Microstructures and metal-ceramic bond properties of Co-Cr biomedical alloys fabricated by selective laser melting and casting

Abstract

Selective laser melting (SLM) technique, based on a layer-by-layer production, was used to produce biomedical Co-Cr devices specifically developed for dental restoration applications. However, information on the metal-ceramic bond strength of SLM-manufactured parts was limited. In this study, SLM-fabricated Co-Cr alloy specimens were subjected to subsequent heat treatments (holding at 1150 °C for 1 h). The alloy microstructures were characterized by scanning electron microscopy (SEM) coupled with energy-dispersive x-ray spectroscopy (EDX), x-ray diffractometer (XRD), and transmission electron microscopy, whereas the bond properties were evaluated by 3-point bend tests and coefficient of thermal expansion (CTE) tests, as compared to casting (CAST) alloys. Results showed that SLM specimens exhibited fine grains and homogeneously dispersed intermetallic compounds, and the resultant XRD analysis revealed a predominated face-centered cubic γ-Co phase (72 %). Student's t-test demonstrated that bond strengths of SLM specimens (45.80 ± 1.91 MPa) were significantly lower than that of CAST specimens (54.05 ± 6.77 MPa) (P < 0.05), and SLM specimens with debonded surfaces analyzed via SEM/EDX analysis revealed a mixed fracture type of adhesive and cohesive fractures and lower area fractions of adherence porcelain. While porcelain firing, the microstructural evolution and thermal CTE matching seriously affected the metal-ceramic bond properties. SEM/EDX analysis of the metal-ceramic interface showed that reaction diffusion layers of CAST specimens were clearly thicker than that of SLM specimens, indicating that CAST specimens had a stronger chemical bonding force. Martensite transformation (γ →ε) occurred in the CAST metal matrix close to the interface, which was not found in the SLM specimens. Additionally, the dynamic thermal process revealed that CAST specimens exhibited superior CTE matching. Overall, Co-Cr alloy fabricated by SLM exhibited inferior metal-ceramic bond properties; however, it still meets the requirements for applications as dental prostheses (>25MPa specified in ISO9693).