Biosci. Biotech. Res. Comm. 10(3): 424-430 (2017)
Bond strength of porcelain to cobalt chromium dental
alloy fabricated by selective laser melting and casting
Fariborz Vafaee
, Farnaz Firouz
, Parisa Alirezaii
, Kusha Gholamrezaii
and Sara Khazaei
Associate Professor, Department of Prosthodontics, School of Dentistry, Hamadan University of Medical
Sciences, Hamadan, IR Iran
Assistant Professor, Department of Prosthodontics, School of Dentistry, Shahid Beheshti University of
Medical Sciences, Tehran, IR Iran
This study aimed to compare the bond strength of porcelain to cobalt-chromium dental alloy fabricatedby selective
laser melting (SLM) and casting methods.Twelve rectangular barsmeasuring 25x3x0.5 mmwere fabricated of cobalt-
chromium alloy for each of the SLM and casting groupsaccording to ISO9693:1999. Porcelain was appliedat the
center of each bar measuring 3 8 mm with 1 mm thickness. Three-point  exural bond strength test was performed
to assess the bond strength of porcelain to alloy. Data were analyzed and compared between the SLM and casting
groups via Independent sample t-test (alpha=0.05). Mode of failure was also determined by visual inspection of
samples. The mean bond strength of porcelain to SLM alloy (35.26±1.22 MPa) was signi cantly higher than that to
casting alloy (33.21±3.02 MPa) (P<0.05). Most failures in both groups were mixed and no sample showed adhesive
failure. The SLM metal-ceramic system showed higher bond strength than the required threshold by ISO9693:1999.
Compared to alloys fabricated by the casting method, the SLM method showed higher bond strength to porcelain.
This relatively new technology is promising for dental application and can serve as a suitable alternative to the
conventional casting method for the fabrication of metal-ceramic restorations.
*Corresponding Author:
Received 23
June, 2017
Accepted after revision 24
Sep, 2017
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DOI: 10.21786/bbrc/10.3/15
Fariborz Vafaee etal.
Metal-ceramic systems for dental restorations are avail-
able since the 1960sand the success of metal-ceramic
restorations depends on the strength and uniformity of
bond at the metal-porcelain interface, which is the most
sensitive area in terms of crack formation (Bowers 1985)
and (Drummond and et al 1984, Pavlovic
2017 and
Abduo 2017). Cracks mainly occur at the metal-ceramic
interface or within the veneering porcelain (Daftary
1986) and (Zhukovsky 1996). Factors such as trauma,
fatigue, occlusal loads and incompatibility between the
mechanical properties of metal and porcelain may result
in porcelain fracture especially of cohesive type (Pamei-
jer 1996 and Kelsey 2000,Ren 2016 and Kaleli 2017).
Metal-ceramic frameworks are conventionally fabri-
cated using the lost wax casting (Anusavice 2003) and
(Kaleli 2017).
However, problems associated with casting of base
metal alloys such as high melting point, high oxidation
potential during casting, time consuming nature of cast-
ing and  nishing of these alloys, limit their application.
Thus, application of a technique to eliminate the casting
process may enhance the use of these alloys (Bhaskaran
2013) and (Akova 2008).
The selective laser melting (SLM) technology, also
known as the three-dimensional (3D) printing, was
introduced for the fabrication of metal copings in metal-
ceramic crowns. This technique does not have many of
the limitations of the conventional waxing technique, so
thatthe restorations are fabricated by incremental appli-
cation of 0.02 μ layers (Xin 2012). This technique works
based on the computer aided design (CAD) data obtained
from the framework design, and uses high temperature
laser beams for selective melting of metal framework,
which is in the form of powder. The metal framework
is formed by incremental addition of these thin layers
(Akova 2008).
The success of SLM is related to its ability in fabri-
cation of metal components with complex geometrical
shapes from a 3D CAD model. Moreover, this technique
can produce components with mechanical properties
comparable or superior to those of conventionally fabri-
cated components (Örtorp 2011) and (Quante 2008).Thus,
it can be an alternative to the conventional process. Only
a few studies have assessed the bond strength of porce-
lain to alloys fabricated by the SLM technique. Some of
these studies have reported superior mechanical proper-
ties for copings fabricated by this method and higher
bond strength of porcelain compared to those fabricated
by the conventional casting method ,while some others
found no signi cant difference in bond strength of por-
celain between the two groups (Liu YH 2010) and (Wu
2014) and (Ren 2016).
Considering the gap of information and the contro-
versies in this regard, this study aimed to assess the bond
strength of cobalt-chromium (CoCr) alloy fabricated by
the SLM technique and the conventional wax burnout
and casting method.
This in vitro, experimental study was conducted on 24
samples (12 fabricated by conventional casting and
12 fabricated by the SLM technique). In the casting
group,12rectangular bars were fabricated measuring 25
mm ± 1mm × 3 mm ± 0.1 mm × 0.5 mm ± 0.05 mmac-
cording to the ISO9693-1 (The International Organiza-
tion for Standardization 2012) using green wax sheets
(Berg, Karl Berg GmbH, Engen, Germany). They were
spruedand  asked using phosphate-bonded investment
material. After wax burnout at 650°C, melted CoCr was
poured into the space created by wax burnout using a
standard broken-arm centrifugal casting machine (Kerr,
Orange, CA, USA). Metal sheets were cleaned from
investment material and the sprues were cut. Dimensions
were adjusted and the thickness of sheets was ensured
by a caliper and gauge.
In the SLM group, 12 samples were fabricated with
the same dimensions as those in the casting group using
EOSINT M270 machine (Manufacturing system; Bego,
Bremen, Germany).This device was equipped with ytter-
bium laser  ber with less than 200 W laser power, scan-
ning speed of 5-10 mm/second, wavelength of 1060-1100
nm, beam diameter of 100-500 μm and powder feed rate
of 5-7 g/minute. Using data obtained from a CAD  le
with STL format, the device fused metal particles and
after scanning each cross-section, the alloy thickness
decreased by one layer and a new layer of alloy was
applied on the upper surface. This process was repeated
until the entire restoration was formed. Table 1 shows
the composition of CoCr alloy used for the casting (WBC
9581TM, Bego, Germany) and SLM (EOS Cobalt Chrome
SP2; EOSGmbH, Germany) techniques.
The samples in both groups were then subjected to
air-borne particle abrasion for  ve seconds (with 110-
250 μ aluminum oxide particles at 3-4 MPa pressure for
the SLM alloy and 120 μ aluminum oxide particles at
0.35 MPa pressure for the casting alloy) to provide sur-
face roughness similar to the standard process practiced
in the clinical setting. The samples were then cleaned
with vapor spray for six seconds. Pre-oxidation was
then performed according to the manufacturer’s instruc-
tionsat 950-980°C for  ve minutes for SLM and 980°C
for eight minutes for the casting alloy. Air-borne particle
abrasion was then performed again and before porcelain
application, the samples were cleaned in an ultrasonic
Fariborz Vafaee etal.
Table 1. Composition of CoCr alloy used for the casting and SLM
techniques according to the manufacturers (wt%)
Group Co Cr Mo W Si Nb V Fe Mn Ce
SLM alloy 63.9 25.3 5.2 5.5 1 Max 0.5 Max 0.1
Cast alloy 61 26.6 5 <2 <2 <2
Co, cobalt; Cr, chromium; Mo, molybdenum; W, tungsten; Si, silicon; Nb, niobium; V,
vanadium;Fe, iron; Mn, manganese; Ce ,Cerium; SLM, selective laser melting; max, maximum.
FIGURE 1. Casting group samples
FIGURE 2. SLM group samples
bath containing 95% ethanol (Elmasonic, Elma Hans
GmbH & Co, Koln, Germany) for  ve minutes.
Next, equal layers of opaque and body porcelain
(VMK 95, Vita, Bad Säckingen, Germany) with 8 mm
length, 3 mm width and 1 mm thickness (0.2 mm opaque
porcelain and 0.8 mm body porcelain) were applied and
baked according to the manufacturer’s instructions
FIGURE 3. Silicon index used for porcelain
FIGURE 4. Three-point  exural test
(Figures 1 and 2). For the purpose of standardization of
porcelain thickness, a silicon index was used for all sam-
ples (Figure 3). To minimize the effect of confounders,
all procedures were performed by one operator.
A three-point  exural strength test was performed
according to the ISO9693-1 standard using a universal
testing machine (Z020, Zwick Roell, Germany) .Surface
of the sample with porcelain over it was placed down-
ward and the two ends of each rod were placed on the
supporting fulcrums with 0.9 mm diameter and 20 mm
distance from each other. Load was applied to the center
of each sample by a crosshead with 0.9 mm diameter at
a speed of 1 mm/minute( gure 4). Load application was
continued until failure (abrupt drop in the stress-strain
curve). The load at failure was recorded and reported by
the computer. Bond strength in megapascals (MPa) was
calculated using the formula A= k x F (N/mm2) where k
Fariborz Vafaee etal.
FIGURE 5. the surface morphology of metal matrix in casting sample(the upper) and in
SLM sample (the lower)
is a constant related to thickness and modulus of elastic-
ity of the alloy and F is the maximum load in Newtons
causing debonding.
After conduction of  exural test, the samples were
visually inspected to determine the mode of failure.
Mode of failure was divided into three groups of adhe-
sive( at the metal-porcelaininterface), cohesive (com-
pletely within the porcelain) and mixed (a combination
of adhesive and cohesive types).
Finally, Data were analyzed by Independent samples
Table 2 shows the mean and standard deviation of bond
strength in the two groups.
Independent samples t-test was used to compare the
mean values between the two groups, which showed
that the mean bond strength of porcelain was signi -
cantly higher to alloy in SLM group compared to the
casting group (p=0.04).
Inspection of the surface of samples in the SLM group
before porcelain applying showed that the surface of bars
in the SLM group was dark gray without metal shine.
Figure 5 shows photography of the surface morphology
of metal matrix of the one sample of each group after
porcelain debonding. Visual inspection of each sample
during  exural strength testing showed that debonding
cracks between metal and ceramic occurred at one end
of the ceramic layer and not at the center.
In terms of mode of failure, in the SLM group, most
failures were mixed and no adhesive failure was noted.
In the casting group, all failures were mixed (Table 3).
Failure of the metal-porcelain bond can occur as the
result of a combination of factors such as different coef-
cients of thermal expansion of metal and ceramic, pres-
ence of micro-cracks within the porcelain and occlusal
loads or trauma. Loss of porcelain to metal bond is the
second most common cause of replacement of metal-
ceramic restorations [18]. The reason for use of SLM
technology in this study was the recent introduction of
this technique and its numerous advantages. The SLM
technique enables the fabrication of restorations with a
more uniform quality and standardization of restoration
shaping process with lower cost, requiring less human
force, saving time due to elimination of many proce-
dural steps (compared to the conventional method) and
elimination of human errors. Moreover, CAD, compared
to manual waxing, results in more accurate control of
the porcelain thickness and decreased risk of fracture.
SLM scanners have disadvantages as well including
limitation of scanning systems in terms of resolution
and creating rounder margins as well as creation of
cloudy spots caused by scanning, which result in inter-
nal mismatch (Walton 1986) and (Huang 2015) and (Li
2014). High cost of laser device used in this technique is
another disadvantage of this method.Also, the CoCr alloy
Table 2. Mean and standard deviation of bond
strength in the two groups (MPa)
Group Mean± standard deviation
SLM 35.26 ±1.22
Casting 33.21 ± 3.02
Table 3. Mode of failure in the two groups
Group Adhesive
SLM 0 5 7
Casting 0 0 12
Fariborz Vafaee etal.
fabricated by the SLM method has not shown any cyto-
toxic potential and evidence shows that ithas higher
corrosion resistance and lower release of cobalt ion
compared to the casting group (Xin 2012).
Among the suggested tests for assessment of bond
strength, Schwickerath crack initiation test or three-
point  exural strength test was  rst suggested by Lenz
et al, and is alsorecommended by the ISO9693:1999(E)
for determination of debonding strength of metal-
ceramic systems(Lenz 1995). Although several mechani-
cal tests such as  exural, torsional, shear, tensile or a
combination of  exural and shear tests can be used for
assessment of the bond strength at the metal-ceramic
interface, the validity of the bond strength assessment
tests is still questionable (Kontonasaki 2008).
The three-point  exural strength test reveals the
modulus of elasticity,  exural strain,  exural stress and
exural stress-strain response of a material. The main
advantage of this test is easy sample preparation and
conduction. However, it has drawbacks as well. One
drawback is that the test result is affected by the geom-
etry of samples, load application and speed of strain
application (Kontonasaki 2008). For this reason, in the
current study, we tried our best to control for all the
confounding factors that had the potential to affect
the results such as speed of load application (crosshead
speed), distance between the two fulcrums and dimen-
sions of the samples. All samples were fabricated with
the same dimensions. Also, a custom-made silicon index
was used for control of the length and thickness of
ceramic layer when applying the porcelain.
In order to acceptably compare the bond strength of
ceramic to metal between restorations fabricate by two
different techniques, the materials used should be the
same in terms of behavior with regard to the formation
of oxide layer, coef cient of thermal expansion, etc. as
much as possible (Kontonasaki 2008) and (Zinelis 2003).
Therefore, in the current study, alloys with the most
similar composition were chosen for SLM and casting
groups in order to minimize the effect of elements in
the composition of alloys and assess the pure effect of
fabrication method on the results .
The bond between the porcelain and metal can be
achieved bythree mechanisms of van der Waals forces,
mechanical retention and chemical bond; chemical bond
is the dominant factor responsible for metal-porcelain
bond. Chemical bond is in uenced by the composition
of elements in the alloy and formation of oxide layer on
the metal surface. Due to the signi cance of the thick-
ness of oxide layer in a successful bond, the pre-oxida-
tion phase of alloy was performed for both groups in our
study prior to the addition of porcelainaccording to the
manufacturer’s instructions (Bagby 1990) and (Jochen
In this study, the SLM group showed higher porce-
lain bond strength compared to the casting group, which
was in line with the results of Liu et al [14]. One pos-
sible reason for higher bond strength of the SLM group
was the surface properties of this alloy. The SLM sys-
tem works based on incremental application of melted
powders. The surfaces of fabricated components by the
SLM technique often have sticky powder. In other words,
relatively melted powders are added to the surface of
components and make them rougher (Zhang 2014). Due
to the inherent roughness of the surface ofSLM sam-
ples and according to the results of a study by Zhang
(Zhang 2014), it may be stated that these surfaces can
increase the contact area of framework and porcelain.
At the same time, surface porosities can provide strong
micromechanical retention for porcelain and enhance
the bond strength (Zhang 2014).
Another possible explanation is penetration of porce-
lain into the gaps created between layers as well as the
balling phenomenon during incremental fabrication by
the laser sintering process, which increase the contact
area and bond strength by creation of undercuts (Eun-
Jeong 2014) and (Gu D 2007).
The fact that all tested samples in our study yielded
higher bond strength than the minimum strength
required by ISO9693:1999(E), which is 25 MPs shows
that the alloy fabricated by the SLM technique can not
only provide an acceptable bond for clinical applica-
tion, it even yields higher bond strength than the cast-
ing method. Akova and Serra-Prat in separate studies
showed that despite higher bond strength of the casting
group, the difference between the casting and SLM tech-
niques did not reach statistical signi cance (Serra-Prat
2014). Controversy between their results and ours may
be attributed to the different form of samples (cylindri-
cal) and assessment of shear bond strength instead of
exural bond strength.
Xiang and Ren found no signi cant difference in
bond strength of the SLM and casting groups; although
the values obtained in the SLM group were higher in
both studies (Xiang 2012). Difference between their
results and ours may be due to difference in the load
application speed for  exural test, difference in laser
system and composition of alloys used and size of sand-
blasting particles.
Inspection of the surface of samples in the SLM group
before porcelain applying revealed that the surface of
bars in the SLM group was dark gray and had no metal
shine in our study. The reason may be that the metal
powder particles are fused on the surface of components
fabricated by the SLM method; thus, the samples show a
color similar to that of metal powder particles.
Visual observation of each sample during the  exural
strength testing revealed that the debonding cracks at
Fariborz Vafaee etal.
the metal-ceramic interface occurred at one end of the
ceramic layer and not at the center of it, which was in
agreement with ISO9693:1999(E) standards. Evaluation
of surface morphology of metal matrix after porcelain
debonding showed that the color was different at the
center and at the ends of the samples. This indicated that
the ceramic layer was still present on the entire surface
of samples. Moreover, mode of metal-ceramic debond-
ing may indicate the fracture energy that degrades the
metal-ceramic bond. The higher the amount of porce-
lain body remaining on the metal surface, the higher
the adhesion of porcelain body to metal, the higher the
fracture energy and the lower the risk of porcelain frac-
ture would be in the clinical setting (Wagner 1993) and
(Lavine 1966). Five samples in the SLM group showed
cohesive failure while in the casting group, all fractures
were mixed. The reason is probably the higher bond
strength of the SLM group. Since after debonding high
amounts of porcelain remained on the alloy surface,
it may be concluded that a strong bond was created
between dental porcelain and SLM alloy at the interface.
Thus, this technique can increase the success rate of por-
celain fused to metal restorations.
Since this study had an in vitro design, the SLM alloy
should also be evaluated in the oral cavity. Also, it is
recommended to assess the marginal  t of copings fab-
ricated by this technique. The bonding interface should
be evaluated using scanning electron microscope and
energy-dispersive X-ray spectroscopy in future studies.
The microstructure of SLM alloy must also be analyzed
under a metallographic microscope.
The SLM alloy system not only provides a clinically accept-
able bondstrength , the bond strength of restorations fab-
ricated by this method to porcelain was even higher than
that of restorations fabricated by the conventional casting
method. Thus, this relatively new technology can serve as
an alternative to the conventional casting method for the
fabrication of metal-ceramic restorations.
The work described in this paper has been retrieved from
Dr. ParisaAlirezaii’sthesis. The authors would like to
thank Dr. MarziehMahmoodi for useful statistical con-
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