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Available online at www.sciencedirect.com
Research Paper
The effect of thermal cycling on the shear bond
strength of porcelain/Ti–6Al–4V interfaces
Isabel A. Senda˜oa
, Alexandra C. Alvesa
, Rodrigo Galob
, Fatih Toptana,c,n
,
Filipe S. Silvaa,c
, Edith Arizaa,d
a
Centre for Mechanics and Materials Technologies (CT2M), Universidade do Minho, Azurém,
4800-058 Guimarães, Portugal
b
Department of Dental Materials and Prosthodontics, School of Dentistry of Araraquara—UNESP,
14801-903 Araquara, SP, Brazil
c
Universidade do Minho, Dept. Eng. Mecânica, Azurém, 4800-058 Guimarães, Portugal
d
Universidade do Minho, SEMAT/UM, Azurém, 4800-058 Guimarães, Portugal
a r t i c l e i n f o
Article history:
Received 6 June 2014
Received in revised form
23 December 2014
Accepted 3 January 2015
Available online 12 January 2015
Keywords:
Dental porcelain
Hot pressing
Shear bond strength
Thermal cycling
Titanium alloy
a b s t r a c t
The aim of the study was to evaluate the effect of thermal cycling on the shear bond
strength of the porcelain/Ti–6Al–4V interfaces prepared by two different processing routes
and metallic surface conditions. Polished and SiO2 particle abraded Ti–6Al–4V alloy and
Triceram bonder porcelain were used to produce the interfaces. Porcelain-to-metal speci-
mens were processed by conventional furnace firing and hot pressing. Thermal cycling was
performed in Fusayama’s artificial saliva for 5000 cycles between 571 and 6072 1C. After
thermal cycling, shear bond tests were carried out by using a custom-made stainless steel
apparatus. The results were analyzed using t-Student test and non-parametric Kruskal–
Wallis test (po0.01). Most of the polished-fired specimens were fractured during thermal
cycling; thus, it was not possible to obtain the shear bond strength results for this group.
Sandblasted-fired, polished-hot pressed, and sandblasted-hot pressed specimens presented
the shear bond strength values of 76.2715.9, 52.2723.6, and 59.9722.0 MPa, respectively.
Statistical analysis indicated that thermal cycling affected the polished specimens processed
by firing, whereas a significant difference was not observed on the other groups.
& 2015 Elsevier Ltd. All rights reserved.
1. Introduction
The modern types of ceramic-to-metal dental prostheses
have been used since the 1960s and they are still important
and reliable restorations in prosthodontics where a good
adhesion of the ceramic to metallic framework is required
to ensure the durability of the restoration (Anusavice et al.,
2007; Henriques et al., 2011). More than 20 metals and their
http://dx.doi.org/10.1016/j.jmbbm.2015.01.004
1751-6161/& 2015 Elsevier Ltd. All rights reserved.
n
Corresponding author at: Centre for Mechanics and Materials Technologies (CT2M), Universidade do Minho, Azurém, 4800-058
Guimarães, Portugal. Tel.: þ351 253 510231; fax: þ351 253 516007.
E-mail address: ftoptan@dem.uminho.pt (F. Toptan).
j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 4 4 ( 2 0 1 5 ) 1 5 6 – 1 6 3

2. alloys are available for dental metal–ceramic restorations,
however, commercially pure titanium (CP Ti) and some of its
alloys have gained attention during the recent decades due to
their excellent corrosion resistance, high strength-to-weight
ratio, adequate mechanical properties and good biocompat-
ibility (Bieniaś et al., 2009; Boeckler et al., 2009; Chakmakchi
et al., 2009; Furuhashi et al., 2012). Ti–6Al–4V is the most
widely used Ti alloy in biomechanical applications due to its
higher strength (Fonseca et al., 2003; Ho et al., 2009).
When a metal and a ceramic having different coefficients of
thermal contraction (CTC) values are fused, residual stresses
can concentrate at the interface during the cooling from the
processing temperature due to the difference in their thermal
contraction rates. Thus, depending on their magnitude, these
stresses may lead to cracks at the interface and eventually
may cause debonding of the dental porcelain (Henriques et al.,
2011). On the other hand, while a thin titanium oxide film
forming on titanium surfaces during firing is accepted to
increase the bond strength, thickening of this film at tempera-
tures above approximately 750 1C result in decreased bond
strength (Adachi et al., 1990; Akagi et al., 1992; Atsü and
Berksun, 2000; Hautaniemi et al., 1992; Mackert et al., 1984;
Oshida and Hashem, 1994). In order to avoid the CTC mis-
match, low-fusing porcelains have been developed to obtain
closer CTC values to that of the Ti, as well as to minimize the
high-temperature oxidation (Hussaini and Wazzan, 2005; Kim
and Cho, 2009; Vásquez et al., 2009).
In addition, residual stresses that may be occurred during
processing, thermal, physical and chemical changes routinely
that occur in the oral cavity may also cause failure of dental
structures (Bishara et al., 2007; Henriques et al., 2012; Tróia et al.,
2003; Vásquez et al., 2009). Thermal stresses may compromise
the integrity of the dental restorations by inducing tensile
stresses that can further induce cracks at the metal/ceramic
interface or by changing the gap dimensions between metal and
ceramic that can result in pumping oral fluids into the gaps
(Gale and Darvell, 1999).
Hot pressing is considered as a promising technique for
producing the ceramic-to-metal dental prostheses where an
enhancing diffusion between bulk metal and porcelain pow-
der interface leads to superior chemical bonding, as well as
reduced porosity and cracks (Henriques et al., 2012, 2011). The
previous study (Toptan et al., 2013) on the influence of the
processing route (i.e. firing and hot pressing) of porcelain/Ti–
6Al–4V interfaces on shear bond strength showed that pro-
cessing route was significantly important on the shear bond
strength of the interfaces having polished metal surfaces
whereas it was not significant on rough surfaces. Besides, it
has also been shown that while surface roughness was a
significant factor on the shear bond strength of the fired
interfaces, it was not significant on the strength of the hot
pressed specimens. However, since alterations in intraoral
temperatures induces repeated stress on metal/porcelain
interfaces (Vásquez et al., 2009) the effect of the thermal
cycling on the shear bond strength of porcelain/Ti–6Al–4V
interfaces should also be evaluated. Very few studies are
available in the literature that investigate the thermal cycling
behavior of the porcelain/Ti–6Al–4V interfaces. To the best of
our knowledge, no study has been reported on these inter-
faces processed by hot pressing. Thus, the present work aims
at evaluating the effect of thermal cycling on the porcelain/
Ti–6Al–4V interfaces processed by firing and hot pressing
techniques having both polished and airborne-particle-
abraded metal surfaces. The null hypotheses tested in this
study were that after thermal cycling, there would be sig-
nificant differences in quantitative shear bond strength and
differences in morphology of the porcelain/Ti–6Al–4V inter-
face between the two different processing routes and metallic
surface conditions.
2. Materials and method
2.1. Materials
Ti–6Al–4V alloy (Bunting Titanium, UK) and Triceram bonder
porcelain (Esprident, Germany) were used to produce the
metal/ceramic interfaces. The chemical compositions of the
materials are given in Tables 1 and 2. SiO2 particles with an
average particle size of 125 mm (Bego, Bremen, Germany) were
used for airborne particle abrasion.
2.2. Processing
Prior to the processing, one group of Ti–6Al–4V cylinders
4.4 mm in diameter and 4 mm in height, were ground
through 1200 mesh SiC paper and polished to 0.04 mm by
using colloidal silica suspension (Struers OP-S) and another
group were ground through 1200 mesh SiC paper and air
abraded by SiO2 particles under a pressure of 2 bars at a
distance of 10 mm to the surface. After both surface treat-
ments, specimens were ultrasonically cleaned in isopropyl
alcohol for 10 min.
Porcelain-to-metal specimens were prepared by two dif-
ferent routes, namely conventional furnace firing (porcelain-
fused-to-metal) according to the porcelain manufacturer’s
procedure (Table 3) and hot pressing by following the cycle
given in Fig. 1. The details for this processing procedure are
Table 1 – Chemical composition (weight %) of Ti–6Al–4V
alloy (from supplier’s datasheet).
Elements Al V Fe Sn Ni
Ti–6Al–4V 6.1 4.21 0.2 0.003 0.01
Table 2 – Composition of Triceram powder as elements and oxides (wt%) (Chakmakchi et al., 2009).
Elements (Oxides) O Si (SiO2) Sn (SnO2) Na (Na2O) K (K2O) Ba (BaO) Al (Al2O3) Ca (CaO)
Triceram 39.5 20.4 (45.0) 15.8 (20.7) 6.0 (8.2) 5.7 (7.0) 5.1 (5.9) 4.8 (9.2) 2.7 (3.9)
j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 4 4 ( 2 0 1 5 ) 1 5 6 – 1 6 3 157

3. explained elsewhere (Toptan et al., 2013), and the testing
groups are given in Table 4.
2.3. Thermal cycling
After processing, thermal cycling was performed in Fusaya-
ma’s artificial saliva having the composition given in Table 5
with bath temperatures of 571 and 6072 1C in order to
simulate the extremes in intraoral temperatures (Gale and
Darvell, 1999). The dwell time for each bath was maintained
for 15 s and the transferring time between the baths was 10 s
and each thermal cycling was completed after 5000 cycles.
2.4. Shear bond tests
After thermal cycling, porcelain-to-metal cylindrical speci-
mens 4.4 mm in diameter and 8 mm in height were subjected
to shear bond tests at room temperature by using a custom-
made stainless steel apparatus and a universal testing mac-
hine (Instron 8874, MA, USA), with a load cell having 25 kN
capacity at a crosshead speed of 0.5 mm/s. The details of the
shear test apparatus were explained elsewhere (Henriques
et al., 2011). Shear bond strength values were calculated using
the following formula:
SBS ¼
Fmax
A
ð1Þ
where SBS is the shear bond stress (MPa), Fmax is the maximum
load (N), and A is the cross-section surface area (mm2
).
2.5. Characterization
Before processing, the roughness of the airborne-particle-
abraded metallic surfaces was measured on three different
areas of each randomly selected group of five specimens using
a mechanical profilometer (Perthometer S5P, Mahr-Perthen).
After thermal cycling, specimens were perpendicularly cut into
two halves by a diamond saw, mounted into a resin, ground to
1200 mesh SiC paper and polished with colloidal silica suspen-
sion (Struers OP-S) to 0.04 mm in order to characterize the metal/
ceramic interfaces by field emission gun scanning electron
microscope (FEG-SEM), FEI Nova 200, equipped with energy
dispersive X-ray spectroscopy (EDS), EDAX. Broken surfaces after
the shear bond tests were also investigated by FEG-SEM/EDS.
2.6. Statistical analysis
t-Student test was performed to evaluate the differences
between the shear bond strength values obtained before
(Toptan et al., 2013) and after thermal cycling (po0.01). The
results for the PF group were analyzed using a non-parametric
Kruskal–Wallis test (po0.01) because the data was not normally
distributed. Statistical analysis was performed with SPSS soft-
ware for Windows, version 12.0 (SPSS Inc., Chicago, IL, USA).
3. Results
Shear bond strength as a function of thermal cycling is sum-
marized in Fig. 2, along with the results obtained in the previous
study (Toptan et al., 2013) before thermal cycling. Most of the PF
specimens were fractured during thermal cycling thus it was not
possible to obtain the shear bond strength results for this group.
The other groups, namely SF, PH, and SH had shear bond
strength values of 76.2715.9, 52.2723.6, and 59.9722.0 MPa,
Table 3 – Producer’s firing procedure.
Base temperature (1C) Heat rate (1C/min) Vacuum
start (1C)
Vacuum
end (1C)
Final temperature (1C) Holding time
under
vacuum (min)
Cooling
rate (1C/
min)
500 65 500 795 795 1 Ambient
air
Fig. 1 – Hot pressing cycle.
Table 4 – Testing groups.
Group Alloy surface Processing
PF Polished Firing
SF Silica-blasted Firing
PH Polished Hot pressing
SH Silica-blasted Hot pressing
Table 5 – Chemical composition of Fusayma’s articial
saliva (Henriques et al., 2012).
Composition Quantity (g/L)
NaCl 0.4
KCl 0.4
CaCl2
. 2H2O 0.795
Na2S . 9H2O 0.005
NaH2PO4
. 2H2O 0.69
Urea 1
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4. respectively. Statistical analysis indicated that thermal cycling
affected PF specimens, whereas a significant difference was not
observed on the other groups.
4. Discussion
Several mechanical tests are being used in order to evaluate
the bond strength of dental porcelain to metal but the failure
of brittle materials is technique-sensitive. The most common
methods are shear bond tests and three-point bending tests
(DeHoff et al., 1995; Melo et al., 2005; Vásquez et al., 2009;
Zinelis et al., 2010). Three-point bending test is considered as
a reliable method, however, maximal tensile stresses created
at the surface of the porcelain and the dependence of the
ceramic breakage on the Young’s modulus of the metal have
been considered as the limitations of the method (Hammad
and Talic, 1996; Saito et al., 2010; Tholey et al., 2007; Zinelis
et al., 2010). Shear bond testing, on the other hand, has been
received criticism due to the development of nonhomogen-
ous stress distributions within the adherence zone, that can
affect the failure mode (Saito et al., 2010; Scherrer et al., 2010;
Sun et al., 2000; Tholey et al., 2007). Even though, shear bond
testing is being widely used due to its easy and rapid test
protocol (Iseri et al., 2011). The design of the testing device
that was used in this work allows to apply the load precisely
to the metal/ceramic interface, that has been considered as
very similar to the intraoral incident forces on metal–ceramic
crowns (Bondioli and Bottino, 2004) (similar testing device
has also been used by Bondioli and Bottino (2004), Henriques
et al. (2011), Melo et al. (2005), Vásquez et al. (2009).
The cross-sectional SEM images of the metal/ceramic inter-
faces after thermal cycling are given in Fig. 3. It was observed
from the SEM images that similar to the interfaces before
thermal cycling (Toptan et al., 2013), PF interfaces presented
relatively thicker and continuous oxide layers as compared to
the interface of the PH interfaces (Fig. 3a and c). The bonding of
porcelain to dental alloys occurs with the contribution of four
mechanisms, namely, chemical bonding, mechanical
interlocking, van der Waals forces, and, in some systems,
compression forces (Bagby et al., 1990; Oshida et al., 1997;
Reyes et al., 2001; Venkatachalam et al., 2009). For the porcelain
adherence to the metal, there must be both an oxide layer at
the interface and the oxide layer must be adherent to the metal
(Mackert et al., 1988). By this way, chemical bonding will form
as a result of a thermodynamic equilibrium at the metal/
porcelain interface consisting three layers, namely metal, oxide
layer, and porcelain (Venkatachalam et al., 2009). However,
thickening of the oxide layer can cause damage at the interface
due to the induced stresses by the volume differences between
Ti and its oxide (Bowers et al., 1985; Gilbert et al., 1994;
Hautaniemi et al., 1992; Hussaini and Wazzan, 2005; Pang
et al., 1995). Therefore, one of the reason of having fracture
on the PF specimens during thermocycling can be attributed to
the thicker oxide layer formed at the PF interfaces (Fig. 3).
Besides, since the porcelain applied as powder on hot pressing
technique and sintered under spontaneous pressure, the
absence of wettability problems can also improve the chemical
bonding (Henriques et al., 2011). Another contribution to the
bonding of porcelain to metal occurs by van der Waals forces
that takes place with the physical attraction between charged
particles without an actual exchange or sharing of electrons.
These forces are weak, secondary forces but their effect
increase by increased wetting of the porcelain on the metal
(Bagby et al., 1990; Oshida et al., 1997). Therefore, the difference
on the shear bond strength values between PF and PH speci-
mens after thermal cycling may be mainly explained by
decreased thickness of the oxide layer and improved chemical
bonding and van der Waals forces on hot pressed specimens.
Thus, the null hypotheses tested in this study was accepted.
However, further studies are needed to measure the residual
and thermal stresses in order to understand the contribution of
each to the bond failure. Besides, shear bond tests should be
performed after lower numbers of thermal cycles for obtaining
shear bond strength from PF samples in order to evaluate the
influence of three experimental variables (i.e. processing route,
metallic surface condition, and thermal cycling).
Oxides were also observed on the interfaces of the silica
blasted specimens (Fig. 3a and c). However, they were often
disturbed by the embedded silica particles, thus formation of
continuous thick oxide layers were inhibited.
Airborne particle abrasion is being used in dentistry as a
standard procedure not only to increase the porcelain to metal
bond strength by mechanical interlocking due to the rough-
ened metallic surface, but also due to the increased surface
area (Kern and Thompson, 1994; Wang et al., 2010). The mean
roughness of the silica blasted surfaces was 0.5270.12 mm.
However, a considerable amount of embedded particles was
observed (Fig. 3) (Toptan et al., 2013). Contamination by air-
borne particle abrasion has also been reported by several
authors (Bondioli and Bottino, 2004; Johnson et al., 2006;
Kern and Thompson, 1994; Papadopoulos and Spyropoulos,
2009; Piattelli, 2003; Wang et al., 2010). However, the effect of
the embedded particles on the bond strength is still not clear.
On one hand, the embedded particles may lead to a decrease
in bond strength by inducing local cracks on the metal surface,
by creating stress concentration points, or by limiting the
metal surface area for porcelain bonding (Gilbert et al., 1994;
Hussaini and Wazzan, 2005). But on the other hand, if the
Fig. 2 – Shear bond strength; values before thermal cycling
are taken from Toptan et al. (2013) ((*) Since most of PF
specimens were fractured during thermal cycling, it was not
possible to obtain the shear bond strength results).
j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 4 4 ( 2 0 1 5 ) 1 5 6 – 1 6 3 159

5. particles are well attached to the surface, they may make a
positive contribution to the bonding strength (Wang et al.,
2010). Besides, as Johnson et al. (2006) reported, since most of
the dental porcelains contain alumina, alumina particle
embedded metallic surfaces may lead to an increase on the
bonding strength. Accordingly, since approx. 45% of Triceram
bonder porcelain consists of SiO2, one may expect that SiO2
particle-attached metallic surfaces may lead to an increase in
bond strength. However, in the previous study, no statistically
significant difference was observed between the shear bond
strength of SiO2 and Al2O3 blasted surfaces, both processed by
firing and hot pressing (Toptan et al., 2013).
As contrary to the as-processed specimens, cross-
sectional SEM images after thermal cycling revealed some
cracks within the metal/embedded silica particle interfaces
(Fig. 4). While the coefficient of thermal expansion (CTE) of
the Ti–6Al–4V alloy (9.2 Â 10À6
KÀ1
(20À315 1C) (Toptan et al.,
2013)) is very similar to that of the Triceram bonder porcelain
(9.2 Â 10À6
KÀ1
(25À400 1C) (Toptan et al., 2013)), there is a
significant difference from that of the silica particles
Fig. 4 – Backscattered SEM images of a broken embedded
silica particle at the metal/porcelain interface of the SF
specimen.
Fig. 3 – Backscattered SEM images of the metal/ceramic interfaces after thermal cycling for (a) PF, (b) SF, (c) PH, and (d) SH
conditions, together with EDS spectra taken from the marked zones representing the oxide layer, Ti–6Al–4V, porcelain, and
the silica particles embedded in the alloy surface.
j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 4 4 ( 2 0 1 5 ) 1 5 6 – 1 6 3160

6. (0.5 Â 10À6
KÀ1
(0À1000 1C) (Park et al., 2013)). The cracks may
be nucleated due to the sandblasting impact or during
processing due to the CTC mismatch between the Ti–6Al–4V
alloy and the silica. Although this behavior did not create a
statistically significant difference on the bond strength,
further studies are needed to clarify the effect of the CTC
mismatch between the embedded silica particles and the
metal on the bond strength after increased number of
thermal cycles.
Fig. 5 presents the broken metal surfaces after shear bond
tests. No methodological study was performed to evaluate
the fracture mode, however, similar to the as-processed
broken metal surfaces (Toptan et al., 2013), the presence of
only a small fragments of the porcelain suggested mainly
adhesive failure. A similar failure mode was also reported by
Vásquez et al. (2009) after studying the effect of 6000 thermal
cycles (between 5 and 55 1C with an immersion time of 13 s)
and mechanical cycling (20,000 cycles under 50 N load and
immersion in distilled water at 37 1C) on the shear bond
strength of Triceram porcelain to CP Ti. Furthermore, Tróia
et al. (2003) studied the effect of thermal cycling (3000 cycles
between 472 and 5572 1C with a immersion time of 10 s) on
the bond strength of low fusing ceramic to Ti–6Al–4V alloy
and reported that after three-point flexural tests, most of the
failure types observed were adhesive.
5. Conclusions
The effect of thermal cycling on the Triceram bonder porce-
lain/Ti–6Al–4V interfaces processed by conventional firing
and hot pressing having both polished and sandblasted metal
surfaces was investigated after 5000 cycles between 571 and
6072 1C. It can be concluded within the limitations of this
study that, thermal cycling affected the shear bond strength
of PF specimens, where relatively thick oxide layers were
present, whereas other groups did not exhibit a statistically
significant difference.
Acknowledgements
This study was supported by the Portuguese Foundation for
Science and Technology (FCT-Portugal), under the project
EXCL/EMS-TEC/0460/2012, and The Calouste Gulbenkian
Fig. 5 – Backscattered SEM images of the broken metal surfaces of the thermocycled specimens after shear bond tests for
(a) PF, (b) SF, (c) PH, and (d) SH conditions.
j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 4 4 ( 2 0 1 5 ) 1 5 6 – 1 6 3 161

7. Foundation through “Programa de Mobilidade Académica
para Professores”.
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