide layer. Baran found the same
result.15 The oxide on the surface of the metal provided
a bridging link for adherence of the porcelain to the
metal.23,24 The bond between oxide and porcelain was
strong but the loss of oxide adherence to the metal
substrate could cause separation of porcelain. The
formation of excessive oxide layers was found to be
enhanced by repeated firings of the base metal alloys.
25,26
The oxide layer thickness increased significantly
after each stage of porcelain firing in the current study
(Table 1).
In an in vitro study, the bond strength of a titaniumporcelain system in two firing conditions was evaluated
and compared with a Ni-Cr alloy. The result showed the
Ni-Cr conventional porcelain fired in an argon
atmosphere had significantly higher bond strength. The
SEM-EDS analysis of the metal oxide region revealed
that the amount of Ni2o and Cr2o3 was less in the argonfired specimens than in the vacuum-fired ones.27
Pask and Tomsia using Ni-Cr alloys, and Wanger et
al using palladium alloys found that vacuum firing
resulted in an excessive oxide layer thickness and an
argon atmosphere limited the oxide formation at the
metal-porcelain interface.28,29
Sandblasting has been shown to affect oxide layer
thickness; the oxide layer formed before sandblasting
differed from the one obtained after sandblasing.30
Sandblasted samples had significantly higher beam
failure loads than non sandblasted samples which
showed detachment of the porcelain from the metal.31 In
this current study, the mean thickness of oxide layer was
increased after firing under vacuum and decreased after
sandblasting for 10 seconds with a 4 bar force.
The amount of force required for the fracture of the
porcelain applied to the samples was significantly
different between groups, and the result indicated that
there was a relation between oxide layer thickness and
porcelain to metal bond strength (Table 3). In group A
(no oxidation technique), a thin oxide layer was present,
and this resulted in the least bond strength. When the
thickness of oxide layer was controlled by oxidation
under vacuum followed by sandblasting, bond strength
was the highest. Oxidation in air resulted in a thick
oxide layer and lower bond strength in comparison with
oxidation under vacuum. Excessive oxidation of the
alloy could lead to fracture through the metal oxide. The
The Effect of Oxide … (Shahin Rezaee Rokni, et al)
Ni-Cr alloys were expected to exhibit more oxidation
than noble alloys.32
It has been noted that an ideal bonding oxide should
consist of a monomolecular layer of oxide that is part of
the metal and the porcelain.9 The current study
confirmed that the oxide layer should be controlled to
improve bond strength.
Conclusion
Within the parameters of this study, the following
conclusions were drawn:
1. The thickness of oxide layer was increased after
oxidation. The greatest thickness resulted from
oxidation in air.
2. After porcelain application, the oxide layer was
sequentially increased.
3. There was a statistically significant relation
between the oxide layer and bond strength.
4. Degassing in vacuum followed by sandblasting
improved the bond strength of porcelain to basemetal alloy.
Acknowledgment
The authors wish to acknowledge the support given
by the Mashhad University of Medical Sciences.
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