Comparison of microstructures and mechanical properties of 3 cobalt-chromium alloys fabricated with soft metal milling technology

Statement of problem: Although several manufacturers market soft metal milling blanks and systems, comprehensive comparative studies of differences in properties across commercially available soft metal milling alloys are lacking. Purpose: The purpose of this in vitro study was to compare the microstructures and mechanical properties of 3 soft metal milling cobalt-chromium (Co-Cr) alloys (Ceramill Sintron, Soft Metal, and Sintermetall). Material and methods: Disk-shaped specimens (for surface characterization and hardness test) and dumbbell-shaped specimens (for tensile test as per International Organization for Standardization (ISO) 22674) were prepared by following each soft metal milling manufacturer's instructions. The crystal structures and microstructures of the 3 alloys were evaluated with optical microscopy, X-ray diffractometry (XRD), and scanning electron microscopy with electron backscattered diffraction (EBSD). The mechanical properties were investigated with a tensile test and Vickers hardness test (n=6). The results of the mechanical (tensile and hardness) tests were analyzed with 1-way ANOVA and the post hoc Tukey multiple comparison test (α=.05). Results: The Sintermetall specimen showed a finer microstructure and more porosity than the other 2 alloys. The XRD and EBSD analyses showed that the γ (face-centered cubic, fcc) matrix phase was predominant in the Ceramill Sintron alloy and the ε (hexagonal close-packed, hcp) matrix phase was predominant in the Soft Metal alloy. The Sintermetall alloy showed a slightly higher amount of ε phase than γ phase, with more chromium carbide formation than the other 2 alloys. The Ceramill Sintron alloy showed a significantly higher tensile strength than the other 2 alloys (P<.05), but a significantly lower 2% offset yield strength than the other 2 alloys (P<.05). The highest elongation was found in the Ceramill Sintron alloy, followed by the Sintermetall and Soft Metal alloys. The elastic modulus was the highest in the Sintermetall alloy, followed by the Soft Metal and Ceramill Sintron alloys. No significant differences in Vickers hardness values were detected among the 3 alloys (P=.263). Conclusions: The different commercially available soft metal milling blanks and systems produced dissimilar alloys in terms of crystal structures and microstructures and, as a result, different mechanical properties.