Keywords Bond strength - surface treatment - veneering ceramics - zirconia
Introduction
Numerous possibilities of materials are available for oral rehabilitation. Porcelain
esthetic level and the mechanical reliability of metallic frameworks have provided
the longer lifespan of these restorations.[[1 ]] However, the use of metals and the release of ions frequently cause gingival coloration
and allergic and inflammatory reactions.[[2 ]] This factors motivating the replacement of metallic alloys by biocompatible materials.
Esthetical solutions are increasingly challenging due to both patient demand and new
materials and techniques available. To this end, ceramic materials have been developed
with optical properties similar to natural teeth,[[3 ]],[[4 ]] besides excellent mechanical properties.
Among the ceramic materials used in frameworks, zirconia is notable for its tenacity.[[5 ]] For use in dental practice, it is usually stabilized on the tetragonal phase with
yttrium addition (Y-TPZ).[[4 ]],[[5 ]]
Clinical evidence about the tetragonal zirconia polycrystalline stabilized by Y-TZP
performance challenge the high frequency of coating ceramics chipping in comparison
to the metalloceramic system.[[6 ]],[[7 ]] Possible causes are related to adhesion ceramic materials to zirconia surface,
thermal and mechanical compatibility, coating ceramics thickness, technical control
of manufacture, among others.[[6 ]],[[7 ]],[[8 ]] The clinical consequence of a fracture in coating porcelain depends on its length
and location.[[9 ]]
When associating metalloceramics to zirconia ceramics, both of them need thermal and
mechanical compatibility. However, the metal is less damaging to the ceramic coating
because of its elastcity modulus. Due to its hardness, zirconia produces destructive
stress on the coating ceramics, and hence, the mechanical properties of coating ceramics
are essential for the restoration long-term success.[[10 ]],[[11 ]]
In view of exposed seems pertinent to assess, with the three-point flexural test,
the bond strength between Y-TZP and two coating ceramics, with two surface treatments.
Materials and Methods
This study evaluated the bond strength between Y-TZP (yttria zirkonzahn) and coating
ceramics (Ice Zirkon Keramik color A3 dentine and Vision zircon color A3 dentine)
also altering surface treatment (treated and nontreated).
Forty bar-shaped samples with dimensions of 25 mm × 3 mm × 0.5 mm were submitted to
ceramics application on its central portion with dimensions of 8 mm × 3 mm × 1 mm
for the flexural test and DIS.[[12 ]],[[13 ]] The D is test is characterized by the three-point flexural assay where the bonding line
of materials is simultaneously under compression, tensile, and shear strength.[[14 ]]
The cutting machine was set to 300 rpm (low speed). After cutting, the bars were then
sinterized for 8 continuous h at a temperature of up to 1700° in the Zirkonofen 600/v2
oven (Zirkonzahn GMBH - Bolzano - Italy). Subsequently, the surface treatment and
the coating ceramic application were carried out.
Surface treatment
The forty sinterized bars were divided into four groups (n = 10). Two groups were submitted to mechanical treatment (aluminum oxide blasting
25 μm, the pressure of 4.5 bar, and the other two groups received no mechanical treatment).
All samples were cleaned with water before application in an ultrasonic sink for 4 min
and 30 s.
Stratification
Two groups, treated and nontreated, were applied ceramics Ice ZirkonKeramik (Zirkonzahn
GMBH, Italy) color A3 dentine (same manufacturer as zirconia). The remaining groups
were applied vision zircon (Wohlwend AG - Liechtenstein) color A3 dentine.
A hollow matrix of dense addition silicone (Elite HdPutty Soft Normal Set-ZhermackSpA,
Italy) measuring 8 mm × 3 mm × 1 mm was prepared to receive the coating ceramics and
assure the samples standardization [[Figure 1 ]].
Figure 1: Matrix used in the application of coating ceramics. (a) Silicone matrix that standardize
stratification. (b) Visualization of a matrix with a sample after polishing
A ceramic paste was applied to the matrix and taken to an oven for burning at 490°C
for 1 min of precuring. In the following, vacuum was turned on with a heating rate
of 55°C/min until reaching the final temperature of 840°C. The sample was once again
placed in the matrix and assessed to check the need of material addition due to the
ceramics contraction. A second paste was molded and taken to the oven for a second
burning, which reached the final temperature of 830°C. Finally, the sample was finished
and polished [[Figure 2 ]].
Figure 2: Samples ready for testing. (a) Lateral view of the finished sample. (b) Group of
samples prepared for the testing
Three-point flexural test device
A device was designed (USIMAQ, Brazil) with a distance of 20 mm between supports and
support cylinders with 1 mm of radius. From this, a three-point flexural partially
articulated device was adapted [[15 ]] [[Figure 3 ]].
Figure 3: Flexural test device. Superior and inferior support view
Three-point flexural mechanical assay
The groups were named according to the following: vision zircon/without treatment
(VZ/woT), vision zircon/with treatment (VZ/wT), ice/without treatment, ice/with treatment
(I/wT).
The samples were mounted placing the ceramics opposite to the load application. The
universal testing machine EMIC DL 2000 (Instron-Equipamentos Científicos Ltda, Brazil)
was loaded with 50N load cell and 1 mm/min speed until failure.
The results were recorded by the software (Tesc version 3.01, EMIC- INSTRON BRASIL,
São José dos Pinhais/ Paraná/Brasil) in the assay machine.
Results
Statistical analysis
Shapiro-Wilk’s test showed normal distribution of the D is bonding strength test data (P = 0.994). Levene’s test also showed homogeneity between the experimental groups (P = 0.923). Both tests used α = 0.05.
One-factor variance analysis showed that the technical effects of coating ceramics
application on zirconia in a laboratory setting significantly influenced adhesive
strength between these materials (P = 0.019) [[Table 1 ]]. Tukey’s test showed that the group VZ/woT (31.26 ± 4.18 MPa) presented higher
DIS values in comparison to the groups VZ/wT (25.23 ± 4.52 MPa) (P = 0.027) and I/wT (25.27 ± 4.07 MPa) (P = 0.034) [[Table 2 ]]. Two-factor variance analysis revealed that the effect “surface treatment” before
application significantly affected the bonding strength of coating ceramics and zirconia
(P = 0.018). On the other hand, the effect “type of ceramics” showed no significance
(P = 0.138). Interactions show a significant influence of the surface treatment with
vision zircon ceramics and no statistical significance with ice ceramics (P = 0.000) [[Table 3 ]] and [[Figure 4 ]].
Figure 4: Group comparison
Table 1:
One-factor ANOVA (α = 0.05)
Variation source
Sum of squares
df
Mean square
F
Significant
Between groups
198.746
3
66.249
3.858
0.019
Within groups
497.974
29
17.172
Total
696.721
32
Table 2:
Tukey's test for variance difference (α = 0.05)
Groups (I)
Groups (J)
Mean difference (i-J)
Std.
Error
Sig
Tukey
Vision Zircon
Vision Zircon
6.02828*
2.01355
0.027
wo/Treat
w/Treat
Ace wo/Treat
4.69399
2.07193
0.130
Ace w/Treat
5.98988*
2.07193
0.034
Vision Zircon
Vision Zircon
–6.02828*
2.01355
0.027
wo/Treat
w/Treat
Ace wo/Treat
1.33428
2.01355
0.910
Ace w/Treat
–0.03840
2.01355
1.000
Ace wo/Treat
Vision Zircon wo/Treat
-4.69399
2.07193
0.130
Vision Zircon
1.33428
2.01355
0.910
w/Treat
Ace w/Treat
1.29589
2.07193
0.923
Ace wo/Treat
Vision Zircon wo/Treat
–5.98988*
2.07193
0.034
Vision Zircon
0.03840
2.01355
1.000
w/Treat
Ace wo/Treat
–1.29589
2.07193
0.923
Caption: *Mean difference is significant at the level of 0.05. Source: own authorship
Table 3:
Two-factor Anova (α = 0.05)
Source
Type III sum of squares
Df
Mean square
F
Sig
Model
24256.472a
3
8085.491
445.854
0.000
Effect type of ceramics
42.070
1
42.070
2.320
0.138
Effect treatment
114.558
1
114.558
6.317
0.018
Interactions
24100.491
1
24100.491
1328.962
0.000
Error
544.045
30
18.135
Total
24800.517
33
Failure analysis
All failures occurred in two stages: visible delamination of coating ceramics and
fracture at the midpoint of the zirconia bar. The analyzes were performed in the 40
× Binocular Stereo Microscope (Tecnival, São Paulo, Brazil).
Discussion
In the last years, new dental ceramic materials have been developed with the aim of
increasing the durability of metal-free reconstructions while keeping the esthetic
benefits.[[3 ]]
Ceramic materials are friable, and their irregular shapes make it difficult to standardize
the tests to evaluate their properties. Quinn and Goulet [[15 ]] developed an aticulated device for the flexural test. This study used a partially
articulated device and during the assay, the strength curve showed a constant oscillation
before loading, showing an adjustment of the device to the sample.
The three-point flexural strength test evaluates the adhesion of porcelain to metals.
It has also been used to assess porcelain adhesion to zirconia.[[14 ]]
The average result of 27 MPa of bonding strength between porcelain and zirconia in
this study corroborated the results found in other works, regardless of the test applied.[[1 ]],[[11 ]],[[12 ]],[[13 ]],[[16 ]],[[17 ]],[[18 ]],[[19 ]],[[20 ]],[[21 ]],[[22 ]],[[23 ]],[[24 ]],[[25 ]]
Zirconia surface can be treated with aluminum oxide, diamond drills, or silica coating
blasting. Some authors [[13 ]] reported that surface treatment with aluminum oxide blasting results in roughness
and increases zirconia’s surface energy. Better bond strength was observed when a
single ceramic coating was used on zirconia.[[13 ]] Other authors [[25 ]] suggested a chemical bond between zirconia and porcelain and ruled out the need
for pretreatment.[[6 ]],[[25 ]] Contrasting the aforementioned studies, in the present study, the surface treatment
of Y-TZP with aluminum oxide sandblasting when associated with vision zircon ceramics
significantly reduced bonding strength. In agreement, other authors also mention a
significant reduction in this adhesion; however, the roughening was performed with
diamond drills.[[21 ]]
It is of great relevance to study the types of coating ceramic and its mechanical
properties. Thermal compatibility and mechanics of materials are essential and can
influence results. Some authors have reported that the bigger the difference in the
thermal expansion coefficient of a ceramic, the bigger its bond strength.[[11 ]] When correlating stress caused by different coefficients of thermal expansion (CTE)
of coating ceramics, authors show that there is a limit on the difference in CTE,
which if exceeded, the stress caused in the cooling can damage the porcelain. Moreover,
this difference can be better defined by correlating the bond strength of the ceramics.[[10 ]] In the present study, the ceramic was burned following the technical instruction
provided by the manufacturers.
In this study, the type of ceramics did not alter the results; however, the surface
treatment of Y-TZP Zirkonzahn with vision zircon ceramic (different manufacturers)
allowed a significant reduction in bond strength. Notwithstand, ice showed similar
strength. The same result was found by Cömlekoglu et al. [[26 ]] using ceramics from different manufacturers. Differences in bond strength can occur
since the type of grain, CTE, and capacity to resist stress are properties that can
change depending on the manufacturer. These variations can show different behaviors
according to the application techniques.
The liner or intermediate ceramics have been presented by manufacturers as an enhancer
of adhesion of coating ceramics to zirconia. However, some authors do not reported
differences in bond strength when using liner,[[12 ]] while others showed worse results with their use.[[20 ]] Here, no intermediate ceramics were used since the ceramics paste have a larger
volume than the liners and therefore have a greater influence on adhesive strength
than the liner.[[11 ]]
In the present study, the failures found were delamination and fracture at the midpoint
of the zirconia bar. Prosthetics restorations involving zirconia are more sensitive
to technical errors, contrarily to those based on metals. However, a 5-year clinical
study did not show high failure rates with monolithic zirconia restorations. The global
rate of fracture for this restorations, regardless of position and type, was 1.09%.[[27 ]] Hence, it is important that the clinical stages are compiled to allow the material’s
adequate thickness, as well as the correct technical process, to result in a proper
function of the prosthesis in the oral cavity, absence of trauma, and long lifespan.
It can be seen, therefore, that this study is in addition evidence for a better understanding
of the possibilities of the materials studied.
Conclusion
According to the results, it was concluded that the type of coating ceramics evaluated
have no influence on the bonding strength, and the surface treatment decreased the
bond strength depending on the type of ceramic used.
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