INTRODUCTION

Titanium possesses several qualities, such as relatively low elastic modulus compared to base metal alloys, low density (4.5 g/cm3), low thermal conductivity, resistance to corrosion, and biocompatibility1,2). However, the material’s strong chemical reactivity, especially in high temperatures, and its high melting point (1,668°C), create difficulties with casting and porcelain firing which are considered to be negative aspects of the material3,4). Titanium has two common crystal structures. At room temperature, the titanium has a hexagonal close-packed crystal structure known as the α-phase. At temperatures above 882°C, an allotropic transformation occurs from the hexagonal to a centered cubic crystal structure, called the β-phase5,6,7).

Since titanium is a widely used material in the field of odontology, different manufacturing techniques are used, i.e. casting, milling and electron beam melting (EBM)8). The most commonly used casting technique is the lost-wax method2) where the casting is performed in an argon environment to prevent the absorption of gases9,10). There are several technical problems associated with the casting of titanium, such as its reaction with the investment material and the incidence of porosities10). The casting creates a hard, oxygen-rich surface on the titanium, a so-called α-case, which arises due to the absorption of oxygen, hydrogen and nitrogen. This leaves the titanium with undesirable surface properties, reduced ductility and low fracture strength. Consequently, post-processing of the object is required to remove the surface layer9).

Milling is another processing technique based on

CAD/CAM technology. After designing an object using CAD, it can be milled from a titanium blank/disc in a CAM machine. The advantage of this technique is that there will not be any α-case formation, promoting increased bond strength between the titanium and the porcelain, without the need for extensive post-processing. The disadvantages, however, are that although the milling technique offers the ability to produce complex shapes, the technique is time-consuming and leaves a significant amount of waste in the form of material swarf after milling2).

Unlike milling, which is a subtractive process, EBM is an additive manufacturing technique. The advantage of this technique is that the surplus material can be recycled and almost no material need to be wasted. EBM describes a technology in which an electron beam is used to fuse together titanium alloy grains (Ti6Al4V, grade 5), grain by grain, layer by layer into the desired shape8).

Regardless of the method used, after the titanium framework has been produced, it may be veneered by fusing porcelain to the metal. During the firing process, the increase in temperature may lead to the development of a brittle surface layer, growing spontaneously on the titanium surface and thus forming a weak interface between the porcelain and the titanium5,11). It has been shown that if this layer reaches a critical thickness of approximately 1 µm, it will fracture cohesively within the oxide layer due to the shear stresses that may develop due to differences in thermal expansion [coefficient of thermal expansion (CTE)] between the alloy and the oxide layers12,13). Thus, titanium requires low-fusing porcelain to prevent the oxide layer from becoming too thick and to prevent titanium phase transformation6,14). Bonding between titanium and porcelain is accomplished by: 1) chemical bonds between

Cast, milled and EBM-manufactured titanium, differences in porcelain shear bond strengthEvaggelia PAPIA1, Pernilla ARNOLDSSON1, Ayna BAUDINOVA1, Ryo JIMBO2 and Per VULT VON STEYERN1

1 Department of Materials Science and Technology, Faculty of Odontology, Malmö University, 205 06 Malmö, Sweden2 Department of Oral and Maxillofacial Surgery and Oral Medicine, Faculty of Odontology, Malmö University, 2015 06 Malmö, SwedenCorresponding author, Evaggelia PAPIA; E-mail: evaggelia.papia@mah.se

The objectives were to analyze the oxide layer generated between titanium and porcelain during firing and compare it in different manufacturing techniques: cast, milled and EBM-technique. Seventy two specimens were manufactured, subdivided according to surface treatment: time of passivation (P) and no time of passivation (NP) before porcelain firing. Specimens from each group were analyzed with scanning electron microscopy: one only fired once, and one subjected to six firings. Remaining specimens were subjected to shear bond strength test. The EBM-produced NP-group had highest mean value (25.0 MPa) and the milled P-group showed lowest mean value (18.5 MPa) when all factors were compared. No significant difference was detected according to time of passivation. SEM showed consistent and well-defined boundary between the different layers. Time of passivation and impact on oxide growth was not detected. The bond strength of porcelain to milled titanium is lower when compared to cast titanium and EBM-produced titanium.

Keywords: Bond strength, Porcelain fused to metal, Oxide, Surface properties, Titanium

Color figures can be viewed in the online issue, which is avail-able at J-STAGE.Received Dec 6, 2016: Accepted May 26, 2017doi:10.4012/dmj.2016-404 JOI JST.JSTAGE/dmj/2016-404

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surface oxides of the titanium and of the porcelain, 2) mechanical retention, and 3) compression forces that arise when the metal contracts and the porcelain is therefore pulled towards the metal, due to minor differences in thermal expansion (CTE) between the materials. To achieve a compact adhesive oxide layer prior to porcelain firing, the dental technician oxidizes the titanium framework at 800°C for a short time. During this time, the temperature may increase, the adhesive forces may decrease and the oxide layer may become more porous and simultaneously thicker1,11). Following the recommendations from the manufacturers, the subsequent surface treatment is to apply the bonding agent on the oxidized framework, created by limiting the time of oxide passivation. The process to let the surface to oxidate is included in the procedure to produce a thin and stable oxide layer on the surface. If the time of passivation is extended, the oxide layer may grow and become more porous and decreases the bond strength between the titanium and porcelain. In order to avoid a thick and porous oxide layer, low-fusing porcelain is used. Firing at a low temperature, however, can result in incomplete sintering of the porcelain. It is acknowledged that this subsequently leads to a weakened bond to the titanium. In addition, debonding might occur if there is a mismatch in CTE between titanium and porcelain2).

In a comparison of bonding between porcelains and titanium manufactured using different production techniques —cast, milled and laser-sintered— it was shown that the porcelain presented significantly higher bond strength to laser-sintered titanium than the other two techniques. The shear bond strength of the laser-sintered titanium varied between 32–33 MPa, compared to 25–28 MPa for the cast titanium and 18–25 MPa for the milled titanium. It was speculated whether disparities in the oxide layer and the adhesive ability might be the reason for the differing results2).

It has been suggested that oxides possess a complex microstructure and that information about the microstructure and composition of the reaction zones is very important for optimizing the metal-ceramic systems of titanium, their production and processes6). Thus, there is great interest in determining whether different manufacturing techniques can influence the oxide layer growth kinetics.

The objectives of the current study were to analyze the oxide layer that was generated between titanium and porcelain during firing and to compare this oxidation in three different manufacturing techniques. It was hypothesized that the manufacturing technique and time of passivation have no influence on the growth of the oxide layer and there would therefore be no influence on the bond strength between titanium and porcelain.

MATERIALS AND METHODS

A total of 72 specimens were manufactured and used for each manufacturing technique: cast, milled and EBM

technique. Each specimen measured approximately 10×10×10 (±0.2) mm. They were further divided in two groups according to surface treatment: time of passivation (the bonder was applied after the oxidation procedure and with further oxidation at room temperature) (P) and no time of passivation, (the bonder was applied immediately after the oxidation procedure) (NP) before porcelain firing. Two specimens from each group were later randomly selected for scanning electron microscopy, SEM analysis: one with only a bonding agent applied and therefore only fired once, and one with porcelain that was subjected to six firings. Remaining specimens were subjected to a shear bond strength test (Fig. 1).

Manufacture of the specimensThe cast specimens were manufactured in titanium grade 1 (Tritan, (cp) Grad 1, Dentaurum, Ispringen, Germany, batch 0151) according to the lost-wax method. A template was used to ensure dimensional standardization of the wax-ups which were then embedded (Trinell investment, Dentaurum, batch 091134 and Trinell, liquid, Dentaurum, batch 071132) with three specimens in each mold and cast with a vacuum pressure machine (Autocast, Universal230, Dentaurum). The cast was allowed to cool down slowly to room temperature, embedded and separated. Casting surplus and α-case were manually abraded with a carbide cutter (Elephant Dental, Hoorn, The Netherlands, LOT 8056510).

The milled specimens were manufactured in titanium grade 2 (Titan (cp) Grad 2, T-Ronde (100/16), KaVo Everest®, Biberach, Germany). A model with the exact dimensions was waxed up and a double scan was performed (KaVo Everest®, Scanpro and CAD-CAM KaVo Everest®) where the line of the preparation was placed around the specimen’s lower outer ledge. A file containing the information from the scan was sent to the CAM unit where it was processed and the items were milled out of titanium discs. EBM-produced specimens in titanium alloy (Ti6Al4V) grade 5 were provided by Dentware Scandinavia (Kristianstad, Sweden), from the same file as the milled specimens.

The bonding surfaces of all specimens were pretreated for porcelain veneering using the following procedures: 1. ground in one direction at 15,000 rpm with carbide cutters for titanium, with the exception of the cast group which was ground at 10,000 rpm, all in accordance with the relevant metal manufacturer’s recommendations, 2. airborne particle abrasion using 125 µm aluminum oxide was applied for ten seconds at two bar pressure from a distance of three centimeters and at a 45° angle, 3. finally, the specimens were steam blasted for ten seconds from a distance of ten centimeters.

The veneering porcelain GC Initial (GC Europe, Leuven, Belgium) for titanium was used for all the specimens. The porcelain was given a cylindrical shape measuring 5±1 mm in diameter, with a height of 3±0.5 mm. To standardize and facilitate the application of

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Fig. 1 Flowchart of groups and subgroups included. SBS-test: Shear bond strength test *one specimen with only a bonding agent applied (one firing), and one specimen with

complete porcelain system applied (bonding agent, opaque, porcelain and glaze firing, six firings in total).

the porcelain, a custom-made template of silicone (DC Dublier Silicone, Dental Central, Bremen, Germany) was used. The bonder (Ti Bonder, batch 200904301 and liquid, batch 200906161, GC Initial) was applied after a passivation time of five minutes on the milled and EBM-produced specimens, while the cast specimens had a passivation time of ten minutes, all in accordance with the relevant metal manufacturer’s recommendations.

Passivation was carried out by allowing oxidation at room temperature (20°C) and atmosphere. To avoid as much passivation as possible on the NP group, the bonder was applied directly after steam blasting and the specimens were fired (Programat EP5000, Ivocar Vivadent, Schaan, Liechtenstein) one by one. To ensure the same conditions for all the specimens, even the groups with passivation were fired one by one.

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Table 1 The results based on all included factors

Groups (n) Mean value (MPa) SD (±) Minimum value (MPa) Maximum value (MPa)

C-P 10 24.1a 3.71 19.2 30.6

C-NP 10 23.3 b 4.30 15.0 30.6

M-P 10 18.5 a-e 4.17 12.4 25.4

M-NP 10 21.3 c 1.90 17.3 24.1

EBM-P 10 22.5 d 4.53 16.6 30.0

EBM-NP 10 25.0 e 4.57 15.7 30.3

C-P: Cast with time of passivation, C-NP: Cast with no time of passivation, M-P: Milled with time of passivation, M-NP: Milled with no time of passivation, EBM-P: EBM-produced with time of passivation, EBM-NP: EBM-produced with no time of passivation.a,b Groups denoted by the same superscripted letter indicate a significant statistical difference in bond strength (p<0.05) between the groups.

After the bonder, one specimen from each group was randomly selected for SEM analysis and the remaining specimens were coated twice with opaque (Ti Opaque, batch 5773 and liquid, batch 200608211, GC Initial) according to the production process for porcelain firing, and fired one by one. Two layers of dentin (Ti Dentin A3, batch 200201111 and liquid, batch 201106011, GC Initial) were applied to compensate for the shrinkage and three specimens were fired at the same time. Finally, the specimens were run through a self-glaze program, no extra glaze was applied. One specimen from each group, with porcelain, was randomly selected for SEM analysis.

Testing procedureA shear bond strength test was performed using a universal testing machine (model 4465, Instron, Norwood, MA, USA). Each specimen was carefully positioned with the shearing blade in contact with the titanium body and the cutting edge as close as possible to the interface. The applied load was set to 0.5 mm/min until failure occurred and the fracture load was recorded in Newton. Thereafter, each porcelain bonding area was measured with a digital caliper and the bond strength (MPa) was calculated by dividing the load at which fracture occurred by the area of the porcelain. The fracture surfaces were examined visually and the type of fracture was noted as adhesive, cohesive or combined.

SEM analysisEach specimen that was set for SEM analysis was embedded in clear acrylic (Permodent, Forshaga Dentaldepå, Stockholm, Sweden) before it was cut in half (Precisian Saw, IsoMet 5000, Buehler, Lake Bluff, IL, USA) with a diamond-coated blade (diamond wafering blade, 15HC diamond, Buehler) at low speed with water cooling. The cut surfaces were polished using a polisher (Phoenix 4000, Buehler) with progressively finer sandpaper (silicon carbide grinding paper, 1200 grit, 2500 grit, and finally 3000 grit, Buehler). Finally,

the specimens were ultrasonically cleaned (BioSonic 4C100XD, Colténe Whaledent, Cuyahoga Falls, OH, USA) for 15 min to remove any abrasive residue.

Analysis of the structure and chemical composition was performed on the interface with an SEM coupled to an energy dispersive spectrometer (EDS) (X-max 80 mm2, Oxford Instruments, Abingdon, UK). The microscope was equipped with a field emission gun (FEG) and an environmental scanning electron microscope (ESEM) (Quanta™ scanning electron microscope FEG-ESEM, 200, FEI, Eindhoven, The Netherlands). To obtain sharp images, the images were taken in low-vacuum mode with magnification ranging from ×400 to ×16,000 with HV 10.0 kV. Mapping and point analysis was conducted for the definition of titanium oxide.

Statistical analysisStatistical analysis of normal distribution (Shapiro-Wilk test) and one-way analysis of variance (ANOVA) was performed for statistical comparison of all factors: the fabrication process, passivation (P) and no time of passivation (NP), followed by a post hoc Tukey’s test. The level of significance was set at α≤0.05.

RESULTS

Bond strength testWhen all factors were compared, the EBM-produced, with no time of passivation gruop (EBM-NP) had the highest mean value (25.0 MPa) and the milled, passivated group (M-P) showed the lowest mean value (18.5 MPa) with significant difference in bond strength (Table 1). When a statistical analysis was performed between the fabrication processes, the difference in mean values between milled titanium and the other two groups was not as pronounced as when all the factors were compared. It was sufficient, however, to show a significantly lower bond strength of the milled titanium (Table 2). No significant difference could be detected between the groups with time of passivation and no time

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Table 2 Summary of bond strength between the different manufacturing processes: cast, milled and EBM-produced

Groups (n) Mean value (MPa) SD (±) Minimum value (MPa) Maximum value (MPa)

Cast 20 23.7 a 3.93 15.0 30.6

Milled 20 19.9 a,b 3.45 12.4 25.4

EBM 20 23.8 b 4.60 15.7 30.3

a,b Groups denoted by the same superscripted letter indicate a significant statistical difference in bond strength (p<0.05) between the groups.

Table 3 Summary of bond strength between the different time of passivation regardless of manufacturing processes

Groups (n) Mean value (MPa) SD (±) Minimum value (MPa) Maximum value (MPa)

Passivated 30 21.7 4.65 12.4 30.6

Non-passivated 30 23.2 3.96 15.0 30.6

Fig. 2 SEM images between the different layers and the boundary between titanium (T), bonding agent (B), opaque and porcelain (P).

a) surface of a C-P specimen with magnification ×500, b) surface of an M-P specimen, magnification ×500, c) surface of an EBM-P specimen, magnification ×400, d) boundary between titanium, bonding agent, opaque and porcelain of C-P specimen, magnification ×8,000, e) boundary between T, B, P of M-P specimen, magnification ×8,000 and f) boundary between T, B, P of EBM-P specimen, magnification ×8,000.

of passivation (Table 3). The results of bond strength showed a normal distribution.

Three fracture types were observed after the shear bond strength test: adhesive, cohesive and combined, where the fracture had gone through the bonder, opaque and/or porcelain. The most common type of fracture was adhesive. Cohesive fracture was observed in five cases: four in the EBM group and one in the milled group. Combined fractures occurred in four cases: two in the milled group, one in the cast group and one in the EBM

group.

SEM analysisSEM images showed consistent and good adhesion between the different layers, and the boundary between titanium bonder, opaque and porcelain was well defined in all specimens (Figs. 2a–f). There were no visible differences between the manufacturing groups. The time of passivation and the impact on oxide growth were not detected. Point analysis of the porcelain showed

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Fig. 3 Mapping showing the boundaries with the presence of O across the interface between the titanium and bonding agent.

a) structure of the boundary, titanium (T), bonding agent (B), opaque and porcelain (P), magnification ×6,000, b) presence of O (white area), c) presence of T (white area).

the presence of Si, Na, O, Ti, Al, Ca, C and K. The same analysis showed Ti, O and a small quantity of Si particles on the titanium surface. Si, Ti, Al, O, Ca, K, Na and La were present at the interface (Figs. 2d–f). Mapping illustrated indistinct boundaries with the presence of O across the interface between titanium and bonder. The presence of oxide was noted. However, its placement in the interface could not be identified or determined (Figs. 3a–c).

DISCUSSION

The oxide layer on the surface of titanium has been identified as the primary problem when bonding porcelain to titanium13). Although studies have analyzed the titanium oxide and the bond between the porcelain and metal, there is a lack of knowledge regarding the mechanisms and modifications that occur at the interface. The aims of the present study were therefore to analyze and define titanium oxide at the interface between titanium and porcelain as well to evaluate whether differences in the thickness and composition of the oxide layer exist based on fabrication process and passivation time.

The results in the present study showed few differences in bond strength between porcelain and titanium fabricated using the different production methods under observation. One finding, however, was that milled titanium showed significantly lower bond strength to porcelain than cast titanium and EBM-produced titanium. This finding could be due to differences in the alloy composition of the alloys used,

changes in the metal due to reactions at high temperature during melting, or reactions with the investment material during casting. Considerable changes in important properties have been reported to take place in titanium during the melting procedure15-17).

Milling is often considered to be a cleaner process compared to technologies based on melting since there will not be any α-case formation on the milled surface. This is often believed to promote increased bond strength between the titanium and the porcelain without the need for any post-processing2). In the present study, however, the results showed the opposite which might be caused instead by another mechanism, such as fewer micromechanical retentions on the milled surfaces than on the other two materials. Iseri et al., 2011, studied the impact of manufacturing technology on the bond strength of porcelain to titanium2). They evaluated cast, machined and laser-sintered titanium, and their results showed significantly lower bond strength for the milled group compared to the other groups, results that were confirmed by the present study with a mean of 19.9 MPa (milled) compared to 23.7 MPa (cast) and 23.8 MPa (EBM).

It was decided that the pre-treatments of the titanium specimens should follow the metal manufacturers’ recommendations prior to porcelain firing. However, two of the manufacturers, the producers of the milled and the cast groups, referred to the porcelain manufacturers’ recommendations for pre-treatment of the metal. These recommendations were followed accordingly for those two groups. Only the EBM manufacturer presented guidelines for the titanium

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pre-treatment and for that group, the EBM titanium recommendation was followed. As a result, differences arose in the pre-treatments between the different groups. However, when comparing the EBM and the cast groups, representing the two different pre-treatment modalities, it was not possible to see any difference in bond strength between the groups. The porcelain was fired according to the porcelain manufacturers’ recommendations in all the groups, with no differences between the groups. Passivation was carried out according to the manufacturers’ recommendation but did differ between the groups. Five minutes’ passivation time was used for the milled and EBM groups and 10 min was used for the cast group. No significant difference was found in bond strength between the groups with regard to time of passivation before porcelain firing, nor when comparing the groups that were passivated with the groups that were not. In other words, the time of passivation shows no influence on the shear bond strength and can be excluded as a factor. Moreover, due to the limitations of different processing techniques and the materials availability, three different grade of titanium was used for each manufacturing method, which could have an impact on the oxide formation. The study design could also have included further surface characterization, to clarify the factors that could affect the bond strength. Future studies using the same grade of titanium is warranted for further evaluations to limit the different factors that could have an impact on the bond strength. When searching in previous literature, no study was found that compared the effects of passivation time on the bond strength of porcelain and titanium.

The majority of the specimens showed adhesive failure at the interface between bonder and titanium. Four specimens showed combined failure with a fracture distribution through several layers/interfaces. Four out of five cohesive failures occurred in the EBM-produced group where the fracture occurred within the porcelain. This may indicate a stronger interfacial bond between the titanium and the porcelain than the cohesive bond within the porcelain itself. No previous studies have been found that compare the bond strength and the oxide layer using titanium that has been manufactured by casting or machining or using EBM. EBM manufacture of titanium is still a relatively new technology.

SEM analysis focused primarily on the time of passivation, and specimens from the groups with passivation were analyzed. Since the oxide layer could not be defined in the no time of passivation groups, specimens from these groups were not further analyzed with the SEM. One specimen in each group had only bonder, i.e. had only undergone one firing cycle, and was compared to one specimen with porcelain that had completed all six firing cycles. This was to see if and to what extent the oxide layer grew from the first to the last firing cycle. The test specimens which were analyzed by SEM showed close-fitting and good adhesion between the different layers of the porcelain system (Figs. 3a–c). Those specimens, however, represent only a small proportion of all the specimens tested. Considering

previous studies that analyzed the bond between titanium and porcelain, SEM and EDS was used to enable an analysis of the chemical composition and to define the oxide layer1,18,19). Point analysis of “known” regions, i.e. an analysis on the titanium surface and on the porcelain surface, was carried out to provide a comparison20) and was continued with mapping. Mapping is a series of analyses at selected points where the presence and orientation of the content can be seen. This was done at selected points in the interface where the titanium oxide was expected to be present. The result of analyzing the interface indicates a non-homogeneous compound without clear boundaries with the presence of more or less oxygen over the entire surface (Figs. 3a–c). A small quantity of silicon particles was identified on the titanium surface and the same content, with the exception of lanthanum, was identified on the porcelain surface. This may be a contamination arising from the polishing procedure during preparation of the specimens for the surface analysis. A large amount of aluminum was present and in addition was embedded in the titanium. This has also been identified in previous studies. It is a well-documented phenomenon that aluminum particles are left over from blasting, despite the cleaning procedure with water rinsing and steam blasting4,12,18,21). However, few studies have been conducted to establish the effect of aluminum particles on the bond between the different interfaces21,22). The theory, however, is that the particles in the titanium and at the interface may well play a role in enhanced or impaired bonding12,21).

To have images for SEM and EDS analysis, the specimen must be read in low vacuum. This means that oxygen may be present in the chamber, making it difficult to identify the oxide layer because it may give rise to a misrepresentation of the chemical composition from the analysis. Another difficulty encountered with EDS analysis is that the detector receives radiation from surrounding surfaces outside the set analysis point. This may mean that contents are recorded that do not actually belong to that particular point20). Most studies have images showing titanium oxide from above but a few images have been found showing titanium dioxide in cross-section19,23). A comparison has been made with the images in the present study but these analyses are not actually sufficient to provide real answers. Kajima et al. evaluates the bond strength between porcelain and different preheating treatments of a Zr-14Nb alloy compared to titanium. They concluded that preheating treatment did not significantly improve the bond strength and that prolonged preheating even might reduce the bond strength due to extensive oxide layer growth. The decreased bond strength may also be a result of a prolonged preheating treatment as for titanium, which have a highly reactive nature and poor adherent oxide layer19).

Shear bond strength test with a cross-head speed of 0.5 mm/min was selected and thermocycling was excluded to enable comparisons with a previous study2). Several methods are available for testing the bond strength between porcelain and metal but no ultimate

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test exists. Several authors prefer shear bond strength test and consider it an appropriate method11,18,24). Others claim that the three-point flexural strength test is preferable due to it being a less complex test method7,25). The limit value for the bond strength of metal-ceramic systems is 25 MPa as specified in ISO/FDIS 9693-1:2012. However, this is based on the three-point flexural strength test and is calculated in reference to the modulus of elasticity and the thickness of the metal13). In order to obtain results from the three-point flexural strength test, the precise value of the modulus of elasticity must be given. Shear bond strength test was chosen in the present study to exclude this potential bias-factor. Generally speaking, any comparison with other studies should be performed cautiously since the materials and methods differ.

CONCLUSION

Within the limitations of this study, the bond strength of porcelain to milled titanium is lower than the bond strength of porcelain to cast titanium and EBM-produced titanium which may be due to differences in the alloy composition of the alloys used, reactions with the investment material during casting or changes in the metal due to reactions at high temperature during melting or differences in the surface structure due to the manufacturing techniques (the milled titanium may have been with fewer micromechanical retentions). Further studies are needed to clarify the influence and mechanisms of passivation and passivation time on the porcelain bond to titanium and the oxide formation, as well as the differences of manufacturing techniques and different grade of titanium. Thus the null-hypothesis was partly rejected.

ACKNOWLEDGMENTS

Milling material was kindly provided by KaVo and the EBM-manufactured specimens were kindly provided by Dentware.

CONFLICTS OF INTEREST

The authors report no conflicts of interest.

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