Abstract
Purpose: To compare the mechanical properties and fracture behaviour of laser-sintered/3D-printed cobalt chromium (LS CoCr) with soft-milled cobalt chromium (SM CoCr) to assess their suitability for use in high-stress areas in the oral cavity. Material and Method: Two computer-aided manufacturing methods were used to fabricate dumbbell specimens in accordance with the ASTM standard E8. Specimens were fractured using tensile testing and elastic modulus, and proof stress and ultimate tensile strength were calculated. Fracture surfaces were examined using scanning electron microscopy. Plate specimens were also fabricated for the examination of hardness and elastic modulus using nanoindentation. Unpaired t-test was used to evaluate statistical significance. Results: LS CoCr specimens were found to have significantly higher ultimate tensile strength (UTS) and proof stress (PS) (p < 0.05) but not a significantly higher elastic modulus (p > 0.05). Examination of the dumbbell fracture surfaces showed uniform structure for the LS CoCr specimens whilst the SM CoCr specimens were perforated with porosities; neither showed an obvious point of fracture. Nanoindentation also showed that LS CoCr specimens possessed higher hardness compared with SM CoCr specimens. Conclusion: LS CoCr and SM CoCr specimens were both found to exhibit uniformly dense structure; although porosities were noted in the SM CoCr specimens. LS CoCr specimens were found to have superior tensile properties, likely due to lack of porosities, however both had mean values higher than those reported in the literature for cast CoCr. Uniformity of structure and high tensile strength indicates that LS CoCr and SM CoCr fabricated alloys are suitable for long-span metallic frameworks for use in the field of prosthodontics.
Highlights
Dental prostheses require a material with minimum physical properties to be able to function structurally in the harsh oral environment
Cobalt chromium (CoCr) is a common base metal alloy used in dental prostheses
CoCr alloys were associated with fabrication of metallic frameworks in removable prosthodontics; today its use is frequently extended to fixed prostheses
Summary
Dental prostheses require a material with minimum physical properties to be able to function structurally in the harsh oral environment. In addition to the high level of waste involved in milling techniques [10], the high hardness of CoCr results in extensive wear of milling components when cutting a fully sintered CoCr block [11]. This problem can be alleviated by using a low-stress subtractive technique on softer, wax-bound CoCr powder milling blocks known as soft-milling (SM). The densification that occurs during this process raises the material to its optimal strength [12]
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