J Adv Prosthodont.  2013 Nov;5(4):471-478. 10.4047/jap.2013.5.4.471.

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

• Ayyildiz S
• Soylu EH
• Ide S
• Kilic S
• Sipahi C
• Piskin B
• Gokce HS

• KMID: 2176541
• DOI: http://doi.org/10.4047/jap.2013.5.4.471

Abstract

PURPOSE
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.
MATERIALS AND METHODS
Five groups of Co-Cr dental alloy samples were manufactured in a rectangular form measuring 4 x 2 x 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 A).
CONCLUSION
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.

Keyword

Dental alloys; Hardness test; X-ray microanalysis; Metal ceramic alloys

MeSH Terms

Alloys
Argon
Atmosphere
Ceramics
Dental Alloys*
Dentists
Electron Probe Microanalysis
Hardness Tests
Hardness*
Humans
Metal Ceramic Alloys
Nanostructures*
Oxygen
Prosthodontics
Alloys
Argon
Ceramics
Dental Alloys
Metal Ceramic Alloys
Oxygen

Figure

• 

  Fig. 1 SAXS patterns of the groups that were analyzed; q, magnitude of the scattering vector.

• 

  Fig.2 Structure of materials shown schematically according to fractal dimension analysis. (A) mass fractals (1< di < 3, small spherical entities that have volumetric mass fractals with covered surfaces), (B) surface fractals (3 < di < 4, differently oriented and disjointed planks), (C) regular surfaces (di=4).

• 

  Fig. 3 Statistical results of the Rockwell measurements of the groups. Maximum hardness was measured in Group III (non-annealed ) and minimum value was seen in Group IV (argon-annealed).

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