Metallurgical and interfacial characterization of PFM Co–Cr dental alloys fabricated via casting, milling or selective laser melting

Abstract

Objectives

Bulk and interfacial characterization of porcelain fused to metal (PFM) Co–Cr dental alloys fabricated via conventional casting, milling and selective laser melting.

Methods

Three groups of metallic specimens made of PFM Co–Cr dental 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 evaluated 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 Holm–Sidak’s method ( α = 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 γ phase matrix, a second phase, believed to be the Co 3 Mo phase, was also observed by SEM and subsequent XRD analysis. Cr 7 C 3 and Cr 23 C 6 carbides were also identified via XRD analysis in the CST and MIL groups. The hardness values were 320 ± 12 HV, 297 ± 5 HV and 371 ± 10 HV, and statistically significant differences were evident among the groups.

Significance

The microstructure and hardness of PFM Co–Cr 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.

Introduction

Technological developments have led to the implementation 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 . These technologies can be classified as based on subtractive manufacturing, such as the milling of pre-manufactured materials assisted by computer-aided design/computer-aided manufacturing (CAD/CAM) systems , or on additive manufacturing, such as the recently developed selective laser melting (SLM) technique . 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 technology.

As a recently introduced technique, SLM has attracted the worldwide interest of research groups for the manufacturing of dental metallic structures. In prosthetic dentistry, most studies on SLM have focused on Co–Cr dental alloys ; significantly fewer studies have been performed on Ti alloys or the costly precious alloys . These studies have primarily focused on the evaluation of the marginal and/or internal fit of the restorations , whereas other studies have tested the bond strength with dental porcelain , internal porosity , effect of surface treatments on microroughness and electrochemical properties . However, the comparative analyses of specific properties among metallic structures made using SLM and conventional techniques are limited ; thus, the effect of the SLM technique on mechanical, electrochemical, microstructural and other properties is still unknown. Given the large differences in the manufacturing 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 powder, large differences in microstructural characteristics are anticipated. These microstructural differences may also differentiate the interfacial characterization of metallic elements at the metal–porcelain interface. Although common for other prosthetic dental alloys, interfacial analyses of Co–Cr alloys cast or milled with porcelain are still absent from the dental literature. Therefore, the aim of this study was to metallurgically and interfacially characterize Co–Cr dental alloys prepared by casting, milling and SLM techniques. The null hypothesis was that there would be significant dissimilarities among the groups prepared by different manufacturing techniques.

Materials and methods

Specimen preparation

Three groups (CST, MIL, and SLM) were prepared using Co–Cr dental alloys as indicated by the manufacturers. The specimens of the CST group were fabricated by the traditional casting technique using Co–Cr raw material; those of the MIL group were milled off a prefabricated block and the specimens of the SLM group were fabricated by the SLM technique using Co–Cr mixed powder. The brand names, manufacturers and elemental composition of the alloys tested are presented in Table 1 .

Table 1
Brand names, elemental compositions and coefficients of thermal expansion (CTE) of the tested alloys, as provided by the manufacturers.
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 <1 <1 <1
VI-COMP Dentsply, Elephant Dental, Hoorn, The Netherlands. 61.1 ± 2.0 32.0 ± 2.0 5.5 ± 1.0 <1 <1 N/A
N/A, not available.

In the CST group, 12 wax patterns (IQ sticks, Yeti Dental, Engen, Germany) were invested with phosphate-bonded investment (GC Stellavest, GC Europe NV, Belgium) with dimensions of 0.5 mm × 3 mm × 25 mm. The mold was pre-heated at 910 °C and cast with VI-COMP alloy at 1450 °C using a centrifugal casting machine (Ducatron S3, Ugin’Dentaire, Seyssins, France). The mold was left to cool down to room temperature and the specimens were then divested and cleaned by sandblasting with alumina particles (100 μm).

A prefabricated block of a commercial Co–Cr 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 (0.5 mm × 3 mm × 25 mm) using the Organical Multi Milling/grinding machine (R+K CAD/CAM Technologie).

The laser-sintered specimens were prepared from commercial Co–Cr powder (ST2725G) using a dental laser sintering device (PM 100 Dental System, Phenix Systems, Clermont-Ferrard, France) equipped with a 500 W Yb-fiber laser, at a temperature of 1650 °C; the laser system had the ability to weld across a controlled ( XY )-axis coordinate system with a Z -axis tolerance of ±0.0254 mm. The Co–Cr powder was applied to a stainless steel plate and was laser-sintered upwards in subsequent layers after a 20-μm-thick layer was completed until the final product was generated. Following laser sintering, the sintered parts were cooled down to furnace temperature. In total, 12 specimens with dimensions of 0.5 mm × 3 mm × 25 mm were fabricated using this technique.

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 70 kV and 5 mA with an exposure time of 15 s. Digital images were collected from all specimens, and the X-ray images were assessed by the naked eye.

SEM-EDX characterization

For microstructural characterization, three specimens of each group were examined using a SEM (JSM 6610 LV, Jeol Ltd., Tokyo, Japan) equipped with an X-ray EDS microanalysis (EDX) unit (Oxford Instruments, Abingdon, UK). All examined surfaces were ground using SiC paper (220–2000 grit) under continuous water cooling and polished in a grinding polishing machine (Ecomet III, Bueler, Lake Bluff, IL, USA) using a diamond 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 mm × 3 mm × 25 mm) were examined on the 3 mm × 25 mm surface. The examined surfaces were imaged using a backscattered electron detector (BSE) under an accelerating voltage of 30 kV and a beam current of 48 μA at a working distance of 10 mm. The elemental composition was determined by EDX. EDX spectra were acquired from each specimen using the area scan mode under an accelerating voltage of 30 kV, a beam current of 48 μA, a sampling window of 430 μm × 430 μm and an acquisition time of 1000 s. Moreover, EDX spot analyses were performed under these conditions in a dispersed phase under mean atomic number contrast. Quantitative analysis of the acquired spectra was conducted using INCA software (INCA Suite v 4.13) in a non-standard analysis.

XRD analysis

Three specimens from each group were ground using SiC paper of up to 2000 grit, cleaned with acetone and subjected to analysis using an XRD machine (D8 Advance, Bruker, MA, USA) with an accelerating voltage of 40 kV, a beam current of 40 mA, a 2 θ angle scan range of 30–110°, a scanning speed of 0.02°/s, a sampling pitch of 0.02° and a preset time of 1 s.

Hardness

Vickers hardness (HV10) testing (Diatronic 2RC, Wolpert, Germany) was performed in five specimens of all groups, with 12 measurements obtained per specimen under a load of 10 kg for 30 s of dwell time. The average value of the 12 measurements was used to characterize the hardness of each specimen.

Interfacial characterization

Four specimens from each group were equally divided in two subgroups. Before the veneering procedure was initiated, the surfaces of the specimens were ground with SiC paper of up to 2000 grit, polished with diamond pastes up to 1 μm (DP Paste, Struers) and cleaned in an ultrasonic bath for 10 min in 95% ethanol. The first subgroup was fused with two layers of opaque, followed by two layers of dentin and one layer of dental porcelain (GC Initial MC, Europe N.V., Leuven, Belgium) according to the manufacturer’s instructions ( Table 2 ). The second group was initially covered with bonding agent (GC Initial INmetalbond) before the application of the opaque and other layers. The specimens were then embedded in resin, ground with SiC paper of up to 2000 grit and polished with 6 μm, 3 μm and 1 μm diamond pastes (DP Paste, Struers). Following platinum sputter coating (JFC 1600 Auto Fine Coater, Jeol), the metal ceramic interface was imaged using a BE detector at a nominal magnification of 2000×. For each specimen, an EDX spectrum was acquired for the bulk of the opaque or INmetalbond (indicated by the dotted rectangle in Fig. 4 ) using an accelerating voltage of 30 kV and a beam current of 53 μA at a working distance of 10 mm over an acquisition time of 200 s. EDX line profile analysis was performed across the interfaces of the two subgroups under an accelerating voltage of 30 kV and a beam current of 48 μA at a working distance of 10 mm, a line profile scanning distance of 2.56 μm, a magnification of 50,000× and an acquisition time of 1280 s. The curves were smoothed using the local smoothing technique with bi-square weighting and polynomial regression.

Table 2
Firing schedules of the veneering procedure for GC Initial MC porcelain (according to the manufacturer’s instructions).
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 1
Glaze 480 2 45 No 850 1

Statistical analysis

The hardness results were statistically compared using one-way ANOVA and Holm–Sidak’s multiple comparison test ( α = 0.05).

Materials and methods

Specimen preparation

Three groups (CST, MIL, and SLM) were prepared using Co–Cr dental alloys as indicated by the manufacturers. The specimens of the CST group were fabricated by the traditional casting technique using Co–Cr raw material; those of the MIL group were milled off a prefabricated block and the specimens of the SLM group were fabricated by the SLM technique using Co–Cr mixed powder. The brand names, manufacturers and elemental composition of the alloys tested are presented in Table 1 .

Table 1
Brand names, elemental compositions and coefficients of thermal expansion (CTE) of the tested alloys, as provided by the manufacturers.
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 <1 <1 <1
VI-COMP Dentsply, Elephant Dental, Hoorn, The Netherlands. 61.1 ± 2.0 32.0 ± 2.0 5.5 ± 1.0 <1 <1 N/A
N/A, not available.

In the CST group, 12 wax patterns (IQ sticks, Yeti Dental, Engen, Germany) were invested with phosphate-bonded investment (GC Stellavest, GC Europe NV, Belgium) with dimensions of 0.5 mm × 3 mm × 25 mm. The mold was pre-heated at 910 °C and cast with VI-COMP alloy at 1450 °C using a centrifugal casting machine (Ducatron S3, Ugin’Dentaire, Seyssins, France). The mold was left to cool down to room temperature and the specimens were then divested and cleaned by sandblasting with alumina particles (100 μm).

A prefabricated block of a commercial Co–Cr 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 (0.5 mm × 3 mm × 25 mm) using the Organical Multi Milling/grinding machine (R+K CAD/CAM Technologie).

The laser-sintered specimens were prepared from commercial Co–Cr powder (ST2725G) using a dental laser sintering device (PM 100 Dental System, Phenix Systems, Clermont-Ferrard, France) equipped with a 500 W Yb-fiber laser, at a temperature of 1650 °C; the laser system had the ability to weld across a controlled ( XY )-axis coordinate system with a Z -axis tolerance of ±0.0254 mm. The Co–Cr powder was applied to a stainless steel plate and was laser-sintered upwards in subsequent layers after a 20-μm-thick layer was completed until the final product was generated. Following laser sintering, the sintered parts were cooled down to furnace temperature. In total, 12 specimens with dimensions of 0.5 mm × 3 mm × 25 mm were fabricated using this technique.

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 70 kV and 5 mA with an exposure time of 15 s. Digital images were collected from all specimens, and the X-ray images were assessed by the naked eye.

SEM-EDX characterization

For microstructural characterization, three specimens of each group were examined using a SEM (JSM 6610 LV, Jeol Ltd., Tokyo, Japan) equipped with an X-ray EDS microanalysis (EDX) unit (Oxford Instruments, Abingdon, UK). All examined surfaces were ground using SiC paper (220–2000 grit) under continuous water cooling and polished in a grinding polishing machine (Ecomet III, Bueler, Lake Bluff, IL, USA) using a diamond 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 mm × 3 mm × 25 mm) were examined on the 3 mm × 25 mm surface. The examined surfaces were imaged using a backscattered electron detector (BSE) under an accelerating voltage of 30 kV and a beam current of 48 μA at a working distance of 10 mm. The elemental composition was determined by EDX. EDX spectra were acquired from each specimen using the area scan mode under an accelerating voltage of 30 kV, a beam current of 48 μA, a sampling window of 430 μm × 430 μm and an acquisition time of 1000 s. Moreover, EDX spot analyses were performed under these conditions in a dispersed phase under mean atomic number contrast. Quantitative analysis of the acquired spectra was conducted using INCA software (INCA Suite v 4.13) in a non-standard analysis.

XRD analysis

Three specimens from each group were ground using SiC paper of up to 2000 grit, cleaned with acetone and subjected to analysis using an XRD machine (D8 Advance, Bruker, MA, USA) with an accelerating voltage of 40 kV, a beam current of 40 mA, a 2 θ angle scan range of 30–110°, a scanning speed of 0.02°/s, a sampling pitch of 0.02° and a preset time of 1 s.

Hardness

Vickers hardness (HV10) testing (Diatronic 2RC, Wolpert, Germany) was performed in five specimens of all groups, with 12 measurements obtained per specimen under a load of 10 kg for 30 s of dwell time. The average value of the 12 measurements was used to characterize the hardness of each specimen.

Interfacial characterization

Four specimens from each group were equally divided in two subgroups. Before the veneering procedure was initiated, the surfaces of the specimens were ground with SiC paper of up to 2000 grit, polished with diamond pastes up to 1 μm (DP Paste, Struers) and cleaned in an ultrasonic bath for 10 min in 95% ethanol. The first subgroup was fused with two layers of opaque, followed by two layers of dentin and one layer of dental porcelain (GC Initial MC, Europe N.V., Leuven, Belgium) according to the manufacturer’s instructions ( Table 2 ). The second group was initially covered with bonding agent (GC Initial INmetalbond) before the application of the opaque and other layers. The specimens were then embedded in resin, ground with SiC paper of up to 2000 grit and polished with 6 μm, 3 μm and 1 μm diamond pastes (DP Paste, Struers). Following platinum sputter coating (JFC 1600 Auto Fine Coater, Jeol), the metal ceramic interface was imaged using a BE detector at a nominal magnification of 2000×. For each specimen, an EDX spectrum was acquired for the bulk of the opaque or INmetalbond (indicated by the dotted rectangle in Fig. 4 ) using an accelerating voltage of 30 kV and a beam current of 53 μA at a working distance of 10 mm over an acquisition time of 200 s. EDX line profile analysis was performed across the interfaces of the two subgroups under an accelerating voltage of 30 kV and a beam current of 48 μA at a working distance of 10 mm, a line profile scanning distance of 2.56 μm, a magnification of 50,000× and an acquisition time of 1280 s. The curves were smoothed using the local smoothing technique with bi-square weighting and polynomial regression.

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Metallurgical and interfacial characterization of PFM Co–Cr dental alloys fabricated via casting, milling or selective laser melting
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