Effect of two heat treatments on mechanical properties of selective-laser-melted Co-Cr metal-ceramic alloys for application in thin removable partial dentures.

Statement of problem. Heat treatment has been used to reduce the residual stress of alloys fabricated by selective laser melting (SLM) to avoid deformation. Co-Cr metal-ceramic alloys are used to fabricate metal-ceramic restorations and removable partial dentures (RPDs) on the same substrate by SLM. A heat treatment that enables the fabrication of metal-ceramic restorations and RPDs with excellent mechanical properties should be evaluated. Purpose. The purpose of this in vitro study was to determine the effects of 2 heat treatments on the mechanical properties of SLM Co-Cr metal-ceramic alloys intended for the fabrication of thin RPDs. Material and methods. Tensile bars were manufactured using cast metal-ceramics (C-MC group), RPD alloys (C-RPD group), and SLM Co-Cr metal-ceramic alloys. The SLM specimens were subjected to 2 different heat treatments, L1 at 880 degrees C and L2 at 1100 degrees C, and were further divided into subgroups (L1-MC, L1-RPD, L2-MC, and L2-RPD). Thirty-six tensile specimens were prepared in C-RPD, L1-RPD, and L2-RPD (simulated partial denture alloys for clinical use) and in C-MC, L1-MC, and L2-MC (simulated metal-ceramic alloys); 18 metal-ceramic bond strength specimens were prepared in C-MC, L1-MC, and L2-MC groups (n=6). The tensile test and 3-point bend test were conducted using a universal testing machine. The fracture surfaces of the L2-RPD tensile bar were examined using a scanning electron microscope. The Student t test (alpha=.05) was used for statistical analysis. Results. No significant differences were observed between the bond strengths of L1-MC and C-MC (P=.74) or between those of L2-MC and C-MC (P=.124). The 0.2% yield strength (sigma(0)(.2)) and elongation of all SLM specimens exceeded the minimum requirements required for the fabrication of thin RPDs as prescribed in ISO 22674:2016. The sigma(0.2) value of L1-MC and L2-MC was significantly higher than that of C-MC. Significant differences in sigma(0.2) values were found among the 3 RPD groups, L1-RPD>L2-RPD>C-RPD. For the elongation, significant differences were found among the 3 groups, L2-RPD>C-RPD>L1-RPD. The fracture surface of L2-RPD showed clear submicroscale dimples with fusion defects. Conclusions. When Co-Cr metal copings and RPD frameworks were fabricated on the same substrate simultaneously using SLM, heat treatment at 1100 degrees C was found more suitable than at 880 degrees C to release residual stress, considering the toughness required for dental prostheses.