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Abstract
Purpose
The purpose of present study is to compare mechanical properties and microstructural characteristics of fractured surface for cast, 3-D printing laser sintered and CAD/CAM milled cobalt-chromium (Co-Cr) alloy specimens and to investigate whether laser sintered technique is adequate for dental applications.
Materials and methods
Thirty six flat disc shape Co-Cr alloy specimens were fabricated for surface hardness test and divided into three groups according to the manufacturing methods; 12 specimens for casting (n=12), 12 specimens for laser sintered technology (n=12) and 12 specimens for milled technology (n=12). Twelve dumbbell shape specimens for each group were also fabricated for a tensile test. Statistical comparisons of the mechanical properties for the alloys were performed by Kruskal-Wallis test followed by Mann-Whitney and Bonferroni test. The microstructural characteristics of fractured surfaces were examined using SEM.
Results
There were significant differences in the mean Vickers hardness values between all groups and the cast specimen showed the highest (455.88 Hv) while the CAD/CAM milled specimen showed the lowest (243.40 Hv). Significant differences were found among the three groups for ultimate tensile strength, 0.2% yield stress, elongation, and elastic modulus. The highest ultimate tensile strength value (1442.94 MPa) was shown in the milled group and the highest 0.2% yield strength (1136.15 MPa) was shown in the laser sintered group.
Conclusion
Different manufacturing methods influence the mechanical properties and microstructure of the fractured surfaces in Co-Cr alloys. The cast Co-Cr alloy specimens showed the highest Vickers hardness, and the CAD/CAM milled specimens revealed the highest tensile strength value. All alloys represent adequate mechanical properties satisfying the ISO standards of dental alloy. (J Korean Acad Prosthodont 2014;52:67-73)
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Fig. 2.
Scanning Electron microscopy images of fractured surfaces for alloys. The characteristics of the cast specimen; unique loose pattern and typical casting porosity, A (×1000, ×2500 original magnification), the laser sintered specimen; coarse granular structure with flat facets like characteristic features, B (×1000, ×2500 original magnification), and the CAD/CAM milled specimen; homogenous and regularly dense microstructure, C (×1000, ×2500 original magnification) are represented.
Table 1.
Chemical composition of cast, laser sintered, and milled Co-Cr alloys as a percentage according to the manufacturer's instructions (wt %)
| Alloys | Co | Cr | Mo | W | Si | Fe | Mn |
|---|---|---|---|---|---|---|---|
| JEWOOS02 (Cast) | 63 | 28 | 5.5 | etc. max. 3.5 | |||
| EOS CobaltChrome SP2 granule (laser sintered) | 63.8 | 24.7 | 5.1 | 5.4 | 1 | max. 0.50 | max. 0.10 |
| Starbond CoS (milled) | 59 | 25 | 3.5 | 9.5 | 1 | max. 1.5 |
Table 2.
Means and standard deviations of Vickers hardness test
| Group | N | Mean (Hv) | SD |
|---|---|---|---|
| Cast | 12 | 455.88 | 37.08 |
| Laser sintered | 12 | 413.10 | 8.77 |
| CAD/CAM milled | 12 | 243.40 | 8.97 |
| Total | 36 | 370.79 | 95.61 |
Table 3.
Means and standard deviations of tensile strength test for the cast, laser sintered, and CAD/CAM milled specimens
| Group | Ultimate tensile strength (MPa) | ∗0.2% Yield strength (MPa) | Elongation (mm) | Elastic modulus (GPa) | |
|---|---|---|---|---|---|
| Cast (n=12) | Mean | 831.51 | 620.67 | 0.59 | 59.0 |
| SD | 41.10 | 20.06 | 0.11 | . | |
| Laser sintered (n=12) | Mean | 1411.12 | 1136.15 | 0.87 | 67.0 |
| SD | 17.00 | 49.10 | 0.14 | . | |
| CAD/CAM milled (n=12) | Mean | 1442.39 | 1014.94 | 1.87 | 61.0 |
| SD | 13.25 | 48.29 | 0.24 | . | |
| Total (n=36) | Mean | 1228.34 | 923.92 | 1.11 | 62.33 |
| SD | 286.06 | 226.78 | 0.59 | . |
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