Medical

Communication

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

methods

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

ABSTRACT

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.

KEY WORDS: SHEAR STRENGTH; METAL-CERAMIC; LASER MELTING

424

ARTICLE INFORMATION:

*Corresponding Author: sarakhazaei_83@yahoo.com

Received 23

June, 2017

Accepted after revision 24

Sep, 2017

BBRC Print ISSN: 0974-6455

Online ISSN: 2321-4007 CODEN: USA BBRCBA

Thomson Reuters ISI ESC and Crossref Indexed Journal

NAAS Journal Score 2017: 4.31 Cosmos IF: 4.006

reserved.

Online Contents Available at: http//www.bbrc.in/

DOI: 10.21786/bbrc/10.3/15

Fariborz Vafaee etal.

BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS BOND STRENGTH OF PORCELAIN TO COBALT CHROMIUM DENTAL ALLOY FABRICATED 425

INTRODUCTION

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.

MATERIALS AND METHODS

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.

426 BOND STRENGTH OF PORCELAIN TO COBALT CHROMIUM DENTAL ALLOY FABRICATED BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS

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

application

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.

BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS BOND STRENGTH OF PORCELAIN TO COBALT CHROMIUM DENTAL ALLOY FABRICATED 427

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

t-test.

RESULTS AND DISCUSSION

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).

DISCUSSION

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

failure

Cohesive

failure

Mixed

failure

SLM 0 5 7

Casting 0 0 12

Fariborz Vafaee etal.

428 BOND STRENGTH OF PORCELAIN TO COBALT CHROMIUM DENTAL ALLOY FABRICATED BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS

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

1986).

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.

BIOSCIENCE BIOTECHNOLOGY RESEARCH COMMUNICATIONS BOND STRENGTH OF PORCELAIN TO COBALT CHROMIUM DENTAL ALLOY FABRICATED 429

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.

CONCLUSION

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.

ACKNOWLEDGEMENTS

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-

sultations.

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