title: Annealing of Co-Cr dental alloy: effects on nanostructure and Rockwell hardness.
abstract: OBJECTIVE
  The 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.
  
  
  METHODS
  Five 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.
  
  
  RESULTS
  The 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).
  
  
  CONCLUSIONS
  After 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.
authors.affiliation: Department of Prosthodontics, Dental Health Sciences Center, Gulhane Military Medical Academy, Ankara, Turkey.
authors.forename: Simel
authors.initials: S
authors.lastname: Ayyıldız
authors.affiliation: Department of Physics, Karadeniz Technical University, Faculty of Science and Literature, Trabzon, Turkey.
authors.forename: Elif Hilal
authors.initials: EH
authors.lastname: Soylu
authors.affiliation: Department of Physics Engineering, Faculty of Engineering, Hacettepe University, Ankara, Turkey.
authors.forename: Semra
authors.initials: S
authors.lastname: Ide
authors.affiliation: Department of Public Health, Gulhane Military Medical Academy, Ankara, Turkey.
authors.forename: Selim
authors.initials: S
authors.lastname: Kılıç
authors.affiliation: Department of Prosthodontics, Dental Health Sciences Center, Gulhane Military Medical Academy, Ankara, Turkey.
authors.forename: Cumhur
authors.initials: C
authors.lastname: Sipahi
authors.affiliation: Department of Prosthodontics, Dental Health Sciences Center, Gulhane Military Medical Academy, Ankara, Turkey.
authors.forename: Bulent
authors.initials: B
authors.lastname: Pişkin
authors.affiliation: Dental Health Service, Beytepe Military Hospital, Ankara, Turkey.
authors.forename: Hasan Suat
authors.initials: HS
authors.lastname: Gökçe
keywords: Dental alloys; Hardness test; Metal ceramic alloys; X-ray microanalysis