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Please cite this article in press as: Al Jabbari YS, et al. Metallurgical and interfacial characterization of PFM CoCr dental alloys fabricated viacasting, milling or selective laser melting. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.01.008

ARTICLE IN PRESSDENTAL-2311; No.of Pages 10

d ent al m at eri al s x x x ( 2 0 14 ) xxx.e1xxx.e10

Available online at www.sciencedirect.com

ScienceDirect

journal homepage: www.int l .e lsevierheal th.com/ journals/dema

Metallurgical and interfacial characterization of

PFM CoCr dental alloys fabricatedvia casting,

milling or selective laser melting

Y.S. Al Jabbaria,b,, T. Koutsoukisa, X. Barmpagadakic, S. Zinelisd,a

a Dental Biomaterials Research and Development Chair, College of Dentistry, King Saud University, P.O. Box 60169,

Riyadh 11545, Saudi Arabiab

Department of Prosthetic Dental Sciences, College of Dentistry, King Saud University, P.O. Box 60169, Riyadh11545, Saudi Arabiac Private Practice, Githiou 81 Str, Pireas 18544, Greeced Department of Biomaterials, School of Dentistry, University of Athens, 2 Thivon Str, Goudi 11527, Athens, Greece

a r t i c l e i n f o

Article history:

Received 29 April 2013

Received in revised form

25 September 2013

Accepted 16 January 2014

Available online xxx

Keywords:

SLM

Fabrication techniques

CoCr dental alloy

Metalceramic interface

a b s t r a c t

Objectives. Bulk and interfacial characterization of porcelain fused to metal (PFM) CoCr

dental alloys fabricated via conventional casting, milling and selective laser melting.

Methods.Three groupsof metallic specimensmade of PFMCoCrdental alloys were prepared

using casting (CST), milling (MIL) and selective laser sintering (SLM). The porosity of the

groups was evaluated using X-ray scans. The microstructures of the specimens were evalu-

ated via SEM examination, EDX and XRD analysis. Vickers hardness testing was utilized to

measure the hardness of the specimens. Interfacial characterization was conducted on the

porcelain-covered specimens from each group to test the elemental distribution with and

without the application of INmetalbond. The elemental distribution of the probed elements

was assessed using EDX line profile analysis. Hardness results were statistically analyzed

using one-way ANOVA and HolmSidaks method (= 0.05).

Results. X-ray radiography revealed the presence of porosity only in the CST group. Different

microstructures were identified among the groups. Together with the phase matrix, a sec-

ond phase, believed to be the Co3Mo phase, was also observed by SEM and subsequent XRD

analysis. Cr7C3and Cr23C6carbides were also identified via XRD analysis in the CST and MIL

groups. The hardness values were 32012HV, 2975HV and 37110HV, and statistically

significant differences were evident among the groups.

Significance. The microstructure and hardness of PFM CoCr dental alloys are dependent

on the manufacturing technique employed. Given the differences in microstructural and

hardness properties among the tested groups, further differences in their clinical behavior

are anticipated.

2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Correspondingauthor at: Dental Biomaterials Research and Development Research Chair, College of Dentistry, King Saud University, P.O.Box 60169, Riyadh 11545, Saudi Arabia. Tel.: +966 1 4698312; fax: +966 1 4698313.

E-mail addresses: [email protected], [email protected] (Y.S. Al Jabbari).

0109-5641/$ see front matter 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.dental.2014.01.008
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ARTICLE IN PRESSDENTAL-2311; No.of Pages 10xxx.e2 dental mater i al s xxx ( 2 014 ) xxx.e1xxx.e10

1. Introduction

Technological developments have led to the implementa-

tion of novel manufacturing processes in everyday dental

practice. In recent decades, digitalized technologies have been

employed for the production of metallic structures, mainly in

prosthetic dentistry [15]. These technologies can be classifiedas based on subtractive manufacturing, such as the milling

of pre-manufactured materials assisted by computer-aided

design/computer-aided manufacturing (CAD/CAM) systems

[1,2,57], or on additive manufacturing, such as the recently

developed selective laser melting (SLM) technique [3,4,810].

Although CAD/CAM has long been directly associated with

the milling procedure in dental literature, it should be

mentioned that SLM is also classified as CAD/CAM technol-

ogy.

As a recently introduced technique, SLM has attracted

the worldwide interest of research groups for the manufac-

turing of dental metallic structures. In prosthetic dentistry,

most studies on SLM have focused on CoCr dental alloys[3,8,9,1120]; significantly fewer studies have been per-

formed on Ti alloys [4,21] or the costly precious alloys

[3,12]. These studies have primarily focused on the evalua-

tion of the marginal and/or internal fit of the restorations

[3,6,9,13,1517,20], whereas other studies have tested the

bond strength with dental porcelain [8,18,21], internal poros-

ity [4], effect of surface treatments on microroughness [14]

and electrochemical properties [19]. However, the compara-

tive analyses of specific properties among metallic structures

made using SLM and conventional techniques are limited

[13,21]; thus, the effect of the SLM technique on mechani-

cal, electrochemical, microstructural and other properties is

still unknown. Given the large differences in the manufac-turing process between casting, which uses the complete

melting and overheating of casting materials, the milling of

a prefabricated metal block and SLM of a fine metallic pow-

der, large differences in microstructural characteristics are

anticipated. These microstructural differences may also dif-

ferentiate the interfacial characterizationof metallic elements

at the metalporcelain interface. Although common for other

prosthetic dental alloys, interfacial analyses of CoCr alloys

cast or milled with porcelain are still absent from the dental

literature. Therefore, the aim of this study was to metal-

lurgically and interfacially characterize CoCr dental alloys

prepared by casting, milling and SLM techniques. The null

hypothesis was that there would be significant dissimilari-ties among the groups prepared by different manufacturing

techniques.

2. Materials and methods

2.1. Specimenpreparation

Three groups (CST, MIL, and SLM) were prepared using CoCr

dental alloys as indicated by the manufacturers. The spec-

imens of the CST group were fabricated by the traditional

casting technique using CoCr raw material; those of the MIL

groupwere milledoff a prefabricated block andthe specimens

of the SLM group were fabricated by the SLM technique using

CoCr mixed powder. The brand names, manufacturers and

elemental composition of the alloys tested are presented in

Table 1.

In the CST group, 12 wax patterns (IQ sticks, Yeti Den-

tal, Engen, Germany) were invested with phosphate-bonded

investment (GC Stellavest, GC Europe NV, Belgium) with

dimensions of 0.5mm3 mm25mm. The mold was pre-heated at 910 C and cast with VI-COMP alloy at 1450C using

a centrifugal casting machine (Ducatron S3, UginDentaire,

Seyssins, France).The mold was left to cool down to room tem-

perature and the specimens were then divested and cleaned

by sandblasting with alumina particles (100m).

A prefabricated block of a commercial CoCr dental

alloy (Okta-C) was milled to fabricate a dental restoration

using the Organical Multi Milling/Grinding CAD/CAM system

(R+K CAD/CAM Technologie, Berlin, Germany). A rectangular-

shaped wax pattern was digitized and the specimens were

cut to their final dimensions (0.5mm3 mm25mm) using

the Organical Multi Milling/grinding machine (R+K CAD/CAM

Technologie).

Thelaser-sinteredspecimens were prepared fromcommer-

cial CoCr powder (ST2725G) using a dental laser sintering

device (PM 100 Dental System, Phenix Systems, Clermont-

Ferrard, France) equipped with a 500W Yb-fiber laser, at a

temperatureof 1650C;the laser systemhadthe ability toweld

across a controlled (XY)-axis coordinate system with a Z-axis

tolerance of0.0254 mm. The CoCr powder was applied to a

stainless steel plate and was laser-sintered upwards in subse-

quentlayers after a 20-m-thick layer was completeduntil the

final product was generated. Following laser sintering, the sin-

tered parts were cooleddown to furnace temperature. In total,

12 specimens with dimensions of 0.5mm3 mm25mm

were fabricated using this technique.

2.2. X-ray testing

All specimens of all groups were then examined for internal

porosity using a dental X-ray unit (Orix 70, Ardet, Milan, Italy)

operating at 70kV and 5mA with an exposure time of 15s.

Digital images were collected from all specimens, and the X-

ray images were assessed by the naked eye.

2.3. SEM-EDX characterization

For microstructural characterization, three specimens of each

group were examined using a SEM (JSM 6610 LV, Jeol Ltd.,

Tokyo, Japan) equippedwith an X-ray EDS microanalysis (EDX)

unit (Oxford Instruments, Abingdon, UK). All examined sur-

faces were ground using SiC paper (2202000 grit) under

continuous water cooling andpolished in a grinding polishing

machine (Ecomet III, Bueler, Lake Bluff, IL, USA) using a dia-

mond paste (DP Paste, Struers, Copenhagen, Denmark). The

specimens were then cleaned in an ultrasonic water bath for

5 min. Specimens fabricated by the casting, milling or SLM

technique (0.5 mm3 mm25mm) were examined on the

3 mm25mm surface. The examined surfaces were imaged

using a backscattered electron detector (BSE) under an accel-

erating voltage of 30 kV and a beam current of 48A at a

working distance of 10mm. The elemental composition was
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dental mater i al s xxx ( 20 14 ) xxx.e1xxx.e10 xxx.e3

Table 1 Brand names, elemental compositions and coefficients of thermal expansion (CTE) of the tested alloys, asprovided by themanufacturers.

Brand names Manufacturer Co Cr Mo Si Mn Fe

Okta-C Sae Dental Products

Inc. Bremerhaven,

Germany

61.6 30.0 6.5 0.8 0.8 N/A

ST2725G SINT-TECH, Riom,France. Bal (max 62.5) 29 5.5

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Table 2 Firing schedules of the veneering procedure for GC Initial MC porcelain (according to themanufacturersinstructions).

Product name Pre-heating temp. (C)

Drying time(min)

Heating rate(C/min)

Vacuum Final temp. (C) Holding time(min)

INmetalBond 550 6 80 Yes 980 1

Opaque 550 6 80 Yes 940 1

Dentin 580 6 55 Yes 900 1Glaze 480 2 45 No 850 1

Fig. 1 RepresentativeX-ray images of the (A) CST, (B) MIL and (C) SLM groups. Porositywas identified only in the CST group.

enriched mainly in Mo and less in Cr, which was also the

case for the Am phase. In the MIL group, the Bm was heavily

enriched in Mo but depleted in Cr relative to the matrix con-

tent. All of the dispersed phase primarily had low Co content

(Table 3).

3.3. XRD analysis

Following the XRD analysis, the diagrams recorded from all

groups were indexed as presented in Fig. 3. In addition to the

face-centered cubic (fcc) phase of Co and Cr, the hexagonal

close-packed (hcp) Co3Mo phase was identified in all speci-

mens. Additionally, Cr7C3and Cr23C6carbides were identified

in the CST and MIL groups.

3.4. Hardness

In total, 12 measurements of Vickers hardness were acquired

for the specimens of each dental alloy. The mean values

and the standard deviations were calculated as 32012 HV,2975HV and 37110HV for the CST, MIL and SLM groups,

respectively. Statistically significant differences were found

among all the groups tested (p< 0.05).

3.5. Interfacial characterization

Representative BEI from the interface of all materials tested

are presented in Fig. 4, and they show a well-formed and

defined interface between the CoCr alloy and the opaque or

INmetalbond. Image contrast revealed that the opaque is a

Fig. 2 Representative BE images of the microstructure observed in (A) cast, (B) milled and (C) SLM specimens. Note the

dispersion of a second phase in the cast and milled specimens (white contrast indicated with white arrows) and the

absence of such a phase in the SLM specimen. Precipitates of a third phase were also detected in the milled specimen (BM

in Fig. 2B). Porosity can also be observed in the images.

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Fig. 3 Indexed XRD diagrams of all groups included in the study. SLM showed only the presence of(Co,Cr) and Co3Mo

phases, in contrast to the CST and MIL groups, in which two Cr carbides were also identified.

Fig. 4 Representative BE images of the (A) metalINmetalbondopaque and (B) metalopaque interfaces. EDX area analyses

were performed as indicated on the images.

Table 3 Quantitative results of EDX analysis (wt%) of alloys (area analysis) and different phases (spot analysis) based onthe mean atomic contrast of BEI (Fig. 2). For the sake of clarity, the accuracy intervals of the EDX analysis are given onlyfor the alloy composition.

Element CST MIL SLM

Alloy Mc Ac Alloy Mm Am Bm Alloy

Co 59.9 [56.962.9] 62.7 46.9 59.1 [56.162.1] 60.2 47.9 43.2 62.7 [59.665.8]

Cr 32.2 [30.633.8] 31.7 35.8 33.1 [31.434.8] 33.3 37.9 26.9 29.2 [27.730.7]

Mo 6.2 [5.66.8] 4.3 15.3 6.0 [5.46.6] 4.9 12.3 28.2 6.3 [5.76.9]

Si 1.0 [0.81.2] 0.9 1.3 0.9 [0.41.3] 0.6 1.0 0.6 0.9 [0.41.3]

Mn 0.5 [0.20.7] 0.4 0.7 0.4 [0.20.6] 0.5 0.5 0.8 0.8 [0.41.2]

Fe 0.2 [0.10.3] 0.2 0.1 0.5 [0.20.7] 0.5 0.4 0.3 0.1 [0.10.2]

Table 4 EDX area analysis (wt%) of the INmetalbond and the opaque.

Material Element

O Si Ti Zr Fe Na Al K Ca Mg Zn

INmetalbond 22.9 19.4 29.2 12.3 1.7 3.7 3.4 6.6 0.8

Opaque 24.7 21.9 1.5 30.0 0.2 3.1 5.7 9.3 2.3 0.3 1.0

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Fig. 5 EDX line profile analysis performed across the interface between the alloy and the opaque (A, C and E) or

INmetalbond (B, D and F). The white horizontal line represents the scanning route. The plateau observed in the Cr profile is

indicated by an arrow.

multiphase material, whereas the INmetalbond exhibited a

matrix with a dispersed phase that had a higher mean atomic

number. The elemental composition of both materials as

determined by EDX analysis is presented in Table 4. INmetal-

bond showed increased Ti content relative to the opaque,

mainly at the expense of Zr content. The results of the EDX

line profile analysis, recorded across either the metalopaque

interface or the metal-INmetalbond interface, are presented

in Fig. 5. All probed elements demonstrated a steady decrease

(Co) or increase (Si, K and Ti) from the alloy toward the porce-

lain or INmetalbond. An exception to this general trend was

observed for the Cr profile line in the groups with opaque,

which demonstrated an obvious plateau at the interface, as

indicated by the arrows in Fig. 5A, C and E.

4. Discussion

This study focused on the microstructural and interfacial

characterization of CoCr PFM alloys fabricated using casting,

milling and selective laser sintering. All of these manu-

facturing processes are currently used for the production

of dental prosthetic restorations. Based on the data pre-

sented above, the null hypothesis must be rejected, as

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significant dissimilarities were found among the groups

tested, apart from the interfacial distribution of metallic

elements at the metalceramic joint with or without INmetal-

bond.

Although great effort was invested in finding a single alloy

that could be used for the three manufacturing processes,

suchan alloy was notavailable at thetime of this study. There-

fore, the best matching nominal compositions as presentedin Table 1 were selected from a large pool of commercially

available CoCr dental alloys. EDX results indicated similar

values for all elements among different groups within the rel-

ative internal error of the quantitative standardless analysis

(Table 2), although a slightly higher Co/Cr ratio was identified

for the SLM group relative to the rest of the alloys. Although

Fe was identified in all groups, it is only known to exist in

the nominal composition of the ST2724G alloy. Interestingly,

the presence of Fe in the nominal composition of the ST2724G

CoCr alloy hasbeen both reported[9] and not reported[14,15].

An explanation for this inconsistency may be that the manu-

facturers slightly modified the formulations without changing

the brand name. Alternatively, the presence of these traces

may be attributed to the contamination of the raw materials

and/or contamination during the manufacturing process of

the alloys.

BEI demonstrated microstructural differences among the

tested groups. The CST group showed the typical cast struc-

ture of a CoCr dental alloy [2225], as the specimens were

composed of a matrix (Mc) and a heavier dispersed phase

(Ac) that occupy the interdendritic spaces. Although the den-

dritic structure was not clearly outlined, this magnification

was kept constant among the different groups for compar-

ison purposes. This phase was expected to be rich in the

heavier available elements, primarily Mo (Table 1), because of

its lighter contrast relative to the matrix in the BEI mode, and

this phenomenon was confirmed by EDX analysis (Table 2).

Because of its different composition and crystal structure, the

formation of this bulky phase is considered to be undesirable

because it increases the brittleness and deteriorates the cor-

rosion resistance of the alloy by removing Mo from the solid

solution [26]. A second phase (Am) with a higher mean atomic

number relative to the matrix (Mm) was also observed in the

microstructure of the MIL group (Fig. 2B), similarly to the cast

material, but with a more bulky morphology. Similarly to the

CST group, Am demonstrated increased Mo and Cr content,

andan even higherMo content was detectedin the third phase

Bm(Table 2), which agrees with previous observations in sim-

ilar alloys [26]. The formation of this phase, which preferably

nucleates and coarsens at the interface of the second phase

and the matrix, promotes the further depletion of Mo in the

matrix, which implies the deterioration of the corrosion resis-

tance [27]. Mo is also added to CoCr alloys to achieve a finer

grain structure, thereby enhancing the mechanical proper-

ties of the material [26]. This favorable effect is diminished

when Mo is segregated in Mo-rich compounds rather than

being dispersed within the matrix. In contrast to the CST and

MIL groups, no mean atomic contrast was revealed for the

SLM material a finding which is in agreement with a recently

published study [25]. This difference implies a completely

different solidification and/or thermomechanical history of

the tested groups [22,27,28].

XRD analysis results (Fig. 3) indicated that the microstruc-

ture of all groups consisted of the face-centered cubic (fcc)

phase, which mainly comprised Co and Cr which is in agree-

ment with a previous study [29]. As an allotropic element,

Co has a fcc crystal structure above temperatures of approx-

imately 417C and a hexagonal crystal packing (hcp) below

this temperature, but in CoCr alloys, the fcc structure is

maintained at room temperature because of the low fcchcp

transformation rate. The fcc phase should be attributed to

the matrix of the microstructure for the CST and MIL groups

(Fig. 2A andB), whereas theCo3Mo phase, whichhas also been

identified in previous studies of cast alloys [27,30] as a Mo-

rich compound (Table 2), should be attributed to the dispersed

phases Ac and Am of the CST and MIL groups, respectively.

Although this phase was identified by XRD analysis in the

SLM group, no mean atomic number contrast was found in

BEI, possibly because of therapid solidification of fused metal-

lic particles that led to a very fine phase size that was below

the resolution of backscattered electron imaging. Additionally,

the previously found carbides [22] Cr23C6and Cr7C3were also

identified by XRD analysis only in the CST and MIL groups

(Fig. 3). Even when the manufacturer produces a raw mate-

rial with a low C content, carbide formation may occur during

the manufacturing process and thus affect the nominal prop-

erties [24,26,28,3033] of the produced alloys. In addition to

the detected carbides, it is possible that other carbides could

have formed in the microstructure of the examined materi-

als [24,30], but the amount of these carbides is most likely

below the detection limit of the technique. The application of

advanced electron microscopy techniques, such as transmis-

sion electron microscopy (TEM), could provide further crucial

information, especially on the microstructural characteriza-

tion and the formation mechanism of the developed phases.

Despite the detrimental effect of internal porosity, which

is a common complication of the casting procedure for CoCr

alloys [34,35], the significantlyhigher hardness observedin the

CST group (32012HV) relative to the MIL group (2975HV)

may be attributed to the finer distribution of the dispersed

phase (Fig. 2). However, the increased hardness observed in

the SLM group (37110HV) could be attributed to the sinter-

ing technique,which not onlydiminishes undesirable porosity

but also provides a much more fine-grained structure [1]. This

is in agreement with a recently published study indicating

that SLM technique provides CoCr alloys with enhanced ulti-

mate tensile strengthand elongationcompared to casting[29].

Additionally, the presence of residual stresses during sinter-ing is another possible explanation for the increased hardness

in the SLM [36,37].

The surface of the three tested groups was intentionally

polished using a 1-m diamond paste prior to the veneering

procedure. Despite the manufacturers recommendations for

surface roughening before porcelain application, a flat surface

is required to obtaina very small area forEDX line profile anal-

ysis (Fig. 5). In addition, the retained alumina fragments on

metallic surfaces after sandblasting[38] mask the real distri-

bution of Al and O at the interface. Interfacial analysis with

BE imaging revealed that both the opaque and INmetalbond

adhered well to the metallic substrates for all groups tested

(Fig. 4), thereby providing a continuous interface with thesubstrate. In addition to the typical layering procedure with

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Fig. 6 Standard free energy of different reactions with O for Co, Cr, Si and Ti, commonly known as Ellinghams Diagrams.

A lower position reflects higher chemical affinity of the element with O. Both axes have the same range for comparison

purposes.

opaque directly applied on the metallic surface, various prod-

ucts have been introduced to themarket that claim to increase

either the bond strength or the esthetics of PFM restorations

[39]. Specifically, INmetalbond is advertised as a material that

aims to block the escaping metal oxides and neutralize differ-

ences in the thermal expansion coefficient between porcelain

and metal. The application of this bonder is optional and

is recommended by the company for all precious and non-

precious PFM dental alloys.

The line profile analysis did not reveal any concentra-

tion gradient at the interface, but interestingly, the Cr profile

demonstrated a plateau at the interface for all groups coveredwith opaque, but not with INmetalbond. An explanation for

this behavior may include the chemical affinity of the involved

elements to O. This information is provided by typical Elling-

ham diagrams presented in Fig. 6 [40], which show the free

energy of a given oxide as a function of temperature. A lower

position reflects higher chemical affinity of an element to O

and thus a higher tendency to react with O. Given that Cr has

a higherchemical affinity to O than Co andMo (the latteris not

shown in Fig. 6), it reacts first, thereby increasing the contribu-

tion of Cr compounds at the interface. It should be mentioned

that even if the other metal reacts first because of kinetic rea-

sons, Cr ions can still reduce the oxides of the other metals

based on themetallothermic reaction, in whicha metal with a

higher chemicalaffinityto O (MH) reduces theoxides of metals

with a lower affinity to O (ML) based on the following general

formula:

(MH) + (ML)xOy (MH)Oy+x(ML)

Surprisingly, this behavior vanished when the INmetal-

bond was applied for all groups tested, which may be

associated with a higher chemical affinity of Ti with oxygen,

as Ti is the predominant element of INmetalbond. Of course,

this mechanism requires that Ti be capable of reacting with

O or participating in metallothermic reactions. In contrast,

although Si has a similar chemical affinity to O as Ti, it is

widely known that Si is anchored in the glassy matrix as an

oxide [41], and thus its capability for further reaction is elim-

inated. Although this thermodynamic approach agrees with

the data presented above, it is worthwhile to note that these

calculations represent the reaction of pure elements with O

and must be corrected when they are used for alloying ele-

ments in different alloys. However, such data are not available

and their calculation requires extensive research. In conclu-

sion, it is clear that the application of INmetalbond affects the

interfacial distribution of involved elements, and therefore its

effect should be further examined.

These findings regarding microstructural properties may

have clinical implications. Mechanical properties (i.e., thefatigue resistance of dental clasps), electrochemical proper-

ties and other properties may be altered by microstructural

changes, and further research is thus required in this field,

especially for the recently introduced SLM technology. How-

ever, no differences were found for the elemental distribution

of the probed elements at the metalporcelain interface,

which agrees with previous data that showed no difference

in bond strength between cast and laser-sintered structures

[8,18].

Given that the microstructure and hardness were signif-

icantly different in the SLM group, further differences in

the clinical behavior of prosthetic restorations manufactured

using this technique are anticipated. SLM technology has

recently been introduced in the dental field, and a vast spec-

trum of factors should be tested and/or optimized to increase

its efficacy in the production of metallic dental restorations.

5. Conclusions

Within the limitations of this study, the following conclusions

can be derived:

CoCr dental alloys fabricated via casting, milling or SLM

techniques show significant differences in microstructure

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and hardness but not at the elemental distribution level of

metalceramic interface.

The application of INmetalbond has a profound effect on

the elements at the interface between the metal and the

opaque porcelain.

Acknowledgments

This study has been funded by a research grant (RGP-VPP-

206) from the Research Group Program, Deanship of Scientific

Research, King Saud University, Riyadh, Saudi Arabia.

The authors would like to thank Phenix Systems and Mr

Briakos Dental Lab for the manufacturing of metallic speci-

mens.

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