Annealing of Co-Cr dental alloy: effects on nanostructure and Rockwell hardness

Abstract

PURPOSEThe aim of the study was to evaluate the effect of annealing on the nanostructure and hardness of Co-Cr metal ceramic samples that were fabricated with a direct metal laser sintering (DMLS) technique.MATERIALS AND METHODSFive groups of Co-Cr dental alloy samples were manufactured in a rectangular form measuring 4 × 2 × 2 mm. Samples fabricated by a conventional casting technique (Group I) and prefabricated milling blanks (Group II) were examined as conventional technique groups. The DMLS samples were randomly divided into three groups as not annealed (Group III), annealed in argon atmosphere (Group IV), or annealed in oxygen atmosphere (Group V). The nanostructure was examined with the small-angle X-ray scattering method. The Rockwell hardness test was used to measure the hardness changes in each group, and the means and standard deviations were statistically analyzed by one-way ANOVA for comparison of continuous variables and Tukey's HSD test was used for post hoc analysis. P values of <.05 were accepted as statistically significant.RESULTSThe general nanostructures of the samples were composed of small spherical entities stacked atop one another in dendritic form. All groups also displayed different hardness values depending on the manufacturing technique. The annealing procedure and environment directly affected both the nanostructure and hardness of the Co-Cr alloy. Group III exhibited a non-homogeneous structure and increased hardness (48.16 ± 3.02 HRC) because the annealing process was incomplete and the inner stress was not relieved. Annealing in argon atmosphere of Group IV not only relieved the inner stresses but also decreased the hardness (27.40 ± 3.98 HRC). The results of fitting function presented that Group IV was the most homogeneous product as the minimum bilayer thickness was measured (7.11 Å).CONCLUSIONAfter the manufacturing with DMLS technique, annealing in argon atmosphere is an essential process for Co-Cr metal ceramic substructures. The dentists should be familiar with the materials that are used in clinic for prosthodontics treatments.

Laser and electron‐beam powder‐bed additive manufacturing of metallic implants: A review on processes, materials and designs

“…Oyague et al and Kim et al separately evaluated the fit of dental prostheses produced by SLM and reached different conclusions about the suitability of SLM technology in producing dental prostheses . In terms of hardness, elastic modulus and strength, Ayyildiz et al concluded that CoCr produced by laser AM is suitable for dental applications …”

A novel coping metal material CoCrCu alloy fabricated by selective laser melting with antimicrobial and antibiofilm properties

“…Among the bio-active functions proposed for dental materials, antibacterial activity seems the most promising [17,29]. The use of CoCr based alloys has become popular in recent years as a coping material for use in restorative dentistry [6,7]. As a widely used coping material, CoCr based alloy is bio-inert which does not possess inherent antibacterial properties, and therefore is easily colonized by bacteria with subsequent development of a biofilm.…”

“…Currently, CoCr based alloys are widely used in prosthodontics due to a combination of favorable properties, including mechanical strength, ductility, corrosion resistance and biocompatibility, which makes them broadly applicable in a variety of applications with fixed and removable restorations. CoCr based alloys, based on SLM technology, were recently introduced for manufacturing base metal coping [6,7].…”

The place of direct metal laser sintering (DMLS) in dentistry and the importance of annealing

“…The authors' findings for phase transformation status and microstructural characterization of the samples that were manufactured by the DMLS technique provide novel data for academics in this field. However, there are some important points I would like to add, as we previously published a DMLS study in another journal [2].…”

“…However, this technique is still very new for many clinicians manufacturing Co-Cr metal ceramic substructures, and therefore, the steps of the procedure and the factors that affect product quality are not well known. In our paper, we aimed to present the micromechanical properties of Co-Cr samples that were manufactured by the DMLS technique and the importance of the laboratory procedures for biological and mechanical factors [2]. As we outlined in our study, the annealing process is an inevitable step for the clinical use of these products.…”