Effects of the rare earth element lanthanum on the metal-ceramic bond strength of dental casting Co-Cr alloys

Statement of problem. The metal-ceramic bond strength of dental casting Co-Cr alloys may be improved by the addition of the rare earth element lanthanum (La), but studies are lacking. Purpose. The purpose of this in vitro study was to evaluate the effects of the rare earth element La on the metal-ceramic bond strength of dental casting Co-Cr alloys. Material and methods. Four groups of specimens with different La content (0; 0.02 wt%; 0.04 wt%; 0.06 wt%) were prepared using conventional casting methods. The metal-ceramic bond strength was assessed by using the 3-point bend test. The microstructures were characterized by metallurgical microscopy, scanning electron microscopy, and X-ray diffraction analyses. The morphology and element distribution of metal-ceramic interfaces were evaluated by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The results of bond strengths were statistically analyzed by the Tukey honest significant difference (HSD) test (alpha=.05). Results. The specimens showed typical dendritic microstructures, few defects, and island-shaped intermetallic compounds rich in Mo and Cr, mainly consisting of alpha-Co phase of a face-centered cubic (FCC) structure and epsilon-Co phase of a close-packed hexagonal (HCP) structure. As the amount of La increased, the thickness of the native oxide layer and the diffusion layer at the interface increased, and the wettability between the oxide layer and the metal matrix improved. The mean +/- standard deviation values of the metal-ceramic bond strengths were 28.11 +/- 4.53 MPa for group 0%, 33.13 +/- 5.65 MPa for group 0.02%, 37.48 +/- 7.86 MPa for group 0.04%, and 40.70 +/- 5.17 MPa for group 0.06%. The statistical analysis indicated that significant differences (P<.05) were observed among all groups tested, except for group 0.04% and 0.06% (P>.05). The debonded surfaces of Co-Cr specimens showed a mixed type of adhesive and cohesive fractures. Conclusions. The microstructures, morphologies, and compositions of oxide films were influenced by the La content, and the La addition could significantly improve the metal-ceramic bond strength of dental casting Co-Cr alloys.