Effects of posttreatment on the metal-ceramic bond properties of selective-laser-melted Co-Cr dental alloy-Part 2: Heat treatment after porcelain firing.

STATEMENT OF PROBLEM: The selective laser melting (SLM) technique has been a promising method of fabricating Co-Cr metal-ceramic restorations; however, the lower metal-ceramic bond properties of SLM Co-Cr restorations have become a major issue in clinical use. PURPOSE: The purpose of this in vitro study was to propose and verify a method of improving the metal-ceramic bond properties of SLM Co-Cr alloy with heat treatment after porcelain firing (PH). MATERIAL AND METHODS: Forty-eight (25x3x0.5 mm) Co-Cr specimens, divided into 6 groups (Control group [CG]; 550 degrees C; 650 degrees C; 750 degrees C; 850 degrees C; 950 degrees C) according to PH temperatures, were prepared by using SLM techniques. The 3-point bend tests were performed to evaluate the metal-ceramic bond strengths; subsequently, the fracture feature was assessed by using a digital camera and scanning electron microscope (SEM) coupled with an energy-dispersive X-ray spectroscopy (EDS) detector, to determine the area fraction of adherence porcelain (AFAP). The interface morphologies and element distribution were determined with SEM/EDS detectors. Phase identification and quantification were examined with an X-ray diffractometer (XRD). A 1-way ANOVA and the Tukey honestly significant difference tests were used to analyze bond strengths and AFAP values (alpha=.05). RESULTS: The bond strengths were 35.33 +/-1.25 MPa for the CG group, 34.53 +/-3.20 MPa for the 550 degrees C group, 38.20 +/-2.60 MPa for the 650 degrees C group, 42.85 +/-2.31 MPa for the 750 degrees C group, 33.28 +/-3.85 MPa for the 850 degrees C group, and 29.09 +/-2.86 MPa for the 950 degrees C group. Significant differences were not observed among the CG, 550 degrees C, and 850 degrees C groups (P>.05) but were found among the other groups (P<.05). Fracture and AFAP results displayed a mixed fracture mode of adhesive and cohesive fracture. The thicknesses of native oxide films across the 6 groups were relatively close as the temperature increased, but the thickness of the diffusion layer increased as well. Excessive oxidation and massive phase transformation caused holes and microcracks to appear in the 850 degrees C and 950 degrees C groups, reducing bond strengths. XRD analysis evidenced that the phase transformation of gamma-->epsilon occurred at the interface during PH treating. CONCLUSIONS: PH treatment significantly affected the metal-ceramic bond properties of SLM Co-Cr porcelain specimens. The 750 degrees C-PH-treated specimens displayed higher mean bond strengths and improved fracture characteristics among the 6 groups.