Comparison of the bond strength of ceramics to Co-Cr alloys made by casting and selective laser melting

Effect of laser etching on surface characteristics and porcelain bond to soft milled and direct metal laser sintered cobalt chromium alloys

STATEMENT OF PROBLEM

The surface topography of metal substrate can affect its bond to porcelain. A neodynium-doped yttrium aluminum garnet (Nd:YAG) laser has been introduced to modify the metal surface topography and improve porcelain bond strength. However, studies on the effect of laser etching on metal to porcelain bond strength are lacking.

PURPOSE

The purpose of this in vitro study was to determine the effect of Nd:YAG laser etching on the surface roughness and wettability of and the porcelain bond strength to cobalt chromium (Co-Cr) substrate fabricated by milling and direct metal laser sintering (DMLS).

MATERIAL AND METHODS

Thirty-two 0.5×3×25-mm Co-Cr specimens were fabricated by milling soft Co-Cr (M group) and DMLS Co-Cr metal powder (DML group). The surface topography of representative specimens from each study group was assessed under a scanning electron microscope (SEM) and an atomic force microscope (AFM). All specimens were assessed for surface roughness using a contact profilometer, and for wettability with a contact angle goniometer. Half of the specimens of each study group (n=8) were subjected to surface laser etching by using a Nd:YAG laser. The specimens subjected to etching were assessed again for surface topography and wettability. All specimens in both study groups were veneered with porcelain. The porcelain bond strength was tested with a 3-point bend test in a universal testing machine. The results were statistically analyzed with 2-way ANOVA test followed by the post hoc Tukey test for pairwise comparisons (α=.05).

RESULTS

After etching, the M group had a higher mean ±standard deviation Ra and Rz of 2.9 ±0.6 and 17.7 ±3.2 µm and significantly better wettability and bond strength of 79 ±6 and 52 ±13 MPa. In contrast, after etching, the DMLS group had a significantly lower Ra and Rz of 7.9 ±2.4 and 41.8 ±9.3 µm and significantly lower wettability and bond strength of 87 ±4 and 70 ±10 MPa. The DMLS group had a significantly higher roughness and bond strength than the M group before and after laser etching. The SEM and AFM showed different surface topography in the study groups.

CONCLUSIONS

The manufacturing process of Co-Cr substrate had a significant effect on surface characteristics and porcelain bond strength. Laser etching improved the surface topography and bond strength of milled Co-Cr but not of DMLS Co-Cr.

Micro-shear bond strength of a novel resin-modified glass ionomer luting cement (eRMGIC) functionalized with organophosphorus monomer to different dental substrates

Objectives

This study aims to assess and compare the micro-shear bond strength (μSBS) of a novel resin-modified glass-ionomer luting cement functionalized with a methacrylate co-monomer containing a phosphoric acid group, 30 wt% 2-(methacryloxy) ethyl phosphate (2-MEP), with different substrates (dentin, enamel, zirconia, and base metal alloy). This assessment is conducted in comparison with conventional resin-modified glass ionomer cement and self-adhesive resin cement.

Materials and methods

In this in vitro study, ninety-six specimens were prepared and categorized into four groups: enamel (A), dentin (B), zirconia (C), and base metal alloys (D). Enamel (E) and dentin (D) specimens were obtained from 30 human maxillary first premolars extracted during orthodontic treatment. For zirconia and metal alloys, 48 disks were manufactured using IPS e.max ZirCAD through dry milling and Co-Cr powder alloy by selective laser milling. Each group was further subdivided into three subgroups (n = 8) according to the luting cement used: (1) Fuji PLUS resin-modified glass ionomer luting cement (FP) as a control cement, (2) modified control cement (eRMGIC), and (3) RelyX U 200 (RU 200) self-adhesive resin cement. The two-way analysis of variance and Tukey's HSD were used to assess the data obtained from measuring the μSBS of the samples.

Results

The results of this study showed that the mean μSBS values of eRMGIC were statistically higher compared to FP in all tested groups (p < 0.001). The mean μSBS results of eRMGIC were non-significantly different from those recorded by RU 200 for all substrates except for the dentin substrate, where the RU200 cement produced significantly higher strength (p < 0.001). The failure modes were limited to a combination of mixed and adhesive failures without pure cohesive failure.

Significance

The functionalization of FP with an organophosphorus co-monomer (2-MEP) directly affects the adhesion performance of the functionalized cement, which may be utilized to develop a new type of acid-base cement. It exhibited a performance comparable to that of resin-based cement and should serve well under different clinical conditions.

Comparison of tensile properties and porcelain bond strength in metal frameworks fabricated by selective laser melting using three different Co-Cr alloy powders

STATEMENT OF PROBLEM

The selective laser melting (SLM) manufacturing technique has been widely employed to produce Co-Cr dental metal frameworks. The selection of Co-Cr alloy powders has the potential to influence the microstructure and tensile properties, consequently impacting the bond strength of the metal-porcelain. However, limited information is available regarding the effect of Co-Cr alloy powder on these properties when all other factors remain consistent.

PURPOSE

The purpose of this in vitro study was to assess how the choice of Co-Cr alloys during SLM manufacturing influences the microstructure, tensile properties, and bond strength of metal-ceramic combinations.

MATERIAL AND METHODS

Three different Co-Cr alloy powders, Co-Cr-Mo, Co-Cr-Mo-W, and Co-Cr-W were selected in this study. The powder characteristics and chemical compositions were analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis, respectively. Subsequently, 12×12×15-mm cube specimens, cylindrical tensile test specimens, and 25×3×0.5-mm metal strips were fabricated using the SLM technique. Microstructural investigations of the cube specimens were conducted after metallographic preparation using SEM. The cylindrical tensile specimens (n=8) from each composition were subjected to tensile tests at a deformation rate of 0.5 mm/min. Following the application of ceramic to the metal specimens (n=10) in each group, the strength of the metal-ceramic bond was evaluated through a 3-point bend test conducted at a crosshead speed of 1 mm/min. Mechanical properties obtained from the tensile tests and bond strength values were statistically analyzed using a one-way ANOVA and Tukey post hoc comparison tests (α=.05).

RESULTS

Melt pool boundaries, columnar and equiaxed grains, and precipitates were observed in the microstructures of 3 different alloys produced by SLM. The Co-Cr-Mo-W alloy had more uniformly dispersed and finely distributed precipitates compared with other alloy compositions. The Co-Cr-Mo-W alloy had exhibited the highest yield strength (1068.0 ±41.2 MPa) and ultimate tensile strength (1263.4 ±10.7 MPa) while showing the lowest ductility under the tensile tests (6.1 ±0.9%) among all 3 alloys. Significant differences in the tensile mechanical properties were observed in the alloys except between the yield strength of the Co-Cr-Mo and Co-Cr-W alloys. The highest elongation (8.9 ±1.2%) was seen in the Co-Cr-Mo alloy. However, no significant differences were detected regarding the bond strength of all 3 groups (P>.05). The mean bond strength values were approximately 42 MPa for all the alloys.

CONCLUSIONS

The results indicate that the selection of different Co-Cr alloy powders used in SLM production may influence both microstructure and tensile properties. However, the strength of the metal-ceramic bond of Co-Cr alloys remained unaffected by this selection.

Evaluation of the relationship between metallurgical properties and metal-ceramic bond characteristics of Co-Cr alloys manufactured by different techniques

STATEMENT OF PROBLEM

Metal-ceramic restorations made from cobalt chromium (Co-Cr) alloy have been increasing, but studies on the effects of different manufacturing techniques on metal-ceramic interface characteristics and metal-ceramic bond strength are sparse.

PURPOSE

The purpose of this in vitro study was to examine the metal-ceramic interface and the metal-ceramic bond strength of a Co-Cr alloy produced by casting, milling, and selective laser melting (SLM) with or without thermal cycling.

MATERIAL AND METHODS

Co-Cr alloys were prepared by casting, milling, and SLM. Two different SLM devices were used. Ninety-six specimens (25×3×0.5 mm) were manufactured. The structure of the oxidation surface of Co-Cr specimens was examined by scanning electron microscopy (SEM) and by X-ray fluorescence spectroscopy (XRFS). After porcelain application, selected specimens were thermal cycled, and the strength of the metal-ceramic bond was measured by the 3-point bend test. All specimens were analyzed for failure type with a stereomicroscope. The elemental composition and morphology of the metal-ceramic interface were examined by XRFS and SEM with energy-dispersive X-ray (EDX). The results of bond strength were analyzed using a 2-way analysis of variance (ANOVA) for manufacturing methods and testing conditions and the Tukey honest significant difference (HSD) test (α=.05).

RESULTS

The main effect of the interactions of the testing condition and manufacturing method variables on the bond strength variable was not statistically significantly different. No significant differences were found among the testing conditions tested (P=.638). Significant differences were found among the manufacturing methods tested statistically (P<.001). Statistically significant differences were found in the CAD-CAM and CONSEPT LASER groups, the CAD-CAM and SLM LASER groups, the CAD-CAM and CAST groups, and the CAST-SLM LASER groups (P<.05). Differences were observed among the interface morphologies of casting, milling, and the two SLM groups.

CONCLUSIONS

The bond strength between Co-Cr alloy and ceramic is affected by the manufacturing method. The metal-ceramic bond strength is independent of thermal cycling. The bond strength value in all groups was over 25 MPa, which has been considered clinically acceptable. The interface metallurgical structures of Co-Cr alloys were affected by different manufacturing techniques.

Adhesion of veneering porcelain to cobalt-chromium dental alloys processed with casting, milling, and additive manufacturing methods: A systematic review and meta-analysis

STATEMENT OF PROBLEM

Selective laser melting (SLM) additive manufacturing (AM) technologies provide an alternative to conventional casting and milling procedures in fabricating metal-ceramic dental prostheses. However, the quality of porcelain bond strength to the SLM AM cobalt-chromium (Co-Cr) metal framework of a dental restoration is unclear.

PURPOSE

The purpose of this systematic review and meta-analysis was to identify in vitro studies that reported the porcelain bond strength to SLM AM Co-Cr dental metal alloys and compare the porcelain bond strength values to cast, milled, and additively manufactured Co-Cr dental alloys.

MATERIAL AND METHODS

An electronic systematic review was performed in different databases: MEDLINE/PubMed, EMBASE, World of Science, Cochrane, and Scopus. A manual search was also conducted. Studies that reported the porcelain bond strength to SLM Co-Cr metal alloys and in the English language were included. Two investigators evaluated the quality assessment of the studies by applying the JBI critical appraisal checklist for quasi-experimental studies (nonrandomized experimental studies). A third investigator was consulted to resolve lack of consensus. Two subgroups were created based on the test used, 3-point bend and shear bond strength tests. The porcelain bond strength of cast, milled, and AM Co-Cr dental alloys were compared. The I2 statistic and its associated P value were used to assess the heterogeneity between studies. The Eger test was used for determining significance of the funnel pots.

RESULTS

A total of 216 studies were collected from the electronic and manual searches. After independently evaluating the titles and abstracts by the reviewers, 26 articles were identified. Three of these were excluded after full-text revision. The porcelain bond strength comparison between the cast and AM alloys for the 3-point bend subgroup revealed a significant result for overall effect (P<.001) favoring the SLM method with considerable heterogeneity (I2=83%, P<.001). Furthermore, the porcelain bond strength comparison between cast and milled alloys for the shear bond strength subgroup revealed a significant test for overall effect (P=.04) favoring milled procedures with a nonsignificant unimportant heterogeneity (I2= 0%, P<.47) and for the 3-point bend subgroup (P<.001) favoring milled specimens with a significant considerable heterogeneity (I2=79%, P<.001).

CONCLUSIONS

The metal manufacturing method had no effect on the porcelain bond strength to Co-Cr dental metal alloys.

Effects of Bonding Agents on Metal-Ceramic Bond Strength of Co-Cr Alloys Fabricated by Selective Laser Melting

Bonding agents have been developed to improve bond strength between ceramic and Co-Cr metal. The aim of this study was to investigate the influence of two bonding agents on bond strength of Co-Cr metal fabricated by selective laser melting (SLM). Bond strength was determined by a three-point bending test, and the interfaces of the metal and ceramic, before and after the bending test, were observed by optical microscopy and scanning electron microscopy (SEM) to determine the thickness of the oxide layer and amount of ceramic remaining. To analyze the elemental composition of the bonding agents and fractured surfaces, energy dispersive X-ray spectroscopy (EDS) was used. Co-Cr specimens with bonding agent showed significantly higher bond strength than Co-Cr specimens without bonding agents. The fractured surfaces of most specimens showed mixed failure, but failure mode varied according to bonding agent and fabrication type. Specimens from groups treated with bonding agents had significantly higher remaining ceramic fractions on fractured Co-Cr alloys than specimens from groups that did not receive bonding agent. Mass amounts of silicone (Si) and titanium (Ti) on the fractured alloy surfaces were also different among specimens according to method of fabrication and presence of bonding agent. Together, the results suggest that application of bonding agent to 3D printed Co-Cr metal increases bond strength with ceramics.

Comparative In Vitro Study of the Bond Strength of Composite to Carbon Fiber Versus Ceramic to Cobalt-Chromium Alloys Frameworks for Fixed Dental Prostheses

Purpose: The aim of this comparative in vitro study was to assess the bond strength and mechanical failure of carbon-fiber-reinforced composites against cobalt–chrome structures with ceramic veneering. Materials and methods: A total of 24 specimens (12 per group) simulating dental prosthetic frameworks were fabricated. The experimental specimens were subjected to a thermocycling aging process and to evaluate bond strength. All specimens were subjected to a three-point bending test to fracture using a universal testing machine. Results: The cobalt–chrome/ceramic group yielded a bond strength value of 21.71 ± 2.16 MPa, while the carbon-fiber-reinforced composite group showed 14.50 ± 3.50 MPa. The failure assessment reported statistical significance between groups. Although carbon-fiber-reinforced composite group showed lower bond strength values, the chipping incidence in this group was as well lower. Conclusions: The chrome–cobalt/ceramic group showed greater bonding strength compared to the carbon-fiber-reinforced composite; most of the fractures within the cobalt–chrome/ceramic group, had no possibility of direct clinical repair.

Chemical composition, surface roughness, and ceramic bond strength of additively manufactured cobalt-chromium dental alloys

STATEMENT OF PROBLEM

Selective laser melting (SLM) additive manufacturing (AM) technology is a current option to fabricate cobalt-chromium (Co-Cr) metal frameworks for dental prostheses. However, the Co-Cr alloy composition, surface roughness, and ceramic bond strength values that SLM metals can obtain are not well-defined.

PURPOSE

The purpose of this in vitro study was to compare the chemical composition, surface roughness, and ceramic shear bond strength of the milled and SLM Co-Cr dental alloys.

MATERIAL AND METHODS

A total of 50 disks of 5 mm in diameter and 1 mm in thickness were fabricated by using subtractive (control group) and AM with each of following SLM providers: SLM-1 (EOS), SLM-2 (3D systems), and SLM-3 (Concept Laser). The milled disks were airborne-particle abraded with 100-μm aluminum oxide particles. All the specimens were cleaned before surface roughness (Ra), weight (Wt%), and atomic (At%) percentages were analyzed. Three-dimensional profilometry was used to analyze the topographical properties of the surface parameters Ra (mean surface roughness). The chemical composition of Co-Cr alloy specimens was determined by using energy dispersive X-ray (EDAX) elemental analysis in a scanning electron microscope (SEM). Thereafter, the specimens were bonded to a ceramic (Dentine A3 and Enamel S-59; Creation CC) interface. Specimens were stored for 24 hours at 23 °C. The bond strength of the SLM-ceramic interface was measured by using the macroshear test (SBT) method (n=10). Adhesion tests were performed in a universal testing machine (1 mm/min). The Shapiro-Wilk test revealed that the chemical composition data were not normally distributed. Therefore, the atomic (At%) and weight percentages (Wt%) were analyzed by using the Kruskal-Wallis test, followed by pairwise Mann-Whitney U tests between the control and AM groups (AM-1 to AM-4). However, the Shapiro-Wilk test revealed that the surface roughness (Ra) and ceramic bond strength data were normally distributed. Therefore, data were analyzed by using 1-way ANOVA, followed by the post hoc Sidak test (α=.05).

RESULTS

Significant differences were obtained in Wt%, At%, and Ra values among the Co-Cr alloys evaluated (P<.05). Furthermore, the control group revealed significantly lower mean ±standard deviation Ra values (0.79 ±0.11 μm), followed by AM-3 (1.57 ±0.15 μm), AM-2 (1.80 ±0.43 μm), AM-1 (2.43 ±0.34 μm), and AM-4 (2.84 ±0.27 μm). However, no significant differences were obtained in the metal-ceramic shear bond strength among the different groups evaluated, ranging from mean ±standard deviation 75.77 ±11.92 MPa to 83.65 ±12.21 MPa.

CONCLUSIONS

Co-Cr dental alloys demonstrated a significant difference in their chemical compositions. Subtractive and additive manufacturing procedures demonstrated a significant influence on the surface roughness of the Co-Cr alloy specimens. However, the metal-ceramic shear bond strength of Co-Cr alloys was found to be independent of the manufacturing process.

Adhesion of porcelain to three-dimensionally printed and soft milled cobalt chromium

PURPOSE

To investigate the adhesion strength and bonding interface of layered porcelain to powdered cobalt-chromium (CoCr) processed by two different computer-aided manufacturing methods.

METHODS

Sixteen specimens were manufactured from each of Three-dimensionally -printed/laser-sintered (LS) CoCr and milled pre-sintered (SM) CoCr. The specimens were layered with porcelain and 4-point bending was carried out. Nanoindentation was used to calculate changes in elastic modulus and hardness before and after porcelain firing along with adhesion energy. Fracture surface and microstructural changes were examined before and after porcelain firing observed using scanning electron microscopy.

RESULTS

The adhesion energy of the LS specimens bonded porcelain were higher than the SM specimens (P<0.05). Analysis of the fracture surfaces showed a predominantly adhesive mode of failure. Elastic-modulus and hardness of the CoCr specimens increased post porcelain firing. Examination using electron-backscatter diffraction (EBSD) showed a fine grain structure for both manufacturing methods. Significant localized changes in the crystal structure post firing were only observed at the surface of the SM specimens.

CONCLUSIONS

Both manufacturing methods showed regular microstructures prior to porcelain firing. Laser-sintered CoCr had stronger bonding to porcelain than milled pre-sintered CoCr and was also more stable microstructurally post-ceramic firing. However, both manufacturing methods were deemed to have satisfactory adhesion strength to porcelain. It was also found that increased hardness of CoCr had an inverse relationship with bonding strength. High strength porcelain bonding and stability following multiple ceramic firings indicate suitability for use of these CoCr materials with implant or tooth supported long-span frameworks.

Cobalt-chromium alloys in fixed prosthodontics in Sweden

Aim: The aim of this study was to compile the usage of Co-Cr alloys in fixed prosthodontics (FP) among dental laboratories in Sweden.

Methods: From March to October 2015, questionnaires were sent to 542 registered dental laboratories in Sweden. The questionnaires were divided in two parts, one for fixed dental-supported prosthodontics (FDP) and one for fixed implant-supported prosthodontics (FIP). Reminders were sent three times.

Results: In total of 542 dental laboratories, 55% answered the questionnaires. Most dental laboratories use Co-Cr in FP, 134 (74%) in FDP and 89(66%) in FIP. The laboratories used Co-Cr alloys of various compositions in the prostheses, 35 for FDP and 30 for FIP. The most commonly used Co-Cr alloys for tooth-supported FDPs were (a) Wirobond^® 280, (b) Cara SLM and (c) Wirobond^® C. For implant-supported frameworks the frequently used alloys were: (a) Cara SLM, (b) Cara Milled and (c) Wirobond^® 280. Except for the difference in composition of these alloys, they were also manufactured with various techniques. In tooth-supported prostheses the dominating technique was the cast technique while newer techniques as laser-sintering and milling were more commonly reported for implant-supported constructions. A fourth technique; the ‘pre-state’ milling was reported in FDP.

Conclusion: More than 30 different Co-Cr alloys were reported as being used in FP. Thus, there is a need for studies exploring the mechanical and physical behavior and the biological response to the most commonly used Co-Cr alloys.