Effects of post-treatment on metal-ceramic bond properties of selective laser melted Co-Cr dental alloy. Part 1: Annealing temperature.

Statement Of Problem: Dental cobalt-chromium (Co-Cr) alloy manufactured by selective laser melting (SLM) is not recommended for clinical applications before annealing because of excessive residual stress. However, limited information is available regarding the relationship between annealing temperature and the metal-ceramic bond properties of SLM Co-Cr alloys.

Purpose: The purpose of this in vitro study was to investigate the effects of annealing temperature on the metal-ceramic bond properties of SLM Co-Cr alloys.

Material And Methods: Four groups with different annealing temperatures (850 °C; 950 °C; 1050 °C; 1150 °C) were prepared by using SLM techniques. Bond strengths were measured by using a 3-point bend test; subsequently, debonded surface morphologies and elements were assessed by using a scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The area fraction of adherence porcelain (AFAP) value was introduced to analyze fracture characteristics. Microstructural and interfacial characteristics were characterized by SEM/EDS and X-ray diffraction analysis. The coefficient of thermal expansion (CTE) test was used to analyze thermal matching. A 1-way ANOVA and the Tukey honestly significant difference tests were used to analyze bond strengths and AFAP values statistically (α=.05).

Results: The mean ±standard deviation values of the metal-ceramic bond strengths were 40.68 ±4.34 MPa for the 850 °C group, 37.54 ±5.34 MPa for the 950 °C group, 45.97 ±2.18 MPa for the 1050 °C group, and 50.79 ±1.79 MPa for the 1150 °C group. Significant differences (P<.05) were observed among all groups. Debonded surfaces and AFAP analysis displayed a mixed fracture mode of adhesive and cohesive fracture, and 1150 °C-annealing specimens exhibited better fracture characteristics close to cohesive fractures. As the temperature increased, native oxide film thicknesses remained unchanged; the 850 °C group had the thinnest diffusion layer, while the other 3 groups had similar thicknesses. Although the 1050 °C and 1150 °C groups displayed higher CTE values, their microstructures were more conducive to atomic diffusion and improved chemical bonding. Microstructure analysis found that ε phase and second-phase precipitates jointly affected metal-ceramic bond strength.

Conclusions: Annealing temperatures affected the metal-ceramic bond strengths of SLM Co-Cr porcelain specimens. 1150 °C annealing SLM Co-Cr specimens displayed higher bond strengths and improved fracture and interface characteristics among the 4 groups.